8 Jun
2021
8 Jun
'21
3:43 p.m.
Not strictly a Linux problem, but I hope for advice.
We're using a Behringer X32-Rack mixer for our church PA system. One of
the inputs that we need is
for people to plug in laptops - usually via the headphone socket into a
Stagg dual DI box. We seem
to get a lot of hum pickup on that, which I think is down to the
proximity to the laptops. We have
another identical DI box at the other end of the room which takes
keyboard and bass and I've had no
problems with that. Is there an alternative method we can try? We do
raise the ground lift switch on
the DI box, which reduces the hum somewhat but still get enough hum to
be noticeable when nothing
else is playing. I wondered if the matching transformers in the DI box
are acting as pickups for the
RF noise generated by the laptops. The mixer on EQ display with RTA
shows noise across the audible
spectrum, but most of the audible sound is mains hum.
Bill
--
+----------------------------------------+
| Bill Purvis |
| email: bill(a)billp.org |
+----------------------------------------+
8 Jun
8 Jun
4:21 p.m.
Hello,
Have you tried plugging in the laptop without the DI in between? I think
laptop outputs are balanced, low-impedance, so you shouldn't need the DI
in between. Even if this is not true, it's at least worth a shot.
Brandon Hale
On 6/8/21 7:43 AM, Bill Purvis wrote:
Not strictly a Linux problem, but I hope for advice.
We're using a Behringer X32-Rack mixer for our church PA system. One
of the inputs that we need is
for people to plug in laptops - usually via the headphone socket into
a Stagg dual DI box. We seem
to get a lot of hum pickup on that, which I think is down to the
proximity to the laptops. We have
another identical DI box at the other end of the room which takes
keyboard and bass and I've had no
problems with that. Is there an alternative method we can try? We do
raise the ground lift switch on
the DI box, which reduces the hum somewhat but still get enough hum to
be noticeable when nothing
else is playing. I wondered if the matching transformers in the DI box
are acting as pickups for the
RF noise generated by the laptops. The mixer on EQ display with RTA
shows noise across the audible
spectrum, but most of the audible sound is mains hum.
Bill
4:44 p.m.
Headphone outputs are a strange beast. Unbalanced, for sure (a single signal
lead for each channel, with a common ground return), but much lower impedance
than you would expect for an unbalanced line out.
I've struggled as well, trying to find a consistently reliable DI for this
application. There are several on the market which specifically address this
issue. Search for 'laptop interface' or 'A/V adapter' to find some of the
ones
I've tried.
I've also resorted on occasion to abandoning the headphone jack altogether,
and using an external USB soundcard/interface instead, which provides proper
balanced outputs.
Rick Green
On Tue, 8 Jun 2021, Brandon Hale wrote:
Hello,
Have you tried plugging in the laptop without the DI in between? I think
laptop outputs are balanced, low-impedance, so you shouldn't need the DI in
between. Even if this is not true, it's at least worth a shot.
Brandon Hale
On 6/8/21 7:43 AM, Bill Purvis wrote:
--
Rick Green
We, the People of the United States of America, reject the U.S. Supreme Court's
Citizens United ruling, and move to amend our Constitution to firmly establish
that money is not speech, and that human beings, not corporations, are persons
entitled to constitutional rights.
http://www.MoveToAmend.org
Not strictly a Linux problem, but I hope for
advice.
We're using a Behringer X32-Rack mixer for our church PA system. One of the
inputs that we need is
for people to plug in laptops - usually via the headphone socket into a
Stagg dual DI box. We seem
to get a lot of hum pickup on that, which I think is down to the proximity
to the laptops. We have
another identical DI box at the other end of the room which takes keyboard
and bass and I've had no
problems with that. Is there an alternative method we can try? We do raise
the ground lift switch on
the DI box, which reduces the hum somewhat but still get enough hum to be
noticeable when nothing
else is playing. I wondered if the matching transformers in the DI box are
acting as pickups for the
RF noise generated by the laptops. The mixer on EQ display with RTA shows
noise across the audible
spectrum, but most of the audible sound is mains hum.
Bill
_______________________________________________
Linux-audio-user mailing list
Linux-audio-user(a)lists.linuxaudio.org
https://lists.linuxaudio.org/listinfo/linux-audio-user
7:14 p.m.
On 08/06/2021 13:43, Rick Green wrote:
Headphone outputs are a strange beast. Unbalanced,
for sure (a single
signal lead for each channel, with a common ground return), but much
lower impedance than you would expect for an unbalanced line out.
I've struggled as well, trying to find a consistently reliable DI
for this application. There are several on the market which
specifically address this issue. Search for 'laptop interface' or 'A/V
adapter' to find some of the ones I've tried.
I've also resorted on occasion to abandoning the headphone jack
altogether, and using an external USB soundcard/interface instead,
which provides proper balanced outputs.
Rick Green
There are various groups who use the building, and bring their own
laptop/mobile phone, so most of
the interface devices are a non-starter. It has to be something working
from a 3.5mm TRS plug (and
as it's only a single channel to the mixer, it is effectively a 3.5mm TS
plug!)
Bill
--
+----------------------------------------+
| Bill Purvis |
| email: bill(a)billp.org |
+----------------------------------------+
11:39 p.m.
On Tue, Jun 08, 2021 at 08:21:20AM -0400, Brandon Hale wrote:
Have you tried plugging in the laptop without the DI
in between? I think
laptop outputs are balanced, low-impedance, so you shouldn't need the DI in
between. Even if this is not true, it's at least worth a shot.
Don't do this - ever. You could easily destroy your mic input(s) if the laptop
power supply isn't as isolated as it should be (quite common), or the laptop's
headphone output if there is phantom power on the mic input.
Connecting laptops to a PA or recording system can be one of the most frustrating
things you may come across as a sound engineer. In particular if the laptop's mains
power supply is being used.
You need a DI box designed for this task. Those made for guitar etc. usually
won't do. Transformer isolation and a 'ground lift' switch are a must. This
doesn't mean the DI-box has to be 'passive'. There are a few which have both
transformer isolation AND phantom powered active amplification.
The one I used when working at the Casa della Musica in Parma was the Radial
JPC <https://www.radialeng.com/product/jpc>. It worked perfectly, and I can
really recommend it. Not the cheapest at around $200, but worth every penny.
Ciao,
--
FA
11:54 p.m.
Don't do this - ever. You could easily destroy
your mic input(s) if the laptop
power supply isn't as isolated as it should be (quite common), or the laptop's
headphone output if there is phantom power on the mic input.
That was my bad in
assuming he's not using the microphone inputs, but
the Behringer's aux inputs that have quarter inch jacks.
Brandon Hale
On 6/8/21 3:39 PM, Fons Adriaensen wrote:
> On Tue, Jun 08, 2021 at 08:21:20AM -0400, Brandon Hale wrote:
>
>> Have you tried plugging in the laptop without the DI in between? I think
>> laptop outputs are balanced, low-impedance, so you shouldn't need the DI in
>> between. Even if this is not true, it's at least worth a shot.
Don't do this - ever. You could easily destroy
your mic input(s) if the laptop
power supply isn't as isolated as it should be (quite common), or the laptop's
headphone output if there is phantom power on the mic input.
>
> Connecting laptops to a PA or recording system can be one of the most frustrating
> things you may come across as a sound engineer. In particular if the laptop's
mains
> power supply is being used.
>
> You need a DI box designed for this task. Those made for guitar etc. usually
> won't do. Transformer isolation and a 'ground lift' switch are a must.
This
> doesn't mean the DI-box has to be 'passive'. There are a few which have
both
> transformer isolation AND phantom powered active amplification.
>
> The one I used when working at the Casa della Musica in Parma was the Radial
> JPC <https://www.radialeng.com/product/jpc>. It worked perfectly, and I can
> really recommend it. Not the cheapest at around $200, but worth every penny.
>
> Ciao,
>
9 Jun
9 Jun
12:09 a.m.
On Tue, Jun 08, 2021 at 03:54:31PM -0400, Brandon Hale wrote:
Wouldn't make much difference. If the laptop is mains powered and happens
not to be grounded you could easily have a hundred volts on the laptop's
'ground'. High impedance, almost 'static' and hence not lethal to humans,
but more than enough to kill an audio input. Watched it happen more than
once, the more spectacular case was when a certain university professor
deciced to cut of the ground connection of his laptop's power supply in
order to get rid of the hum. He didn't do that again.
Anyway, without transformer isolation and separate grounds you will never
get rid of the broadband interference generated by both the laptop itself
and its switching power supply.
--
FA
Don't do
this - ever. You could easily destroy your mic input(s) if the laptop
power supply isn't as isolated as it should be (quite common), or the laptop's
headphone output if there is phantom power on the mic input.
That was my bad in assuming he's not using the microphone inputs, but the
Behringer's aux inputs that have quarter inch jacks.
12:15 a.m.
I'll definitely remember this, thank you Fons!
Brandon Hale
On 6/8/21 4:09 PM, Fons Adriaensen wrote:
On Tue, Jun 08, 2021 at 03:54:31PM -0400, Brandon Hale
wrote:
Wouldn't make much
difference. If the laptop is mains powered and happens
not to be grounded you could easily have a hundred volts on the laptop's
'ground'. High impedance, almost 'static' and hence not lethal to
humans,
but more than enough to kill an audio input. Watched it happen more than
once, the more spectacular case was when a certain university professor
deciced to cut of the ground connection of his laptop's power supply in
order to get rid of the hum. He didn't do that again.
Anyway, without transformer isolation and separate grounds you will never
get rid of the broadband interference generated by both the laptop itself
and its switching power supply.
Don't
do this - ever. You could easily destroy your mic input(s) if the laptop
power supply isn't as isolated as it should be (quite common), or the laptop's
headphone output if there is phantom power on the mic input.
That was my bad in
assuming he's not using the microphone inputs, but the
Behringer's aux inputs that have quarter inch jacks. 
1:05 a.m.
Fons Adriaensen <fons(a)linuxaudio.org> writes:
On Tue, Jun 08, 2021 at 03:54:31PM -0400, Brandon Hale
wrote:
Wouldn't make much difference. If the laptop is mains powered and happens
not to be grounded you could easily have a hundred volts on the laptop's
'ground'. High impedance, almost 'static' and hence not lethal to
humans,
but more than enough to kill an audio input. Watched it happen more than
once, the more spectacular case was when a certain university professor
deciced to cut of the ground connection of his laptop's power supply in
order to get rid of the hum. He didn't do that again.
Puts me back when a physics professor in a lecture did not get a
volunteer for entering a Faraday cage connected to a band generator
producing impressive sparks and then went in himself.
That was at a time when Lavalier microphones were not yet customarily
wireless...
Static electricity, sure, and the assistant stopped the band generator
almost immediately again. It didn't help getting volunteers...
Don't
do this - ever. You could easily destroy your mic input(s) if the laptop
power supply isn't as isolated as it should be (quite common), or the laptop's
headphone output if there is phantom power on the mic input.
That was my bad in assuming he's not using the microphone inputs, but the
Behringer's aux inputs that have quarter inch jacks. Anyway, without transformer isolation and separate
grounds you will
never get rid of the broadband interference generated by both the
laptop itself and its switching power supply.
--
David Kastrup
6:04 a.m.
Le 2021-06-08 à 17 h 05, David Kastrup a écrit :
Puts me back when a physics professor in a lecture did
not get a
volunteer for entering a Faraday cage connected to a band generator
producing impressive sparks and then went in himself.
"Prof Jim Al-Khalili survives nearly a million volts of electricity!"
https://www.bbc.co.uk/programmes/p00l30l1
That was at a time when Lavalier microphones were not
yet customarily
wireless...
Static electricity, sure, and the assistant stopped the band generator
almost immediately again. It didn't help getting volunteers...
Professors have special powers: they know...
Marc
13 Jun
13 Jun
9:01 a.m.
On 08/06/2021 21:09, Fons Adriaensen wrote:
On Tue, Jun 08, 2021 at 03:54:31PM -0400, Brandon Hale
wrote:
Wouldn't make much
difference. If the laptop is mains powered and happens
not to be grounded you could easily have a hundred volts on the laptop's
'ground'. High impedance, almost 'static' and hence not lethal to
humans,
but more than enough to kill an audio input. Watched it happen more than
once, the more spectacular case was when a certain university professor
deciced to cut of the ground connection of his laptop's power supply in
order to get rid of the hum. He didn't do that again.
Anyway, without transformer isolation and separate grounds you will never
get rid of the broadband interference generated by both the laptop itself
and its switching power supply.
I bought a Behringer ULTRA_DI DI20 active DI box
and that seems to work
well.
Only thing is the Ground Lift switch seems the wrong way round to me. In the
'lift' position I get hum when feeding from the laptop with mains power, but
in the ground position the hum disappears. No hum either way on battery
power.
Thanks for all the responses!
Bill
--
+----------------------------------------+
| Bill Purvis |
| email: bill(a)billp.org |
+----------------------------------------+
Don't
do this - ever. You could easily destroy your mic input(s) if the laptop
power supply isn't as isolated as it should be (quite common), or the laptop's
headphone output if there is phantom power on the mic input.
That was my bad in
assuming he's not using the microphone inputs, but the
Behringer's aux inputs that have quarter inch jacks.
1:44 p.m.
On Sun, Jun 13, 2021 at 06:00:59AM +0100, Bill Purvis wrote:
I bought a Behringer ULTRA_DI DI20 active DI box and
that seems to work well.
Only thing is the Ground Lift switch seems the wrong way round to me. In the
'lift' position I get hum when feeding from the laptop with mains power, but
in the ground position the hum disappears. No hum either way on battery
power.
That's to be expected when you don't have transformer isolation. It also means
you've been lucky this time, things may not work out so well with another
laptop or in other conditions (e.g. laptop on stage on a different mains
circuit and light dimmers all around).
Ciao,
--
FA
8 Jun
8 Jun
4:48 p.m.
Bill Purvis <bill(a)billp.org> writes:
Not strictly a Linux problem, but I hope for advice.
We're using a Behringer X32-Rack mixer for our church PA system. One
of the inputs that we need is
for people to plug in laptops - usually via the headphone socket into
a Stagg dual DI box. We seem
to get a lot of hum pickup on that, which I think is down to the
proximity to the laptops.
Unlikely. You are connecting the Lo-Z outputs via two XLR cables to the
mixer, right?
The main question is just _how_ you are connecting your laptop to the
DI since the connectors are not identical and loads of adapters exist.
The correct way would use a 3.5mm TRS connector into the laptop (it is
important that it be TRS and not TRRS because laptops with combined
microphone/headphone socket sometimes have the microphone on the second
ring, sometimes on the shield and misusing the semi-floating microphone
input as ground would cause serious hum) and two 6.3mm TS plugs into the
DI (with S taken from the 3.5mm TRS plug, and T of one 6.3mm TS plug
taken from T of the TRS plug, and T on the other taken from R of the TRS
plug). Sometimes one needs to piece a few adapters together to arrive
at such a configuration, and it is important that those adapters don't
just have the right plugs but also the correct wiring including what to
use for shielding.
So what adapters _are_ you using here?
We have another identical DI box at the other end of
the room which
takes keyboard and bass and I've had no problems with that. Is there
an alternative method we can try? We do raise the ground lift switch
on the DI box, which reduces the hum somewhat but still get enough hum
to be noticeable when nothing else is playing. I wondered if the
matching transformers in the DI box are acting as pickups for the RF
noise generated by the laptops. The mixer on EQ display with RTA shows
noise across the audible spectrum, but most of the audible sound is
mains hum.
--
David Kastrup
7:01 p.m.
On 08/06/2021 13:48, David Kastrup wrote:
Bill Purvis <bill(a)billp.org> writes:
3.5mm TRS plug both ends into a
3.5mm-1.4" TRS jack, into the (mono) DI
box input.
I've also tried a lead with 3.5mm TRS wired to a 1/4" mono jack plug
(effectively
just the TS connections). Positioning of the DI box affects matters,
moving it
well away from the desk reduces the hum to negligible level, but this is
not
acceptable for the regular usage. Usage is mono-only.
Bill
--
+----------------------------------------+
| Bill Purvis |
| email: bill(a)billp.org |
+----------------------------------------+
Not strictly a Linux problem, but I hope for
advice.
We're using a Behringer X32-Rack mixer for our church PA system. One
of the inputs that we need is
for people to plug in laptops - usually via the headphone socket into
a Stagg dual DI box. We seem
to get a lot of hum pickup on that, which I think is down to the
proximity to the laptops.
Unlikely. You are connecting the Lo-Z outputs via two
XLR cables to the
mixer, right?
The main question is just _how_ you are connecting your laptop to the
DI since the connectors are not identical and loads of adapters exist.
The correct way would use a 3.5mm TRS connector into the laptop (it is
important that it be TRS and not TRRS because laptops with combined
microphone/headphone socket sometimes have the microphone on the second
ring, sometimes on the shield and misusing the semi-floating microphone
input as ground would cause serious hum) and two 6.3mm TS plugs into the
DI (with S taken from the 3.5mm TRS plug, and T of one 6.3mm TS plug
taken from T of the TRS plug, and T on the other taken from R of the TRS
plug). Sometimes one needs to piece a few adapters together to arrive
at such a configuration, and it is important that those adapters don't
just have the right plugs but also the correct wiring including what to
use for shielding.
So what adapters _are_ you using here?
7:46 p.m.
Bill Purvis <bill(a)billp.org> writes:
On 08/06/2021 13:48, David Kastrup wrote:
That doesn't parse. What does "both ends" mean? What does
"3.5mm-1.4"
TRS jack" mean, apart from 1.4" probably being a typo for 1/4"? What
kind of socket is there, what kind of plug?
Bill Purvis <bill(a)billp.org> writes:
3.5mm TRS plug both ends into a
3.5mm-1.4" TRS jack, Not strictly a Linux problem, but I hope for
advice.
We're using a Behringer X32-Rack mixer for our church PA system. One
of the inputs that we need is
for people to plug in laptops - usually via the headphone socket into
a Stagg dual DI box. We seem
to get a lot of hum pickup on that, which I think is down to the
proximity to the laptops.
Unlikely. You are connecting the Lo-Z outputs via two
XLR cables to the
mixer, right?
The main question is just _how_ you are connecting your laptop to the
DI since the connectors are not identical and loads of adapters exist.
The correct way would use a 3.5mm TRS connector into the laptop (it is
important that it be TRS and not TRRS because laptops with combined
microphone/headphone socket sometimes have the microphone on the second
ring, sometimes on the shield and misusing the semi-floating microphone
input as ground would cause serious hum) and two 6.3mm TS plugs into the
DI (with S taken from the 3.5mm TRS plug, and T of one 6.3mm TS plug
taken from T of the TRS plug, and T on the other taken from R of the TRS
plug). Sometimes one needs to piece a few adapters together to arrive
at such a configuration, and it is important that those adapters don't
just have the right plugs but also the correct wiring including what to
use for shielding.
So what adapters _are_ you using here? into the (mono) DI box input.
Any TRS plug into the mono DI box input is unpredictable since there are
no guaranties which of RS will (if any) make a reliable connection to
the T of the socket and you have no guarantee that R will not get
shorted.
I've also tried a lead with 3.5mm TRS wired to a
1/4" mono jack plug
(effectively just the TS connections).
That should work for the left channel. What does "wired" mean in this
context? You need to make this connection with a shielded cable, with
the T-T connection being the core of the cable and the S-S connection
being the shield. If you just put in wires, that's where your noise
injection will happen.
Positioning of the DI box affects matters, moving it
well away from
the desk reduces the hum to negligible level, but this is not
acceptable for the regular usage. Usage is mono-only.
Meaning you want to attach to both L and R of the laptop (creating a mid
signal) or to just L?
--
David Kastrup
8:15 p.m.
On 08/06/2021 16:46, David Kastrup wrote:
I've been wiring thing for over 70 years. I know enough
to use screened
wiring. In fact the cable came with the 3.5mm plug, I chopped the other end,
checked which one goes to the tip and wired that the T of the 1/4" jack.
Not really bothered which, but I guess it's usually the
L. A mix would
be nice,
but clearly needs more subtle treatment. No-one is going to know the
difference
here. The setup makes no pretense of being stereo! Four channels out,
all the same!
Bill
--
+----------------------------------------+
| Bill Purvis |
| email: bill(a)billp.org |
+----------------------------------------+
Bill Purvis <bill(a)billp.org> writes:
1.4"->1/4" typo!
It's a combined 3.5mm socket and 1/4" plug, both TRS.
On 08/06/2021 13:48, David Kastrup wrote:
That doesn't parse. What does "both ends"
mean? What does "3.5mm-1.4"
TRS jack" mean, apart from 1.4" probably being a typo for 1/4"? What
kind of socket is there, what kind of plug? Bill Purvis <bill(a)billp.org> writes:
3.5mm TRS plug both ends into a
3.5mm-1.4" TRS jack, Not strictly a Linux problem, but I hope for
advice.
We're using a Behringer X32-Rack mixer for our church PA system. One
of the inputs that we need is
for people to plug in laptops - usually via the headphone socket into
a Stagg dual DI box. We seem
to get a lot of hum pickup on that, which I think is down to the
proximity to the laptops.
Unlikely. You are connecting the Lo-Z outputs via two
XLR cables to the
mixer, right?
The main question is just _how_ you are connecting your laptop to the
DI since the connectors are not identical and loads of adapters exist.
The correct way would use a 3.5mm TRS connector into the laptop (it is
important that it be TRS and not TRRS because laptops with combined
microphone/headphone socket sometimes have the microphone on the second
ring, sometimes on the shield and misusing the semi-floating microphone
input as ground would cause serious hum) and two 6.3mm TS plugs into the
DI (with S taken from the 3.5mm TRS plug, and T of one 6.3mm TS plug
taken from T of the TRS plug, and T on the other taken from R of the TRS
plug). Sometimes one needs to piece a few adapters together to arrive
at such a configuration, and it is important that those adapters don't
just have the right plugs but also the correct wiring including what to
use for shielding.
So what adapters _are_ you using here? into the (mono) DI box input.
Any TRS plug into the mono DI box input is unpredictable since there are
no guaranties which of RS will (if any) make a reliable connection to
the T of the socket and you have no guarantee that R will not get
shorted.
The 1/4" sockets o9n the D/I box are the standard TRS plastic
sockets,
with the R
wiper omitted. I did check!
I've also
tried a lead with 3.5mm TRS wired to a 1/4" mono jack plug
(effectively just the TS connections).
That should work for the left channel.
What does "wired" mean in this
context? You need to make this connection with a shielded cable, with
the T-T connection being the core of the cable and the S-S connection
being the shield. If you just put in wires, that's where your noise
injection will happen. Positioning of
the DI box affects matters, moving it well away from
the desk reduces the hum to negligible level, but this is not
acceptable for the regular usage. Usage is mono-only.
Meaning you want to attach
to both L and R of the laptop (creating a mid
signal) or to just L?
8:29 p.m.
We have the Behringer x32 at the place of my work. Why not just use a
3.5mm male to RCA L+R male out and plug into the aux input on the
Behringer? That's what those inputs are for, right? That's what we do at
work and have had success with it running small conferences with little
to no audible noise.
I hope I'm not muddying the waters more here,
Brandon Hale
On 6/8/21 7:43 AM, Bill Purvis wrote:
Not strictly a Linux problem, but I hope for advice.
We're using a Behringer X32-Rack mixer for our church PA system. One
of the inputs that we need is
for people to plug in laptops - usually via the headphone socket into
a Stagg dual DI box. We seem
to get a lot of hum pickup on that, which I think is down to the
proximity to the laptops. We have
another identical DI box at the other end of the room which takes
keyboard and bass and I've had no
problems with that. Is there an alternative method we can try? We do
raise the ground lift switch on
the DI box, which reduces the hum somewhat but still get enough hum to
be noticeable when nothing
else is playing. I wondered if the matching transformers in the DI box
are acting as pickups for the
RF noise generated by the laptops. The mixer on EQ display with RTA
shows noise across the audible
spectrum, but most of the audible sound is mains hum.
Bill
9:12 p.m.
Brandon Hale <bthaleproductions(a)gmail.com> writes:
We have the Behringer x32 at the place of my work. Why
not just use a
3.5mm male to RCA L+R male out and plug into the aux input on the
Behringer? That's what those inputs are for, right? That's what we do
at work and have had success with it running small conferences with
little to no audible noise.
I hope I'm not muddying the waters more here,
That's what I'd use at well except that "church PA" and "other end
of
the room" sounds like a significant distance that benefits from
symmetric transmission.
If the laptop is close to the mixer and only one laptop at a time needs
to be connected, that might be the most straightforward way.
--
David Kastrup
11:01 p.m.
That's what I'd use at well except that
"church PA" and "other end of
the room" sounds like a significant distance that benefits from
symmetric transmission.
If the laptop is close to the mixer and only one laptop at a time needs
to be connected, that might be the most straightforward way.
These are great
points. I don't think running RCA cables across the
entire room would be a good idea. Haha. :D Bill said he can't get to the
aux ins on the mixer anyways.
On 6/8/21 1:12 PM, David Kastrup wrote:
> Brandon Hale <bthaleproductions(a)gmail.com> writes:
>
>> We have the Behringer x32 at the place of my work. Why not just use a
>> 3.5mm male to RCA L+R male out and plug into the aux input on the
>> Behringer? That's what those inputs are for, right? That's what we do
>> at work and have had success with it running small conferences with
>> little to no audible noise.
>>
>> I hope I'm not muddying the waters more here,
That's what I'd use at well except that
"church PA" and "other end of
the room" sounds like a significant distance that benefits from
symmetric transmission.
If the laptop is close to the mixer and only one laptop at a time needs
to be connected, that might be the most straightforward way.
>
10:53 p.m.
On Tue, June 8, 2021 6:43 am, Bill Purvis wrote:
Stagg dual DI box.
Is it this device?
https://staggmusic.com/en/products/view/SDIST-2-channel-passive-di-box-with…
I don't see any specifications, so I'll just guess that it is a pair of
low cost transformers inside the case.
David Kastrup's advice about verifying the adapters cables is probably the
first place to start.
to get a lot of hum pickup on that, which I think is
down to the
proximity to the laptops.
The only mains frequency noise around a laptop would be the power supply,
which you should be able to move farther away since it is on a cord, and
probably the LCD screen refresh rate is running at 60Hz.
If it is a proximity problem because the DI transformer is not shielded,
you should be able to tell by using a longer cable between the laptop and
the DI box and just moving the box farther away. Put the AC power supply
farther away, or just remove it temporarily and run on battery, and if
that is not the problem put the box farther away from the laptop screen
(or change the power settings to only power off the screen when you close
the laptop, not put the entire laptop to sleep, and just close the lid).
keyboard and bass and I've had no problems with
that.
Bass should be easy because it does not have a connection to AC power
supply, and most keyboards are low enough power that they have an isolated
power supply with only two pin power entry, no third earth pin, so they
are effectively floating sources and so pretty easy as well. Computers
are almost all high enough power that they have a connection to safety
earth, which you can get rid of temporarily for troubleshooting by running
on battery power.
raise the ground lift switch on the DI box, which
reduces the hum
somewhat but still get enough hum to be noticeable when nothing
else is playing.
Have you made sure that the volume controls on the laptop are at maximum
so you can optimize the gain structure? If you have not, you may be able
to make the hum less noticeable by turning down the gain at the mixer and
turning up the output from the laptop.
I wondered if the matching transformers in the DI box
are acting as pickups for the RF noise generated by the laptops.
If you are seeing mostly 60Hz noise probably not RF, but could still be
magnetically coupled from the laptop or the laptop power supply.
--
Chris Caudle
11:12 p.m.
On Tue, 8 Jun 2021 13:52:59 -0500
"Chris Caudle" <6807.chris(a)pop.powweb.com> wrote:
On Tue, June 8, 2021 6:43 am, Bill Purvis wrote:
A while ago I was surprised to find that some very cheap USB modules (mic in
phones out) to my great surprise *totally* isolated the audio from the USB. I
don't use the mic, just the stereo out (at 48k) Even though it's on a 3.5 TRS
the quality is good enough for all but the most demanding purposes. They go by
the name ugreen.
--
Will J Godfrey
https://willgodfrey.bandcamp.com/
http://yoshimi.github.io
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.
Stagg dual DI box.
Is it this device?
https://staggmusic.com/en/products/view/SDIST-2-channel-passive-di-box-with…
I don't see any specifications, so I'll just guess that it is a pair of
low cost transformers inside the case.
David Kastrup's advice about verifying the adapters cables is probably the
first place to start.
to get a lot of hum pickup on that, which I think
is down to the
proximity to the laptops.
The only mains frequency noise around a laptop would be the power supply,
which you should be able to move farther away since it is on a cord, and
probably the LCD screen refresh rate is running at 60Hz.
If it is a proximity problem because the DI transformer is not shielded,
you should be able to tell by using a longer cable between the laptop and
the DI box and just moving the box farther away. Put the AC power supply
farther away, or just remove it temporarily and run on battery, and if
that is not the problem put the box farther away from the laptop screen
(or change the power settings to only power off the screen when you close
the laptop, not put the entire laptop to sleep, and just close the lid).
keyboard and bass and I've had no problems
with that.
Bass should be easy because it does not have a connection to AC power
supply, and most keyboards are low enough power that they have an isolated
power supply with only two pin power entry, no third earth pin, so they
are effectively floating sources and so pretty easy as well. Computers
are almost all high enough power that they have a connection to safety
earth, which you can get rid of temporarily for troubleshooting by running
on battery power.
raise the ground lift switch on the DI box, which
reduces the hum
somewhat but still get enough hum to be noticeable when nothing
else is playing.
Have you made sure that the volume controls on the laptop are at maximum
so you can optimize the gain structure? If you have not, you may be able
to make the hum less noticeable by turning down the gain at the mixer and
turning up the output from the laptop.
I wondered if the matching transformers in the DI
box
are acting as pickups for the RF noise generated by the laptops.
If you are seeing mostly 60Hz noise probably not RF, but could still be
magnetically coupled from the laptop or the laptop power supply.
11:44 p.m.
On June 8, 2021 9:11:56 AM HST, Will Godfrey <willgodfrey(a)musically.me.uk> wrote:
On Tue, 8 Jun 2021 13:52:59 -0500
"Chris Caudle" <6807.chris(a)pop.powweb.com> wrote:
I have a powered USB2 hub here. When its power supply is connected, I get some hum or
noise. I don't know if that's a possibility here.
My old band's sound set up - 12-channel Firewire audio, compressor, amp, in a single
rolling cabinet - had a rackmount power conditioner that powered everything, including the
mixer. Maybe something like that would help?
---
David W. Jones
gnome(a)hawaii.rr.com
authenticity, honesty, community
http://dancingtreefrog.com
Sent from my Android device with F/LOSS K-9 Mail.
On Tue, June 8, 2021 6:43 am, Bill Purvis wrote:
of
>low cost transformers inside the case.
>
>David Kastrup's advice about verifying the adapters cables is
> probably the first place to start.
>
> > to get a lot of hum pickup on that, which I think is down to the
> > proximity to the laptops.
>
>The only mains frequency noise around a laptop would be the power
> supply, Stagg dual DI box.
Is it this device?
https://staggmusic.com/en/products/view/SDIST-2-channel-passive-di-box-with…
I don't see any specifications, so I'll just guess that it is a pair
11:53 p.m.
On 08/06/2021 19:52, Chris Caudle wrote:
50 Hz here in the
UK!
I don't see it much but it's the main sound source. The mixer RTA
display shows
sound input across the spectrum.
I'm going to try a direct input, but need to make up a 3.5mm - XLR cable
for that.
Something to try tomorrow!
Bill
--
+----------------------------------------+
| Bill Purvis |
| email: bill(a)billp.org |
+----------------------------------------+
On Tue, June 8, 2021 6:43 am, Bill Purvis wrote:
That's the one!
Stagg dual DI box.
Is it this device?
https://staggmusic.com/en/products/view/SDIST-2-channel-passive-di-box-with…
I don't see any specifications, so I'll just
guess that it is a pair of
low cost transformers inside the case.
That's what is inside.
David Kastrup's advice about verifying the
adapters cables is probably the
first place to start.
That does
help. I'm investigating if I can place the DI box at the base
of the desk
which is as far as I can get and still have it within the confines of
the desk.
to get a lot of hum pickup on that, which I think
is down to the
proximity to the laptops.
The only mains frequency noise around a laptop would be
the power supply,
which you should be able to move farther away since it is on a cord, and
probably the LCD screen refresh rate is running at 60Hz.
If it is a proximity problem because the DI transformer is not shielded,
you should be able to tell by using a longer cable between the laptop and
the DI box and just moving the box farther away. Put the AC power supply
farther away, or just remove it temporarily and run on battery, and if
that is not the problem put the box farther away from the laptop screen
(or change the power settings to only power off the screen when you close
the laptop, not put the entire laptop to sleep, and just close the lid). keyboard and bass and I've had no problems with
that.
Bass should be easy because it does not have a connection to AC power
supply, and most keyboards are low enough power that they have an isolated
power supply with only two pin power entry, no third earth pin, so they
are effectively floating sources and so pretty easy as well. Computers
are almost all high enough power that they have a connection to safety
earth, which you can get rid of temporarily for troubleshooting by running
on battery power.
Slight problem there - the laptop that controls the mixer is so
ancient
the battery
is kaput! It will only work with the power cable in.
raise the
ground lift switch on the DI box, which reduces the hum
somewhat but still get enough hum to be noticeable when nothing
else is playing.
Have you made sure that the volume controls on the laptop are at
maximum
so you can optimize the gain structure? If you have not, you may be able
to make the hum less noticeable by turning down the gain at the mixer and
turning up the output from the laptop.
I wondered if the matching transformers in the DI
box
are acting as pickups for the RF noise generated by the laptops.
If you are seeing
mostly 60Hz noise probably not RF, but could still be
magnetically coupled from the laptop or the laptop power supply.
11:43 p.m.
On Tue, June 8, 2021 6:43 am, Bill Purvis wrote:
a lot of hum pickup on that, which I think is down to
the
proximity to the laptops.
The advice from David Kastrup about verifying the wiring of your cables is
a good place to start.
But it is certainly likely that a low cost passive DI would have
transformers with no or poor shielding. Proximty related problems should
be really easy to test, though, get a longer cable and move the box
farther away.
else is playing. I wondered if the matching
transformers in the DI box
are acting as pickups for the
RF noise generated by the laptops. The mixer on EQ display with RTA
shows noise across the audible
spectrum, but most of the audible sound is mains hum.
Not likely to be RF pickup if it is mostly 60/120/180 Hz noise. Could be
multiple things going on, so you will just have to be systematic and note
which things help, and does it help all of the noise, just the mains hum,
or just the higher frequency components of the noise.
There are two main places you find 60Hz in a laptop, the power supply, and
the screen refresh rate. If you don't have a longer cable for the laptop
to DI box, you do have a cable between the power supply and the laptop, so
start by moving that farther away from the DI box. Does it help or not?
Unless your battery is worn out, you can also remove the AC power supply
from the situation entirely, so there is no mains power involved. Does
that completely get rid of the problem or not? Does it get rid of the
60Hz hum, but not higher frequency buzzes?
To check screen refresh signals you can move the DI box farther away,
change the orientation between the DI box and the laptop, and modify the
laptop power settings so that when you close the lid it only turns off the
display and doesn't put the entire laptop to sleep. Close the lid and see
what happens to the noise when the screen goes off.
One other thing to verify, and this really should be first: either get
someplace quiet, or use some sensitive headphones, and make sure the noise
isn't always present on the headphone outputs. On my laptop I hear all
kinds of noise on the headphone output when I use Shure in-ear phones,
which are both relatively sensitive and also block out external sound
pretty well. There is no way I would want to take that signal and amplify
it to play over a large sound system, I can hear varying noise levels as
different applications start and stop, as the screen pattern changes, all
kinds of things leak into the headphone signal at a low level. Mostly
covered by the audio when playing, but on quiet sections or when nothing
is playing it is defintely noticeable, so no magic DI box is going to get
rid of something that is present in the source.
--
Chris Caudle
1351
days inactive
1356
days old
linux-audio-user@lists.linuxaudio.org
23 comments
9 participants
participants (9)
-
Bill Purvis -
Brandon Hale -
Chris Caudle -
David Kastrup -
David W. Jones -
Fons Adriaensen -
Marc Lavallée -
Rick Green -
Will Godfrey