[LAU] Ground lifts, TRS, etc. to PA? USB/other signal crossover to audio?
len at ovenwerks.net
Thu May 8 21:06:11 UTC 2014
On Thu, 8 May 2014, Jonathan E Brickman wrote:
> This has to do with my MIDI synth, which is a headless Linux box, often used
> with three Yoshimis running simultaneously. Input is just MIDI, either
> through USB direct from keyboards or through USB MIDI adapters. The problem
> is other really odd high-pitched non-static signals which are contaminating
> the audio, when plugged into any of three different PAs.
The first thing to mention, is that for this discusion, a keyboard that
has USB out that looks to your computer like a midi port is not the same
as midi itself. A 5 pin MIDI out and in are electrically isolated. The
ground in the cable should only be connected at one end (output I think
but some here may know better) and the input end is optical exactly to
make sure there can be no ground problems. USB has a ground and +5 v both
of which can cause signal loops. Most keyboards I have seen are not
grounded... a lot of them being powered by "wall warts". My 80s DX7 does
have a grounded PS, so you want to check that. Also, It will matter if you
use the audio out on the keyboard or not (if you have any).
> About three days ago, reviewing for the umpteenth time the whole
> memory-history of this project, I remembered one thing I did a long time ago
> which seemed to knock out the extra signal altogether (back then, it was a
> 60-cycle hum): I used a Radio Shack inline-audio ground lift device, a 2"
> black barrel with two short double-RCA-male cables. I wore out several sets
Is that what they called it? The normal way of isolating things is with an
audio transformer (passively). often call a DI (direct injection) box with
1/4 inch in and xlr ballanced out. (There are active ones out there too,
but I hate batteries so I won't mention them any more :) ) They come with
ground lift, phase change, pads, etc.
> then (might have been ten years ago), found that Behringer among others is
> making one using 1/4" TS/TRS, ordered the Behringer, tried it last night,
> and voila, lovely gorgeous silence until the Great Key-Pressing!!!
> 1. I would love to be using 1/4" jacks built into the box. But clearly
> clear grounding is a terrible problem. Sounds like I have to have the 1/4"
> jacks on a non-metal plate; do you think?
That would make no difference as most computer audio IF is a signal
against ground. The only way to deal with it is an audio transformer to
create an isolated signal to begin with in which case you want to go to
XLR that can plug right into a PA mic channel. The TRS jacks in the
computer are not ballanced but Stereo.... so next question:
are you using Stereo out? If so you want two audio transformers to two XLR
connectors. You can use adaptors to get two ballanced TRS lines (some PA
line inputs will deal well with these) or a different adaptor if you
really need an unballanced signal. Normally you would have a ground lift
just in case it actually made less noise without the ground lifted :) a
phase switch is probably of no use to you unless you also have your own
amp you use for monitoring.
> 2. What is the real advantage of TRS vs. TS? Some of the audio
> ground-lifts advertise TS/TRS conversion and/or problem resolution; what is
> the deal??? Does TRS handle longer runs better? Whenever I have the case
> plate reengineered, should I have it done with TRS?
Better to use the term ballanced or not. TRS doesn't, on it's own, mean
anything... it could just mean you have stereo out. What you want is
isolation... the ability to have no electrical connection from one unit to
the next. This can be done optically or inductively (I guess capacitors
can work too, but they can have other problems)
> 3. The 96 kHz audio available via Firewire is clearly helpful in the upper
> registers. Fitting it along with a 1/4" double inline audio ground lift
> device into my box, is somewhat less than appealing, especially because the
> metal cases of both the Fireware and the ground lift devices, will be in
> contact with PC case metal, possibly resulting in more contamination. What
> to do ???!???
Assuming you are using stereo, something like this could be mounted to
whatever ground is around
The output connectors are isolated from the case. Many TSR connectors made
for PCboard mounting have no connection to the case except what you
connect as the barrel is plastic. But the costly part is the transformer
in any case.
Any music store (not electronics store or stereo store, but one that sells
music instruments) will have at least one kind of DI box as they have been
a part of connecting powered outputs to PA inputs forever... they were not
new in the 70s when I first started playing.
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