[linux-audio-dev] [ann] unmatched - a LADSPA amp tone

[Tim Goetze]

Lamar Owen wrote:

>> > http://quitte.de/3x-driven.gif
...
>> Hmmm... if that is sag from the powersupply then it kicks in much lower
>> than I though. Another thing I guessed wrongly. Should make it easier to
>> implement though :) - more margin of error.
>
>I should have read this one before replying previously.  This most certainly 
>looks like the result of undersize interstage coupling caps -- a high pass 
>filter.

ok, with all of you pointing in that direction, i took a look
at the waveforms a valve_1209 and a hipass produce for a sine. 
at lower distortion levels, they do in fact come really close. 
no surprise there, the combination is proven to sound good.

comparing the frequency spectra of our setup to the amp output
reveals that there is quite a difference. it is visible at low
gain settings and, as expected, becomes emphasized at higher
distortion levels. 

the amp output frequency spectrum changes quite noticeably the
more gain we apply. i've uploaded plots relating to the same
waves that are shown in '3x-driven' to

http://quitte.de/3x-spectrum.gif

(the first spike is unrelated 50 Hz hum if i'm not badly 
 mistaken.)

maybe it would be possible to use two or more chebyshevs to
create the harmonics needed to emulate this behaviour, and then
blend between them according to the simulated gain?

>If the power supply was sagging, the 6L6's would drop first, being that they 
>need more current.  You can measure the voltage rails to see (using 
>appropriate caution, since there is lethal voltage in this circuit) if they 
>sag.

the waves shown in the above image were taken from the amp's 
line-out with the main volume at min, so it's very unlikely 
power supply sag is an important factor i come to think.

>Can a trace be made in the middle?  I'm looking for a signal trace situation 
>here -- the signal after each active component.  Of course, you need a scope 
>that can handle the voltage there, or build a series capacitor audio probe 
>first.

i'm afraid that's not an option -- the only scope i call my own
is a pure software solution; therefore i'm limited to what i
can feed an audio interface. would this series capacitor thing 
be capable of scaling down the voltages appropriately?

>While much of the 'tube sound' comes from tubes, much more comes from the 
>types of components used in tube circuits -- the circuit impedance dictates 
>the technology of the capacitor, for instance.  The 'tube sound' can be had 
>with FET's, as long as they are biased right and you use depletion mode 
>devices.

in fact there are some pretty good analog stomp boxes out there
that sound really valvy. i bet they make use of FETs then.

tim