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

[Lamar Owen]

On Tuesday 29 October 2002 10:27 am, Steve Harris wrote:
> On Tue, Oct 29, 2002 at 02:20:32 +0100, Tim Goetze wrote:
> > i've uploaded another image showing how the line output from
> > the amp changes as the incoming sine increases in volume:

> > http://quitte.de/3x-driven.gif

> > admit i hope that frank's explanation holds, and that the
> > valve_rect can be made to produce similar shaping.

> 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.

With the currents in those tubes being at most a few milliamps, and the power 
supply filter caps at 16 microfards, with a choke in the PI arrangement no 
less, it is unlikely this is sag, unless the third filter cap (there's a 
double-pi power supply filter here -- the first stage of the pi, the one with 
the choke, feeds the output tubes.  The second stage, since the current is so 
much lower, uses a resistor in the top leg of the pi, which is perfectly fine 
for this application) is very leaky.

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.

Now, there are two interstage coupling caps of interest.  One is between the 
12AY7 first stage and the cascade connected 12AX7, and the other follows the 
12AX7 cascade.  I can't read the value on the second one. (I know the first 
stage is actually a mixed twin stage -- it is simpler to drop one side of the 
mixer and treat it as a single stage, from the signal's point of view).

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.

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.
-- 
Lamar Owen
WGCR Internet Radio
1 Peter 4:11