On Jan 24, 8:51*am, Engineer wrote:
On Jan 24, 10:24*am, "Stephen Cowell"
wrote:
"Morris Slutsky" wrote
...
I don't know if it's original or not, in the context of a guitar amp,
but I think it's pretty neat.
http://img718.imageshack.us/img718/8...edfeedback.png
I like it! *Two thoughts:
1.) *If you replace one gain stage with this, you'll
* * invert the signal... this can cause problems sometimes.
2.) *Switching the cathode cap out also rolls off lows.
I'd like to hear it in action.
__
Steve
.
Looks good! *More thoughts... (for a flat frequency response
application)
-- use a much larger cap than 0.44 uF, e.g. 100 uF, for cathode
decouplng (and feedback) over a much wider frequency range
Beware of leakage currents in the 100uF if you go this way. You will
hear the click as the leakage current gets sent to the output node.
Since the plate resistor of the second stage is 100K, this is the
impedance you need to assume for the short term effect. IIRC the
12AX7 has a higher plate impedance than this.
A two device amplifier with the feedback like shown is not a
novel idea. It has been used quite a bit.
The wiring of the switched to ground will add some capacitance etc
to that node when the switch is open. This will cause peaking of
the high frequencies. It is better to run wiring at the low
impedance points of the circuit.
With the switch closed, the first section is running the tube at
its full gain. This means that unit to unit variations and the
nonlinear effects of this part will show up in the results.
-- plate load on the 2nd triode is 100K//47K (at least when switch is
closed), or about 32K; this seems a bit low.
-- bypass the 2nd triode cathode resistor (and make it smaller, e,g.
2.7K) for more gain, then increase the above 47K for the right amount
of feedback
This design might serve to switch a single audio input from microphone
gain to AUX gain (not tried)
Cheers,
Roger