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What is the purpose of the feedback diode that i see in distortion pedal schematics?
Old 12th October 2013
  #1
Gear Head
 

What is the purpose of the feedback diode that i see in distortion pedal schematics?

It looks like the output of the transitors are fed back into the base of the transistor. Why is this?

Sent from my SAMSUNG-SGH-I777
Old 12th October 2013
  #2
Deleted User
Guest
Do you mean the clipping diodes? Post a schematic of what you are looking at.
Old 12th October 2013
  #3
Gear Head
 

http://home-wrecker.com/bazz.html

Any of these

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Old 12th October 2013
  #4
Quote:
Originally Posted by Awesome View Post
It looks like the output of the transitors are fed back into the base of the transistor. Why is this?

Sent from my SAMSUNG-SGH-I777
The diode limits the current fed to the base, thereby controlling the negative feedback path. Usually, in distortion pedals, you'll see it in parallel with some mid value resistor...
Old 12th October 2013
  #5
Gear Head
 

Why is there any feedback at all though. Cant you just bias the transistor to make it clip? Also dont diodes allow fairly high current for a given v over.7ish

Sent from my SAMSUNG-SGH-I777
Old 12th October 2013
  #6
Lives for gear
 
Grant Ransom's Avatar
 

Diodes in the feedback path are there to clip "softer" or more like a tube, allegedly. I think the combination with a filter/cap enables this in a different way to just level clipping.

The TS (Si) scheme is the most well known AFAIK.
The Distortion+ and Rat etc. simply have the diodes after the opamp, which merely decapitates an amplified signal, though the Distortion+ is germanium and the Rat silicon.

Clipping a transistor with bias would also hard clip, though it'd be "better" with Ge.


http://www.geofex.com/article_folder...ch/tsxtech.htm
Old 12th October 2013
  #7
Quote:
Originally Posted by Awesome View Post
Why is there any feedback at all though. Cant you just bias the transistor to make it clip? Also dont diodes allow fairly high current for a given v over.7ish

Sent from my SAMSUNG-SGH-I777
Excuse me, the RESISTOR limits the current, the diodes clip off the top/bottom (they are usually back to back) of the waveform. I suppose you could just bias a transistor to clip one side of the waveform...think about this: with a 9V supply, you'd usually bias at 4.5, providing for .6-7 on the way in to the base...what would you get if you bias at 6V? Try it... You'll get asymmetry in your waveform with the positive swings clipped, negative left alone. This WILL give you more 2nd harmonic...but I don't think you'll care for the sound, as it'll get too gnarly, too quickly.
Old 12th October 2013
  #8
Also, negative feedback is to provide stability, both reducing the gain and increasing the frequency response
Old 12th October 2013
  #9
Gear Head
 

Ok thanks so much guys. Im gonna do some simulations and experiment. Maybe start building next week. Ill post back with some results

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Old 12th October 2013
  #10
Lives for gear
 
Grant Ransom's Avatar
 

Out of interest, what is your rough goal?

I built a few (simple) pedals, and use them maybe more than any commercial ones... I spent a lot of time experimenting. Great fun.
Old 13th October 2013
  #11
Lives for gear
 

Quote:
Originally Posted by Awesome View Post
Why is there any feedback at all though. Cant you just bias the transistor to make it clip? Also dont diodes allow fairly high current for a given v over.7ish

Sent from my SAMSUNG-SGH-I777
that would be od distortion. It would be interesting to find a semiconductor that would no self distruct clipping that way.

Tubes on the otherhand could take that abuse.

but starving a tube or clipping the signal with a diode is normal.

one fwd and backwards in the emitter circuit is another variation.
seen LED's in this circuit sometimes.
Old 14th October 2013
  #12
Gear Maniac
 
theBF's Avatar
 

I have never built this exact circuit, but here is how I understand it.

First remove the diode: this is a simple class C amplifier. The transistor is turned off hard ( no bias) until a signal at the base exceeds the barrier voltage at the base/emitter junction. Then current will flow for the positive portion of the guitar input signal. The output at the collector then looks like it went through a rectifier but it will be inverted from the input, Flat on the top, half a sine wave on the bottom ( or so )

Add the diode: when the voltage at the collector is higher than the base by barrier voltage of the diode, current flows through the diode into the base. This should limit the gain of the transistor at that point, clipping the bottom of the wave form, now making a square wave at the output.

I will mock this up to confirm.

BF
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