Playing with Joe's Diode Compression OpAmp

Started by mac, April 12, 2006, 10:26:48 PM

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I had some extra time so I built a clone of the Red Fuzz at GGG. After changing the jfet for a ge transistor, I decided to use Joe Davisson's diode compression opamp instead of the LF351, but using ge diodes and transistors... just for fun.
I picked a couple of 2N3906 hfe: 150 and a 2N3904 hfe: 265 first. Then instead of the si diode I used a 1N60 ge diode and two series 4K7 instead of the 6K8 to get 4.6V at the ouptut. I wonder if I had to modify the 100k because of the lower voltage drop but it seems to work fine.  With two series 1N60 the 6K8 is ok.
Then I changed the transistors. A ge npn 2SD352 hfe: 100 leak: 300uA and two almost identical ge pnp 2SB175 hfe:155 leak: 110uA and two series 1N60. I had two change the 6K8 for a 3K3.
Ge diodes gives more fuzz, and even the lower gain than specified transistors, 2N5087 & 2N5089, the amount of gain is good. There is a subtle difference when using one or two ge diodes, but I can not say which I like most.
The all ge version has lower gain as expected, as the npn ge has not much gain. I wish I had a high gain low leakage npn ge.
Maybe I should have to modify the opamp to be npn input, pnp output to use my ge high gain pnp.
Because of the lower gain I can not tell If using three germs is justified.
Despite the ge flavour, the opamp behaves much like a std ic, but the diode in its guts smooth the sound even without external clipping leds. Nice.

I like the opamp as is, but ge diodes gives that ge fuzz tone I am always looking for. But this is my personal taste.
Come and taste the ba... eemmhh ge diodes.

mac@mac-pc:~$ sudo apt-get install ECC83 EL84



Today I tried something different but with a TL071. OK, this not Joe's opamp, but uses one of his techniques. A diode in the signal path just before the TL071 as in the Vulcan:

                    |  470K + 47K (or 470K + trim pot)
                    /        |-
                    |        |   -
    ------|<----------|  ic  ------ ~4.5V
        |                    |   -
        /                    |-
        /  470K
       -- gnd

Adjust the 47K or trim pot to get vcc/2 at the ouput.

The si or ge diode compress the signal entering the opamp, so the behaviour is different than Joe's version. But for those who want to reduce parts or lack enough board space, it is an alternative.

mac@mac-pc:~$ sudo apt-get install ECC83 EL84