Discrete Op Amp: Why is This Noisy?

[Ge_Whiz]

Anybody here know of a schematic for using a quad op-amp to emulate a germanium transistor? 

[Joe Kramer]

Hi Friends,

Glad to see this thread enjoying some longevity.  Finally got some time to remedy the bias/noise problem and here's what the circuit looks like now:

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               |            +------------------+
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               |      +---- | -----------+     |
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          1uF  |   b|/       \|b         |  b|/
   IN >---||---+----|\       /|---+      +---|\
                     e|     |e    |            |c   1uF   36K
                      +--+--+     |            +----||---N N N-+---> OUT
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                    100K Z        |            Z 3.3K          Z 12K
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                         +--------+-----+------+---------------+
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                     (2SC1583)          G    (2N5087)


The bias network on the input xstr is the same ratio as before, only lowered by several decimal points. There is also a 10uF decoupling cap from the divider junction to ground, not shown.  The bias point slipped up a small amount, as per the formula RG writes about, and gauss-markov brought to my attention--thanks!  The whole thing is over-biased by about one volt, but this is not a problem, since the circuit is very forgiving in that respect.  FWIW, I've often found myself over-biasing many xstr circuits anyway, because they sound a little sweeter to my ears that way.

As promised, this fix greatly lowered the noise, and now it's virtually inaudible.  This also lowered the overall noise of the analog delay, of which the above circuit forms the input amp.  Because it feeds a compressor going into the BBD, it's noise tended to be magnified even more, and on long delays, there was a sort of "noise halo" around the repeats. That problem is also solved now.

BTW, if you replace the output resistor divider on this circuit with 50K pot, it makes a very sweet booster.  I'd recommend a trimmer arrangement for the bias, to accommodate whatever xstrs you happen to have on hand.  I don't have a circuit for emulating a Ge xstr with a quad op amp (Ge Whiz), but you could build four of these and emulate a quad op amp with Ge xstrs!  Just substitute a 47K for the 100K input emitter resistor, and a 2.2k for the 3.3K output collector resistor.

Thanks everybody for your help in optimizing this circuit--sounds great now!

Regards,
Joe

[mojotron]

Is anyone interested in ROD-10 clips - I can make some if anyone is interested - let me know.

The ROD-10 is a decent (above average) sounding OD, fairly average distortion in distortion mode, and really nasty fuzz. What I really love about this unit is the EQ - to me is always seemed to be the most intuitive to have a mid-freq and mid-level control.