Preamp from Marshall Compressor Bass Amp 4160

[kagliostro]

The idea is to copy the whole preamp and use one SS Power Section

the problem is I didn't understand well the Compression (Whidth & Ratio) circuit

is correct the reduced circuit attached ?

connecting R26 to a tap in the secondary of the OT

is enough to have the proper functioning of the whole preamp ?

or the Compression circuit also uses the PI circuit ?

Thanks

K

[sluckey]

connecting R26 to a tap in the secondary of the OT is enough to have the proper functioning of the whole preamp ?

yes

or the Compression circuit also uses the PI circuit ?

no

Point "F" is the signal source from the OT secondary. That signal feeds into two totally unrelated circuits. It feeds into the LTP PI for NFB loop just like in many other familiar NFB circuits.

But it also feeds the compressor circuit which operates totally independent of the NFB circuit. The signal is rectified and the resulting dc voltage (whose amplitude is proportional to the signal amplitude sent to the speaker) is applied to a FET. The FET acts like a voltage controlled resistor to vary the gain of V1a.

[kagliostro]

Many thanks Steve

Now all is more clear

---

talking about the FET, there is a resistor near it, R7, the value of this resistor in the list is --

is this referred to the resistance of the FET that vary during function ??

Franco

[sluckey]

talking about the FET, there is a resistor near it, R7, the value of this resistor in the list is --

is this referred to the resistance of the FET that vary during function ??

R7 "may" be in reference to the FET. I can't say for sure from just looking at the schematic. But, R7 could also be a "real" high value resistor and the FET1 simply shunts that resistor. You would probably need a good picture or the actual amp in front of your eyes to know for certain. But, the compressor should work with or without a real resistor for R7.

[kagliostro]

Thanks Again

Franco

[PRR]

I think R7 was put on the schematic drawing but isn't rreally needed so is not installed.

The compressor needs about 5V peak (3.5V rms) into a load which could be as low as 1K, or could be much higher. (I don't really like how they do that; the attack time is heavily affected by the "ratio" setting.) (Which affects much more than ratio.)

V1 has an over-size cathode resistor, so its gain is less than 7. If we bypassed that resistor, gain would be over 40. There's a cap, but blocked by the FET. At no-signal, the FET Gate is near Source voltage, FET is low resistance, cap bypass is effective, gain is 40. With signal, diodes make Gate positive of Source, FET resistance increases, gain drops toward 7. 15dB gain reduction.

It's not a tube compressor. It's an FET compressor that happens to use tubes for the other functions.

There's several pedal-size compressors which are at least as good as this one. Orange Squeezer is one. Merlin Blencowe has a new one intended to be better than OS.

[kagliostro]

Many Thanks for this clarification PRR

Franco