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Unknown bass amp+preamp schematics help

Started by Endeo, February 07, 2024, 02:51:21 AM

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Hello everyone,

Long time lurker here, first of all thanks for this beautiful forum for both help and inspiration!

I need your help in a preamp schematic which was reverse engineered, but first a little history....I found it near a dumpster and since I felt sorry for it (especially since it seems someone's handwork/modified commercial work) I decided to restore it.

Here are some pics:


The amp section is a quasi complementary AB class amplifier with two NPN 2N3055 (common for the 90s/00s here if not earlier in Croatia) but that is not of significant interest to me as it is rather simple, the VU meter and speaker delay pcb are also simple and not of interest right now, the interesting thing to me is the preamp section (in order to determine switch and pot functions) which when reverse engineered showed that it used a parametric EQ as shown in the schematic can anyone confirm my suspicions on the pot functions? also there is a omitted cable and a hole in the front plate in the schematic is at the output of the preamp section with a voltage divider is this possibly a recording out thing?

Here is the schematic of the preamp section:


Thanks in advance!!



Did you reverse engineer the amp and make this schematic?  If not, where was it obtained?
Life is what you make it.
Still rockin' the Dean Markley K-20X



@joecool85 yes the preamp section was reverse engineered (this is the schematic of the preamp the amp as mentioned is a simple quasi complementary amplifier and not particularly of interest for investigation for restoration)

@g1 yes the parametric eq section and calculations were found on ESP's page (as it was not a familiar topology for me)

Can anyone confirm if the output of the voltage divider is a recording out or something? (marked headphone/rec out? on the schematic)

Thanks everyone for the replies!


Quote from: Endeo on February 07, 2024, 03:03:47 PMCan anyone confirm if the output of the voltage divider is a recording out or something? (marked headphone/rec out? on the schematic)
Yes as it has a Preout then the other is likely a head phone.


I think it might be a line out since it is not variable, the one marked Pre Out has a volume pot.
Headphones would also need a level/volume pot.
The output might also be for the VU-leds so you can see if you are over-driving the gain stages before the master volume ?
There are no stupid questions.
There are only stupid mistakes.


@phatt I rather doubt it's a headphone out as it is not advised to drive headphones directly from the preamp and there is no practical (or easy) way to switch-out the speaker output when the headphones are connected.

@Tassieviking I also rather think it is a recording out as the speaker would still be in use as a "monitor", the VU meter circuit has its separate input (from the main amp - speaker output), and there is a covered hole on the frontplate.

Thanks everyone for the replies!