Help Wirring Bass Guitar on board preamp

[MarkusBass]

Please note that I confused VIOLET and BLUE_BLACK wires in my previous message. The post is edited and it is correct now.
POT1 is: VIOLET, GREEN, WHITE
POT2 is: BLUE_BLACK, YELLOW, ORANGE

Mark

[dwmorrin]

Breadboarded the basic circuit today with success.
Reposting the schematic I followed. It is my sketch again with components values now, and I changed the Blue/Black and Grey wire positions to match some info I read in one these posts regarding clues as to what was originally connected.
However, on the breadboard, it did not matter if I swapped over Blue/Black and Grey.
For my experiment here, I just left off the "loose" components for now, and I used a 12k resistor for the "Q" resistor. (the 18k resistor was not used.)

It's a great wah circuit. Replacing the dual gang pot with something voltage controlled definitely comes to mind while fiddling around.

I don't have time to go much further with it right now. In messing with the "Q" path (that's the feedback path from pin 14 to pin 5), you can open circuit this for some super resonance action... possibly interesting for a psychedelic bass solo, but probably not in a typical design. It's just on the verge of self oscillation when I open circuit it.
As you go down in resistance, the boost bump gets smaller and flatter down to about 2k or so, and then you can do down to a short connection and basically have a "filter cancel." So I'm leaning towards THAT as the "bypass" device. i.e. the switch shorts out the path between pin 14 and pin 5, or at least makes it much less than 2k so to remove the boost effect.

Edit... after looking back at the schematic, a short isn't on the table... but paralleling the 18k and 12k will give the lowest boost setting, so that is probably one of the switch combinations. The OFF position probably doesn't open circuit it, so one of those resistors should make a path even when the switch is in the OFF setting.

[MarkusBass]

In messing with the "Q" path (that's the feedback path from pin 14 to pin 5), you can open circuit this for some super resonance action... possibly interesting for a psychedelic bass solo, but probably not in a typical design. It's just on the verge of self oscillation when I open circuit it.

I wouldn't go that far. This was a bass build most probably in 70-ties or 80-ties and possibly used for slap playing. Here you have a contemporary example of such playing technic (check solo at 3 min.):

As you go down in resistance, the boost bump gets smaller and flatter down to about 2k or so, and then you can do down to a short connection and basically have a "filter cancel." So I'm leaning towards THAT as the "bypass" device. i.e. the switch shorts out the path between pin 14 and pin 5, or at least makes it much less than 2k so to remove the boost effect.
Edit... after looking back at the schematic, a short isn't on the table... but paralleling the 18k and 12k will give the lowest boost setting, so that is probably one of the switch combinations. The OFF position probably doesn't open circuit it, so one of those resistors should make a path even when the switch is in the OFF setting.

Here is what I think about the switch:
If you solder 12k and 18k resistors to the middle lugs of the switch, in one position they are connected in parallel (~7.2k), and in the opposite only one resistor is in the circuit. I assume that it was 12k because with 18k the filter does not sound good. And this makes sense. The only problem is with the middle position (OFF). In this case the resistors are disconnected from the circuit but I don't think that this disables the filter (I will check it in the evening).

Mark

[MarkusBass]

OK, I checked what happens when the resistors are disconnected from the switch: the gain raises to +40dB but the filter becomes very narrow. This may sound either very bad, or is not audible at all (to be verified with live circuit).
So my hypothesis is the following:
resistor R18k (BLUE_STRIPED wire) to lug #2 of the switch,
resistor 12k ((BLUE wire) to lug #5 of the switch,
GREY wire to the "shorted" lugs of the switch.

In the "up" position both resistors are in parallel and the gain of the filter is +18dB.
In the "down" position only the 12k resistor is in the circuit and the gain of the filter is +23dB.
In the "middle" position both resistors are disconnected from the circuit and gain raises to +40dB but the filter becomes very narrow.

I would disregard previously posted drawings of the switch, especially that on the drawing there is the following information about lug #2: "had wires connected", "never had wires connected" - both cannot be true at the same time .

Mark

[dwmorrin]

OK, I checked what happens when the resistors are disconnected from the switch: the gain raises to +40dB but the filter becomes very narrow. This may sound either very bad, or is not audible at all (to be verified with live circuit).

Already verified Mark. That's what I was saying this morning. The 18k/12k and 560Ω resistors make up a basic voltage divider for the series feedback and set the gain and Q of the filter. More feedback = lower boost, broader Q. Less feedback = higher boost, narrower Q.
I was playing through it, and my open circuit test was a very narrow boost with a bit of self oscillation. That's the psychedelic sound I mentioned. Obviously not the original design, but thought it was fun to mention. I only tested the open circuit response because I have this on my breadboard and I had it open while changing resistor values.

Most of my testing was actually with my sweep generator and dual trace oscilloscope.

[MarkusBass]

Thanks, it means that most of the problems are solved now. The only thing that we don't know is the OFF position of the switch and possibly switching the preamp active-passive. Everything else is cleared.
Now, let's give the OP day or two to assemble the preamp and check the results. I hope he is still with us.

Mark

[dwmorrin]

I either forgot about or ignored post #33 with the original pot and switch wiring clues. Also realized I had drawn the DPDT jumpers wrong.

I changed my breadboard to match those notes from #33.
Specifically:
50k pot section A:
CW = YELLOW
W = BLUE/BLACK
CCW = BLUE/BLACK

50k pot section B:
CW = GREEN
W = VIOLET
CCW = VIOLET

Nice sweep from 145Hz to 4kHz. I had a 12k Q resistor when I measured that.

Still stumped on the DPDT, and what to do with the GRAY, ORANGE, and WHITE wires. But good thing is: you don't need them to make the filter work.

[PaulBass]

looks like a nice basic SVF circuit but those TL061 & 64 are nasty noisy chips. it must hiss like crazy. TL071 & 74 would sound a lot better with less noise.