Help Wirring Bass Guitar on board preamp

[ilcaccillo]

I will try to conduct today new testes on this circuit.
So I will do I "connect 3rd lug to the lone resistor (1st pot) and to the lone cap (2nd pot)" and see how it goes.

Any other suggestions or connections I should try out?

Thank

[MarkusBass]

Hi Mark,
Green was not connected to the input jack but to the 1st lug of the 50K pot.

Or maybe I understood wrongly what you wanted to explain

For a moment I thought that there are input and output jacks on your drawing . Now I see that there are pots on the drawing. Sorry for the confusion.

Mark

[MarkusBass]

Any other suggestions or connections I should try out?

The simplest test is to connect the pickups to the input buffer input (PAD???) and the output of the input buffer (after C5 capacitor) to the output jack. Make sure you know which ground should be connected to the output. In this way you will check if TL061 is OK.

Mark

[dwmorrin]

Tested and there's sound coming out of the bass, although the signal is quite low and the low end is filtered out.

That would immediately make me suspicious of C5 and C4. These are 30+ year old electrolytic caps. It is a possibility that they now have low capacitance, and are thus acting as a high pass filter by themselves.

Let's take a break from the reverse engineering of the filter, and focus on the input->buffer->mixer->output path.

Quick reality check first: always check your dc conditions first. With power supplied, but no signal, measure around all the pins. Power pins should have the right power, and the other pins should be all near 0 volts. Anything funny there is worth noting.

Next let's troubleshoot the "clean" path.
IC2 input and output (pins 3 & 6) should have the same level and timbre.
Both sides of C5 should have the same level and timbre (i.e. no high pass effect)
The node of C5, R12, and R17 is the input to IC1C (summing amp). R12 and R10 set the gain to x2. Expect the input voltage x2 at pin 8 of IC1.
Check that both sides of C4 have the same level and timbre.

If there is any question as to whether the filter is influencing the signal coming out of the mixer, I would either open circuit the controls or just alligator clip the open grey wire to ground.

Ideally, you'd be injecting a small sine wave (100mVp-p, 440Hz or whatever audio freq. you like) into the input (PAD??) and then comparing this to each step of the clean path using a scope.
If no scope, an audio probe will do.
If no sine wave, go ahead and strum on the bass. But this needs to be checked as well. Does the passive wiring work? (Sorry if you verified this way back when...) Weak, high pass sound is also a symptom of bad pickups... so if any doubt here I would alligator clip the pickups to a raw 1/4" jack and audition them first, then connect passive controls and audition, and then present the vetted bass guitar signal to the PAD?? input.

Or just forget troubleshooting and go ahead and replace C5 and C4 and check again. 30+ year old electro caps should be replaced for reliability anyways. Especially if you are repairing this for someone else.

Ignore the filter part until the clean path is verified working good.

[ilcaccillo]

For a moment I thought that there are input and output jacks on your drawing . Now I see that there are pots on the drawing. Sorry for the confusion.

Mark

No worries, that drawing is just to represent the wires that were connected to the Pot when I received the Bass.
Maybe I didnt represented the dual gang pot quite well in picture.

[MarkusBass]

Maybe I didnt represented the dual gang pot quite well in picture.

I would call it "symbolic". But there is a label "Dual gang pot" - I should look at it more carefully - my mistake. On the drawing the first lug of the pot is shorted with the second. Are you sure it was like this? This would mean that the pots were connected like on Alembic schematic and "lone components" should be connected differently.
But in any case the version discussed here will work and the connection of the lone parts may be left for future. First make the preamp work.

Mark

[ilcaccillo]

I will conduct new tests, and will post a longer reply.

Just to say really quickly that the passive part works great and pickups sound really good wired passively so they are fine. The low output and filtered sound is just from the active circuitry and the way it is connected at the moment.

Thanks

[MarkusBass]

I was curious how the preamp works so I made two "virtual" recordings.
The first one has the filter set to 400Hz and gain to 18dB: out_400Hz_18dB.mp3

The second one has the filter set to 600Hz and gain to 22dB: out_600Hz_22dB.mp3

I think that they sound OK.

The extreme setting is 27dB and 1kHz. In this case the sound is not nice.
I wonder how the switch was connected. I think it was 18, 22 and 27dB but the latter one is too much for me.

PS: I decreased the general gain of the preamp from +6dB to 0dB.

Mark

[ilcaccillo]

I was curious how the preamp works so I made two "virtual" recordings.
The first one has the filter set to 400Hz and gain to 18dB: [ATTACH]32754[/ATTACH]

The second one has the filter set to 600Hz and gain to 22dB: [ATTACH]32755[/ATTACH]

I think that they sound OK.

The extreme setting is 27dB and 1kHz. In this case the sound is not nice.
I wonder how the switch was connected. I think it was 18, 22 and 27dB but the latter one is too much for me.

PS: I decreased the general gain of the preamp from +6dB to 0dB.

Mark

Hi Mark,
thank you so much for posting the sound samples, that's really impressive I really wish I can make this circuit work like that.

I conducted new tests today.

I re soldered all the wires, as some of them were coming loose from the pcb and making interminent connections.

I have sound going in and out of the preamp now, the sound is fine, not filtered, good gain to noise ratio also.

The only thing that is not working its the switch and the dual gang pot.

I tried different the combinations of wiring discussed here:

__________
Dual gang Pot
gang1
yellow to pin1, grey to wiper and pin3
gang2
violet to pin1, green to wiper and pin3

Blue/black on Switch changing connection from Blue stripped to Blue or Disconnected (middle position of switch)

_________


Dual gang Pot
gang1
yellow to pin1, Blue/black to wiper and pin3
gang2
violet to pin1, green to wiper and pin3

Grey on Switch changing connection from Blue stripped to Blue or Disconnected (middle position of switch)

_________

Dual gang Pot
gang1
yellow to pin1, Blue/black to wiper, Pin 3 White lone resistor (and then reversed orange here and white on the other gang)
gang2
violet to pin1, green to wiper, Pin 3 orange lone cap (and then reversed white here and orange on the other gang)

Grey on Switch changing connection from Blue stripped to Blue or Disconnected (middle position of switch)

_________

Dual gang Pot
gang1
yellow to pin1, grey to wiper, Pin 3 White lone resistor (and then reversed orange here and white on the other gang)
gang2
violet to pin1, green to wiper, Pin 3 orange lone cap (and then reversed white here and orange on the other gang)

blue/black on Switch changing connection from Blue stripped to Blue or Disconnected (middle position of switch)

_________

In none of the above configurations the dual gang pot affects the Tone/EQ of the bass sound, I rotate it and nothing happens, the exception is when the Blue/black wire is connected to the Wiper there's some High pitch squeal when the pot is rotated more than half way.
Changing the switch positions also doesn't alter anything sound-wise.

Markus how did you made this connections in the circuit you used to do the virtual recordings?

Maybe I didn't understand well all the info you provided and I'm missing something.

Thank you so much,
Even if everything is not still working properly I'm really happy with the progress it was possible because of your help.
Made me really happy when I connected the 2 9V batteries today and the jack and for the first time I heard a Good Bass Sound coming out of the preamp.

Thanks

[MarkusBass]

I resoldered all the wires, as some of them were coming loose from the pcb and making interminent connections.

You are on a good track but in this case nothing will work (even if you make correct connections).

I haven't check all your connections but I see that your are making a mistake with GRAY wire (this one should go to the switch and not to the pot) and with BLUE_BLACK (this one should go to the pot).

The analysis of the preamp is not yet completed because we don't know how batteries should be connected (you already modified this to symmetrical supply), we are not sure about the dual pots (on your last drawing they had two lugs shorted) and we are not sure about the switch (especially that it has middle position - OFF) and we are not sure about lone components.
So "my version" is kind of my vision how I would connect it if I'd got the preamp to be fixed. I does not mean that this is the way it was actually connected. Please keep it in mind.

Since the preamp is already passing the signal it means that the power supply, input and output wires are connected correctly.
The connections are (just to recap):
PAD?? - input (from pickups),
ORANGE_STRIPED - tip of output jack,
RED - V+,
GND of batteries - GND,
RING_OUT_JACK (-9V) - ring output jack,
BROWN (-9V) - ring output jack.
It looks like they are connected correctly (although the power supply is already changed).

Other wires (filters):
VIOLET (output of the filter mixer) - POT1 lug1,
GREEN (input of the first integrator) - POT1 lug2 (slider),
WHITE (27k resistor) POT1 lug3,

BLUE_BLACK (output of the first integrator) - POT2 lug1,
YELLOW (input of the second integrator) - POT2 lug2 (slider),
ORANGE (22nF capacitor) - POT2 lug3,

Other wires (switch) - being frankly I don't know how it should be connected (for simplicity I assumed that this is SPDT switch).
For a simplest test just connect the GRAY wire to BLUE wire (without the switch and leave BLUE_STRIPED unconnected).
In the second step I would connect it in the way that GRAY is connected either to BLUE, or to BLUE_STRIPED.

If you connect the wires in this way, the preamp will work and it will sound like the samples posted previously. Let me know if it works.
The actual connections are still a mystery for me. I hoped that someone could help you with this problem.

The mistake that you made in the very beginning was the initial drawing of the preamp. Now we know that IC2 is the input buffer (and it should be on the left on the schematic) and IC1B is input mixer of the filter (so it also should be on the left on the schematic). Please take a look at my schematic, which I think is much more readable .

Mark

[MarkusBass]

Switch details:
In my version of the preamp I would connect the switch in the way that GRAY wire is connected either to BLUE (12k resistor), or to BLUE_STRIPED (18k), or to both resistors connected in parallel. But I have no idea how this can be realized with ON-OFF-ON switch.
I assume that the switch can be wired completely different and it could be used to disable the filter (to act as active-passive switch). This is open for discussion and I hope that dwmorrin may have some idea about it.

Mark

[MarkusBass]

Two more changes:
- the preamp as it is has +6dB gain. I changed it to 0dB by changing R12 on your schematic from 4k7 to 10k. You may check it with live sound which gain is better.
- the 2n7 cap on the output causes HF roll off at 5kHz. I thought that this was to much and changed it to 1n. Are you sure it is 2n7 and not for example 270pF?

Mark

[dwmorrin]

I think it's time to throw this together on my breadboard and mess around with it.
Should have time tomorrow morning for that.

Great to hear your got the basic preamp part working!

[MarkusBass]

Two wires are missing on my list but I think that you already know that they are connected to the ground:
- BLACK - ground to the output jack,
- YELLOW_STRIPED - ground to the pots.

Mark

[dwmorrin]

I just did a SPICE simulation using the last sketched schematic I submitted and without either of the two "loose" components connected, and I am getting a good band pass filter in simulation.

Do get near 0Vdc on all inputs and outputs of the op amps?