Help Wirring Bass Guitar on board preamp

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Whoops
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Re: Help Wirring Bass Guitar on board preamp

Post by Whoops »

It came to my attention that there's an Alembic preamp with a similar SVF topology
here it is the Alembic Schematic

Image
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JR.
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Re: Help Wirring Bass Guitar on board preamp

Post by JR. »

Whoops wrote:It came to my attention that there's an Alembic preamp with a similar SVF topology
here it is the Alembic Schematic

Image
yes that's a SVF, and while all are similar topology, I see some differences (i think but I am still guessing about the other circuit), at least this one appears complete.

The pot P1 varies the BPF gain which also varies the filter Q, but since the output of that signal chain is taken from the LPF output the Q adjustment and changing BP gain does not change output volume... BUT running the bandpass section with 20dB more voltage gain than the rest of the bandpasses means it can clip way before the rest of the audio path. This is a rookie mistake for designing a clean SVF, but perhaps they are intentionally clipping the BP to dirty up the sound? But once again the distortion from clipping the BP will generally fall on harmonic overtones that will like wise be higher frequency and end up in the HPF output not being listened to. Perhaps the designer does not know his way around SVF design, or is just happy with what he got.

The use of a V/2 makes sense for single 9V supply, I suspect your apparent finding of actual grounds suggests the two batteries are center tapped to grab a hard ground, unless your ground is really a divided V/2. The spare cap and resistor may be parts of a V/2 bias supply. The tapped battery string is inconsistent with switching power off using a single pole of jack switch.

Maybe this was the inspiration for yours, or both were inspired by some other SVF, they are not that rare.

JR
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mediatechnology
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Re: Help Wirring Bass Guitar on board preamp

Post by mediatechnology »

My wife and I put a puzzle together recently.
It was the "Leg Lamp" in "The Christmas Story."

Image

Putting this back together reminds me of that puzzle.
Even though we had a picture to work from there were so many dark areas and hard transitions airbrushed out it was very difficult.

At least we now have a picture to work from though it's not the same scene as the puzzle we're working on.

I'm thinking that gain boost in the SV may be intentional.
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JR.
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Re: Help Wirring Bass Guitar on board preamp

Post by JR. »

Yup, but it's not a picture of the actual lamp, just a picture of another lamp.

JR
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Whoops
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Re: Help Wirring Bass Guitar on board preamp

Post by Whoops »

mediatechnology wrote:My wife and I put a puzzle together recently.
It was the "Leg Lamp" in "The Christmas Story."

Image

Putting this back together reminds me of that puzzle.
Even though we had a picture to work from there were so many dark areas and hard transitions airbrushed out it was very difficult.

At least we now have a picture to work from though it's not the same scene as the puzzle we're working on.

I'm thinking that gain boost in the SV may be intentional.
LOLOLOLOL

The thing is that every puzzle has an ending, you cannot give up in the middle.

Thanks for you patience.
Whoops
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Re: Help Wirring Bass Guitar on board preamp

Post by Whoops »

I conducted new tests today,
batteries connected in series, middle of the batteries connected to ground, this is good enough for the testing procedures.
Both IC's are getting the right voltages now.

I re soldered all the wires, as some of them were coming loose from the pcb and making intermittent 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 :

__________
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.

Am I'm missing something? do you have any suggestions on how to wire this?


Thank you so much,
Even if everything is not still working properly I'm really happy with the progress so far.
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
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JR.
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Re: Help Wirring Bass Guitar on board preamp

Post by JR. »

As I posted back in Jan--
dual gang? OK the wipers will connect to Yellow and Green to shift the center frequency of the SVF. for proper operation the bottom of both pots are grounded through an end limit resistance (to be a variable divider) or left floating to be a variable resistance.
--and--
This is painful to make sense of. but best guess for frequency pot is violet wire goes to top of one section of frequency pot with the wiper of that section going to green wire. The top of the other pot section connects to the blue -black wire with it's wiper to the yellow wire. I will guess for now that the other two leads of the dual pot are left floating since I do not see two same value resistors to ground.
====================

I do not see a connection for the other two pot leads. so pots are probably used like a variable resistance (rheostat).

If pots are connected wrong the op amps may peg to one rail or the other due to lack of DC feedback... when correct the op amps will be centered on 0V and pass audio

JR
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