Cowpoke preamp  [schematic]

Pickups, wiring schemes, switch techniques and onboard active electronics for guitars and basses
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cars10
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Post by cars10 »

I am the proud owner of what is called a cowpoke bass. This is a bit of an odd Fender bass having 22 frets and an active preamp. It has been produced in Mexico for only about two years (94/95). Some belive the preamp was designed by Mr. Kubicki as this was the time Fender had a collaboration with him. Other say it was designed by Fender themselve. This thread over at talkbass is about the best source of information for those basses.

Well, mine did show a problem with the output signal being very low some times and being good others. So I decided to have a look and, while I am at it trace the schematic as I did not find it elsewhere on the net.

So here is the schematic I came up with:
cowpoke_schematic.jpg

The pots are stacked pots (mix/volume and bass/treble).
I am not an expert for preamps so first it seemed odd to me that the output of the opamp1 would not be directly connected to the input of opamp2. But I checked several times. This is how it is build. I did a quick ac simulation to see its frequency response. Be aware I did not use the right op amp and the output is in volt. The input signal was a sine with a peak voltage of 1V. Bass and treble pots have been in the middle.
freq_mid.jpg

This might explain why most of people feel the preamp sounds rather bright.
If you sweep the pot position you get this response so there is quite a range to adjust.
freq_sweep.jpg

If someone is able to explain more on how this preamp and especially the eq works I would be very interested.

My initial problem with the output being sometimes low turned out to be a problem of the pickup connectors. On the pickup and on the preamp there have been some connectors which seemed to be rather strangely paired to me. When shaking a little on them the signal dropped. In the end I simply soldered the pickups directly into the pcb. The connector seems like a nice idea but as the bridge ground wire is directly soldered to the output jack the preamp cant be removed without desoldering anyway.

pic of the preamp (sorry, the 9V clip covers the op amp):
preamp.jpg
pic of the opamp:
opamp.jpg
opamp.jpg (37.06 KiB) Viewed 957 times

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tonyharker
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Post by tonyharker »

The circuit around the second op amp is just a Baxandal feedback tone control and is correct.

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george giblet
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Post by george giblet »

For the schematic, there is a trace on the web which was posted in this thread,
about 1/3 the way down page 11 I put up links to all the Kubicki designs.

viewtopic.php?t=3818&hilit=fodera&start=200

The schematic is here,
https://elektrotanya.com/files/forum/20 ... sugori.jpg

I have trace the board as well and I found R12 & R13 were 270k like your schematic.

The main differences between the two schematics is the pot values
- Your schematic all 50k pots
- Older trace has 50k bass but 100k treble and 100k volume.
From my comments in the old thread it looks like I could not read the pot values. If you *measured* the pot values in-circuit then it's likely your pot readings are both low, in fact both pots might actually be 100k.

For the sound of the unit, the preamp has some unconventional fixed EQ built in. Apart from that it's pretty conventional in its structure and design.
- The 3n3 & 47k on the first stage provides a 6dB boost starting around 500Hz then shelving of again at 1kHz. That's going to be the cause of the bright sound. You could lift one leg of the 3n3 cap to see what it sounded like.
- The 10k & 6n8 at the output roll-off above 2kHz. Sometimes that's OK but you can restore some highs by adding a resistance in series with the 6n8 cap. That's what some of the later Ibanez preamps do.
- Lastly is some low-end roll-off with the 150nF cap. That's going to roll-off below 20Hz or 10Hz depending what the volume pot actually is. It's totally fine to do that and it's a subtle effect. Probably necessary due to the lack of cap between the first and second stages.

It is OK to DC couple the first and second stages. The only down side is sometimes it can promote pot scratchiness when the pots get old. That can also happen when the two stages are AC coupled as well.

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george giblet
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Post by george giblet »

I haven't made any progress on the pots but I'm really starting to think the bass and treble pots are both 100k's.

Suppose both pots are 100k and we measure in circuit.

Measuring across the treble pot we see 100k in parallel with the circuit around and through the bass pot (10k + 18k + 100k + 18k + 10k) = 156k.
100k in parallel with 156k = 60.9k That's surprisingly like the 60k on your schematic.

Measuring across the bass pot we get 100k for the bass pot in parallel with (18k + 10k + 100k + 10k + 18k) = 156k
So that to ends up with a measured value of 60.9k

It does make more sense the bass and treble pot values are the same since the pots are ganged but you can get custom pot made
so it's not 100% conclusive,

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cars10
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Post by cars10 »

tonyharker wrote: 01 Nov 2021, 16:07 The circuit around the second op amp is just a Baxandal feedback tone control and is correct.
Thank you. I looked that up and learned a lot.
I am sure I have seen a Baxandall circuit before but I did not bring it together.
Now I will remember. Hopefully :D

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Post by cars10 »

george giblet wrote: 01 Nov 2021, 21:49 For the schematic, there is a trace on the web which was posted in this thread,
about 1/3 the way down page 11 I put up links to all the Kubicki designs.

viewtopic.php?t=3818&hilit=fodera&start=200

The schematic is here,
https://elektrotanya.com/files/forum/20 ... sugori.jpg

I have trace the board as well and I found R12 & R13 were 270k like your schematic.
Great, thank you. I searched a lot for the schematic, but this one did not turn up.
For some reason I cant watch the thread you link, only the schematic.
george giblet wrote: 01 Nov 2021, 21:49 The main differences between the two schematics is the pot values
- Your schematic all 50k pots
- Older trace has 50k bass but 100k treble and 100k volume.
From my comments in the old thread it looks like I could not read the pot values. If you *measured* the pot values in-circuit then it's likely your pot readings are both low, in fact both pots might actually be 100k.
Actually I have a 100k Pot for the Mix, a 50k Pot for the volume and 60k for treble, bass in my schematic.
You comment about the treble bass below and I think you are right.
For mix and volume I am rather sure I have measured at least twice as I was wondering also why they would use a dual pot with different values.
Especially here at this positions. Also, measuring here should be fine as the pots are not in parallel with other resistors.
The preamp is mounted again, maybe I can sometimes check if I can at least reach the volume pot to measure again.
Mix pot will give no good reading anyway as the pickups are now connected by soldering the wires.
george giblet wrote: 01 Nov 2021, 21:49 For the sound of the unit, the preamp has some unconventional fixed EQ built in. Apart from that it's pretty conventional in its structure and design.
- The 3n3 & 47k on the first stage provides a 6dB boost starting around 500Hz then shelving of again at 1kHz. That's going to be the cause of the bright sound. You could lift one leg of the 3n3 cap to see what it sounded like.
I tried this in the simulation. But this disconnects the stages. Maybe one should shout cut the cap?
Anyway, the treble pot can take care for this if turned down I think.
george giblet wrote: 01 Nov 2021, 21:49 - The 10k & 6n8 at the output roll-off above 2kHz. Sometimes that's OK but you can restore some highs by adding a resistance in series with the 6n8 cap. That's what some of the later Ibanez preamps do.
Interesting. But as heights are rather to much here I will leave it as is.
But goof to understand.
george giblet wrote: 01 Nov 2021, 21:49 - Lastly is some low-end roll-off with the 150nF cap. That's going to roll-off below 20Hz or 10Hz depending what the volume pot actually is. It's totally fine to do that and it's a subtle effect. Probably necessary due to the lack of cap between the first and second stages.
I simulated this also and it can be seen.
Need to listen to it to understand better the effect probably.
george giblet wrote: 01 Nov 2021, 21:49 It is OK to DC couple the first and second stages. The only down side is sometimes it can promote pot scratchiness when the pots get old. That can also happen when the two stages are AC coupled as well.
If it would be AC coupled, would there be another cap between the first op amp output and R6/R8?
george giblet wrote: 16 Nov 2021, 05:07 I haven't made any progress on the pots but I'm really starting to think the bass and treble pots are both 100k's.

Suppose both pots are 100k and we measure in circuit.

Measuring across the treble pot we see 100k in parallel with the circuit around and through the bass pot (10k + 18k + 100k + 18k + 10k) = 156k.
100k in parallel with 156k = 60.9k That's surprisingly like the 60k on your schematic.

Measuring across the bass pot we get 100k for the bass pot in parallel with (18k + 10k + 100k + 10k + 18k) = 156k
So that to ends up with a measured value of 60.9k

It does make more sense the bass and treble pot values are the same since the pots are ganged but you can get custom pot made
so it's not 100% conclusive,
I think you are right. I measured in the circuit and your math makes sense.
I checked my notes the actual values have been 62.4k Ohm and 61.2k Ohm. But I did not measure all the resistors.
I would think this changes the frequencies above a bit.

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george giblet
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Post by george giblet »

For some reason I cant watch the thread you link, only the schematic.
Some parts of the forum are only visible when you are logged in.
Actually I have a 100k Pot for the Mix, a 50k Pot for the volume and 60k for treble, bass in my schematic.
You comment about the treble bass below and I think you are right.
For mix and volume I am rather sure I have measured at least twice as I was wondering also why they would use a dual pot with different values.
So we can confident those values are correct. The other schematic may have an error. Another possibility is at some point the pot value got changed in later models.
I tried this in the simulation. But this disconnects the stages. Maybe one should shout cut the cap?
Anyway, the treble pot can take care for this if turned down I think.
It shouldn't disconnect the stages. It kind of looks that way on the schematic but it's actually not. Where R8 connects to the second opamp it also connects to C7. C7 is a filter for the DC 4.5V rail and it block signals from R8 to the second opamp.

Where you might have trouble with simulator is you can't just leave components with one lead hanging. The easier way to trick the simulator would be to set R2 to 10MEG (not 10m). Otherwise you would have to delete C1 and R2 then replace R1 with a wire.

I think you should do test to see how lifting C1 on the sounds. It is a source of brightness and would be the best mod to make that preamp act more normally. When you have fixed EQ you can't always undo its effect with things like the treble control because the treble control might not act on the same frequencies as the fixed EQ.
If it would be AC coupled, would there be another cap between the first op amp output and R6/R8?
Yes, that's it. You can see the idea in this schematic,
https://www.petervis.com/record_players ... ircuit.gif

The low-cut filter cap here vs the existing low-cut filter cap at the output do behave differently. However, it's quote a subtle difference.
I think you are right. I measured in the circuit and your math makes sense.
I checked my notes the actual values have been 62.4k Ohm and 61.2k Ohm. But I did not measure all the resistors.
I would think this changes the frequencies above a bit.
OK cool. Thanks for confirming it. I think we have a very good idea of what the pots are now.

If you redo you simulation with 100k bass and treble pots you will notice the amount of boost/cut now more that with the 60k's. Also the Bass/Treble frequencies will change.

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