freestompboxes.org

Hi,

I'm working on a bass that will have 2 MM humbuckers. Although the parallel impedance is around 2kOhm for each pickup, I plan to have each pickup switchable between parallel/series/single+ghost coil. On top of that there will be a 4-way rotary pickup selector with series/parallel option.
So the resulting impedance can vary between 1kOhm and 16kOhm, depending on switch settings.
Would it be possible to use a buffer with an output impedance of 2kOhm between pickups and 2-band EQ board to retain the original tone control timbre?

marsbel wrote:Hi,

I'm working on a bass that will have 2 MM humbuckers. Although the parallel impedance is around 2kOhm for each pickup, I plan to have each pickup switchable between parallel/series/single+ghost coil. On top of that there will be a 4-way rotary pickup selector with series/parallel option.
So the resulting impedance can vary between 1kOhm and 16kOhm, depending on switch settings.
Would it be possible to use a buffer with an output impedance of 2kOhm between pickups and 2-band EQ board to retain the original tone control timbre?

It's actually DC resistance, not impedance. Impedance increases with the frequency.

The output of the buffer is not important here. The preamp has a rather low input impedance. This works fine for the pickup with the coils in parallel, but if you wire them in series it rolls off some top end. But since you are planning on having them switchable, that's not a problem. It will enhance the series setting.

Two pickups in parallel is also not a problem, as the resistance drops by half, and will give you a nice bright tone.

If you wanted to retain a brighter tone with the pickup in series (8k) you need a buffer with a higher input impedance. Buffers generally have low output impedances, so that doesn't matter.

Also I think you have your parallel resistance mixed up; two 2k pickups in parallel is 1k. If both of those pickups are wired in series, then two 8k pickups in parallel is 4k. Were you planning on running the two pickups in series with each other? That's always a pretty dark, fat tone, so once again I wouldn't worry about a buffer.

marsbel wrote: Would it be possible to use a buffer with an output impedance of 2kOhm between pickups and 2-band EQ board to retain the original tone control timbre?

This will unfortunately not work, you'll kind of "lose" treble knob function...

Regards
Andy

That's what I was afraid of.
Any suggestions on how to keep the treble function with variable PU DC resistances?
As I mentioned, I have 2 MM PU's with 4K coils and all switching options.
Lowest DCR=1k (4x 4k in parallel), Highest DCR= 16k (4x 4k in series).

The variable DC resistances will of course alter the treble peak frequency. I'd expect the circuit to be almost useless with 16k (the treble peak will shift by about factor 8-10 down).

Maybe altering the preamp might work: if you use individual ops, one for each pickup and either two individual treble pots or one stereo pot. If you can't find 1 meg rev log you could try linear. These two (buffered) signals could then be fed to a balance pot followed by another stage doing the bass. This solution will have "as much stingray" as possible and might be very useful (especially when you play mixed with individual treble adjustement).

Other possibility: two complete individual preamps (each with bass and treble control) then mixpot eventually followed by a simple buffer.

You should also respect the input voltage of the 16k wiring - that's four sources in series - maybe a bit too much for the preamp.

Another approach could be: omit the treble pot(s) and wire two bufferd input stages as if the pots would be maxed and use a simple high cut to tame down the peaks.

You can use a buffer before the preamp for high impedance pickups. That works fine. It sounds the same.
I've also used a 1M linear pot for the treble control. That works fine too.

I guess I'll built the circuit and experiment and listen. I can always build another 2-band preamp if it's not right.
And I'll try to get more educated on pickup theory.

There are much better preamp designs out there. This one does has a certain tone due to the way the treble control is implemented, but you can get a similar tone with more conventional circuits.

I build the 2-Band Stingray preamp on perfboard.
Just finished debugging and tested it. It seems to work OK, but either due to the pickups I tested it with, or an undiscovered mistake, the treble function doesn't work well.
When the pot is fully closed, I get a good sound, but as soon as I turn up treble, it attenuates the level to a certain point.
Bass works as it should.
The pickup is a Schaller JBX (copy of Dimarzio Model J), with a DCR of about 8k.
Next step wil be to test with a proper MM pickup and see what that does.
I also decided to not go with all the switching options I planned originally. When would I ever need all coils in series anyway.
So I worked out a switching diagram with a 4 pole 5-way switch and a 4PDT switch, giving me several sensible options, and saves me from winding 2 phantom coils. (Similar switching diagram can be found on the internet, but is really wrong)
By the way, the original Sabre Bass also had 2 humbuckers and seemed to do well with the 2-band preamp.

sounds like you wired the treble pot incorrectly. its a common mistake because it connects to 3 points on the PCB board.

PaulBass wrote:sounds like you wired the treble pot incorrectly. its a common mistake because it connects to 3 points on the PCB board.

No, that was OK.
It turned out to be the 1n8 ceramic cap went bad. Strange, because before the assembly, I selected it out of 10 or so to be closest to 1n8 (all were higher than the indicated value).
Anyway, I replaced it and all is good now.
This will be the last time too that I'll use perfboard. In the same time it takes me to connect the dots on the board, I can etch and drill a proper PCB.
I'll make a second one on PCB.
Quite happy with the results and how good it sounds with a MM pickup.

I found a PCB marked as Bajaman Preamp MKII .Is it tested by any one?Please sugggest.

all surface mounts. good luck soldering that

Can you give a link for this surface mount version that I could open in Eagle? Or at least a shot of both sides of the board. I'd be happy to test it out.

John

Oops, never mind! Found it here: https://oshpark.com/shared_projects/L31gTGNL

I recently sold a Musicman Stingray control panel manufactured in 1979.
I made some shots and traced the circuit.
I made a layout clone of the board for SprintLayout6.0,
you can find it here packed as Zip file.

33uF is incorrect. they are 10uF

PaulBass wrote:33uF is incorrect. they are 10uF

Hi, I have to contradict you here.
I was very surprised about these values, because I had never before seen that in some reference schematics.
The Tantals were indeed labeled with 33 uF.
To be sure I measured the values with my PEAK altlas ESR 70 tester.
I do not know exactly the values, but I do rember that all capacitances were within the range of tolerance.

In my opinion, the value differences of these capacitors do not affects the circuit function very much.
The roll-of frequencys are as follows:
R = 10 kOhms, C = 33uF -> ~0,5 Hz
R = 10 kOhms, C = 10uF -> ~1,5 Hz
R = 25 kOhms, C = 33uF -> ~0,2 Hz
R = 25 kOhms, C = 10uF -> ~0,6 Hz

looks like they stuffed the wrong caps in there or ran out of 10uF's. my 77 Stingray had 10uF tants

... or ran out of 10uF's

I still clearly recall how, in the time at the end of the 1970s,
I was, among other things, developer for test facilities.
At that time, the first computers for the private use cames massive into the market
and this has led to a lack of electronic components.
It may well be that 10uFs were not available and a quick substitute had to be found.