Low-Z pickups and JFET buffers, HELP!!

[Biarnel]

Gosh, I'm in trouble.
I made a low impedance bass humbucker (2 coils of 1.5kOhm each, ergo 3K hen in series) for a customer:
this was supposed to be wired to an artec on board eq, a 3-way + volume 9V powered thing.
Well, that artec eq is not boosting the signal enough... the output is really too thin.

I decided to hook an "alembic stratoblaster" circuit (the one often posted here, with a J201 JFET) to the humbucker (temporarly removing the artec eq), but..
The output is still too thin.
Something wrong? Should I expect more boost from that circuit?
In the schematics there is a potentiometer, 50k, but if i turn it the output shows no relevant changes.

Please help. I'm in trouble... should I buy some "branded" onboard eq/preamp, something that will boost the signal?
Wich one?

[TGD]

Biarnel, if you decided to use low impedance pickups and you want just buffer it out, device as an Alembic Stratoblaster does not change inductance frequency response.

[Steve Conner]

Yeah, that isn't "thin", it's "accurate" Ordinary high-impedance pickups have a resonance that boosts the midrange and rolls off the high end.

[Biarnel]

Ok, let's say.. I just would like to have more output from those Low-Z coils.
Sure, they are accurate, but I have to raise the gain to the max on the amp!
I need some circuitry to boost the signal, being that something commercial or something (simple!) I have to build myself.

Someone send a distress signal to David Schwab please!

[TGD]

Depending to your pickup project, you have to measure how much modulated current is sent to the input preamp state. If it's not much and your electronic device have a low gain, it's normal have few current in output and general volume is low.

So you have two ways to fix that problem: or you encrease pickup output adding inductance or you have to encrease preamp gain, that's it.

Try pickup output with and without preamp; if signal isn't boosted, you have to revise electronic project, it doesent works propely.

[Albert Kreuzer]

Hi Biarnel,

instead of the stratoblaster, try Don Tillman's preamp

http://www.till.com/articles/GuitarPreamp/index.html

and add a 100µF cap in parallel to R2 (2k2).

HTH
Albert
www.albertkreuzer.com

[TGD]

Much better an other preamp project with variable gain trim control.

[Biarnel]

Maybe I made a mistake with the J201's pins.
How may I identify the S, G, D pins?
I assumed the G was the central, but..

To be sure, looking to the stratoblaster schematics, how should I wire it?

And again, what's that trimpot for? Gain? Tone?

[TGD]

Biarnel, something wrong in your assembly, I guess.

FET preamp project is really easy to do; it has few components at all, so it's easy to find mistakes. Alembic project is a very old one, IMHO not the best for bass pickups. You can find many project on the net with variabile gain control too, so you can adjust output level and you can drive your amp propely.

Pay attention: low impedance pickups have high frequency peak and large Q, you risk to overclip high end and system will autooscilate.

[Mark Hammer]

My understanding is that the optimum output-to-input impedance ratio is something like around 1:10. That is, if the output impedance of a device is noticeably less than or more than 1/10th the input impedance of the device it is going to, then signal transmission will be less than optimal. Feeding pickups with an impedance of a couple of kilohms to a FET stage with an input impedance on the order of several megohms likely does nto fall within those guidelines. Many after-market on-board preamps are expecting pickups on the order of 8-10k.

Instead of a high-impedance device like a FET, maybe what you want is something with a lower input impedance, like a 5534 op-amp. Alternatively, consider using a suitable step-up transformer that can turn your modest pickup impedance into something a FET is more comfortable with.

[Sock Puppet]

My understanding is that the optimum output-to-input impedance ratio is something like around 1:10. That is, if the output impedance of a device is noticeably less than or more than 1/10th the input impedance of the device it is going to, then signal transmission will be less than optimal.

I've always thought that so long as the the input impedance of the following device is higher than or equal to the source output impedance you'd be fine.

Ne'er mind. Here's some more jfet Geofex stuff:

http://www.geofex.com/Article_Folder.../foolwfets.htm

S.

[Sock Puppet]

[QUOTE=Biarnel;36461]Maybe I made a mistake with the J201's pins.
How may I identify the S, G, D pins?
I assumed the G was the central, but..
QUOTE]

Just noticed that bit. The TO92 package has the source in the centre:

http://www.fairchildsemi.com/ds/J2/J201.pdf

S.

[David Schwab]

Maybe I made a mistake with the J201's pins.
How may I identify the S, G, D pins?
I assumed the G was the central, but..

To be sure, looking to the stratoblaster schematics, how should I wire it?

Well I've seen some wired differently but generally it's like this: with the flat side up, and the pins facing down, it should be Drain, Source, Gate.

Check out the attached picture. The pins on the schematic symbol is also listed.

And again, what's that trimpot for? Gain? Tone?

Did you try and turn it? It's the gain pot. That circuit gets a lot of gain. I don't remember if it's 10 or 100, but it can get loud. When I use that with pickups would about the same as yours I usually crank the gain all the way up. My bass has a line level output.. I probably should turn it down a bit. I was trying out an Eden amp the other day and the bass was a bit too much for it.

Here's a few things to note, some Stratoblaster circuits on line show a 68K resistor in series with the input. That's supposed to be for suppression of radio frequencies, but you don't need it and the original circuit didn't have it. I never use it.

The electrolytic cap in the gain pot circuit should be 10MF, if you make it smaller it will boost highs. The output cap should be increased to 5MF for better low end.

A low impedance pickup will be bright, but it shouldn't be thin. It wont sound like a "vintage" pickup with the big lower mid hump, but I get a lot of low end from mine.

[Biarnel]

I just tried a Don Tillman preamp I built for a piezo pickup, hooked it to the bass humbucker and booom, now I got sound.

Too bad, I got a lot of hiss & hum from fluorescents lamps and if I stay near the amp.

Anyway, I thought Don Tillman preamp was good forh HIGH impedance pickups (such as upright basses' piezo), and not for these low impedance one.
This pickup will be on a high-end bass, so I would like to give the customer something really good, with very low noise and good output.

Any ideas?

[David Schwab]

I just tried a Don Tillman preamp I built for a piezo pickup, hooked it to the bass humbucker and booom, now I got sound.

Too bad, I got a lot of hiss & hum from fluorescents lamps and if I stay near the amp.

Anyway, I thought Don Tillman preamp was good forh HIGH impedance pickups (such as upright basses' piezo), and not for these low impedance one.
This pickup will be on a high-end bass, so I would like to give the customer something really good, with very low noise and good output.

Any ideas?

I use the Stratoblaster circuit in my basses. I get no noise at all. Very high quality tone. Did you shield the pickups?

Any pickup will hum if you get too close to certain things because humbuckers don't reject magnetic field noise, as from transformers, just electrical field noise.

You can reduce the input impedance on the buffer by changing some resistor values, but the easiest way is to put a resistor from the input to ground. Try a 100K to 220K and see if that helps. It will smooth out the top end a bit also.