OT: onboard differential input preamp

I was about to add shielding to both my electric bass and guitar because I get buzz that goes away when I touch the strings or knobs. But, then I found some info about how installing a simple onboard differential input preamp is the ultimate, best way to eliminate this kind of noise, both in the studio and even in live situations where you might have power supply issues or wind up standing next to flourescent lights or neon signs.

I'm interested to know what others more knoweledgable than myself think about this concept.


Two 9V batteries for power and 5532 or similar opamp.

If this is a good concept for eliminating this kind of noise, might a discreet jfet design have some benefits versus this opamp version?

BTW - I'm probably still going to do the shielding anyway. :wink:

Thanks!
~ Charlie

I'm interested to know what others more knoweledgable than myself think about this concept.

Click to expand...

That's not me, but here are my misc ramblings anyway:

I've been thinking a bit about using balanced transfers as well, but skipped it so far because the plugs/cables would soon become quite non-standard - which maybe undesirable at a gig. Spares can solve this of course.

If your PU's are passive/unbuffered you'll like a higher input impedance, like 1M in total for each pickup.


The setup above is promising for all kinds of processing/possibilities etc, like you indicate. Could even do Rick-O-Sound ! :wink:

And as I take it still has the advantage of 'sounding passive', I mean, you'll still get the cable-losses so you wont get an 'active' sound. (Active buffering inside the instrument combined with cap between PU & buffer would do the trick as well of course).

Oops, I realize you want to install the active arrangement inside the instrument ? Then however critical those few precious centimeters are, there'll be less need to pass those short distances in balanced mode I guess.

But, then I found some info about how installing a simple onboard differential input preamp is the ultimate,

Click to expand...

Do you have a link ?


Bye,

Peter

[quote author="clintrubber"]Do you have a link ?[/quote]
... to an old thread I searched up over at TalkBass.com...
http://www.talkbass.com/forum/showthread.php?t=112934&highlight=differential+input+preamp

~ Charlie

Charlie,

Charlie here (great name, btw :grin

I would think that you would want to use FET input OAs instead of the 5532 in this location for the input impedance. You'd want higher value resistors as well, on the order of 100k. Either that or you could use a FET buffer in front of the 5532.

And as I take it still has the advantage of 'sounding passive'

Click to expand...

I don't think this will be true myself. This will make your passive pickups into active.

HTH!
the other Charlie (you were here first!)

Hey Charlie, completely off-topic: Where in New England are you?

Peace,
Al.

Charlie - I don't know if I was here first. You've got a lot more posts than I do. But, whatever...

Yeah, JFET's is actually kind of where I was going with one of my questions... about making the whole thing discrete instead of using opamps. I don't know how to do a differential input with transistors, tho. But, I'm guessing that it might allow for more tweaking to get the best performance with specific instrument, pickups, etc...

Al - I'm in southeastern New Hampshire. You?

~ Charlie (the other one)

[quote author="moosapotamus"]Al - I'm in southeastern New Hampshire. You?[/quote]

I'm in northeastern Mass... We're practically neighbors! :grin:

Peace,
Al.

> I get buzz that goes away when I touch the strings or knobs.

So ground the strings and knob-bushings.

AL - Howdy, neighbor.

PRR - Thanks for chiming in! Strings and knob bushings are already grounded. I've checked and verified continuity throughout, between the bridge, strings, pots, bushings, knobs, and internal signal ground.

I am planning to get to shielding the thing, and will probably be satisfied if that solves most of the noise. But, currently one side of the PU's are wired to ground. I understand that can cause problems. Hence, the idea of the onboard differential input preamp.

Of course, there's another way that is also very effective... I've wrapped a little piece of bare wire around my pinky finger, then attached a small lead with alligator clips on either end between the wire on my pinky and the bridge on my bass. Works great, but I'm not sure if that's the best permanent solution, tho. :roll: :razz:

~ Charlie (the other one, again)

Coffee shortage. Wasn't thinking straight.

All the signal leads and parts should be surrounded with shielding. If the pickups pots switches caps are in cavities in the body, line the inside of the cavity with tin-foil. There is heavy-duty aluminum self-stick tape for serious heating-duct work: expensive, but a roll is a lifetime supply. The foil must be connected to ground. In some cases you can get the metal shell of the pickup to clamp the foil. If the pots are on a plastic plate, take the pots off, line the back with foil, and when you put the pots back they will ground the foil.

Since millions of guitars do work unbalanced without much trouble, I suspect that balancing is the $100 way to do what $10 of shielding would do.

Also: the plan you present has a low (and unbalanced) impedance compared to pickup impedances. It will strangle the highs. Also the noise level is high (though volume-pot noise may be the bigger issue). I think this is one case where the 3-opamp Instrumentation Amp would be called for, even though that comes to 6 op-amps and 2 chips and significant battery drain. Yes, TL074 opamps should be fine at these levels and impedances so it isn't any money, just work and batteries.

[quote author="PRR"]Coffee shortage. Wasn't thinking straight. [/quote]
I get that too ...
put the kettle on, time for a cuppa ...

GuitarNuts.com has a page with the best illustrations and instructions I have seen on this subject. Loads of info there and all that I have tried works well. HTH!

A while back there was an article in EE Times that showed a prototype strat pickguard that was actually a multi-layer PCB with the pickup "wiring" built in. Very cool!

PRR changed his avatar...what's up with that? I was becoming rather accustomed to the puppy dog!

Peace!
Charlie

> GuitarNuts.com has a page

Excellent guide.

> I was becoming rather accustomed to the...

...puppy dog!

No puppy: that bitch is 12 years old in the picture and passed-away recently at the age of 13 (too weak to pose for a picture). I switched to her pic at that time, but she's been gone for months now. I went back to an older avatar of another creature.

But one last blast:


This one is still around (though it is often hard to tell....)

This one is still around (though it is often hard to tell....)

Click to expand...

That looks like a good avatar choice too, PRR!

Peace!
Charlie

from Charlie (Son of Charlie):

Quote:
And as I take it still has the advantage of 'sounding passive'

I don't think this will be true myself. This will make your passive pickups into active.

Click to expand...

As I see it there is no question of whether PU's are active or passive. In the beginning all the usual ones are passive, and by putting the buffering close to it people usually call it 'active'. Certainly makes sense.

But in a simplified view on it (and note, it'll be an oversimplification), the only difference between (1) a gtr with passive PUs plus a cord and the buffering at the amp and (2) a gtr with say EMGs and a cord and an amp is that the buffering is now before the cable-influence (say 100pF/meter).

See for instance C1 in:
http://www.jensentransformers.com/as/as004.pdf

(If you go for a circuit i.s.o. shielding etc, this circuit might be right on the money for your application).


So when having a buffer inside the instrument, I figure the passive-taste of it can be brought back by rolling of highs again (add cap-to-gnd between PU & buffer-input and to introduce the LPF-peak etc, as drawn in Jack Ormans article on PUs http://www.muzique.com/lab/pickups.htm.


Bye,

Peter