BMP Tonestack after preamp is a post-tonestack gain stage necessary

[joeychickenskin]

Hi

My friend has a stick on piezo pickup for his acoustic guitar which he normally plugs into a PA. he has a lo-fi aesthetic and wouldn't listen to suggestions about installing an under-bridge pickup with a built in pre-amp, it's probably a big step for him to have stuck a piezo on the front of the guitar.

I'm looking at making him this pre-amp,

http://tagboardeffects.blogspot.co.uk/2014/04/acoustic-piezo-preamp.html


so that the signal going into the PA is less sickly and while the sound engineer will still have to do their best to get something that isn't too horrible out of it, was going to put a BMP tonestack after the pre-amp to roll off some of the treble.

A lot of BMP layouts suggest a gain stage/boost after the tonestack but would this be necessary after a pre-amp?

I did think of a more elaborate eq circuit but he'll want to keep things as simple as possible.

[anotherjim]

Well, if it's driving a mixer line in, the low impedance of that (compared to Hi-Z guitar inputs) is going to load down the tone stack. In the Big Muff, the last stage is there to buffer the tone stack from the following load (so its response doesn't change whatever is plugged in next) and make up it's insertion loss. If your pre-amp will be the only thing before the PA input, a buffer stage would be advisable.

[PRR]

That preamp seems to have gain of 3(?).

Most tonestacks have loss of 1:10.

So the amped+toned level is -lower- than you started with.

Also the output impedance of most tonestacks is much higher than you want on a long cable. Tonestack ~~100K, cable wants to see <1K.

The real problem with hi-Z on a long cable is loss of treble. So it "could" come out alright. But different cables, different PA boards, I'd expect it to be not-right more gigs than not.

If you JUST want a treble cut, skip the tonestack. Put 1K in series with the output. Put 10nFd to 100nFd across the output jack. This gives a high-cut at 17KHz to 1.7KHz. You almost certainly do not want 17KHz from a string instrument. Get a handful of 22nFd caps, wire five across the jack (110nFd). When he says "too dull!", snip them out one by one (88, 66, 44nFd) until it balances.

[joeychickenskin]

That makes sense.

Can I remove the pot on the preamp and run the output straight into the tonestack with the tonestack's tone and volume pots as the only pots?

My worry is that the volume pot on the output of the pre-amp would then be 'on full', going into the Tonestack and buffer overdriving them or will the tonestack limit this enough. The comments for the preamp say that the gain isn't huge so maybe it would be Ok

[joeychickenskin]

Sorry, that was a reply to anotherjim.

If the tonestack is likely to create problems, I might leave it out. Even the most rudimentary board will have some tone-shaping and with a stick on piezo, maybe it's over-thinking it to try to provide anything more than a pre-amp. I might make the tonestack/boost combo, try it (I'm sure it will be useful for something else), explore adding the capacitors as another option.

When you say 'across the output jack, PRR, do you mean accross the two lugs of the jack between the signal and ground?

Thank you both for the information and suggestions

[anotherjim]

One thing I like about the big muff tone stack, is the built in ability to have mid range cut that's always there. Piezos are quacky & peaky at the best of times, they usually need a notch filter somewhere in the response. I was assuming that's the reason it was chosen?
Paul's right about the top end. It is unnatural for a guitar to put out anything much over 5kHz, so it's unlikely you would need to turn the knob of the BMP tone to the treble side, but the high-pass components need to be there to get the notch.

For conversations sake, lets assume you can't experiment too much. It's not your guitar, you don't know what it'll be plugged into, and you won't be able to keep tweaking the design - the guy only wants it when its ready.
My instinct is to mod that pre-amp design a little. Make R3 a 1k in series with a new 100k "gain" pot wired as a variable resistor. It can now provide plenty of gain. Delete it's volume control. Follow it with the a copy of the big muff tone stack AND it's last stage and volume control.

Make the tone pot 47k and insert a 47k resistor between the top pot track and C9/R5. This will simulate a 100k control that can only swing between mid-cut and bass boost and never treble boost.
On the pre-amp, put a 100pF cap across R5 to provide some radio suppression and limit the high end response somewhat, because I'd expect the bare piezo to be highly exposed to interference, especially on stage.

Got to love it when someone makes a kludge and then expects you, the electronics guru, to sort it out for him. I have friends like that. It doesn't mean they're bad people