Taming piezo pickups

Has anyone played with the problem of "harshness" from piezo pickups on acoustic giutars. I have three guitars with bridge piezo's and they all suffer with this. It is not just a case of too much treble response which would be easy to fix with EQ. It's more that the output is not linear at all frequencies and is much higher on the leading edge of the signal. I made up a little FET comp in which the gate is fed with signal that is passed through a tone control network making the compression frequency selective. This is a big improvement but it needs to be dynamicaly selective too. a combination of the two.
I may not have explained this too well but I am sure that there are many who will be familiar with this problem.

Steve
PS I think fender has something they call "string dynamics" for this.

You problem can be, that you use bridge piezo.
It (as every acceleration pick up) have direction of
best sensing in direction perpendicular to you use by simply
gluing on the resonant plate:

you must glue bridge pick up perpendicular, as it on the
bridge of real violin. On acoustic guitar, there are some pieces of wood
inside is, which oscilates in plane you want.

xvlk

I discovered (as a sound guy dealing with a lot of acoustic guitarists) that a lot of problems are caused by particular piezo pickups which need a very high impedance input - something like 5-10M. A very simple FET circuit can fix that. The biggest result is more bass, but often some midrange and treble weirdness gets cleared up as well.

Turns out I need to revise a couple capacitor values on this circuit, but it still sounds a lot better for some guitars.
http://www.scotthelmke.com/Mint-box-buffer.html

I have made a couple of these and found them very good.
http://www.uwm.edu/People/msw/ActivePlug/BootStrap.html
Stephen

Thanks for that guys. It is not an impedance match problem, they all have FET buffers. Our friend from Prague makes a good point about using contact pickup's rather than under-bridge, but only under-bridge gives that zing I like, just want to tame it down a bit.
Steve

Like Scodiddly posted they like high input R values for min loading.

One thing I have been wanting to build is a tube based unit. Someone that plays a upright asked me about that.

I think a Tube input could be better because of the extra headroom .provided by the higher voltage power supply. Maybe something like 1/2 a 12at7 200V B+ a 47K plate and 2.2K cathode R good cap bypass the other 1/2 a CF out.

I think you're talking about piezo quack as its called, i've read a valve preamp takes the harsness and quack away by a greater degree than a solid state unit.
Vikki(uk)

I have a couple of the $29 B-ringer GI-100's and believe it or not, the speaker emulator on that does an excellent job of taking the "spit" sound out of my Fishman piezo.

I worked with a guy who had a problem like this, his Fishman sounded hard & harsh. We tried a couple of hi Z FET designs with varying results.

He came to work from a gig really excited. He had plugged his axe into a Blackface Twin & loved it. I built the first stage from a twin for him & this became his Pre. Sounded very nice.

Peter

I know it's not DIY, but the Raven Labs stuff does WONDERS to my upright bass with only a bridge transducer. I opened the box up and there were no markings on the ICs...it does the same for acoustic guitar. This pre is the best piezo amplifier I have ever used. I wonder what the secret is.

-BH :guinness:

I've a Raven Labs PMB1 and the markings on the chips have been sanded off but the front end chip in each channel has a black pen mark on them,where the other chips have no pen mark. I think the input chip must be some some sort of instrumentation amp? Anyone know what the chips are as a matter of interest?
V

Any schematics about for the Pendulum audio SPS1? :twisted:
V

I'm skeptical about any particular tube-based circuit reducing "quack", because that quack is really a result of where the pickup is located.

Mixing with a mic will make a huge improvement.

I also follow the Acoustic Guitar forum over at Harmony Central, and one of the tweaks periodically mentioned is adding some kind of "clay shim" (probably baked clay?) to the under-saddle pickup to smooth out the response a bit. I've also seen people recommending a softer playing technique, to avoid mechanically overloading the pickup.

The Calrec eq that many have built on this site, works great on acoustic guitar. I use a PMB1 preamp and feed it into the calrec, great sound shaping tool. Nice one Jacob, i've built 6 of these units now.
Vikki(uk) :green:

Sorry to plug my own circuit, but Don in California reports that the circuit I posted in the "high-gain amplifier" thread makes a great DI. It uses inexpensive tubes and offers shitloads of gain.

Found some blurb on the Fender "String dynamics". They say that the quack comes in at about 6.5khz which is outside of the fundamentals of a guitar so I guess they are talking harmonics. Their answer is to reduce gain arround the 6.5khz by frequency selective compression. Also that the comp has a very fast rise time and about 1/2 second decay. Maybe a tube circuit with a "vactrol". The LED is easy to drive, just got to sort out the filtering of what drives it.
Steve

I didn't know ducks came into the equation - more end of year jargon?
Stephen

Since making the original post I have trawled the net for anything on this. There is a school of thought along the lines that when a string is hit a little harder than just gently the piezo generates spikes of output in the 5-8KHz range.
I have come up with this idea, dont suppose it's really new most things are not, but it's new to me!
I have inserted into a comp sidechain a mixer channel which has a sweepable mid eq with a "Q" control. By sweeping arround 6KHz with plenty of "Q" I got a hefty gain reduction at the "quacking" point and by playing with attack and decay I pretty well got what I was after.
Like most things like this I allready had the tools for the job sitting under my nose.
Steve

The installation is really important. I use a Fishman on my violin bridge, and I found the amount of pressure exerted on the pickup changes the tone greatly. I experimented with mylar shims between the pickup and the slot of the bridge until I achieved a full rich tone (or as rich as you can get from a piezo). In the context of a guitar pickup, i don't know if pressure can be changed. This is probably a function of string pressure on the saddle..

My experience with piezos has been that the only acceptable sound I've heard from them came when they were fed into a preamp with a vacuum tube input stage, and very high impedance. Anything else sounded, well, plastic, for want of a better term. Everyone's comments about placement are apropos, but once you've got it in the right place, it still needs to drive a tube.

Why? Why not a FET? Well, piezos are very high-impedance devices, and FETs have an unfortunate tendency to generate distortion when fed by high impedances.

Peace,
Paul