My band has a violin player. She uses a fairly standard stick-on type pickup for her instrument, and has been running it into a Fuhrman (I *think*) preamp. The Fuhrman has been giving us no end of problems. I suspect shoddy construction, and would like to replace it. I'd love to see suggestions for a straightforward preamp (an EQ section would be great, but not required, and a volume pot would be great, but of course that's easy enough to tack on to the end (as is the EQ I suppose...)).
9V power is a must, and I do not need balanced ins/outs. Found a few things online but I wanted to see if anyone here had any good suggestions for this application. Simple would be a plus, too, as I'd like to just perfboard this thing and get it in a box for our next gig....
I recommend going to AMZ Q&D and built comp2, a compressor built based on the SSM2166 IC. When compression is turned all the way down, the IC is a linear amplifier, no compression. It also has a noise gate and can have up to 60 dB gain. I built a compressor based on the SSM2166 for my acoustic guitar. This is an excellent choice for the amplifier section.
Now for the EQ, I think it was GEOFEX that had some great suggestions. A simple EQ is presented in National Semiconductor's TL082 datasheet application notes for what I would consider a very quality 3-band EQ.
This can be found at this link:
http://cache.national.com/ds/TL/TL082.pdf
3 band active tone control found on page 9.
Hmmm....very cool suggestions. Was hoping to just to do it with parts on hand...looks like I'd have to hunt down the SSM2166....Definitely thanks for the suggestion, though.
A resonant instrument like a violin will provide no end of difficulty in a miked situation without any sort of feedback control.
The obvious solution to feedback issues is the use of very steep notch filters. There usually aren't all that many frequencies at which feedback is occurring, and even fewer if you are applying anti-feedback to a single sound source, as opposed to 20 mics around a room.
There are a bazillion parametric equalizer schematics around on the net, and any of those will do nicely. You can also check out some of the anti-feedback solutions found in circuit fragments of the assorted acoustic-instrument processors in Fender Acoustasonic amps, or at the Rane or Rolls websites. RG Keen has a nice little circuit and article on DIY -EQ at geofex.
At the most basic, all you really need for feedback control is a simple tunable bandpass filter. Go to generalguitargadgets and look at the Bi-Filter Follower for an example. This filter is inverting. If you split the input signal, feed half to such a filter then to a mixer, and the other path directly to the mixer stage, you'll get cancellation at the centre-frequency of the filter, precisely because it is inverted, relative to the unaffected signal. Stick a mix-amount pot in there so you can trim back on the amount of filter signal at the mixer stage and you can adjust how much cancellation/cut you get.
Why do that instead of just using a graphic equalizer? The EQ certainly doesn't use any more parts or introduce any more noise. The advantage to the dip-only arrangement is that ALL of the pot's rotation is used for cut (nbo boost capabilities), which gives you more precision in dialing out ONLY as much of the offending frequency band as you need to to stop the howling.
In theory, you should be able to cascade two identical filter sections for greater selectivity in what you cut, although making the two sections track each other perfectly for purposes of tuning the anti-feedback filter to different frequencies might be a tad tricky. You can also have several different such filters in parallel, each tuned to different bands. A quad op-amp, like a TL074 would let you have a two-band tunable anti-feedback filter (one op-amp for input/splitter, one for mixer/ouput, one for bandpass filter 1, another for bandpass filter 2). Two pots for tuning, two more for adjusting amount of cancellation/cut, and you're in business.
Another very different approach that has been used with some modest success in the past is phase adjustment. Acoustic instruments and mics feed back when the output of the speaker system is in phase with the audio input and the signals can sum. If what enters the mic is NOT in phase with the speaker output, then they can't sum. Many acoustic instrument processors nowadays have "invert" built into the preamp to be able to avoid input and speaker output summing. Not perfect, but certainly simple enough since all it needs is a single transistor or op-amp.
You can also be more selective by using something very much like a phase shifter. For instance two fixed allpass stages, exactly like what you would see in a phase shifter, without the sweep, would get you a notch if mixed with a nonaffected signal, but if used without a dry signal would insert a given amount of phase shift from the original signal, starting at some frequency F and increasing from there. A simple dual-ganged pot used to adjust the tuning of the allpass sections would determine where that phase-shift starts in the spectrum and prevent input and speaker output from summing.
I will also put in a vote for a barebones "exciter" circuit as a way of spicing up acoustic input. Look for the "Harmonic Sweetener" in the schematics here, or look at the trbele/high circuit fragment in my "Woody" acoustic simulator.
Honestly, if you want something simple that will work, build an Ibanez TS9, only change the clipping stage .047 uF (? the feedback cap in series with the 470 ohm) to a 10 uF. Get rid of the diodes, remove the 51k resistor in series with the pot, and use the pot as your Volume control. You probably won't need that much gain, so it will be easier to use if you use a 100k pot instead of what is in the TS schematic. This will give you an amplification range from unity to 100-200 ish depending on the pot you use.
Now, to get a better response, take out the TS tone control stage (including the parallel cap at the clipping stage input) and replace it with the EQ from the datasheet page I posted.
This is a kind of a simple cut and paste method for making a preamp.
The only thing this won't do for you is offer feedback cancellation EQ, as Mark Hammer presented. You may want to build a seperate EQ for that later if the gigging is coming soon.
If you decide to use this "TS9" mod idea and can't follow my explanation, I'll pirate a TS schematic from somewhere, touch it up in MS paint to make it clear what I was talking about, and email it to you.
Otherwise, take care, there are, as I'm sure you have found, many simple solutions available on the net.
Transmog: I hear ya. I think I am going to make just a very simple amp as per your suggestion for the time being (we have a show within 2 weeks) and see how that works out. That TL082 active circuit looks plenty simple enough for perfboard as well... The feedback control is definitely something I'll start working with on the breadboard. I've been itching for a good excuse to start really getting my hands dirty figuring something out, and I can't think of a better oppurtunity.
Mark: Thanks so much for the post on feedback control. You are absolutely correct in that midrangey feedback has been an absolute *nightmare* particularly in the rehearsal space, which is very small. To compound the problem, we play very loud, and the singer/songwriter likes delay a little too much, so both the violin and his vocal mic get the reverb and delay laid on pretty thick.