New preamp for electric violin

[mayfly]

Here's a cool one.  my old friend Theresa bought an electric violin for her prodigy violinist daughter.  It's a cool piece of modern design, but unfortunately it sounds like crap.

The issue appears to be the pickup, or more specifically the active electronics after the pickup.  It is totally frapping out.

Theresa and I isolated the piezo pickup and the audio is clean from there.  So now onto looking at the guts of the electronics!

... More to come.

 

[stratamania]

Maybe you need something like this device.

http://www.radialeng.com/pzdi.php

 

[timezon3]

Neat project.  You planning on trying to fix the unit, replace it with something off-the-shelf, or (since I know you used to build amps), build something custom?

TZ

 

[mayfly]

All options are on the table!  However after having a look at the electronics I figure I'll end up building something custom.

 

[BigSteve22]

Another thread I'll be following! It's a miracle I get anything done.........  ???

 

[mayfly]

Gentlepeople,

Today I decided to see just how much level the pickup could put out on its own.  So I pulled the electronics out of the back, hooked up my probes directly to the piezo outputs, and had a look on the scope.

Here's the setup:

I directly connected the pickup output to the scope:

Here's what the scope sees when plucking the string:

Holy crap! over 9V?!?  When plucking with vigor I could get over 12V on the thing.  It was pretty crazy.  Bowing was less, but still I could get over 9V without too much effort. 

The preamp is a typical 9V job.  Sending 12V into the thing would no doubt drive it into clipping, generating that nasty distorted tone.  I tried it with a 1M resistor across the leads and there was no change that I could see.

So!  Now we know why.  Now - how to fix it and create a useful musical tool out of it?

... more to come...

 

[Cagey]

Holy crap is right! I would never have expected levels like that. Was there any load on the output? Because you can see ridiculous levels just on your fingertips since the input impedance of the 'scope is so high. If you didn't have the instrument plugged into an amp, try looking its output levels when it is. I expect they'll be back down in the mv range where you'd expect.

 

[mayfly]

Holy crap is right! I would never have expected levels like that. Was there any load on the output?

I put 1M across it. 

 

[Cagey]

Interesting. I wonder why the output is so high? Guitars don't generate anything like that.

 

[mayfly]

Time to look at the electronics!  The input impedance I measured to be around 2M.  Looking at the unit closely it appears to be a re-purposed acoustic guitar unit.  I suspect that whoever made it never suspected that the unit would be subjected to over 12V on the input.  So I decided to give that a try!!

I got my beautifully restored Heathkit signal generator, ran it into the input, and put a scope on the output.  Here's the test rig:

Here's what the scope sees.  The yellow trace is the input, and the blue trace is the output.  As you can see, it starts to clip well before 9V!

The gain appears to be about 1, but there is considerable group delay there...

 

[mayfly]

Cranking things up a bit, we can see clipping on both sides when the input reaches about 6V P-P

When we go further, things really begin to freak out!

Nice!  It's pretty clear that the preamp just can't handle those kinds of input levels.

 

[Cagey]

So, it's real after all. Hmm. Antonio Stradivari noticed the same thing back when he was making violins back at the dawn of time. He blamed it on his oscilloscope; friends and family blamed it on side effects from his addiction to Dr. Strangelove's Miracle Elixir, which was basically just liquid opium mixed with water, sugar, and vanilla extract. Of course, oscilloscopes back then were subject to all sorts external interference. There were more gods and demons on the loose then than there are now, so who knows what he was seeing? It's been posited that oscilloscopes didn't even exist in those days, that they were also figments of his overactive imagination, but that's just silly.

 

[stratamania]

Some of the Piezos on violins have an impedance up to 10 Megs. That's what that radial box I linked to deals with.

 

[mayfly]

Gentlemen,

Here's what I ended up doing.  I traced out the input circuit to see what was on there.  As typical of single supply circuits, there was a coupling capacitor, leading to 1M input resistor, then going to a bias network between power and ground.  The bias network was 900K for each leg, with an equivalent AC resistance of around 450K.  This is intended to bias the input to half the supply level.  What it means from a practical point of view is that there is a voltage divider on the input of 1M to 450K, possibly as an attempt to lower the input level to something more practical for the preamp. 

My strategy was to replace that initial 1M resistor by something larger.  I decided on 3.3M!  I actually did some calculations and determined that it should tame the input to something around 5 V on a 12V transient.

I soldered it up and put it all back together.  Here's the what the scope sees:

Input is Blue, and the output is Yellow.  Note that I managed to get the input to over 18V P-P with some excessive plucking!  The output is a reasonable 4V P-P.  Amplitude aside, the input and output waveforms match.  Well, pretty close.

I buttoned it up, hooked it up to the PA, and it sounds pretty good - at least to me.  Before I declare victory though I'll need to have the player have a go at it and see what she thinks.  Stay tuned.

 

[mayfly]

Gentlemen,

Rehearsal was tonight and both Theresa and Deanna tried the violin.  In fact, Deanna used it for the entire rehearsal.

Results:  No sign of the original distortion and general 'frapping out' of the electronics.  In fact, the tone was pretty good by everyone's ears.  Theresa took it home to her daughter who subsequently gave it the thumbs up. 

I managed to turn the thing into a musical instrument!  I hereby declare victory! 

 

[Cagey]

Good job, and congratulations! For your trouble...

 

[mayfly]

Thanks man! <sniff>

 

[stratamania]

A Canadian Victory deserves a song...
[youtube]
https://www.youtube.com/watch?v=rMYDuPWHFAo[/youtube]

 

[BigSteve22]

I second that "Attaboy" !
Ya done a good thing for a friend.  :icon_thumright:

 

[mayfly]

Thanks man! 

I hope the owner (Theresa's daughter) goes out there and tears up the stage with it!