Stompbox for Chapman Stick

[StickMan]

******************** Background *************************

OK, so if you don't know what a Chapman Stick is, go here:

http://www.stick.com

The only issue with Stick is that it needs two of everything, since the output is stereo and one side is voiced like a bass and the other like a guitar.  At home, I've got a custom rack rig that has a stereo preamp, built for Stick running into a 300 watt power amp and then a 4x10 bass cabinet.  It's heavy as hell.

What I'd like to be able to do is walk into a situation with a minimum of gear, just like a guitar play, plug in to whatever is there and go.  So that means being able to get good sound out of both the bass & melody sides of the Stick from a transistor bass amp.

What I'm working on is a stereo preproccessor that will also serve as a low power amp, DI box and headphone amp and can put out both stereo and mono.  I'm looking at a pair of LM386 amps, with EQ, compression, delay and distortion (customized for each side).

*************** Question ************************

I played with Ruby and Little Gem layouts and ended up going with a hybrid, it actually pushes the 4x10 cabinet pretty loudly.  I put an AMZ mini booster at the front end and I was happy with the sound.  For the EQ I'm looking at a blend of the Anderton Parametric Equalizer and the Geofex EQ.  The Anderton design has a standard two knob tone stack in the feedback loop of an op amp in front of a mid-range EQ built with MBP filters.  I like the idea -- tone stack in front of mid-range EQ -- but the Anderton design has a complicated arrangement where you have a knob for Q, and two knobs for frequency that seems counter-intuitive.  The Geofex design is very straightforward, with a bunch of 5088 based gyrators and Q of about 1.5.  I figured that three bands of that, set up at the right frequencies should do the trick.

The booster and amp were working, and I set up the 3 band EQ first.  It uses the +/- inputs of an opamp to do the blending.  I plugged it in and....nothing.  Eventually, I disconnected the gyrators and just tried going through the opamp (which is a TL072).  I reconfigured it to use the 10 gain amplifier application from the TL072 information sheet...nothing.  Not quite nothing, it oscillates if I push the output from the booster high enough.  I fiddled with the power setup; I ran straight from the +12V and to ground, I used the 1/2V on pin 4 instead of ground.  I disconnected the booster and when straight from input -> opamp -> LM386....nothing.  I swapped chips...nothing.

Does anyone have any suggestions?  I'm stumped.

thanks,

dave

[Peter Snowberg]

Welcome to the forum!

The Stick has got to be one of the most amazing instruments ever invented.

Without a schematic to work from it's a little hard to guess, but I'll give it a shot. Since you've swapped chips it sounds like it could be a power problem. I don't understand the 1/2V connection on pin 4? Pin 4 should be getting the most negative supply voltage available, which in this case would be ground. If you do have pin 4 connected to your 1/2V supply (I'll call it Vref from this point on), then the opamp can't operate because of the high impedance of your Vref generator which is usually a two resistor divider.

Connect pin 4 to ground, pin 8 to V+, and then measure the voltages on pins 2, 3, 5, and 6. Each of those input pins should be sitting at a voltage 1/2 of the way between the voltage on pins 4 and 8. If one of those inputs is sitting at V+ or ground, that's probably where the problem lies.

If you still can't get sound after connecting pin 4 to ground, Please measure and post the voltages on each opamp pin, relative to ground. I'm sure an answer can be found without too much work.

Two tips on 386 amps.... The first is that the connection between pins 1 and 8 (if used) should have a 10uF cap in series with it. I've seen lots of designs around here that use a gain pot just connected between those pins which does work, but it upsets the DC balance of the 386 input stage and I don't think it sounds as good as when you add the cap. The second tip is that the New Japan Radio JRC386BD is the best sounding 386 that I've tried. You can also run them on 18V to get more volume. Since I'm rambling already, another addition I like to 386 amps is a 500 ohm rheostat in series with the speaker. With the rheostat turned all the way down, I get a really good bedroom volume from a single 10" or 12".

[Mark Hammer]

Don't mean to rain on the parade but isn't the Stick output a little on the headroom-demanding side for a 386?

Not sure what it is equal to in the JRC series, but if you want to stick with a 386 maybe try an LM386-4 which will handle higher outputs at higher supply voltages.  As well, maybe the TBA820M or LM380 might be more to your tastes in terms of delivering relatively "clean" audible sound.  Both take kindly to 12v supplies and  deliver surprising levels into 8 ohm loads.

[StephenGiles]

This might give you an idea or 2.
http://www.rane.com/sp13.html
Stephen

[spudulike]

TBA820M Clean stereo power amp (2W) - pretty loud at 12v into a 2x12.

http://tacashi.tripod.com/elctrncs/poweramp/poweramp.htm

[StickMan]

Thanks for the replies!

The Rane SP-13 is cool device, but....

1.  It's very dry sounding.
2.  It's rack mounted.

Lots of player's use them, and they're good in a rack rig where you have the luxury of using all kinds of other gizmo's to tweak the sound.  A lot of player's use them coupled with Boss SE-50's or SE-70's.  Personally, I prefer tubes where possible, analog SS if you can't do that and digital only if all else fails.  SP-13 + SE-50 just doesn't do it for me.  I've got a custom made two channel tube preamp, which I think sounds way better than an SP-13.  It's also rack mounted.

What I wanted was something that could fit into a gig bag; so that you could walk into a jam situation, pull out the Stick and the stompbox and plug it into any old generic SS bass amp and get good sound.

I'm not sure what was meant by "headroom" for the 386.  The Stick has several available pickups, the one on mine is a EMG active pickup which has decent output levels (but still lower than a guitar).  The two passive pickups have significantly lower output than a standard guitar.  The AMZ booster at the front end is supposed to deal with that, and allow the levels to come up high enough so that the standard circuits (compressor, distortion, etc...) will perform as expected.

With just the booster and the 386, the thing sounded OK.  It was noisy, but I'm assuming that's mostly because it's all open on one of those white breadboard things, with the pots hovering over top like antennae.

I'm confused with the TL072 power hookup stuff.  I've seen all kinds of schematics that do all kinds of things.  I'll try the simplest setup which is pin 8 almost straight off the +ve of the battery, and pin 4 straight to ground.  Then I'll check the voltage on the pins, as suggested.
I'll disconnect the booster, and run the TL072 as a unity gain buffer and see what happens.  Thanks again for that advice.

dave.

[StickMan]

At the end of the day, I'm not sure how many things I had wrong.  I ended up stripping everything out and just trying to get a unity buffer working with the opamp. 

Of all the stupid things, I left out the DC blocking cap between the opamp output and the LM386 input.  Once I had that going, I just kept going on building a Shaka circuit out of it.


dave.