GGG Foxx Tone Machine build questions

[Csquare4]

Hi folks,

My teenage son built an FTM clone based on the GGG kit and schematic here http://www.generalguitargadgets.com/pdf/ggg_ftm_sc.pdf using the dual footswitch layout http://www.generalguitargadgets.com/pdf/ggg_ftm_lo_2fsw.pdf

This is his 4th pedal build so I pretty much left him alone to do it himself and he did pretty good job.  Only thing is when he fired it up the first time using first one, then another 9V power supply we use for any number of other pedals without issue, there was a massive ground hum.  I had him check all his solder joints, wiring layout and to make sure nothing was "touching" that shouldn't be - all of which were fine.  He checked it with a 9V alkaline and the hum was gone.  So I had him get a regulated power supply from my pedal board and try it.  That one worked like a charm, so we now know that the hum was from the wall warts.

However, when we went to the regulated supply, even though the hum was gone, we had some major hiss on the output when the circuit was in the quiescent no signal stage, less so when the octave was engaged, but still pretty high.  Is this normal?  He checked the voltages on the transistors, all were within a few % of the GGG guidelines, so I am not sure if it is a bias issue.  Just wandering if this is pretty normal for this circuit.

On the hum issue using "cheaper" supplies, I am wondering if it would make sense to put some kind of conditioning filter on the +9V power, as there isn't anything there currently.  I am thinking maybe a 10k in series and a 10uF to ground on the power supply might help with a cheaper 9V supply.  Any thoughts?

On the hiss, is there something I could maybe do to quiet it down a bit, or should I just expect to have to live with it in this circuit?

Thanks for the help!
Craig

[amptramp]

There is no power supply capacitance in this circuit, so it would help to add some.  A 10K resistor in series with the power as you have typed, I assume is a typo - 10 ohms would be good and would be enough to isolate the added capacitance from the wall-wart cap.  Some wall warts use a resistor in series with the power filter cap internally to limit the inrush current, so they are not to be considered as low hum sources.

Hiss is a completely different issue.  It is often from a bad transistor or one that has been brought up to soldering temperature for too long.

[Mark Hammer]

A better solution is a diode, like an 1N4001, and a 220uf-470uf cap to ground.

I've posted in past about a "power distribution block", which you might want to consider.  It is essentially a little plastic box that the wallwart plugs into with an indicator LED (so you know there is power to distribute), a big-ish smoothing cap and diode, and additional output jacks so you can run power patch cables to the various effects on your board.  The patch cables have to be made, so there's a bit of nuisance there.  But the cables can be custom tailored to the location and jack-mounting of the various pedals on the board, leading to a neater board.  Plus, the distribution box can be situated off to a corner, and once you unplug the wallwart from it, the pedalboard has no wires hanging off or needing to be tucked away.

Not for everybody, but it works well in some contexts.

[Csquare4]

A better solution is a diode, like an 1N4001, and a 220uf-470uf cap to ground.

Makes sense...that should be easy enough.  I will have my son try that when he gets home from school - he wouldn't be happy if I played around in his "sandbox" while he wasn't here.

I've posted in past about a "power distribution block", which you might want to consider.  It is essentially a little plastic box that the wallwart plugs into with an indicator LED (so you know there is power to distribute), a big-ish smoothing cap and diode, and additional output jacks so you can run power patch cables to the various effects on your board.  The patch cables have to be made, so there's a bit of nuisance there.  But the cables can be custom tailored to the location and jack-mounting of the various pedals on the board, leading to a neater board.  Plus, the distribution box can be situated off to a corner, and once you unplug the wallwart from it, the pedalboard has no wires hanging off or needing to be tucked away.

Not for everybody, but it works well in some contexts.

I have often thought about building a power block using transformers for isolated power.  I will search out your posts and see what is all in there, might at minimum give me some ideas and the incentive to move.  Thanks for that.

Hiss is a completely different issue.  It is often from a bad transistor or one that has been brought up to soldering temperature for too long.

I have an oscilloscope available to try and isolate the hiss...this may be a good opportunity to teach him how to use it.  He needs to know how to use a probe as well, so I will have him check the transistor outputs and resistors to see if we can figure out where it originates.  I just didn't know if this circuit is inherently noisy.  Btw, we used the 2N3904 transistors.  I may try to swap some of those out to see if we can nail it down to a particular active device if its not the resistors.

Thanks for the comments guys.
Craig

[Ronan]

These 2.1mm DC plug leads might help reduce the cost of a power block. I bought some recently, look like reasonable quality and seem to work fine.

search ebay for "Power Splitter Adapter Cable for CCTV Camera"

$2.19 shipped example:
http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=320808816508

[DavenPaget]

These 2.1mm DC plug leads might help reduce the cost of a power block. I bought some recently, look like reasonable quality and seem to work fine.

search ebay for "Power Splitter Adapter Cable for CCTV Camera"

$2.19 shipped example:
http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=320808816508

http://www.dealextreme.com/p/1-to-8-splitter-cable-for-cctv-security-system-camera-dc-12v-111787 

[azrael]

I recently made a layout for this circuit, and added a 100 pF cap, one at the IN of the circuit, should also help cut down a bit on noise.

Maybe if you're using 2N3565s, try a lower noise transistor, too.

[PRR]

> maybe a 10k in series and a 10uF to ground

The LED alone draws 6mA. The rest of the circuit 1.5mA-2.0mA. Say 8mA total.

Assuming 10K series resistor, we calculate 10K*8mA= 80 volts dropped.

That's obviously impossible on a 9V supply. What you will really get is 3V or less on the power rail, poor-to-crappy audio, no LED.

Assume you can spare 1V. 1V/8mA is 125 ohms. Round down to 100 ohms series.

For good filtering use:

10 ohms - 1,000uFd
100 ohms - 100uFd
1K ohms - 10uFd

Therefore 100 ohms and 100uFd. If a 47 ohm and a 220uFd are hanging around, that's fine also.

Hisss could be because the unit has maxed-out gain of 3,500. I'm pretty sure you never want both Sustain and Volume turned way up in a small room.

> less so when the octave was engaged

If everything were matched and signal level could be dead-zero, the "Octave" mode should have "no gain". Gain happens when signal happens. So it is reasonable that hiss would be less.

[Csquare4]

Thanks for the input Paul.

The LED alone draws 6mA. The rest of the circuit 1.5mA-2.0mA. Say 8mA total.

Assuming 10K series resistor, we calculate 10K*8mA= 80 volts dropped.

That's obviously impossible on a 9V supply. What you will really get is 3V or less on the power rail, poor-to-crappy audio, no LED.

Assume you can spare 1V. 1V/8mA is 125 ohms. Round down to 100 ohms series.

For good filtering use:

10 ohms - 1,000uFd
100 ohms - 100uFd
1K ohms - 10uFd

Therefore 100 ohms and 100uFd. If a 47 ohm and a 220uFd are hanging around, that's fine also.

Hisss could be because the unit has maxed-out gain of 3,500. I'm pretty sure you never want both Sustain and Volume turned way up in a small room.

> less so when the octave was engaged

If everything were matched and signal level could be dead-zero, the "Octave" mode should have "no gain". Gain happens when signal happens. So it is reasonable that hiss would be less.

So here is what I have done so far.  I put a 100R in series from the power plug, with a 220uF to ground at the +9 connection on the board.  I used my breadboard and o-scope with the wall wart to test it before installing.  Without any filtering across a 100k load resistor, the 9V Wall wart was showing about a 30mV p-p sawtooth on top of the ~10V DC.  Using a 100R and 100uF, I knocked it down to ~10mV p-p sinusoid.  With the 220uF and the 100R, it dropped to about 5mV...so I went with that.  Btw, the Boss regulated 9V had no real ripple at all, I was pretty well pleased with that one.

So I thought I had the problem of hum through the noisy PS pretty much licked.  I installed the 100R and 220uF into the Foxx TM and...well, no change.  Still a loud and very annoying hum using the unregulated wall wart. Makes me wonder if I have a bad ground somewhere, even though I checked them all again.  Also, with the Boss, the hum was non-existent for the most part. 

The cool thing about this additional cap and resistor is I think I turned my son's pedal into a short wave receiver!  How lucky is that?  As I played with the volume/sustain knobs, I started hearing the voice of some "end-of-the-worlder" coming from my amp.  I have no idea where he is, but it was quite entertaining for about 1 or 2 minutes.  How this was the last year for mankind and the world as we know it.  Pretty neat stuff for a while.  Now I don't know whether the extra cap and resistor have anything to do with it - it may have been there all along and we just didn't notice it - but I am sure my son probably is not real interested in having a short wave receiver on his pedal board.

So -- back to the drawing board on this one!  I am thinking that regulated multi-tap power block is looking mighty nice right about now...will be a good next project.

Oh, and one more thing, the hiss actually goes down significantly when using battery power.  I was confused by that, but I haven't opened it up to cross check voltages on the transistors with the two power options.  I will do that later.

[azrael]

battery is usually quieter.

Did you try adding a 100pF cap to the input?

[Csquare4]

Azrael -

Did you put the 100pF in series with the input or across it?  I will try that as well. 

Funny thing is with the shortwave radio, I was probing around the circuit and seems to be originating in the final transistor, coming out of the emitter.  Obviously there is a resonance there at whatever frequency this DOOM NOW guy is broadcasting on.

[azrael]

Put one end to the input, the other end to ground. It's just a little low pass filter, may block some hum and that rF you're dealing with.

[Csquare4]

Put one end to the input, the other end to ground. It's just a little low pass filter, may block some hum and that rF you're dealing with.

Ok - thanks for that.  Will give it a try.