Geofex style ABY box with mods -- hum in unselected amplifier.

[Paul Fawcett]

Hi Guys,

I've just completed an ABY box that rolls together a couple of circuits found on RGs site.  Basically, it combines, in a straightforward way,  RG's Humless ABY splitter box schematic http://www.geofex.com/FX_images/spltr2.gif with his charge pump http://www.geofex.com/circuits/+9_to_-9.htm, so that I can get the bipolar supply required in the first schematic with just a single battery.  The design incorporates the input jack switching arrangement.  I've also taken care to separate the audio and power grounds.

When I bread-boarded this, I noticed some hum in the unselected amp, but I couldn't find anything wrong, in principle with the circuit, so I figured that it would go away once the effect was boxed up. 

The only problem is, of course, that it didn't go away.

Note that the selected amp has no noise problems -- the one that is currently engaged has a noise floor as good as if I were plugging it straight in.  There is also no noise problem if I use the "both" setting -- so the common ground problem, at least, is being properly addressed.   There is only some buzz in whichever amp is unselected... it's a bit like if there was a cable plugged into the amp, but not into a guitar.

Incidentally, I haven't drawn a schematic yet, since it's so derivative from RGs work, but it's simple enough that you can easily see what is going on from the drawing of the PCB: http://wombatamps.blogspot.com/2012/02/hum-splitter.html

If anyone has any advice, or experience with the Geofex splitter, I'd like to hear about it.  It's like the unconnected transformer (which is grounded only on the primary side) is picking up crappola.

[trixdropd]

Hi Guys,

I've just completed an ABY box that rolls together a couple of circuits found on RGs site.  Basically, it combines, in a straightforward way,  RG's Humless ABY splitter box schematic http://www.geofex.com/FX_images/spltr2.gif with his charge pump http://www.geofex.com/circuits/+9_to_-9.htm, so that I can get the bipolar supply required in the first schematic with just a single battery.  The design incorporates the input jack switching arrangement.  I've also taken care to separate the audio and power grounds.

When I bread-boarded this, I noticed some hum in the unselected amp, but I couldn't find anything wrong, in principle with the circuit, so I figured that it would go away once the effect was boxed up. 

The only problem is, of course, that it didn't go away.

Note that the selected amp has no noise problems -- the one that is currently engaged has a noise floor as good as if I were plugging it straight in.  There is also no noise problem if I use the "both" setting -- so the common ground problem, at least, is being properly addressed.   There is only some buzz in whichever amp is unselected... it's a bit like if there was a cable plugged into the amp, but not into a guitar.

Incidentally, I haven't drawn a schematic yet, since it's so derivative from RGs work, but it's simple enough that you can easily see what is going on from the drawing of the PCB: http://wombatamps.blogspot.com/2012/02/hum-splitter.html

If anyone has any advice, or experience with the Geofex splitter, I'd like to hear about it.  It's like the unconnected transformer (which is grounded only on the primary side) is picking up crappola.

Sounds to me like you're not grounding the un-selected amp. This would sound exactly like a guitar cable plugged into amp but not guitar.

[Paul Fawcett]

If you take a peek at the Geofex schematic, you'll see that the point of RGs circuit is to avoid explicitly grounding the amp inputs - each is galvanically isolated from the guitar.  The unselected amp is however always connected to the secondaries of the interstage transformer, which always has one leg grounded on the primary side. 

[R.G.]

One dodge that works sometimes is to tie the two output grounds together through a resistance. How big? Try 100-ish, 1K-ish.

There are so very many ways for hum to get in - as you're finding out.   

[trixdropd]

If you take a peek at the Geofex schematic, you'll see that the point of RGs circuit is to avoid explicitly grounding the amp inputs - each is galvanically isolated from the guitar.  The unselected amp is however always connected to the secondaries of the interstage transformer, which always has one leg grounded on the primary side. 

I understand the design. When I built mine, I did my own switching setup. When a specific amp is deselected, that amp gets shorted to "it's" ground. So I/m not mixing the grounds between amps, but I am shutting up the amp that is not being used.

[PRR]

Stay away from hum sources! The audio transformers will pick up hum. Some wall-warts, guitar amps, rack-gear emit significant hum. Some lamps buzz atrociously. These fields fall very quickly with distance, and often with orientation. Wave the ABY around the area. Where hum/buzz gets worse, note that area as "keep away".

Try loading the circuits. A 2K circuit catches less crap than a 200K circuit (open guitar amp input). The LM833 can easily drive a 2K or two.



As R.G. says, sometimes it aint easy.

[Paul Fawcett]

One dodge that works sometimes is to tie the two output grounds together through a resistance. How big? Try 100-ish, 1K-ish.

There are so very many ways for hum to get in - as you're finding out.  

I just tried this.  It does help a bit...but at the cost of greatly increased bleed-through between A and B, even with a largish resistor.  There is already a very low but perceptible bleed-through (acceptable though), presumably from the transformers talking to one another.   I guess I'll give Paul's suggestion a shot, although it's less clear how I'll shoehorn that in if it turns out to be curative...

Thanks all for your suggestions!

I'm a bit late to the party, I guess, but I also just discovered RGs improved ABY design http://www.geofex.com/FX_images/humfree2.gif  Wish I had noticed that before plunging in!

[R.G.]

I just tried this.  It does help a bit...but at the cost of greatly increased bleed-through between A and B, even with a largish resistor.  There is already a very low but perceptible bleed-through (acceptable though), presumably from the transformers talking to one another.   I guess I'll give Paul's suggestion a shot, although it's less clear how I'll shoehorn that in if it turns out to be curative...

Bummer.

It occurs to me (DOH!) that I ought to rework that design to eliminate a few other hum mechanisms. I think you're right - you're getting bleedthrough from the transformers talking to each other, and the unselected amp is humming because its input is open(ish).  It's probably sidestepping the hum created by connecting two grounds together and dodging into other noise issues. Lemme think on it a bit.

I think I'll work into it a way to clamp the off-amp input to ground when it's deselected. That should cure some issues.

[R.G.]

Take a look at http://www.geofex.com/FX_images/TransformerSplitter.pdf.

If you have a 3PDT, you can rewire the a/b/y switching to ground the unused primary when both are being used. This ought to help with off-side hum and crosstalk.

I haven't had a chance to try it yet. I you do try it, let me know how it works.

[PRR]

Are both 2.2Meg input resistors necessary? One leftover? Or is there a coupling cap missing?

The gain of 2.1 is intentional? I undertand the point-one (coil-loss) but why the 2?  (Nobody complains about gain?)

[trixdropd]

Take a look at http://www.geofex.com/FX_images/TransformerSplitter.pdf.

If you have a 3PDT, you can rewire the a/b/y switching to ground the unused primary when both are being used. This ought to help with off-side hum and crosstalk.

I haven't had a chance to try it yet. I you do try it, let me know how it works.

I just insert the switch between the transformer and jack. It works fine for me with no mods.

[Paul Fawcett]

Take a look at http://www.geofex.com/FX_images/TransformerSplitter.pdf.

If you have a 3PDT, you can rewire the a/b/y switching to ground the unused primary when both are being used. This ought to help with off-side hum and crosstalk.

I haven't had a chance to try it yet. I you do try it, let me know how it works.

Thanks, R.G.  As it transpires, I don't have a 3PDT to hand at the moment, so it'll be a bit before I can try this out, but I'll include a few in my next parts order and surely will give it a whirl.  I'll report back when I do.

Trying Paul's loading mod turns out to be awkward at the moment, since I didn't discover the problem until it was already boxed up.  Before I tear things apart to try it, could you comment on any likely effect on the bass response?  I know that circuit depends on the low impedance output from the opamp to achieve an acceptable bass response out of those cheap transformers, so I'm leery of adding additional loads.....  If it's likely to be problematic, I'll give it a pass for now and try the switch trick first.

Cheers,

Paul

[Paul Fawcett]

Take a look at http://www.geofex.com/FX_images/TransformerSplitter.pdf.

If you have a 3PDT, you can rewire the a/b/y switching to ground the unused primary when both are being used. This ought to help with off-side hum and crosstalk.

I haven't had a chance to try it yet. I you do try it, let me know how it works.

I just insert the switch between the transformer and jack. It works fine for me with no mods.

Hi trixdropd,

I guess this is the switching mod you referred to earlier?

It's unclear to me whether you are talking about a DPDT or 3PDT switch, and exactly how you wired it up.  Can you draw a quicky diagram?

Cheers,

Paul

[R.G.]

Are both 2.2Meg input resistors necessary? One leftover? Or is there a coupling cap missing?

Kinda leftover, kinda just in case. Originally there was a coupling cap. It's not strictly needed for a bipolar supply; however, it's a really good idea. It stops DC from whatever drives the input from affecting it. I took it out because I expected the first question to be "...but why is there a cap there? couldn't you just DC couple the input?" The ready-prepared answer was " yes, but...". But I wanted the circuitry in place for inserting the cap when needed. Probably I should not have economized on typing time and just explained both options.

The gain of 2.1 is intentional? I undertand the point-one (coil-loss) but why the 2?  (Nobody complains about gain?)

Yep.

[trixdropd]

Take a look at http://www.geofex.com/FX_images/TransformerSplitter.pdf.

If you have a 3PDT, you can rewire the a/b/y switching to ground the unused primary when both are being used. This ought to help with off-side hum and crosstalk.

I haven't had a chance to try it yet. I you do try it, let me know how it works.

I just insert the switch between the transformer and jack. It works fine for me with no mods.

primitive drawing.


Hi trixdropd,

I guess this is the switching mod you referred to earlier?

It's unclear to me whether you are talking about a DPDT or 3PDT switch, and exactly how you wired it up.  Can you draw a quicky diagram?

Cheers,

Paul

[Paul Fawcett]

Take a look at http://www.geofex.com/FX_images/TransformerSplitter.pdf.

If you have a 3PDT, you can rewire the a/b/y switching to ground the unused primary when both are being used. This ought to help with off-side hum and crosstalk.

I haven't had a chance to try it yet. I you do try it, let me know how it works.

R.G. -- Finally got my 3PDT switches yesterday, and had a chance to wire this up this morning.  I'm happy to report that it's a winner - the modification has eliminated both the "noise in unselected amp" AND the transformer cross-talk.

Many thanks. 

Paul

[R.G.]

Thanks for the follow up. It's always nice when a guess turns out to be correct.

Now go make LOUD noises. 

[Paul Fawcett]

Well, maybe I'm not to the promised land with this one quite yet.....  The guy I gave it to to test out in a band environment is reporting that it makes loud pops for him when switching.... which is weird, since it didn't make any objectionable popping noises when I tried it at home.  Sigh.  I'll double check the way I wired it to make sure that it's not LED current related or something like that.  If it's strictly from the mechanical switching I'm a bit at a loss what to try this late in the game.

He's also reporting that it whines when run from one of his wallwarts - though I'll bet he was using some noisy cheapo switching thing. I guess I'll have a crack at RGs "Spyder" - an eight output regulated linear supply.

[R.G.]

Popping is *always* a change in DC level when the switch moves. Especially if it didn't pop for you, does for him, then he may have DC on his signal lines in the setup.

Or could be a broken wire, I guess.

As to the whining, could be noise on the power supply, in which case filtering will help. Could also be received EMI from the power supply radiating RF, in which filtering won't. Could be magnetic pickup from the switching power supplies picked up in the transformer windings.

Today's electronic environment is so much more complex than the one in the "Golden Age".

[Paul Fawcett]

Yes, there certainly a lot more HF crappola floating about the ether these days, that's without question.

Since I used a charge pump, I did, in fact, use some modest power supply filtering... 470R/47uF RC filters on both the positive and negative rails.  Although I think I've spotted a small potential problem in that the 2.2K resistors feeding the LEDs +Ve supply is coming off the filtered node shared with the opamp positive rail. Probably it would have been slightly smarter to put these on the raw DC side?

I realize that popping comes from a change in DC level, but I'm struggling a little bit to understand where that could be happening.

Certainly, the transformer primaries are isolated from the opamp output with the parallel 100uF and 100nF caps.

On the other hand, the transformer secondaries are always connected (whether switched into the circuit or grounded on the primary side), so I don't see how the DC level there could change, or even where it would come from....   

But well....come to think of it, my amps are tube amps with traditional 1Meg pull down/grid leak and grid stop straight to the grids.   But he -- let's see -- he plays a couple of funky old Alamo SS/tube hybrid amps (surprisingly nice sounding, BTW) that have BJT input sections.  If I recall, these have from the input, a series resistance, into a coupling cap, into a fixed bias arrangement on the base of an NPN.  No pull down resistor on the input side of the cap.  But even if the cap was leaking DC back out -- since the secondary is always attached there's no DC level change going to happen on the secondary side in any case, right?  Similarly, adding a pull down resistor to his amps would be unlikely to make any difference, right?