Going crazy with hum free ABY and transformer isolation

[pb3000]

I'm trying to build an ABY box to run my guitar signal to 2 tube amps, or switch between either.  All attempts have resulted in tremendous amounts of new hum.

I've been building effects since the early 90s and the Anderton book.  I've built tube amps without hum.  Built complicated delays and modulation pedals.  I've never had this kind of problem with such a simple circuit.

I'm working off of a breadboard since I am trying different buffering/splitting options including the latest RG Keen hum free ABY but also the AMZ buffer/splitter.  Since I have had such troubles with the "Both amps" situation I am now just simply trying to get my guitar signal through a buffer and out one isolation transformer and into one amp without humming.  I have not been successful in many days of trying.

I have my guitar input going into the bread board, and then through either just one section of the AMZ buffer/splitter (http://www.muzique.com/lab/splitter.htm)  or the Keen aby circuit (http://www.geofex.com/FX_images/spltr2.gif - yes I know about the updated one but other than routing switches it's the same and I'm not yet at the routing portion).  After either buffer, the signal goes through the mouser 42TM018 transformer out to a single amp.  This hums loudly independent of guitar volume, like there is an open ground.  I have the amp and a one spot power supply plugged into same outlet.  I have the circuit grounds and the one spot power supply ground tied together on the breadboard (tried just a 9v battery, too btw, no change).  the input jack is grounded on the breadboard, the output jack is grounded on the mouser transformer.

after trying all sorts of things to fix it, I notice that if i tie both jacks' grounds together, the hum goes away and all is well.  but isn't that defeating the purpose of the transformer, by now no longer isolating the output ground?

I hope i've provided enough info and context.  Thanks to anyone with any ideas to help.

[JohnForeman]

lift the ground on one of the amps by using a plug adapter. 

[alfafalfa]

lift the ground on one of the amps by using a plug adapter.

Imho the whole idea of the aby humfree  box is not having to do what you suggest because it is a dangerous practice.

Especially if you use tubeamps with high voltages , keep both amps grounded.

Alf

[R.G.]

I'm trying to build an ABY box to run my guitar signal to 2 tube amps, or switch between either.  All attempts have resulted in tremendous amounts of new hum.

I've been building effects since the early 90s and the Anderton book.  I've built tube amps without hum.  Built complicated delays and modulation pedals.  I've never had this kind of problem with such a simple circuit.

I'm working off of a breadboard since I am trying different buffering/splitting options including the latest RG Keen hum free ABY but also the AMZ buffer/splitter.  Since I have had such troubles with the "Both amps" situation I am now just simply trying to get my guitar signal through a buffer and out one isolation transformer and into one amp without humming.  I have not been successful in many days of trying.

If all the fixes you've tried have changed nothing, that is telling you that you haven't touched the part that's causing the hum yet.

I have quit using breadboards entirely. They were useful, but sometimes maddening, as they had isolated sections of connectors that I was always forgetting about and the contacts got loose over time with use. One possible cause is that your breadboard "ground" may not be going where you think it is.

I have my guitar input going into the bread board, and then through either just one section of the AMZ buffer/splitter (http://www.muzique.com/lab/splitter.htm)  or the Keen aby circuit (http://www.geofex.com/FX_images/spltr2.gif - yes I know about the updated one but other than routing switches it's the same and I'm not yet at the routing portion).  After either buffer, the signal goes through the mouser 42TM018 transformer out to a single amp.  This hums loudly independent of guitar volume, like there is an open ground.  I have the amp and a one spot power supply plugged into same outlet.  I have the circuit grounds and the one spot power supply ground tied together on the breadboard (tried just a 9v battery, too btw, no change).  the input jack is grounded on the breadboard, the output jack is grounded on the mouser transformer.

The AMZ "splitter" offers no ground isolation at all. If that hums, your circuit has grounding issues completely outside whether or not the transformer isolator is working properly.

When things are confusing, simplify. What happens when you make your breadboard be a resistive divider? Say a 100K from the input jack to another 100K to ground, with the output jack hot connected to the junction of the two 100ks? If that hums, all is not well with the "ground" you think you have from the input jack to the 100K to the output jack.

after trying all sorts of things to fix it, I notice that if i tie both jacks' grounds together, the hum goes away and all is well.  but isn't that defeating the purpose of the transformer, by now no longer isolating the output ground?

The issue may well be that the input jack ground has to go SOMEWHERE. Your input jack has to have continuity to the circuit's ground so the input circuit has a complete loop. If that's not true, anything you connect to the output jack is merely a large and possibly complex hum antenna. So connect the input jack to the circuit ground. Then work on isolating the output jack ground, since that's where the amp isolation has to happen.

[pb3000]

I have quit using breadboards entirely.

I have been wondering if that's a problem, so thanks for that tip i will hardwire as much as I can.

The AMZ "splitter" offers no ground isolation at all. If that hums, your circuit has grounding issues completely outside whether or not the transformer isolator is working properly.

Thanks, this i know...I am simply considering using the AMZ splitter as the spitting portion, and then run its outputs through transformer isolation.  This doesn't hum in and of itself into one amp - when I try it into two, i get hum and volume loss on one side.  I will troubleshoot that later...

When things are confusing, simplify. What happens when you make your breadboard be a resistive divider? Say a 100K from the input jack to another 100K to ground, with the output jack hot connected to the junction of the two 100ks? If that hums, all is not well with the "ground" you think you have from the input jack to the 100K to the output jack.

Exactly, hence trying to solely focus on a buffer and isolating transformer into one amp, and getting that working as desired before trying to split to another amp.

The issue may well be that the input jack ground has to go SOMEWHERE. Your input jack has to have continuity to the circuit's ground so the input circuit has a complete loop. If that's not true, anything you connect to the output jack is merely a large and possibly complex hum antenna. So connect the input jack to the circuit ground. Then work on isolating the output jack ground, since that's where the amp isolation has to happen.

Thanks for this! You've given me a few things to think about.  I will reply after trying out some more.

[R.G.]

OK. Hum problems seem simple because if you happen to fix the one preeminent issue, it seems like magic - until you get hit with one of the many subtle ways it can creep in.

Let me know how you get on.

[wavley]

If you are experiencing volume loss, you may have yet another problem on top of the hum.  Not all amps are the same phase input to output (or in the case of fenders with reverbs, the two channels are out of phase with each other)  I had to install a phase reversal switch on the primary of one of my transformers to make my fender play nice with my traynor, but I have to flip it back for the cornford to play nice with the fender because both amps flip the phase 180 degrees input to output.

[pb3000]

Thanks for the phase tip...i'm aware of that problem but ignored it for now because I was still just trying to get one amp going through the transformer.

I got it working well yesterday, first with one output.  For breadboarding, I have a setup like this:

- small enclosure with two jacks, wired to a DPDT switch (floating, not secured to the enclosure) that will either connect them or sent them to leads (this is for testing effects circuits), and a 9v jack connected to 2 leads (plastic so not grounded to enclosure). 
- I typically run the 9v/gnd leads from power to the bread board, and the signal/gnd leads from the switch to the board to test an effect circuit.

For this project, though, I have the jacks connected by wire but physically pulled off the enclosure so they are isolated (in the final box they'll be shoulder washer isolated from enclosure).  the power i send to to the breadboard (Another reason i'm using AMZ splitter for now, which is a 9v circuit rather than RGs, which is a bipolar circuit).  The signal in jack I ground on the breadboard, and the signal out i ground on the floating transformer secondary.

I got everything to stop humming by simply grounding the enclosure itself on the breadboard.  Not sure why, since no components are actually touching it!

So then I moved on to the splitting scenario.  it works pretty well, but only under some strange situations. 

For starters, I do not get the same signal strength going to the amps through the transformer isolated splitter circuit as directly plugging in.  it's not a phase thing either.

I get a little hum with this setup, not horrible but not what i'm looking for either.

Here's the weird thing:

if I add my diy Super Hard On pedal in front of the splitter, and daisy chain power to it off the same power supply going to the splitter, then:

with SHO bypassed, I get a little hum that almost all goes away if i touch the metal sleeve of one of the output cables (or i touch any metal switch on one amp, like the power or standby).  it all goes away if i also hand mute the guitar strings (guitar is properly grounded itself, and i tried multiple guitars too) at the same time.

with SHO engaged, all hum stops completely.  virtually silent amps. the amps sound good but there is a signal loss on both, NOT phase related.  this can be mitigated by turning up the SHO, but that's not exactly what I'm looking for.  I will try the RG LM833 bipolar buffer next.  maybe that helps with the signal loss but i doubt it will fix the hum.

What could cause another pedal to silence the hum?  Especially when it's on?  An impedance thing?  I find this confusing because I'm now daisy chaining power, making a new ground loop! 

FYI, working concurrently on the RG Spyder, using 10 inexpensive, double secondary transformers that should give me at least 16 independent isolated 9v power taps plus a -9v or two, an 18 or 20v tap, and a bipolar +/-9v tap.   looking forward to finishing that one for the ultimate, massive, hum free pedal board feeding a two amp, a/b or stereo rig...

Thanks for continued discussion and help!!

[Willow]

G'day PB.
Did you have any luck?
I have been trying a similar thing (different buffer circuit but similar principle). I got to the stage where, with breadboard circuit, everything was working ok.
Put it into the Alloy case and got a lot of hum and actually tuned into the local commercial radio station (I think this was the most disheartening part) when I flicked to drive channel.
I removed the transformer, ran it as a dual buffered output and everything was fine. No noise at all. Even with both channels engaged.
So I tried a different transformer - I'm using a 10k 1:1 Tranny - and the same noise comes back when on drive channel. Note - it is quiet on clean channels.
I haven't tried the ABY after all my other pedals so don't know if it is an impedance issue / filter issue.
Id love to know if you found a solution!
I can only attribute my noise to the transformer itself, so can only assume it is acting as an antenna! Even in an enclosed case.
I sort of proved this by connecting only the transformer output to a socket, disconnected the transformer input. still had the resistor/capacitor circuit between output poles of tranny.
plugged a cable to my amp from the socket. Same noise!
Also, have tried it powered by a 9v Battery and Power supply with no significant difference.
Scratching my head too!

[anchovie]

I removed the transformer, ran it as a dual buffered output and everything was fine. No noise at all. Even with both channels engaged.

At the expense of never knowing fully what was wrong with the transformer-based circuit, you've found a solution that works for your setup. You could stop here and get on with enjoying playing guitar! 

[blackieNYC]

But wait!  I think RG's circuit will prepare you for the most demanding amp split - where they are on two different AC power circuits, with the ever present difference in ground potential.  (Unless there is studio or "hospital" wiring.)
If you're not using the transformer, but your amps are on the same circuit, you might indeed get no hum.  I used to split two amps onstage, and yes I initially lifted the third prong (US ground) on one tube amp - bad, not smart. Eventually I wired a female and a male jack to a 1:1 xformer and it was fine just like that - I didn't know there was a better way, so of course it worked well. Ignorance being bliss, I never had to build that circuit. Anyway,
1. You might not be prepared to split to to amps in this highly likely situation.
2. I have always wondered why RG's circuit requires two transformers.  Why isn't one enough to split to the amps? (I am only slightly less blissful nowadays.)

[karbomusic]

I removed the transformer, ran it as a dual buffered output and everything was fine. No noise at all. Even with both channels engaged.

I built similar with a 4-way splitter a few months back using the AMZ quad buffer. I used "out 1" with a 10k transformer and often had hum. I traced it back to the transformer picking up EMF from power sources outside the pedal. Easy to diagnose because when I used any two outs other than Out 1, no hum. I had the same complaint as you, "I added the transformer to stop the exact same thing it is causing".  

Here's the thread where I was originally troubleshooting it in the rare event our issues are the same (they may not be).

http://www.diystompboxes.com/smfforum/index.php?topic=107139.0

[R.G.]

I think I mentioned that there are many ways that hum creeps in. One of them is through an emitter of magnetic field hum, like a leaky power transformer. In this case, isolation transformers that are not excruciatingly shielded can act as hum pickups and make hum worse. It's a completely different source of hum, but sounds the same.

I begin to suspect this is the issue with the OP, especially if removing the transformers made hum worse.

Hum is a problem that comes from outside the guitar-FX-amp chain, so hum fixes are ways to make the signal chain more immune to it. In the electrical/magnetic field sense, most buildings are cages of field-emitting wires, all spitting out hum frequencies. An amp is a powerful hum emitter (the power transformer, wiring and heater wires, if any) that's right on top of your signal. Fluorescent lights spit out RF pulses at 2x line frequency, rectifier diodes making DC from AC can spit out pulses of RF, and switching power supplies can spit out RF fields on many frequencies which are modulated with 1X, 2X, 3X, etc.  the line frequency as the PWM regulation "rides" the ripple and hum it's compensating out to get static DC. And that's only the ones I can think of before finishing the first cup of coffee. It's everywhere.

There are reasons that the really high-dollar input transformers are expensive. One is fancy core materials and winding. Another is corporate greed, perhaps. But a big one is that the really good ones will have several nested layers of electrical and magnetic shielding to try to shield the transformer itself from radiated fields. Shielding is something to which there's no easy short cut; shielding out interference is difficult and expensive.

The transformer isolator is a great deal for some of the more likely hum sources, specifically the ones that come from electrical leakage and offset issues in two- or three-amp setups. But it can't stop all hum; otherwise the professional sound world would be littered with those $3 transformers and we would never discuss hum. Getting rid of hum is like peeling layers off onions - you peel off the big outer ones and get to the next, and hopefully smaller ones. At some point, you feel ike you've peeled enough and live with what's left.

As to why there are two: much of Geofex is stealth education. In this case, there are two to show the way to do isolators for more than one.

[Willow]

Anchovie, I agree that I should be playing more, which I managed to get to tonight. But, as Blackie says, I'm preparing myself for the many and varied conditions I'll face on stage with 2 amps.
It is currently working ok. Have very minor hum on Hard overdrive with 2 amps. Still with no other pedals in the chain.
I have one socket on the output of buffer circuit 1.
I have one transformer following buffer circuit 2 which runs to a socket with r/c between signal and ground. I have also run a lead from the negative on the transformer output socket to ground pin of the other socket which is disconnected from ground when plug inserted and connects to ground when plug removed.
This way I get no hum when either is disconnected. Which was a problem before this addition!
I also reversed the polarity of the transformer, so signal is reversed on output. I don't think this had a major impact though.
Anyway, I'll try do drill down the problem and articulate it when I'm not so tired!

[Willow]

I'm hearing(reading?) you R.G.
I have peeled enough onions for now!
I was very sceptical about the transformer from the start due to the lack of shielding, but I didn't want to throw dollars at a more expensive one until I'd proven the circuit and poked around with a few curiosities.
I have to say that the tone retention is very good though with the cheap tranny I bought.
Thanks for the info and the schematics! Awesome to be able to rock in here and nut out problems!

[wavley]

What RG said.

I used his Humfree as a jumping off point and adapted it to my needs, as designed it's fantastic I just wanted a couple of different features/parts I had on hand.  

What I learned during this experience is that while it broke my ground loops it didn't have enough low end for my Bass VI and if I fed it enough gain the transformers saturated (enter the $6 edcor transformers), also it's fantastic to be able to flip the phase of one of the transformers so if you have two amps out of phase with each other (like a traynor which is phase coherent from input to output and the reverb channel of a fender which isn't), I also learned about where the other hum sources are on my pedalboard... so I solved the ground loop, but now I have electromagnetic hum from where the ac comes into my board, the transformer for my multi power supply, wall warts, etc.  So then we start learning about shielding, but even with mu-metal and copper bands on my transformers in a junction box wrapped in mu-metal distance from all of these things is still pretty necessary, luckily I have a corner of my pedalboard that is away from all that stuff and all pedalboard layouts have to revolve around these electromagnetic relationships (like my tube preamp has to be at the other far reach of my board for the same reason).

RG's onion analogy is perfect but incomplete because with removing each layer you also cry a little because you have to learn about how to remove the next layer.

[karbomusic]

What RG said.

I used his Humfree as a jumping off point and adapted it to my needs, as designed it's fantastic I just wanted a couple of different features/parts I had on hand.  

What I learned during this experience is that while it broke my ground loops it didn't have enough low end for my Bass VI and if I fed it enough gain the transformers saturated (enter the $6 edcor transformers), also it's fantastic to be able to flip the phase of one of the transformers so if you have two amps out of phase with each other (like a traynor which is phase coherent from input to output and the reverb channel of a fender which isn't), I also learned about where the other hum sources are on my pedalboard... so I solved the ground loop, but now I have electromagnetic hum from where the ac comes into my board, the transformer for my multi power supply, wall warts, etc.  So then we start learning about shielding, but even with mu-metal and copper bands on my transformers in a junction box wrapped in mu-metal distance from all of these things is still pretty necessary, luckily I have a corner of my pedalboard that is away from all that stuff and all pedalboard layouts have to revolve around these electromagnetic relationships (like my tube preamp has to be at the other far reach of my board for the same reason).

Exactly the same idea here, phase switch on the transformer. I even have dip switches to isolate the grounds individually per output if needed. In the end it is great but I still have the issue of the transformer wanting to pick up power hum anytime it is anywhere close to any such signals.

[pb3000]

It's been a while, but I've finally returned to this.  I have hardwired the input opamp section of RGs ABY circuit together, and connect it through the breadboard to one amp directly and one amp via the mouser transformer.  It hums about at bad as simply using a stereo pedal, of which i have several.  This is alot better than where I was before. 

Here's a confusing part - I have the switching and jacks enclosed in a metal box, with the transformer isolated output jack going through a nylon isolation washer (right? can't have grounds of the 2 output jacks connected through the box).  Yet they are connected regardless (i.e. ~0 ohms) I assume through the fact that grounds on the amps are connected through the signal grounds in the instrument cable shields.  I'm asking myself why this is all the case and what I am missing.  I can measure resistance between jack grounds, amp chassis, circuit board, etc and they are all interconnected.

Despite not being a gigging musician, I'm almost at the point of just ponying up and buying a Lehle box to see if that will solve this all.  I love DIY but I'm spending more time that I need to on this and the Lehle comes with a transformer specifically tuned for guitar signals so why not? 

Does the mouser transformer need some sort of shielding?

Thanks

[trixdropd]

The circuit works fine. I've built it.

[PRR]

> Yet they are connected regardless (i.e. ~0 ohms) I assume through the fact that grounds on the amps are connected through the signal grounds in the instrument cable shields.

Divide and conquer. Then measure Ohms. Try with the guitar cords unplugged. Try with the amplifiers unplugged from the wall.

Yes, in the "real" situation there *will* be "zero Ohms" from amp to amp. For user safety we MUST connect any/all chassis to each other and to all touchable metal in the building (and dirt if playing near the stuff). That's what third-pin plugs and 3-hole outlets are for.

These power-outlet grounds are pretty hefty wires, but may have 0.1 Ohm in them, and often carry stray leakage currents. They often have hum-levels as large as small guitar levels.

The "isolator" is to avoid having ANOTHER ground path between the two amplifiers on the signal side of the system. That's why you want to break the known wall-ground path, and then the audio cable path, and be sure there isn't a A-B ground connection in the isolator or the cables.