Quad Buffer Mod Issue

[karbomusic]

Not a huge issue but probably worth solving at some point since this build is so close to being perfect for me. I built Jack Orman's Quad Buffer off of the CD. Great circuit btw and I added 3 main mods:

1.  Added a Edcor PC10k on output 1 so I'd have at least one isolated output.
2.  Polarity switch for the Edcor/Out 1
3.  4 SPST DIP switch on the outputs that act as ground lifts (one of those if you ever need it, it's there to try options). The outputs are isolated from the rest of the circuit casing to accomplish and everything should be star grounded.

It works great, love it but I noticed Out 1 (transformer isolated) has a slight hum which ironically was added to prevent ground loop/hum issues. It sounds exactly as if the input has an unconnected cable in it just not as bad. So much so I have mistaken it for such on two occasions. The rub is that I have already tried various grounding tests to see if that was it and it doesn't "seem" to be. It doesn't seem to be proximity related either, IOW in the pic below you'll notice the transformer has it's own daughter board, removing it and moving it doesn't affect anything. I also shielded the cable run from the transformer to the output (not in this pic) and that wasn't it either. Edcor appears to be wired correctly best I can tell and also occurs when using a battery.

What am I missing? Was this just a silly idea? My 4th mod (passthrough switch on out 4 was silly which is irrelevant but happy to explain if anyone doesn't already know what problem that causes).  .

[PRR]

If the transformer is picking up room hum, moving it around the room will make it hum more (and maybe less).

A closed metal box helps. An iron box may help more.

That transformer is awful close to input, output, and power jacks.

Are those audio jacks insulated from case? If not, the ground-lift doesn't lift.

[karbomusic]

If the transformer is picking up room hum, moving it around the room will make it hum more (and maybe less).

A closed metal box helps. An iron box may help more.

That transformer is awful close to input, output, and power jacks.

Are those audio jacks insulated from case? If not, the ground-lift doesn't lift.

Metal case properly shielded, outputs = isolated from case, ground lifts because if I throw the switch for channel N and there isn't a ground loop, the signal dies in channel N (and hums due to the open circuit) because there literally is no ground at that point. Occurs with a battery for power, relocating the transformer internally = no difference. Need to double check moving the box around the room though but only happens on the transformer output with outputs 2,3,4 being quiet as a mouse. That being said, thanks for the suggestions and I'm still trying to think of anything else I can think of or to describe.

Edit: I did notice the hum is slightly more pronounced if I flip the polarity switch I made for the transformer which makes me wonder if I'm creating some situation where the transformer output and only the transformer output is susceptible. The output side of the transformer is truly isolated from the case and I did go back and manually shield the + output because it was the longest. hmm... seems like the case being sort of a faraday cage and grounded would still catch that but maybe I'm thinking about it wrong. Any other takers/comments welcome before I take the steps of completely shielding both + and - transformer outs as I'd like to be more sure; it will be a bit tedious to do. Maybe the outputs on the tranny need true shielded wires on both+ and - with the shield going to the casing.

[karbomusic]

Any more takers?  Trying to get additional input before I rip it apart or better said, I don't want to rip it apart unless I have something distinct to test/try.

[mth5044]

How are those jacks isolated from the enclosure?

[karbomusic]

Rubber for lack of a better term, plasti-dip + some other stuff. I've checked the outputs with a meter so they are truly isolated from the case with the possible exception of the input. If I throw the ground lift on an output, it completely disconnects the signal if there is no ground loop. Also, just to be clear only the transformer output has the issue everything else works exactly as intended.

Short story, checking with the meter shows isolation where I expect/want isolation to be assuming I'm not simply thinking about it wrong.


[EDIT]

Also note.... the ground lift switches are somewhat irrelevant here, that's a separate feature that isn't being used while testing this issue. Additionally the ground loop switch on Out 1 is even more irrelevant because I added the transformer on Out 1. The transformer is a mod I added after building. Sorry if I confused anyone. I'm really only troubleshooting the hum I get on output 1 after the addition of the tranny on output 1.

[karbomusic]

OK, testing tonight I was able to move it around and found it is picking up the hum externally not generating it. Anytime it is near power it picks it up the 60Hz hum. This is from Output 1 the channel with the transformer. If I switch from output 1 to output 2 it is gone. Meaning that when using the non-transformer output instead, I can literally set the pedal on my power supply and it will be dead silent.

What I don't know is how to fix it or if it can be fixed. Is this the wires going from the output of the transformer or is the transformer itself inductively picking it up? It get's better when I flip the polarity switch but I don't know if that is because the - wire is much shorter than the + wire or if some of it is simply cancelling out somehow. Halp meh!  

This is very close to becoming an ideal tool for me. It is already but I'd really like to get rid of that small hum on out 1 as it would be perfect then.

[karbomusic]

^Any ideas on my latest findings? Not sure if I mentioned cake and ice cream is involved.

[Quackzed]

ironically was added to prevent ground loop/hum issues.

if thats the only reason its there, but the other output-without the transformer- is hum free, then just remove it?

not really a solution, but ,if it ain't broke...
you could see if getting it far from in/out and power lines/jacks fixes it, but that'll only tell you that its induced from those sources, it won't help to make more room for the transf. still if 'why' is what you wanna know you could try it and find out if thats it, THEN remove it...

[karbomusic]

ironically was added to prevent ground loop/hum issues.

if thats the only reason its there, but the other output-without the transformer- is hum free, then just remove it?

not really a solution, but ,if it ain't broke...
you could see if getting it far from in/out and power lines/jacks fixes it, but that'll only tell you that its induced from those sources, it won't help to make more room for the transf. still if 'why' is what you wanna know you could try it and find out if thats it, THEN remove it...

Well thank you for answering!! I was starting to feel a bit lonely in here; I have the Ice Cream and Cake people on the horn as I type this.

It's not an internal location problem that I can prove; it is literally picking up ac mains power and transformers in the room through the air not through wires or internally as far as "I" know from my testing. Makes me feel like I missed a silly step somewhere.

I do need it because I'm extending the original functionality to allow running it to multiple destinations not just sending to parallel FX lines. AKA, it is now a multi-use box, I can throw it on a pedal board and run parallel stuff, I can run it to multiple amps etc, I can run to two mic'd amps and a third direct line to my DAW simultaneously (already tested, works phenomenally). Or, it can just be a simple ole buffer.

Once it leaves the parallel pedal scenario I have to be able to deal with the inevitable ground loop and polarity issues that will almost always show up when using a splitter box with two amps etc. and the tranny fixes that. So, I need to know if it is unsolvable or I'm missing something. I'm hoping that I'm missing something since I'm basically using the transformer to do the exact job one might be used for - isolation.

Either way, I do need to know so I can decide what to do next. Maybe I need to make it out of MU Metal. Here is an enticing picture of the top view carefully chosen to elicit a wave of empathetic responses:

[PRR]

> Anytime it is near power it picks it up the 60Hz hum

So keep it away from 60Hz power.

Magnetic shielding would help.

However *distance* is by far the cheapest shielding for magnetic fields.

You could try an iron box. However you don't have the space, and DIY mag-shield is always iffy.

How does it connect to the buffer? Short low-resistance connection or is there a series resistor?

Edcor probably makes a shielded transformer. And IMHO a 10K:10K at guitar level does not need to be all that big.

[karbomusic]

Good info, thanks. Great call on keeping it away because my room is bad for stray 60hz power.

From the output of the buffer there is nothing but a straight connection to the input of the transformer. It is the short orange wire up top going from the main board to the daughter board pictured in the initial post. I'd say it's about 1 inch long. The other longer orange wire going to the jack is the return from the transformer.

Is the series resistor something I might want?

[PRR]

> resistor something I might want?

NO!!

Sufficiently strong driver with near-zero resistances *could* reduce hum flux in the core. However you are already at the reasonable limit.

I looked at Edcor and oddly they do not seem to offer any form of shielding, certainly not at your price-point. Jensen has small shielded line Xformers but they cost 10 to 20 times more.

[karbomusic]

> resistor something I might want?

NO!!

Sufficiently strong driver with near-zero resistances *could* reduce hum flux in the core. However you are already at the reasonable limit.

I looked at Edcor and oddly they do not seem to offer any form of shielding, certainly not at your price-point. Jensen has small shielded line Xformers but they cost 10 to 20 times more.

Cool, I already have two or three Jensens actually but they aren't 1:1. Hmm... Maybe a 1:1 Jensen? I could do that if it A) fits and B) works. Anybody got a part#? Budget isn't a big constraint for this build but they are too pricey to order the wrong thing.

[karbomusic]

Maybe one of these if I didn't miss a spec?

JT-11P-1HPC

[PRR]

> Maybe one of these

For what Jensen charges, you don't want to pick the wrong lump. I have heard their support is excellent. Call them and describe your application. (They may have more suggestions; they've actually been doing this longer than I have, and for many-many more clients.)

[karbomusic]

Will do. Thank you! This helps immensely.