Guitar amp effects send balancer/de-balancer

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grid_stopper

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Mar 2, 2016
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My band recently moved into a new rehersal space where the power is quite bad, and on top of my amp just being a bit noiser over all than it is elsewhere-im getting all kinds of interference when i use the effects loop. I also have to stand a reasonable amount away from the amp, so i usually need to run about 15-20 feet of cable to/from my efx send and return. Im hoping to turn lemons into lemonade here, and workout a balancing/de-balancing system for me to use, which would allow me to keep the unbalanced runs a few inches long, and do the bulk of that cable run balanced. Ive included the idea below. It is for the most part cobbled together from various datasheets and was hoping people could point out any glaring errors/areas for improvment. The buffer opamp is a TL072, and the Balancing/de-balancing will be done by the appropriat THAT corp chips. any/all advice is greatly appreciated!

EDIT: there will be a duplicate of this ciruit at the pedal board end, to debalance the signal, then enter the pedal chain the rebalnce back to the amp where it enters the BALIN terminal in the schematic shown

interface idea.png
 
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That's not going to have the result you're looking for because guitar pedals need to be grounded. So you can't use ground as a cold line. At least not at both ends.

But you don't need balanced in / out anyway. An impedance balanced output might be good enough. More specifically, you add a 33-100 ohm resistor on each leg of a conventional unbalanced out.

More generally, if you have power issues, you should study the problem before just throwing more circuitry at it. There are things that you can do to help diagnose the issue. For one, take one pedal that has true bypass and run your cables out and back and see what kind of noise you have with just the one pedal. If you don't have a lot of noise with the one pedal bypassed, then the problem is not the long cable runs at all.
 
That's not going to have the result you're looking for because guitar pedals need to be grounded. So you can't use ground as a cold line. At least not at both ends.
This is a very good point that i had not considered, thank you for pointing that out. I conceptually understand what youre pointing out, but im having trouble wrapping my head around a way to remedy of that. perhaps it is bedtime lol.

EDIT: d'oh. im realizing i forgot to include-I will have 2 of these made, one at the amp end, one at the pedalbaord end. that should clear up this issue, correct?

But you don't need balanced in / out anyway. An impedance balanced output might be good enough. More specifically, you add a 33-100 ohm resistor on each leg of a conventional unbalanced out.
I considered the impedance balanced approach and might still explore that option, but truthfully-id also like a chance to work with the THAT series stuff. haven't had a reason to employ them, and this seemed like a good opportunity.

More generally, if you have power issues, you should study the problem before just throwing more circuitry at it. There are things that you can do to help diagnose the issue. For one, take one pedal that has true bypass and run your cables out and back and see what kind of noise you have with just the one pedal. If you don't have a lot of noise with the one pedal bypassed, then the problem is not the long cable runs at all.

This was actually the first thing I tried, and unfortunately it would seem i am picking up noise through the cable.
 
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Guitar pedals are typically unbalanced. Driving them from low impedance buffers might help but 20' long runs seems too long, can you make that shorter?

JR
 
Guitar pedals are typically unbalanced. Driving them from low impedance buffers might help but 20' long runs seems too long, can you make that shorter?

JR
unfortunatley due to circumstances of the physical layout of the room the best i can do is about 15 feet, but that keeps me pretty close to the drums. 20 would be a bit comfier, physsicaly. im realizing i left some details out, which should hopefuly clear things up- I would make the schematic shown twice. One at the amp end, one at the pedal end. ive made a rough block diagram of the signal flow, red wires being unbalanced, blue being balanced:
block.png
 
Can you put the amp head next to you and then run a long speaker cable?

If you really need to go balanced /unbalanced I would be more inclined to try something simple first, like some inexpensive transformers.
 
Can you put the amp head next to you and then run a long speaker cable?

If you really need to go balanced /unbalanced I would be more inclined to try something simple first, like some inexpensive transformers.
a long speaker cable would likely be the simplest way, and if all else fails I will likely resort to this.

that being said- even if this solution is...complicated(even needlessly so), part of me wants to go through with it anyways as a learning experience
 
This is a very good point that i had not considered, thank you for pointing that out. I conceptually understand what youre pointing out, but im having trouble wrapping my head around a way to remedy of that. perhaps it is bedtime lol.

EDIT: d'oh. im realizing i forgot to include-I will have 2 of these made, one at the amp end, one at the pedalbaord end. that should clear up this issue, correct?
In this case, you would have to use a two conductor balanced cable like TRS or XLR and then make sure the shield is connected to earth ground at either end.

I considered the impedance balanced approach and might still explore that option, but truthfully-id also like a chance to work with the THAT series stuff. haven't had a reason to employ them, and this seemed like a good opportunity.
Ok. But if you're interesting in actually solving your problem, there is a process and throwing new powered circuits at the problem is not necessarily going to fix it.

Personally I would try a few things first:

1) Try connecting send directly to return with each cable and with the cable fully extended or coiled close to the amp.
2) Try cleaning all connectors with deoxit and a rag. Put some deoxit on a plug and put it into the jacks and twist them around to try to clean the plug contacts.
3) Try different cables. If the cable shielding is not a fully enclosed wrap, you will get noise.
4) Try very short cables with everything close to the amp. Or use the same cables but neatly coil them at the amp.
5) Temporarily hook up a 10K audio input transformer in front of the amp return (or maybe an ISO-MAX if you can borrow one from a rich person) and use a good impedance balanced pedal to drive the return cable. Note that you need to make sure the pedal board is grounded through the send. All pedals need a solid earth ground.

Study all of the above for changes in noise to help diagnose the issue.

If you still have noise after doing the above, you're dealing with some EMI from maybe power cables in the floor or something of that nature.
 
In this case, you would have to use a two conductor balanced cable like TRS or XLR and then make sure the shield is connected to earth ground at either end.


Ok. But if you're interesting in actually solving your problem, there is a process and throwing new powered circuits at the problem is not necessarily going to fix it.

Personally I would try a few things first:

1) Try connecting send directly to return with each cable and with the cable fully extended or coiled close to the amp.
2) Try cleaning all connectors with deoxit and a rag. Put some deoxit on a plug and put it into the jacks and twist them around to try to clean the plug contacts.
3) Try different cables. If the cable shielding is not a fully enclosed wrap, you will get noise.
4) Try very short cables with everything close to the amp. Or use the same cables but neatly coil them at the amp.
5) Temporarily hook up a 10K audio input transformer in front of the amp return (or maybe an ISO-MAX if you can borrow one from a rich person) and use a good impedance balanced pedal to drive the return cable. Note that you need to make sure the pedal board is grounded through the send. All pedals need a solid earth ground.

Study all of the above for changes in noise to help diagnose the issue.

If you still have noise after doing the above, you're dealing with some EMI from maybe power cables in the floor or something of that nature.
ive actually tried some of these things- i cleaned all the jacks with deoxit quite thoroughly(this actually slightly improved the noise floor of the amp as a whole) but ive still had trouble with noise in the loop. additionally, i made the cables myself, and know the shielding is solid. I also tried it with 3ft cables and the pedals on top of my amp, which worked a lot better but isnt a practical length. i hadnt considered using aome 10k 1:1 transformers(but i do have a few lying around...), then again, given that the problem went away when i played a show this week with the exact same setup, but had returned at a band practice m yesterday i felt pretty confident about what i experiencing
 
Make sure power for everything is all sourced from the same plug and preferably the same power strip at the amp. If you power the pedals from a socket on the other side of the room, you will get noise because of the differences in ground potential between the amp ground and the pedal power ground creating currents in the shield of the cable between the pedal board and the amp.
 
hoping people could point out any glaring errors/areas for improvment.
So far there is only one "glaring" mistake. The voltage follower in the debal has no galvanic connection at the input. It should have a resistor to ground.
However, this stage is utterly dispensable, since the THAT line receiver is perfectly capable of driving any of the usual unbalanced inputs in a guitar rig. You may want to move the 100R resistor to the output, though.
The balancer is fine, although the 10uF/10k between the input buffer and the THAT line driver is also dispensable.
Ther are commercial units that do that, I think Radial.
There is one point that should be considered. Adding gain to the balancer, compensated by complementary attenuation on the debal runs the balanced lines at a higher level, which makes them somewhat less sensitive to some forms of interference. Turning teh follower into a non-inverting gain stage just takes 2 resistors.
The THAT line driver already has 6 dB gain. Adding a little additional gain to the input buffer provides some noise reduction. Of course it must be compensated by some passive attenuation after the receiver in the debal. It takes just an additional resistor.
The same circuit would make a perfect -10/+4 interface.
 
The voltage follower in the debal has no galvanic connection at the input.

Same for the balanced input, no resistors on the input so if there is not a DC-coupled output stage connected that stage will likely saturate. No part numbers on the schematic, so forgive me if I have forgotten about a ThatCorp part which includes the resistors in the package.

The likely problem which occurred to me in the general idea is this (please excuse the length, to make clear I want to cover the basic assumptions):

The general problem with unbalanced interfaces is that the shield is forced to carry signal return current as well as whatever chassis-to-chassis power related current flows. The input receiver then detects the inter-chassis current I*R drop as a voltage superimposed on the signal.

Balanced interfaces can avoid that problem by forcing all inter-chassis currents to flow only on the chassis (or an isolated low impedance trace that connects all connector shells if the chassis is not sufficiently conductive). In the past some designs have created the same problem that unbalanced equipment is forced to have by connecting the connector shell/shield to the internal circuit reference node (aka "ground"), well known now as the "pin 1 problem."

So to avoid the problems of chassis current flowing through an unbalanced circuit, you should have the cable shield connect directly to the chassis, and the unbalanced circuit should be inside the chassis connected directly to the balanced input stage output reference connection (i.e. the reference pin on the ThatCorp balanced-to-unbalanced stage should connect directly to the ground node of the first stage of the unbalanced circuit). The entire combined circuit should then have one connection between the circuit reference and the chassis/cable shields connection.

In the proposed scheme as I understand it the guitar pedals will remain unmodified, and the balanced-to-unbalanced and unbalanced-to-balanced circuits will be in external enclosures. In that setup, wouldn't the chassis-to-chassis currents flow across the new boxes, and still flow through the (now short) unbalanced cables and through the guitar pedals just as before? Built in pin 1 problem with no good way to avoid.
The new balancing boxes would help for any noise that is magnetically induced into the cables, but chassis-to-chassis power supply related currents seem more likely to cause problems than induced noise.

I am open to any counter arguments, I'm not positive my analysis is correct, but wanted to bring it up before the OP spent time building the boxes and then realized it solved a different problem than needed.
 
In the proposed scheme as I understand it the guitar pedals will remain unmodified, and the balanced-to-unbalanced and unbalanced-to-balanced circuits will be in external enclosures. In that setup, wouldn't the chassis-to-chassis currents flow across the new boxes, and still flow through the (now short) unbalanced cables and through the guitar pedals just as before? Built in pin 1 problem with no good way to avoid.
The new balancing boxes would help for any noise that is magnetically induced into the cables, but chassis-to-chassis power supply related currents seem more likely to cause problems than induced noise.

Not sure that induced noise isn't a significant issue in the scenario the OP outlines. Likely to be power transformers etc around giving rise to H-Fields.
But with the ground / screen connection issue - yes - you might not want to have a 'solid' screen connection all the way through.
Interrupting the screen with eg 100R // 100n is a typical solution.
See Bo Hansen DI for example.
 
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The THAT receivers are referenced internally. Thereis no need for galvanic resistors.
As for the ground differential currents, I don't think it's an issue, as long as one of the interface is power from the same outlet as the amp, and the other is powered from the same outlet as the pedal board, which just makes sense.
The potential effects of differential ground voltage would not be more significant than between an outboard and a mixer. Actually lifting some connectors may improve the noise situation, or not.
 
So far there is only one "glaring" mistake. The voltage follower in the debal has no galvanic connection at the input. It should have a resistor to ground.
However, this stage is utterly dispensable, since the THAT line receiver is perfectly capable of driving any of the usual unbalanced inputs in a guitar rig. You may want to move the 100R resistor to the output, though.
The balancer is fine, although the 10uF/10k between the input buffer and the THAT line driver is also dispensable.
Ther are commercial units that do that, I think Radial.
There is one point that should be considered. Adding gain to the balancer, compensated by complementary attenuation on the debal runs the balanced lines at a higher level, which makes them somewhat less sensitive to some forms of interference. Turning teh follower into a non-inverting gain stage just takes 2 resistors.
The THAT line driver already has 6 dB gain. Adding a little additional gain to the input buffer provides some noise reduction. Of course it must be compensated by some passive attenuation after the receiver in the debal. It takes just an additional resistor.
The same circuit would make a perfect -10/+4 interface.
This is excellent, thank you-and good catch on the missing resistor. I have made most of these changes, and can post an updated schematic later tonight. I also like the idea of adding gain, so I added footprints for that to try it. For now, the passive attenuation is set via a trimmer, as I dont want any physical knobs that can be bumped during a performance. as you pointed out though, this would be a handy -10/+4 interface and having an external control might be more useful in that situation.

I did actually consider the Radial unit(as an added bonus, they are a local company!) but, then i miss out on the fun of learning to do it myself!
 
Not sure that induced noise isn't a significant issue in the scenario the OP outlines. Likely to be power transformers etc around giving rise to H-Fields.
But with the ground / screen connection issue - yes - you might not want to have a 'solid' screen connection all the way through.
Interrupting the screen with eg 100R // 100n is a typical solution.
See Bo Hansen DI for example.
From the tests ive done, it does seem to be a problem. Short runs work reasonably well, but longer lengths are very problematic. It doesnt help that Our jam space shares a building(which is quite old, and probably wired less than ideally...) with a heavy machine shop, and about a dozen other jam spaces. The power situation is less than ideal for sure.

I have a reel to reel setup in the room, and it has also been prone to picking up stray rfi on occasion as well(all unbalanced interconnects, about 10ft in length.) If this circuit fixes my effects loop problem, i may investigate adapting it to use with the reel to reel setup as well.
 
As for the ground differential currents, I don't think it's an issue, as long as one of the interface is power from the same outlet as the amp, and the other is powered from the same outlet as the pedal board, which just makes sense.
The potential effects of differential ground voltage would not be more significant than between an outboard and a mixer. Actually lifting some connectors may improve the noise situation, or not.

Not sure that would be the case. Since the 'Ground' (and I hesitate to use the term !) ie Screen is directly tied between the Pedalboard and distant Amp in the scenario that I think is being outlined. The amplifier has an unbalanced input so using that Screen connection as the 'Signal Reference' by definition. Looks like a baked in 'Pin 1 Problem'
The situation "between an outboard and a mixer" seems fundamentally different if we are considering a connection that is 'balanced' at both output and input.
A "Pin 1 Problem" may exist depending on the designs but is likely to be more easily fixed internally to the kit.

I may be misunderstanding the exact situations being considered - an example of a picture = 1000 words maybe. (Cue Telly Savalas)

Sort of 'ironically' a passive guitar/bass pickup is essentially a floating source except where it is deliberately 'grounded' at one conductor.
But attempts at exploiting this 'Balanced Hi-Z' type output have never proved very popular ( a bit more so in Bass but not in 'Six-String').

Apologies in advance for any misconceptions here. Long day and I already had to edit something I'd posted that I realised was missing the point as I parked up at Waitrose on the way home from work :)
 
From the tests ive done, it does seem to be a problem. Short runs work reasonably well, but longer lengths are very problematic. It doesnt help that Our jam space shares a building(which is quite old, and probably wired less than ideally...) with a heavy machine shop, and about a dozen other jam spaces. The power situation is less than ideal for sure.

I have a reel to reel setup in the room, and it has also been prone to picking up stray rfi on occasion as well(all unbalanced interconnects, about 10ft in length.) If this circuit fixes my effects loop problem, i may investigate adapting it to use with the reel to reel setup as well.

Yeah - those are long lengths of unbalanced connections.
Without wanting to get into the debate about transformers vs active solutions I'd suggest you try a few simple 1:1 transformer "Hum Stopper" type solutions before getting into something very technical. Just to prove the point if nothing else.
Between recorder and mixer the source impedance should be quite low (?) so transformer spec' not so critical ? (flame thrower retardant suit being put on now :oops:)
 
Yeah - those are long lengths of unbalanced connections.
Without wanting to get into the debate about transformers vs active solutions I'd suggest you try a few simple 1:1 transformer "Hum Stopper" type solutions before getting into something very technical. Just to prove the point if nothing else.
Between recorder and mixer the source impedance should be quite low (?) so transformer spec' not so critical ? (flame thrower retardant suit being put on now :oops:)
I do actually have some 10k:10k transformers that i intend on using as one final check that i do in fact have an rfi problem, but ultimately i would like to pursue this active solution, even if its only a learning exercise. the parts count is low enough, that i dont mind giving it a try
 
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