When is transformer isolated signal splitting necessary?

[iandy4]

Hi Guys,

I'd like to build a signal splitter to have my guitar going to two amps.  One of the first schematics I came across was this one:
http://www.geofex.com/FX_images/splitter.gif

I was hoping someone has some good info or links about isolating grounds via transformers. When is it necessary? What happens when you don't do it?

Also if I were to add the transformers to this schematic http://www.muzique.com/lab/splitter.htm it should work just the same right?  I'd like to build a jfet based splitter simply because I haven't really messed around with them yet and everyone seems to agree that they sound warmer than opamps.

[R.G.]

I was hoping someone has some good info or links about isolating grounds via transformers. When is it necessary?

The amplifiers we use for musical equipment have essentially zero "common mode signal rejection". What that means is this:
- in theory, your guitar amplifier amplifies the difference between signal on the tip of the cord coming in and the ground/shield that also comes in on that cord.
- in practice, whatever is on the other end of that cord can cause its "ground" to waver around compared to the "ground" on the amplifier.
- the amplifier receiving this double entendre gets confuses and amplifies everything, including the ground movement.
The most common way for this ground movement to happen is if the sending unit and receiving amplifier are plugged into two different AC outlets. The sheer current and voltage drops on the AC power line make them nearly always have some "ground" difference. This only has to be a few millivolts to make for an ugly hum.

So ground isolation is needed any time the two pieces of equipment have "ground" voltages which are different. As you're thinking right now, that's really hard to tell about ahead of time. Sometimes they do. Well, I guess the right way to say it is "sometimes they don't hum."

Net result: It's needed if you have hum and can't solve it other ways. Sorry - it's an ugly and thorny problem.

What happens when you don't do it?

You get AC hum in varying degrees.

Also if I were to add the transformers to this schematic http://www.muzique.com/lab/splitter.htm it should work just the same right?

Probably not. The simple JFETs there probably can't drive the transformers in the way that's needed to get good results from them. That "splitter" is just a signal buffer which can drive more than one load.

I'd like to build a jfet based splitter simply because I haven't really messed around with them yet and everyone seems to agree that they sound warmer than opamps.

Depends on what you mean by "warmer". They have easily measurable second harmonic distortion even when they are nominally not distorting. The human ear seems to like up to about 1% of second harmonic without really noticing it as distortion. It's this effect and the voltage coefficient of resistance that's behind the fabled sweeter sound of carbon composition resistors, by the way. Only it takes some special conditions for that to work.

"Warmer" also gets bandied around in a way that has to mean "treble loss", as well. "Warmer" and "duller" are essentially the same situation, described by different people depending on whether they are trying to sell you something or have you NOT buy something else. 

[Brymus]

So does that mean when running stereo FX out to two amps,that they should both be plugged in the same outlet?
I tried stereo amps at pratice a few weeks ago and it hummed,but was attributed to the drummers neon sign plugged into the same outlet as one amp,and my FX.

[R.G.]

So does that mean when running stereo FX out to two amps,that they should both be plugged in the same outlet?
I tried stereo amps at pratice a few weeks ago and it hummed,but was attributed to the drummers neon sign plugged into the same outlet as one amp,and my FX.

It means that hum is pernicious, and comes in many ways. To get freedom from hum, you have to be persistent and lucky. Or at least not unlucky. Not all "bad"situations hum, just like not all otherwise good ones are quiet. Hum is not one animal.

[iandy4]

Thanks R.G.,

It's nice to talk to the man who designed the circuit in the first place! I'm a big fan of your website.  Thanks for the detailed response.

[amptramp]

As R.G. has said, you may have to isolate signals to eliminate ground noise issues.  It can also be a good way to prevent unsafe conditions such as AC voltage being present on the ground line.  A number of amplifiers have a capacitor going to chassis ground that can be switched to one side of the power line or the other.  This is intended to eliminate spike noise coming in from the power lines.  But it can also put the user at line voltage with the current limited only by the capacitor.  Guitar players have been zapped by touching the microphone and guitar at the same time when the ground for one was at line and the other at neutral.  You should always try to avoid this situation, but you can never be sure if the venue you are playing at has been wired correctly.  If you are using isolation for this purpose, you should ensure that the transformers used can tolerate line voltage from one winding to the other.

Audio transformers are expensive and even with the best design and materials, you get short-changed on both bass and treble and it may introduce distortion of its own.  Maybe not such a big problem with a guitar, which has a limited bottom end, but there are ways to isolate and split a signal that avoid this problem.  One is to use an isolation amplifier.  These use either optoisolation or RF isolation, the latter meaning that you modulate a signal and transformer couple it to a demodulator.  If you have the old National Semiconductor Applications Manual, AN-146 shows a carrier current intercom that FM modulates a signal onto the power lines and recovers it from them.  You could simplify this circuit since you do not need to use the power line.  Similarly, you could use AM modulation for a cheaper implementation.  At the frequencies in question (100 KHz to 200 KHz) you could use a ferrite bar antenna or IF transformer ccosting no more than $2 and it would not limit the audio frequency response.  Here is one device with 2 pF capacitors used to couple signal:

http://focus.ti.com/lit/ds/symlink/iso121.pdf

I don't think we have seen the optimum design yet.  Although the geofex design is relatively simple and not that expensive, there are a lot of ways to perform the same function with less audio limitation and the easiest involve getting away from baseband audio frequencies.

[Processaurus]

Guitar players have been zapped by touching the microphone and guitar at the same time when the ground for one was at line and the other at neutral.  You should always try to avoid this situation, but you can never be sure if the venue you are playing at has been wired correctly.  If you are using isolation for this purpose, you should ensure that the transformers used can tolerate line voltage from one winding to the other.

Transformers are only useful for DC isolation (and step up/step down in AC signals).  AC goes right through, that's why power transformers work.  If there was 110 on the mic ground relative to your amp's ground, a transformer in your signal chain to your amp does not help, the circuit when you're getting shocked would just go 110 on mic ground > resistance from your lips to your fingers > guitar strings ground directly to transformer primary ground > transformer secondary ground to your amps 3rd prong chassis ground.  I believe the original post is about hum isolation, not a solution to the unfortunate situation of musicians getting shocked by mics.

The RF modulated thing sounds interesting, but would anything like that that takes power on both ends need transformer isolated power supplies for either end?

There was a circuit at geofex I never totally got (maybe in that article?), that used an opamp to drive the ground connection to a second amp, achieving some cheap ground loop rejection.

[R.G.]

Guitar players have been zapped by touching the microphone and guitar at the same time when the ground for one was at line and the other at neutral.  You should always try to avoid this situation, but you can never be sure if the venue you are playing at has been wired correctly. 

One can never be sure, but one can  buy one of those $5 three-prong outlet testers to plug into the outlet and know many of the wrongly-wired conditions. It's very cheap insurance. I keep harping on this issue to try to get guitarists to save being shocked. It eliminates a lot of them - well, at least it lets you know you're at risk. Having a $5 AC-reading multimeter with you also lets you probe your guitar strings and the mike stand, bass strings, etc. at sound  check and know if something is badly wrong and dangerous. These things are too cheap and easy to ignore in favor of ditzing with isolators for electrical safety.

Signal isolators are for reducing hum, not electrical safety. Different objective, as you so eloquently state.

If you are using isolation for this purpose, you should ensure that the transformers used can tolerate line voltage from one winding to the other.

It is a bad idea to depend on NON safety approved/tested transformers for shock avoidance. On the other hand, none of the small transformers I've tried have ever failed to withstand line voltage. They're probably fine, but don't bet your life on them.

Isolators work well with hum voltages of a few to maybe 10V, which is a HUGE hum signal.

Audio transformers are expensive and even with the best design and materials, you get short-changed on both bass and treble and it may introduce distortion of its own.

I'd say it this way: "even with expensive ones you MAY lose bass or treble". The best ones can be very good, with response outside human hearing. But that is VERY expensive.

but there are ways to isolate and split a signal that avoid this problem.  One is to use an isolation amplifier.  These use either optoisolation or RF isolation, the latter meaning that you modulate a signal and transformer couple it to a demodulator. ...
I don't think we have seen the optimum design yet.  Although the geofex design is relatively simple and not that expensive, there are a lot of ways to perform the same function with less audio limitation and the easiest involve getting away from baseband audio frequencies.

I'm not sure how I'd pick an optimum one from the many, many ways to do it. The very word "optimum" implies some way to evaluate and rank the results based on a criteria. The criteria you select make huge differences in the results you choose.

The design at geofex is in fact pretty much optimal for the major design objective I had: most bang for the buck in a beginning builder's hands. It's been hard enough to convince people that a meter than can measure DC voltages is necessary; I can't imagine the problems the average forumite would have with aligning an RF link. There are ways to make it cheaper, and ways to make it easier to build, but these also subtract performance. There are ways to make it perform much, much better, but these add cost pretty quickly.

Transformers are only useful for DC isolation (and step up/step down in AC signals).  AC goes right through, that's why power transformers work.

Transformers are pretty good for ignoring any signal below their passband. You can effectively eliminate  AC power line frequencies by designing a transformer that has almost no response at those frequencies. The isolation transformers from the power-line signal carrying transformers noted in the National Semi appnote are a good example. Also, transformers are good at transforming only the difference in voltages between their input wires, and ignoring the common mode wobbling that both inputs are doing. This makes it good for eliminating common mode ground hum from one unit to another. I think you may be confusing the differential mode signal, which a transformer does pass, and the common mode signal, which it eliminates pretty well.

If there was 110 on the mic ground relative to your amp's ground, a transformer in your signal chain to your amp does not help, the circuit when you're getting shocked would just go 110 on mic ground > resistance from your lips to your fingers > guitar strings ground directly to transformer primary ground > transformer secondary ground to your amps 3rd prong chassis ground.  I believe the original post is about hum isolation, not a solution to the unfortunate situation of musicians getting shocked by mics.

An isolator transformer freeing you from direct connection to the offending ground would help, probably eliminating the shock hazard. It's good to know how big the shock hazard is; a small isolator transformer will prevent the passage of shocks from common mode hum. However, it's a bad idea to count on this for safety if the shock voltage difference is more than maybe 10-20V. That's where the multimeter becomes your close personal friend; knowing what's death waiting to happen.

The RF modulated thing sounds interesting, but would anything like that that takes power on both ends need transformer isolated power supplies for either end?

Depends on the power. Battery powered RF transmitters are available. Could work, but is horribly complex for beginners.

There was a circuit at geofex I never totally got (maybe in that article?), that used an opamp to drive the ground connection to a second amp, achieving some cheap ground loop rejection.

It used an opamp to make a signal equal to the common mode hum voltage and subtract it from the signal it put out, giving a signal at its output which eliminated the common mode hum. It works well within the limits of the resistor tolerances and common mode rejection of the opamp, up to the limits of how big the opamp can swing its output. It can't subtract out a bigger hum than its output can swing. In practice, that's down to about 3V peak as a limit for 9V battery operation.

[iandy4]

Guitar players have been zapped by touching the microphone and guitar at the same time when the ground for one was at line and the other at neutral.

I've often wondered why I'm getting shocked!  I'm sorry if the answer to this question is obvious, but what do you mean when you say that the ground was "at line"?  Is this when you have a wall outlet with only two prong inputs rather than three?  I know the third round prong is for ground and have often wondered why some outlets only have two prongs.  In this case, does it mean the ground is "at line"?

[amptramp]

@iandy4:

If you check these schematics:

http://www.lynx.bc.ca/~jc/YBA1Schems.pdf

You may notice that Traynor schematics have the Association of Professional Engineers of Ontario stamp on them.  This is supposed to be a guarantee of safety.  It doesn't necessarily work that way.

You will see that there is a capacitor going from the line cord to chassis ground ahead of the power transformer that can be switched to either AC input.  So as R.G. says, it pays to check the venue wiring with a circuit tester and a multimeter, but there is an additional element of chaos here in that you could have, say, a microphone amp with the chassis ground switched to the neutral via the capacitor switch and a guitar amplifier with the chassis ground switched to line voltage via its capacitor switch.  The switch is usually set for minimum noise, not safety.  The pair of 0.05 µF capacitors in series are 0.025 µF and at a line frequency of 60 Hz, this is about 106 K of capacitive reactance, allowing over 1 milliamp at 120 VAC.  Transformer isolation as per the geofex design should be adequate because this is a common-mode voltage, but transformers have some winding-to-winding capacitance - just not as much.  It is entirely possible to have an op amp drive two linear optoisolator LED's in series, one of which is used to provide feedback to the driving stage and the other to drive a phototransistor across the voltage barrier.  This makes the design linear.  (I would post a picture, but I have not found a schematic on the net for it - maybe I just have not been looking in the right places.)

P.S. If you do find your venue has been miswired, what do you do?  Refuse the gig?  Your setup should be able to accommodate any wiring issue.

[R.G.]

I've often wondered why I'm getting shocked!  I'm sorry if the answer to this question is obvious, but what do you mean when you say that the ground was "at line"?  Is this when you have a wall outlet with only two prong inputs rather than three?  I know the third round prong is for ground and have often wondered why some outlets only have two prongs.  In this case, does it mean the ground is "at line"?

In the USA and Canada (I think) the electrical power company uses a power-pole transformer to convert whatever the incoming high voltage line runs at (and this can be up to 13kV) down to (generally) 240Vac three phase. Each 240V is center tapped,  and in residential wiring usually supplies up to 200A.

The center tap of the 240V secondary is tied to a ground wire at the utility pole. This is the copper wire which runs down the side of the pole and ends up in a neat circle on the bottom of the pole before it's planted. So the "120VAC" is generally two halves of 240Vac, with the center of this 240V tied to ground. This is brought into the house distribution box, and there is usually another ground wire in the box, which is tied to a ground rod driven into the ground at the house.

The outer ends of the 240Vac are the two "lines" and the centertap is "neutral". Back when we didn't know as much about electricity, it was thought to be OK to have that neutral be one of two prongs in every slot. This is how all two-prong receptacles are wired. Well, OK, how they should be wired. Electricity always obeys Ohm's law, so the neutral at a given plug is not at local ground potential, because all those amperes running through the neutral wire back to the pole cause a few volts of resistive losses, so the prong "neutral" can never be the same as the ground either back at the pole or in the house. People got shocked and some killed.

Amps built before this change to three-wire (in the 50s, I think, perhaps later by smaller companies) had two-wire cords, one "hot"/110Vac and one "neutral". But which one was neutral? It was impossible to tell, so clever electrical guys made one blade wider so it could only be plugged in one way round. But UN-clever electricians had been wiring outlets either way round for so long, you never knew which way the plug would be inserted. This mattered, because if the chassis of an amp were connected to the "neutral" wire by a ceramic capacitor, it "ate" most of the RF interference and buzzing. Some clever guy figured this out and provided a capacitor, followed immediately by the "line reverse" switch which moved the capacitor to either line, to get the lowest buzz/hum. However, all that had to happen was that single capacitor going leaky or shorted (like by a refrigerator turning off with a messy high voltage arc on the line) and now the line reverse switch connected one or the other AC power line to chassis, and you had a 50-50 chance of being on the deadly side of the switch. This is how people have been killed. Chassis connected to power line by bad "death cap" (or other wiring short), connected to guitar strings, and all you have to do is grab a grounded mike stand and your heart goes into ventricular fibrillation. Say a last prayer if you can ...

Enter third-wire ground. It was noticed that if you made electrical things with a metal housing, and then tied that metal housing to an independent ground rod, it was much more difficult to kill people with electrical equipment, because a short to the metal housing was conducted to a local ground potential and blew a fuse or breaker. It was so much better that it was written into the electrical wiring code. First, there's no need for the "death cap" because with the chassis grounded to a solid, non-current-carrying ground, the RF protection is much better to start with. Second, any internal short makes the fuse blow, and does not kill you, as long as the fuse is in the ***hot*** incoming AC power line, not the neutral. This is why rewiring two-wire to three-wire amps need to get persnickety on this point. Unfortunately, now the properly grounded chassis and signal line is an unwitting participant in the electrocution of someone else who has an old two-wire amp that is leaky. Tell your friends to get their amps converted over: it's quieter for them and safer as well.

Unfortunately, this did not stamp out the UN-clever electrician. They sometimes "upgraded" outlets by putting in a three-wire outlet, but NOT the ground wire. They simply tied the neutral line to the third prong. Or swapped the prong wires. Or just in general did anything to get their pay and get out of there. Mostly this would work OK, powering the equipment, at least until someone using that outlet grabbed something that was properly grounded, and also happened to have a leaky amp cap... then they died, or were slapped across the stage by the shock if it was a mild one. Improper wiring of outlets, and accidentally having one or more of the wires in an outlet go high resistance simply happens. This is why I consider a guitarist at least naive and possibly as bad as unteachably stupid if they don't have at least access to a tester plug. It's almost a Darwinian condition. 

It is possible for that neutral line to be high resistance, in which case, most of the current goes from hot wire to safety ground. When that happens, the balance of that centertap depends on whatever is on the OTHER side of the AC line, like beer coolers, stage lighting, refrigerators, welders, etc., and the AC power your amp gets can vary from nearly zero to as much as 240Vac, out of that one "120Vac" socket. It is possible for the safety ground to be open (no protection) or either accidentally or deliberately wired to "neutral", which puts several volts of AC on your chassis ground, and makes your amp hum if anything is plugged into it from a DIFFERENT ground level. It is possible for hot and neutral to be reversed AND to have a high impedance neutral or open safety ground, which is an electrocution waiting to happen, especially if you have a two-wire "death cap" amplifier.

The cure for not having to go learn all this is to (a) get a tester plug and use it and (b) get a multimeter and get in the habit of testing plugged-in/powered-on ground to ground voltages for everything you can touch on stage.

It's much better than shrieking "SH&T" into a microphone when you touch it. 

[iandy4]

The cure for not having to go learn all this is to (a) get a tester plug and use it and (b) get a multimeter and get in the habit of testing plugged-in/powered-on ground to ground voltages for everything you can touch on stage.

It's much better than shrieking "SH&T" into a microphone when you touch it. 

Thanks again for the detailed description!  I was recently electrocuted with 240V because a faulty kitchen top water boiler was leaking water on to the metal counter top which I happened to be touching as well as the faucet (I assume connected to ground via the water pipes).  I got out my multimeter and it was indeed 240.  Although this conversation has veered away from my original question I am absolutely fascinated.

As an example to test what you've described, I would set the multimeter to AC and simply touch one lead to my guitar strings and the other to the microphone right?  Since both are grounded I assume any voltage showing on the multimeter would be a difference between the ground voltages.  If this is the case, what do you do to fix the problem?  Is it as simple as plugging the amp and PA into the same circuit on the houses wiring?

[R.G.]

As an example to test what you've described, I would set the multimeter to AC and simply touch one lead to my guitar strings and the other to the microphone right?  Since both are grounded I assume any voltage showing on the multimeter would be a difference between the ground voltages. 

Yes, that will show you the voltage difference. Differences of a few volts will cause a tingle, more than a few will cause a shock of increasing intensity with the voltage.

If this is the case, what do you do to fix the problem?  Is it as simple as plugging the amp and PA into the same circuit on the houses wiring?

That's the problem. There are so many ways it can be wrong, in both the house wiring all the way back to the pole and in the various amps that it can be very difficult to say what's "right". The three-prong tester tells you something about the house wiring, and tells you most of the flaws the outlet itself can have. Plugging into the same outlet is a good start, if that's possible. It may not always be possible.

There are problems existing in some house wiring you can't easily and quickly fix. I went through a number of these in some articles I wrote for Premier Guitar on the topic of doing gigs powered by generators, like some outdoor festivals. Generator power is just an extreme example of the kinds of things that you may find anywhere; just more likely because the generators are set up temporarily, so they're more prone to setup and wiring errors happening. 

A slam-dunk way to fix it is to get yourself (and even better, the whole band) a harmonic-neutralized constant voltage transformer to run from and connect a separate ground wire to ... something ... at the venue, like a cold water pipe as long as the place is not piped with plastic pipe. The CVT takes in from about 85 to 140VAC and turns out clean sine waves at about 115 to 120, as well as isolating you from the AC power line, even if it's wired wrong. The ground wire lets you KNOW that your safety ground is actually tied to a local earth ground - if you can find one. This fix forces the whole band to be insulated/isolated from the house wiring, safe from voltage extremes, and all grounded to one point, at least if you do it right. The problem is that a 1KVA CVT is expensive and heavy, as much as 50-75 pounds. They can be cheaper on ebay, but ebay doesn't make them any lighter. A less expensive way to go is to get a suitable isolation transformer that breaks your power from the AC house wiring and lets you establish a safety ground independently of it. No regulation and transient filtering, but the same ground safety here. Both versions of isolation transformer will dramatically cut ground-related noise for everything attached to them that's in good operating order.

One problem you can get into is that some venues have a captive PA that has its own power. In that case, the mikes are grounded to the PA and that leaves you open to shocks from the PA ground even if your equipment ground is correct. In that instance,  you need to put an isolator in the mike signal cable to break the grounding of the mike shell to the PA.

One mod worth doing to your guitar is to find the wire going from signal ground on the jack to the bridge (and hence strings) and insert a large resistor paralleled by a small capacitor. Values like 100K to 220K and 0.01 to 0.001 work. What this does is to break the solid connection at 60Hz so any shocks you get are severely current limited and not as severe, and also to shunt RF hum and fluorescent buzz through the capacitor to signal ground as though there was no resistor there. This works by insulating YOU and not increasing hum very much.

Most people will not go to the expense and effort of hauling around a 75 pound lump of iron and copper just to protect their lives.    They get to the point of reading through all of what I have typed because they hate the hum that bad grounding causes and that's a bigger motivator than sometimes getting shocked.

[familyortiz]

Persistent and lucky?
All I had to do was build RG's Hum Free ABY and even with the $2.50 Xicon tranformer, got absolutely great performance with this simple design. Why reinvent the wheel?

[R.G.]

Persistent and lucky?
All I had to do was build RG's Hum Free ABY and even with the $2.50 Xicon tranformer, got absolutely great performance with this simple design. Why reinvent the wheel?

Kewl~!

It does prevent a lot of them. But there are still situations that can let hum sneak in.

[wavley]

Persistent and lucky?
All I had to do was build RG's Hum Free ABY and even with the $2.50 Xicon tranformer, got absolutely great performance with this simple design. Why reinvent the wheel?

Kewl~!

It does prevent a lot of them. But there are still situations that can let hum sneak in.

Like if you build a Hum Free and don't shield the transformer and set it down next to your power supply to test it

I had even better performance with the hum free and the $3.89 Triad transformer ( pretty flat down to 40Hz, maybe a little lower) and superb performance with the $6 Edcor transformer (it's physically much larger so that's a trade off, but it does sound a lot better clarity wise in the low end)

Any way it goes, it's a great circuit, but it's true that there are many other ways to make your guitar rig hum and I'm working very hard to find all of them!

[iandy4]

@ wavley

I definitely want to try this project with R.G.'s suggested transformers and the Edcor's that you mentioned to compare.  The quality of the sound is more important to me than the size and and price of the build.

I was on their Edcor's website but I can't find the transormer you mentioned.  Do you remember which model it is?  I am sure I need a 1:1 ratio but I don't really know what values I need to use in terms of Ohms and Henries.

[R.G.]

I was on their Edcor's website but I can't find the transormer you mentioned.  Do you remember which model it is?  I am sure I need a 1:1 ratio but I don't really know what values I need to use in terms of Ohms and Henries.

I would guess it's this one:
http://www.edcorusa.com/category/19-pcseries.aspx?turnsratio=11&primary%28input%29impedance=136

[wavley]

They also have these, I haven't tried them yet because they're new @ $3.81 http://www.edcorusa.com/products/877-com23.aspx

R.G. is right with the series, but I actually used the 15k:10k one because it's 1.2:1 and I wanted a little attenuation plus I was a little curious how it would sound into 15k vs. the 10k transformers I had tried previously. http://www.edcorusa.com/products/126-pc15k-10k.aspx but the 10k:10k one is perfect drop in for his circuit.

The quality of the sound is more important to me than the size and and price of the build.

Same here

[iandy4]

Thanks guys! Just ordered them!  Can't wait!