Splitter with a transformer

[P-Trash]

Hi. I built a simple splitter to plug my guitar in two amps and now sometimes have a ground loop hum. So i think to add a transformer (I only have TM018 right now). Maybe someone could help me. Will it work right?

[Kevin Mitchell]

Wouldn't running your guitar through a transformer seriously alter the sound of the signal? Is that something you want in a passive circuit?

[P-Trash]

Wouldn't running your guitar through a transformer seriously alter the sound of the signal? Is that something you want in a passive circuit?

Maybe. I want to try do that and also add a switch to turn off a transformer. So wanted to find out how bad is this idea

[noisette]

It is a perfect splitter without the transformer, with it, it is a dubious bandpass filter 

[R.G.]

I did a couple of articles on splitters at geofex. I believe that these articles are the origin of the circuit you show.

I suggest that you read those articles for some more background information.

The short version is that
1. magnetic pickup guitars are not as difficult to transformer-isolate as full-spectrum audio. An electric guitar has a lowest fundamental in the 60-82Hz range, depending on tuning, not the 20Hz hifi would have you achieve. And magnetic pickups have nearly no output above 7kHz. There is some, on some pickups, and piezo pickups are different, but in general, 7Khz. That's way down from the 20+kHz that hifi rigs have to achieve. So yes, it is -possible- to use a transformer, even an inexpensive one, for electric guitar, but with some special caveats.

2. Wide-bandwidth transformer design is HARD. That's why the design uses opamps to present the guitar a high input impedance. It preserves the remaining high frequency of an electric guitar without losing even more of the high frequencies that magnetic pickups have problems with. The opamps also use a trick to extend the the low frequency response of cheaper transformers down to the range of 60Hz or so. Cheaper transformers usually only quarantee a low frequency rolloff of 300hz, and that definitely would change the tone of an electric guitar. So that circuit uses the opamps to help the electric guitar pickups stay wide range, and also helps the transformer stay wide range.

3. Defeating power line frequency hum is HARD. Using two or more amps makes it even harder. the culprit in multi-amp setups is usually different amounts of AC mains leakage current. Every AC-powered devices leaks some amount of the AC power line into space. Exactly how much depends on how carefully the design has been done to prevent this. Amplifiers vary in how well they do this. This situation gets even worse with amplifiers with only two-wire line cords on the amps, as they have no hard "reference" of the AC power safety ground. It's not pefect even with modern, three-wire powered amps. So yes, you man need a transformer isolator on your guitar. It does not isolate the -guitar- from the amps so much as it isolates the ground of one amp from the other.

[P-Trash]

I did a couple of articles on splitters at geofex. I believe that these articles are the origin of the circuit you show.

I suggest that you read those articles for some more background information.

The short version is that
1. magnetic pickup guitars are not as difficult to transformer-isolate as full-spectrum audio. An electric guitar has a lowest fundamental in the 60-82Hz range, depending on tuning, not the 20Hz hifi would have you achieve. And magnetic pickups have nearly no output above 7kHz. There is some, on some pickups, and piezo pickups are different, but in general, 7Khz. That's way down from the 20+kHz that hifi rigs have to achieve. So yes, it is -possible- to use a transformer, even an inexpensive one, for electric guitar, but with some special caveats.

2. Wide-bandwidth transformer design is HARD. That's why the design uses opamps to present the guitar a high input impedance. It preserves the remaining high frequency of an electric guitar without losing even more of the high frequencies that magnetic pickups have problems with. The opamps also use a trick to extend the the low frequency response of cheaper transformers down to the range of 60Hz or so. Cheaper transformers usually only quarantee a low frequency rolloff of 300hz, and that definitely would change the tone of an electric guitar. So that circuit uses the opamps to help the electric guitar pickups stay wide range, and also helps the transformer stay wide range.

3. Defeating power line frequency hum is HARD. Using two or more amps makes it even harder. the culprit in multi-amp setups is usually different amounts of AC mains leakage current. Every AC-powered devices leaks some amount of the AC power line into space. Exactly how much depends on how carefully the design has been done to prevent this. Amplifiers vary in how well they do this. This situation gets even worse with amplifiers with only two-wire line cords on the amps, as they have no hard "reference" of the AC power safety ground. It's not pefect even with modern, three-wire powered amps. So yes, you man need a transformer isolator on your guitar. It does not isolate the -guitar- from the amps so much as it isolates the ground of one amp from the other.

Thank you for so detail answer. One more question: for what in circuits like that uses a 10k and 0,001uF between jack's pins?

[Kevin Mitchell]

When l was looking into this trying to find out if using a transformer like this is common practice I've found that it is - but from what people have reported it's a tricky one unless your guitar has active pickups to compensate.


You said sometimes it has a hum. One thing I'd consider is swapping out the mains input with one with a built in filter. These help clear out the noise from the mains line. I've been using these with all of my amplifier builds.
Jameco Listing


Just more to consider!

[P-Trash]

When l was looking into this trying to find out if using a transformer like this is common practice I've found that it is - but from what people have reported it's a tricky one unless your guitar has active pickups to compensate.


You said sometimes it has a hum. One thing I'd consider is swapping out the mains input with one with a built in filter. These help clear out the noise from the mains line. I've been using these with all of my amplifier builds.
Jameco Listing


Just more to consider!

Interesting solution, will try to find one

[Kevin Mitchell]

Sorry I had the wrong url attached to the jameco hyperlink. I've updated it if you're interested in US stock - not sure where you reside.

[noisette]

Sorry for my trashy (and unhelpful) reply btw.

Check this:
https://www.neutrik.com/en/product/ntl1
While this one may be overkill, that xicon is definitly underkill, afaik it is intended for telephone or intercom uses.
While useful for effects stuff (AMZ, Escobedo use it) I wouldn´t count on it to pass an undamaged clean guitar signal
Of course trying out is the key! 

[Rob Strand]

Thank you for so detail answer. One more question: for what in circuits like that uses a 10k and 0,001uF between jack's pins?

I would suspect it tames any crazy peaking in the frequency response with high impedance loads in parallel with variable capacitive loads.  There may have been a specific reason it's there like flattening the response.  It enforces a 10k resistive load on the transformer output above 15kHz.

[blackieNYC]

I needed a passive isolation so I went with the Edcor transformer. 10k:10k I think. Nice piece of iron for the price.since you are building an active splitter, you should build RG's splitter.

[wavley]

Definitely read RG's stuff about this at Geofex, his circuit really works well.  I tinker with iso transformers and my rig a lot, it all started with RG's article.  I've tried purposely saturating them to get that API-ish sound, tinkered with all sorts of variations of his circuit.  I play a lot of baritone and Bass VI so I went with $6 edcor transformers because the little guys choked out on the low notes, but were fine for regular guitar.  Recently I was messing with a buffer I built using an OP275 and found it ran my transformers wonderfully off a single 9V supply and sounded great, so I guess that's what I'm building into my new widget weird solutions box on my board.

[PRR]

> for what in circuits like that uses a 10k and 0,001uF

A transformer is a LOT of reactances (inductances and capacitances) and resistances.

Drawing the whole Equivalent Circuit would fill a chalk-board.

Trust R.G. While it is pretty sure to work without the R-C network, it is likely to have a peak at the top of the audio band. For the type of transformers _he_ worked with, these R-C values tame the peak.

[Prehistoricman]

When l was looking into this trying to find out if using a transformer like this is common practice I've found that it is - but from what people have reported it's a tricky one unless your guitar has active pickups to compensate.


You said sometimes it has a hum. One thing I'd consider is swapping out the mains input with one with a built in filter. These help clear out the noise from the mains line. I've been using these with all of my amplifier builds.
Jameco Listing


Just more to consider!

Would this do much to help a guitarist with two amps? Perhaps the ground loop hum would be more clean  but the ground loop is still there.

[PRR]

Mains filter does *nothing* for 50/60Hz hum and related buzz.

It cuts radio waves coming up the power wire. That is (almost) never a problem in guitar-world. When we do have Peruvian Polka Radio or cellfone bzzzt, it usually comes in the guitar cord, not the power cord.

[Rob Strand]

It cuts radio waves coming up the power wire. That is (almost) never a problem in guitar-world. When we do have Peruvian Polka Radio or cellfone bzzzt, it usually comes in the guitar cord, not the power cord.

These days it's more it's to do with not radiating or conducting junk out of the device.

The other thing is mains filters usually connect the earth straight through.   There are some which have chokes in the earth but it's not going to do much at 50hz.  I have used common mode chokes to stop crap getting in at 2MHz (but I actually did on the DC side).

[Rob Strand]

Mains filter does *nothing* for 50/60Hz hum and related buzz.

The ground-line choke has been used on medical grade Line-Filters and power supplies for *many* years.  In fact in the early days the ground-line choke was always marketed at Medical Device applications.  It largely comes about because medical devices have lower leakage requirements which limits the size of the caps to ground (and hence limits the filtering).

For the hell of it I scanned over some recent documents and surprisingly the Schurter documents mentioned it can help be used for audio ground loops.

http://medical-technology.schurter.com/kfa-with-ground-line-choke
"Audio equipment for instance is known for being prone to ground loop problems. Often referred to as hum or buzz, the KFA, with ground line choke, is well suited for suppressing interferences that lead to diminished sound quality. "

Also stated is the traditional use,
"The KFA is also well suited for medical equipment required to use low leakage filters for increased patient safety. The reduced Y capacitance to ground, or absence thereof, leads to reduced asymmetric attenuation in higher frequencies. The KFA with ground line choke helps to compensate for this reduced performance. "

From memory the ground-line chokes are about 3mH (maybe upto 10mH), so they have an impedance of XL = 1 ohm.  So it will have some effect but it's not like the 10ohm ground isolating scheme you might put in an amp. eg.
http://sound.whsites.net/earthing.htm

[amptramp]

Op amps are not that fond of reactive loads and tend to see trouble when there is a capacitive load.  Since the transformer can be modeled as a tuned circuit, there will be a (low) frequency where it sees capacitance.  Perhaps an isolating resistor from the output of the op amp to the transformer will make things better.  It could be anything from 100 to 1000 ohms.

[PRR]

> 100 to 1000 ohms.

For driving naked stage-lines, good plan.

The effect of 1,000s of pFd on a typical op-amp is usually squeal *above* the audio band. You don't hear it, but it mangles your audio, may burn your power amps.

If forced to drive low-price 600:600 transformers, a series resistor becomes a tradeoff with bass response. IIRC R.G. was using <100r to drive these things, to get better than rated 300Hz bass limit. Generally 60Hz is possible on modem transformers with lo-lo-Z drive.