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?
(https://i.postimg.cc/z31dRNSY/Qtn.png) (https://postimg.cc/z31dRNSY)
Wouldn't running your guitar through a transformer seriously alter the sound of the signal? Is that something you want in a passive circuit?
Quote from: Kevin Mitchell on January 04, 2019, 08:22:45 AM
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 :)
It is a perfect splitter without the transformer, with it, it is a dubious bandpass filter ;)
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.
Quote from: R.G. on January 04, 2019, 09:46:47 AM
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?
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 (https://www.jameco.com/z/1EGG1-2-TE-Connectivity-1-Amp-250-Vac-Power-Entry-Filter-With-Fuse-Chamber_1172613.html)
(https://www.jameco.com/Jameco/Products/ProdImag/1172613.jpg)
Just more to consider!
Quote from: Kevin Mitchell on January 04, 2019, 11:49:22 AM
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 (https://www.jameco.com/Jameco/Products/ProdImag/1172613.jpg)
(https://www.jameco.com/Jameco/Products/ProdImag/1172613.jpg)
Just more to consider!
Interesting solution, will try to find one
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.
Sorry for my trashy (and unhelpful) reply btw. :icon_rolleyes:
Check this:
https://www.neutrik.com/en/product/ntl1 (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! :P
QuoteThank 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.
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.
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.
> 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.
Quote from: Kevin Mitchell on January 04, 2019, 11:49:22 AM
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 (https://www.jameco.com/z/1EGG1-2-TE-Connectivity-1-Amp-250-Vac-Power-Entry-Filter-With-Fuse-Chamber_1172613.html)
(https://www.jameco.com/Jameco/Products/ProdImag/1172613.jpg)
Just more to consider!
Would this do much to help a guitarist with two amps? Perhaps the ground loop hum would be more clean :icon_biggrin: but the ground loop is still there.
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.
QuoteIt 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).
QuoteMains 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
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.
> 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.
Quote from: amptramp on January 05, 2019, 05:25:31 PM
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.
Yep. Opamps and even emitter followers get downright ugly with some capacitive loads. In fact, many opamps today included a maximum capacitance they will drive.
@OP; the geofex versions of these things all use a 100R resistor to isolate funny loads from the opamp and keep it stable.
On other topics mentioned: Active drive for a transformer in applications like this is a way to cure many of the flaws of cheap isolation transformers. It won't so it all, but it helps. The bigger defects in cheap trannies can be sorted into buckets: poor low frequency response, usually about 300Hz; poor tolerance of high signal levels; and peaky, ringing or harsh high end frequency response.
A continuous problem is the low input impedance. Jensen likes their 10K:10K isolator for a passive guitar signal, and maybe it will do OK, as suggested in their app note. But I never wanted to buy a $50 transformer to try it out. Magnetic pickup guitars tend to lose highs badly with impedances under 300K, and really badly into 10K to 100K.
Of course, it's hard figuring out what impedance an isolator transformer presents to a guitar. Transformers don't have impedances (at least not directly), they have RATIOs. The stated impedance and frequency response tell you indirectly about the internal inductances, and a ltttle about the internal capacitances. A spec on a trannie of 10K:10K only tells you that if you put a 10K resistor on the secondary, the impedance looking into the primary will be mostly a 10K resistance to AC signals >> in the mid band of the frequency response << and that since the impedance ratio is the square of the voltage ratio, the voltage transformation will be 1:1, as the square root of 10K/10K is one.
You can back calculate some things. If the maker tells you the frequency response and the impedance ratios, you can assume that if the trannie is loaded with a resistor on the secondary, that the low frequency rolloff point will be determined by the primary inductance's impedance reducing down to equal the stated impedance. This is a somewhat nuanced thing in terms of transformer practice, so at worst, the transformer's stated low frequency rolloff frequency can be taken as the frequency where the primary inductance impedance equals the stated impedance. So for a 10K:10K with a rolloff of 100Hz, you could infer that the manufacturer is telling you that the primary inductance's impedance is no worse than 10K ohms at 100Hz, and since Zl = 2*pi*F*L, you can calculate L >= 10K/(2*pi*100Hz). L is actually likely to be better than that, or the part would not always meet its specs.
This is the approach I took with the cheapo Xicon trannies. They're all specified at a low frequency rolloff of 300Hz. If you think about it, the primary inductance is eating an equal amount of the incoming AC signal CURRENT as the reflected secondary impedance if they're equal. It is possible to simply give the primary inductance more current to eat and keep the voltage across the reflected secondary impedance the same. That's one thing the amplifier at the input does. It takes the input voltage, then drives the transformer primary (through the 100R to keep it stable) and just shovels all the current out to the primary it can to keep the signal voltage right. As frequency drops, more and more of this current goes into the primary inductance, but as long as the opamp can shovel out enough current, the reflected secondary impedance sees the same signal voltage in spite of the primary inductance current hogging. So the overall actual low frequency rolloff as seen at the secondary is much lower. How much lower depends on where the opamp gives up on shoveling current. My measured result was that I could get those $2-$3 Xicons down to 60Hz for a -3db point.
That brings up signal level tolerance. The little cores can't live with too many ampere-turns shoved into them. The cores will eventually saturate. Not much to be done about this, as the core size is what it is. But one of the artifacts of core magnetization does get better with active primary drive. The magnetic core actually has a non-linear inductance, so that the signal current lost to primary inductance also varies instantaneously depending on how hard it'd driven. Fortunately, driving it with a low freqency source (opamp plus resistor) also partially fixes this, because the opamp will sit there and shovel in current to the mag field's dips and quirks to keep the signal voltlage (more) constant at the reflected secondary impedance. Not a cure, but it helps.
The high end is probably best tamed with that resistor/capacitor load. It is, as speculated, to damp the high end resonance of a cheap transformer and an unknown, unspecified load. The Xicon I messed with was specified +/-3db at 300Hz to 3kHz - that is, voice quality. In fact, it had quite a flat response out to about 14kHz, where response started rising. It ran up to a peak in the mid-20khz region. The cap value was chosen to let the load resistor start sucking energy out of the trannie's secondary above X frequency (can't remember right now) so that it would damp that ring-y peak. The peak wouldn't matter with normal guitar inputs, but somebody is sure to stick their MegaFire Octave Belter and IcePick emulator pedal in front of an isolator and let it rip. The cap/resistor load helps keep the insect-attracting tones down.