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Isolation Transformer

D

DB56

Aug 1, 2021
12
6
#1
Hi All,

Wasn't sure if this was the right forum for this question but thought I would start here.

I play through a GK Legacy 800 and I have a problem with a high pitched noise (like a whine) I get when playing a certain club that doesn't happen anywhere else. It only seems to happen when the DI is plugged in. I have tried the ground lift switch, changing out cords, different outlets, running through a power conditioner, removing my effects pedals from the chain and anything else I can think of to stop it. I'm sure the club power is dirty, but I haven't had this problem in the past when using older amplifiers (like 20+ years older) in the same place. The only thing that seemed to make a change was when we started unplugging some of the club's lights, etc.

After talking with an engineer at Black Lion Audio, he thought I needed an isolation transformer. I figured that some of you have had this type of problem in the past so I thought I would ask your opinions on this.

Any help would be greatly appreciated.

Thanks in advance!
 
#2
Isolation transformer can definitely help if the problem is caused strictly from connecting the PA to DI box. You would put that transformer between the DI and PA. But make sure it is not just a ground lift adapter. I used Art DTI before. It is here we've is about $80 and maybe more than what you need.
 
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#4
It depends on how the noise is entering the signal path. IF it’s radiated noise from the SMPS in the lights than no it won’t make a bit of difference.

IF the noise is due to common mode conducted emissions than maybe. IF it’s due to noise injected into the power grounding system, it may help but probably only if pin 1 is lifted also.

IF it’s a combination of the above, it may help but eliminate entirely.

Also be sure that the shell and pin 1 are not connected on the cable. There should be no shell connection, that can bypass the pin 1 lift switch.
 
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#5
agedhorse said:
It depends on how the noise is entering the signal path. IF it’s radiated noise from the SMPS in the lights than no it won’t make a bit of difference.

IF the noise is due to common mode conducted emissions than maybe. IF it’s due to noise injected into the power grounding system, it may help but probably only if pin 1 is lifted also.

IF it’s a combination of the above, it may help but eliminate entirely.

Also be sure that the shell and pin 1 are not connected on the cable. There should be no shell connection, that can bypass the pin 1 lift switch.
Click to expand...

Thanks for your reply! It looks like they may have neon lights as well as video games on the same circuit. This place is an older bar with questionable outlets that we plug into. I can check the outlets with a multi-meter, but that's about it. At this point, can you think of any device that would help the situation?

Thanks again!
 
#6
Mike Caldwell said:
If the pin 1 switch did not help an ISO transformer would be the next thing, Whirlwind, Sesscom, Radial all make in line transformers.

That said it could be the XLR cable, some cheaper "import" cables have the outer shell tied to pin1 that can cause strange grounding issues.
Click to expand...


Thank you! I can check the cable ground easily. As far as the transformers go, I just need to be sure that it's not just a ground lift device.
 
#7
DB56 said:
Thanks for your reply! It looks like they may have neon lights as well as video games on the same circuit. This place is an older bar with questionable outlets that we plug into. I can check the outlets with a multi-meter, but that's about it. At this point, can you think of any device that would help the situation?

Thanks again!
Click to expand...
Does the noise change with the master volume control? The volume on the bass? Your position in the room? All of these point to radiated noise entering your bass.
 
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#9
kgbeast said:
How about this
"It only seems to happen when the DI is plugged in. "
Click to expand...
At high frequencies (that Switchmode power supplies operate at), ground loops do not necessarily behave like they do at low frequencies. Because of this, pin 1 lift circuitry can behave differently when there are 2 sources of high frequency interference (the smps in the lights combined with the smps and class D PWM frequencies in the amp).

I was trying to rule out this possibility rather than guessing.
 
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#11
agedhorse said:
At high frequencies (that Switchmode power supplies operate at), ground loops do not necessarily behave like they do at low frequencies. Because of this, pin 1 lift circuitry can behave differently when there are 2 sources of high frequency interference (the smps in the lights combined with the smps and class D PWM frequencies in the amp).

I was trying to rule out this possibility rather than guessing.
Click to expand...
The volume controls on neither my base nor amp seem to make a difference. I have stood out in front of the stage using my wireless and still have the noise as long as the DI is plugged in. Do you think the close proximity to my wireless setups could possibly cause this?
 
#13
FWIW, I've tried a lot of solutions that normally work to kill EMI at my apartment on my little tube guitar amp. The only thing that makes me not care about it is a Darkglass noise gate, but that's definitely more difficult to apply on a DI out.

I finally got a Radial Twin City to run two bass amps at once and thought "why not try the isolated transformer output" on the guitar amp. Didn't do a thing. I can only surmise that much like your club, it's the location itself (ancient circuit breakers, daisy chased AC outputs, common area CFL bulbs running 24/7, high current transmissions line across the street, solar power inverter on the roof, 111 apartment units grounded together, rusted out LADWP transformers, etc.).
 
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#14
DB56 said:
The volume controls on neither my base nor amp seem to make a difference. I have stood out in front of the stage using my wireless and still have the noise as long as the DI is plugged in. Do you think the close proximity to my wireless setups could possibly cause this?
Click to expand...
I have seen noise like (rarely but confirmed) this from UHF transmitters that operate at a multiple of the frequency that either the SMPS or the class D's PWM is operating at. There are also the harmonics that must be considered, what you can get then is heterodyning between the two signals, or even something as simple as sum/difference frequencies that fall within the audio band. Sometimes the noise will change in frequency as you play because the PWM frequencies vary with load.

Remove the wireless and see if things change.
 
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#15
Why not just put a DI after the pedal board, balanced out to the console, and parallel unbalanced out (aka "Thru") to the Legacy? Or (O the horror, God save the Queen, &c.) close-mic the bass cab just so the gig can be completed, albeit suboptimally, without all these headaches?

bayer3.jpg
 
#17
musicmansf said:
FWIW, I've tried a lot of solutions that normally work to kill EMI at my apartment on my little tube guitar amp. The only thing that makes me not care about it is a Darkglass noise gate, but that's definitely more difficult to apply on a DI out.

I finally got a Radial Twin City to run two bass amps at once and thought "why not try the isolated transformer output" on the guitar amp. Didn't do a thing. I can only surmise that much like your club, it's the location itself (ancient circuit breakers, daisy chased AC outputs, common area CFL bulbs running 24/7, high current transmissions line across the street, solar power inverter on the roof, 111 apartment units grounded together, rusted out LADWP transformers, etc.).
Click to expand...

Thanks for your reply. I definitely think the age of the place has something to do with it. As well as a total lack of maintenance.
 
#18
Rule of thumb is to rule out the easy things first.

It’s probably not ground noise between the two grounds because the ground lift in the amp didn’t solve it, but it could be common mode noise.
 
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#19
Most XLR DI out interfaces are electronically balanced driven by a dual opamp. There are coupling capacitors on the hot and cold pins to block DC. Common mode noise could be your issue, to which agedhorse mentioned too.

The pin 1 ground lift switch in an electronically balanced DI won't disconnect from earth, rather "isolate" by connecting through a 10k resistor and 10nF bipolar cap in parallel. This is to bleed off any radio frequencies as the long XLR cable acts like an antenna if not tied to earth.

Very few XLRs these days have an 1:1 600 ohm isolation transformer built into them.

You can try a HOSA Pin 1 lift adapter, but chances are it will make the noise worse.

Rolls, Whirlwind, ART, and many others with Jensen being the top brand in my book for freq response and low insertion loss.

Try a Rolls MS20C. I have two of them. It has a transformer inside. Not the best as it does attenuate the bass frequencies a bit, but it's a cheap way to determine if breaking the ground loop would fix the noise.
 
#20
Thanks for your reply. I appreciate your insight. I will check into that and hopefully we will be able to eliminate that noise on our next gig there. All the suggestions I have received are excellent! I'll keep everybody posted as to the outcome of the problem.
 
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