I use standard line level voltage of 300mVAC RMS.
Still, THESE isolation transformers distort at any level.
Tested using a loopback and signal analyzer, with my audio interface and DAW.
Closed loop: 0.003% -90db highest harmonic.
Through ebay isolation transformer : 10% THD (-20db) @ 30Hz, 1% (-40db) @60Hz.
Clearly audible especially the bass and unacceptable.
For now, PE lift via diodes is what I'm doing, even though I trade safety.
Still, THESE isolation transformers distort at any level.
Tested using a loopback and signal analyzer, with my audio interface and DAW.
Closed loop: 0.003% -90db highest harmonic.
Through ebay isolation transformer : 10% THD (-20db) @ 30Hz, 1% (-40db) @60Hz.
Clearly audible especially the bass and unacceptable.
For now, PE lift via diodes is what I'm doing, even though I trade safety.
Last edited:
learn how to build Class II it's not very hard. non SELV needs other things so it doesn't work out that easily in all cases.
See what you did there...
Read it again. Mark Whitney didn't say anything about having no ground.
The problem with Class II is the limit on power, 100VA - 30VAC which I translate to 15-0-15VAC and limits its usefulness for a power amp. But not a problem for a pre-amp or CD player.
Okay I have a solution for my problem.
I found an Optical/SPDIF to RCA small conversion box I had plugged to my TV.
Before I run music through it I made sure it has low noise floor and low distortion at peak volumes.
Luckily these small digital to analog converters are quite good, with at least -95db noise floor and 0.003% distortion across all frequencies, surprisingly just as good as my Focusrite audio interface.
It's probably even better (lower) because I may just see the the noise floor of the inputs and the distortion too.
This mini DAC is a Class II device with a 5V or USB powering it, I used a 5V adapter to prevent using a the same ground as my PC.
Connected an optical cable from my audio interface to this mini DAC and from it via RCA to the amp... Silence.
THESE little DACs are quite useful and even better in specs than I've expected.
Anyway, PE is now connected as it should to dangerous Class 1 devices.
The DAC which is a Class 2 device and don't have PE, now connected to the Class I (Amp) which do have a PE, they share the same ground.
Now the other Class 1 devices (PC and Audio Interface) from which the music comes, are optically isolated from the first Class 1 device (Amp).
No mixing of Class 1 devices = No ground loops.
All Cool.
I found an Optical/SPDIF to RCA small conversion box I had plugged to my TV.
Before I run music through it I made sure it has low noise floor and low distortion at peak volumes.
Luckily these small digital to analog converters are quite good, with at least -95db noise floor and 0.003% distortion across all frequencies, surprisingly just as good as my Focusrite audio interface.
It's probably even better (lower) because I may just see the the noise floor of the inputs and the distortion too.
This mini DAC is a Class II device with a 5V or USB powering it, I used a 5V adapter to prevent using a the same ground as my PC.
Connected an optical cable from my audio interface to this mini DAC and from it via RCA to the amp... Silence.
THESE little DACs are quite useful and even better in specs than I've expected.
Anyway, PE is now connected as it should to dangerous Class 1 devices.
The DAC which is a Class 2 device and don't have PE, now connected to the Class I (Amp) which do have a PE, they share the same ground.
Now the other Class 1 devices (PC and Audio Interface) from which the music comes, are optically isolated from the first Class 1 device (Amp).
No mixing of Class 1 devices = No ground loops.
All Cool.
Eh? Most any integrated amp or HTR from the big Japanese manufacturers that you'll run across over here will be Class II, with rated power well into the 100s of VA.
It's a bit of a different story for switching power supplies, which do tend to include a grounded mains lead at higher wattages (75-ish W up, I think?). That's more for EMC reasons though (so you can connect the mains filter properly), and the secondary side is not necessarily connected to PE at all. I've seen a few different versions on notebook power supplies. 1 kOhm seems to be the Dell and IBM way, an MSI supply went for a hard PE connection, and a Toshiba had no Ohmic connection at all if memory serves.
Class II SMPS' are nasty stuff and good for all kinds of trouble. Common-mode RF noise, hum in microphones and sensitive earphones due to capacitive coupling to supply through the mains filtering (so leakage currents will travel from mains filtering over circuit ground over you to ground).
For Class-I you MUST connect PE wire to chassis (AND metal parts which a "test finger" can touch, e.g. a RCA connector shell and parts which are reachable through holes in the housing).
There is no scope for interpretations here.
For Class-II a rated transformer with reinforced isolation is only one important part.
In Class-II there is more to pay attention to than the transformer.
For example consider the case where the live cables break or a loose screw or metal part gets inside the chassis.
Good practice is that the live parts have a second plastic cover which prevents contact even if the housing is openened by a diy.
The isolation distances too are stricter depending on pollution class.
If you have problem with ground loops:
- use isolation transformers (e.g. for digital signals)
- use a low ohmic wire to connect equipment to make a path for ground current
There is no scope for interpretations here.
For Class-II a rated transformer with reinforced isolation is only one important part.
In Class-II there is more to pay attention to than the transformer.
For example consider the case where the live cables break or a loose screw or metal part gets inside the chassis.
Good practice is that the live parts have a second plastic cover which prevents contact even if the housing is openened by a diy.
The isolation distances too are stricter depending on pollution class.
If you have problem with ground loops:
- use isolation transformers (e.g. for digital signals)
- use a low ohmic wire to connect equipment to make a path for ground current
Does this mean that Amplifiers (practically all Class I) meant to be used with Class II (CD/DVD Players, Turntables, Game Consoles) or Class III (Wall-wart or Battery powered, small DACs, CD/DVD Players) devices only?
What about a PC setup which is always class I?
Laptop (Class III)?
What about a PC setup which is always class I?
Laptop (Class III)?
Last edited:
I was wrong here by confusing Class II with Class 2 (NEC). Class II has no power limit and is a construction method.
I don't understand why this thread became an Appliance Class discussion when all I wanted is an advice about what people do to prevent ground loops and whether it's safe to lift the PE via diodes.
Anyway, I understand from Bonsai that even top quality amplifiers have problems with hum and ground loops and this is a very common problem.
The question still stands, how people combat this with maximum safety?
Hi scobham, Thanks.
The only isolation transformer that is good enough (distortion) is from Jensen but it costs 200$ which is unfairly expensive.
The next best solution I found is a mini DAC (SPDIF input -> RCA output) which is optically isolated and have quite good performance considering its price.
I guess there is no other (safe) way for unbalanced signal.
The only isolation transformer that is good enough (distortion) is from Jensen but it costs 200$ which is unfairly expensive.
The next best solution I found is a mini DAC (SPDIF input -> RCA output) which is optically isolated and have quite good performance considering its price.
I guess there is no other (safe) way for unbalanced signal.
Hi James, you should be able to get them for a lot less. Try Jensen JT-11-YMPC that one is fairly exotic and still only about 30 USD each I seem to recall and there will be lower cost versions. I used a pair of those between my DAC and amp (actually to do the I/V conversion but the side effect is to break the ground loop). My hum levels are now non existent. You can also use Lundahl transformers as an alternative.
James i have been studying the photos posted on your tubes/valves thread . IF! you are prepared to modify the amp I believe I have a cure for you .The chassis / PE earth needs to be separated from the amp signal earth with your Hum-X type circuit . The signal earth appears to meet the chassis earth at the centre tap of the 2 rail smoothing capacitors where this centre tap is screwed down to a metal stand off pillar. You would need to interrupt this connection to insert your circuit between the two.
Apologies I missed post 26 you have already found a cure
Apologies I missed post 26 you have already found a cure
Last edited:
Yes, but the problem is with my SS amp which is a Behringer A500, an excellent amp if the volume pot is maxed (I replaced it with permanent resistor) and attenuated externally using stepped attenuator.
The tube amp is silent.
I guess that my SS has much more power than the tube therefor I here the noise better with the SS.
In any case, the small DAC is a huge value for its cost.
Until I decide to invest in some proper isolation transformers.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.