I'm getting a parts list together for a headphone amp that I'm working on and would like to incorporate an audio source ground loop isolator using a couple of matching transformers. The circuit is simple enough but I'm seeing several different impedance recommendations. Generally 600 and 10k ohm. They have some reasonable wide frequency response models over at Edcor in several impedances. Any of you done this? Can anyone recommend a proper impedance for use with a tube amp? I'm using a 6DJ8 on the input and I don't know what input impedance it likes to see (or how to calculate it for that matter.) It'll take months for them to get here so if I find out I ordered the wrong transformers, I'm probably gonna break down and cry... I'm also hoping that this will solve a little problem I've run into. My phone has no headphone jack. I have gotten two of those USB-C to headphone dongles with the integrated DACs in them and they positively REFUSE to turn on unless actual headphones are plugged into them. No aux input works, either on my headphone amp or any other commercial equipment that I've tried. I'm assuming that the DACs in those things are looking for an inductive load before turning on to protect themselves or something. I'm just not sure. Anybody have experience with that?
If you're trying to drive it with a standard 6DJ8 gain stage you'll want the highest impedance you can get. 10k as a minimum. To drive lower impedances you would want to thinking about using a more schmancy driving circuit, like a white cathode follower or SRPP.
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Why? I would recommend not doing this.
The 6DJ8 input stage isn't what's important, but rather the source driving the transformer.
This is probably just a power saving feature. Make yourself a 1/8" to RCA cable and solder in a 50 ohm resistor across each RCA plug on the output end. That should solve the problem and not introduce new ones.
My wife was trying to use an Apple lightning headphone adapter with the aux input on her car. The only thing it ever did was buzz a bit. I soldered a 50 ohm resistor across each output. It worked better but still intermittent. You may have to invest in a better source.
I'm interested in getting the matching transformers because some sources that I have plugged in introduce a ground loop through the audio source cable. Not all devices. Those transformers are only about $12 so I figured I wouldn't be risking much if it didn't wind up working. Pretty decent frequency response (20hz ~20khz or thereabouts) and they have a wire range of impedances to choose from. I don't want to buy two of each one though. I'll try the resistors and see if the dongle works. Just irritating since the phone would be able to play or access anything I'd want to listen to. I tried to buy a cheap mp3 player this past weekend just to get a baseline and not a single store in town still stocks one. Neither cheap nor expensive. Nothing. 😡
You could debug why they are doing that. It may be that you need to separate earth and audio ground by a pair of antiphase diodes to kick out the noise that you're hearing. This is an opportunity to do some learning...
Decent input transformers will not be $12. I just priced some today and they were more like $60.
Decent input transformers will not be $12. I just priced some today and they were more like $60.
At my desk amp I use 8 different sources for audio, a couple of DACs, a DJ mixer, a bluetooth adapter, tuner, a DVD player, computer, another receiver, and a few gadgets that use wall warts, Alexa and Google devices.
For the sources that hum it's almost always due to how those sources are powered. The easiest thing to try, is if the source has a three prong line cord is to lift the ground on that source with a two prong adapter keeping the neutral/hot properly polarized. That fixes it for me 80% of the time. Also if a source is powered by USB power and you also have the option of a wall wart, many gadgets have dual USB or wart power, try both, frequently one or the other won't hum.
I've had a desktop amp project on my radar for a while and specifically drew it up using an input transformer because of all the hum headaches with many sources.
Another option is to change your source selection rotary switch from a single wafer to a double wafer and have it switch both the shield and the signal for each source. This way only one source at a time is galvanically using the ground.
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I've used the Jensen JT-11P-1 with good results. The datasheet has an explanation of everything needed to get good results.
I don't know anything about Edcor's input transformers. Do they have good datasheets?
I don't know anything about Edcor's input transformers. Do they have good datasheets?
Edcor seems to make a decent product from what little I know on the subject of transformers. I've been happy with their output and power transformers and they significantly undercut Hammond. Hammond gets a lot of press and they make great quality stuff but it seems to me that they're maybe a tad..hmm.."proud" of it.
The problematic audio source is my bench PC. It does have a functional earth pin. As a test, I also clipped everything to a 10-foot grounding rod with a test lead. No difference whatsoever. I just figured that I'd try an isolator more as an insurance policy of sorts rather than a "must have" feature just in case I plug in a problematic device in the future. It'll be on my night stand in operation, so no PC. If it dinks up the sound then I won't use it. They make their stuff to order though and it takes 2-3 months to arrive (part of why it's cheap) so I didn't want to just take a shot in the dark as to specifications.
Edcor USA WSM Series - Multiple available impedances
Wattage 0.5W
Input Maximum Voltage 10Vrms
Turns Ratio Depends on model.
Primary (input) Impedance Depends on model.
Secondary (output) Impedance Depends on model.
Frequency Response 20~20K Hz., <1dBu
THD+Noise <0.05% @ 1K Hz.
Insertion Loss 0.5 dB
Bobbin Material Nylon 6/6 GF-30
Flammability Rating Class B 130°C
Core Material M-6 29 ga. grain oriented lamination steel
Secondary Insulation (transformer) Clear air dry polyester varnish
Termination 0.187" (3/16") quick disconnects
Mounting Zinc plated steel channel/frame
Weight 0.4 lbs.
Compliance RoHS & REACH
The problematic audio source is my bench PC. It does have a functional earth pin. As a test, I also clipped everything to a 10-foot grounding rod with a test lead. No difference whatsoever. I just figured that I'd try an isolator more as an insurance policy of sorts rather than a "must have" feature just in case I plug in a problematic device in the future. It'll be on my night stand in operation, so no PC. If it dinks up the sound then I won't use it. They make their stuff to order though and it takes 2-3 months to arrive (part of why it's cheap) so I didn't want to just take a shot in the dark as to specifications.
Edcor USA WSM Series - Multiple available impedances
Wattage 0.5W
Input Maximum Voltage 10Vrms
Turns Ratio Depends on model.
Primary (input) Impedance Depends on model.
Secondary (output) Impedance Depends on model.
Frequency Response 20~20K Hz., <1dBu
THD+Noise <0.05% @ 1K Hz.
Insertion Loss 0.5 dB
Bobbin Material Nylon 6/6 GF-30
Flammability Rating Class B 130°C
Core Material M-6 29 ga. grain oriented lamination steel
Secondary Insulation (transformer) Clear air dry polyester varnish
Termination 0.187" (3/16") quick disconnects
Mounting Zinc plated steel channel/frame
Weight 0.4 lbs.
Compliance RoHS & REACH
G'day, as it so happens I'm on the same search for a decent input transformer. And I don't want to pay $300 for a Jensen Iso-Max. My problem: I've hung a TV on a steel bracket connecting to the steel frame of the house. The TV is connected to power by two-wire, the amp is grounded / earthed by 3 wire connection. Amp and TV are connected by HDMI... and it hums. Take TV off bracket, hum is gone. So I've done some research and come across this: UNDERSTANDING, FINDING, & ELIMINATING GROUND LOOPS. I found it fascinating reading.
These might work for you. They are cheap and work well.
https://www.mouser.ca/ProductDetail...VRCibyEbxzgqzuKVlDMRbzGCn0ckUhjRoC-08QAvD_BwE
https://www.mouser.ca/ProductDetail...VRCibyEbxzgqzuKVlDMRbzGCn0ckUhjRoC-08QAvD_BwE
Couldn't wait... so connected the amp via optical cable. No hum 😉
On the weekend the TV will be isolated from the bracket with some nylon washers...
Lo-tech all the way!
On the weekend the TV will be isolated from the bracket with some nylon washers...
Lo-tech all the way!