[DutchGFX]

Hey gang,

I built an OTL very similar to DIY Audio Projects Forum • Another Completed OTL Headphone Amp. It sounds fine, other than a few small issues



1. There is a slight hum even when the amplifier is off, when it is plugged it. If I unplug it completely the hum goes away, but if it's plugged in and off (via a power strip) then there is a slight hum/buzz. If I touch the ground pin on the plug (even tho my amp isn't connected to the ground pin) it stops.

2. Sort of building on that point, if the headphones are like half plugged in, then I get pronounced hum, even when the amp is off. If they are fully plugged it, the hum is faint but noticeable. If I touch the non-grounded terminals of the jack (tip or ring) I get a loud hum aswell.

3. When I turn the amp on, it hums very loudly for a few minutes, then that hum goes away (the hum from #1,2 remains)

I'm using an Antek Toroid. I currently have the tubes removed. Disconnecting the PSU ground solves the problem, but I'm measuring 0V AC on the transformer so I'm not sure how the PSU is doing anything. I also checked with a DMM, my Live-Gnd voltage is .8V more than my Live-Neutral, so I think my plug is wired properly.i m using a regulated B+

Any help is greatly appreciated. Thanks so much.

 

[MisterX]

Fix the grounding.
Detailed troubleshooting without a schematic and photos is nearly impossible and typically = a headache for the readers.

 

[DutchGFX]

Fix the grounding.
Detailed troubleshooting without a schematic and photos is nearly impossible and typically = a headache for the readers.

The amp schematic is definitely okay. I would think it's grounding but how could it be if the live is disconnected that I'm still getting hum? I used a star ground to a terminal strip. Especially since the tubes are disconnected and the PSU B+ isn't connected to the amp, I don't know how any current could flow

The amp is currently constructed on cardboard.

I can send a schematic and wiring in a bit

 

[DutchGFX]

How could it possibly hum with only the ground wire connected to the amp? I definitely don't have ground loops on the amp. Maybe the PCB has a ground loop? But even so, how would current flow in a ground loop without being on?

 

[Paladin79]

I would like to see the schematic if if becomes available. Any thoughts of mounting the the amp inside a metal case? It sure sounds like something is acting as an antenna and picking up 60 cycles. Do you happen to have access to an isolation transformer? They can be beneficial when troubleshooting shielding issues.

 

[NA Blur]

Some amp designs actually require sending the internal grounds to the chassis especially the main ground from the power supply.

 

[Paladin79]

There should be no floating ground with what info I see so far so hopefully the plans did call for a chassis ground.

 

[DutchGFX]

I made the plans, it's currently built on cardboard but will have a metal top plate. I believe I have remedied the issue by switching to DC heaters, have to test with a regulator above 5V (all I have available at the moment) but it is dead quiet with the 5V regulator on the heaters.

I'll keep the thread updated until it's ready for final build, but I think it's going to work.


The hum when off is due to the heater tap while my switch only switched the Live wire, so neutral was still conducting

Thanks for the help so far. I'll post back in a few days

 

[Paladin79]

I am glad you figured something out. Do you have the heater circuit live the whole time? The part about turning the switch off and still having the hum puzzled me a bit.
 
thanks

 

[DutchGFX]

Nah it was off, but there was a tiny amount of hum leaking in on my Neutral line, which was getting into the heater and somehow making it to the headphone jack


Is it worth it to go regulated heater supply, or should I just rectify and use dropping resistors?

 

[Paladin79]

Nah it was off, but there was a tiny amount of hum leaking in on my Neutral line, which was getting into the heater and somehow making it to the headphone jack


Is it worth it to go regulated heater supply, or should I just rectify and use dropping resistors?

It should be easier to go with a voltage regulator and less circuitry to allow for.
 
I was getting some hum the other day on a DIY pre-amp but luckily it was just a microphonic tube. I had been doing some tube rolling and got into some low quality tubes out of China with which I was experimenting. Just touching the tip of one tube was enough to stop the hum. I replaced the remaining two tubes and the sound got better progressively. I also found that using RCA cables with a third shield wire helped as well.

 

[Sunshine55]

I would check the neutral and ground to see if one or the other is not connected properly. Use a neon or meter and measure ; 1- from gnd to hot yes. 2- from neutral to hot yes. 3- from gnd to neutral NO. If you get any other result the problem is in the house wiring not the amp or system.

 

[JDBrian]

Just a thought. I'm not sure what the amp schematic looks like but is it a typical OTL amp with a cathode follower output? If so, have you biased the filament supply to prevent leakage. Typically you would bias to about 1/4 of the B+ supply to prevent exceeding the output tube filament to cathode voltage limit. I'm currently building an amp like this and have AC filament power. If I get hum I will do a regulated supply. 

 

[DutchGFX]

I no longer have hum due to the filaments. The DC fixed the issue. However, when I don't short the input, I get loud hum in both channels. Even with a grid leak resistor (330k, but hum even with a 10k) I get hum. As soon as I short the inputs, the hum goes away if I move my hand near the wires. It's pretty odd. If I position the wires correctly then shorting the input kills the hum

 

[JDBrian]

It's always hard to troubleshoot something like this remotely. Do you have the circuit ground connected to chassis ground? This should be done at a single point if possible.