Need Help eliminating hum on a Aikido headphone amp

[Stixx]

I have his manual but that is not noted.

Err, it is noted in RevA... in the paragraph "Configuring the PCB as Headphone Amplifier". But you are right, he changed a few things to RevC...?

So RevA says: You will need R17 and have to replace R8 and R15 with jumpers...

But then it should work hum free following RevC as well...

 

[Brit01]

Could the incorrect value of R15 cause such a hum?

I do need to increase R15 a little I was calculating. From 79K to 82.5K. I could use a variable resistor and fiddle.

I will check all grounding again.

 

[Stixx]

I do need to increase R15 a little I was calculating. From 79K to 82.5K. I could use a variable resistor and fiddle.

There even was a thread about using a variable resistor for R15 to tweak for max performance, but I very much doubt that a value just a little off would cause massive hum...

Good luck again!

 

[Brit01]

Yep you are right Stixx.

For sure it's a grounding or transformer issue injecting this much hum.

 

[Brit01]

Last night I was rearranging the wiring so the HV and input signals were well separated. I changed tubes, checked all grounding again.

BUT still loads of hum.

C3 (1800uf) connected. Jumper for R8.
C11 (0.1uf).

Lifted ground (0.01uf).

Maybe C5 could be removed to see the effect.

Might try removing the PIO bypass cap (maybe faulty?). Thinking of anything here.


If all fails guess I'll have to try a new power transformer.

 

[Stixx]

Brit,

maybe you can post a highres image of your built...
many eyes see more than two, and it makes thinking about what it could be much easier...

Lifted ground (0.01uf).

Just that or did you elevate the heaters with a voltage divider...?

 

[Brit01]

Heaters elevated with ~1/4 B+

I'll try to get a pic in a day or 2.

 

[Brit01]

Was testing with my valve psu last night. Hum present and realised I had the virtual tap connected to the ground instead of the heater bias

Hum diminished significantly when fixed connection.

So decided to try the solid state rectifier I designed originally. Hum back again

Increased heater bias to 50V but still hum. Seems to be eminating from around the volume pot also. I have it grounded and with shielded cable

Would it help if I increased the heater bias? 90V instead if its current 50V?

Also I will try putting back the valve psu tonight.

 

[Brit01]

QQ:

Never understood the purpose of C5 (to ground) on the Aikido board.

Can someone explain the benefit of this cap?

I've never had a cap in this position on the B+ side preceding the anodes on other designs.

 

[Stixx]

As I see it C5 (or C3 in RevA) is a decoupling cap... can't tell you in scientific terms what it does but it doesn't affect hum.

In my pre amplifier (not built by me) this decoupling cap is an entire bank of highest quality capacitors, sort of as an energy reservoir before the actual circuit.

I have it grounded and with shielded cable

Have you grounded the shield, and on one side only? Sorry for asking, but I did that (grounding both sides) in my very first amplifier and had trouble to find the source of the resulting hum...

 

[Brit01]

HI Stixx,

As I see it C5 (or C3 in RevA) is a decoupling cap... can't tell you in scientific terms what it does but it doesn't affect hum.

okay good to know.

Have you grounded the shield, and on one side only? Sorry for asking, but I did that (grounding both sides) in my very first amplifier and had trouble to find the source of the resulting hum...

Grounded on one side only.
With the alps I found I had to also ground the metal casing.

This humming is making me lose sleep.

think I will breadboard everything and clearly separate all parts and wires.

 

[Brit01]

Now this is fustrating.

Re-wired, separated all heater wires, HV wires and signal wires. Tried with a valve power supply and solid state but both hum. Tried different tubes also.

I'm thinking its the transformer causing this noise now.

Maybe time for a new one.

 

[Stixx]

Seems you found a very persistent kind of hum...

 

[Brit01]

Yep

Pulled out my aikido board this morning.

Going to throw together another circuit, one I tried before with great results.
6CG7 with a 6080 (wheatfield I believe).

If this hums then I will order a new transformer.

 

[Brit01]

Just finished my wheatfield with a 6N6P input tube and 6080 second stage.

Sounds great but still got the hum.

Guessing it's from the power transformer now.

I'll try to pick up a new one.

 

[Wieslaw Lipowsk]

>>Grounded on one side only>>

Make sure it's right at the input.

 

[Brit01]

Yep checked everything 4 times over. Never had this hum before so I wouldnt be surprised if its the transformer.

One time a burning plastic smell came from it for a couple of seconds when a diode blew on my psu.