Need Help eliminating hum on a Aikido headphone amp

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wonsup

Member
2007-11-23 11:53 pm
Finally got it up and running and it has a great sound, but I have a couple issues with hum.

BTW, the chasis holds a current sourced cathode follower tube buffer stage as well as the Aikido in headphone configuration (in that order). Both are powered by a same power supply. I can serilzie the two or bypass either one by a switch.

1. If I do not plug anything into the RIGHT channel RCA plug, there is a massive hum on the RIGHT channel (when I use both aikido and the tube buffer). As soon as I plug in the sourceto the right channel (just the tip. the ring/ground does not even have to connect at all) the hum goes away. What causes this and where shall I look into?

2. If everything is plugged in, it still has very faint 60 hz hum (just aikido, or both buffer and aikido has the same hum. So the hum must come from Aikido). I do not notice it until the music stops. It seems to come from more on the left channel. I am running AC 6.3v from my transformer directly to heat the tubes. If I switch over to a DC power, would that improve the silence? (I didn't quite understood the 1/4 B+ voltage thing in the instruction, but I thought it did not apply if you are using AC heater? Correct me if I am wrong)

Note - the chasis is grounded, but the input, output and any other parts of the amps are not grounded to the chasis, but to a ground bus only. Isolating the input and output so far has not helped with the hum at all.
 
For reference, shown below is an octal Aikido circuit.

Please post what tubes you're using, the value of the voltage divider resistors, and the cathode resistors that set the current.

6bx7_1.png


-- josé k.
 

wonsup

Member
2007-11-23 11:53 pm
I am using the stereo 9 pin board. I am using the values that the manual recommends for 6CG7/6DJ8 tube configuration for headphone.

R2/R4 = 470 ohm
R8 - no resistor
R11 - 300 ohm
R15 - 82K ohm
R16 - 100K ohm
B+ is set at 250 V

I re-read the instruction manual for the 10th time, and I think I do need to reference the heater power supply to the 1/4 of B+ voltage by using my center tap (CT) of the 6.3V AC secondary. Currently by 6.3 V AC CT is just grounded.

Mine looks more like below, except I do not have R17 (it was removed in revison B PCB)
 

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Wonsup, the 'massive hum' on the input that doesn't have anything plugged into it is called open input hum, and is universal as far as I know.  Don't select a channel that doesn't have anything plugged into it.  When I test for hum+noise, I plug a shorting RCA (hot directly connectd to ground inside the plug) into that channel.  If you're going to use only one channel of a source, plug a shorting RCA into the other one to avoid this.

I have limited eggsperience with the Aikido (I breadboarded one up, and didn't get the hum out in a couple attempts, gave up), but as I understand it, the 'hum cancellation' feature is that pair of resistors (with a capacitor in series) split across the grid of the 'lower' tube of the second stage.  In yours, the top resistor is 64.9K and the bottom one is 100K.  You might try changing the value of one of those resistors about 20% and see if the hum increases or decreases.  This might point to a solution for you.

Spearmint on,

Poinz
 

wonsup

Member
2007-11-23 11:53 pm
Thanks Pointdexter. I think I do have a "open input hum". Basically I believe it is induced by my input RCA jacks?

I have 2 RCA jack sets used for input for this amp. I use a switch to use one input or the other input.

Input A - goes through the current sourced buffer and then the output of that goes through Aikido.

Input B - goes stright through Aikido and skips over the buffer section.

The above input is controlled by a switch. When I put the switch to Input A and have nothing plugged in, I get a massive hum, although it only hums on the right channel and not left channel.

Is this the symptom of "open input hum"? How do I get rid of it?
 

wonsup

Member
2007-11-23 11:53 pm
OK, I got the hum out.

Hum A (Loud Hum from the cahtod follower) - I put a resistor on the input to the ground that is used as a reference voltage. This removed the hum when nothing is plugged in.

Hum B (Quiet hum from Aikido) - When I did the 1/4 B+ thing to the heater wire, it removed the hum that I had. This thing is asboultely quiet, it is quite amazing. Probably the quietest amp I've ever built.

The Aikido sounds pretty awesome, considering that it is not even broken in yet. It has a lot of slam on the bass and great details. Right now the mid sounds a bit grainy, but I think it will smooth out after the break in.
 

Brit01

Member
2008-07-04 10:35 am
Just converted my Aikido pre to the headphone configuration.

But I'm getting a constant hum with or without input/music.

Now I've got both R13 and R14 1M to ground resistors connected. 200uF cap in C2 and bypass 0.47uF PIO C1.

Could the 2 x 1M resitors be creating a ground loop?

The only reason I haven't disconnected one of them is because of the design and would take some work to get at the resistors.
 

Brit01

Member
2008-07-04 10:35 am
I tried floating the ground. Raised the heater referance to 35 volts. Floated the ground on pin 9 of the 6N6P with a 0.1 uf cap.
Still Hum.:(

Think I need to change the power supply. Pretty sure its coming from the basic solid state psu I threw together.


I have a valve psu on a buffer I will utilize to see if it eradicates the hum.
 

Brit01

Member
2008-07-04 10:35 am
Used a different psu. Solid state instead. But I used this solid state power supply on another design I had and no hum. It doesn't agree with my aikido:(
Maybe I can try resistor/cap combination over the 1N4007 diodes.


Will revert back to the tube rectifier and see the outcome.

Was driving myself crazy checking the grounding system. Lets see how it sounds once I get the tube rect. connected.
 

Brit01

Member
2008-07-04 10:35 am
:no::no:

used my valve psu (Geek's well tested/proven psu), and still the hum. Actually a bit stronger.

If it's not a ground loop;
Could there be a problem with the transformer?

I remember when one of the diodes blew on the psu, a slight burning smell came from the power transformer :spin::spin::spin:
 
Just converted my Aikido pre to the headphone configuration.

But I'm getting a constant hum with or without input/music.

Now I've got both R13 and R14 1M to ground resistors connected. 200uF cap in C2 and bypass 0.47uF PIO C1.

Could the 2 x 1M resitors be creating a ground loop?

The only reason I haven't disconnected one of them is because of the design and would take some work to get at the resistors.
200uF seems way too much here. Couldn't it be a hum problem?
 
Could there be a problem with the transformer?

I remember when one of the diodes blew on the psu, a slight burning smell came from the power transformer.

It could well be the transformer though I would suspect there also would be some kind of vibration when you touch the transformer...

Otherwise I can only ask stupid sounding questions..:eek:
Everything grounded to ONE point including the headphone jack?

Tracking down hum sure is one of the most obnoxious tasks in tube DIY... :D:D
 

Brit01

Member
2008-07-04 10:35 am
Tracking down hum sure is one of the most obnoxious tasks in tube DIY...

Sure are right here. :rolleyes:

All grounded to one point. I will check again.

Maybe its the layout. I might try stripping it down and laying it out on a breadboard again.

Apart from the loud hum, the music coming out was superb. Very nice headphone design indeed.

I'll check the NOS PIO caps I'm using as bypass for the 200uF electros.

Could an incorrect value of the R15 cause a hum here?


Ah one more thing: When I touch the power transformer the tone of the hum changes very slighty, maybe this is normal as I'm grounding it with my body. Not sure.
 
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Brit01

Member
2008-07-04 10:35 am
Wired as a headphone amplifier R15 should be just a jumper (see Boskie's Aikido.revA.pdf!) The output stage is different than wired as a pre so this might cause some kind of misbehaviour...

Are you sure about that? I've replaced R8 with a jumper but I thought R15/R16 were critical to the noise cancelling design even as a headphone amp.

Aikido Detour and R15 & R16 math and coupling capacitor issues

Thks Stixx
 
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