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Old 14th January 2008, 09:56 AM   #1
wonsup is offline wonsup  United States
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Default Need Help eliminating hum on a Aikido headphone amp

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.
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Old 14th January 2008, 03:05 PM   #2
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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.

Click the image to open in full size.

-- josť k.
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Old 14th January 2008, 11:39 PM   #3
wonsup is offline wonsup  United States
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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)
Attached Images
File Type: png aikido%20headphone%20amplifier.png (62.5 KB, 477 views)
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Old 15th January 2008, 02:01 AM   #4
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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,

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Old 15th January 2008, 07:09 AM   #5
wonsup is offline wonsup  United States
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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?
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Old 16th January 2008, 08:21 AM   #6
wonsup is offline wonsup  United States
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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.
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Old 8th November 2009, 05:20 PM   #7
Brit01 is offline Brit01  United Kingdom
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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.
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Old 8th November 2009, 07:23 PM   #8
Brit01 is offline Brit01  United Kingdom
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OK removed one of the 1M to ground resistors and still loads of hum

Not sure where its coming from now.
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Old 9th November 2009, 03:54 PM   #9
Brit01 is offline Brit01  United Kingdom
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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.
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Old 9th November 2009, 04:29 PM   #10
Stixx is offline Stixx  Germany
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Are you using the same psu that you used for the pre configuration?
Strange that it has hum as a headphone amplifier...

Have you checked all ground connections and that you don't accidentally have a loop somewhere ?

Good luck!

Edit: my Aikido headphone amp is dead quiet (heaters lifted but even without so it had no hum...)
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