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Aikido hum

kacernator

Member
2008-01-09 10:38 pm
Hi,

first of all, yes I have searched the forum, but didnt find the same symptoms I have.

I have built my first stereo Aikido preamp with 4xECC88.
My PS is SS rectifier then split for each channel: 47uF, 120 Ohm, 440uF to preamp.

According to Duncan PS Designer I should have audible ripple, which I have when I turn the volume on poweramp up high. Turning the volume on Aikido causes no change in hum. BUT when I disconnect the 100nF cap coming from B+ and going to the tube output stage grid to cancel the PS noise/hum I get LESS hum. Why is that? I thought the Aikido should remove any hum/noise coming from PS supply.

I aim waiting for my chokes to arrive so I can replace the 120 Ohm power resistor with them. Then I will have NO ripple coming from PS supply, so disconnecting the 100nF is not problem.

Is 100nF required for the Aikido sound? Or it is just there for the hum canceling, which doesnt work for me?

Thank you.
 
Not sure I am looking at the same 100nF cap you referred to.
In my Aikido diagram, I suspect C6 is the one. It looks like

B+ ----- C6 ------- R15 ------ Grids of both channels ---- R16 --- Ground

If so and you indeed removed C6, then you literally grounded the grids which is no longer the intend of Aikido topology.

Keep in mind, this topology can help canelling the power supply noise, but it doesn't mean eliminating any kind of noise. For example, if R15 and R16 are not closely matched per John's formula, then the cancellation won't work as well.
 

kacernator

Member
2008-01-09 10:38 pm
Yes it is the C6 cap. I should have R15 87K, but I have there 91K. I will put there 10k pot to adjust the right value.

You write (and also in the schematics of stere version) is that BOTH channels share the same PS canceling circuit - C6, R15, R16. BUT I have these seperate for each channel. Is it wrong? If so, how should I do it, when I have this PS supply: http://satanda.aspweb.cz/schem/aikido_psu.png

Should I just take the B+ from one channel from 47uF then to C6, R15 and R16 to both grids?

Thank you.
 
OK, this is the same cap we talked about, I just happened to have the stereo diagram with me. That's why I used the stereo description. In your case, you use the B+ from the 47uF of each channel. Don't share them.
You didn't say what R16 value you had. I guess it is 100K. If so, 91K might be a little too high for R15. In this case, you want to add the 10k pot to the R16. Roughly, you are looking at the R15:R16 ratio of 87K:100K +/- 500 on the R15 side. Use the pot to tune to the lowest hum.
 
Ok, so just to clarify. I will have twice R15, R16 and C6 on its own channel coming from B+ on dedicated channel, right?
Yup, yup.

Last question, why is the hum louder when the C6 cap is disconnected?
In your first post, you said
when I disconnect the 100nF cap coming from B+ and going to the tube output stage grid to cancel the PS noise/hum I get LESS hum
so with or without the cap has louder hum? I will expect noisier if C6 is missing because 1/2 is the PS noise is injected into the output upper triode's grid, and the other half goes to the lower triode's grid via the R15/R16 voltage divider. When C6 is removed, you only have the 1/2 PS noise from the upper. Therefore, noisier because no cancellation. If your experience is the other way around, I dunno why would that be.
 

kacernator

Member
2008-01-09 10:38 pm
Sorry my mistake. I meant when there IS cap the hum is LOUDER.

Maybe I have found the mistake...I dont know right now, because I am at work now, so I do not have here my preamp, but I possible forgot totaly R12 (1M) resistor. Will check that tomorrow and will report back.
 

kacernator

Member
2008-01-09 10:38 pm
I checked the circuit and everything is ok.
I installed the 100k trimpot instead of R15 and when turning with it the hum is the same, there is just a LITTLE change when the pot is fully clockwise and anticlockwise.

I dont know now what to do, or how to discover whats wrong. :xeye: Any ideas?

Thanks.
 

kacernator

Member
2008-01-09 10:38 pm
It has been built point-to-point.
I noticed when the C6 cap is disconnected and I get less hum, the sound is dirtier. When I connect it back, the sound is clearer, but I get more hum. Really dont understand it.

When I switch the stand by off I get no hum, so the hum must come from B+. When I remove just first tube and leave the second I get MORE hum.

Grounding is made as - each tube share local ground point which then connects to the only one connection to the chassis, also where the PS is grounded.

Could it be, that this preamp is VERY sensitive to the B+ ripple?

I also substituted R15 for the100k trimpot, adjusting it didnt make any difference.

I hope installing chokes will help a lot, if not I do not want to add another filter capacity, because the sound would be then too stiff for my tastes.

I now have for each channel separate 47uF -- 120 Ohm -- 440 uF -- preamp channels.
 
Are all your ground points just connected right to the chassis at one point?

If you temporarily disconnect the circuit ground from the chassis does the hum stop?

If so you may want to consider putting in a ground breaker. Some back to back diodes paralleled with a 10R power resistor and say a 0.01uF cap should work nicely.

Fran
 

kacernator

Member
2008-01-09 10:38 pm
I lifted the ground from the chassis. The HUM was still there without any change. It seems to me, that the PS rejection circuit is amplifying the ripple instead of canceling it.

I also tried to ground on different spots, diferrent grounding scheme, but the problem is still the same.
 
Let's use this user guide, http://www.tubecad.com/Aikido_9-pin_Mono_Rev-A_PCB.pdf
for reference.
Your wire-to-wire should be based on the line amplifier on page 8.
In this schematic, the cap you referred to is C4 now.
The last paragraph of page 3 says, John says R15 must be used. If C4 is removed, then it is like R15 is not used. Then what white cathode follower described in page 5 is no longer the case.

If it works as describe, then the "equal" ps noise injected into the grids of the upper and the lower triodes should be cancelled because they are supposed to be same amplitude but out of phase. So, one possibility is tha they are in phase which ends up doubling the noise.
Can you remove R5, R6, and R15 to see it is less hum than just disconnect C4? Before removing R5, R6, R15, please take the voltage of the junctions of R5/R6 and R15/R16. Also, compare your wire-2-wire to schematic in page 8 to see there is any wiring errors. My gut feeling is that it could be just a small wiring error.
 
R4 (=R2) and R8 (=R11) should be of values equal to the Rk column in the user guide. So, at 300 B+, it should be around 480R, 3.4K is way too big. Notice that increasing Rk reduces the current drawn. Once you correct it, you will see your B+ to drop below 330. Start with 470R. R4/R2 and R8/R11 are used for bias. So, you have a far-from optimal bias, but I don't that is the cause of the symptom you have. Correct it first, I can be wrong. Also, did you bias your heat to 1/4 of the B+?