Hypex UCD400 hum problem tube pre

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ACF Hum

I just finished ACF last weekend, using glassware PCB(not the all-in-one),6922 tube, and AC heater, hook to my class D AMS1000-2600 AMP, dead silent. The key is to twist the wire, and lift the heater. Use 2X510 resister to divide AC heater, and 47k/200K resisters on B+. Here show you how to do it, take a look a PS part

Excellent! Thanks for the explaining picture. You have lifted the DC heater voltage to +42,6 Volt if i am not mistaking. I 'll trie it right away.
It is the same principle Rayma explained and it has to do with max ratings of tubeheater kathode, right? Which makes the tube (over) sensitive for HUM. In my case, the tube is a 6N6P.
I find it strange it is not mentioned at all in the ACF building manual, as the way the ACF is designed, it will often meet the same problem, won't it?

Anyway, thanks very much for your help. I love the sound of (preamp)tubes, but I don't like annoying hum.:smash:

Peter
 
Yes, this is the right circuit, except the bias voltage should be negative in this case.
Just use a DC voltage of about -50V, half the H-K rating.

This afternoon I have connected it as in picture beneath. But before I power it on, I just wanted to make sure if I am doing the correct thing and not ruining my poweramp and/or the heatersupply.

Can you pls take a look at it? Peter van Doornum

Is this okay?
 

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This afternoon I have connected it as in picture beneath. But before I power it on, I just wanted to make sure if I am doing
the correct thing and not ruining my poweramp and/or the heatersupply. Can you pls take a look at it? Peter van Doornum Is this okay?

Ok, except there must be a capacitor from the -53VDC heater tap to ground, with a value of 0.1-10uF,
with polarity observed (if it's an electrolytic). Make sure the heater supply is really floating, with its chassis isolated
from any other electrical contact. Once you connect it, verify that the -53VDC is still correct.
 
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Elevate heater supply did not work

Unfortunately the elevation of the heater supply did not work for me.
I elevated the supply to _50VDC, thus to reduce the h-kathode value., but it did not lead to the reduction of HUM.:confused::confused::confused:. It did not change, sadly.

There must be something else. I hope I can find it. I'll try to implement the other recommendations brought forward by you.

I'll let you know.

Peter
 
Hi Peter,

(serie 50k to middle of 50k trimpot to ground ) // (6.8 K* should not be + something)
Turn the trimpot to cancel 2nd harmonics or lower noise.

you remove the 150uf, can put a 100R after the main caps in + and - supplies, then another cap in each supplies, then the voltage drop 6.8k* in each supplies.

Try an input capacitors after the volume, just before the 1M this could be another spot.

Finally just get rid of the tubes :) is our source bad enough that sound deteriorates if going directly to uc modules ???

*** cathode follower are almost noise proof due to cancellation, so your noise must come at the grid input, some input wire interference.

Hi Gabdx,

Unfortunately the elevation of heater supply did not work. There was no change in hum, frequency and loudness.

Can you please explain or clarify by drawing or so what you mean and how i can implement what you propose. I really want to try.
I don't know what is going wrong. I have the same ACF in another class-D (Abletec 0300) amplifier, but the same hum occurs. Maybe it has got something to do with unbalance in the HT + and - of the class-D amps which the ACF can not handle??


I hope you can help me.

Peter
 
Thanks Rayma i will go on and let you know of it works
Peter

Hi Rayma,

I listened again today on my hornspeakers and I must say; the hum did reduce considerably. There is still hum but it is a lot less then there used to be. This morning I placed 2,2uF MKP parallel on the 470kOhm to GND but now I have placed 2 extra uF in parallel, so now 4,2uF total. Do you think the hum reduces if I increase the capacitor to max 10uF?

Peter
 
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Hi Rayma,

I listened again today on my hornspeakers and I must say; the hum did reduce considerably. There is still hum but it is a lot less then there used to be. This morning I placed 2,2uF MKP parallel on the 470kOhm to GND but now I have placed 2 extra uF in parallel, so now 4,2uF total. Do you think the hum reduces if I increase the capacitor to max 10uF?

I would guess that the hum will be reduced further if the total amount is doubled. Bear in mind that some
of the hum could be from a different mechanism. Is the filament bias voltage correct?
 
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Ground looping UCD 400

Hi guys,
I think it might be some ground looping problems :) check that!

Hi ionutzxpo,

Thanks for your answer. I was thinking the same but i just couldn't find it. Now i elevated heater voltage to 50 Volts and hum reduced considerably.

Still i believe, there is ground looping to a certain extent, but i don't know where to pinpoint it. I have consequently used star grounding.

Cheers Peter
 
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