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Old 21st February 2008, 06:18 AM   #2031
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Quote:
Originally posted by ahegyes
Here is a drawing of the 2 f4's and the buzz...
Hi...you can try the following:

1. Check the resistors for correct values. If you have a faulty ground resistor (as I had with my A30) you will have exactly this problem.

2. Isolate the humming channel, so it is the only active channel. trace with a scope where the humm is comming from. Is is 50 or 100Hz? 50Hz could indicate humm pickup from mains; 100Hz pickup after rectification. 100Hz could indicate a bad cap or high bias current.

3. Compare the DC voltages on every node of the amp and compare them with the other channels. Try to find the origine of each deviation.

4. Use cold spray / solder iron to heat/chill the Jfets Junction problems can cause this as well.

5. Inspect the PCB for parasitic traces of solder.

6. Only use the amp with teh humming channel. Leave the other disconnected. Is the amp still humming? if not...are you sure about your grounding?

I hope this helps. This can be quite hard to find...
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Old 21st February 2008, 06:46 AM   #2032
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Quote:
Originally posted by ahegyes
Helloo there!
I have built a pair of F4's and they work just fine, but i have a little problem with one of the chanels. It has a very little buzz. It is so little that it is bearly hearable with the ear next to the speaker - but it is there and it is annoying. The othere 3 chanels are dead quiet.
Anyoane any ideea of what shoud be the problem????

PS: I do not have ground loops and all of the other chanels work in the same conditions.
- try to move cables (left, up, right, down) connected to this channel
- move transformer or the buzzing board - maybe the distance is to short
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Old 21st February 2008, 06:52 AM   #2033
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ooops, I did not see that there is the next page (84) already, with drawing, sorry...
a.
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Old 23rd February 2008, 12:04 AM   #2034
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I've read the earlier part of the thread where early builders had problems trying to dial in enough bias current when adjusting P1. Some circumvented the problem by changing R9 to 6.8k instead of the original 10k.

But I wondered if it is because the earliest schematic had R8 at 22.1k instead of the latest schematic which had R8 at 27.4k.

So did any of the recent builders using the latest schematic had issues dialing in the bias current with R9 at 10k?

thanks

her shann
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Old 23rd February 2008, 12:21 AM   #2035
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I'm using Peter's pcbs and the latest schematics. I mistakenly fired it up with P1 running full tilt--there's was ample bias current. Seems to me I had a 400mV reading across the 0.47 ohm resistors.
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Old 23rd February 2008, 12:26 AM   #2036
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Quote:
Originally posted by carpenter
I'm using Peter's pcbs and the latest schematics. I mistakenly fired it up with P1 running full tilt--there's was ample bias current. Seems to me I had a 400mV reading across the 0.47 ohm resistors.
Thanks for the info - am using Peter's PCB too.

Will use 10k for R9 then.

her shann
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Old 24th February 2008, 07:44 PM   #2037
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Her Shann,
I had no problems with bias and offset.
Set every P1 and P2 in the middle.
Put a small piece of sticker in different colors to distinguish P1 and P2, see attached photo, it makes tunning easier.
Good luck!
Andrzej
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File Type: jpg imgp0720_.jpg (73.5 KB, 1090 views)
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Old 24th February 2008, 07:53 PM   #2038
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Quote:
Originally posted by Andrzej Sochon
Her Shann,
Set every P1 and P2 in the middle.

Set every P1 and P2 in the middle before you attach supply voltages for the very first time. Then begin to turn P1 to adjust 130 mV on one of the 0.47R, while all is cold.

Regards,
a
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Old 24th February 2008, 07:57 PM   #2039
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Hi Andrzej,

no isolation between fets and sink?

c.
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Old 24th February 2008, 08:18 PM   #2040
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Quote:
Originally posted by jameshillj
Nichicons need to "run-in" for quite a few hours - at least 24 to 48, before they start to sound good - MKPs taken longer.
If you are patient with them, you should get a very clean sound overall with a heavier, more solid base....
Hello JH,
I try to be patient, not easy. I see I have some problems with my ears,
a kind of slight infection. Normally I do not hear the differences but while listening my f4 I discover sometime better perception of trebles at the left, sometime at the right side. From few days I plug my ears with cotton wool when I am outdoor, it helps me to keep acoustic symmetry of audio perception later, at home. I am sensitive for a cold weather.

I also observe not so deep ambient acoustic scene as before Nichicons with MKP Vishay caps. Sound is perfectly clear now, maybe to much highest frequencies, maybe I will detach MKPs later.
Did you have similar phenomena?
Regards,
a.
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