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Old 22nd August 2013, 05:34 PM   #13301
sjhomey is offline sjhomey  United States
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Yes kasey197. That is where I will start.
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Old 7th September 2013, 08:46 AM   #13302
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Default F5 Voltage question

Hi Guys,

I have just built an F5 and it biases up OK with no or very little DC offset once warmed up, and sounds very good with no issues.

However i noticed that when measuring the DC supply to the Amp PCBS i get +21.5v/-21.5v on both my PCBS (I run them as a monoblocks pair)

The offload PSUS give 24.5v (when not connected to the amp PCB and all PSU parts are the same values as papas design except i used smaller value bleeder resistors cuz that's all i had to hand !)

i was under the impression that most people got from 23.5 v to 24v on there creations?-- just wondering if this is an issue-- but it dosent sound like it when listening to music.

thanks boys

johnny
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Old 7th September 2013, 09:08 AM   #13303
Zen Mod is offline Zen Mod  Serbia
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what you have is normal

DC voltage can't be same for unloaded and loaded

when unloaded , pretty much as 1.41 x Vac

when loaded typically as FW amp , then you're in ballpark of 1.2 to 1.25 x Vac
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Old 7th September 2013, 09:24 PM   #13304
TTo is offline TTo  Finland
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I seem to have a problem with the left channel of my newly built F5. I can only get about 0.13V across R11 and R12, when P1 and P2 are set full clockwise. Any advise on how to troubleshoot this?

The right channel seems to be OK with 0.59V across R11 and R12.
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Old 7th September 2013, 11:56 PM   #13305
dazed2 is offline dazed2  Canada
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Quote:
Originally Posted by TTo View Post
I seem to have a problem with the left channel of my newly built F5. I can only get about 0.13V across R11 and R12, when P1 and P2 are set full clockwise. Any advise on how to troubleshoot this?

The right channel seems to be OK with 0.59V across R11 and R12.

If you really set p1and p2 clockwise then its not going to work.
Remember p1and p2 are reversed on the boards . so if you turn one clockwise the other has to turn counterclockwise.
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Old 8th September 2013, 12:20 AM   #13306
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Quote:
Originally Posted by dazed2 View Post
If you really set p1and p2 clockwise then its not going to work.
Remember p1and p2 are reversed on the boards . so if you turn one clockwise the other has to turn counterclockwise.
Since you have one channel working I assume you know the biasing routine. But just in case here is what I have found works best after having done this numerous times.

Biasing should always start with the pots set to zero and and making small turns. You alternate the two pots in the direction that increases bias and keeps DC offset down. You need 3 multimeters connected to the two resistors and the speaker terminals, so you can keep an eye on all three numbers. Set it to about 400mv first and leave it for an hour and then set it to where you want it.

Incidentally if this doesn't work, I would recheck the soldering on the board. My f5 after about a year of flawless operation suddenly started fluctuating bias on one channel. I replaced all the semis on the board as well as the pots. No luck. Finally I just reheated every solder point on the board and that fixed it.
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Old 8th September 2013, 07:25 AM   #13307
TTo is offline TTo  Finland
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Thanks for your responses.

Yes, I was able to bias the right channel with that procedure.

On the left channel, P1 and P2 run out of turns at about 0.13V across R11 and R12.

I have desoldered, measured, and replaced P1 and P2. They are 5Kohm trimmers as they should.

Any ideas on what to do next?

Last edited by TTo; 8th September 2013 at 07:51 AM.
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Old 8th September 2013, 07:38 AM   #13308
Marra is offline Marra  United Kingdom
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Here's the link to 6L6's excellent build thread where he had the same problem.Although he was able to bias higher than you have.Hope it turns out to be as simple as changing the resistors.I had to do the same on my F5T build.

An illustrated guide to building an F5

Last edited by Marra; 8th September 2013 at 07:41 AM.
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Old 8th September 2013, 08:47 AM   #13309
TTo is offline TTo  Finland
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Thank you Marra and others!

With 10Kohms at R3 and R4, I now have 0.4V across R11 and R12. I will wait for the amplifier to warm up and see if the bias stabilises.
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Old 8th September 2013, 01:41 PM   #13310
Marra is offline Marra  United Kingdom
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Good to hear.Enjoy the music.
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