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-   -   Weird F5 bias issue (http://www.diyaudio.com/forums/pass-labs/190377-weird-f5-bias-issue.html)

JZatopa 6th June 2011 11:38 PM

Weird F5 bias issue
 
So I have my F5 playing using a bench power supply that has 27V rails and can put out up to 10 Amps. I have read 2 different ways to bias the amp (Nelsons and someone else's post). I am using Cvillers V2 boards and TechDIY parts. I do have current limiting resistors and thermistors in the circuit. When I try to set the bias using what is outlined in the Pass manual I can get .59V over R11 and R12 but my DC offset is around 230mv. The other way described has you set bias so it is ok on one resistor and then use the pots to 0 out DC offset. When I do this the highest voltage I can get on R11 and R12 is around .3V. Each time I let the amp warm up and adjusted settings, inputs were shorted and the speaker terminals were left open.

Am I doing something wrong? Could I have put a resistor in the wrong place? If so which location should I check first? It looks like everything is ok visually but some of the resistors are turned so I would have to remove them to make sure the values are proper. I would think that if I did make a mistake I must have made the same mistake on both boards.

Any help would be greatly appreciated.

JZatopa 6th June 2011 11:49 PM

Would this happen if I put a pot in the wrong way?

Zen Mod 6th June 2011 11:57 PM

increase input Jfets drain resistors for , say , 20%

repeat biasing procedure from start

use " one source resistor , offset " procedure

JZatopa 7th June 2011 12:38 AM

Zen Mod - Thanks for the help but I need you to break it down into ultra-amp noob speak for me. I think it was your post that explained the bias method but I didn't want to say for sure because I didn't remember.

Which resistor is the jfet drain resistor? What is it's R#?

Zen Mod 7th June 2011 12:49 AM

R3 & R4 at Papa's schmtc

go to 2K7

Andersonix 7th June 2011 02:13 AM

Or 3k3

JZatopa 8th June 2011 12:13 AM

1 Attachment(s)
Before I try different resistors I read somewhere that the pots on the board should be mirrored not oriented the same way. Do I have one of the pots going the wrong way on this board? I have measured R3 and R4 and one has the proper V and one is around 3V. This is true for both boards. Should I switch the one on the low V side around?

Zen Mod 8th June 2011 12:34 AM

having pots oriented is just matter of CW or CCW for increasing resistance

I'm not bothering usually with that - I always use ohmmeter to pre-set them ,prior to powering up ( if needed) .

in case of F5 -just be sure that you have them set to zero resistance , later voltage meters will tell you what's right direction to turning them

JZatopa 8th June 2011 02:02 AM

OK then I am not going to worry about trim pots.

Should I be worried about low voltage across the resistor or should I just up the resistance and try to bias again?

JZatopa 8th June 2011 04:38 AM

Both boards

R3 = 4.08V
R4 = 4.5V

The highest I can bias R11,R12 to and keep ~ 0mV offset is .25V

This is roughly the same for both boards. One side I could bias up to .3V and still keep offset. Are there other Voltages to check?


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