F5 power amplifier

[Tea-Bag]

Originally posted by woodturner-fran
Ok, my experience so far is that both are at zero... you need to bring both up a few turns to get any reading. I think what I'll do is start all the way clockwise (ie zero) and then turn both back up say 5 turns. I think the bourns pots are 20 turn AFAIK. See what I get and report back.

Fran

I had to go many turns before the voltage across R11 and R12 would do anything, then it actually is fairly steady (not jumpy) to set at this point up to 500mv or so, then had to do more fidling over time to get it to stick at .6v and <10mv offset. Mine does not drift much. Once it's sub <.05v, I let it warm up for a few hours to Willie Nelson from Laptop connection to cheap bench speakers. Then did more setting for it. I will adjust it agian once it warms up in a chassis.

[tms0425]

Originally posted by Tea-Bag



I had to go many turns before the voltage across R11 and R12 would do anything, then it actually is fairly steady (not jumpy) to set at this point up to 500mv or so, then had to do more fidling over time to get it to stick at .6v and <10mv offset. Mine does not drift much. Once it's sub <.05v, I let it warm up for a few hours to Willie Nelson from Laptop connection to cheap bench speakers. Then did more setting for it. I will adjust it agian once it warms up in a chassis.

I did the same thing without problems. Note, as ZM said, these are both bias settings for each half, not one bias and one offset. Once I got the 20t pots halfway, just alternate slight adjustments on either one to get it up to .59-.60V and small offset of < 10mV or so. After 3 hours mine only moved about 1-2mV.

[woodturner-fran]

ok..... so I should be able then to adjust P1 upwards and see a voltage develop across R11. R12 stays at zero? Then bring up P2 to match and get the same voltage across R12. At that point I should have zero offset, right?




On a side issue. There are 2 other things here I need to watch. One is the PS setup, dual mono but with all the grounds tied together at the speaker terminals. Second thing is that all my grounds are brought together here, thats PS, input and output. in a star...

I wonder if I would now be better to run shielded twisted pair from inputs to boards, then separate twisted pair to the outputs. Run GND from PS to the boards rather than the "star" at the speaker terminals.

*************

I'll get back out tonight after the kids hit the hay and investigate more. I really appreciate all of your patience in this......


Fran

[tms0425]

I think you're going to need to bring both pots up together gently, enough to turn on both output transistors.

You can look at my pictures for grounds, though I am using CViller's boards. Even on Peter's F4 boards I brought a ground wire (not necessarily shielded) from each floating RCA connector to the SG input on each PC board. Speaker grounds and DC power grounds are all tied together as star, and there is a thermistor between that DC ground to chassis ground (the way Nelson does it). Chassis metal ground is tied directly to the mains ground at the IEC connector.

[woodturner-fran]

Well I was like this:



But now I'm like this:


but also like this:


Yeah, I'm a gobshite. Officially.

On Peters boards there are 2 set of through holes for mounting R5-8. Well, I had mine stuck in so that the 2 legs were in the "same" track. So essentially there were no R5-8 as the amp saw it.

Now I have rock solid 590mV bias and 5mV offset and if I played with that a bit more I can go to 0 offset. On one channel I have a little imbalance on R11/R12, of the order of about 15mV. After things settle a while I'll go back and look at that again.


So it onwards now. One question remains though. Should I refit all the current limiting stuff or not bother? What do you guys think? Would leaving it out be a danger to speakers?


Thank you all so much for the help.... I knew something was wrong when the bias was off like that. Just had to stare at it a bit longer.


Fran

[tms0425]

I doubt 15mV is much difference to worry about.

The current limiting will just protect you from near dead short kind of stuff. The way I look at it, it's only 2 inexpensive non-matched complementary Mosfets and 0.47r's, so if I cook one or two while doing something stupid with the output wires they're easy enough to replace.

[woodturner-fran]

mmm and I note that Peter seemed to indicate that the amp sounded a bit better without the current limiting bits. In any case, I'm gonna leave them out for now until I get her all cased up and fitted. I still need to do some tidying up inside, things like connecting the GND to chassis earth, power LEDS and such niceties. I got some nice binding posts and neutrik RCAs in teh mail today and I might just go back and swap them in instead of what I have there.


Again, my heartfelt thanks to all who helped me out here.


Fran

[tms0425]

Originally posted by woodturner-fran
mmm and I note that Peter seemed to indicate that the amp sounded a bit better without the current limiting bits. In any case, I'm gonna leave them out for now until I get her all cased up and fitted. I still need to do some tidying up inside, things like connecting the GND to chassis earth, power LEDS and such niceties. I got some nice binding posts and neutrik RCAs in teh mail today and I might just go back and swap them in instead of what I have there.


Again, my heartfelt thanks to all who helped me out here.


Fran

Fran,

Glad to hear it's finally singing properly. Now go listen to some music and you won't even care about some cheezy connectors.

Tom

[juluska]

What do you think about that pcbs that sold on ebay?
I's worth a try to build that kits?

[cviller]

Originally posted by juluska
What do you think about that pcbs that sold on ebay?
I's worth a try to build that kits?

I don't know about the pcb - it looks fairly nice, but I haven't looked into it in detail. It does however seem to violate the PASS logo.

Any particular reason why you want to buy those boards instead of those offered at the two group buys?