F4 power amplifier

[Andrzej Sochon]

Originally posted by Andrzej Sochon
Good news from me too: yesterday my balanced f4 was born, the twin should be ready next weekend...

Hi,
I finished twin balanced f4 yesterday evening. I couldn’t bear the tension, couldn’t wait till the weekend.
First impressions: fantastic! Deep stage. Very clean sound and very good resolution. Finally I can listen with real pleasure music difficult to reproduce before. I discovered new sounds, details, backgrounds absent before.
Gentlemen, chapeau bas, here is a genius: Mr. Nelson Pass!
Remembering problems I had with reaching super symmetry at one of variations of exceptionally simple SOZ (thermal problems with potentiometers and probably hidden cold solders), I was afraid about f4 symmetry – a lot of electronic parts. Now is okay, I don’t know what will be with higher levels of input signals, now my f4’s are driven by DAC only. But symmetry is perfect even in spite of accidental parts I collected. For instance all 12 IRFP240 are matched by myself for 30mV (from bunch of 30 pieces), IRFP9240 are better: I got 12 matched for 20mV. Low power resistors are 1%, no-name. C1 and C2 capacitors are accidental (Philips, 105oC).
Only Q1 and Q2 have been professionally matched (all for 8.8-9.7 mV). I would like to thank Mr. Erno Borbely who properly decipher my intentions and perfectly realized my order.
I would like to thank Nelson Pass for his design, Ian Studley for Peter Daniels boards and all of you for advices.
What next:
I must replace old PS capacitors (8 years of work at SOZ), I discovered one of them is slightly swelled causing a little hum (one of supply voltages disappears much faster).
And build pre-amp.
Best Regards,
Andrzej

[carpenter]

Congratulations!

[Andrzej Sochon]

Carpenter,
Thank you very much!
What time is at your place? You do not sleep?
javascript:smilie('')
wink
a.

[jacco vermeulen]

No room for more biasing higher than 1.5 Amps ?

Sensible practice is placing output devices on the sink and checking for a short one-by-one.

(the state of Oregon is located above California, Pacific time zone, 10 hour difference with central european time. Means Woody likely still hits the sack earlier than some in this time zone)

[Andrzej Sochon]

Originally posted by jacco vermeulen
No room for more biasing higher than 1.5 Amps ?

Hi Jacco,

My heatsinks are as you see at my avatar, approx. 245(H) x 200(W) mm, so there is a lot of space for additional MosFets or higher biasing, but the heatsinks are no black. MosFets are located on two opposite side heatsinks walls, front and back walls are free and are separated by oak wood. I set 260mV bias and after 2 hours of listening music I got 61C, so I think it is better to go down to 250 mV and make top cover more ventilated. And blacken side walls.
Best Regards,
a.

[SCD]

Hello Andrzej:
I too struggled with the bias setting. I originally set the bias at 250mv when the amp was cold. I let it run for an hour or so and checked the setting, it had drifted to +-300mv. The amp was too hot to hold for more than a few seconds. I reset the bias to 200mv with the amp warm, it seems to be fine now. I think I can could raise the bias a bit but I am not sure if what the advantage would be.
I am making a pre amp right now and once completed I will be able to decide if the bias needs to go up a bit.

A novice question for the group:
What would be the minimum bias setting and what are the indicators of it being set too low?
Thanks for the help

[carpenter]

Originally posted by Andrzej Sochon
Carpenter,
Thank you very much!
What time is at your place? You do not sleep?
javascript:smilie('')
wink
a.

1:45 in the A.M. is a bit late, but I'm off with a shoulder injury (strained rotater cuff).

I just like to hang with the men whenever I can...

[Andrzej Sochon]

Originally posted by SCD
I too struggled with the bias setting...What would be the minimum bias setting and what are the indicators of it being set too low?

Hi,
As far as I understand voices on f4 Forum: lower bias may cause distortions at lower frequencies.
I think that for f4 approx 0.5 – 0.6A bias, Vds/Vgs voltages and temperature >50C define the best point at IRFP240/9240 characteristic curve resulting in optimal compromise between the largest output amplitude (large clipping margin) and lowest distortions.
The best formula for novices: proceed as close as possible to the Nelson Pass First Watt model F4 Operation and Service Manual:
“Output device bias is approximately .43 amps per device, which measures 200 mV across the .47 ohm source resistors....Cold, bias is adjusted low at 130 mV and readjusted as the amplifier warms up...The proper value is achieved after 1 hour with the heat sinks at about 50 deg C...”
So yours 200mV is perfect, now you can start to experiment observing bass at lower bias.
rgds, a.

[Andrzej Sochon]

Originally posted by carpenter
I'm off with a shoulder injury (strained rotater cuff).

I just like to hang with the men whenever I can...

Hi Carpenter,
wow, pain, I am sorry to hear that; can you walk and do typical daily activities or you stay in bed watching TV and surfing Internet. What happened? An accident? To small ax, to heavy transformer, too much
screwdriver operations during bias/offset experiments?
Wishing you a good health,
Andrzej

[chrismercurio]

I have the first run of boards that Peter Daniel made for this project and am about to order the parts to do the stuffing. I know that the current BOM is a little different than when those boards were created...but has the circuit layout changed?

I apologize in advance for being lazy and not studying before asking my question, and thanks to any and all who respond,

C