A guide to building the Pass F4 amplifier

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#1,041
Zen Mod said:
if you can cool it - crank it to have up to 35W heat per mosfet ........
Click to expand...

Ha, nice advice 😀 35w per device times 12 devices lands you in the territory of my Aleph-X monoblocks that each burn ~425w of power... 😱 This requires a bit more DIY effort to ditch that much heat - makes for a cozy room in the winter months, though!
 
#1,044
Pegasus- no, running the amp balanced and getting 100w does not increase the heat sink requirements. (aside for the entire 2nd amp...) You are using the same power much more efficiently, by being able to swing twice the voltage across the load.
 
#1,045
6L6 said:
Pegasus- no, running the amp balanced and getting 100w does not increase the heat sink requirements. (aside for the entire 2nd amp...) You are using the same power much more efficiently, by being able to swing twice the voltage across the load.
Click to expand...

I am considering changing my current 30V rails single ended F4 to a balanced F4 configuration with both channels in a 5U case. 2 boards per heatsink. I'll reduce the rails to about 20-24V and still have higher power in the end.

Impasse needed for such a build though!
 
#1,046
Eric said:
..................

Of course, if you are looking to make this determination BEFORE HAND, multiply the cumulative power burn for each transistor that will be mounted on a single heatsink by the thermal dissipation (c/w) of that sink.

For example, the following are measurements I made with my F4:
200mV = 56w per channel
225mV = 63w per channel
250mV = 70w per channel
275mV = 77w per channel
300mV = 84w per channel

My F4 uses two 3U sinks from the DIYAudio store per channel. Each sink is rated at 0.50c/w, thus I have sinking capability of 0.25c/w per channel. If I run at 300mV bias, I get 84w per channel that is dissipated by 0.25c/w worth of heatsinking. Thus, expected heat sink temp rise will be somewhere near 21c over ambient. Assuming "standard" room temperature somewhere near 20-25c, this results in a final temp of somewhere between Blimey and Crikey hot - about the upper limit of two 3U sinks per channel.

The 4U chassis uses two sinks, each rated at 0.31c/w. Dissipating 84w (300mV bias) on a 0.31c/w sink results in a temp rise of 26c - likely crossing the 50c barrier, so larger sinks are desirable for longevity of your output devices.
If you want more bias from the F4, you'll need to move to greater levels of sinking.
Click to expand...
Heatsink manufacturers rate their heatsink Rth s-a when DeltaT is 70C to 80C degrees.
If you are predicting a temperature rise of 21Cdegrees, then your DeltaT is massively less than the typical 75C and you need to follow the manufacturer's de-rating for different DeltaT.
Typically you will find that going from 75C to 25C you need to apply a De-rating Factor of around 1.4 to 1.6, i.e. your 0.31C/W becomes 0.31*1.5 = 0.46C/W when DeltaT~25Cdegrees.
That gives a temperature rise of ~0.46*84W = ~38Cdegrees.
Now that you have a more accurate prediction, look again and correct the DF for 38C DeltaT. You will find that DF has gone down slightly.
Add on Ta of 25degreesC and to arrive at Ts~63degreesC.

You must de-rate for DeltaT !
 
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#1,047
6L6 said:
Pegasus- no, running the amp balanced and getting 100w does not increase the heat sink requirements. (aside for the entire 2nd amp...) You are using the same power much more efficiently, by being able to swing twice the voltage across the load.
Click to expand...
??
Changing to lower supply rails and bridging or balanced, reduces the efficiency of the amplifier, even into a load that is doubled.
More of the input power is wasted as heat compared to the amplifier that is not bridged/balanced driving the appropriate load.
 
#1,049
one enclosure for stereo regular F4

that same one , if delivered with balanced signal , is becoming mono 100W bridge amp ...... thus needing two of these cases/amps for stereo

simple as that

to have stereo 100W/ch F4 in one case means putting 2+2 regular channels in that same case ...... having at least 360W of heat

that's BIgguns league
 
#1,050
Quite tough but not impossible...
 

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#1,055
Hi guys,

Any opinions or tips about the values I should use for the universal psu v3 and torroidal Tx recommendation to power the F4 boards? This is my first serious SS amp build coming from building valve amps.

The BOM gives a range, I was wondering if anyone had a 'recipe' of values they used for the PSU that'll be a starting point for me to just follow. I'm being cautious before collecting parts as I don't have a parts bin to dip in for this and want the project as straightforward assembly as possible so I thought I'd ask this first.
 
#1,057
Hi jervill
Regarding the tx, I can highly highly recommend the torroidy supreme audio tx. I think it's a 2x18v you need but check that. 400w is plenty. I previously spent best part of 100 quid on a 500w sedlbauer and it hummed.... The torroidy was a little more but had several transformers which haven't been great (most hummed) I'm just going to go straight for them in the future. Potted in chromed steel case with an electrostatic shield.

Regarding capacitors I used some 22000uf 35v. As it's going in a hot amp I always try to find 105 degree rated. Also check the ripple current rating is adequate. I can't remember how to work that out but it's important. I don't think you need to go nuts regarding capacitance, 8x 15000 or 22000 is plenty. I've never once felt like this amp has struggled and it's gone plenty loud enough with bass heavy music.
 
#1,058
Hi Zen, I've read it. Just canvassing from the collective experience here that have built the setup if there are any strong opinions on parts choice and values that have developed over the years.

mrdave45 thanks. Your info was what I had in mind to know.

Are there any cap brands I should avoid maybe?
 
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