A guide to building the Pass F4 amplifier

[Eric]

if you can cool it - crank it to have up to 35W heat per mosfet ........

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!

 

[Zen Mod]

In other words the 5U is awesome but overkill for my needs.......

not as much as you have impression

there is a loooong way to that chunk of 'luminum , which you can call overkill

 

[drpro]

I have been running a pair of F4s balanced using the BA-3 pre and now the Impasse pre both amps in the deluxe 4U chassis. They play well, both are on an open shelf, so plenty of air circulation.

 

[6L6]

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.

 

[MASantos]

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.

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!

 

[AndrewT]

..................

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.

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 !

 

[AndrewT]

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.

??
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.

 

[lehmanhill]

(aside for the entire 2nd amp...)

Pegasus,

I'm not sure your intention. I have the impression that you are thinking you can put a 100W balanced F4 (that is, 2 F4's) into a single enclosure. On the other hand, I think everyone else is assuming you are using 2 separate enclosures for each F4.

Or maybe I'm just reading to much into it......

Jac

 

[Zen Mod]

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

 

[rolandong]

Quite tough but not impossible...

 

[lehmanhill]

Forced air heat sinks. Very cool.

A diyAudio friend also went the liquid cooling route.

Jac

 

[Zen Mod]

plenty of sinking , but Iron can't keep up ......

 

[rolandong]

Zenmod,
Thats a 1.5" center leg traffo also diy that fit the limited space. I had built 4 channel Leach superamp and I had serious trouble lifting it

 

[Zen Mod]

if that's the case , even better

 

[jervill]

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.

 

[Zen Mod]

read F4 manual , from First Watt site ?

 

[mrdave45]

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.

 

[jervill]

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?

 

[mrdave45]

I'm not sure about brands to avoid, most companies who make high grade bits also make some really cheap low grade bits. The component filters on the mouser and Farnell sites are quite good for narrowing down the parts.

 

[jervill]

I'm not sure about brands to avoid, most companies who make high grade bits also make some really cheap low grade bits. The component filters on the mouser and Farnell sites are quite good for narrowing down the parts.

Thanks!