F5 power amplifier

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#12,121
albertNL said:
Apologies for my ignorance, but I have a question about R11 and R12.

I am building an F5 using standard diy-audio store boards, 24V rails, tech-diy parts kit plus optional Caddock MP915 and MP930 resistor kit (also tech-diy).

I understand that R1 and R2 (Caddock MP915 10R) will not require heatsinks.

I also understand that R5 and R6 (Caddock MP930 50R) will dissipate up to 5.5W (post 11134) and hence require sufficient heatsinking. I asked about placing on the main heatsink and extending the leads and Nelson replied that is OK (post 11128).

I considered placing R11 and R12 on the main heatsink as well, but am wondering if a small heatsink would also be an option. I am not sure how much these resitors will dissipate. I know that the schematic shows a 0.6V drop, so they would see a current of (I = V/R = 0.6 / 0.5 =) 1.2A (surprise, surprise, the bias current). What I do not know (shows the level of understanding, I am afraid) if this current is a constant value or depends on the load. Intuitively I would assume it to be constant, but would hate to blow up the amp on this assumption. 🙂

Assuming the current of 1.2A is correct and constant, R11 would dissipate (P = VI = 0.6 x 1.2 =) 0.7W. I am unsure of the thermal characteristics of the small heatsinks I have (they were no-name brand so no data sheet) but I would expect them to be about 20-30C/W. This should then be sufficient.

Could someone confirm this for me please?

Thanks,

Albert
Click to expand...
I bolted R5/R6 with big washers to the heatsink. [Did the same with these in the power supply].
They can dissipate enough, left alone, but do get hot. It is not 5.5 W continous, but about 1 watt continous. That is what counts.
 
#12,123
triode_al said:
I bolted R5/R6 with big washers to the heatsink. [Did the same with these in the power supply].
They can dissipate enough, left alone, but do get hot. It is not 5.5 W continous, but about 1 watt continous. That is what counts.
Click to expand...

Yes, 5.5W for R5/R6 is not continuous, but I did not know what a normal "worst case scenario" (playing loud at a party) would be. Anyway, I will also place them on the main heatsink. Can't go wrong there.

But now R11/R12! Main heatsink or a small heatsink? I think that going by the answers from Andrew so far, any heatsink (to lower the thermal resistance) will do. I hope so anyway, otherwise I understand less than I hope I do!
 
#12,124
triode_al said:
.........They can dissipate enough, left alone, but do get hot. It is not 5.5 W continuous, but about 1 watt continuous. That is what counts.
Click to expand...
what are you doing to your F5?

That 5.5W figure was a peak figure when the output stood at 20Vpk.
That equates to 2.8W continuous, but only when the amplifier is delivering 14Vac continuously. We don't listen to music like that.

To dissipate 1W continuously in the feedback resistors, would require the average output to be ~8.5Vac. That is only -4.4dB below maximum power.
How do you abuse your F5?
 
#12,125
does anyone try build F5 turbo v1 ?

i have a problem with the bias voltages.

i used 24vac to supply 32vdc.

at the r bias 1ohm/3w there are no voltage, no matter how many times i turn the 5k trimpot, there still no voltage here, always zero.
and the mosfet irfp is always cool.

from the power supply the voltage is good, shown +32v - 0 - -32v

what is happen? thx.
 
#12,126
That's not enough information to troubleshoot your problem. First, we need to see photos of your build. There are guys here that can spot the smallest problems in a photo.

Also it would be helpful to give more information about your build. But first, take a bunch of photos and post them here.
 
#12,137
Got one channel working

Thanks to all got 1 channel working...
R11&R12 --- .623V
VGS9240---4.39V
VGS240---3.87V
Offset --- 25mV(1 2.2k...will experiment)
R3/R4--5.02V/4.51V
Rails -- +24.8V/-25V

Heatsink warm...washers on top of mosfet hot...readings taken after 3 hours.
Coming from AVR's the difference is as evident as bald pate v/s shaggy mane (never missed the other channel)

Any cause for concern with the above values?
 
#12,140
Assembly is screw->bolt->washer->mosfet->silicon paste->mica sheet->silicon paste->heatsink.

Can put my hand on heatsink for minutes but 5 seconds on washer is uncomfortable.

I do have Berquist Silpads (TO220) but they seem to be undersized for Mosfets. My understanding is if using Silpads then thermal paste is not necessary?
 
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