F5 power amplifier

Jump to Latest
#7,281
Hello,

sorry if it has been covered before but I've read about half of this thread and this took me two weeks worth of my spare time 🙂

I wonder if any of the semiconductors in standard F5 have to be matched? I think there's no need to match output devices cause I can adjust their bias with P1-P2.
How about input jfets?
Can I substitute transistors from protection circuit(Q5-Q6) with something more available like BC550-560? If not what makes zetex devices so special for our application?

looking forward for your help/explanation,

thanks
Michał
 
#7,284
cviller said:
You don't need to match the jfets, but I seem to remember to have read somewhere that jackinnj made some tests where he got better thd with matched jfets.
Click to expand...

Thank you for your fast replies. I think I'm gonna make first version with cheapo components and then, if I like the overal sound put some more money into fancier, matched devices/resistors.

One more thing: In your guide, Cviller, there's written in the PSU section:
"1-2 thermistors for soft starting the toroid"
If I'm not plannig to use any thermistors at all should I use slo-blo fuse or just bigger one?
 
#7,286
cviller said:
You don't need to match the jfets, but I seem to remember to have read somewhere that jackinnj made some tests where he got better thd with matched jfets.
Click to expand...

I think the results were only a gnat's hair better -- and there was certainly no rigorous scientific method involved. You can use a BL as one JFET and a GR on the other and the amplifier will still work.

The Zetex's can be substituted, but be mindful of orientation (which Cviller has attended to.)

The output devices are of their own mind and don't have to be matched.

If you're building the amp just to measure THD% the 10K/22K resistors on the limiter can be swapped around, the source resistors on the JFETs, etc.

The most important factors to consider are an adequate heat sink and patience in setting up the bias and offset voltages.
 
#7,289
jackinnj said:
I think the results were only a gnat's hair better -- and there was certainly no rigorous scientific method involved. You can use a BL as one JFET and a GR on the other and the amplifier will still work.

The Zetex's can be substituted, but be mindful of orientation (which Cviller has attended to.)

The output devices are of their own mind and don't have to be matched.

If you're building the amp just to measure THD% the 10K/22K resistors on the limiter can be swapped around, the source resistors on the JFETs, etc.

The most important factors to consider are an adequate heat sink and patience in setting up the bias and offset voltages.
Click to expand...

thank you for detailed explenation. I can start my iron now, can't wait to listen to it.

M.
 
#7,290
jackinnj said:
The most important factors to consider are an adequate heat sink and patience in setting up the bias and offset voltages.
Click to expand...

Amen to that brother Jack!


Question for the masses:

Do anyone else's heatsinks just barely get warm?
I haven't scanned them with the laser temp gun, but when I put my hand on them after hours of playing they are barely warm. Is this nOrmaL? 😕
R11 / R12 .59V with DC offset at ~+/- .05V that fluctuates a little as it warms up.

Silent as a snowflake. 😀

Ron
 
#7,291
Hi Ron, I find that with the type of heatsink you have, this is very dependent on how you place the amp. If your wife forces you to put the amp inside a shelf, then it will be much warmer. 😉

One word of caution though - make sure that your mosfets have a good thermal contact with the heatsinks, because they can only handle the heat on their own for a very limited time...
 
#7,292
Heatsink Temps

Hey Ron,
Number one - like Christian said - make sure you have good thermal contact between outputs and the sink.

However -- I'm not done building mine yet. But, I do have an Aleph J that I built and I notice a difference in the heatsink temeratures with respect to the ambient temp. The heatsinks always get warm. But, they are noticeably hotter when the ambient temp is hotter.
So, I don't know where you have your amp. But if the surrounding temp if it's kind of low, then maybe that would explain the barely warm sinks.

Steve
 
#7,293
haven't scanned them with the laser temp gun, but when I put my hand on them after hours of playing they are barely warm. Is this nOrmaL?
Click to expand...

It will get hot eventually. It runs real cool, until BLAMO! It starts cooking. It shouldn't get so hot that you can't keep you hand on it. Ambient temp has a lot to do with it. I found mine gets real hot during the summer.

Run some bass heavy material thru it and see if it heats up more.
 
#7,295
Thanks guys,
My ambient room temp is 66*F. The amp is on the floor with 2 feet of free air space above it. (the wifee is good about component placement, speakers ect.)
I am using silicone pads for insulators. Should I use a compound like Artic Silver between the heatsinks // insulators // Mosfets ? I suppose it couldn't hurt. I am using a large washer to displace the clamping force from the bolt.

Thanks,
Ron
 
#7,299
Hi Andrew!
I also want to know about the temperature differences:

In my F5 Between heatsink and FET body is a 10-degree difference approx.(heatsink about 40-50; body about 60 - 65-i measured with IR thermometer some time ago) I use the silicone insulation pad which not the best. It is thought to replace the Mica or kapton but I can not pull oneself again for rebuilding 😀
Do I have to worry about it ?

thank you,

regards
 
#7,300
Hi,
the FETs are dissipating about 30W each (1.3A * 24V).
A temp difference of 10 to 15Cdegrees is about normal.
Ts-Tc=15Cdegrees indicates an insulator resistance, Rth c-s = 0.5C/W
Ts-Tc=10Cdegrees indicates an insulator resistance, Rth c-s = 0.33C/W
I doubt the best insulator can better Rth c-s = 0.2C/W
 
You must log in or register to reply here.
Top Bottom