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Old 10th March 2013, 10:21 PM   #12951
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Quote:
Originally Posted by Barniboy View Post
Sorry, me again, regarding cascoding resistors. I just noticed that the diyAudio store F5c (based on cviller design, but new board) changed the schematic for these voltage deviding resistors.
Your design was 4K7 for R101 thru 104.
They changed it to 10 ohms for R 101 thru 104 ! That's gotta be an error !
Why pull 1 amp just for the bases of the cascode resistors ?
I guess I should stick to 4K7, right ?
(They also made the emitter resistors in half without changing the current sensing circuit. So now these limit at 20 amps for EACH O/P device....).
(I should say at this point that I dont want to use dual parallel O/P devices in my build, just the input cascoding which I expect will help to reduce distortion. I learned long ago that it is difficilt to parallel semiconductors. The hottest one will carry the load. Unless you use large emitter resistors, which is not were you wanna move).
Schematics in the attachment and here:
http://www.diyaudio.com/forums/blogs...e_forguide.png
it's no problem running dual outputs. and there are no need for cascoding with +/-23V rails. only when you have around +/-30V and up. and for that, you will need dual outputs. otherwise you will destroy your outputs. unless you plan on running it with low bias.

Last edited by AudioSan; 10th March 2013 at 10:34 PM.
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Old 10th March 2013, 10:55 PM   #12952
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Quote:
Originally Posted by AudioSan View Post
it's no problem running dual outputs. and there are no need for cascoding with +/-23V rails. only when you have around +/-30V and up. and for that, you will need dual outputs. otherwise you will destroy your outputs. unless you plan on running it with low bias.
As I understand it, cascoding also has other advantages, wrt. reducing distortion, than just beefing up the voltage, as Mr. Pass explains here: http://www.passdiy.com/pdf/articles/cascode.pdf
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Old 14th March 2013, 08:54 AM   #12953
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Question Tranny at 22V secondaries

Hi,

I have just finished my F5 amp and I am very excited how it sounds. It is awesome!!

But I have a little hiccup .... I used a 300w 22V dual secondaries I had on hand ( searched the forum and I found that it was right to use it but at the high point limit) and with the Universal power supply and 44000 uF each channel capacitance gives roughly +- 30 Volts. With this psu voltages and the recommended bias of 0.59 volts across R11 and R12, the temperature of the heatsink is 55 C and the bolt securing the output transistor is at near 110 C. I am worried if I am going to fry the output transistors or something else.

I have bought a 18 V dual sec tranny rated at 500 W but I am still waiting for it.

Maybe I have to reduce bias in my actual setup? Or is it wrongly set the bias and one transistor is working harder than the other in the same channel?

Thank you all and sorry for my English...

Jorge
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Old 14th March 2013, 09:08 AM   #12954
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Jorge, I would reduce the bias to about 0.38 for now if I was you.
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Old 14th March 2013, 09:49 AM   #12955
sangram is offline sangram  India
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Hi Jorge

55C difference between bolt and the sink means either your sink base is too thin or you have improper mounting. At 40W dissipation 55C delta temperature means ~1.4C/W between device and sink surface, which is extremely high. A good mount should not be more than 0.4, an excellent mount can be as little as 0.2.

Can you post a picture of the setup?
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Old 14th March 2013, 09:49 AM   #12956
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Quote:
Originally Posted by valvesound View Post
Jorge, I would reduce the bias to about 0.38 for now if I was you.
Hi,

As I understand, due to the higher voltages of my psu, the bias adjustment should be lower. The value stated of 0.38 is obtained from?

Thank again for your support, I appreciate.

Best

Jorge
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Old 14th March 2013, 09:53 AM   #12957
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Quote:
Originally Posted by sangram View Post
Hi Jorge

55C difference between bolt and the sink means either your sink base is too thin or you have improper mounting. At 45W dissipation 55C delta temperature means ~1.2C/W between device and sink surface, which is extremely high. A good mount should not be more than 0.4, an excellent mount can be as little as 0.2.

Can you post a picture of the setup?
Hi,

I have some photos but I am afraid I have to learn how to upload them....

For now, my case is a HiFi 2000 Pessante Dissipante 3U. And the channel boards are in the middle of each heatsink.

I will post photos soon, thank you all folks...

All the Best,


Jorge
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Old 14th March 2013, 10:06 AM   #12958
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the "standard" F5 has an idle current of around 1.3A when biased at 0.6V on a 23V rail. The power dissipated is 30W through each FET.

These are rough numbers but if you worked on the power dissipated you could bias at 0.47V which will be 1A through 0.47ohm.

I suggested 0.38V as it still gives you reasonable class A performance for now. After you have some more mileage (hours) on your new amp and have sorted out the thermal coupling and the corrected rail voltage you can ramp it up again.

Its a value I played with in the begining and the sound was still great. Only a suggestion...

Last edited by valvesound; 14th March 2013 at 10:10 AM.
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Old 14th March 2013, 10:12 AM   #12959
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i would not use the amp until the thermal coupling is corrected.
did you mount the outputs to the sinks before or after you soldered them?
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Old 14th March 2013, 10:16 AM   #12960
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Quote:
Originally Posted by valvesound View Post
the "standard" F5 has an idle current of around 1.3A when biased at 0.6V on a 23V rail. The power dissipated is 30W through each FET.

These are rough numbers but if you worked on the power dissipated you could bias at 0.47V which will be 1A through 0.47ohm.

I suggested 0.38V as it still gives you reasonable class A performance for now. After you have some more mileage (hours) on your new amp and have sorted out the thermal coupling and the corrected rail voltage you can ramp it up again.

Its a value I played with in the begining and the sound was still great. Only a suggestion...
Hi,

Thanks again for the info. I thought erroneously that increased rail voltages did not affect too much to the bias setting. And as I learn from all of you and also from experience (I work in a office and this is a hobby along with 3 kids.... ) I wanted to know where was obtained.

Best,

Jorge
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