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Old 27th June 2013, 01:47 PM   #751
PMI is offline PMI  United States
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Quote:
Originally Posted by Tyimo View Post
PMI!

I see in the picture, that you are using only one 0.33R 5W resistor. Are you sure is it enough? In this value it can drive less than 4 ampers.

Tyimo
You are correct.

A 100% safe resistor value would be 0.2R/5W. Also, the inductor is probably too small to use at continuous full power, and so is the transformer. For best possible performance @ full power & maximum voltage that this amplifier is capable of, you would need two transformers and two power supplies this size, and a much better filter, which is where I am going with my new supply.

However (and please do check my math):

I did not size the power supply components for continuous maximum power. I may do that with my new power supply, but I have to do some testing first.

Even fused at around 1/2 max power level using fast-blow fuses, this setup can get crazy loud in a normal size space (not the point of your q., but that is the reason why I wrote low-to-moderate power above).

As far as the resistor goes, it is a good quality wire-wound 5W resistor, which can easily take 50% more than its power rating for short peak-power use. Not for very long, but long enough (if the fuses do not pop first, saving my eardrums from destruction... )

Transformer is 250VA, 25V AC output, so that gives around 35V DC rail voltage (nominal, and less after diode drop in the bridge). Neglecting all other losses, that would be 125 watts maximum per rail output. So at 35V DC, the current can only reach 125W/35V ~ 3.6A per rail. After that, the transformer field will start to collapse, main fuse will blow, etc, etc...)

So, the maximum power in the resistor would be ~ 4.2 watts, I think.

We can do a similar calculation the other way, in other words, how much peak, and how much continuous power do YOU want to design for, and what will the components have to be?
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Old 27th June 2013, 01:59 PM   #752
PMI is offline PMI  United States
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Originally Posted by Idefixes View Post
That's not really an issue as you can mount 2k2 under board so you have enough room faor heatsink.

Marc
Related to how you mount the resistors, I wonder if it is worthwhile to bring up the single heatsink (again) for the pair of VAS transistors.

LC has everything mounted to the one large heatsink, so the issue does not arise.

I did originally design my board for this, but I could not tell any difference, with my ears, or on the oscilloscope, when I first tried it. After that, I got busy testing other things... (and the individual heatsinks are more convenient)

As an aside, I see a temp rise of 20-22C above room temperature, measured on the case of the transistor, with a small u-shaped heatsink.
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Old 27th June 2013, 02:04 PM   #753
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Quote:
Originally Posted by PMI View Post
Related to how you mount the resistors, I wonder if it is worthwhile to bring up the single heatsink (again) for the pair of VAS transistors.

LC has everything mounted to the one large heatsink, so the issue does not arise.

I did originally design my board for this, but I could not tell any difference, with my ears, or on the oscilloscope, when I first tried it. After that, I got busy testing other things... (and the individual heatsinks are more convenient)

As an aside, I see a temp rise of 20-22C above room temperature, measured on the case of the transistor, with a small u-shaped heatsink.
That's right but through doing that he was necessary to add temp. compensation (not sure it's the right word) through TLV431 adding...so not in Shaan initials statements....

Marc
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Last edited by Idefixes; 27th June 2013 at 02:17 PM.
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Old 27th June 2013, 02:37 PM   #754
PMI is offline PMI  United States
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Originally Posted by Idefixes View Post
That's right but through doing that he was necessary to add temp. compensation (not sure it's the right word) through TLV431 adding...so not in Shaan initials statements....

Marc
Yes, I remember. I read most of that thread, but it is easy to misunderstand the reason why something was done (for me anyway).

The temperature of the VAS transistors can rise very quickly. It will stabilize at a reasonable level, even with a single small-ish heatsink, but it changes much faster than the temperature of the big heatsink with the mosfets, and so does the bias current.
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Old 27th June 2013, 04:33 PM   #755
Tyimo is offline Tyimo  Hungary
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Thanks... maybe this one?
Yes, it looks better. Did you checked all the components in the reality, how much space are they need?

Quote:
That's not really an issue as you can mount 2k2 under board so you have enough room faor heatsink.
It is a good idea.

Btw: your pcb looks very nice.
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Old 27th June 2013, 04:35 PM   #756
Tyimo is offline Tyimo  Hungary
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Hi PMI!
Quote:
Transformer is 250VA, 25V AC output, so that gives around 35V DC rail voltage (nominal, and less after diode drop in the bridge). Neglecting all other losses, that would be 125 watts maximum per rail output. So at 35V DC, the current can only reach 125W/35V ~ 3.6A per rail. After that, the transformer field will start to collapse, main fuse will blow, etc, etc...)
I understand you. I was asking this because I am coming from the class A world. There are different rules....
Greets:
Tyimo
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Old 27th June 2013, 04:46 PM   #757
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Originally Posted by PMI View Post
Related to how you mount the resistors, I wonder if it is worthwhile to bring up the single heatsink (again) for the pair of VAS transistors.
I think that two small U shaped heatsinks is very practical solution and it is much easier to buy them than to make and drill single one.
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Old 27th June 2013, 05:00 PM   #758
PauloPT is offline PauloPT  Portugal
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Quote:
Transformer is 250VA, 25V AC output, so that gives around 35V DC rail voltage (nominal, and less after diode drop in the bridge). Neglecting all other losses, that would be 125 watts maximum per rail output. So at 35V DC, the current can only reach 125W/35V ~ 3.6A per rail. After that, the transformer field will start to collapse, main fuse will blow, etc, etc...)
Isn't peak current greater than 3.6A, somewhere near 8A?
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Last edited by PauloPT; 27th June 2013 at 05:03 PM.
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Old 27th June 2013, 07:13 PM   #759
PMI is offline PMI  United States
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Originally Posted by PauloPT View Post
Isn't peak current greater than 3.6A, somewhere near 8A?
You mean during the short time the caps charge through the bridge, or maximum current through the filter resistor, averaged over several AC cycles?
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Old 27th June 2013, 07:55 PM   #760
shaan is offline shaan  India
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Default HV test...

Me finally back home(this time to stay)... 'been six long months wandering...

Back to test bench and connected my +/-35V PS to peeceebee.

(Changes: "U" shaped heatsinks for VAS, two parallel 33K as current sources on each side(13mA VAS+input, 190mA total, offset 5mV), FB cap reduced to 1000uF, added zobel(oscillation with speaker cable(3meter long))).

Result: Nothing special.
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