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Old 7th February 2011, 07:38 AM   #111
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Quote:
Originally Posted by NYCOne View Post
Can you show an example? I'm not sure how one calculates U.
V=IR

IF R = 8 Ohms and you want the maximum Class A Power using a bias of 2A (=4A class A for push pull)

Then V=4x8 =32V

IF R=4

Then V=4x4 =16V

That does not mean you have to use those values. Some people bias higher knowing that their speakers drop to 3Ohms even though the nominal impedance might be 8 Ohms (espescially important for single ended amps). Other people might bias higher because it sounds better or they might bias lower because it sounds better.
There are always exceptions to the rules.
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Old 7th February 2011, 07:41 AM   #112
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Quote:
Originally Posted by NYCOne View Post
I backed off the bias a bit. Now I'm at 2A and I'm 48C. Room temp is 27C. Better.
How long after turn on, are you taking these temperature measurements?
What are you using to measure?
It does not seem right at all. Your only 21 degrees above ambient.
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Old 7th February 2011, 07:54 AM   #113
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Nelson designed the F5 for 25w/ 8 ohms using 1 pair.
I t seems reasonable to me, when using two pairs, to limit the classA power to 50w.

In your example, you are reaching 64W

Max voltage swing is about 36v-4v=32v (4v = losses)
32*32/2*8=64W classAB
4*4*8=128W peak =64w ClassA

ClassA power= classAB one which is fine but a bit high.
It's possible but allows no error margin.
Personally, i prefer 33v rails.

Edit: crossed posts.

Last edited by bobodioulasso; 7th February 2011 at 08:09 AM.
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Old 7th February 2011, 08:07 AM   #114
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Quote:
(espescially important for single ended amps)
*

Melon Head,
could you develop?
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Old 7th February 2011, 08:21 AM   #115
NYCOne is offline NYCOne  United States
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I'm measuring continuously. I have a digital thermometer, I put the tip in one of the thread holes in the heat sink. I touch the tip to the Mosfet cases for that measurement.

When I thought I was 30C over room temp, I may have mis-measured the room (temp changed as time went on).

I'll try again tomorrow and log all the temps vs time.


Quote:
Originally Posted by Melon Head View Post
How long after turn on, are you taking these temperature measurements?
What are you using to measure?
It does not seem right at all. Your only 21 degrees above ambient.
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Old 7th February 2011, 09:54 AM   #116
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Quote:
Originally Posted by bobodioulasso View Post
*

Melon Head,
could you develop?
Maybe best to use an example.

Amp a) Push Pull biased at 2A
Amp b) Single ended biased at 4A

Lest assume we connect both up to an 8 Ohm speaker.

Amp a) Will produce (4x4)x8 =128W pk =64W avg
Amp b) Will produce (4x4)x8 =128W pk =64W avg

So far so good, except this speaker has impedance dips down to 2Ohms.

Lets assume both Amps have 32V rails with no losses.

At 2 Ohms Amp a) simply goes into class B operation and produces 16A (32/2=16) =16^2 x 2 = 512W pk =256W avg

Amp b) does not go into class B operation and will only output a maximum of 4A =4^2 x 2 = 32W pk = 16W avg

So Amp b) experiences severe current clipping while Amp a) just goes into class b operation and is still happy.

So the point here is when considering the bias point for a single ended amp don't just think about the nominal impedance but also consider what is the minimum impedance. The Aleph current source gets us out of trouble a bit here because it is capable of exceeding the bias current by a certain percentage depending on how it is setup. However Nelson has said it does not sound that great while operating in this mode. So best not to rely on it.
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Old 7th February 2011, 10:14 AM   #117
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Thx, i had understood single ended as opposite to balanced.
eg: F5 single ended.
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Old 7th February 2011, 10:38 AM   #118
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I should also point out that this was an extreme example for illustration purposes.

For my loudspeakers, I know I don't exceed 16V music peaks and I know the impedance doesn't fall below 3Ohms. So if I were to build a single ended Amp then I would bias around 5A. Even 4A would probably be enough knowing that it would only be on rare occasions that there would be a Voltage peak at the impedance dip, and this rare occasion could be tackled by the Aleph current source without significant audible effects.
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Old 7th February 2011, 02:41 PM   #119
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Melon Head View Post
Maybe best to use an example.
hey Melon Head, are you forgetting that the P-P(balance) amp experience a 4ohm speaker as being a 2ohm load ?
meaning that the "worst case" situation actually happens to the balanced amp with any "normal" 4ohm speaker
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Old 7th February 2011, 03:27 PM   #120
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Quote:
Originally Posted by tinitus View Post
the P-P(balance) amp experience a 4ohm speaker as being a 2ohm load
A singled ended balanced output amplifier has the same loading effect.
The difference is that someone designing a balanced amplifier does so for the intended loading.
Assembling a pair of unbalanced amplifiers in bridged topology is where mistakes could be made and often are.
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regards Andrew T.
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