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Old 19th November 2010, 08:54 PM   #1
Yarpen is offline Yarpen  Poland
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Default Musical Fidelity A-1 in A/B class?

Hi

I have a question Does anyone of you have tried to polarize the Musical Fidelity A-1 Class AB for example to 70 - 100mA? I like this amp but it now needs something that much less warms . Is this a good idea?
Listened to it in many configurations and almost always played very well.
I wonder if his secret is a class A or topology?

Looking for a power amplifier schematic about 20 watts to a very small room (about 10 m2 with speaker > 90dB) . I like amplifiers symmetrical design but I can not find anything in bipolar technology
Can you help me?

Cheers

Yarpen
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Old 19th November 2010, 09:37 PM   #2
Zen Mod is offline Zen Mod  Serbia
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best specimens of A1 have Iq in range of ~700mA , and that's ( by memory , I'm too lazy to calculate now ) no more than 7-8W of class A at 8R

just double values of collector resistors ( hoping that you have 0R22 in , not 0R47 as in some ) and you'll decrease Iq to half
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Old 20th November 2010, 10:14 AM   #3
AndrewT is online now AndrewT  Scotland
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The A1 is a ClassAB amplifier set up with high bias.

700mA bias allows ~1.4A of ClassA output. This is equivalent to 7.8W of ClassA.

100mA of bias allows ~200mA of ClassA output. This is equivalent to 0.16W of ClassA.

If you double the value of the emitter resistors, you will very roughly halve the bias current. This will drop the Maximum ClassA current to ~700mA, equivalent to ~2W of ClassA output.
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Old 20th November 2010, 10:19 AM   #4
Zen Mod is offline Zen Mod  Serbia
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just a slight difference

these are collector resistors , not emitter ones
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Old 20th November 2010, 10:46 AM   #5
AndrewT is online now AndrewT  Scotland
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oops.
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Old 20th November 2010, 11:33 AM   #6
Zen Mod is offline Zen Mod  Serbia
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I know you know , and there is no difference in rest of we both wrote

but - for sake of knowledge of OP
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Old 20th November 2010, 01:01 PM   #7
jaycee is offline jaycee  United Kingdom
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ugh.. every time i see this circuit it makes me cry. what on earth was TdP thinking.
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Old 20th November 2010, 01:20 PM   #8
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by jaycee View Post
ugh.. every time i see this circuit it makes me cry. what on earth was TdP thinking.
I agree. Double feedback loops, CFP output stage . . . .
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Old 20th November 2010, 01:25 PM   #9
Yarpen is offline Yarpen  Poland
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I know that technically there is no problem to change the quiescent current. My question was different - if someone tried this and how does that sound? How much is the original sound and what is changing / worse?

I am thinking of cloning the power amp with other (tube?) pre.
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Old 20th November 2010, 03:15 PM   #10
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Originally Posted by Bonsai View Post
CFP output stage . . . .

Yes, confused at CFP. Did Tiefbass.. draw that wrongly too? Anyway, there's no problem with CFP in class A. The THD is exemplary without crossover artifacts and better efficiency for such low rail voltages. Just don't ask it to work in class AB too where I think most agree that it's not..errr..not always good.
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