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Old 30th November 2004, 11:29 AM   #1
pro is offline pro  Italy
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Default Op amp class A biasing.

Hi All.
I am building a 3 way active crossover with double op-amp (NE5532).
A silly question.
Because I use 1/2 op amp af filter and the other 1/2 as output buffer (voltage follower), if I want bias in class A the op-amp with a resistor or a CCS, witch one must I bias: the first one, the buffer or both? (or nothing).
Thanks a lot to averybody.
Ciao.
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Old 1st December 2004, 08:33 AM   #2
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By this connection you get higher distortion ( " musical sound " ) and manufacturer can low distortion . Any manufacturer don't claim, which distortion is " nice ".
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Old 1st December 2004, 08:36 AM   #3
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I suppose the reason for wishing to force the opamp in Class A is for improving the sound of the signal.

To me it would make no difference whether one of both amps in the 5532 acts as a buffer stage, as the signal passes both forcing both in class A would be the logical choice.

Personally i would go for a 634 combined with an opamp with a much higher slewrate than the 5532.
I admit it adds considerable to the cost with a 3-way filter but compared to overall cost the relative increase in cost is justifiable.
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Old 1st December 2004, 08:38 AM   #4
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Default Re: Op amp csall A biasing.

Quote:
Originally posted by pro
Hi All.
I am building a 3 way active crossover with double op-amp (NE5532).
A silly question.
Because I use 1/2 op amp af filter and the other 1/2 as output buffer (voltage follower), if I want bias in class A the op-amp with a resistor or a CCS, witch one must I bias: the first one, the buffer or both? (or nothing).
Thanks a lot to averybody.
Ciao.
Just try and listen.

A good starting point whould be about 2k2 - 4k7 to - supply both outputs. The exact value depends on signal current drawn from output. You choose that resistor that the current thru it is at least equal to signal current calculated from load and Vout max.

Uli



PS: Your only criteria should be your ears!!!
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Old 1st December 2004, 08:40 AM   #5
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Do you mean that biasing in A class an op-amp, it seems to sound better because of the increased distortions? So it is better to don't do that.
Ciao.
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Old 1st December 2004, 08:41 AM   #6
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Quote:
Originally posted by jacco vermeulen
Personally i would go for a 634 combined with an opamp with a much higher slewrate than the 5532.

634 is a good choice at output. I recommend swapping the aged NE5532 with OPA2134

Uli

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Old 1st December 2004, 08:46 AM   #7
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Make it like Uli wrote, uprising of sound will be surpraised for you , Uli is correct .
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Old 1st December 2004, 08:46 AM   #8
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Default not necessarily

Quote:
Originally posted by pro
Do you mean that biasing in A class an op-amp, it seems to sound better because of the increased distortions? So it is better to don't do that.
Ciao.
Biasing class A changes the kind of distortion.

Look at peranders site -> the GC with class A biasing.

Go to www.passdiy.com and read NPs articles about biasing single ended class A which is the same as biasing an opamp with a resistor (CCS) at the output.

Uli

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Old 1st December 2004, 08:53 AM   #9
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Or build discrete opamp - example you can find on Bryston's pages ( they are using only eight cheap transistors ) .
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Old 1st December 2004, 09:10 AM   #10
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Well, I am gonna using a 2134 for the high section, two M5238P for the mid section and a 5532 (the slowest) for bass section.
Now, if the filter drives the buffer, it means that looks at an input inpedence of the buffer very high. Isn't it 2k2 to 4k7 to high current for that (at 12V rail it's 5.4 and 2.7 mA), and the buffer with those values, must drive a 2k2 hom in parallel with 34K of the amp, that is about 2K. Isn't it a value a bit low for an Op-amp?
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