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Old 1st December 2004, 09:20 AM   #11
uli is offline uli  Austria
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Quote:
Originally posted by pro
Isn't it a value a bit low for an Op-amp?
Ciao.

you arent screwed to 2k2, just use the "appropriate" value.

This is about 50% of the load.

If you got a load of say 10k, a 6k8 for bias would be sufficient.

Better is a ccs . 10k load full swing about 15V peak -> 1,5mA.

build a 2mA ccs and youre done.

Uli

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Old 1st December 2004, 09:21 AM   #12
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Quote:
Originally posted by Upupa Epops
Or build discrete opamp - example you can find on Bryston's pages ( they are using only eight cheap transistors ) .


Pavel, I strongly advise you to try to be more helping (if you really want to help that is) not telling "pro" to do something totally different.

We have seen pretty many posts of yours lately where you are to no help at all. So please Pavel.....

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Old 1st December 2004, 09:37 AM   #13
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Default Re: Op amp class 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.

Pro, it's normal to have a class A circuit between an opamp output and a buffer input but how much good this circuit does is hard to know if you don't know how the output stage is designed in the opamp. It's not certain at all that you really will get an improvement, just by adding a circuit not knowing if you should tie it to V+ or V- or how much current you should have.

My advise here is that you start with no class A circuit to begin with.

From what I have seen not very many have bothered to used this class A ciircuit on this rather (or very!) old opamp. I have mostly seen it together with an OPA627, AD8610 or LT1115.

Notice also that it's not suitable to have this circuit if you have a normal load. It's best suited if you have a high-ohmish buffer as load.

Currents for the current generators are normally 1-2 mA, not more.

Inspiration can be found here:
http://tangentsoft.net/audio/meta42/misc/schematic6.pdf
http://www.sjostromaudio.com/hifi_fi...0schema_p1.pdf
http://www.linear.com/prod/datasheet.html?datasheet=202
http://www.sjostromaudio.com/hifi_fi...0schema_p1.pdf
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Old 1st December 2004, 10:01 AM   #14
pro is offline pro  Italy
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OK. Thank you to all for the advices.
I think i'll start without class A, and then I'll arrange a small module where insert a j-fet or bjt CCS with the op-amp, inset it in the op-amp socket , maybe only for high and midrange section, and then listen to the difference.
I didn't find the Class A GC in the Peranders site. Because I am interested to it, could somebady give me the right link?
Thanks again.
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Old 1st December 2004, 10:19 AM   #15
uli is offline uli  Austria
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look at his QRV4 project

Uli

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Old 1st December 2004, 11:20 AM   #16
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Quote:
Originally posted by pro
I didn't find the Class A GC in the Peranders site. Because I am interested to it, could somebady give me the right link?
I'm afraid there aren't any such Gainclones

I can say so much that I haven't been able to pick any difference with and without this class A circuit but others can I'm sure.

You have the lnks above in those two project I have used this class A circuit. You may notice that Tangent has used a cascoded JFET, slightly better than a single transistor but you will loose some voltage.
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Old 1st December 2004, 11:45 AM   #17
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Hi,

Yeap, I'm going to get burned for this one!! lol..

I've heard of certain respected engineers rave about doing this to a 741 for audio purposes, I guess if you have nothing else around. So now I'm thinking for anyone who's really curious about this, it might be neat to experiment first with a totally abhorrent op amp like a 741, should be able to tell real fast what kind of difference it makes.

Though I of course agree for your actual circuit, do try it without first, it's not a hard addition to make later.

Also, I have it on good authority you should tie your CCS, or resistor, to the power rail which displays the least PSRR according to the data sheet, and that all it really does to improve things is bypass this 20dB worse PSRR, but then again I'd think since it is working class A the improvement should be two fold.

Keep an eye on the temperatures too.

Regards
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Old 1st December 2004, 11:53 AM   #18
uli is offline uli  Austria
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Quote:
Originally posted by classd4sure
Also, I have it on good authority you should tie your CCS, or resistor, to the power rail which displays the least PSRR according to the data sheet, and that all it really does to improve things is bypass this 20dB worse PSRR, but then again I'd think since it is working class A the improvement should be two fold.

Keep an eye on the temperatures too.

Regards
Depends on the design of the outputstage of the op.
As normally OPs arent complementary in design for best result you should take a look into the datasheet. Normally there is a circuit shown.

Uli

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Old 1st December 2004, 12:28 PM   #19
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I was under the impression that you need to connect it to move current from the output stage device that is in the emitter follower mode, in cases of quasi-complementary design. Is this right? e.g. in a classical NPN output stage the top transistor will be EF and the bottom CFP. You connect from output to -ve rail.
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Old 1st December 2004, 12:29 PM   #20
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Quote:
Originally posted by uli


Depends on the design of the outputstage of the op.
As normally OPs arent complementary in design for best result you should take a look into the datasheet. Normally there is a circuit shown.

Uli


Hi,

To what end?

I'm just guessing you would do that with the intention of using the NPN in class A and biasing off the PNP, however it doesn't seem to make sense if the NPN has -20dB less PSRR.
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