OPA627 - Class A Biasing?
Apr 8, 2004 at 6:09 PM Thread Starter Post #1 of 28

Duncan

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Hiya... not too often that I post here
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Quick question to all you DIY gurus... looking at converting the 627s that I have put into my CD player into class a biasing, but am not sure of the value of the resistors to get...

I have a service manual / schematics for this player if there isn't a straight forward answer, and is dependent on the rest of the circuit...

Thanks all!
 
Apr 8, 2004 at 6:14 PM Post #2 of 28

Glassman

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you need to get around 2mA per opamp, the equation is quite simple:

your negative supply voltage divided by 2 = resistor value in kiloohms..

put that resistor from the opamp's output to that negative supply..
 
Apr 8, 2004 at 6:20 PM Post #3 of 28

Duncan

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15v... so thats a 7.5k ohm resistor per channel...

Sweet, thanks Glassman
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(btw... any particular grade or type that I should get, or just general purpose?)
 
Apr 8, 2004 at 8:33 PM Post #6 of 28

PinkFloyd

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That's what I initially thought but Duncan pointed out "or would it be two... if its a feedback 'device' then it'd be two rather than 4"

So....... 6-2 or 6-4 ? I still think 6-4 but it's always best to get a third and 4th opinion :)
 
Apr 8, 2004 at 8:44 PM Post #7 of 28

KTpG

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6-4.

Class A biasing is for the output... and I don't think it would do well on the inverting input (pin 2).

BTW- 2mA per chip may not be exactly what you want. You can use other biases. You might try different values to see what works best- perhaps as low as 2.21k ohms and as high as 10k or so.
 
Apr 8, 2004 at 8:46 PM Post #8 of 28

00940

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Quote:

Originally posted by Glassman
put that resistor from the opamp's output to that negative supply..


or also from the HA-1 project :
meier6a.gif


the biasing resistor is the 1.5k, 0.6W one

or even, from Tangent's article on opamp biasing :

opamp-bias2.png
 
Apr 8, 2004 at 8:54 PM Post #9 of 28

PinkFloyd

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Quote:

Originally posted by Duncan
Hiya... not too often that I post here
wink.gif


Quick question to all you DIY gurus... looking at converting the 627s that I have put into my CD player into class a biasing, but am not sure of the value of the resistors to get...

I have a service manual / schematics for this player if there isn't a straight forward answer, and is dependent on the rest of the circuit...

Thanks all!


So the question is fourfold from where I stand.

1: Will converting the 627's into class A biasing bring about any sonic improvements and, if so, why?

2: Is it worth spending money on milspec resistors or will standard 1% metal film be ok ( I know the answer to that one..... use standard 1% varieties in this application)

3: connect between pins 6-2 or 6-4 ?

4: Is Glassman's reply 100% on the button or are there any other views on this "you need to get around 2mA per opamp, the equation is quite simple:

your negative supply voltage divided by 2 = resistor value in kiloohms..

put that resistor from the opamp's output to that negative supply.."

Comments appreciated and anything else that should be taken into consideration welcomed.

Pinkie.
 
Apr 8, 2004 at 9:05 PM Post #10 of 28

guzzler

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1. It should provide a small improvement in sound, as the opamp is always loaded, and so there is no crossover distortion

2. 1% resistors will be fine there

3. 6 and 4. 6 and 2 would make it act as a feedback device, I'm not sure what affect it would have, but it wouldn't be nice!

4. 2mA is a decent starting value, some opamps will respond better at lower a lower bias, but that is something experimentation determines.

A more advanced approach would be to use a JFET cascode (see PIMETA/PPA schematic) or a CRD (see MINT schematic)

g
 
Apr 8, 2004 at 9:13 PM Post #12 of 28

PinkFloyd

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Quote:

Originally posted by guzzler
1. It should provide a small improvement in sound, as the opamp is always loaded, and so there is no crossover distortion

2. 1% resistors will be fine there

3. 6 and 4. 6 and 2 would make it act as a feedback device, I'm not sure what affect it would have, but it wouldn't be nice!

4. 2mA is a decent starting value, some opamps will respond better at lower a lower bias, but that is something experimentation determines.

A more advanced approach would be to use a JFET cascode (see PIMETA/PPA schematic) or a CRD (see MINT schematic)

g


Cheers Guzzler.

Pinkie.
 
Apr 8, 2004 at 9:28 PM Post #13 of 28

PinkFloyd

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attachment.php



This is all very interesting
wink.gif
Can anybody work out what Graham slee has done by placing a 33K resistor between pins 7-8 and 1-8 on the AD823 op amp?

This was one of the circuit mods for the 2004 Solo :wink:

Pinkie.
 
Apr 8, 2004 at 9:54 PM Post #14 of 28

PinkFloyd

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That was probably an unfair question as the mod also involved replacing a BC337 with a BC184C, a 4k7 with a 33K, a 10uT with a 220nF, aa 100pF with a 220pF, a 47K with an 18K, a 1k with a 4K7 and a 220uF with a 1000uF........... oh and a 4K7 was replaced with a 66K.


here's the consumers view:
proto%20solo%202.jpg


I've got the full schematic of the 2004 Solo but I would be really interested to hear your opinions on the 33k op amp bridge ..... I know the answer as I've got the blueprints in front of me but it's an interesting exercise
wink.gif


The answer will follow
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Apr 8, 2004 at 10:59 PM Post #15 of 28

Glassman

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positive bias, nothing more.. most opamps sounds better when biased to negative, ie. they are sourcing current, but a few others sound better when they sink current.. this guy apparently believes AD823 is one of them
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