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Old 7th December 2011, 05:27 AM   #1
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Default Powering Opamps???

Can someone explain to me why people are saying this sounds better on AD825 opamps than without the "pass transistor/resistors" on the rails?

Cheers George
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Old 7th December 2011, 10:26 AM   #2
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Mmmmm . . .

Absolutely no idea. Now, if there were capacitors connected from the transistor bases to ground, that would constitute a ripple eater and there I could see a possible reason for your findings.
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Old 7th December 2011, 10:31 AM   #3
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Mmmmm. . .

Similar drawing appeared here, post #1904
The best sounding audio integrated opamps
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Old 7th December 2011, 10:36 AM   #4
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I'm missing to as Bonsai said couple of capacitors each one connected from base of transistor to GND. Then you get the basic capacitance multiplier, which is defined by the capacitor capacitance and transistor gain. R supplies the charging current as well as the transistor's (denoted by Q) base current. I'm olmost shure, that capacitors are missing from the circuit. IMO this is for stabilizing the pover to the IC.
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Old 7th December 2011, 10:42 AM   #5
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Nope no caps missing, this is what I though a cap multiplier, here they are on Ebay.
eBay Australia: Buy new & used fashion, electronics & home d?r

Cheers George
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Old 7th December 2011, 11:04 AM   #6
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It seems to me Chris Daly is the author or designer of this, I think he should be asked what the benefits are, if any.
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Old 7th December 2011, 11:28 AM   #7
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Q1 and Q2 act as resistance dividers, so the main effect is to insert a resistance in the supply rail of R/beta. I can't see what benefit this will bring; why would anyone want to deliberately degrade their DC supply?
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Old 7th December 2011, 12:29 PM   #8
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Hi
Thanks for your interest, the benefit with an emitter follower on each rail in op amp circuits is in a number of areas firstly if used in a stereo installation left right information is correctly represented. Direct comparison with the same op amp in this case AD825 without pass transistors stereo is a muddle appearing from the centre soundstage.
Secondly turn on and off thump is very much reduced.

I started this exploration examining dreadful implementation of op amp power supply in a Quad 405 which led to thoughts of locating transistors directly on the DIP adapter. Because there are many different locations of ground in op amp circuits. It dawned on me that leaving the capacitor out is still a valid circuit, obtaining the benefits of negative feedback arising from collector to emitter. It was then a matter of tailoring the base current to suit. Much experimentation found 6ma to be ideal.

The circuit in practice on a DIP Brown Dog adapter is extremely consistent delivering the same improvement time after time.

In October after much work I perfected a Dual version using a single adapter, Construction details are at my blog site opamp - opamp construction

Cheers / Chris

article here I believe validly explains an emitter follower:
Emitter Follower

"Due to this deep negative feedback, the voltage gain of the emitter follower is smaller than unity. However, the circuit is drastically improved in terms of its input and output resistances. In fact the emitter follower acts as an impedance transformer
Comparing this with the input resistance and output resistances of the common-emitter transistor circuit, we see that the emitter follower circuit has very favorable input/output resistances.

Although the emitter follower does not amplify voltage, due to its high input resistance drawing little current from the source, and its low output resistance capable of driving heavy load, it is widely used as both the input and output stages for a multi-stage voltage amplification circuit."
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Old 7th December 2011, 12:37 PM   #9
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Emitter followers may be fine, in the right circumstances. That circuit is not an emitter follower. An emitter follower has a voltage supply to the base; that circuit has a current supply. As I said, it is a resistance divider. It adds resistance to the supply. Given the low supply current of most small-signal opamps even an emitter follower would not work because the emitter impedance would be relatively high. I suggest you go and do some more reading about emitter followers.
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Old 7th December 2011, 12:56 PM   #10
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In the attached article an Emitter follower diagram is provided: note Rb is a resistor delivering current to the base from Vcc, An emitter follower as provided has a resistor from Vcc to the base. In the article a AC coupled signal is shown. Ignoring complexity of
AC coupling to process audio. The voltage source in my implementation is Vcc which also delivers current to the base exactly as the article provides. The load is provided on the emitter - in the article lifted from ground. In my implementation the load is also fed by the emitter to the relative op amp pin.

Cheers / Chris
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