A different way of powering I/V opamps

georgehifi · 08:41 PM

I would like some I/V guru's input to this way of powering an I/V opamp or output buffer, as it has been getting some good feedback from those who have implemented it.
Post 16 says it's a Gyrator circuit and would definately be better/sound better because of the improved PSRR figures.

Powering Opamps???

Cheers George

AndrewGM · 7th December 2011, 10:04 PM

The feedback of his e bay customers says it all - not one complaint.
Assuming of course they all tried it before they left the feedback in the first place.

Interesting indeed.

Oh and.. hello George - it's been a while
Hope all is well

martin clark · 7th December 2011, 11:00 PM

No. The posts criticising this usage are correct - those two transistors ONLY add a diode voltage drop in series with each rail. No benefit in that alone, at all.

martin clark · 7th December 2011, 11:19 PM

Edit to add: the only way this could help is if the intrinsic impedance of this diode juntion made a useful filter with the decoupling caps at the opamp supply pins.

For something like AD825, with 5mA/rail supply current that means an effective added supply impedance of about 5-6ohms (=26mV/I,mA - the same as any emitter-follower output Z) and so assuming say 0.1uf bypass caps at the opaamp, a supply filter point above 100Khz; maybe significantly lower with larger local decoupling caps.

In other words simply adding say 5ohms in series in each supply line will provide at least the same effect, very cheaply. And without the BS.

AndrewGM · 7th December 2011, 11:39 PM

BS Martin ?

Not here... surely ?

Joachim Gerhard · 8th December 2011, 01:31 AM

I simply think it is drawn wrong. With a capacitor in the base to ground it is a capacitance multiplier. You could argue that this is a kind of Gyrator then but there are better circuits that simulate a coil.

Joachim Gerhard · 8th December 2011, 02:02 AM

I have now seen the website where this is promoted. When it works, fine. I would add the cap for sure.

qusp · 08:24 AM

hmm, well considering the usually quite massive PSRR of modern opamps, is this really something to bother with? or is it simply a way to avoid proper regulation and local decoupling for each chip? wouldnt it be perhaps more useful to add ferrite beads, or the newish NFM C-L-C filter network types to address the problem at higher frequency, but still leave the DC low impedance?

SoNic_real_one · 8th December 2011, 10:31 AM

It is frequent on ebay not to show the complete schematics, to prevent "stealing" ideeas

Even if they are 20 years old.

7th December 2011, 08:38 PM	#1
georgehifi diyAudio Member Join Date: Sep 2003 Location: Manly Australia (Jewel of the Pacific)	A different way of powering I/V opamps I would like some I/V guru's input to this way of powering an I/V opamp or output buffer, as it has been getting some good feedback from those who have implemented it. Post 16 says it's a Gyrator circuit and would definately be better/sound better because of the improved PSRR figures. Powering Opamps??? Cheers George __________________ Avatar : Production Lightspeed Attenuator www.lightspeedattenuator.com Last edited by georgehifi; 7th December 2011 at 08:41 PM.

8th December 2011, 08:17 AM	#8
qusp Sometimes a square peg fits a round hole just fine diyAudio Member Join Date: Oct 2009 Location: Brisbane, Australia	hmm, well considering the usually quite massive PSRR of modern opamps, is this really something to bother with? or is it simply a way to avoid proper regulation and local decoupling for each chip? wouldnt it be perhaps more useful to add ferrite beads, or the newish NFM C-L-C filter network types to address the problem at higher frequency, but still leave the DC low impedance? Last edited by qusp; 8th December 2011 at 08:24 AM.

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7th December 2011, 10:04 PM	#2
AndrewGM diyAudio Member Join Date: Jun 2008	The feedback of his e bay customers says it all - not one complaint. Assuming of course they all tried it before they left the feedback in the first place. Interesting indeed. Oh and.. hello George - it's been a while Hope all is well

7th December 2011, 11:00 PM	#3
martin clark diyAudio Member Join Date: Jul 2003 Location: Bath, UK	No. The posts criticising this usage are correct - those two transistors ONLY add a diode voltage drop in series with each rail. No benefit in that alone, at all.

7th December 2011, 11:19 PM	#4
martin clark diyAudio Member Join Date: Jul 2003 Location: Bath, UK	Edit to add: the only way this could help is if the intrinsic impedance of this diode juntion made a useful filter with the decoupling caps at the opamp supply pins. For something like AD825, with 5mA/rail supply current that means an effective added supply impedance of about 5-6ohms (=26mV/I,mA - the same as any emitter-follower output Z) and so assuming say 0.1uf bypass caps at the opaamp, a supply filter point above 100Khz; maybe significantly lower with larger local decoupling caps. In other words simply adding say 5ohms in series in each supply line will provide at least the same effect, very cheaply. And without the BS.

7th December 2011, 11:39 PM	#5
AndrewGM diyAudio Member Join Date: Jun 2008	BS Martin ? Not here... surely ?

8th December 2011, 02:02 AM	#7
Joachim Gerhard diyAudio Member Join Date: Mar 2009	I have now seen the website where this is promoted. When it works, fine. I would add the cap for sure.

8th December 2011, 10:31 AM	#9
SoNic_real_one diyAudio Member Join Date: Jan 2008 Location: Virginia	It is frequent on ebay not to show the complete schematics, to prevent "stealing" ideeas Even if they are 20 years old.

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