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Old 30th August 2012, 06:04 PM   #551
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Quote:
Originally Posted by RNMarsh View Post
All true. Just met my goals for audio line stage or headphone amplifier.
Main point -- no servo and thermally stable. And, cheap and simple to make. Why is it thermally so stable? Does answere have utility in opamp?
Never heard of an op-amp with a servo, maybe chopper but that's for 50 nano-volts/C. Bi-polars typically do 250nV/C matched pair.

Why do you keep mentioning this point, at G = 12dB many of these circuits would be fine with a simple offset trim.
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Old 30th August 2012, 06:34 PM   #552
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Default Non traditional ckt -

Quote:
Originally Posted by hitsware View Post
> Except it's not an op-amp and not an easy drop in in lots of places.
> Does not scale easily to phono use either.

Isn't it a ' current feedback ' op-amp ?
It isnt the traditional high open loop gain amp used for opamp service. Scott's is more versatile especially for phono preamp application.

But, I find it's topology to be most useful for audio line level stages and it doesnt need a dc servo added on to it.

I also thought it could bridge the gap between consumers who are weary of the term High Neg Feedback. And, still have the low distortion often associated with high neg feedback.... it gets the thd down without large amounts of high gnfb. It is super cheap for most any DIY budget or commercial.

Thx - RNM

Last edited by RNMarsh; 30th August 2012 at 06:46 PM.
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Old 30th August 2012, 06:42 PM   #553
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by scott wurcer View Post
Never heard of an op-amp with a servo, maybe chopper but that's for 50 nano-volts/C. Bi-polars typically do 250nV/C matched pair.

Why do you keep mentioning this point, at G = 12dB many of these circuits would be fine with a simple offset trim.
it doesnt take much searching on this site and others to see how popular the dc servo has become -- discrete and opamp.. many topologies -- line level and power amp level. They (servo) are used with all manner of IC circuits, filters - all over the place. That is why i keep mentioning this point. An out of the box idea has become entrenched and sometimes it appears as a crutch.

Anyway -- Where would you put the offset trim in your opamp circuit topology that doesnt affect the performance/specs? Thx - Dick

Last edited by RNMarsh; 30th August 2012 at 06:55 PM.
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Old 30th August 2012, 07:13 PM   #554
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Quote:
Originally Posted by scott wurcer View Post
Why do you keep mentioning this point, at G = 12dB many of these circuits would be fine with a simple offset trim.
I am extremely glad to read this.
IMHO much more than 12db can be handled in a simple way, for audio I tend to say 30-40db.
As long as our OP amp is not intended for a fully DC coupled system from phono to speaker, the entire discussion about DC-offset and a good portion of the discussion about the thermal drift thermal appears irrelevant for audio.
Let's assume to start DC coupling from line level. This simplifies things extremely and allows to focus on the AC propteries, which are most likely more relevant for our audio perception.
A simple adjustment of the offset (please with fixed resistors, not trimmers) is absolutely fine with me.

P.S:
No, please nobody try to tell me that also a phono stage must be DC coupled.
Vinyl is not flat, at least mine never was. And there is no way to convince me that having the woofer cone moving at 0,55Hz or 0,75Hz would impact the sound reproduction less than one more MKP in the signal path.
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Old 30th August 2012, 07:57 PM   #555
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Quote:
Originally Posted by RNMarsh View Post
it doesnt take much searching on this site and others to see how popular the dc servo has become -- discrete and opamp.. many topologies -- line level and power amp level. They (servo) are used with all manner of IC circuits, filters - all over the place. That is why i keep mentioning this point. An out of the box idea has become entrenched and sometimes it appears as a crutch.

Anyway -- Where would you put the offset trim in your opamp circuit topology that doesnt affect the performance/specs? Thx - Dick
I'd make the left resistor in one of the current mirrors trimmable, you can match the JFET's just enough to get within a reasonable range (+-10% or so).
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Old 30th August 2012, 08:01 PM   #556
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Are we talking of SOTA audio or MKP cap coupled - anything will do?

Looking at the bigger picture of the whole recording/playback chain, multiple roll-off at the bass end has its accumulated consequences. DC-coupled all the way thru will make the sound a lot better than it is now. but, hay! Do what you like. Me? I dont listen to records at all any more. gave it all away to Kavi. That doesnt mean I always like the sound of CD better or LP any less. For me LP systems are just way, way too fussy to use and I'm not into that level of screwing around to get the best sound... and it Still didnt sound close to 'real' live music... and it wears out with use (!). They are like tires -- great when new. So try something else. On to the next great Hope. -RNM

Last edited by RNMarsh; 30th August 2012 at 08:10 PM.
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Old 30th August 2012, 08:03 PM   #557
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Originally Posted by scott wurcer View Post
I'd make the left resistor in one of the current mirrors trimmable, you can match the JFET's just enough to get within a reasonable range (+-10% or so).
OK. Great. Done with this project. See you on the next one.
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Old 30th August 2012, 08:24 PM   #558
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For the record, I once added a chopper servo to my studio board design (shown earlier) and got very low offset, enough to drive an analog record head, directly, without a coupling cap. Try that, sometime, without a servo! '-)
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Old 30th August 2012, 08:50 PM   #559
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Originally Posted by EUVL View Post
> Especially now that I found my blunder!

So at least we can still trust Toshiba quality.

Patrick
BTW, I never doubted Toshiba quality. Their discretes (sometimes their ICs not so much, and I've used literally millions of their power amps) have been generally superlative. The question was one of how they handled the isolation of the dual halves. And it now seems that they did it in about the best way possible --- they are probably two separate chips! I'd still have appreciated a six lead package, but grateful for the versatility of the five.

It's just a shame they see no profit in continuing some of the products.
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Old 30th August 2012, 08:53 PM   #560
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Originally Posted by john curl View Post
For the record, I once added a chopper servo to my studio board design (shown earlier) and got very low offset, enough to drive an analog record head, directly, without a coupling cap. Try that, sometime, without a servo! '-)
I'll venture that you paid a lot of careful attention to power cycling transients!
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