Sort of, the performance of the non-inverting input would be problematic in many places. Those PFET's are pretty noisey too sort of like the J177's I looked at.
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2145 gate leakage measurement
OK: with a much-improved setup, chopping the input gate-to-common resistor between 2.7kohm and 2Gohm, I see output delta V of only 800uV, indicating Ig of only 400fA. Much bettah! This is with +9V on the upper drain, -9V on the lower Idss-connected current sink. Drain current is 2.73mA for this setup.
Knew that Victoreen resistor would come in handy someday.
With this result I'm leaning still further toward the theory that these are separate chips inside.
OK: with a much-improved setup, chopping the input gate-to-common resistor between 2.7kohm and 2Gohm, I see output delta V of only 800uV, indicating Ig of only 400fA. Much bettah! This is with +9V on the upper drain, -9V on the lower Idss-connected current sink. Drain current is 2.73mA for this setup.
Knew that Victoreen resistor would come in handy someday.
With this result I'm leaning still further toward the theory that these are separate chips inside.
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Of course, trimming can get you zero offset at the output. but, is it thermally stable - no drift with time/temp? That is the subject brought up and being discussed by others here.... getting both..... without a dc servo circuit added.
Scott
I'm not sure that Dick is saying that it's an op amp, I guess this is the RNM headphone amp.
It looks like an early current feedback amp, but with comp jfets at the input.
As I see it it is more like a buffer with some gain and cfb.
With local compensation and used close to unity gain it could be designed as a OK ops for the "No Global NFB folks", (use the available gain to lower THD and output Z).
Then the "frontend" wich is what most here are discussing could do the voltage amplification and the current is left to the "buffer".
Or it could be used as a part in a "nested" fb design.
(a bit Krill)
Cheers
Stein
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?
John, could you get same results with your toplogy in current use w/o dc servo? You indicated you wanted the external dc servo [to allow optimum performance without compromise of circuit?.] That isnt clear to me.
I know trimmer contacts causes trouble but ignoring that for a second -- if you unbalance diff stage in some topologies, the thd goes up.... at least in low-modest gnfb designs. Need dc trim that doesnt do that and still be thermally stable.
With dc trim shown in my simple circuit... it wasnt needed in practice as offset was so close without it due to matching devices. It is thermally stable into 30 Ohm loads. Better performance into higher Z loads, of course.
Is it possible that many topologies dont need dc servo if properly balanced?
What about opamp circuit of conventional appl as Scott points out. Can it be done there? We shall see. -RNM
PS - I just put up the example that dc servo isnt always needed and to answere the reader about my whinning on the matter and not showing anything myself. It makes a point. Gets the job done. (BTW - other popular jFEt were also tried with similar results).
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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.
Non traditional ckt -
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
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
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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
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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.
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).
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
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
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> Especially now that I found my blunder!
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.