Frequency response of the amplifier was not mentioned, but cutoff frequency of inverting low pass filter (DC servo) which is important performance parameter of DC servo circuit.
Sorry, but no meaningful discussion with you is possible, because of evident language or knowledge barrier.
As seen in many other threads, you don’t completely understand what was said and your replies are usually missing the point being discussed.
Ohhh, you must be someone great, as companies are sending you gifts. It’s called free samples here. 🤣
Bye bye
This only means that you play DIY later.
Before 2008. All companies Ti BB ns will give free samples. I have almost all models.
Many people know this.
I opened mark Levinson No.33 .
Accuphase E210 E405. A60
PROCEED AMP2
They all use ordinary 4580。5532 operational amplifiers.
Attachments
That's it? You should have read all the discussion in this thread about the requirements for a servo opamp.
Jan
Jan
OK, see OPA192 datasheet: THD+n 0.00008%, full 36-V differential input range, true high-impedance differential input capability for highvoltage applications {patented input protection architecture), integrated electromagnetic interference (EMI) filtering, internal phase-reversal protection, patented output stage capable of driving up to 1 nF of pure capacitive load, ... and I repeat again 5 microvolts (not milli) offset with jfet 5 pA input current and noise 5.5 nV/√Hz. Power ±2.25 V to ±18 V with 1 mA quiescent. And 3.5$. What else do you want for DC servo and is anything better?
I would readily give up the 5uV and 1nF driving capability, for instance, for a <1$ price point.
Always dangerous to state ´the best´ or ´the worst´. It´s so absolute 😎
But I agree the full supply diff mode input is a big plus.
Jan
Always dangerous to state ´the best´ or ´the worst´. It´s so absolute 😎
But I agree the full supply diff mode input is a big plus.
Jan
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For a servo, an op amp with good DC specs is needed. AC specs need not be all that good. You especially want low DC offset and low input bias currents that could cause a DC offset to occur, since the sole purpose of a servo is to null out DC offset.
It depends. Some circuits have the servo op-amp output straight in series with the lower feedback resistor, or with a branch of it (when it is split up in two resistors, one resistor straight to ground, the other to the servo op-amp output). The feedback resistor then injects signal current into the op-amp output, and any distortion components or noise at the op-amp output directly affect the distortion or noise of the amplifier.
Hi Jan
I’ve used the opa1656 on an home made adapter to replace a opa2134 in a Dip-8. This was tested on Bob Cordell’s Dh-220c design. Recall he uses both sections for his servo. The adapter has a 100n 0805 mlcc across the supplies.
I’ve used the opa1656 on an home made adapter to replace a opa2134 in a Dip-8. This was tested on Bob Cordell’s Dh-220c design. Recall he uses both sections for his servo. The adapter has a 100n 0805 mlcc across the supplies.
Bob Cordell use a nice circuit idea preventing these shortcomings, in its own way by connecting the output of the servo-op-amp not to the input stage, but to the output of the second stage of the voltage amplifier. See Fig.6 and Fig.7 of his VinylTrak @ [ Linear Audio, volume 4, September 2012, pages 139-140 ]. I have also use this nice idea in my electrolyteless phonopreamp [ https://c10.patreonusercontent.com/4/patreon-media/p/post/60580885/ec6a031a104046318a56e87f0e9b0a42/eyJxIjoxMDAsIndlYnAiOjB9/1.jpg?token-time=1660953600&token-hash=0a-9KpgHL2hS-BzdBTYPBZobnMtqGAGUHctZxiYblIA= ]. No added noise or distortion at all.
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Yes, many ways to skin that particular cat. It's called 'designing'. 😎
But connecting the bottom of the feedback shunt resistor to the output of the servo opamps doesn't strike me as a good one though.
Jan
But connecting the bottom of the feedback shunt resistor to the output of the servo opamps doesn't strike me as a good one though.
Jan
Nick, I understand how Bob Cordell's circuit works: he has an open-loop voltage-to-current converter and adds a correction current at its output.
I don't understand how yours works: you have a multistage negative-feedback amplifier with shunt feedback at the output and pull a correction current from that output. Ideally that should do almost nothing until the amplifier's output stage runs out of current.
In general I'm not so keen on injecting servo currents halfway a feedback amplifier because you then get sensitive to the open-loop nonlinearity of the stages up to the point where you inject the current (just calculate it back to the input to see why).
I don't understand how yours works: you have a multistage negative-feedback amplifier with shunt feedback at the output and pull a correction current from that output. Ideally that should do almost nothing until the amplifier's output stage runs out of current.
In general I'm not so keen on injecting servo currents halfway a feedback amplifier because you then get sensitive to the open-loop nonlinearity of the stages up to the point where you inject the current (just calculate it back to the input to see why).
Seems that DC servo is used as virtual ground for biasing resistor of the CCS at SE output stage. Varying current through diodes provides enough Vbe variation on CCS, to compensate for any change at the output.
The input offset control range is the difference between the maximum and minimum current that the DC loop can draw from the output divided by the open-loop transconductance of the amplifier. Normally one would expect a three-stage amplifier to have a large transconductance, so the control range gets quite small. I'm surprised it is supposedly still enough.
Jan,
I have not looked through all the post three time before posting.
But I have also been looking for alternative opamps for DC servo because of IC shortages.
To me, apart from high input impedance (FET input) and low DC offset, an important criteria for me is LF noise.
The DC servo opamp has the most impact on the output at LF, as it should.
So any LF noise from the opamp will get through to the output also.
And comparing OPA1656 to say OPA1642, OPA2140, etc., it has at least 5x more noise at 10Hz and below.
And the latter are not so expensive as OPA627 or ADA4627.
Cheers,
Patrick
I have not looked through all the post three time before posting.
But I have also been looking for alternative opamps for DC servo because of IC shortages.
To me, apart from high input impedance (FET input) and low DC offset, an important criteria for me is LF noise.
The DC servo opamp has the most impact on the output at LF, as it should.
So any LF noise from the opamp will get through to the output also.
And comparing OPA1656 to say OPA1642, OPA2140, etc., it has at least 5x more noise at 10Hz and below.
And the latter are not so expensive as OPA627 or ADA4627.
Cheers,
Patrick
In fact, the offset voltage is often overrated. In a power amp, a few mV offset is totally irrelevant.
So paying a premium for 10uV offset opamp is money wasted for me.
Low LF noise could be a factor, not sure because the integration cap has a higher impedance at LF and thus also generates noise.
I haven't done the sums, maybe no factor, maybe dominates. And what about the usual 1M R?
Jan
So paying a premium for 10uV offset opamp is money wasted for me.
Low LF noise could be a factor, not sure because the integration cap has a higher impedance at LF and thus also generates noise.
I haven't done the sums, maybe no factor, maybe dominates. And what about the usual 1M R?
Jan