Let's Discuss Servos (Previously OPA188 Thread)

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mediatechnology
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Let's Discuss Servos (Previously OPA188 Thread)

Post by mediatechnology » Wed Aug 17, 2011 11:34 am

Just saw the press release for these in Planet Analog. They have 36V capability, low Vos, fairly low Ib, low drift and are available in singles, duals and quads. If available and resonably priced (both big "ifs") these might make good servo amps particularly when duals are needed.

http://www.ti.com/lit/ds/symlink/opa2188.pdf

Just checked and the dual is ~$1.40 at 1K pcs so not too bad. Looks like they're sampling.
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by JR. » Wed Aug 17, 2011 12:02 pm

the slew rate seems a little weak for use in audio path..

while the servo output doesn't literally need to swing much distance quickly, it needs the speed to counter fast HF inputs. IMO. I am speculating or extrapolating and it may be more a GBW not pure rate of change issue, but I would be nervous.

I wrote a column back in the '80s about how caps in a servo are still in the audio path so need to be good.. that goes for the opamps too within reason.

JR

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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by mediatechnology » Wed Aug 17, 2011 12:12 pm

It's about 3X faster SR (0.8 V/uS vs. 0.3 V/uS) than an OP07 which work fine as servos. (Though the OP07's DC performance could be better which is the point of this post.)

I wouldn't be nervous about using this part in an integrator with <<< 20 Hz cutoff having to slew a few 10's of millivolts. You may be spending too much time with the propeller heads over at the other place. :D

I'm suspicious of Safari's roll in this cookie thing.
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CDC: "For 6% of the deaths, COVID-19 was the only cause mentioned. For deaths with conditions or causes in addition to COVID-19, on average, there were 2.6 additional conditions or causes per death." https://www.cdc.gov/nchs/nvss/vsrr/covi ... /index.htm

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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by JR. » Wed Aug 17, 2011 12:49 pm

I'll take your word for it, I used a fast device in the only servo I ever did and that was 20+ years ago.

I do tend to approach such question from an absolute performance perspective. Looking at the servo more closely I guess it depends on the topology of the servo. If the audio input goes through a passive LPF at something like 20Hz first, I agree no worries. If the servo is inverting overall using a simple integrator fed by the wideband audio the output must supply wideband current to match the input, admittedly not much wideband voltage but the current needs to be fast. This suggests to me there is only one "good" way to make a servo. 8-)

I apologize if this answer sounds like I'm weaseling.. I just had to look up my old design to see what I actually used in my old design. It turns out i did the servo right (passive LPF on input) so I could have used a slower opamp, but I still needed the bifet for the 10M impedances and it was part of a quad. The speed of the servo opamp may have some subtle impact depending on what that servo output connects to, but I expect most servo outputs feed through a pretty high impedance and drive low voltage nodes. (My old servo has a passive LPF in series with it's output too, so once again I really didn't need the speed.)

Sorry if the servo topology difference was common knowledge. I think I've seen them both ways, but I don't study a lot of servos.

JR

PS: Yup it may be a safari issue... whatever... Now that I know what it is I can delete all the proaudiodesign cookies every time I'm blocked and get back in...just a minor PIA
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by mediatechnology » Wed Aug 17, 2011 1:00 pm

The LT1012 is only 0.1 V/uS.

I do like the idea of the passive LPF ahead of the integrator.
My old servo has a passive LPF in series with it's output too
I think I used that in a 1512/1570 summing amp similar to the SSM2017 application note.

Too bad (affordable) BIFETS lack the DC precision for many preamp servo applications.
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CDC: "For 6% of the deaths, COVID-19 was the only cause mentioned. For deaths with conditions or causes in addition to COVID-19, on average, there were 2.6 additional conditions or causes per death." https://www.cdc.gov/nchs/nvss/vsrr/covi ... /index.htm

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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by mediatechnology » Wed Aug 17, 2011 1:19 pm

OK, you made me look. I had a hunch I'd find it in the phono preamp post. For the benefit of our readers it's A2. R17 and C9 form the passive input LPF. R14 and C11 form the output LPF.

Image

There's a lot of LF gain in A1 which makes the Vos requirements of A2 far less demanding.

Just like the improved 1510 servo here: viewtopic.php?f=6&t=256 where we take advantage of the 1510's gain to reduce the servo's Vos requirements. There is no passive input LPF. In this circuit there is significant attenuation of the servo output ~2M2/1K5. The output LPF of the servo may not be immediately obvious.

Having all that gain inside A2's loop doesn't hurt...
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by JR. » Wed Aug 17, 2011 1:39 pm

I suspect i have a ton of DC gain in that servo... the output feeds a current source so those servo resistors are working directly on the RIAA feedback net of probably 400kOhm+. A servo was the only practical way to bias up that input kluge without nasty caps in the audio path, C7 my dominant HPF is a film cap. .

Today we have better opamps available but I'd still put that preamp up against all comers.

JR

PS: My audio path is actually LPF by the RIAA 75uSec pole (C2xR3) so my speed requirements are even lower yet.. :lol:
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by mediatechnology » Wed Aug 17, 2011 1:46 pm

Today we have better opamps available but I'd still put that preamp [P100] up against all comers.
You like that one better than the P10? Probably a better question posed in that thread but I had to ask.
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by JR. » Wed Aug 17, 2011 4:26 pm

mediatechnology wrote:
Today we have better opamps available but I'd still put that preamp [P100] up against all comers.
You like that one better than the P10? Probably a better question posed in that thread but I had to ask.
Phono preamps were already mature when I did the P-10 and that was actually my second, I published one in '78 in a DJ mixer that involved a little cleverness too (IMO).

In my judgement the (middle design) was way better than the vinyl media and carts... The P100 was an interesting adventure in gross over engineering. So much of that later effort was into obscure or esoteric tangents that don't show up on the spec sheet. The power supply alone was a work of art (mild exaggeration).

I am a normal design engineer, so yes i could improve every design I ever did, given one more cut, even the p100... but nah, no mas vinyl, :lol:

JR
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by mediatechnology » Wed Aug 17, 2011 4:40 pm

I have some 2SK389s I want to use. I moved my vinyl a couple of weeks ago and just got around to pulling out about 20-30 albums I wanted to re-visit. As I mentioned in another thread the turntable got hooked up before the DVD player. The phono preamp in the Kenwood is a 4560 IIRC. :oops:

The servo in the P100 looks like the "Forsell" servo. The more I think about it you make a good point WRT the input passive LPF filter. The "virtual" ground at a simple integrator's inverting input is only "ground" as much as the op amp can make it. Sort of like "virtually white" in laundy detergent commercials. Not really, but almost.

Were you actually able to measure a difference in the overall performance? I suppose that with RIAA pre-emphasis the bandwidth limits of A2 may have been more apparent.
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Plandemic Series: "Pland3mic - Indoctornation" https://proaudiodesignforum.com/content ... nation.mp4
Plandemic Series: "Plandemic - Doctors In Black" https://proaudiodesignforum.com/content/Plandemic.mp4

CDC: "For 6% of the deaths, COVID-19 was the only cause mentioned. For deaths with conditions or causes in addition to COVID-19, on average, there were 2.6 additional conditions or causes per death." https://www.cdc.gov/nchs/nvss/vsrr/covi ... /index.htm

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