Let's Discuss Servos (Previously OPA188 Thread)

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

Post by JR. »

I tried google and didn't find a Forsell servo... but I recognize the name (fred?) . Any similarity between his and mine are coincidental at least for my part.

I used JFET inputs for my last two pre and like them for MM carts. They can be pretty high Z at 20kHz.

Are you talking measured performance of the servo? or of the whole preamp? I never did a lot of bench work on the servo and with the way it is configured doubt i would see anything.

For the P10-vs the P100 I suspect the p10 is lower distortion on paper because I run the input JFET open loop in the p100 but with NF in the P10. This was a cognizant trade off made to get an input topology that would literally shrug off the fastest rise time signal that you could throw at it. I don't recall the details but a 1V square wave at 20kHz or higher would just nicely be LPF. The p100 measures around .05% at clipping, and I suspect it is all low order, at nominal levels the distortion is below my old bench residual which was decent.
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Just for jollies I might be tempted to drop in a modern opamp in place of the 4560... The RIAA curve calls for 60 dB at LF but drops to only 20 dB at 20 Khz, so closed loop gain and open loop gain both have a falling characteristic. A decent low noise JFET modern opamp could do well, the old tl07x is marginal for the whole gain, and isn't quiet enough anyhow. The bigger issue is the quality and tolerance of the caps used in the RIAA EQ and maybe the topology. But a MM phono preamp is not heavy lifting for modern silicon.

I don't have any P100 PCBs left...:-( but i may have some P-10 pcbs if you want to mess with that...:-) I also have some decent polystyrene caps and 1% resistors for the RIAA EQ if you use one of my topologies.. :-) neither of which is simple one opamp,

Revisiting p100 with a different output stage wouldn't suck... I scribbled some ideas for how to improve the input stage linearity using two JFETS but at some point more active devices could start increasing noise... and I never proved out any improvements. I have a stock p100 in my system that i haven't turned on in a while.

I would prioritize in order...

#1 accurate RIAA EQ (or just not too far off)
#2 decent noise, but not hard to do wrt 1.5k source, and record surface noise
#3 actually dial in the capacitive termination for the actual cart, and turntable, and cables... almost free but this actually matters.


blah blah.. blah blah

JR

PS: I think i may have found the link that is trashing my cookies...,http://www.ka-electronics.com/images/jpg/Loftec_TS1.jpg or not...is this the old links you were talking about?
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mediatechnology
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by mediatechnology »

PS: I think i may have found the link that is trashing my cookies...,http://www.ka-electronics.com/images/jpg/Loftec_TS1.jpg or not...is this the old links you were talking about?
No, I think the link would be one that looked like this: "posting.php?mode=reply&f=6&t=419"

or "http://www.padforum.com/forum/php/posti ... &f=6&t=419" etc.

If you enter the site with one of those links and then login, which it will let you attempt to, it'll plant a cookie with http://www.proaudiodesignforum.com. Safari will think it's two different sites and that were pulling some kind of security trick. The board will think the same thing and throw you out of the login. I should load Safari on this machine and check it.

Were those JFETS in the P10 duals? I've got those 2sK389s... What I like about the P10 is the ground-sensing differential input.

Does the P100 might have a better HF overload point? Does C2 loading the drain and current source form an RIAA pole? Since that may occur in the first stage it looks like it may have a little more HF headroom.
#3 actually dial in the capacitive termination for the actual cart, and turntable, and cables... almost free but this actually matters.
The Kenwood has 47K and 100K switchable termination and it makes a huge difference in sound. Don't know if the Cload gets switched.
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JR.
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by JR. »

mediatechnology wrote:
damn this is getting old.. it kicked me out twice this morning, trashing my answers both times.

I'll talk with you off line about this no need to add clutter here.

Were those JFETS in the P10 duals? I've got those 2sK389s... What I like about the P10 is the ground-sensing differential input.
2sk117 singles... 1nV/rt Hz, not too shabby for the '80s

OK I wrote a longer answer before, but short answer I would experiment with better 2 conductor shielded wiring. Again being aware of cable capacitance.

Does the P100 might have a better HF overload point? Does C2 loading the drain and current source form an RIAA pole? Since that may occur in the first stage it looks like it may have a little more HF headroom.
indeed c2/r3 form 2kHz real pole... that attenuates forever. Even decent alternate topologies stop at unity gain. The p10 exhibits a flat fixed front end gain. Still ok in practice, but p100 is tweakierer
#3 actually dial in the capacitive termination for the actual cart, and turntable, and cables... almost free but this actually matters.
The Kenwood has 47K and 100K switchable termination and it makes a huge difference in sound. Don't know if the Cload gets switched.
Transducer designers target 47k termination.. 100k can alter interaction with cart inductance.. I stick to 47k and tweak C for best 20kHz response.

Let me hurry up and post this before it disappears again.

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

Post by JR. »

OK, something that I said in my lost post... that disappeared.

There was a guy in TX who got a patent on balanced input (differential input?) phono preamps.. He sent me a cease and desist letter, I sent him a copy of an old tube manual circuit for a phono preamp using a transformer input :lol: and then put a 0 ohm jumper in my design. Since i didn't feel like paying lawyers, even though I believed I would win. His pockets probably weren't any deeper than mine, but I was already moving on to my next project... they weren't going to recall old pop electronics issues, or print a correction for that matter.

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

Post by mediatechnology »

This is a schematic of the Jensen Twin Servo from around 1984. I see that it's also fully-differential servo with a passive LPF (edit: 16K9/100nF) at the input. I found it in the circuit collection JR sent me.

Image
Jensen Twin Differential Servo Microphone Preamp Using 990, 1984.

Hi Res: http://www.ka-electronics.com/Images/jp ... p_1984.JPG
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by JR. »

Not to quibble but there is no (RxC) LPF on that input, while there is a transformer in series that is in fact a BPF.

The RxC shunt across the input is dialing in the transformer transfer function, one of Deane's (RIP) rigorous engineering practices.

The inductor(s) in his opamp input LTP went a long way toward mitigating extreme input edge rates.

We can observe he did the servo right with real pole on it's input.

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

Post by mediatechnology »

Not to quibble but there is no (RxC) LPF on that input, while there is a transformer in series that is in fact a BPF.

We can observe he did the servo right with real pole on it's input.
Image

Yes, there is. I was talking about the servo input, not the preamp.

EDIT: The Jensen transformer really deserves it's own thread.
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by mediatechnology »

Image
Jensen Twin Servo 1997 Version

The AD706 has a SR of 0.15 V/uS. There is no passive LPF at the servo input nor is there a passive LPF at the servo output.
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by mediatechnology »

the slew rate seems a little weak for use in audio path..
OK, let's clarify for our readers and for the sake of discussion that the servo itself is not directly in the audio path. Let's also stipulate for sake of discussion that the servo topology we're about to discuss is a typical simple inverting integrator, 1M Rin, Cfb 100 nF with a cutoff of <2 Hz.
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.
Well the bench is where speculation hits the road.

Does SR limiting in simple integrating servo amps matter?

I tried an OP07 SR ~0.15 V/us, Rin 1M, Cfb 100n in the aforementioned inverting integrator. Obviously with large values of Rin, the peak current and peak current slew rate (in A/us or uA/us) is quite small. 1M/100nF are typical values and are often used to keep Cfb small and non-electrolytic.

I wrapped this servo around an inverting 5532 to close the loop, Rin=Rfb=10K. No Cc 'cause I felt like walking on the wild side.

The output of the OP07 fed a 220K/10K voltage divider to the non-inverting input. The output of the OP07 is attenuated by a factor of ~23 and the output of the swing of the OP07 is scaled upward: 2mV of Vos correction becomes 46mV. This was done to provide some magnification.

I set the generator input to 20V P-P and swept the frequency upward to induce slew rate limiting, monitoring both the 5532 an OP07 outputs to see at what point, if any, the OP07 came unglued.

It didn't.

The only point the OP07 did anything unusual was when the 5532 went into slew rate limiting around 200 kHz - that was because the 5532 itself developed a DC component due to asymmetry. The OP07 corrected the baseline error. It did what servos are supposed to do.

Though a passive LPF at the input is a good idea, with large-value servo input resistors, I'm not sure it's required.

A servo having low-value input resistors would have to slew more current. In that situation, the SR of the servo amplifier itself should make you nervous enough to be looked at.
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Re: OPA188, OPA2188 and OPA4188 Low Drift Amps for Servos

Post by mediatechnology »

Interesting article on op amp myths. Several paragraphs into it Gilbert has a section called "“Virtual Ground” Virtually Groundless." It makes John's point about how servo bandwidth limitations could place the servo's errors in the audio path.

http://www.edn.com/blog/Anablog/41272-O ... ilbert.php

Though with high-value input resistors it may be moot since the peak current slew rate is kept very low.
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