I am not getting the same CMRR in my pin-board but there are some issues in implementation. Did get the servo to work but there remains a 3.4mV between DC emitter voltages.
Thought you might need this chart:
Thought you might need this chart:
Hmmm. What are you using for the servo amps? The OPA2188 spec a few uV offset at worst.
The graph is nice - you can easily pick closely matched pairs out like that.
The graph is nice - you can easily pick closely matched pairs out like that.
Yes - just looked at the Vos on that one - +-1mV typical, +- 5mV max - about 100x-200x higher than the 2188. If you're showing +-3.4mV you can bet the servo is working. What you could try is to put two very mismatched ZTX851's in and see if you get the same offset just to confirm its all ok.
OPA2188 is a chopper op amp; I would not use a chopper op amp in a low noise head amp, even as a servo. But then I don't know, it may work without any noise impact.
With the AD8512 there is more than a tenfold improvement.
I have a partial reel (ex Scientific Atlanta) of TLC4502ACDR which is another chopper. Will give that a try tomorrow. If anyone wants me to peel off 10 of these let me know via PM and I will mail them to you. They are intended as low Vcc, single supply, 8SOIC.
Here's a comparison with two different servo amplifiers, OPA2604A and AD8512AR. Some LF stability issues with the latter.
Syn08 I used the OPA2188 in the X-Altra MC input offset servo- that measures at 232pV/rt Hz and is in very close agreement with the sim. The opamp is driving the control point with a massive very low <<<1Hz cut off filter so I think it’s ok. The noise specs on these new chopper amps are superb compared to the old devices that used external caps from the late 80’s
Last edited:
Gain -- for the input I use a transformer and 5.6 Ohm resistor, the input and outputs measured in "balanced" on the AP. I could just use the balanced output of the AP, but after blowing out the front end a few years back I thought some isolation was in order. Curious -- the low point seems to be about 1/(47k*100nF*2pi) . You guys are much smarter than me, so figure it out.
I didn't measure the delta between emitters with a bench meter, only my Fluke. Nonetheless, it was in the low hundreds of micro-Volts.
I didn't measure the delta between emitters with a bench meter, only my Fluke. Nonetheless, it was in the low hundreds of micro-Volts.
My concern is not about noise as in a regular amplifier, but about highly correlated noise (aka spurious) and intermodulation products.
No, I don’t claim these could be anywhere audible, if they appear in the FFT.
Jack so yellow is one channel with one kind of opamp in the servo position and yellow the other channel or is the yellow the ref and the red trace one chan of the MC amp?
Hello, there.
Here is a similar design using ZTX851:
https://www.proaudiodesignforum.com/forum/php/viewtopic.php?t=783
Here is a similar design using ZTX851:
https://www.proaudiodesignforum.com/forum/php/viewtopic.php?t=783
No that’s quite different as the inputs in that design are into the transistor bases. The Zin in mediatechnology’s design is primarily defined by the load resistor across the transistor bases (499 ohms// with transistor hFE*Remitter). The zin of the Transvaal is very low (1/gm) and it’s a current input, not a voltage input topology. Nevertheless, his design is fascinating IMV.
Ignore my post here. I reread your original post Jack and see what you were doing. Quick question though, are you running the opamps off single low voltage supply or at +-15V?
Changing the servo "inverting input" resistor from 100k to 1Meg alleviates the issue with the AD8512 op amp. The voltages on the ZTX851 emitters remain within a few hundred uV
That’s great - looks like solid progress. I think Syn08 also noted the servo resistors were too low. I’ve gone with 330 k but upping to 1 Meg easy enough to do
😊👍
😊👍
I don’t understand your point. One is a voltage input topology and the other a current input topology. I think the mediatechnology circuit will be a bit cheaper to build - fewer parts