does anybody knows PMI SSM2139 opamp? - diyAudio
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Old 22nd March 2006, 03:39 PM   #1
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Default does anybody knows PMI SSM2139 opamp?

I`m performing some simple modifications to my dvd player -a middle of the road Sony, but I don`t remember the model now-; it have an AKM ak4385 dac, with dual differential outputs.
The dac is followed by a LPC/summing stage identical to the one depicted in http://www.akm.com/AppsNotes/Tech%20Tip%203.pdf.
I made a separate linear +/- 15v PSU for the analog stage, with good regulation and filtering.
For the LPF filter, I have used good quality silver mica and styroflex caps.
I have tried a pair of fet-input opamps in that position: opa2134 and opa2604. They performed well -no dc output, no heat, no perceptible noise-, but the sound was very ¨midrangey¨, eufonic at low levels, but annoying when crankin´up, due to an apparent predominance of mids and high mids. Nothing out of the blue, but very noticeable. And barely better than the sound of the stock player.
Then, I put a SSM2139 bipolar dual opamp that some friend gave me, and the output became excellent: detailed sound, nice highs, correct midrange, control and slam in the bass... wow! Not very different to my analog rig!
I have googled everywhere and I can`t find info about that opamp. Only that was once made by AD, previously by PMI and SSM, and that is a bipolar general purpose opa.
Have any of you some additional info -bandwidth, current output, etc-, or idea of the reasons for the stellar performance of that opamp in such configuration.
Thank you very much in advance.
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Old 22nd March 2006, 03:42 PM   #2
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Default sorry, there`s a typo in the URL

is
http://www.akm.com/AppsNotes/Tech%20Tip%203.pdf
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Old 22nd March 2006, 11:24 PM   #3
markp is offline markp  United States
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All the PMI chips are well designed and made to have very low noise especially for audio. You can't go wrong with any of their stuff. I remember they are +-18volts supply but run them at 15 at the most. Slew rate was greater than 15v/us and distortion was below .01% for most gains.
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Old 23rd March 2006, 02:04 PM   #4
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Default ssm2139

Thanks, Markp.
Do you know if ssm2139 is stable in unity gain?
I have no oscilloscope yet, and I´m concerned about stability, cause the circuit has no gain. Apparently, there are no nasties or malfunctions, but I have no way to know if the opa is performing ¨marginally¨. I know that a test would be the best method, but...
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Old 23rd March 2006, 02:33 PM   #5
markp is offline markp  United States
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It is unity stable from what I remember.
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Old 23rd March 2006, 02:54 PM   #6
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by markp
It is unity stable from what I remember.
According to this:

http://www.datasheets.org.uk/specshe...?part=SSM2139S

it is decompensated with a minimum closed loop gain =3 .

It looks like an equivalent of two NE5534 in one package

x-pro
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Old 23rd March 2006, 02:54 PM   #7
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Default ssm2139

Thank you very much, Markp.
Cheers, y hasta la vista!
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Old 23rd March 2006, 03:40 PM   #8
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Default ssm2139

Quote:
it is decompensated with a minimum closed loop gain =3 .
Oooop! thanks, x-pro.
Difficult to obtain info about it; a relatively unknown opa...
Pity that it sounded so good...
Maybe it would be ¨compensable¨ a la opa2228 -a low value cap and matched resistors in the feedback loop and resistor and cap to gnd at the output-, but who knows... it´s in a summing position, with lpf around in differential mode, plenty of components... My desire is to maintain signal path as compact as possible, and should compensation be possible it will need extra components around, a crowded pcb, and the mod will be not simple anymore...
I will try some other unity gain opas and quit ssm2139, saving it for another design; I`m concerned about reliability too.
Thank you very much for your input
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Old 23rd March 2006, 08:19 PM   #9
anatech is offline anatech  Canada
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Hi federico,
I have this in my AD databook from 1992. It is stable for gains greater than or equal to 3. Slew rate is 11V/uS with a voltage noise of 3.2nV/√Hz (5nV @ 1KHz max). I was hoping I had a PDF file for you.

-Chris
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Old 31st March 2006, 06:59 PM   #10
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Default thanks

Thank you, Anatech; I wasn´t noticed the notification of your posting because my mail was saturated with junk.
I believe that now, with the advice of all of you, I have sufficient info to avoid 2139 in that particular circuit.
It`s interesting to note that, being uncompensated, it sounded nice enough despite the low gain...
But, for I have read in this forums, sometimes you never know when an apparently stable -but marginal- design decides to give up and begins to oscillate.
Now I´m playing with an old ADI op275: nice sound, a bit analytical in the treble but with very good mids.

A little question: Is some benefit, soundwise, in buffering the output of a cdp with something more muscular that an opa alone -I believe that op275 is a +/- 40 mA Iout part- ; or the opamp, per se, is ¨macho¨ enough to drive the input of the preamp?
I´m using currently an opa627/buf634 ¨textbook¨ combination as a pre. The input is configured with a 47k shunt resistor at the rca, followed by a 50k pot, then a 2.2 uF MKC ¨Ero¨ input cap, and then a 100k resistor to gnd the opa input pin.

Thank you again. I will save that PMI opamp for a RIAA preamp. I believe it will do good there.

Cheers
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