Op Amp Replacement/Compatibility Question on an Audio Preamplifier

Thread Starter

The Dragon

Joined Feb 8, 2021
5
New to the forum. I work at NASA during the day as a GNC engineer/manager. I have several hobbies and one is building and repairing home and pro audio equipment for many years now. I learned several years backt that some designs use opamps in the feedback loop to manage DC offsets. With this latest project, I have run into a situation with an opamp I have never come across before.

I have a vintage Nakamichi CA-5AII preamplifier which uses several of the old µPC811 opamps as servos. Does anyone have an idea which current opamp, or range of opamps, will replace these? They are not directly in the signal path, however I think these are causing some issues with sound quality. It also has some NE5542s in the phono section that are in the actual signal path. These seem to be fine. Any help is much appreciated.
 

wayneh

Joined Sep 9, 2010
17,152
New to the forum. I work at NASA during the day as a GNC engineer/manager. I have several hobbies and one is building and repairing home and pro audio equipment for many years now. I learned several years backt that some designs use opamps in the feedback loop to manage DC offsets. With this latest project, I have run into a situation with an opamp I have never come across before.

I have a vintage Nakamichi CA-5AII preamplifier which uses several of the old µPC811 opamps as servos. Does anyone have an idea which current opamp, or range of opamps, will replace these? They are not directly in the signal path, however I think these are causing some issues with sound quality. It also has some NE5542s in the phono section that are in the actual signal path. These seem to be fine. Any help is much appreciated.
Welcome to the AAC forums.

Here's a starting point:
https://www.renesas.com/us/en/document/dst/upc811-datasheet

What do you mean by "...uses several of the old µPC811 opamps as servos". You mean to control some servos?
 

Wolframore

Joined Jan 21, 2019
2,273
A DC servo circuit is used to null a DC output offset. I would replace them with TL071 It will be pin compatible, did you try adjusting the offset null? You may need a scope to do it correctly.
 

AnalogKid

Joined Aug 1, 2013
9,254
What do you mean by "...uses several of the old µPC811 opamps as servos". You mean to control some servos?
No. Servo the input DC offset such that there is none at the output, all with no coupling capacitors. If you are a golden-eared wonder who believes that all capacitors are evil, you connect the amplifier circuit output to the inverting input of an opamp integrator stage, with the non-inverting input connected to GND. The integrator output pees the low low lowpass filtered output signal into the amplifier input as a very small offset-correcting current.

Given that this a Nak circuit, it could be a direct-coupled tape head or photo cartridge preamp, with a servo to prevent any DC through the head, again with no coupling capacitor.

ak
 
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Thread Starter

The Dragon

Joined Feb 8, 2021
5
A DC servo circuit is used to null a DC output offset. I would replace them with TL071 It will be pin compatible, did you try adjusting the offset null? You may need a scope to do it correctly.
If I can get the DC offset to null out properly (which I have not tried yet), I suppose it would not hurt to leave these in. I can't help but wonder if replacing them with a newer part might pay dividends in the long run. But, wondering is not always a good thing. Perhaps I should leave well enough alone. The preamp seems to be working, but not up to what I would expect for a high end dedicated audio preamp. Perhaps a re-cap is in order first. That may prove to be rather expensive since most of them are high end Silmic Cerafine electrolytics.
 

Thread Starter

The Dragon

Joined Feb 8, 2021
5
Seems unlikely.
Why do you think that?
The schematic shows the main amp section output signal path bypasses the µPC811C to the output jacks. The main output is connected to pin 3 through a resistor. Pin 6 is routed to the feedback loop of the cascoded input section which is shown at 0VDC. Maybe I am reading this wrong. I can drop a copy of that part of the schematic to an attachment if it would help.
 

Thread Starter

The Dragon

Joined Feb 8, 2021
5
That would be helpful.
So why do you think that particular op amp is a problem?
Fair question. This preamp has an issue of some sort. No way this thing sounded this lame when new. It is hard to describe the sound. In fact "hard sounding" is exactly the way I would describe it. Something about the upper midrange to high frequencies are not very clean sounding. That and the soundstage is closed-in and artificial sounding. This is not the territory of Nak equipment of this vintage. To your question, I had not heard of this µPC811C before. Even though it is in the feed back loop, it can impact sound quality if something is not right. I need to go through the null procedure first, which I will attempt this weekend. The model number is Nakamichi CA-5II. I also have the year prior model, the CA-5, and it sounds completely different. Night and day really. I believe the CA-5 is a Nelson Pass design. The CA-5II is described as a "modified circuit" in the marketing literature of the day. Of course that could mean an opamp troll designed it as a joke. Who knows....
 

AnalogKid

Joined Aug 1, 2013
9,254
Here is a part of the schematic for those who would like to see...
Pretty straightforward circuit (only 11 posts!). No surprises in the audio channel, but the floating voltage regulator is interesting.

Your experience base clearly is greater than mine, but

a) The servo corner frequency is 2.2 Hz. That's a bit higher than I've seen in other gear.

b) My impression (and lotsa reviews) of Nak gear is that it is "bright" or "clean" sounding, as opposed to warm or sweet. Interested in your opinion.

Separate from that - do the two channels sound differently?

ak
 
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sarahMCML

Joined May 11, 2019
39
uPC811 found on ebay
An equivalent might be functionally similar other issues is hard to guess because compatibility can be anything.
https://datasheetspdf.com/pdf/545768/NEC/UPC811/1

'Which op amp used in audio amplifiers uses servo to reduce DC offset?'
Possibly the LF411
Renesas were still making those uPD811 opamps up to mid 2019 according to its data sheet, so you should be able to find new ones somewhere.
 
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