Mod RME Quadmic I?

[opnordahl]

I am considering modifying a RME Quadmic. The microphone preamp uses the JRC 4580 (NJM4580) op amp both in the gain stage and the output stage. Would OPA1612 be a drop in replacement? It seems like this has been done in a very similar modification of the pre amp section of RME UFX link: RME UFX mod

The preamp uses standard Su'scon (SL series) capacitors all over the board. At least the input and output caps could maybe be replaced by Nichicon MUSE ES 100uF/25V bypassed with WIMA MKP 0.1uF or Vishay MKP1837 (or other alternative). The MUSE ES are non-polarized so there might be other low ESR polarized alternatives (maybe Nichicon Fine Gold).

The gain stage uses 2SA1084-E transistors. These are maybe ok?

The PSU uses Lelo RXJ series (105dg) capacitors. These could be upgraded to Panansonic FR. But I do not know if this is needed.


Output section with 4580 op amps:

Gain section with volume, phanthom, Lo-Cut, phase. 4580 op amp and 2SA1084-E transistors

PSU

Input Su'scon SL standard series capacitors

Distortion measurement using a Motu 828 (2024)


Any suggestions?

 

[JohnRoberts]

What is the problem you are addressing?

JR

 

[opnordahl]

Do you think the OPA1612 would work as a drop in replacement for the JRC 4580 (NJM4580) op amp and sound wise how many of the caps should be replaced (lower ESR caps)? The amp is old and the measurement on the different channels are quite different so some of the capacitors probably is a bit dried out.

The JRC 4580 is not the best op amp so I am trying to squeeze out some extra juice of the mic preamp.

 

[opnordahl]

Also, should I replace the 2SA1084-E transistor with something else?

 

[Khron]

As JR mentioned - WHY?

 

[opnordahl]

Just to try to improve the sound quality a bit. The mic preamp is old so some of the capacitors are not in the best shape. The four channels do not measure the same. I thought when I started changing the caps I could also upgrade the op amps.

 

[Khron]

Ok, how is "sound quality" defined? In numbers since you say the channels "don't measure the same"... What parameters are different?

 

[opnordahl]

Two channels show 4 times higher distortion.

Regarding "Sound Quality" I think this term is very subjective and difficult to quantize. For example I did a test pf a loudspeaker cross over using lower and higher quality capacitors of the same value. The sound measurements were the same but the "sound quality" was different and better for the higher cost capacitors.

For a preamp distortion plots tells something about the "sound quality". Especially for a "clean" preamp like the RME Quadmic. I have a Neve 1073 clone. The distortion plots are bad, but the "sound quality" is still good ( pleasing distortion). So "sound quality" is a vague term and very subjective. Still important.

 

[opnordahl]

Still I do not know if changing the cheap chinese caps to higher quality ones and changing the op amps will change the "sound quality" or lower the distortion before I do a test. So yes, I do not know if this project makes sense yet.

 

[Khron]

For example I did a test pf a loudspeaker cross over using lower and higher quality capacitors of the same value. The sound measurements were the same but the "sound quality" was different and better for the higher cost capacitors.

Double-blind? Confirmation bias is a fickle mistress...

Anyway, stick with the capacitors for starters.

For a preamp distortion plots tells something about the "sound quality". Especially for a "clean" preamp like the RME Quadmic. I have a Neve 1073 clone. The distortion plots are bad, but the "sound quality" is still good ( pleasing distortion)

So that implies that the distortion you're measuring (!!!) in your Quadmic does NOT sound "good"...?

 

[micolas]

Just to try to improve the sound quality a bit. The mic preamp is old so some of the capacitors are not in the best shape. The four channels do not measure the same. I thought when I started changing the caps I could also upgrade the op amps.

A tip: Don't make too many changes at once. Changes must be made in steps, to be able to evaluate the effect of each change.
Otherwise, you will never know what caused the change in results.
To begin with, change caps only in the channels that measure a worsening of the distortions. Keep good channels to have a reference in the new benchmarks.
After that, repeat the tests again, compare the good channels (which have original caps) with those in which you changed the caps.
Analyze test results.
Only now you make the decision whether or not to change the opamps.
If you still want to experiment, then change the opamps on one channel only. You have to make tests as complete as possible, analyze not only the distortions but also the other parameters that determine the audio quality.
You proceed in the same way in steps if you change transistors.

 

[tinn]

I would recommend building another 1073 clone before ruining this excellent piece of gear.

Cheers, Tinn

















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