McIntosh C30 preamp measurements

[radix]

@radix Is there an inverse RIAA built-in weighting function for the QA403 generator?

Yes, there is! You can load a custom weighting and they now ship an RIAA encoder and decoder.

User Weighting and RIAA

A new feature in release 1.6 allows you to specify an arbitrary user weighting to the displayed data. This came as a request from those that are using calibrated microphones that wanted to reverse any imperfections in their mic, along with requests from those measuring RIAA pre-amps. In the...

quantasylum.com

 

[restorer-john]

Yes, there is! You can load a custom weighting and they now ship an RIAA encoder and decoder.

User Weighting and RIAA

A new feature in release 1.6 allows you to specify an arbitrary user weighting to the displayed data. This came as a request from those that are using calibrated microphones that wanted to reverse any imperfections in their mic, along with requests from those measuring RIAA pre-amps. In the...

quantasylum.com

That's awesome. I've got a QA403 landing on my bench next week to play with.

 

[BDWoody]

That's awesome. I've got a QA403 landing on my bench next week to play with.

I wondered when you'd decide to get one of these.

 

[radix]

That's awesome. I've got a QA403 landing on my bench next week to play with.

Congrats. They are really fun to work with. I found that good cables and positioning it a good distance from my laptop helped with the noise level.

 

[Minkybut]

The short non-technical answer: apart from the one obviously bad capacitor, replacing other caps and a few of the opamps (with the same model) made no audible difference. The bad cap was making a very audible difference in one channel. I'd say if it sounds OK to you, its not worth the re-cap.

If you are suspicious of something and have an A/D converter, you could do a measurement just with your computer and REW. Otherwise, a repair shop could measure it for you for likely not too much, to see if there's a problem.

The phono section had a bad cap, it was pretty obviously looking at it that it was bad and it showed in the measurements. That was the only one that mattered. Replacing one cap fixed the problem (this was an AC ground cap in the RIAA filter, not even in the direct signal path).

As I was in there, I did re-cap the power supply to try to remove the 60Hz spike in the phono section, but it did not help. I also replaced the AC filter caps on the input board, and that didn't help either. Basically, there's a very low 60Hz spike on the DC and the phono preamp bumps it up to where it looked bad on the THD measurements, but It's not at all audible. I've not gone overboard adding more filter caps.

I also tried replacing a couple of the op amps (same model) to see if that would help with the 60Hz spike, but that did not make a difference either.


Marc

Did you ever fix that spike?
Also. I was wondering did you replace anything in the eq section? I really use those controls a lot, so I was wondering if the caps or electrolytics in the eq section should be replaced while I have it in the shop?

 

[radix]

Did you ever fix that spike?
Also. I was wondering did you replace anything in the eq section? I really use those controls a lot, so I was wondering if the caps or electrolytics in the eq section should be replaced while I have it in the shop?

I did come up with some additional filtering caps to remove that spike, but decided not to mess with the circuits by adding more filters. In my SPICE models and tests (outside the c30), it did help reduce the 60hz. But I would have had to add a new board with caps between the power supply and a C30 card.

The only issue I have had with the EQ section was a flaky potentiometer. Hitting it with some cleaner and wiggling it over the full range a few times cleaned it up and removed the noise issue Intermittently had. I did not replace the pot.

If it's in a shop, the tech should be able to measure the frequency response and distortion through the EQ section. If it's all in spec, I would not do it. But I could understand replacing them just as preventive maintenance if it's hard to get the unit back into the shop. It's an easy unit to work on, so should be quick and low risk to have a tech do it. In any case, the tech should measure the performance before and after the replacement (if done).

I do sometimes oscillate between "dont fix it if it ain't broken" and "fix it until it breaks."

 

[Minkybut]

I did come up with some additional filtering caps to remove that spike, but decided not to mess with the circuits by adding more filters. In my SPICE models and tests (outside the c30), it did help reduce the 60hz. But I would have had to add a new board with caps between the power supply and a C30 card.

The only issue I have had with the EQ section was a flaky potentiometer. Hitting it with some cleaner and wiggling it over the full range a few times cleaned it up and removed the noise issue Intermittently had. I did not replace the pot.

If it's in a shop, the tech should be able to measure the frequency response and distortion through the EQ section. If it's all in spec, I would not do it. But I could understand replacing them just as preventive maintenance if it's hard to get the unit back into the shop. It's an easy unit to work on, so should be quick and low risk to have a tech do it. In any case, the tech should measure the performance before and after the replacement (if done).

I do sometimes oscillate between "dont fix it if it ain't broken" and "fix it until it breaks."

Ok, thank you sir for your help. I think I am going to go through the pain of sending it away to get checked out. I don't think it sounds very good after getting my SAE preamp back from getting recapped. Thinking about sending it to Bob the Audi Tech. I hate to have to ship it, but I have read he was very good with McIntoshs. There is a place I can drive to take it to fairly near me that is listed as a McIntosh service center in Yorktown VA , but I've never heard anything about how competent they are. I was running a Dbx 10/20 with the McIntosh that made it sound better, but I shouldn't have to do that I don't think. Plus using it made the music sound very compressed... I guess I had gotten used to hearing it that way.
Anyways, thank you for your help very much.

 

[Bob from Florida]

I did come up with some additional filtering caps to remove that spike, but decided not to mess with the circuits by adding more filters. In my SPICE models and tests (outside the c30), it did help reduce the 60hz. But I would have had to add a new board with caps between the power supply and a C30 card.

The only issue I have had with the EQ section was a flaky potentiometer. Hitting it with some cleaner and wiggling it over the full range a few times cleaned it up and removed the noise issue Intermittently had. I did not replace the pot.

If it's in a shop, the tech should be able to measure the frequency response and distortion through the EQ section. If it's all in spec, I would not do it. But I could understand replacing them just as preventive maintenance if it's hard to get the unit back into the shop. It's an easy unit to work on, so should be quick and low risk to have a tech do it. In any case, the tech should measure the performance before and after the replacement (if done).

I do sometimes oscillate between "dont fix it if it ain't broken" and "fix it until it breaks."

Nice write up!

Regarding the 60 HZ spike, are you confident your test setup is not picking that up via coupling to AC wiring or internal components coupling to the power transformer? If so, no amount of capacitors added will completely eradicate things. The empirical test I rely on is putting my ear close to the speaker - if I can’t hear any hum, I am happy.

 

[Bob from Florida]

That's awesome. I've got a QA403 landing on my bench next week to play with.

I would be interested to hear your “second opinion” of the QA403 - @radix seems to like it! Visiting the web site almost convinced me to buy one, although I probably should wait another couple of years to get one for retirement hobby work. Just in case an “improved” variant is available.

 

[radix]

Nice write up!

Regarding the 60 HZ spike, are you confident your test setup is not picking that up via coupling to AC wiring or internal components coupling to the power transformer? If so, no amount of capacitors added will completely eradicate things. The empirical test I rely on is putting my ear close to the speaker - if I can’t hear any hum, I am happy.

I don't remember the exact process I went though, but I'm pretty sure it's a real 60 Hz tone in the power lines that bleeds into the NE5534 via the Vcc-/Vcc+. IIRC, I saw it both on the QA403 and my oscilloscope. It was present even when I tested the main power supply disconnected from the daughter board.

I usually also null-test the QA403 with the cables I use and some pass-through shunts to make sure my environment is OK. I cannot swear I did that (too long ago to remember).

I cannot hear it. IIRC, it's only an issue with the phono input as the NE5534 gain is much higher. For a line input the 60 Hz is < -100 dB. For the phono it is <-70 dB.

I do not know the exact numbers, but I suspect the NE5534 is running around 60-80 dB gain at 60 Hz (40dB or so for phono level + 20 dB for RIAA, give or take a bit). Those are all guesses. So that appears to be allowing about 25-30 dB more 60Hz power supply ripple into the audio output.