McIntosh C30 preamp measurements

[radix]

EDIT: The LF/HF Filter button was engaged, which caused the bass and treble rolloff. I'm making new measurements of the multitone and gain/THD vs frequency.

I just picked up a 1-owner McIntosh C30 (1985-87). I believe this was their first preamp to use transistor switching instead of relays. I got it from the son of a guy who collected (or should I say hoarded) hifi gear. He said his dad had 6 pairs of L100's. He has the Mc MCD7000, their first cd player, some Cerwin Vega speakers, several tube amps and really a whole garage full of stuff.

For being about 40 years old, it looks in great shape and isn't awful performance. the only physical issue is a loose fuse holder. I think I'll get a rebuild kit and recap it, then we where I am again (anyone have suggestions where to get a rebuild kit, or are the ones on ebay ok?).

Here's some measurements with my QA403 going into the CD input and out from the Main output. Loudness is set to "flat" and the 5-band EQ is disabled. The unit is rated at 0.007% THD 20-20k. The frequency response is supposed to be within 1/2 dB 20-20k.

Here's 1kHz at 0dBV. The volume control is at 50%, which looks to be about unit gain (1 click above 50 is actually a little closer to unity). This actually looks pretty good!

Here's the same thing at 6dBV (2V). Again, pretty nice.

This is with the volume control at 75% (eyeballing it).

And if I go to 75% (1 more click on the volume control), the distortion jumps up. I'm not sure what causes this yet. EDIT: This is likely clipping because the input level (6 dBV / 2V) was too high for more gain.

Here's a multitone. You can see that < 50Hz and > 10 kHz there is a significant rolloff. This is done with the volume control at 50% (about unity gain). EDIT: This is with HF/LF rolloff button engaged.

And here's the gain vs frequency and THD vs frequency charts. Both done at 50% volume. EDIT: This is with HF/LF rolloff button engaged.

 

[restorer-john]

Nicely done.

The frequency response sweeps should be done at maximum volume position, with the input level set to achieve the rated output. Based on the 22dB rated gain and 200mV sensitivity, that should be around 2.5V out.

 

[radix]

Nicely done.

The frequency response sweeps should be done at maximum volume position, with the input level set to achieve the rated output. Based on the 22dB rated gain and 200mV sensitivity, that should be around 2.5V out.

Ok, I'll give that a try. The rated maximum output is 10V on the main out and tape out.

 

[restorer-john]

Ok, I'll give that a try. The rated maximum output is 10V on the main out and tape out.

So crank the volume, feed it 200mV at a line input and measure the FR and THD. I'll bet the response flattens right out.

Sure, it's not how we use our volumes, but it is the standard for measuring FR.

 

[radix]

Here is the fixed multitone, and gain/thd vs frequency. These are run at 6dBV (2v) input. Also, without the LF/HF filter, the noise in the last octave is gone too.

I am not sure what those dropouts are. I think they are measurement errors with the QA403 automated test. I had them in the THD v Freq too, until I turn off the auto-ranging input, but that was not an option with the FR test.

 

[radix]

Here's the 1kHz at -14 dBV and full volume. I also did a multitone. The 1 kHz is well below the rated 0.007% THD. This shows the 22dB gain @restorer-john cited.

 

[radix]

Here's the phono input. I used -50 dBV output with the volume at max. I applied RIAA weighting in the QA403. The left channel looks OK, but the right gain is low. This looks like it will need some investigating.

Here's the same thing but at -48 dBV input and 50% volume.

 

[restorer-john]

Excellent, well done.

The QA-403 glitches are interesting- they appear on precisely the same spot frequencies which suggests to me, there are only a small number of stepped frequencies it is testing.

How many points is the sweep?

 

[radix]

I checked all the other inputs and the 5-band EQ with the MT pattern. They all looked good. Seems just the phono right channel needs some attention.

 

[radix]

I used the defaults (shown below), which appear to be 3 per octave.

 

[levimax]

Transparent is nothing new...

 

[radix]

I ran the test a few more times. Watching it, sometimes the FFT does not seem to complete. After running it 3-4 times, I did get a good FR curve.

 

[radix]

Transparent is nothing new...

This was more about what I would find in a 36 year old preamp. It looks like the only issue the the R channel of the phono input.

 

[restorer-john]

There's 10 octaves between 20Hz and 20KHz, so that would mean only 30 points over 20,000Hz with only 2-3 points in the last 10,000Hz. The last measurement point would also be off the chart.

The Hz per step at 500Hz seems strange as that would mean it makes a measurement at 20Hz and the next one at 500Hz, but it should make 3 before 40Hz... Usually that would be for a linear sweep and should grey out if you check log. I would think.

I'd crank up the number of points.

My old AudioLab hardware takes 496 measurement points over a 20-20k log sweep.

I used the defaults (shown below), which appear to be 3 per octave.

View attachment 290136

 

[radix]

This is the right phono channel. That large cap (C316, 470 uF) looks bad. it is in the ground path for the RIAA filter. I suspect it's not conducting at all and thus floating the filter, which is what's resulting in the about unity gain I saw.

Vcc is +17.68V / -17.88V. I'm not sure if that's the regulators (7818/7918, LM340T) or the PS caps. The regulator outputs are +17.85/-18.05, so looks like the 7818 is a little weak. The input voltages are +31.24 / -31.24. All the PS caps look OK, except maybe one 0.1uF ceramic input filter on the negative side might have a few flakes coming off a lead.

I ordered a restoration kit and some new bulbs for it. I'll take out that C316 and test it. I'll probably replace the power supply caps too, since I'll have them.

 

[radix]

It's going to be a few days until I get the replacement cap for the phono section, so I setup the system. I put photos in a different related thread. Once I get the phono fixed, I have a B&O turntable to put on it.

Retro speaker recommendations?

Hello, I'm putting together a vintage McIntosh stack for a casual listening space. Why? I've always wanted it and want to do it while I can. The components are mc2205, c30, mr74, B&O turntable, volumio rpi4 streamer. I'm looking for speaker recommendations for it. I'd prefer something that...

www.audiosciencereview.com

 

[radix]

I replaced that bad-looking cap and it fixed the problem with the right phono input.

The main limiting factor on the phone stage is 60 Hz noise, which you can see a bit in the above multi-tone. It looks like the Vcc and Vdd have about -110 dBV 60Hz on them, so when amplified 40 dB its that spike you see. There is not a good 60 Hz filter in the circuit. The input board has a 10-ohm resister and 1 uF supply cap (16 kHz low pass).

I re-capped the power supply, but that made marginal improvement to the 60 Hz noise. I replaced the old NE5534N with a low-noise NE5534AP, but again only marginal improvement.

The line-level inputs are good. This is with the volume control at max and -14 dBV input for a 2.5V output.

This is the headphone amp at max volume (-14 dBV input) into 330 ohms and 33 ohms

And the THD vs frequency for the headphone amp

I would like to kill that 60Hz spike.

 

[Minkybut]

EDIT: The LF/HF Filter button was engaged, which caused the bass and treble rolloff. I'm making new measurements of the multitone and gain/THD vs frequency.

I just picked up a 1-owner McIntosh C30 (1985-87). I believe this was their first preamp to use transistor switching instead of relays. I got it from the son of a guy who collected (or should I say hoarded) hifi gear. He said his dad had 6 pairs of L100's. He has the Mc MCD7000, their first cd player, some Cerwin Vega speakers, several tube amps and really a whole garage full of stuff.

For being about 40 years old, it looks in great shape and isn't awful performance. the only physical issue is a loose fuse holder. I think I'll get a rebuild kit and recap it, then we where I am again (anyone have suggestions where to get a rebuild kit, or are the ones on ebay ok?).

View attachment 290105

Here's some measurements with my QA403 going into the CD input and out from the Main output. Loudness is set to "flat" and the 5-band EQ is disabled. The unit is rated at 0.007% THD 20-20k. The frequency response is supposed to be within 1/2 dB 20-20k.

Here's 1kHz at 0dBV. The volume control is at 50%, which looks to be about unit gain (1 click above 50 is actually a little closer to unity). This actually looks pretty good!

View attachment 290106

Here's the same thing at 6dBV (2V). Again, pretty nice.

View attachment 290108

This is with the volume control at 75% (eyeballing it).

View attachment 290109

And if I go to 75% (1 more click on the volume control), the distortion jumps up. I'm not sure what causes this yet. EDIT: This is likely clipping because the input level (6 dBV / 2V) was too high for more gain.

View attachment 290110

Here's a multitone. You can see that < 50Hz and > 10 kHz there is a significant rolloff. This is done with the volume control at 50% (about unity gain). EDIT: This is with HF/LF rolloff button engaged.

View attachment 290111

And here's the gain vs frequency and THD vs frequency charts. Both done at 50% volume. EDIT: This is with HF/LF rolloff button engaged.

View attachment 290129

View attachment 290113

Hey Radix, I have a c30 also. That's cool you can rest all these specs on the unit. I was wondering if you can tell me in nin technician terms if you saw much difference in performance according to your test equipment after doing the re-cap? Other than the phono section which I don't use on mine.

 

[radix]

Hey Radix, I have a c30 also. That's cool you can rest all these specs on the unit. I was wondering if you can tell me in nin technician terms if you saw much difference in performance according to your test equipment after doing the re-cap? Other than the phono section which I don't use on mine.

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

 

[restorer-john]

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