McIntosh MA-5100: Surprises galore! (a little long)

Posted by M Jarve (A) on January 8, 2005 at 09:04:58

Yesterday I received my first piece of McIntosh gear, namely the MA-5100 preamplifier/amplifier. All and all, it combined a lot of what I expected with some things I did not. While waiting for it to arrive, I was almost more excited about looking at how it was designed (under the hood, so to speak) than I was about how it would sound. This is because, as a rule, McIntosh is supposed to sound good. Indeed, it does.

In any event, here are some excerpts from the email I shot back to the fellow I got it from last night after playing with it at a friend's house. This email more or less constitutes my review and initial impressions of the unit. I will allow the original owner to make himself known, should he choose to.

Excerpts from the email follow:
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I just got back from my friendâ€™s house and listening to the Mac. First I should tell you that it certainly was not quite what I expected, in a good way though. Indeed, the popular belief that this is a capacitor coupled amp is seemingly false, unless they are referring to the pre-amp section only. This is quite as direct coupled as you can get. This also changed my diagnosis of a bad output cap to a bad DC balance circuit, and indeed, I found the DC output problem to be temperature related. When the amp chassis was 23F, DC output on the right channel (as well as the left) was 0.1mV; awesomely good. However, as the amp warmed up to about room temp, the right channel began to ramp up DC voltage starting slowly at about 900mV, then quickly climbing to 27VDC. This, all the while the left channel had stabilized at about 15mV.

I found a resistor in the right channel amp was running out of spec at very high temps (measured 190F) and this caused the DC balance to go wild. After seeing that problem, I could control the DC by (believe it or not) turning up the volume. As the resistor ran cooler at higher volumes, the DC stabilized to an excellent 16mV for the right channel; enough to run it to the speaker without a blocking cap. Tomorrow I will be going through my parts bin to find two new 3,600-ohm, 10 watt resistors. I plan on replacing them in both channels, although the left channel unit seems fine. It does seem that one of the output transistors had been replaced at some point, as it is different than the others. I looked it up, and it was a suitable replacement, but it did indicated that perhaps it has had this DC problem for a long time, and past attempts at repairing it (by replacing the obvious: the output transistor) have failed.

I can say without hesitation that the construction of this unit is extremely clean and well done. Wires are bundled nicely, but not wound together, making replacement very easy. I was also very surprised to see the use of real glass for the front panel. Although more brittle than Plexiglas it is more resistant to scratching. The power supply is one of the most unusual ones I have seen. Where as most power supplies have a medium and low voltage secondary (i.e. 35V-0-35V for the power amp, and about 12V for everything else), this uses the standard 35V-0-35V for the power amps, but an incredible 115V-0-115V for everything else! It uses high-value dropping resistors to bring the voltage down to an appropriate level, but, holy-moly, Iâ€™ve never seen anything like that before, except in a tube amp. This has the effect of creating virtually no distortion, but the resistor that is failing in the right-channel power amp is one of the ones that drops voltage; in that case from 115VDC to 1.3VDC.

Now as to sound. I used my little Wharfedale Diamond 7.2 ALE speakers first with, then without a blocking cap (after I found out how to control the DC problem). The overall sound is quite warm and mellow, with the highs more subdued than they were on the 930, and even more than my 430. Bass is strong, but not overwhelming, and the amp seemed to maintain good control over the Diamondâ€™s woofer; this is a difficult task for many amps with a low DF. The Diamondâ€™s themselves are just warm of neutral on my QSC power amps (which is as generic a sound as you can get) and with my h/k 430, I normally run the tone controls at flat, with the Loudness/Contour engaged. With the 5100, comparable sound was achieved with the bass at center, the Loudness/Contour on, and the treble at about the 3 oâ€™clock position. The highs almost seemed rolled off, in a way. This is likely because of the aged caps in the preamp section, and it seems many people recommend replacing them anyways. The 5100 had no noticeable hum or noise throughout the entire range of the volume control, which is a feature I have really come to appreciate from my 430. In many ways, it sounds like a warmer, more powerful version of my h/k 430, which I consider to be one of the best sounding amps ever made. After repairing the DC balance, and replacing some of the old caps in the preamp, I would not be surprised to see it meet or exceed the sound of the 430. It just needs to brighten up a tad. A very good sounding unit with character and maturity, and worthy of a place among my permanent collection.

[Today] I will attempt repairs and calibration for bias (curse McIntosh for not indicating the bias voltage in the service manual!!!), and perform more extensive listening with a wider variety of speakers, including my Advent/1 system, and 1963 Voice of Music model 65 speakers, as well a the headphone tests with my Koss Pro4-AAâ€™s.
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Those are my initial thoughts and impressions of the MA-5100. I was surprised to see it was not a CC power amp, and I can only assume that I had read some other posts regarding it incorrectly. Indeed, the pre-amp is capacitor coupled to the power amp, but I do not believe that is unusual, even today. I can see where, perhaps, one could mistake it for a CC power amp, as there are two large black caps on the chassis, but those seem to be multi-section power supply caps, not output coupling caps. I may still be wrong about it though, seeing as how until yesterday I would have laughed at the thought of finding +/- 145VDC in a solid state amp, feeding mostly the lower voltage components no less!
/Mike

My h/k site

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Topic - McIntosh MA-5100: Surprises galore! (a little long) - M Jarve 09:04:58 01/08/05 (18)

Ok I bow to you my friend you win :-) - crooner 19:53:36 01/08/05 (2)

It was fun, and we both win! - M Jarve 20:27:05 01/08/05 (1)

very true (nt) - crooner 21:41:42 01/08/05 (0)

So you like-a-da Mc - airtime 10:48:44 01/08/05 (0)

It is capacitor coupled... - crooner 10:43:52 01/08/05 (10)

Not necessarilly... Scan of schematic. - M Jarve 12:08:07 01/08/05 (9)

I don't see a cap in the output - Scholl 14:25:55 01/08/05 (7)

Full sized scan. - M Jarve 15:39:10 01/08/05 (6)

I see an inductor in the output...L201 & L202 hence not direct coupled - crooner 18:28:17 01/08/05 (5)

Re: I see an inductor in the output...L201 & L202 hence not direct coupled - Crazy Dave 09:00:27 01/09/05 (1)

keep us posted on it. Rich Links has one also!!! (nt) - crooner 11:19:12 01/09/05 (0)

Re: I see an inductor in the output...L201 & L202 hence not direct coupled - Scholl 05:30:05 01/09/05 (1)

It should be noted that the Japanese took this concept to the extreme... - crooner 06:05:21 01/09/05 (0)

It's still direct coupled, inductors or no. - M Jarve 18:36:44 01/08/05 (0)

Re: Not necessarilly... Scan of schematic. Also... - M Jarve 12:16:30 01/08/05 (0)

You are absolutely amazing.... - DavidLD 10:10:21 01/08/05 (2)

Re: You are absolutely amazing.... - Grateful 17:26:49 01/08/05 (1)

Re: You are absolutely amazing.... - Grateful 17:32:55 01/08/05 (0)

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