You may be (are) over analyzing. The 2.5mV input sensitivity MM phono spec is bog standard 40dB gain. The Output 2 spec adds the maximum linestage gain. The 80mV input overload spec is generous so a LOMC run through a 12x SUT should be perfect. You say the 24x is fine…to me it seems like the volume range would be better at 12x, but your results speak for themselves. Chill, enjoy!
Deciphering Integrated Specs - Step Up Matching
I have always struggled to make sense of my McIntosh MA352 hybrid tube/ss integrated amplifier’s published specs (see below) when trying to match, at least on paper, with a Moving Coil step-up device. In particular, the "Phono to Output 1 and 2" specs of 84dB and 55dB always throw me off as 40dB seems to be a more traditional voltage gain spec for a MM stage.
Of late, I have been using a Quadratic MC-1 SUT in conjunction with a Dynavector 20X2L cartridge (0.3mV output) with truly excellent results, really what matters in the end. The MC-1 has 22dB (12x) and 28dB (24x) taps. The latter seems to work best with the Dynavector and doesn’t present any volume knob/headroom problems.
I am considering switching to another moving coil cartridge with an 0.4mV output. But, as mentioned, I always struggle with how to interpret the integrated’s specs, and thus which gain setting to use on the SUT. I generally prefer the higher gain setting so would like to keep using that whenever possible.
Based on the attached specs, setting aside specific cartridge loading determinations, what is the best/simplest way (if there is one) to evaluate the MA352’s phono stage for matching cartridges with SUT gain options (e.g., 22dB or 28dB) so as not to cause any obvious matching or overload issues?
For example-
- Does the 2.5mV sensitivity imply a phono stage output gain of 40dB?
- If indeed the phono stage output gain is 40dB, does adding 28dB from the step-up device, for a total of say 68dB on its face present any particular problems when used with an MC cartridge with say 0.5 mV output or lower?
- Would a cartridge with 0.4mV output overload the unit if used with a 24x tap (would think not as 9.6mV seems to be well below the 80mV maximum input signal?
For what it’s worth, my understanding is that "Output 1" is the speaker output and “Output 2" is the output of the preamp section of the integrated amp - but I’m not completely sure.
Any thoughts/suggestions would be greatly appreciated.
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- 12 posts total
The only data that might help to answer your many questions are in the section headed by”voltage gain”, and there are a few different possible interpretations of those data. However, I’d start with your SUTset to 1:12, not 1:24. You can always choose the higher gain setting if 1:12 proves inadequate. Since your linestage seems capable of adding unusually high gain for a linestage, you might even get by with no SUT, depending upon the correct interpretation of the given info. Do you have an owners manual? If so, read it. |
Yeah, since the spec sheet states MM sensitivity of 2.5mV, and the line stage states sensitivity of 0.25V (= 250mV), that is a factor of 100x which is 10x * 10x = 20dB + 20dB = 40dB. That’s assuming the MM output pipes into the unbalanced input line path. Typically input sensitivity means the signal level required to produce full rated output. I’ve used SUTs configured to produce 10mV into the MM stage (even up to 12mV once, I think). It can in fact sound great. You’ll lower the relative noise floor this way, but you are cutting into the MM stage’s overload margins. Sometimes I’ve noticed a bit of harshness when pushing the MM stage like this. If the MM stage is solid state, you might have lower margins than with tubes (but it really depends). As others sad, it’s a give-and take compromise. In *most* cases, you want to shoot for 5mV. But if your system is lacking line gain and you have to really push the volume to get where you want, you might prefer the 0.4mV on 24x tap - and that’s fine! In "most" systems, the 12x tap would be more ideal. Also when doing the calculations, higher step-up ratios are not without losses - due to loading (Ohm’s Law). The losses depend on the coil ohms. So for example, if your 0.4mV cart was 12 ohms (e.g. Benz Glider, Wood), it sees a load of 82 ohms when used with the 24x taps (82 = 47000 / 24 / 24). Then the loss is calculated as 20* log( 82 / (82 + 12) ) = -1.18dB. So you lose just over 1dB from the load the cartridge "sees". Losses are usually bad, but this -1dB actually helps you on the overload margins. The Quadratic is a nice SUT! Really sweet bass, very clean midband, slightly relaxed top end. Just one more note - having ample overload margin is so important because the peak levels off the record can be much higher than what the rated cartridge output indicates. I don’t have any measurements here but IIRC you can expect at least 6dB (4x) higher than the rated output, and possibly more. At 10mV (calculated) and MM overload at 80mV, that's a factor of 8x which should be "ok". Of course, the overload is frequency dependent ;) |
@mulveling, great post. |
- 12 posts total