McIntosh MC240 Question

I took a look at the schematic, and as JJ and TLS indicated the design intentionally provides different sensitivities on the two sets of inputs, to provide flexibility for differing applications.

Zd, looking at the schematic I see that in this particular case there is no difference in the active circuitry that is used for the two sets of inputs. The only difference is in the configuration of some resistors and pots (potentiometers, i.e., variable resistors) that are located right at the inputs of the amp.

As to which set of inputs is best to use, if in fact it makes any difference, it's probably not predictable and would have to be determined experimentally. It depends on various design characteristics of the preamp, including what part of the range of its volume control it performs best at; on the output level(s) of the source component(s); on whether or not any ground loop effects occur between the preamp and power amp, that might cause or contribute to low frequency hum or high frequency buzz (the severity of which might be reduced by increasing the signal level between the components); and on the condition of the 50+ year old pots in the power amp. There are pots on both sets of inputs, btw, a balance control being provided on the low (2 volt) sensitivity inputs, and independent gain controls being provided for each channel on the high sensitivity inputs.

Regards,
-- Al

I really appreciate the thoughtful responses to my question. I'll start running in the direct mode. If anyone owns this amp and has knowledge of tube compliments or any other improvements I can apply, let me know. If I have 1 minor quibble about it, the bass is a little lean. Perhaps a particular tube on the input side would beef it up a little. I'm astonished at how quiet this amp is.

"I would have thought the opposite ... that increasing the gain at the power amp level would sound cleaner as opposed to increasing gain at the preamp level, which would be one level further away from the power source. In a single line source system, you could run the source directly from the power amp ... yes?"

Almost certainly, no. Think of it this way. Would you go out and buy 2 preamps? That's what's going on here. The active gain controls inside the power amp are essentially a preamp. That's why he's getting almost no volume control. He's turning the volume on the preamp inside the amp all the way up leaving almost no adjustment.

A far as using the preamp in the amp or his stand alone preamp, there's no real way to tell what one will sound better. If I had to guess, I would go with the external preamp, and bypass the one in the amp. Most likely the external preamp is a better design. Especially the volume control. The reason they put gain controls on the amp is not to bypass a preamp, but to allow better volume control in situations like multi room installs, home theatre and possibly biamping. But in a high end audio system, I can't think of a reason that you would ever want to use 2 line stages and link them together. You're putting another component in the signal chain for absolutely no reason.

Zd542 ... interesting. I would have thought the opposite ... that increasing the gain at the power amp level would sound cleaner as opposed to increasing gain at the preamp level, which would be one level further away from the power source. In a single line source system, you could run the source directly from the power amp ... yes? I have no experience with the MC240.

Rich

"I'm wondering what the explanation is for the difference given that gain controls wide open or running without those controls should be the same input level."

You're amplifying the signal 2x. First in the preamp and then again when it reaches the active gain stage in your amp. I wouldn't do it that way. The system should sound much better if you just use the preamp to control the signal gain and not the one in the amp.

In addition to Jjrenman's comment, the input sensitivity with the gain controls is 0.5v, and the one without is 2.0v, so the one without will require more output from the preamp for the same volume level.

I'm wondering what the explanation is for the difference given that gain controls wide open or running without those controls should be the same input level.

The same input level for both would only be true if the input impedance of the input pots (amplifier gain controls) was the same impedance of the load resistor on the direct inputs.

Looking at the schematic shows that they are not the same and that the input sensitivities are different.

I have an Mc250 which is I think the solid state version of yours. I too have 2 gain controls which enable me to use a preamp,active or passive or go direct from a CD. I have always interpreted the gains as being like volume pots and used them that way into a pretty or direct. I have a manual and can check later but I know mine operates as you described too.you might want to check for a manual online and see what it says if you don't have it. I think Roger Russell's site has some.