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McIntosh MC240 Question

I'm the owner of an MC240 amplifier. You can run this amp in a couple of different ways. It can be run direct with gain controls for each channel. In this mode with the gain controls wide open, I barely have to raise the volume of my preamp in order to achieve good listening levels. The alternate way to run it is direct without the amplifier gain controls. In this mode I need to raise the preamp volume control considerably more to achieve a reasonable listening 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. If anyone out there owns this amp, maybe you can explain this away for me. I really like the amp for its all around balanced presentation. I plan to find a nice compliment of tubes for it. Any suggestions in this area are most welcome too. Thanks in advance for any advice or suggestions.
frontier1
jjrenman
279 posts
 
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.
tls49's avatar
tls49
1,351 posts
 
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.
zd542
5,339 posts
 
"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.
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