Mixed driver tube for Mcintosh MC275

That may be true as I do not have the gain specs in my manual. It does say that the input sensitivity is 1.2 volts unbalanced and 2.5 volts balanced. The only time I would use XLR is if I needed to use a longer interconnect from the preamp to the amp. It is not a fully balanced amp so what benefits are there to the balanced connection other than if you need a long run to your preamp? it makes absolutely no sense other than spend more money that I could put towards more records.

Well Tzh21y, I don't know how you worded your inquiry to the McIntosh tech, nor did I hear his response. However, I'm a little suspicious of his words as you reported them

their tech support told me that V-1 is definitely in the signal path when using balanced cables on the Mark IV

Could he perhaps have meant "the V-1 tube is (still) in the SINGLE-ENDED signal path, even when you are using balanced cables"? -- which would be true. But the V-1 tube is definitely NOT in the balanced signal path. EVER! Further, "using balanced cables" has nothing to do with whether the V-1 tube is in (even) the single ended signal path or not. It ALWAYS is. It has to be. What determines whether the V-1 tube is in the (single-ended) signal path (before the signal goes to the rest of the amp) is the position of the BAL/UNBAL switch.

And I can tell you this: I'm right now looking at the McIntosh service manual for the MC275 Model IV (which I ordered from McIntosh after purchasing my amp) and the schematic clearly shows there is NO WAY for the balanced input signal to go through the V-1 tube (and on to the rest of the amp,) when the the input switch is in the BAL position. It's not a matter of whether I believe you. Frankly, I think there was some miscommunication going on when you spoke with whomever at McIntosh.

You can order a service manual from McIntosh for $10, or (now that the Mk IV is not a current model ;--) download a pdf version for free at this site (which I just tested for you): http://safemanuals.com/user-guide-instructions-owner-manual/MCINTOSH/MC%20275%20MK4-_E

OR! you can prove it to yourself by sliding the input switch on your amp to the 'BAL' position. I guarantee you won't hear anything out of your amp (which you WOULD if the RCA signal could get to the rest of the amp (via the V-1 tube) with the input switch in the BAL position.

Furthermore, you can 'double-prove' it to yourself by doing the reverse: unplug the RCA cables and plug in balanced cables (carrying a signal of course) and slide the input switch to UNBAL. Again, I guarantee you'll hear nothing out of the amp. The balanced input signal neither goes to, or through V-1. Why would it? There is absolutely no reason for it to do so when driving the amp with a balanced signal -- in fact, going through V-1 would actually screw up the balanced signal!

If you still think driving the amp single ended sounds better than driving it balanced (assuming you have balanced main outputs available on your preamplifier, and have made an honest A-B comparison,) then it has to be for one or another of the reasons I suggested previously, and you should investigate the cause. If, as you say, "others" feel the same way, then they should also investigate! As for balanced cables being a "waste of money" unless the amp has balanced circuit design, you are simply dead wrong! First of all, studios have been using XLR's since 'the dawn of time' because of their proven superior performance, regardless of the circuit design of the various equipment they use. And second, any conventional push-pull amplifier, be it SS or tube, requires a balanced signal in order to amplify! If you feed it a single ended signal, then you need to first create a copy (splitter) and then reverse the polarity of one of the copies (inverter) and then feed that BALANCED SIGNAL to the amp, or it won't amplify! This is what the V-1 (splitter/inverter) circuit does. Nothing wrong with that except it is a lower impedance input, and adds another layer of processing (and noise, and distortion) to the signal. However, if you are already supplying the amp with a balanced signal, the V-1 tube is not needed, WHICH IS WHY IT REMAINS OUT OF THE SIGNAL PATH WHEN DRIVING THE AMP WITH A BALANCED SIGNAL!

It would have been fun to listen in on your conversation with the McIntosh tech, just to discover where the miscommunication started. In any case, I stand by all my previous remarks regarding the basis for your BAL vs. UNBAL listening experiences. So there YOU have it! ;--)

Tzh21y
You can find schematic of MC275 re-issue in http://farm5.static.flickr.com/4037/4204605299_bdfa12827f_b.jpg. You can study from the schematic that V1 is not in the signal path.

Okay, after viewing the schematic, it is clear that V1 is not in the signal path. Then what am I hearing? Voodoo? I am obviously crazy! Now I am convinced as to not buy anymore tweeks because my hearing is somewhat flawed.

I thought I noticed a more full sound with the other tubes I used. This is crazy. I am going to put my one pair of balanced cables back into the amp and see what I hear. I did ask the tech and he did say it was in the signal path and would have an effect on the sound. I felt I was pretty clear on how I asked the question. My mind is obviously playing games with me. This is a very informative thread. Maybe I will try different balanced cables and see what happens. There I go again, tweeking.

Tzh21y --

1.) I don't think there is anything at all wrong with your hearing! I'm 68 and I KNOW my hearing isn't what it used to be (to say the LEAST!!) however my ability to LISTEN is better than ever ;--) I wish I could go over your system with you in person, because I know I could find out what is keeping balanced operation from sounding completely marvelous and far superior to single-ended.

2.) My long experience talking with McIntosh technical people (even those at the top) has been 'highly unsatisfactory' to be diplomatic about it ;--) You would have better luck talking with one of the engineers; at least they understand English, but they are harder to reach. See if you can get hold of Ron Evans, V.P. Engineering.

I have a couple of system specific comments:

1.) JPS interconnects are OK (as in: decent)
2.) Everything "Cardas" sucks! Way too much capacitance (especially for your Merlins) and terrible time-alignment. There may be a couple pieces of equipment on the planet that one could connect with Cardas cables, but nothing I'd want to own! ;--))
3.) I'd be interested in knowing what kinds of (good) tubes you own for the amp, and -- I almost forgot -- what (if any) tube changes you've made to the C220?

Neil
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