Mixed driver tube for Mcintosh MC275

Tzh21y, do you know how to read schematics? I'm guessing you don't, because if you can make a statement like "I don't care what the schematic says" etc., then you'll never understand that there is a cause-effect relationship behind ALL audio experiences. Whether a person prefers one kind of sonic experience over another is their call! But that doesn't change the science.

I've used all kinds of 12AX7A/5751 tubes in the MC275 V-1 slot while driving the amp in BAL mode; and they don't have ANY effect on the sound; a result which understanding the schmatics would make clear for you; unless of course you subscribe to 'audio voodoo' ;--) And if you are in fact being sincere and objective (and good manners demands I assume you are ;--), the only explanation I can offer for your "experiences" it that you've got some 12AX7's with badly damaged filaments, gas or perhaps shorts!

Further, it's been my experience that when a listener says they sonically prefer SE operation over BAL operation, it's because;
1.) Some other hardware in their system is skewing the result for reasons contained in that hardware.
2.) Or the quiet and more accurate BAL operation is revealing some nasty condition or component elsewhere in the system that the less precise and less dynamic ("natural sound"?) of SE mode is masking.

My point here, is that instead of throwing up one's hands and pronouncing that the Gods of Science and Audio somehow or other work differently in THEIR listening room, it would be far more instructive (and productive of better sound) to learn some of the science behind the things we hear (both good and bad) and not just chalk it up to the weather or the color of the listening room ;--) I've looked at your system and would be happy to discuss these things in greater detail if you want to send me an email.

Neil
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I spoke to McIntosh today and their tech support told me that V1 is definitely in the signal path when using balanced cables on the Mark IV so I now I know I am not tone deaf. According to Mcintosh, it is the circuit that determines whether you are using balanced or unbalanced. If you do not believe me, call them, they are easy to reach. I would like to see the schematics for this amp because I can read them. Are they available? Where would one find them? I know you can get the schematics for the original 275. So there you have it!

I know that I am not the only one that feels that RCA inputs sound better. I have spoke to others and they feel the same way. In the case of the MC275 which we are discussing, balanced cables offer more gain, otherwise, a waste of money. The MC275 is not a fully balanced amp so why would I want balanced inputs. If my amp was fully balanced, then it would make a difference, otherwise, it is a waste of money that I could use to buy records. I would not call it masking the sound. I also think the masking many hear is due to other factors such as the power cords. It is amazing what good AC power cords can do.

Tzh21y the balanced inputs on the MC275 have less gain not more as you stated in your last but one post.
MC275 gain: Balanced - 4 ohm = 16.8dB, 8 ohm = 19.8 dB, 16 ohm = 22.8dB
Unbalanced - 4 ohm = 23.2dB, 8 ohm = 26.2dB, 16 ohm = 29.2dB
These figures came from Mcintosh tech dept.

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! ;--)