MC 754

[c_dk]

In order for A/B comparisons to be of any use, it is imperative the comparison is made instantaneously.

Nonsense......you keep stating this but I disagree. I would suggest that most all who tube roll or experiment with cables will attest that a long term listen and swap will show off differences more reliably than quick switches.

I understand that some cannot hear differences......hearing ability has been this way forever (saber tooth tigers were happy) and we showed this when doing a ABX demonstration back in the 80s. That was a quick switch test and 1/3 of the group were oblivious to the differences.........but 1/3 were able to pick out the difference every time.

Personally a extended listen, maybe over days, and then effect a change showed off the most important differences.

Think multiple guess test vs essay.......

 

[62caddy]

Nonsense......you keep stating this but I disagree. I would suggest that most all who tube roll or experiment with cables will attest that a long term listen and swap will show off differences more reliably than quick switches.

I understand that some cannot hear differences......hearing ability has been this way forever (saber tooth tigers were happy) and we showed this when doing a ABX demonstration back in the 80s. That was a quick switch test and 1/3 of the group were oblivious to the differences.........but 1/3 were able to pick out the difference every time.

We'll just have to agree to disagree on that point. There is a distinction between someone claiming to hear differences and the actual presence of differences - audible or otherwise.

1/3 accuracy isn't all that impressive either unless we're talking repetitions in the multiple hundreds at least - double blind administered before being anything close to meaningful. There are a host of other items that need to be controlled as well such as isolation of respondents from the group to prevent "going along with popular opinion" and a zillion other things.

 

[Pio1980]

I think both instantaneous swap and long term listening comparison are valid in context. Short term comparisons can weed out obvious differences and save time, longer term listening can reveal more subtle differences.

 

[62caddy]

I think both instantaneous swap and long term listening comparison are valid in context. Short term comparisons can weed out obvious differences and save time, longer term listening can reveal more subtle differences.

I could see longer term listing being appropriate for comparing speakers because certain response patterns may take some time before becoming objectionable to the listener, but then the differences in speakers is also likely to be on a completely different order of magnitude than that for the situation at hand regarding the CR7.

 

[TSmith8605]

Does anyone have thoughts about leaving these 754s powered on all the time? They do not have power switches and the manual claims leaving them always on is fine as long as they are used frequently. When I press my ear to the ribbon tweeters on my speakers I can hear a very (very) low hiss when my pre is off and the 754s running.

When I had just the one 754 I had it plugged into a Mac CR7 Remote Receiver that allowed me to turn it on/off with a clicker. Now that there are two in the mix that isn't an option (or is it?).

Each time you apply power to your power amps, they are subjected to transient responses (e.g. in-rush currents into the power supply as it charges the filter caps), thermal changes and wear on the power switch. Leaving the amps on continuously keeps them in steady state electrically and thermally. Thermal stability means things are not expanding / contracting at differential rates. Components are less-often stressed by in-rush, recharging. Caps stay formed. The amp is always ready to play at it's best.

I have a Musical Fidelity Power amp that I use for the bass speaker on my stats and full range on the stats for less critical listening (e.g. watching TV) that has been on 24/7 for 25 years with no problems. My former Klyne pre-amp was specifically designed to stay on all the time - the designer maintained that doing this, along with what I already mentioned, would keep the humidity low and constant inside the chassis. The power switch was on the back panel.

I do not leave on stuff with incandescent lighting and / or vacuum tubes, like the C22 / MC2505 and in those cases, I do have to wait about an hour before that system really sounds its best.

One more date point, I toured a recording studio in SF in the late '70s and we passed a pair of MC60 amps that were on and not doing much. I asked and the tour guide said they leave even their tube stuff on 24/7 as that provided the longest lasting / cheapest operation.

So, I will stick with what I said, if the engineers who designed the amp recommend leaving them on between uses (unless you use them infrequently) then I suspect they know what they are talking about.

Having said all that, this is not a religious point with, just a guideline.

 

[TSmith8605]

I think both instantaneous swap and long term listening comparison are valid in context. Short term comparisons can weed out obvious differences and save time, longer term listening can reveal more subtle differences.

I agree with this - especially if you listen to one system for a long time and then change just one component. I think long-term listening gives you more information - more changes to hear as you play a bunch of recordings.

I do believe blind listening testing is more objective but blind testing is usually also A/B. So I sometimes wonder about how one could set up a long-term but blind or double blind listening test - especially for audio reviewers. That would require a dedicated listening room outside the reviewer's house, an acoustically transparent curtain to hide speakers if such is possible, a crew to set things up and operate the system behind the curtain. Not double blind yet but if the operating crew is isolated (can't be seen or heard) by the reviewer, that would be close to double blind. The reviewer would then only know that he is reviewing amp #17 (or whatever) which replaces amp #16 in his otherwise existing reference system. Not sure if that could be pulled off, but would interesting, I think.

 

[Kingsin7]

So, I swapped the rebuilt Marantz 1060 for a Mac C28 I just finished working on and had sketchy results with that pre.

As I mentioned before, after A/Bing, the Marantz and remote unit, I thought the preamp addition to the mix sounded very good compared to the remote unit all by itself, a little more detailed, crisp, etc - but the Mac pre either had too much gain on the volume pot to work with the remote unit or I don't know what, but it caused distortion to creep in when the C28 was at the 12 o'clock volume position...

After that I decided to just use the remote unit alone with the bridged 754's for a few reasons beyond fidelity:

1. Remote changing between inputs (when turned on it always reverts to the "tuner" input, which is the DAC for my TV and what I use the most in that setup).

2. Volume reverts to a very low setting every time the unit powers off (again, good for TV).

3. I prefer listening to CD in that room, and my NAD disc changer has a remote (no getting up like a sucker to change input setting on pre).

4. For phono, I have a cheap little MM/MC guy hooked into the "preamp" input on the remote unit that does the trick.

What I give up is tone control, but I can probably find an EQ for that purpose if need be. The 754's monobridged are performing great. They've never gotten above room temp when pushing that 4ohm load for long periods of time.

It's a very minimalist setup I've come to appreciate... Thanks everyone for your input.

 

[damacman]

I like it when a plan comes together . . . well done.