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Asen 21st February 2001 11:36 PM

In case I'm using an inverting, for example, SRPP tube preamp, is it necessary to make changes at the output of the power amp - to change the speaker (+/-) terminals or something like that, to keep the sourse and the output in phase?
I'm thinking about a mu-follower (SRPP) preamp and Andrea Ciuffoli's SE Follower 99 combo. Do you think they will work well together? Any one with experience with such projects?
Thanks

Freddie 22nd February 2001 08:06 AM

I have built Andrea Ciuffoli's SE Follower 99 (with a 38V 500VA transformator/channel and a bias current of 2A/4A). I have been using it for about 18 months now. It works very good but you need a HUGE heatsink. As preamp i'm using a modified Bride Of Zen.

>is it necessary to make changes at the output of the power amp - to change the speaker (+/-) terminals or something like that, >to keep the sourse and the output in phase?

No, it isn't necessary. 180 degrees of of phase is inaudible.

>I'm thinking about a mu-follower (SRPP) preamp and Andrea Ciuffoli's SE Follower 99 combo. Do you think they will work well >together? Any one with experience with such projects?

It should work good if it's capabel of delivering enough voltage swing! If you use a 38V transformator for the Power Follower, the preamp should be able to swing 15V RMS. And keep in mind that the output resistance should be less than 1000Ohms.

Why don't you build the No Compromise OTL Headphone Amplifier by Andrea Ciufolli. It should be able to drive the Power Follower very good.
Or the BoZ?

/Freddie

Asen 22nd February 2001 08:02 PM

No Compromise OTL
 
It looks good, but I live in Eastern Europe. There are no such exotic tubes around here. The preamp is not such a problem for me. I have home-made McIntosh C-22, which is known as one of the best preamps ever. I don't use it because it needs a Follower - 20V & 600 Ohms output. My question was about the phase shifts, because right now I'm working on a SRPP project.
It's interesting what mods have you made on the BOZ.

[Edited by Asen on 02-22-2001 at 03:36 PM]

Petter 22nd February 2001 09:20 PM

Eastern Europe
 
Asen,

No exotic tubes? Are you kidding?

I am using eastern European tubes myself -- they seem to be the holy grail in Europe and Japan .. are you sure they are not available in your locale ;-)

Asen 22nd February 2001 11:13 PM

Ciuffoli's OTL
 
Ciuffoli says - Raytheon and Mullard tubes after lots of tests. I'll find something close but I'll never be sure about the result.

GRollins 23rd February 2001 03:39 AM

Asen,
I'm afraid I must disagree with Freddie...phase reversal is audible on a decent system. Subtle, perhaps, but audible...and subtlety is what we're all here for, else we'd all go buy Walmart table radios and put our money into something useful, like a dusty collection of stuffed parakeets. It is worthwhile to try to keep the absolute phase correct throughout your system. In your case, that would mean (assuming a phase-inverting preamp) that you'd need a phase inverting amp...or to simply switch the plus and minus leads (as you mentioned in your initial post) at the speaker terminals, which is cheaper, faster, and allows more permutations as you change equipment upline (different preamps, active crossovers, what-have-you).

Grey

Freddie 23rd February 2001 08:10 AM

>It's interesting what mods have you made on the BOZ.

I increased supply voltage to about 100V, decreased the source resistor to 80 Ohms. I'm also using another PSU than the original BoZ but you can use the original, just replace a couple of zener diodes.


GRollins:

Are you really sure that phase reversal is audible?
Here's what I found in an article from http://www.sound.au.com

"It has been demonstrated by many workers in the field of acoustics that absolute phase is inaudible. Indeed, if this were not the case, then moving one's head 300mm closer or farther away from a sound source would give rise to a massive change in the perceived sound. As we all know, this is not the case. "

/Freddie

Asen 23rd February 2001 05:46 PM

Nelson Pass says:
 
"A single common-source gain device will invert the phase of the signal it amplifies, and this must be taken into account when using the circuit in an audio system. The Zen amplifier also does this, but we reversed the phase on its output terminals to compensate, which is not possible here.
Using the Zen preamp usually requires that we adjust the phase of connection between amplifier
and loudspeaker for phase integrity. The importance of proper absolute phase has been discussed elsewhere, including the original Zen amplifier contribution. I will simply say that proper absolute phase is a legitimate factor in the overall quality of sound reproduction. Even if I could not hear phase reversal, aesthetic considerations alone would prompt me to assure that it was correct."
The BOZ article

I think that the problem is rather complex. Yes, a sine wave, shifted to 180 degrees is the same sine wave, but music is something more than a simple sine wave. Just imagine the whole way of the sound (from the microphone to the speaker), when the microphone membrane is pushed by the sound wave, the speaker cone or dome must go out making the same sound wave. Otherwise a distortion could show out. I don't know what kind of distortion it would be and can't give it a name but will be something like that: the horn sound of the train is different when it's aproaching the station and when it's leaving it. The lenght of the wave becomes shorter when it moves in one direction with its source. Now imagine the cone muvement of the driver. If there are two frequences - lets say 20 Hz and 500 Hz, the cone moves forward to make a +90 degree cicle of the 20 Hz; meanwhile it moves faster for the 500 Hz in the corect direction at the moment (t). If the microphone and the speaker are out of phase, the cone muvement will be -90 degrees (the oposite direction, and the 500 Hz will no longer be 500 Hz but less (the horn of the train at leaving the station - remember?...). I don't know whether I'm right or not (these are just my thoughts), but the problem deserves a discution.
Excuse me if my English is not clear enough. I hope you'll get what I mean.

GRollins 24th February 2001 03:07 AM

Freddie,
To answer your question: Yes, I'm sure.
You have to ask yourself a couple of things about reports like that: One is--the absolute phase of *what*? Test tones or music? A pure sine wave is a pretty slippery thing for your ear to try to grab onto. But we don't listen to pure sine waves...we listen to music. Music is much more complex and contains phase relationships that are far more intricate than a pure sine wave. If you don't think you can hear the difference, let me suggest that you focus on the drums. Rock records, incidentally, are (almost always) lousy sonically. They may sound good, in the sense of fun, but they don't sound real, in the context of what actual drums sound like. Try a well-recorded jazz or classical record. Something where they actually attempted to get the sound of a real drum on the record. If you still can't hear it, then try listening to the image. (Again, forget rock, rap, et. al.) There are noticible changes in the images of the instruments in both focus and placement on the stage. This is actually my preferred way to listen for absolute phase.
A great hint when someone starts in with that kind of rot about this or that or the other can't be heard is the phrase "massive change." People who don't hear things, either because they can't, or don't want to, purposefully set up straw men to knock down by overstating the case, then saying,'well, it isn't a knock-you-over-the-head difference, ergo, it doesn't exist.' Remember--there are folks out there who can't hear the difference between resistors, caps, speaker cables, etc. and who tell you that all amps sound alike. Don't believe it...or if you do, then buy the cheap Walmart radio and be happy. As a fringe benefit, you'll save a heap of money.

Grey

GRollins 1st March 2001 04:34 PM

For those who feel that what 'experts' say validates (or otherwise) a given point of view, there is a portion in Audio Power Amplifier Design Handbook on absolute phase. Douglas Self (the author) admits that,"Concern for absolute phase has for a long time hovered ambiguously between real audio concerns like noise and distortion, and the Subjective realm where solid copper is allegedly audible," then goes on to cite references where it has been determined that it is audible (he specifically mentions drums--no mention of image). His references:

Greiner & Melton A Quest For The Audibility of Polarity Audio, Dec. 1993 p40

Greiner & Melton Observations on the Audibility of Acoustic Polarity Journ. Audio Eng. Soc. Vol 42

Grey


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