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Re: Perhaps A Different Idea On DBT's?

Posted by tomservo (M) on June 6, 2006 at 08:40:26

In Reply to: Perhaps A Different Idea On DBT's? posted by thetubeguy1954 on June 5, 2006 at 10:34:54:

Hi

AS the real JJ pointed out ones ears do not always hear "one thing" for what it is.
An old hifi salesman’s trick to promote the brand De-jure was to raise the SPL of the speaker being pushed so that it is about 1 or 2 dB louder than the others.
The average potential customer will interpret the small level difference as that speaker being “clearer” not louder. Curiously, the same trick works on video, have a small treble and bass boost on the brand your pushing and people notice a better picture.

An issue here is that since loudspeakers have so many flaws that they usually all sound quite different the issue is how to compare speakers.. Added to that is the fact that at normal listening distances, one is also getting great deal of room effects which are also often different speaker to speaker.
Compared to even cheap electronics however, loudspeakers have many flaws which because they do not help sell, are not generally mentioned in the magazines.
Of all the elements in the chain, loudspeakers, at least theoretically / by measurement, appear to be by far the weakest link in the re-production chain.

Fortunate, they all have similar problems so we are pretty used to the way they sound vs real life.
Loudspeakers compress dynamically, ALL voice coil drivers begin to show the effects of “power compression” and the shift in parameters that accompanies it beginning at 1 / 10 to 1 / 8 rated power.
Loudspeakers spread out a complex signal in time, some frequencies produced before others, most multiple driver loudspeakers cannot preserve the waveshape of the input signal, there is too much time dispersion..
Loudspeakers produce both even and odd harmonically related and non-harmonically related “free sound” which is not part of the input signal and the degree is also level dependant. Make no mistake, the speaker can’t have any idea what the musicians wanted, its job to to produce the drive signal only, any additional sound is coloration.
Loudspeakers have directivity, if this is not “constant” then the spectrum of the on axis sound is different than the off axis spectrum.
Loudspeakers with more than one driver normally also have self interference in the crossover region which is visible in polar or spherical measurements of the speaker (again, as this does not “help” sell, hifi magazines don’t discuss any of this).
The self interference produces lobes at particular frequencies, pointed in undesired /unexpected directions. Which raises the level of reverberant sound at that frequency.

On the other hand, as one increases the level, the size of the audience, acoustic power and room size all of the problems become critical show stoppers, hence are well known outside of hifi.

ON DBT’s.
Studies have shown peoples acoustic memory is very short, in this kind of testing it was necessary to be able to switch back and forth as quickly as possible.
On the other hand, if one were testing speaker wire, it would be logical to use a pair of heavy duty Relays (one at the amp end, one at the speaker end). These (high current types) have self wiping contacts and have very low R etc and would allow one to rapidly (nearly instantly) switch between one set of wires and another.
This allows one to sit back as long as they want to see if anything pops out later on.
Even when you control which is which through the relays, if there is a difference you will hear it switching from one to the other. Once you are sure (if so) have a second person switch back and forth out of your control and knowledge of which is which and then see if that effects your ability to hear some difference.
If you can reliably hear a difference form one to the other, then you have an audible difference.
You don’t need to be blind, just being open minded to the results (what ever that is) and being able to switch back and forth rapidly gets you 90% of the pie.
If you can hear a difference this way in cables or what ever, it is worth doing, if not, think hard about drawing that wallet out.

The last BT I was in was for commercial power amplifiers, for reference I brought in a Threshold Stasis as well. Using very revealing speakers, it was interesting that the best sounding Pro amp (QSC PL-236) sounded slightly detectable as different than the Crowns and Chevin and about the same as the Threshold.
The differences were in the (what it sounded like to me) low level, decay side of transients if that makes sense.
Curiously too, with some program material, the Threshold began to sound “different” at a modest level. An oscilloscope revealed even at a modest level, the amp was instantaneously clipping due to the very dynamic signal.
One couldn’t hear this (I never noticed anyway) unless you could compare to the “without” condition, until then, the Threshold was “plenty of power” for me.
From that day on, I have used what are “Pro” amplifiers in my stereo and use a PL-236 (max of 1300W /ch) on a pair of speakers with 100dB sensitivity now and never clip (headroom is dynamic ranges friend).
Tubes (depending on the topology) have an edge here as the “max” outout is a more gradual limit as opposed to a Solid State brick wall.
One can “see” why too, if the music had a peak to average level of only 30dB and the amp a max power of 200W, then the average level without clipping peaks is only . 2 Watt. Fortunately for the industry, most “modern” pop music, in an effort to be “loud”, has a P/A ratio of about 10dB (or less from FM).

Anyway, what you want for one channel is 2pc of a DPDT relay or 4pc of SPDT relay.
A DC coil is better here, you need a supply for that too.
For instance, ALL ELECTRONICS (800-826-5432) has cat# RLY-351 a SPDT relay for $2.40 each.

What ever you compare or listen for, it will be easy to hear “if” there is a difference or not when you can switch quickly. Like you suggest, it may take time to be able to hear some feature so being able to switch at any time would be useful here.
A relay switcher like this would also allow you to switch between amplifiers too by only using one end of it.
I think you will find both extreme positions on this issue (as is normally the case) is not the place “where reality dwells”. There are real mysteries and room for improvement in audio, there is an awful lot of BS too.
Have fun, do try this, best listening.

Tom Danley
Danley Sound labs

What (tube I presume) amp do you have?

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