Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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Part 4, DBT's and Tape Monitor Loops

Posted by Jon Risch (B) on May 15, 2003 at 23:01:28

This is sort of a continuation of the earlier posts about DBT's,
(See: http://www.audioasylum.com/forums/prophead/messages/2190.html
and at:
http://www.audioasylum.com/forums/prophead/messages/2579.html
and
http://www.audioasylum.com/forums/prophead/messages/2580.html )
but this one is about a specific aspect, the use of a tape loop for cable comparisons.

This is a method frequently proposed on the rec.audio. news groups, as if it were the ultimate method of testing.

Since I have received several e-mails about it recently, I decided to post my reply here too.

The main scientific flaw with this sort of "test", is that unless ALL the other cables have been changed out in the system to very high quality cables, AND you have familiarized yourself with them over a long period of time, and THEN placed a cable to be tested in the tape loop, you are not necessarily going to be able to tell what the cable in the tape loop is, or is not, doing.
If the speaker cable has not been changed out, or any of the other interconnects that are also in the signal path, then the limiting action of these cables will probably not let you hear what the interconnect in the tape loop is doing.

Let's explain this a little more, so that my premise is clear. Think in terms of a filter built into each and every cable, as well as a grunge generator of distortion that leaks into any signal passed down the cable. In a way, these analogies are true, in that certain common cable configurations do filter some of the audio signal, and some do add a certain amount of distortion to the signal. To quantify these two mechanisms, let us assume that the distortion is so minute, that it takes some of the more sensitive listeners to detect it reliably under controlled conditions, and that the filtering effect is of a greater degree that is a little more readily apparent. Neither will leap out at you, they are both subtle in nature.

Now, what does a typical audiophile do to audition cables? Many times, they just substitute a pair of interconnects or a set of speaker cables into the system, fire it up, and wait to see what leaps out at them. When nothing sonically spectacular or obvious occurs, they erroneously conclude that they can't hear anything, and that the naysayers are right: snake-oil.

However, the problem lies with the nature of cable performance benefits, and how the filtering and minute added grundge affect a listeners perception. If only one cable is substituted, then the other cables in the signal chain (at least a speaker cable set OR one other interconnect for simple systems, and several interconnects for more complex systems) will still have their filtering action and added grundge factor.

Looked at another way, if what makes a high performance cable special is being thrown out by other portions of the system, including other cables that haven't been upgraded, then it is much more difficult for those special qualities to be noticed when only one cable is changed. The filtering has already occured to some extent, and a certain amount of grundge has been added already, so it is that much harder to notice any improvements due to the single cable being changed.

There is not just this aspect to consider. If the listener is not used to hearing the freedom from grundge, and the less filtered signal, then even if some of those improved qualities were making it through the rest of the system due to the cable change, it would hard for the listener to pick up on them.

In order for a cable in the tape loop to be discriminated, it would have to exhibit HIGHER levels of losses and added signal abberations than the sum total of all the rest of the cables, which means it would be a very poor cable indeed.

If each cable added X amount of filtering, and Z amount of grundge, and there was one speaker cable in the system,and two interconnects in the signal path used to audition the tape loop, then the cable in the tape loop might have to exceed 3X and 3Z levels of filtering and grundge, in order to be heard as different.

Normally a good test scenario would have one of the cables swapped out for another, so that the level of X and Z would be reduced, not added to. If we had a cable that had 0.5X amount of filtering, and 0.5Z amount of grundge, and it was added to a 3X and 3Z system, the system would end up 3.5X and 3.5Z, which may or may not be noticable, but certainly would not allow the lowered mount of filtering and grundge to be noticed.

However, if we swapped this cable in for one of the others, now the system would go from 3X and 3Z, down to 2.5X and 2.5Z, which again, may or may not be noticable, but at least would going in the right direction so as to possibly hear something positive occuring.

If ALL the cables in the 3X and 3Z system were swapped out for 0.5X and 0.5Z cables, then it would now be 1.5X amount of filtering and 1.5Z amount of grundge, which is much more likely to be noticable.

As a final example, if we used very good cables, with low values of filtering and grundge addition, say around 0.2X and 0.2Z (note that these numbers do not have to track, there can be more, or less, filtering, and the amount of added grundge can vary differently. I am merely using simple examples), then the system would be 1.4X and 1.4Z for the 1X/1Z cable being swapped out for test, and 0.9X and 0.9Z for the 0.5X/0.5Z cable. Now that there are low filtering and low grundge cables in the other positions, it might be easier to tell what the differences were between two cables that differed by 0.5X and 0.5Z.

Another technical problem with the tape loop monitor type "cable test", is that it is not a real world use type of connection or test.

Buffered tape loops are completely out of the question, and would be disqualified. Not many receivers or intergrated amps specify whether or not they use buffered tape loops. The better ones do, and they would then be disqualified on first principles from use in this type of test .

Inserting a cable in a tape loop connection does not provide the same source or load conditions when it is used in the real world to connect a CD player to a preamp, or to a receiver, nor would it be subject to the same kinds of potential ground loop problems and issues (see: http://www.soundstage.com/articles/pete01.htm ).
This aspect also invalidates this kind of test, as the cable does not see the kinds of typical operating conditions it would encounter while connecting another component.

A cable design that provided excellent shielding and resistance to ground loops and RFI would not show any of its advantages that would exist in the real world, yet the tape loop test would gloss over them completely, due to the common ground potential and common chassis.

One other aspect is the commonly overlooked factor of experience and familiarity with component auditions and cables in particular. Unless you have already learned to distinguish audio cables from one another under other settings, then how could you expect to be able to do so just because you are using a tape monitor loop?
Placebo and bias work both ways, and are just as likely to prevent a (blind) doubter from hearing things as they are to allow a sighted listener to hear things that may not be there.

Of course, there is the little issue of the true science of such a test, and that the failure to produce positive results has no other real meaning, it is JUST a failure, period, and does not mean anything else, including it does not mean that there was nothing to be heard.

Cable swap outs are the only truly valid method of checking audio cables for sonic quality, as all other approaches that are used (mainly for convenience) add extra cables, contacts and unusual operating conditions.

Jon Risch

This post is made possible by the generous support of people like you and our sponsors:

Atma-Sphere Music Systems, Inc.

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