I took a look at the statements in the
manual for the Krell 700cx that pertain to DC. As someone having no dog in this fight, I would offer the following comments:
1)The relevant section on page 14 (pdf page 18) begins with the statement that:
The high DC output of tube preamplifiers may exceed the DC protection circuitry of Full Power Balanced amplifiers. Excessive DC level in a signal can damage amplifiers, speakers, or both. The coupling capacitors in Full Power Balanced amplifiers must be engaged when using a tube preamplifier.
Note that the statement does not refer just to tube preamplifiers having DC coupled outputs, it also encompasses the great majority of tube preamps that have coupling capacitors at their outputs. How are those preamps supposed to output "high DC"?
Also, note that when it refers to "the high DC output of tube preamplifiers," it does not make any reference to fault conditions. As if to say that all tube preamps routinely output high levels of DC, when operating normally. Which is nonsense, of course.
2)On page 19 (pdf page 23):
Q. My speakers are rated for 150 Watts. Are the Full Power Balanced 400cx or 700cx models too powerful for them?
A. No. A speaker seldom is damaged from overdriving. More often, damage occurs when an amplifier that lacks sufficient power is asked to handle heavy demand situations such as high playback levels. These amplifiers may have very high 8 Ohm power ratings, but in heavy demand situations they can be driven into clipping (in which DC current goes to speakers due to loss of amplifier power). Clipping can damage speakers.
This reflects a complete misunderstanding of how overdriving an underpowered amplifier can sometimes damage speakers. It has nothing to do with DC. The reason is that a clipped waveform has increased high frequency content relative to the original signal, which can damage tweeters.
All of the above doesn't speak well, to say the least, for the credibility of whoever wrote the statements in the manual cautioning against the use of tube preamps without the amp's coupling capacitors being in place.
3)I have participated extensively in these forums for more than five years, and I cannot recall a single instance in which an amp or speakers were reported as having been damaged by DC emitted by a tube preamp.
4)I second all of the comments by Charles about the credibility Ralph has established via his lengthy track record as a respected designer and manufacturer of very high quality equipment, and that he has further established as a result of his extensive contributions to these forums.
Regards,
-- Al
