condenser versus direct coupled?


I have an Hitachi 8500 MKII preamp and it has a condenser output and a direct coupled output. What is the difference? THANKS!
anthony
tspec

Showing 6 responses by eldartford

A direct coupled preamp is not going to blow out any speakers unless the power amp is also direct coupled. Chances are that the preamp output goes into an input capacitor of the power amp, so you will still be ac coupled regardless of which preamp output you use. I would go with direct...who needs two capacitors in series?

DC won't burn up speakers any faster than AC, but you would not get the audible warning that something bad was happening. However, direct coupled amps that I am familiar with include circuitry to detect this fault condition, and turn themselves off. Some AC coupled amps (particularly pro sound amps that are often driven very hard) also include protective circitry.

And then there are speaker fuses.
Sean...You really ought to be more specific than the frightening "FAR greater" when describing the damage potential of DC vs AC. Actually, as I think you must know perfectly well, the rms voltage of a sinusoidal waveform (its heating power) is 7/10 that of its peak voltage, which is presumably what you are assuming for DC from a faulty amp. Furthermore, if the amp goes into oscillation, which is the most likely failure mode, the output will be a square wave, (instead of a sine wave) which has exactly the same power as DC. Both levels could cause damage, and the difference in time required for damage to occur would NOT be great. I think that, because there is no loud audible indication of a problem with DC, amplifiers that are DC coupled are more likely to include protective circuitry. (Mine do).

Oh, and by the way, DC transmits over long distances very efficiently. Better than AC. A lot of research work was done on that technology here in Pittsfield Mass by the General Electric Transformer business. (Alas, GE quit the large power Transformer business). The problem with DC is that there is no easy way to step voltage up and down. With either AC or DC long distance transmission calls for voltage of several hundred thousand volts. You can't have that going into your house!
Sean...From an electrical power point of view a square wave is the same as dc. The amount of power radiated in the form of sound would be trivial. True that cone motion with AC might tend to ventilate the driver. However, the oscillation would likely be at a frequency too high for the driver to respond, and therefore no significant cone movement would occur. I only claim that the damage potential DOES NOT DIFFER VERY MUCH, and that your "FAR greater" words were a gross exaggeration.

I was not involved with the DC transmission research, but I seem to remember hearing about 450 KV. If you are really interested in power transmission research results I will see what I can dig up and Email it to you. I guess we could check the web also. Some of the GE work was probably proprietary.
Sean...A quick check of the net suggests that 15 to 50 Kvolt transmission is considered "medium" voltage. 200 to 400 Kvolt is "high voltage". And this is for AC. I know that the research on DC transmission was at higher voltages than used for AC.
Sean...And furthermore... Furukawa Electric (Japan) has a 1,100Kvolt AC line in service, and a 500Kvolt DC UNDERWATER cable as well! WOW.
Sean...Now that we have called a truce, I will point out that my suggestion that an AC coupled power amp would oscillate might be challanged. If output transistors on one side blow, the result will be a DC output, just like a runaway DC coupled amp. However, I do think it is true that DC amps generally have protective circuitry.