Impedance Question: Tube Preamp with SS Power Amp

You can help stack the deck in your favor by keeping the interconnect cable lengths SHORT. 1 or 2m is fine, but if you had to go to 15 ft of interconnect cable with this heavy of a mismatch, I'd say fuhgedabowdit.

Many tube preamps will also have a higher gain, which can offset any power losses from high output impedance. High frequency roll off can be an issue if the input circuit is shunted with a filtering capacitor (R-C impedance) but amps with resistive only impedance are not affected.

In well designed equipment, all impedances should match. This would give us the greatest power transfer, highest levels of linearity and least amount of signal reflections and the associated ringing that takes place.

Since we aren't dealing with well designed equipment, most manufacturers recommend using an amplifier with a much higher input impedance than the output impedance of the preamp being used. If one doesn't follow that basic "recommendation", the end result is typically a softening of highs and bloated, muddy, uncontrolled bass response. How severe this will be will depend on how stable the source or preamp is. The more stable that device is, the less noticeable these problems will be. In some systems with bright and lean sounding digital sources, the colouration / distortion caused by the improper loading conditions mentioned above may actually help the system to sound more bearable.

By playing games with various interconnects and altering the feedpoint impedance that the source or preamp sees, one can reduce these sonic drawbacks somewhat, but probably not completely correct them. As such, if one wants to "go beyong the territory of recommended guidelines", they have to be willing to "explore new horizons" on their own. That's because each situation / exploration will probably be different from the last and you'll have to "make things work" all over again. As such, get ready to do a lot of cable swapping i.e. "band-aiding" of the system to compensate for the selection of mismatched components. Sean
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Hmmm:
So I'm looking to hook up my AR Ref one MK2 with a pair of Krell 350mc's. Is this a mismatch? Must I do the internal jumper changes that Krell recommends?

Thanks
John

John,

The Krell amps are "direct coupled" - that is they don't put
capacitors in the signal path. This is an advantage since
"coupling capacitors" can have a negative impact on the quality
of the signal.

Krell amplifiers also have DC protection circuitry which will
shutdown the amp if too much DC signal is seen at the input.
Too much DC can be damaging.

Unfortunately, many tube amps output a signal with a DC
component above that which will trigger Krell's protection
circuitry.

In order to use a tube preamp with a Krell amplifier - Krell
advises that the normally inactive coupling capacitors in
the Krell amp be activated by a Krell dealer.

Dr. Gregory Greenman
Physicist