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Impedance Question: Tube Preamp with SS Power Amp

Having researched this issue with the archived threads, I am still having difficulties understanding the problems of impedance mismatch between Pre and Power amps. As a budding audiophile moving onto my first separates Pre/Power, I want to know more about impedance matching. I have several questions that I hope veterian audiophiles can help me out with.

1)Is the 10:100 ratio between the output impedance of preamp and input impedance of the power amp an absolute rule?

2)What would happen if impedance mismatch do occur? Will the sound be any less satisfying?

3)I understand that Tube preamps generally have higher output impedance values compared to SS, so does that make it problematic to use it with a SS power amp even though tubed preamps are often priased over their SS counterparts?

I ask these questions because I recently purchased a tubed Preamp (Musical Design SP-2B)that has an impedance of 2K Ohms. I want to match that with a SS power amp with an impedance value of 20K Ohms. Am I cutting it too close? (exactlly 10:100 ratio) Or are all of these issues irrevalent and I should just listen to that setup and hear for myself if it would work or not? Thanks in advance for the answers.
speedracerucr

Showing 4 responses by morbius396c

morbius396c
134 posts
 
John,

I'd contact either Krell or Acoustic Research or both.

I'm sure you're not the first to hook an AR pre to a
Krell power amp.

Perhaps the two manufacturers have some experience - either
directly or garnered from their customers.

Dr. Gregory Greenman
Physicist
morbius396c
134 posts
 
John,

On page 26 of the current FPB-series manual; under the
column labeled FPB 350Mc - the input impedance is
specified as 100 kOhms.

In fact, all the amplifiers, both stereo and mono; and all
powers have an input impedance of 100 kOhms.

Dr. Gregory Greenman
Physicist
morbius396c
134 posts
 
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
morbius396c
134 posts
 
John,

The Krell only has balanced inputs - so the balanced output
of the ARC is 400 ohms. [ Ignore SE = single ended ].

ARC states that it needs a minimum 20K ohm load. The Krell
is a 100K ohm load to the ARC - so it meets the minimum.

In the ARC output section, it states it can output 4 Volts RMS
to a 100K ohm balanced load - which is exactly what the Krell
is.

So as far as impedance matching - the Krell is an acceptable
load for the ARC - the ARC is "happy" with the Krell.

The next question is whether the Krell is happy with the
ARC. The Krell doesn't want to see much DC at its inputs.

A previous poster stated the ARC has filter capacitors in
its output lines - which will block the DC the Krell doesn't
like. As long as one of the two puts the filter capacitors
in the line - the Krell will be happy.

Since the ARC already filters its output - you probably
don't need the Krell to filter its input. The Krell
monitors its input and will shutdown if it is not happy
with the input signal. In the unlikely event that this
happens - the jumpers in the Krell can be removed so that
the filter capacitors in the Krell are in the circuit path.

You should be OK - but if you have any doubts - call Krell.

Dr Gregory Greenman
Physicist
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