[Dave]

I am in the process of building an ultralinear tube amp. I currently use an
HK PM650 integrated amp as my amp/preamp. For the simple sake of cost
savings, when I get my tube amp (no volume control) up and running I'd like
to use this unit as my pre-amp.

I have the schematic, and it appears to me that the everything BEFORE the
balance control is pre-amp (tone controls, filters, etc.) and everything
AFTER is power amp (just transistors and more transistors).

Can I just connect the outputs (L/R) from the balance pot to a pair of RCA
jacks and call them "pre-out" or (and there's probably more to it, I know)
am I going to need to do some impedence matching on these outputs? I guess
this would depend on what the impedence is to start with. What's a
reasonable output impedence for a preamp? Or would that once again depend
on the input impedence of the amp?

Would I need to necessarily disconnect between the pre-amp and power amp
sides?

Thanks for any replies, my apologies for my ignorance in advance, just
tryin' to learn sum'tin', ya' know?

Dave

[Serge Auckland]

Dave wrote:
I am in the process of building an ultralinear tube amp. I currently use an
HK PM650 integrated amp as my amp/preamp. For the simple sake of cost
savings, when I get my tube amp (no volume control) up and running I'd like
to use this unit as my pre-amp.

I have the schematic, and it appears to me that the everything BEFORE the
balance control is pre-amp (tone controls, filters, etc.) and everything
AFTER is power amp (just transistors and more transistors).

Can I just connect the outputs (L/R) from the balance pot to a pair of RCA
jacks and call them "pre-out" or (and there's probably more to it, I know)
am I going to need to do some impedence matching on these outputs? I guess
this would depend on what the impedence is to start with. What's a
reasonable output impedence for a preamp? Or would that once again depend
on the input impedence of the amp?

Is the volume control before the balance control? If so, then you can do
as you say, take a the l/r outputs to a pair of sockets. Depending on
what comes after the volume control, the output impedance could either
be low (if there is some active buffering) or variable depending on the
value of the volume pot and/or how the balance control is arranged. The
output impedance could be as high as several kohms. This won't
necessarily matter, but it does mean that the cable between the pre and
power amps needs to be of low capacitance, and therefore short (say no
more than 1 metre of normal coax cable)

A valve (tubed) amp will normally have an input impedance higher than
most SS amps, 100k is typical.

Would I need to necessarily disconnect between the pre-amp and power amp
sides?

This would be a good idea, as if you don't, the power amp will still be
driven by the preamp. Whilst the power amp presumably will be unloaded,
it is still not good practice in my view to drive a power amp
unnecessarily. You could clip the power amp which could cause power
supply anomalies that affects the sound of the preamp. Can't be sure on
this, but I would err on the side of caution.

Thanks for any replies, my apologies for my ignorance in advance, just
tryin' to learn sum'tin', ya' know?

Dave


If you're able to email me the circuit, I'd be happy to cast an eye over
it and give you a more definitive view. To email, remove nospamplease
from the address above.

S.

[Dave]

"Serge Auckland" wrote in message
...

Would I need to necessarily disconnect between the pre-amp and power amp
sides?

This would be a good idea, as if you don't, the power amp will still be
driven by the preamp. Whilst the power amp presumably will be unloaded,
it is still not good practice in my view to drive a power amp
unnecessarily. You could clip the power amp which could cause power
supply anomalies that affects the sound of the preamp. Can't be sure on
this, but I would err on the side of caution.

What would likely make the most sense is to put in TWO new sets of RCA jacks
on the back "Pre-out" and "Main in". That way they can be connected with
one of those u-shaped bars seen on many integrated amp/pre-amps to restore
full functionality to the unit.

Is there anything special I need to do to minimize interference on my newly
installed leads? I would think as long as they're off the PCB and not
touching or crossing each other too much it wouldn't really matter how the
wires are routed to the jacks on the back...