2. amps-preamps
  3. 129890-81298

Amp Input impedance - old Macintosh C-20 preamp with modern SS amp

I have a pair of old, beloved friends in my system, my Macintosh C-20 preamps.  I've owned  them for over 40 years - they have been upgraded and very well maintained.  They sound wonderful.  I wouldn't part with them for anything.  They have been hooked up to a pair of ADCOM 5500s, which is a match I just love.  Some very recent developments have caused me to start thinking my ADCOMs might not be long for this world.  In the past two years I have lost two excellant local repair shops and I'm now faced with shipping my amps from Milwaukee to either Wyoming or the east coast anytime one them has a hiccup.  And lately, they have started to hiccup.
 
I would need two amps for my system so that limits me with what I can afford.  I was looking at the Parasound 2250s as my most logical choice.  My question regards output / input matching of the preamp & amp.
Number 1 - Does anyone know the output impedance of a Macintosh C-20 preamp? All I can find about the output are these two pieces from the old, 1960s manual:

MAIN OUTPUT 2.5 volts with rated input
    and this
MAIN Output Connected to Power Amplifiers
The MAIN and TAPE output jacks are fed from cathode followers. The input impedance of devices connected to these outputs should be 50,000 ohms or greater, and the capacitive reactance of audio cables connecting these devices should not be less than 8,000 ohms at 20,000 cycles. This is the reactance of a capacity of 1,000 mmf. Audio cable having a capacity of 25 mmf per foot may be 40 feet long; 13.5 mmf per foot cable may be 75 feet long

My ADCOMs were 49,900 ohms a good match, but many of today's amps are rated at 33,330 ohms.

Number 2  Long term /short term is this a mismatch? Parasound has a network of dealers including a few around here. Before buying I plan to take my C-20 in and try it together with a 2250; but are there other considerations, maybe long term, that could impact my decision?

My speakers are Magnepan 1.7 s, they are played at a robust volume level, and no, I can't swing a pair of big Parasound A-21s.

Thanks
Toolbox





toolbox149

Showing 4 responses by kdude66

kdude66
1,200 posts
 
Toolbox,

I don't see any issue with trying one a21 in your system,if your old Mac is up to it.I believe the c20 has a output impedance of 23k.

Only way to know for sure is to try the pairing if you can.

Best of luck,

Kenny.
kdude66
1,200 posts
 
George,

You are correct,
Unfortunately the OP needs a solid state amp with a input impedance of 100k to be optimal.That makes the choices much more limited,the only amps that I can think of off the top of my head are some of the older McCormick dna's like Al mentioned or pass labs.I can't remember the thresholds if they are higher,been way to long since I had one.

Kenny.
kdude66
1,200 posts
 
Toolbox,

I don't know if you have found a good repair shop for your adcoms,
we used to have a really good one near me and the tech was a adcom guru for many years,they are older and retired now.

I really can't recommend anybody unfortunately.

It's good to have all thIs knowledge that the good people on audiogon provide and I wish you the best of Luck.


Kenny.
kdude66
1,200 posts
 
Al,

Looks like you are spot on as usual,
I didn't have access to the schematic.50k does broaden the choices compared to 100k.

I know some amps are 47k,would that be close enough.


Kenny.

More to discover