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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
almarg's avatar
almarg
9,670 posts
 
Hi George,

Thanks for providing the link to the manual. The 23K figure, which Kdude66 also referred to earlier, is stated in the manual to be for a left PLUS right output (in other words a mono output) which the preamp provides. The output impedance of the left channel and right channel main outputs does not appear to be specified, but per my previous post will be more than 17K at 20 Hz, due mainly to the 0.47 uf coupling cap that is utilized at those outputs.

Best regards,
-- Al

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.
georgehifi's avatar
georgehifi
10,312 posts
 
olid state amp with a input impedance of 100k to be optimal.That makes the choices much more limited
There are few, having the Lightspeed Attenutaor really makes me look out for anything over 47kohms which would be ok for him.
I believe 100kohm should be the standard, that way any tube or passive pre can drive it like it wasn’t there, also you would'nt see the big differences between preamps we see, especially if the pre is capacitor coupled output, as 10 or 20kohm input impedance can see the bass roll off too early, if the caps not huge.

Cheers George
almarg's avatar
almarg
9,670 posts
 
Toolbox149 5-26-2017
The whole system is bi-amped using Dahlquist high/low pass which rolls of the high pass at about 80 hz at 18 db an octive....

All of my inputs (mostly my CD equipment) feed into my primary preamp, a Bryston BP-25.  From there the signal goes to the Dahlquist LP-1.  After that, the high pass signal goes through a number of pieces including the Sansui QSD-1 quadraphonic unit. From the QSD-1 it goes to the C-20s and from there to the ADCOM 5500s.  The low pass material goes from the Dahlquist to a EQ, then to an ADCOM 555 and finally to a pair of 15" subwoofers.
The fact that the frequency content of the signals handled by the C20 is sharply rolled off below 80 Hz helps the impedance issue considerably.

The C20 schematic I referred to earlier (which is provided in the service manual that can be found at hifiengine.com) shows that the cathode follower output stage is comprised of a 12AX7 tube used in conjunction with a cathode load resistor of 47K.  The resulting output impedance, aside from the impedance of the output coupling cap, will be in the vicinity of 600 ohms.  The impedance of the coupling cap at 80 Hz is about 4.2K.

The 10x rule of thumb guideline that Bob referred to applied to those impedance numbers, and considering also that the 18 db/octave rolloff will still result in significant frequency content extending somewhat below 80 Hz, suggests a minimum load impedance of about 50K, which is what your Adcom 5500s have been providing.  That guideline is further reinforced by the fact that in the case of a cathode follower it is generally desirable, for reasons other than impedance compatibility, to keep the external load impedance greater than the value of the cathode resistor.

50K broadens your amplifier choices considerably, of course, compared to 100K.

Good luck.  Regards,
-- Al
  
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.

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