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

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.

I looked at the manual online and Roger Russell's site and I only see input impedances. Agree with kdude66 though, and can't imagine that it wouldn't work. Almarg...where are you! You might send him a PM, he knows everything.

Bob,
Thanks for joining in.  My C-20s are in a quadraphonic system.  They sit just before the front and rear channel amps restricted to a line-level input coming into the Aux inputs, and then stright out to the amps.  
Basically, I use the C-20s for l-r balancing and volume control of each amp and to get that sweet tube sound they put out.  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. 
So, how does the biamping affect this.
Do you think MacIntosh may have overstated the output impedance?
Thanks,
Toolbox

Thank you, John (Roxy54).

Toolbox, first, I’m not sure I understand the comment just above about biamping. If it’s a quadraphonic system you’ve got four speakers, and your two Adcom amplifiers provide a total of four channels of amplification, rather than the eight channels that would be required to biamp the four speakers. Can you clarify how the Dahlquist crossover fits into the system?

Second, I found a schematic for the C20 at HifiEngine.com (one has to register there to see it), and it shows that a cathode follower stage drives the output via a 0.47 uf coupling capacitor. The impedance of a 0.47 uf capacitor at 20 Hz is about 16,900 ohms, which is extremely high. And the output impedance of the cathode follower would increase the overall output impedance further. So Kenny’s (Kdude66’s) recollection of 23K, although perhaps a bit on the high side, is in the right ballpark for the deep bass part of the spectrum. The impedance will be much lower at higher frequencies, since the capacitor’s impedance will decrease as frequency increases, but it is that **variation** in output impedance (from relatively low values at mid and high frequencies to very high values at deep bass frequencies) that will cause the deep bass roll-off Bob referred to if the load impedance is not high enough.

Which together with the fact that the C20 was designed and introduced when amplifiers were tube-based and generally had higher input impedances than most modern solid state amps, and when most users probably did not have speakers providing a great deal of deep bass extension, points to the conclusion that the 50K minimum load recommendation is aggressive if anything, by today’s standards, and an even higher impedance would be preferable. I would suggest looking for an amp having an input impedance in the 75K to 100K area. Unless, that is, you would consider modifying the C20s by installing coupling caps having much higher values. Although doing so might have sonic tradeoffs of its own, especially if you want to minimize the cost of the new capacitors.

Some of the older McCormack DNA series of amplifiers come to mind as providing 100K input impedances, and are well regarded for their sonics, and my impression is that they tend to be available used for prices in the same general area as the 2250. Some of those models may be a bit marginal for your application in terms of power capability, but if you could find a DNA-225 for example it would be capable of more than 400 watts into the 4 ohm impedance of your Maggies.

Best of luck. Regards,
-- Al

Thanks Al,
FYI     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.
Thanks for the McCormack suggestion, but that puts me back in the same boat.  Old equipment with nowhere to get it serviced unless I ship it halfway across the country. If I have to stay with older equipment, I might as well try to get the ADCOMs fixed.
I was kinda hoping someone would tell me way back in the 60s they probably used the 20 to 1 ratio, but (just my luck) the intel is headed in the opposite direction.

Thanks,
Toolbox

Looking at the manual for the old Mac C20, it states it has a left and right output of 2.5v from a generator impedance of 23,000ohms (23kohm)!!!!!
Which if I'm reading this right, for it to be a great match for a poweramp, the poweramp would have to have at least 100kohm input impedance or more.
http://www.vintagevacuumaudio.com/schematics-manuals/mcintosh/mcIntosh-c20-owner-manual.pdf

Cheers George
 

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

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.

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

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
  

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.

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

I suppose so.  And if the bass response begins to roll off slightly as a result, it could probably be compensated for in a reasonable manner with a slight tweak of the settings that are used in the low pass signal path, that the C20 is not part of.

Best regards,
-- Al
 

Thanks everybody.  This was a great help!  I'm going to save this.

one last question

What is the downside of a match with an amplifier that has an input impedance of below 50K?   Is it just a rolloff or diminishing of the bass response, or is there a long-term possibility of something more sinister happening?

Thanks again,
Toolbox

BTW

I think this has convinced me to make one last try at a long-lasting repair of my ADCOMs.  I hate the idea of pouring good money into bad, but these amps really haven't had a highly regarded repair shop tackle them.  (And I really love the sound)
It'll probably cost $300-$400 just for round trip shipping, but I think I'll send them off to a well respected shop.  

Thanks everyone!

Toolbox, re your question just above, the only downside is with respect to sonics, especially in the bass region. More sinister consequences, such as damage or degradation of long term reliability, are not concerns, at least for load impedances measured in tens of Kohms.

Best regards,
-- Al
 

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.