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Question About Audio Research Preamp Specs

Hi,

I am trying to understand Audio Research terminology while looking at their product specs.

For example, for the LS28 OUTPUT IMPEDANCE, it reads "600 ohms Balanced, 300 ohms SE Main (2), 20K ohms minimum load and 2000pF maximum capacitance..."

What does the  "20K ohms minimum load" tell me?

I am running a McCormack DNA-500 and it has an input impedance of 10K Ohms. It would seem that the amp would be compatible with this preamp based on the 10X/15X minimum rule of thumb. 

My concern is that the AR spec is telling me use amps with input impedance of 20K ohms and the DNA would not work well.

Thanks for listening,

Dsper
dsper
atmasphere's avatar
atmasphere
11,804 posts
 
1: If you own the AR pre, tell me the size (uF) of the output coupling cap, (it will be the big ones near each output) and what the poweramp input impedance is you want to use it with, and i’ll calculate at what low frequency it’s at -3db for you.
It’s a simple to correct if too small, with a $20 larger coupling cap, don’t go buying another preamp because of it.

2: Or you can go the other way and get the input impedance resistor (20cents) raised to 50kohm on the poweramp, if it’s fet input no problem, even a 100kohm

Cheers George audioconnectionGeorge
 Makes great sense here.
  easy to have a tech make the resistor changes on the power amp.
The reason ARC didn't do this initially was simply because the larger you make a coupling cap the more coloration no matter what type of construction. Larger caps start to get considerably higher inductance so even if you have the best Teflon its still going to be an audible degradation.
A secondary reason might also have to do with space. Adding long leads to make a part fit in a place that wasn't designed for it is a good recipe for noise pickup.

georgehifi's avatar
georgehifi
10,312 posts
 
The reason ARC didn't do this initially was simply because the larger you make a coupling cap the more coloration no matter what type of construction.
Take that with a grain of salt. I do prefer direct coupling (no cap), best cap is no cap mentality. 

But dsper OP one of the best tube preamps hailed even by many tube and solid state preamp manufactures as being the best tube preamp ever made is the  Aesthetix Calypso preamp, and it has massive output coupling caps, as shown in this pic circled in red.
https://ibb.co/4Jb0bSV

Cheers George

    
dsper OP
663 posts
 
Well now.

I like the sound of the CJ pre except for the slight LF rolloff due to the impedance issue.

Maybe I should call CJ and ask them if they can adjust the caps on the 17LS2. 

Or look into the ET5 which has a lower out impedance. 

Thanks for listening,

Dsper
georgehifi's avatar
georgehifi
10,312 posts
 
Maybe I should call CJ and ask them if they can adjust the caps on the 17LS2.

Great idea, tell them you want the "uF" of coupling cap increased so you can have "little or no" rolloff above 20hz into your amps 10kohm input impedance.
This is around $20 for the caps, and a 1hr labour.

My calculation for 17ls2’s new coupling caps into the 10kohm of the amp. Any/All are fine.
4.7uf will be -3db at 3hz
6.8uf will be -3db at 2.3hz
10uf will be -3db at 1.6hz

Cheers George

atmasphere's avatar
atmasphere
11,804 posts
 
Take that with a grain of salt. I do prefer direct coupling (no cap), best cap is no cap mentality.

But dsper OP one of the best tube preamps hailed even by many tube and solid state preamp manufactures as being the best tube preamp ever made is the Aesthetix Calypso preamp, and it has massive output coupling caps, as shown in this pic circled in red.
https://ibb.co/4Jb0bSV
This is why we direct couple... this limitation is eliminated.
In the photo of the Aesthetix its not clear what the actual value of the coupling capacitors is. But obviously the Aesthetix faces the same issue of what is the lowest impedance it can actually drive actually; this information does not show up in their specs or operating manual. 
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