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What is the input impedance of the Sugden A21aP power amp?

I have a wonderfully sounding class A power amp, essentially the power section from the revered Sugden A21a (the 90's version). 

But I have been searching wide and far for specs on the input impedance as well as the power output rating on a 4 ohm load (it's 25 wpc into 8 ohm).

Can anyone help? I don't have access to measuring tools.

The reason I want to know is that I would like to match it with my Audio Note M1 tube preamp, which has a high output impedance of between 2,5 and 5 kiloOhms. This translates to a weak bass performance/bass rolloff when playing into the Sugden. With my cheaper transistor preamp, I don't have this issue. 

If I know the specs of the Sugden, I can have my Audio Note dealer modify the M1 with a feedback loop to lower its output impedance to match. 
9radua1
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kosst_amojan
2,095 posts
 
Stereophile measured the A21ai input at about 24kR dropping off to 21kR at 20Hz. Most like yours is about the same. That definitely too low to be driving it with a 5kR source. 

Adding feedback will change the character of that pre-amp. I think a better solution would be an impedance converting buffer. The right one would give you at least 50kR input impedance and drop the output impedance down to a few hundred ohm while hardly changing the character of the pre-amp. 
georgehifi's avatar
georgehifi
4,966 posts
 
If I know the specs of the Sugden, I can have my Audio Note dealer modify the M1 with a feedback loop to lower its output impedance to match.
That's the wrong way to do it, better off getting the Sugden input impedance changed from 24kohm to 68kohm so long as it doesn't effect the output dc offset too much.

Cheers George
9radua1 OP
1 posts
 
Thank you for the advice. 

@kosst, Stereophiles measurements were for the integrated, whereas mine is the poweramp only version. They would have different input impendances, no?

@george, thanks, I will look into that. 
kosst_amojan
2,095 posts
 
I'd be surprised if they were much different. Sugden seems to prefer simple topologies which means they probably just stuffed a volume pot and source switches in front of a power amp. It's possible there's some kind of front end on it, but given the low input impedance it seems unlikely. 
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