Input Impedance

David Ellis · « **on:** 13 Jan 2005, 01:25 pm »

What is the input impedance for the 55wpc and 100wpc AKSA amps?

Thanks,

WEEZ · « **Reply #1 on:** 13 Jan 2005, 09:00 pm »

Dave,

I think I read deep into the Aspen Circle that the input impedence is 43k. If true, then most tubed pre-amps should be compatable.

WEEZ

AKSA · « **Reply #2 on:** 13 Jan 2005, 10:04 pm »

Hi WEEZ,

My thanks, right on the money!

David,

R1, base bias resistor on both AKSA power amplifiers, is 47K. However, the input goes to the base of a LTP running half a milliamp per side, and this reduces overall input impedance to 43K.

This high input impedance was set up to accommodate tube preamps with common cathode output stages.

Cheers,

Hugh

OBF · « **Reply #3 on:** 14 Jan 2005, 12:42 am »

Hi. Just out of curiosity, a high input impedance is never a bad thing is it? I know a low i.i. can cause problems with a high output impendance source, but is there any downside to a highish i.i.?

JoshK · « **Reply #4 on:** 14 Jan 2005, 01:22 am »

I've heard it said that low i.i. can lower the noise floor, I guess that is why some do it.

What about the input sensitivity of the 55 vs the 100, are they close? IIRC (and correct me if I am wrong here) if the i.i are the same and the gain is the same, then the input sensitivity should be the same. Correct?

*Only on page 250 of my first electronic textbook*

AKSA · « **Reply #5 on:** 14 Jan 2005, 01:49 am »

Josh,

You've nailed it. Both have a fixed gain of 38.3, which is a highish 31.6dB. Their sensitivity is the same (both into 8R, of course).

I chose a high gain so that the amps could be driven via a passive preamp directly from a digital source. This is not normal audio convention, of course, but it accords nicely with the Aspen minimalist approach.

Cheers,

Hugh

JoshK · « **Reply #6 on:** 14 Jan 2005, 01:56 am »

My idea was for minimalist biamping, whether they work well. As I understand it, matching input sensitivity = easy passive biamping, i.e. no attenuation needed on one of the amps.

In my case, owning the IRD Purist, I have indepently driven dual outputs which gives me lots of ideas for placing the passive xo in the line level before the amps and biamping!

OBF · « **Reply #7 on:** 14 Jan 2005, 02:01 am »

Out of curiosity #2, what is the rated sensitivity?

For the demo, I had just enough gain in some cases with a transformer based passive. Usually only an issue with some live recordings that are kind of quiet, but my source is around 2v. Not sure if that's RMS or peak-to-peak, but I would guess the latter (Bolder smART DIO) since I sometimes have to run my pre at full volume w/ my amp and it's rated at a 2v sensitivity.