[dpacella]

Hello;
In the manual for the 4B3 the spec for BALANCED INPUT IMPEDANCE states:
NON-INVERTING: 20k ohms
INVERTING: 6k ohms
In comparison, my previous 4BSST2 was 20k ohms for both +/- inputs.
I had a Pass Labs X2.5 preamp with about 1k output impedance (both legs). I used the combo in balanced operation and the match with the 4BSST2 seemed to be excellent as the bass extension and control was thunderous and full. I noticed no obvious (or even subtle) impedance mis-match issues.

I recently picked up a Pass XP-10 preamp with about the same Zout: 1k balanced. (single ended out is about 150 ohms, much less!)

I am concerned about the low 6k Zin on the INVERTING path; as you likely know an audio rule of thumb is minimum multiple of 10x; better still 20x on the Zin/Zout ratio. With the Pass XP10/4B3 we see 6k/1k =~ 6.

I am currently using the XP10 to the 4B3 single ended; the results are wonderful, but I am waiting for longer (8 foot) balanced cables to arrive. I am curious to hear the results.

I asked Bryston 2x via 2 different methods and received no response regarding the spec; I was curious why the INVERTING input is less than half that of the NON-INVERTING leg, and more importantly if Bryston had any concerns about using a preamp with a high-ish output impedance (balanced). Could there be an error in the spec and both are really 20K?


I appreciate in advance any advice/insight!

[Elizabeth]

James, I sent this fellow over here from Audiogon to ask you.
So I am hoping you can help answer his question!

[James Tanner]

HI Liz,

OK I will find out for him.

james

[dpacella]

James,
Was wondering what the output impedance is (both SE and DIFF) for the BP17 cubed. The manual does not list this information.
I realize i made a slight mistake in my pending question about the 4b3 DIFF input impedance: the "+" input is spec'd as 30K ohms (not 20k); I'd like to know if the "-" input is actually 6k ohms (and not 30k).
If it is 6k, how does this imbalance affect the situation when the upstream source has an output Z (symmetric) of 1k ohms.

Thank you

[James Tanner]

Hi dpacella,

Well that ones over my head so I will send to engineering for you.

I know our preamps typically have less than 100 ohms of output impedance so as long as the input impedance of the amplifier is more than 10 times that amount all is good.

These numbers are from an independent Magazine Review of the 4B Cubed:

Input impedance @ 1kHz

Stereo mode     
Balanced input: 20k ohms
Unbalanced input: 29k ohms

Mono mode
Balanced input: 10.5k ohms
Unbalanced input: 7.8k ohms

james