Ideal Pre Amp for Bryston 3B ST or 4B SST 2

I'm using a 3BST with an ARC SP16 in conjunction with a SCE Harmonic Recovery System device that helps with impedance matching between amp/preamp. These are driving Vandersteen 3A Sigs and I am enjoying the sound. When my finances are right I'd like to get an ARC power amp, maybe a 100.2,D300, D400mkII or 300.2. The ARC amps have a 150k input impedance which would mate perfectly with the SP16.
The SCE HRS is no longer made, but there have been discussions about it here and elsewhere. I'ts an option if impedance matching is an issue. Some may not like the idea of another box, more cables, etc in the chain. I have seen them come up here on A'Gon from time to time. If anyone out there has experience with this device, please chime in on your observations.

12-08-10: Bob_reynolds
Some manufacturers of both solid state and tube gear will ensure that the input impedance of their solid state amps is high enough to mate well with one of their tube preamps. Thus, sticking with the same manufacturer has it's advantages.

i can believe that...

12-08-10: Bob_reynolds
Of course, some solid state amps are designed with a high enough input impedance to mate well with any kind of preamp. You'll notice that Bryston amps have a usefully high impedance on their unbalanced inputs and a less useful impedance on their balanced inputs.

i did notice that input impedance for balanced inputs is lower than that for unbalanced inputs in bryston amplifiers. do you know why it is that the input impedance for the balanced inputs is lower than the input impedance for the unbalanced inputs?

I'm going along with Bob reynolds, You may not call it synergy, but when a designer designs both an amp and preamp, he matches them exactly as he see's they should be, no voltage or impedance mis matches, similar idea's and normally the same brand of gear does quite well together. I can't say if it is synergy or not, but it works. Of course that doesn't mean other things won't work together, i'm just putting in a reason why Bryston just might work well with another Bryston. I haven't listened to them, but I'm told Brystons latest generation pre's are quite good.
As far as another pre that won't cost a mortgage??? In july I picked up a New Coda, bought directly from Coda, they discounted it nicely and it is a very nice piece of equipment, worth looking into. Good Listening, Tim

I did notice that input impedance for balanced inputs is lower than that for unbalanced inputs in bryston amplifiers. do you know why it is that the input impedance for the balanced inputs is lower than the input impedance for the unbalanced inputs?

I took a look at the schematic for one of their amplifiers, the 3BSST, shown here. (Bryston is the only current manufacturer I am aware of that has schematics for many of its products on its website). The amp has an input impedance spec of 50K for the unbalanced inputs, and 20K (described as "10K per leg") for the balanced inputs.

The input stage active device, referred to on the schematic as a DOA68, I believe is an op amp that is custom made for Bryston. In the unbalanced input configuration it is used as a non-inverting amplifier with a gain of either 1 or 2, selectable via a switch. Circuits in which op amps are used in a low-gain non-inverting configuration have extremely high input impedance, in this case such that the overall input impedance of the circuit is essentially determined by the 49.9K resistor R6.

In the balanced input configuration, the op amp is of course used as a differential amplifier. That kind of op amp configuration has a much lower input impedance than a low gain non-inverting configuration.

The overall input impedance in the balanced mode is essentially determined by a combination of the various 20K resistors you will see on the schematic. The input impedance of the Channel 1 inverting leg is equal to the parallel combination of R1 and R1A, which is 10K.

In the high gain (1V) configuration, the input impedance of the non-inverting leg is equal to the parallel combination of R2 and R2A (10K) in series with R4 (20K) resulting in an overall input impedance of 30K.

In the low gain (2V) configuration, the input impedance of the non-inverting leg is equal to the parallel combination of R2 and R2A in series with the parallel combination of R4 and R5, resulting in an overall input impedance on that leg of 20K.

So the input impedance description for the balanced inputs of "10K per leg, 20K overall," I don't think is entirely accurate, but the higher input impedance on the non-inverting leg won't have any adverse effects, as I see it. It is common for differential amplifier stages to have unequal input impedances on the two legs.

I suspect that the reason the 20K resistors were not chosen to be significantly larger, such as 50K, is that in the differential amplifier configuration various non-ideal characteristics of the op amp itself, such as perhaps what are called input offset currents, as well its own input impedance, would then have become significant.

Regards,
-- Al