Pass Labs Aleph Question - balanced vs unbalanced?

Ghostrider45,

I've reviewed the schematic again as well as the service manual for the Pass Aleph2. The input is a differential pair and the output stage is single-ended. This means that the input stage will reject common-mode noise and benefit from a balanced cable. For a single ended input signal the negative input of the differential pair is AC grounded by shorting the negative pin to ground on the XLR input.

Again there are two uses of the term single -ended

1) Single ended vs. Differential (Balanced)
Single ended- There is a positive signal and ground (like an RCA cable)
Differential (Balanced) There are both positive and negative signals as well as the ground. (like an XLR cable)

2) Single-ended vs. Push Pull
Single ended - means that only one type of device is fed the input signal for a stage. (N - type or P-type only)
Push-Pull- means both the N and P device are driven by the signal.

By definition 1) the Aleph amps are Differential (Balanced) inputs and single -ended ouput

By definition 2) the Aleph amps are single -ended only. The input is a PMOS differential pair and the output is an array of NMOS devices.

When talking about XLR vs RCA cables, only definition 1 is meaningful. Since the input stage is a differential pair, a balanced connection will have its common-mode noise rejected.

This is why if you have a differential source, I recommend using the balanced inputs. However some sources and preamps aren't truly differential and use extra circuitry in order to created a differential signal. In this case you would still get the benefit of rejecting noise picked up by the cable. However the extra circuitry in the source may degrade the sound.

Wloeb - I concur. The differential pair is certainly no passive circuit, though. By definition 2, the Aleph does not not require a differential input for its operation, so bypassing the additional circuitry when connected to a source that is not truly differential makes sense. This is especially true for short cable runs.

The Pass XONO that serves as the source to my amps is a non-differential implementation that creates a balanced output via a phase splitter at each output.

Pass the dunce cap - I referred to the Pass "XONO" Crossover in my previous posts - should have been the Pass XVR1 Crossover. Slip of the acronyms...

My Xono sounds better single ended.

Ghostrider45 and Wloeb,
Thank you both for the explaining the design of Aleph amps. As an owner of Aleph 5, I've tried all three types connections: balanced, RCA w/ shorting pin, RCA w/o shorting pin, all from ML No. 39. My question is, from the circuit design point of view, what are the benifits or drawbacks of not connecting the shorting pin on the XLR input when operating throught the RCA input? On the other hand, does the input impedence change at the RCA input when the XLR shorting pin in inserted? Though this may not produce much effect as the 39 has pretty low(<50?) output impedence. Thanks.