Pass Labs Aleph Question - balanced vs unbalanced?

Wloeb - I thought something like that may be the case - because Nelson Pass says in his materials that air is single ended in how it relates to sound and that made me think the words meant something else...

Okay - so the Pass Alephs are internally balanced? So if I go RCA, the RCA input has to be converted to balanced? That makes sense... I had thought they were fully balanced years ago.

If an amp is balanced internally then its got to be at least a hair better with balanced inputs - even if that hair is not discernable. (Because of additional conversion step).

Oh - but then there are those who might say that conversion could improve the sound and we get into our basic audiophile philosophy about equipment...

Bottom line - amp is internally balanced, so balanced should be better - recognizing that one needs to listen for final conclusion as odd things happen in complicated systems. Cool.

If you use RCA (single-ended) interconnects, make sure you keep the jumper pins to connect #1 and #3 on both XLR connections. Otherwise, you will loose some dB when using single-ended cable.

You brought up an interesting point regarding terminology. I remember 10 years ago, I had read on Stereophile referring RCA connections as "sigle-ended" connection and used that on one of the online forums and was laughed at for misusing the term as at that time the term was mostly used to describe tube circuitry as others have stated.

FrankC

A push-pull amp topology requires two signals 180 deg out of phase. Each phase feeds either the push or the pull section of the output stage. When driven by a single ended signal, the push-pull stage is always preceded by a phase splitter to generate the correct signals. A balanced input can bypass the phase splitter since it conveniently contains two signals 180 deg out of phase. This saves a bit of active circuitry.

For single ended amps like the Alephs, the situation is reversed. A single ended signal goes straight to the first gain stage, while a balanced signal must go through additional circuitry to subtract the negative phase of the balanced signal from the positive phase, thus achieving common mode rejection and a doubling of the signal voltage.

Pass claims to do this via a passive network. I can't think of a passive method other than a transformer.

A single ended connection lets you avoid this passive circuitry at the expense of common mode rejection. For short cable lengths I don't think noise pickup is much of an issue and I'd rather avoid the extra circuitry.

My system plays both sides of this equation, A Pass XONO crossover high pass section feeds the Aleph 1.2 monos via a single ended connection, which in turn drive the ribbon panels of a pair of Apogee Studio Grands. The low pass section of the XONO drives a pair of Aragon Palladium II monos via balanced connection, which in turn drive the integral Studio Grand subs. The Paladiums are a tradition push pull design that omits the phase splitter and requires a balanced signal to operate (the single ended models just add a phase splitter).

One last thing - the first generation Alephs used a separate pull stage that took effect when the amp approached clipping. These operated push-pull over part of their range. The second generation (including Aleph 1.2 and 5) dropped the pull stage and simplified to 2 gain stages. They never move into push pull operation under any conditions.

Ghostrider45, Thanks for the explanation . I thought I was going to be a majority of one on this. I always say try both and see what sounds best. In his case, with a tube pre, the balanced might be given the nod as the imput imp of the Aleph is 25k ohms vs 10k ohms of the se.

Do you, or anyone else, know whether the XA or XA.5 series would sound more like an SET?

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.