Pass Labs Aleph Question - balanced vs unbalanced?

I don't necessarily think balanced sounds better but there is less chance of hum.

The Alephs are a single ended design, so the balanced signal must be converted to single ended before driving the power stage. I use a pair of aleph 1.2 monos and have run them both single ended and balanced. They sound terrific either way, and I can't say that balanced is any better.

I tried both ways with my Aleph 0 monos and the XLR input does not sound better. I agree with Ghostrider45.

I don't necessarily think balanced sounds better but there is less chance of hum.

Rwwear, that's precisely one of the main reasons of why balanced sounds better!

This at least with well engineered products. In practice, only a few reach the full benefits of balancing.

Cheers,

Interesting... It says in the manual for the Aleph 5:

"For the lowest possible operating noise in any environment, the amplifier is equipped with balanced inputs featuring a common mode noise rejection of greater than 60 dB. Balanced operation is accomplished through a passive network tied directly into the input stage of the amplifier, not with additional active input circuitry as in other products. This assures that the noise benefits of balanced operation are not accompanied by the degradation of more semiconductors in the gain path.

The input of the amplifier is flexible and can also be operated with unbalanced sources. The input system will exhibit full common mode noise rejection with passive balanced sources, where the negative input is connected to ground at the source through the appropriate source impedance. This allows adaptation of unbalanced sources to balanced operation with passive cable connections in a manner that achieves the noise rejection of active balanced sources. Pin 1 of the XLR input connector is ground, pin 2 is the positive input, and pin 3 is the negative input."

And in the beginning: "The Aleph 5 integrates power Mosfet devices and pure single ended Class A operation in a simple two-gain-stage topology"

Okay - so that means the basic topology is single ended and there is a passive network to direct balanced signal to the single ended amplification pathway? Well, there is no reason to convert to balanced if the amp is just going to turn the signal back into single ended and amplify it there - I have no hum when running under RCA. I was under the impression it was a balanced through-and-through amp. Do I understand this right?

I have experienced a bit of hum using the RCA's on Pass Aleph amps that I have not experienced with other amps.

My preamp had only RCA outputs so I never could run balanced, but I have read on many posts on other forums that an XLR input does infact yield a quieter sound. If your preamp has balanced it would be to your benefit to use it. YMMV

Right - but remember I have no hum in my system on RCA cables at full pre-amp output and CD player having just played music and then hitting pause and turning pre-amp all the way up...

So in that case then there is no reason to convert back and forth, right? If I have no RCA hum, then is there any sonic reason to use balanced?

The only "sonic reason" should be that one way sounds better than the other and that's up to you. Sometimes the differences are very subtle and it's hard to tell. Seeing as your pre is not a fully balanced unit, the signal would have to be converted to balanced, then when it gets to the amp, which is not balanced, it would have to be converted again. Seems to me the rca would stand the best chance of sounding better if there is no noise getting into them. If you don't hear an obvious difference, listen again for a little more soundstage, depth, holography etc, with the rca's. Good luck.

Pass aleph amps are balanced (differential) I've looked at the schematic. They are single-ended in that they are not push-pull. This is a different use of the single-ended terminology. They do benefit from a balanced connection to the preamp.

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.

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.

A quick comment about the shorting pin - I don't know why but my current Comcast HDTV feed is very low in volume. Commercials are normal, which is infuriating as I have the volume super high for the show, and then the commercials are defeaning - but anyway - The Aleph 5 is not powerful enough without the shorting pins in place when used via RCA in for me.

All that to say, 1) I hate Comcast, and 2) with the shorting pins in, you will get more power.

I'll quote the aleph 5 manual:

"The Aleph 5 is shipped with a shorting plug between pins 1 and 3 of the input XLR connectors. This shorting plug sets the gain of the amplifier to 26 db when you are using the RCA unbalanced input. Removal of this plug will set the gain to 20 db. Needless to say, you will remove this plug when using the balanced input. There is no penalty for not using this plug withunbalanced operation, and if you ahve particualrly efficient loudspeakers, you may find that you prefer it this way."

Note that I haven't had the chance to get shorting pins (originals lost long ago) and I am currently using 2 twisted and bent paperclips in each pin, with no noticeable decrease in sound quality. He he.....

Lightminer,
Indeed, with the manual in hand, I've been wondering more than once what Nelson Pass was hinted at when he said "...with...efficient speakers, you may find you prefer it this way". Prefering the extra volume range offered by the 6 dB difference, if one uses, say, horn speakers that simply blast out at lowest volume settings when pin 2/3 are shorted? I doubt. Sounds like a gain in quality instead of quantity. Maybe just ask Mr. Pass...Anyway, it's easier to ask questions than dragging out my Horowitz and pretend I am EE major. The 6 dB is easily adjusted by the No. 39 to compare the effect of shorting pin 2/3, and to tell the truth, just like you said, with the pin, it's more powerful. But it's accompanied by a little edge on transients, which might bring the perceived soundstage closer. Kind of like the passive/active difference in preamps.

Agreed - I know he is big on 'fewer gain stages are better' in general, maybe it skips some amplification circuitry - shorter circuit path? Must be something like that. With pin in place it must somehow participate in circuitry involved with the balanced operation??? Agree that without asking its all conjecture...... (But perhaps the one thing we can be definite about is it skips some amplification stage and should have shorter circuit path).