Pass Labs X0.2 still has issues....

nooshinjohn

I have a new(to me) Pass Labs X1, that was just serviced and given a clean bill of health at the Lab just this past week. It was sent in because it had a loud and very audible "pop" between steps on the volume, and an audible "hiss" similar to what you would hear on a cassette tape back in the day. This hiss gets louder with each step up in volume and once an increment of 10 is achieved, the hiss steps back down again.

A large shift away from factory settings for DC offset were found and corrected, as well as a handful of capacitors found to be out of spec and replaced. The popping sound has been diminished by about 50 percent, but I find that the hissing noise is still there and very much a distraction to listening.

I sit 10 feet away from my speakers, and use Polk Audio SDA-SRS 1.2TL's, which have crossovers that have been recapped. The speakers are 91db sufficient. Driving them are my tube amplifiers. All connections to source components are made through XLR cables made by Audioquest.

My previous preamp was a Pass Labs X1, which exhibited none of the noise issues I have described. I do understand the pop sound is a function of the volume control and is a characteristic of it's design, and I am more than ok with that.

All functions of the pre work as intended and it sounds great otherwise. The only issue that really concerns me is that hissing sound. It is present regardless of source selected and can be heard some 20 feet away from the speaker even if nothing is playing. Is there something that can be done to reduce or eliminate this noise?

audioluvr

And swapping out the pre with another eliminated this?

pitdogg2

Man i was hoping this would be a giant game changer for you John.

Bummed

nooshinjohn

audioluvr wrote: »

And swapping out the pre with another eliminated this?

X1 and no noise at all. Swapping in the X0.2 and the noise is quite prevalent. All cables are the same. The only change was swapping the preamps. I have triple checked everything, swapped cables, disconnected the sda, everything I can think of. I think the troubles are within the volume control itself. I cannot fathom why they did not change it when it was there.

On the plus side.... holy crap this thing produces the largest soundstage I have ever heard come from my speakers. Everything was great before but now it is on an entirely different level. I hope a solution can be found, because I would hate to be on the prowl for a different preamp.
I am listening at low level right now and the nuance and micro detail is astonishing.

BlueFox

Good luck with it. I wonder if they do a listen test, or just measure the specs. I'm still waiting to get my XP-22 back.

nooshinjohn

They did an amazing job, including all new packaging and boxes, cleaned the crap out of it inside and out. AMAZING service all the way around, just can't live with that hissing.

They say it takes 48 hours to warm up and settle in. Maybe I am being too hard on things right now, but something tells me this is just the character of this unit. I can't imagine Nelson Pass would accept this noise coming from a flagship preamp.

The detail this beast is capable of is astonishing. Bud... you had an XP30 before, correct? did you have a similar issue, or way it stone quiet? I might look to acquire an XP20 if I can't get this one sorted out.

BlueFox

I had an XP-20, and it was dead quiet. Plus it never broke.

Clipdat

Send it back, since it hasn't been serviced to your expectations.

nooshinjohn

It costs money to do that. My gut just tells me that it may not like my tube amps, which is a very real possibility. What I need is a solid-state amp to test that hypothesis.

Clipdat

Why does the preamp care if the amp it's outputting to is tube or SS?

nooshinjohn

Might just be a bad fit for them is all. I don’t have all the knowledge, but I am thinking a gain mismatch.

Clipdat

I'm getting my read on: https://www.passlabs.com/sites/default/files/x0.2_om.pdf

Could it be some weird configuration issue or setting that has been overlooked? It does seem strange that Pass wouldn't give it a listening test before sending it back.

monepolk

Do you have another system that you can try the pre in?

audioluvr

Maybe reposition it so its isolated to see if it's some weird RF thing.

tonyb

John,

Your not the only one. I have a hiss too in the upper frequencies after changing things back to what they were. Only thing that changed was the addition of a new older amp, same cables same everything same way everything was hooked up.

Had the amp looked at, it's fine. Changed tubes in the pre, fine, still there. Nothing is more aggravating in this hobby of ours than chasing down an errant sound that appears that wasn't there before. Nothing bothers me more, than an errant sound I can't locate from where it's coming from. So much so...that I'm ready to rip it all apart and start from scratch.

txcoastal1

nooshinjohn wrote: »

Might just be a bad fit for them is all. I don’t have all the knowledge, but I am thinking a gain/impedance mismatch.

This can very much be the cause. Do you have the specs on the amp?

When I was speaking with Dan (Modwright) while pairing up a preamp with the Thresholds he asked for the specs. He tweaked the LS100 slightly to better match up the impedance pairing

https://forum.audiogon.com/discussions/how-to-match-preamp-and-power-amp

http://av2day.com/question/how-do-you-match-a-pre-amp-to-a-power-amp/

and many more on the web

tonyb

Good point Ron, a very important point too because sometimes we lay all the blame on a certain piece without considering it's just a bad match-up impedance wise.

heiney9

Clipdat wrote: »

Why does the preamp care if the amp it's outputting to is tube or SS?

Not all gear plays well together and tube amps can be a bit more touchy.

John,

Knowing the caliber of the "lab", I am more inclined to think it's a gear mismatch more than a bad pre. Have you called and talked to Desmond or anyone else at Pass to bend their ear about your issue? You know how willing and helpful they can be. They would know more than anyone here what the issue might be.

It would be nice if someone close to test it on a SS amp just to see what shakes out.

Good luck and I'm sure you know Pass will bend over backwards to help you.

H9

tonyb

Maybe someone in the area can lend you a SS amp to test this theory.

dromunds

I agree with Heiney, I'd call them and have an in-depth talk about your issue.

marvda1

nooshinjohn wrote: »

It costs money to do that. My gut just tells me that it may not like my tube amps, which is a very real possibility. What I need is a solid-state amp to test that hypothesis.

input/output impedance need to match.

mhardy6647

The ol' rule of thumb (which is no more, nor less, than just that): Input impedance of the load (power amp, in this case) should be at least ten times the output impedance of the source (preamp, in this case).
https://electronics.stackexchange.com/questions/46948/the-10x-rule-for-impedance-bridging

The nominal input impedance of the amp and output impedance of the preamp should be available from their manufacturers' specifications.

FWIW. The most telling symptom, I think, is this (from the first post of the thread:

...This hiss gets louder with each step up in volume and once an increment of 10 is achieved, the hiss steps back down again...

I am assuming that this preamp uses a resistive attenuator, which is not a constant impedance device. The maximum impedance of a resistive type volume control is, I have been told, maximum at an intermediate level of attenuation somewhere in the middle of its adjustment range, and it is not uncommon for a component (preamp, integrated amp, or receiver) to display a maximum level of hum at an intermediate setting of the "volume control".

Now, hum is not hiss*, and perhaps the attenuator in the component in question is a constant impedance network rather than simply adjustable resistance, but I am guessing that the symptom arises from an impedance issue of some sort.

Just a cluster of SWAGs, though.

______________
* FWIW: "hiss", in my experience, is usually thermionic noise (tubes) or "shot noise" (transistors), and is also usually most audible at minimum attenuation ("high volume" settings).

nooshinjohn

The amplifier specs...

2 Hz to 85 kHz (–3 dB)
Distortion: Less than 0.5%
Power Bandwidth: 24 Hz to 28 kHz without filters
Line Inputs: 1RCA
Input Impedance: 100 K ohms (RCA)
Gain: 30 dB (8 ohms)
Tube Complement: One 12AX7, one 12AT7, one 6AL5, six KT120
Hum and Noise: Better than 120 dB, referred to 350 watts, A-weighted

Preamp specs for the X0.2

Gain 14dB or 20dB
Gain adjustment Internal
Number of volume control steps 72
Freq. Response -3dB @ 2Hz, -3dB @ 100kHz
Distortion @ 1KHz <0.1% THD typ, 0.003% @ 2V
Output Voltage 20 Balanced, 7 Single-ended
Output Impedance 750 Balanced, 150 Single-ended
Input Impedance 20K Balanced, 10K Single-ended
Number of gain paths 4
Separate analog channels Yes
Remote control Yes
Input voltage before overload 11
CMRR 80db 20-20kHz
Output noise floor -110dBV ref to 20V output
Crosstalk typ. -90dB 20 - 20kHz
External amp turn on
Single-ended only inputs 5
Balanced / single ended inputs 6
Mono operation setting Yes
Unity gain setting Yes
Power Supply External
Power consumption 30 watts
Number of chassis Dimensions 3
Weight 72

mhardy6647

amp input impedance: 100,000 ohms (unbalanced) -- no spec for balanced*
preamp output impedance: 150 ohm (unbalanced), 750 ohm (balanced)

Certainly no mismatch between the two for an unbalanced connection.
Are you running balanced or unbalanced?

___________
* Presumably also high -- does the amp have balance inputs?

nooshinjohn

Unbalanced between amp and pre. The amp does not have balanced input.

heiney9

The "X" gear is really designed to be run balanced. It can run unbalanced, but perhaps this is part or all of your issue?

If your read closely between the lines in the owner's manuals of most, if not all, X gear, it's suggested to run balanced for best results and performance.

Just throwing that out there.

H9

mhardy6647

Integrating the content of the two posts above
If I were @nooshinjohn, I'd try a different set of cables 'twixt preamp and power amplifier. Couldn't hoit

Oh, I assume it does this independent of source input, yes? In other words, if (for example) the preamp has no inputs at all connected, it will still manifest the hiss at some intermediate attenuation (volume) levels, yes?

nooshinjohn

I did H9... The X0.2 also runs at full output at all times, using a unique resistive attenuator to adjust the volume. The X1 does not, and I think that may be what the speakers are picking up.

bikerboy

Preamp output impedance is 150 ohms and the amp input is 100k ohms. Thats the problem. Unless I am reading it wrong.

nooshinjohn

mhardy6647 wrote: »

Integrating the content of the two posts above
If I were @nooshinjohn, I'd try a different set of cables 'twixt preamp and power amplifier. Couldn't hoit

Oh, I assume it does this independent of source input, yes? In other words, if (for example) the preamp has no inputs at all connected, it will still manifest the hiss at some intermediate attenuation (volume) levels, yes?

Yes, it does this independent of source selected and is still present even with no source connected to the preamp. The hiss is present even if the attenuation is set to zero. It is also present in both left and right channels, with or without the SDA connection. In my mind, that eliminates cables or source components from being an issue.

It would appear to be exclusive to the preamp itself, or in the match between amps and preamp.

heiney9

I am also thinking if they at Pass Labs can isolate your issue, maybe they could offer something or a suggestion to lessen the effects. They're pretty good that way. Have you looked in the Pass Labs forum on DIY audio to see if anyone else with similar gear has experienced this? That's an amazing resource and Papa interacts a lot in the DIY forum.

Might want to have a look see over there

H9