This One has Me Stumped

I recently purchased a Pass XP-12 to replace my aging ML 38s. After installation, I noticed gobs of high frequency distortion, especially in Operatic female voices.

I measured every component in the signal chain separately. All are performing in spec.The XP-12 has very impressive test results.

Re-installing the 38s gets me immediately back to more mellow sound.

In my system, I have an EQ and crossover between the XP-12 and my Bryston 2.5b^3 power amp. I removed the EQ and the XO, plugging the XP-12 directly into the power amp. The distortion remains.

I am puzzled. I am a measurement guy. I use REW extensively. I can measure and troubleshoot almost any problem in my system. This one is a stumper.

Can anybody suggest a measurement or a way to solve this apparent component mismatch?


My system in order from source to output :

Shelter 901 MKIII MC or Koetsu RS MC cartridges

Parasound Zphono XRM phono preamp

Technics SL-1200G TT

Oppo BDP-105 Universal player (digital)

Bryston BDA-3 D/A ( feeds the Oppo signal to the XP-12 or 38s)

Sansui TU-X1 tuner

XP-12 (38s) line amp

dBx 1231 EQ

Rane AC22 2 way crossover

Bryston 2.5B^3 power amp

Usher 604 floorstanding speakers

HSU ULS-15 sub (when using the XO)





128x128Ag insider logo xs@2xkevemaher

I’d go right to the source and try calling Pass.  They have a good reputation for customer service so worth a try I’d think.  

Leave your preamp on for a couple of days.  Doesn't have to play music, just don't turn it off.

well - issue is that the XP-12 has a VERY high output impedance (22k ohms).  That'll certainly cause the distortion you are hearing.  The recommended input impedance for the amplifier should be 10 TIMES the output impedance of the pre and unfortunately, the Bryston's is only 30k ohms.   Send the Pass packing... 

Must be your tuner, you better sell it to me. not that there is anything wrong with my TU-9500, but I do like the TU-X1 looks better ;-)   ok maybe I'm a bit jealous as they are hard to find priced reasonably, mind you probably the best tuner ever made.   

Actually, the input impedance is 22k ohms while the output impedance is 25 ohms RCA and 50 ohms XLR, therefore an easy load for virtually any amp.

Tough one. Have you tried different ICs?

Take the pass to a friends house and see if the problem follows you.  Sounds like you think you can measure what is causing the problems, yet you don't know what is causing the problems.  Right now you have no solid evidence that the Pass preamp is working correctly and lots of evidence that it isn't.


Your source is probably overdriving the preamp. Does this happen with digital sources as well?

I called Pass. They told me to send it in. Since I live about 30 min away from the factory, I will hand deliver it tomorrow.

@audphile1 You have a great suggestion. Please check my analysis below:

Shelter 901 output is 0.5mV at 0 dB. Zphono gain is 60 dB (1000x). So the input to the xp-12 is 0.5V at 0dB.

This is right in the sweet spot for input. 20dB more might cause a problem., but that's 5V. I don't think any LP has peaks that high. Measurements by Super Best Audio Friends shows that the THD is 0.001% at 0.5V in according to those measurements.

I just measured the output of the phono preamp with an LP playing. My AC voltage meter indicates a range of 50mV to 300mV as the LPs are played. This is for one channel measured at the RCA outputs (single ended not balanced).

The input signal level shouldn't be the problem.

I called Pass. They told me to send it in. Since I live about 30 min away from the factory, I will hand deliver it tomorrow.

Ooh you lucky baastard (from Monty Python’s Holy Grail). Good for you! Let us know what the issue was if you could.

Kent even provided a tour of the factory and introduced the team assembling their gears. That was my experience when I picked up Pass items. If Kent has time, he would not mind doing that.

I do not have experience with your models, but in general, and to refrain others:  

1:  Check the source out-put max voltage  specifications to the pre-amp input specifications to assure tolerances are stacked.  Adjust the output voltage accordingly if your phono pre or DAC permits such adjustments.  

2:  Check your preamp output max voltage specifications are stacked to your power amp input specifications.  Adjust if the preamp permits.  

3:  Tolerance stack output/input impedances source to pre and pre to power amp as well. 

I had an experience when I got my current integrated amp as a replacement to separates.  My DAC XLR max output voltage setting exceeded the XLR max input voltage specification for the new integrated.  This caused clipping distortion.  Once I stacked and adjusted the DAC max output to the integrated max input specification, the clipping distortion vanished.  

@glennewdick intros a 'phile phenom we've encountered.....

The Vulture...... "IF that's broke, can I have That?" *L*

(....ready to catch what's tossed in frustration.... ;) ...)

..just teasin'...

If Pass can't Rx the issue, @jsalerno277 seems to have an advanced routine that might cure.....👍🤞   Good variable factors.....

@asvjerry Thou speaketh like you have an engineering or science background (variable factors). I, a quality engineer, Six Sigma Blackbelt, VPQARA, medical devices, retired.  

@jsalerno277 ....*L* No, more a background of having engineers design an 'enhancement' that drove fabricators and installers nutz do to the 'special tool' that didn't seem to hit the site with 'Gadget X'.....;)..... just jestin'   *G*

"Practical Magic" is more my experiential background.... 

You need a tech to put it on a scope to check the bios which could be causing the problem.

If you make a change and that breaks whatever you made the change in, that's a pretty good sign that the change introduced the problem. I think Pass will figure it out.

Just buy a Pass amp. The 30.8 is a great amp. 
a lot of people do not understand you have to have your amps specs line up with your preamp or DAC/Streamer used as a preamp. 
hegel like higher powered amps and weaker preamps. American equipment typically does the opposite. I have a buddy who has the Hegel mono blocks with the VAC Master preamp. Oddly though it has very similar specs to the Hegel Preamp so it works perfectly. I think the VAC is 9 volts. Where as my audio Research Ref 5se has 12 volts output which sounds amazing with my Luxman M10X amp. 

Did you ever get a measurement of the high frequency distortion coming out of the speakers? Was REW showing super low distortion, even though you could hear it coming through the speakers as the sweep was playing? I have found that REW can't always distinguish distortion from background noise or late reflections. 

 Seems like you've covered the compatibility issues and the distortion shouldn't be happening. I'm sure they'll figure it out. 

The reason I mentioned REW getting confused is that I've had highly audible distortion pop up that was apparently so strange as to be unrecognizable by REW. Flutes sounded like a Kazoo was playing along with them. I thought a driver had a loose spider or something. Turned out to be an odd anomaly  in the digital domain. 


The 30.8 may have ideal synergy with the XP-12, but the XP-12 should not be causing the amount of distortion the OP is hearing and measuring with any good amp with reasonable input impedance - which he has. Best to take it in and have it checked. 

Similar experience when my source component was overloading my preamp.  I suggest you try every source and see if it's consistent or just a single source.

I suppose the preamp could also be overloading the EQ or your EQ settings are too high relative to the new preamp's signal and overloading the crossover.  Try turning the gain down on everything a bit, including if you have any variable gain on any source so you can be sure you're not overloading the pre. Basically turn it down young man! 

I've taken the xp-12 into the Pass factory. Kent gave me a quick tour of the production area. I explained my observations and measurements to the tech who will be testing the xp-12.

They've had it for about a week. I do hope they have found something wrong.

@jji666 I've measured signal levels throughout my system with an AC voltmeter, osclloscope and RTA. The measured input and output voltages are well within normal range for each component.

I hope that Pass has an answer.

Pass Labs found a gain module that was out of spec. The preamp was manufactured in 2019. Passed replacd the module with the current production module and tested. My XP-12 is now in spec.

I've not set it up in my system yet.

Pass was very responsive and discovered the problem very quickly and was honest and straightforward about what they found and what they would do to fix it.

Excellent service! Very trustworthy.