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

@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.