2. amps-preamps
  3. 189170-23267

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)

 

Thanks,

Kevin

 

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kevemaher's avatar
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kevemaher OP
106 posts
 

@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.

grislybutter
3,410 posts
 

looking at your system photos, it's pretty obvious: the lamp :)

soix
9,796 posts
 

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.

cfa88's avatar
cfa88
111 posts
 

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.

jsalerno277
234 posts
 

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.  

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