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Audio Research PH5 Phono Preamplifier Input Clipping Problem

I have an ARC PH5 phono preamplifier and most of the time it sounds fine.  But on a few of my favorite LP's with wide dynamic range, it will clip and distort on loud sections, especially those with lots of bass content.  This only happens with use of a step-up transformer, without the SUT in line, there is no clipping. I do prefer to use the SUT, as without it I have to turn the volume setting on my ARC REF 3 preamp to a very high level.   I have to conclude that the PH5 is distorting because of high input signal levels, due to the gain of the SUT.   I have tried two SUT's, one with 13:1 and the other at 7:1, both exhibit the same problem (although the lower gain SUT is much better).  The cartridge is Ortofon A90 with .27 mv output.  Given this, would you expect clipping or do you think there is something wrong with the PH5?  I have talked to ARC about this issue, and they seem to think that there is a chance the PH5's maximum input of 70 mv is being reached on these loud sections.  Would appreciate opinions from those that have experience with a similar setup.  Thanks.
Ag insider logo xs@2xscottwsmith
bobsdevices
100 posts
 
Using a step up transformer of 1:20 will give you an output from the cartridge of 5.2mV, about the same as that of a MM cartridge.  The PH5 overloads at 70mV and typically if you allow for about 10x the input voltage for peak overloads, you have enough headroom.  I would not recommend using an input of over 7mV into it.  If you use a step up of 1:13, as you have described, the effective output from the SUT would be 3.5mV, less than if you used a MM cartridge.  So, something is not right here. 
Ag insider logo xs@2x
scottwsmith OP
60 posts
 
So, it would seem there is something wrong with the PH5, in your opinion?
almarg's avatar
Ag insider logo xs@2x
almarg
9,670 posts
 
Are you sure it is the phono stage that is overloading?  Your Ref3 preamp has a specified maximum input level of 10 volts for its unbalanced inputs (which I presume you are using, since the phono stage provides only unbalanced outputs). 

The 57.5 db specified gain of the PH5 corresponds to a voltage multiplication of 750x.  Therefore: 

The preamp input that will occur when the cartridge is providing its rated output into the 7:1 SUT: 

0.27 mv x 7 x 750 = 1.42 volts

The preamp input that will occur when the cartridge is providing its rated output into the 13:1 SUT: 

0.27 mv x 13 x 750 = 2.63 volts

With respect to the Ref3's maximum rated input neither of those numbers allows for the cartridge to exceed its rated output by the factor of 10 that Bob suggested. 

Also, I note that the PH5's maximum rated output (50 volts) is far greater than the Ref3's maximum rated input (10 volts), which also reinforces the possibility that it is the Ref3 that is overloading.

On the other hand, though, if you have somehow determined that it is in fact the phono stage that is overloading, then as Bob said it would appear that something is wrong.

Good luck.  Regards,
-- Al
 
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