Audio Research PH5 Phono Preamplifier Input Clipping Problem

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
 

Thanks for the responses.  Perhaps my logic is faulty, but I suspect it is the PH-5 simply because the clipping disappears when I remove the SUT.  Without it, and with the REF 3 cranked up to a fairly high volume (about 2/3rds of maximum) there is sweet sound with no clipping.  Now, perhaps I am being paranoid, but I don't particularly like running the REF3 at that high a volume.  So I prefer to use the SUT. 

almarg, I see your point.  It could be the mismatch between the PH-5 and the REF 3 output/input.  Is there an easy solution if this is the case?

My suspicion is that the REF 3 design includes active circuitry that is "ahead" of the volume control in its internal signal path. If so, the fact that the volume can be cranked up to high levels without clipping when a SUT is not being used would still be consistent with the clipping occurring in the REF 3 rather than in the phono stage.

IMO there is no reason to be concerned about using the volume control at 2/3 max or other relatively high settings within its range. In fact that is considered by some to be generally preferable, as in some designs it might minimize whatever sonic side-effects may be introduced by the volume control mechanism itself.

So I would simply go without the SUTs. Keep in mind, of course, that without the SUTs the resistive load setting in the phono stage that is sonically optimal stands a good chance of being lower than the setting(s) you use with them.

Regards,
-- Al

Agree with Al 100%. As long as you can get it to play as loud as you want without excessive noise you are fine. It doesn't really matter where the volume control is. 2/3 of max is fine. You are not going to hurt anything. Generally speaking the more wide open the volume control the better it will sound' There is no need to be paranoid about the volume control. Lose the SUT.