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
 

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

All gains are of course not created equal, even if they are numerically equal. Noise performance and consequently hiss levels can certainly differ among different phono stages that provide similar amounts of gain, with that being likeliest to be an issue if an LOMC cartridge having particularly low output is used. Also, in some cases the sonics provided by the moving coil circuitry in phono stages having both MM and MC capability might not be as good as the sonics that would be provided by the combination of a good SUT and the MM section of the same phono stage.

Enjoy! Regards,
-- Al

Yes, under most circumstances 57.5 db is considerably too much phono stage gain for use with a MM or other high output cartridge. Typically MM phono stages provide gain that is in the area of 40 db or so.

In addition to the possibility of clipping, another downside that can occur in many applications would be having to operate the volume control undesirably close to the bottom of its range. As you may have seen in other threads it is not uncommon for that problem to occur with digital sources having **maximum** outputs in the area of 2 volts.

That much gain would often be suitable, though, for use with MM cartridges in systems employing passive preamps, to cite one example.

In your particular case, given the very high overload margin of the phono stage itself a means of utilizing a high output cartridge without overloading the preamp would be to insert a Rothwell in-line attenuator, or something similar, between the two components. Although reports I have seen over the years on the sonic effects of those devices have been mixed, with some (including me) having found them to be quite transparent, and others reporting significant adverse effects on dynamics or other sonic characteristics.

Rothwell attenuators are available in both balanced and unbalanced configurations, and in 10 db, 15 db, and 20 db versions. I believe that most and probably all of those versions would be suitable with respect to ARC’s loading recommendations for the outputs of your phono stage (quoted below from www.arcdb.ws), and certainly with respect to the 10K minimum requirement. But I know that some similar devices from other manufacturers would not meet that requirement.

ARC PH5: Recommended load 50K-100K ohms and 100pF. (10K ohms minimum and 2000pF maximum.)

Regards,
-- Al

In many designs low settings of the volume control may result in an inability to adjust volume in fine enough increments. I see that your REF3, however, provides 104 steps of volume adjustment, so I suspect that is not an issue in your case.

Also, in some designs channel imbalances can occur at low settings of the volume control. But again I would suspect that is not an issue with the REF3.

Finally, it is conceivable that in some designs having to operate the volume control at low settings may result in whatever sonic side-effects may be introduced by the volume control mechanism itself being worse than at higher settings. I have no knowledge, though, as to whether or not that possibility may have any applicability to the REF3. And even if it does, it would be anyone’s guess as to whether or not the possible sonic side-effects of an in-line attenuator, used with these particular components, would outweigh the possible benefit.

Best regards,
-- Al

Yes, I purchased my VTPH-2 new, directly from Keith after speaking with him on the phone. You would want to confirm with him that the lower gain version is the optimal choice for use with a 0.27 mv cartridge. I suspect that it will be, though, in part because he recommended that version for use with a 0.3 mv cartridge I was considering at the time. I ended up going with an Audio Technica AT-ART9, though, which is rated at 0.5 mv.

Best regards,
-- Al

Scott, yes, inspired by a great many overwhelmingly positive comments I've seen here and elsewhere, late last year I purchased a Herron VTPH-2 phono stage ($3650).  It is beautiful sounding, versatile, very intelligently engineered, absolutely quiet at any volume setting in my system in both LOMC and MM modes, and as many others have commented Keith Herron is a treasure to deal with.

It is offered in a choice of two gain configurations.  For the majority of applications, including mine, the lower gain version is preferable.  It provides gains of 43 db in MM mode and 64 db in MC mode.

The one slight caveat I would cite is that like your PH5 it does not provide balanced outputs, and John Atkinson's measurements of the REF3 indicate somewhat better measured performance in balanced mode than in unbalanced mode.  But even if those measured differences have any audible significance, which I doubt, I would suspect they are mainly attributable to JA having used the preamp's unbalanced outputs for the unbalanced measurements, not just the unbalanced inputs.  And I note the following statement in the body of the review:

ARC states that the long-tailed pair used in the Ref.3's input stage provides identical performance (save for the amount of gain) using either the balanced or the single-ended inputs.

Good luck.  Best regards,
-- Al
 

Hi Scott,

In the specific case of the Herron I would not expect using a SUT into its MM input to be preferable to going directly into its MC input. I say that in part because of the great results many have obtained using LOMC’s directly into the VTPH-2; in part because as I had mentioned I have found the VTPH-2 to be absolutely quiet at any volume setting (even when using my Stax electrostatic headphones, with the volume control at max); and in part because using a SUT into a 47K MM input will result in the cartridge seeing a much heavier load (i.e., a **much** lower number of ohms) than seems to usually be necessary with the Herron. (Keith recommends that with the VTPH-2 it is often preferable to apply no loading at all, with the input impedance of its LOMC input being almost infinite. And as Atmasphere and others have pointed out in past threads here, optimal loading of an LOMC is primarily dependent on the design of the phono stage, not on the cartridge).

To answer your question about gain, based on the relation db = 20 x logarithm(Vout/Vin) it can be calculated that for a 45 db gain setting of the Chinook the corresponding voltage multiplication is 178x, and the 43 db MM gain of the Herron corresponds to a voltage multiplication of 141x.

0.27 mv x 7 x 141 = 266 mv
0.27 mv x 13 x 141 = 495 mv

0.27 mv x 7 x 178 = 336 mv
0.27 mv x 13 x 178 = 625 mv

All of those alternatives would provide the 10x headroom factor Bob recommended, relative to the max input spec of the REF3.

Regards,
-- Al