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On Step Ups and the Importance of Managing Gain

The past several weeks have been some of the most eye opening in my entire audio journey. Despite over 30 years listening to LPs across a range of setups it seems I've only just stumbled across one of the secrets of analog reproduction -- the step up transformer AKA the importance of managing gain across the analog reproduction chain.

To understand what I mean you need to start with an understanding of my gain cascade prior to my most recent changes. My cartridge is an Acoustical Systems Palladian which is a relatively low output MC at 0.33mV. I've been running it into an ARC Ref 2SE Phono and then an ARC 40th pre. Both are connected balanced so the gain is 51dB (for the phono stage) plus 12dB (for the pre) = 63dB at the LO gain setting -- I tried listening to the HI gain (74dB) setting on the phono but found the added FET amplification added an edge to the sound, amplified hum and restricted dynamics. I'd therefore learned to live with having the Ref 40 volume control on the high side (about 70-80% of range) and "thought" I was getting good sound - great clarity, sound-staging and frequency delineation.

However with a pending upgrade in my phono stage to the VOSS phono, a pure 40dB only single ended affair, I was forced to add a step up to my setup. I've always sworn off the added complication and additional switching and cables so had assumed it would not do anything to improve things. How wrong I was. The model I have is the Music First Audio Classic V2 in a custom configuration built for me with two inputs (one for my mono arm and cartridge) and fixed 20dB gain (1:10 ratio). It also has a three way ground lift switch so any hum issues can be addressed at source (which works btw). At about $3K it's not cheap (and MFA have cheaper options) but in the scheme of things actually much less than the list price of the interconnect I needed to add to connect it to my phono stage (so in the context of my system a stone cold bargain). 

My gain cascade is now 20dB (step up) plus 45dB (phono, now single ended) plus 12dB = 77dB and the volume control is at 40-50% of range. Once I'd realized that running a step up into 200 ohms is not going to work at all and switched the ARC Ref2SE to 47K I was off -- and was I surprised! All of the sudden the soundstage opened up by 50% side to side, front to back and up and down. Dynamics on individual instruments suddenly snapped into focus and the overall presentation was transformed in a way I would not believe possible. One instance can serve as a sense of what I mean -- the two opening cuts on Joni's Hejira, "Coyote" and "Amelia", are mostly similar sounds layered over one another and can be pretty hard to sort out. With the new step up in place it's as if everything now makes sense and each instrument is positioned in space perfectly in location and scale with all the others.

So what's my take away and potential advice to any reading this?
  1. High gain (i.e. 60dB plus) phono stages are, as we all know, problematic so if you can avoid going that way consider looking into it, BUT
  2. Don't "push your gain" i.e. try to under drive a stage, you may not know it (as I didn't) but trying to amplify .33mV with "only" 51dB is probably losing a lot of detail into the noise floor of your amplification stage and/or giving you problems down stream if gain is set too high
  3. So given 1 and 2 try a step up -- you may be surprised as I was!
Not sure if this experience matches with others and would love some perspective from amplifier designers as to why I may have had such a bad (in retrospect) experience with my prior gain cascade.
128x128folkfreak
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almarg
8,673 posts
 
Is it true that during loud passages the cart output can increase 10x and run into a ceiling in the phono which also causes distortion?
Yes, brief dynamic peaks on some recordings can significantly exceed the standard 5 cm/second test condition. My understanding is that a phono stage overload margin of 10x (which corresponds to 20 db), relative to a typical LOMC output rating of 0.5 mv, is an ample margin, but that some well regarded phono stages fall at least several db short of meeting that number, at least at some frequencies, and still do fine with all or nearly all recordings.

As you may be aware, the measurements section of Stereophile’s phono stage reviews usually present data on overload margins at various frequencies, and at various gain settings if gain is adjustable.

An additional concern that can arise in some cases, though, would be the possibility of overloading the input stage within the line stage, if the line stage is designed such that its volume control is not "ahead" of the active circuitry in that stage.

Regards,
-- Al

folkfreak's avatar
folkfreak OP
1,149 posts
 
Thanks @almarg so the interesting conclusion is that in my prior setup (51dB gain in phono plus 12dB in pre) I was getting a maximum of 0.47V against 0.75V sensitivity hence I suspect the issues I was having. Now with my step up (20dB plus 40dB or 45dB plus 12dB) I am at 1.31-2.34V so lots of room and the dynamics I'm now enjoying
robd2
44 posts
 
Great thread @folkfreak these are very important measurements when pairing equipment.
lewm
6,770 posts
 
What about "current-driven" phono stages for very low output MCs?  This is either a new bit of jargon conjured up by manufacturers or a really new idea.  So far as I can tell, allowing the cartridge to drive the phono stage by virtue of its current output amounts to building the stage with a very low or zero input impedance.  This would seem to require a solid state device for the first amplification stage; tubes don't do well with very low value grid resistors or no grid resistance (grid connected to ground) unless you drive the cathode.  
hdm
971 posts
 
They’ve been around for quite some time Lew. I’ve owned my Aqvox for at least 10 years now and TNT reviewed the Nibiru about 5 years before that. A few others I think as well were around in that time period. 

Technically I really do not have a clear understanding of how this system works; I was able to do an in home audition with the Aqvox before purchasing. I liked what I heard so I bought it. It was early in the game for the company and they had no formal distribution in North America so the price was a steal for me as well-the current Aqvox sells for more than 3 times what I paid for mine at that time.

Carlos Candeias designed the Aqvox and then moved even further upmarket when he created BMC; both stages have been pretty favourably reviewed.

My personal feeling is that the design does not appeal to many audiophiles because it is plug and play with respect to loading-absolutely no ability to "change" the load on a current input. It is what it is. The Aqvox and the BMC are both fully balanced designs also, requiring a change to a balanced tonearm lead, which is not really a big deal but it’s probably another resistance factor in the general audiophile market.

FWIW, (within the context of this conversation), the Aqvox also has fully variable gain within a very broad range (55-75 dB); I find the optimal gain window to be very small with the low output MC’s (from about .2 to .35 mV) I’ve used with the Aqvox. 2 dB or less actually.

Subjectively, I can understand that some listeners might prefer a gain setting very slightly (say 2 dB) on either side of what I would prefer for example. But if you do not have fully adjustable gain, you really cannot appreciate this IMO. If you are dealing with gain settings on a phono stage beyond 3-4 dB increments, for example, I really do not think you can really dial multiple cartridges in.

So, yes, I would be in full agreement that gain is extremely important.
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