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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
willemj
1,036 posts
 
In the old days amplifiers like my old Quad 33 had interchangeable input boards with gain and other characteristics that could be matched precisely to the cartridge for optimum S/N, frequency response and distortion. Quad supplied them ready made for the most common high end cartridges of the day, and others could easily be made from blank boards.That same amplifier had similar adjustment options for the tape input and output. As you are discovering, this is indeed very important.
The same issue also exists with the analogue output of CD players or DACs. CD Red Book specifies a 2V output, but many amplifiers have much more sensitive line inputs. The result is that the input stage may be driven into clipping, with increased distortion and compression of peak signals as a result. The solution is simple: just fit inline attenuators or an attenuating cable. The result is often a far smoother sound. The giveaway sign that this is indeed a problem is when you only have to turn up the volume control a little bit to have a loud signal.
The commercial reason behind this strange situation is that the human brain interprets louder as better. So if a CD player or DAC is slightly louder than another, this gives an unfair advantage in de demo room because the innocent listener will interpret this difference in level as a quality difference (as that is how the brain works). The same applies to amplifier input sensitivity. The more sensitive amplifier will be louder and hence seem to be better. Proper comparison should be level matched within 0.2 dB, but that requires the serious Volt meter that I at least have never seen in a demo room.
lewm
12,460 posts
 
It's interesting that guys who build equipment from scratch very often talk about building for minimal or just barely enough gain, because in their way of thinking, gain is inevitably accompanied by added distortion and after all you're throwing away energy by sinking it to ground through a resistance, when you use a volume control to tame excessive gain.  (They do not like attenuators.) I have educated myself about circuit design, and I do a lot of tweaking of circuits but not much building from scratch.  Yet I have found in practice in my system that an excess of gain always has sounded "better" to me than having just "enough" gain.  Dynamics are better, signal to noise ratio seems to be better. The trade-off is that you need to have a very excellent and transparent attenuator to realize all the benefits of the surplus of gain; the attenuator becomes a critical piece of the puzzle.  (I have heard my own Atma-sphere MP1 phonolinestage both ways, with barely enough gain for a LOMC and now with prodigious gain, sufficient even to run an Ortofon MC2000 [.05mV output] without a SUT.  I like the high gain version much better but only after upgrading the attenuator.)

On the other hand, I disagree with you in dismissing active gain stages as compared to using a SUT, to achieve sufficient gain.  I am not saying that a SUT cannot give a great result, but so too can a properly devised active gain stage, for dealing with low output cartridges.  The experience you relate here has to do with one particular sample of one phono stage, not enough data to support a general conclusion on the merits of phono gain stages.  (Nor does my anecdote support a generalization, I admit. It only suggests there is more than one path.)
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almarg
9,670 posts
 
Interesting post; thanks! And congratulations on the improvement you’ve realized to what was certainly an already exceptional system.

My feeling is that the root cause of the issue in this case may very well have been ARC’s choices of the gains provided by the Ref2SE, which seem a little strange. 51 db being too low for most LOMC applications (and too high for most MM applications), and 74 db being too high for most LOMC applications.

51 db represents a voltage multiplication of 355 times, which would raise the output of a 0.33 mv cartridge under the standard test conditions to only 117 mv. The very reasonable 12 db gain of your line stage would raise that to about 466 mv if the volume control were turned up to max, which would be much too low to drive most power amps to full power. While 76 db phono stage gain is typically best suited to cartridges having outputs that are considerably lower than 0.33 mv.

More typical for use with a 0.33 mv cartridge would be a phono stage gain of a bit more than 60 db, close to the 20 + 45 db you are now using.

Of course, you’ve changed several things in this process in addition to modifying the gain structure, i.e., inserting a transformer into the path; changing the load presented to the cartridge from 200 ohms to 470 ohms; and changing the interface between the phono stage and the line stage from balanced to single ended; and I suppose all of these things may have contributed to the resulting sonic differences to some degree.

BTW and FWIW, my 64 db Herron VTPH-2 seems to be an excellent match for my 0.5 mv cartridge. And during my discussions with Keith Herron prior to the purchase he indicated that the 64 db configuration of the VTPH-2 would have been a better choice for use with a 0.3 mv cartridge I was considering at the time than the 69 db configuration he also offers.

Best regards,
-- Al

folkfreak's avatar
folkfreak OP
1,349 posts
 
@almarg thanks. One minor correction. I neglected to note that the MFA also has adjustable load so I have maintained the 200ohms seen by the cartridge. The step up needs to see 47k to work so that is how the phono stage is set

@lewm thanks for your post. My observation on high gain was also conditioned by Johns design of the VOSS eschewing the attempt to get more than 40dB as presenting unacceptable trade offs

@willemj agreed on matching to the prior stage. Interestingly my DCS cd system offers adjustable output. The 2v level both sounds much worse and also leads to the volume control being too low. Seems the ARC may work best with the volume control set close too mid range which I suspect is not far off no gain at that stage. One day I’ll try connecting the CD direct to find out where zero gain actually is ...
onhwy61's avatar
onhwy61
5,137 posts
 
I'm not disputing you're equipment preferences, but I think you are over generalizing a specific setup change.  That said, some people absolutely swear by step up transformers.  Thanks for the post.
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