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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.
folkfreak
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.
lewm
6,770 posts
 
I keep forgetting that the Aqvox is one example of a phono stage driven by current output of the cartridge.  It makes sense that it would operate in balanced mode, because my search on the topic led me to designs that are balanced topology too.  It may be a requirement for current drive. As my present phono stage has true balanced inputs, I have already converted to balanced tonearm outputs, in one of my two systems; that's no problem.

You mention that the load cannot be changed.  Actually, the load is fixed at zero ohms, is it not? What troubled me with my very basic understanding of circuits is that the input impedance of the phono stage should need to be a multiple of the output impedance of the cartridge, so the cartridge can drive the phono stage.  But I think this is required in "voltage world".  Current-driven phono stages are in "current world".
What's the BMC?  
lewm
6,770 posts
 
My last post is more than 30 min old, so I am blocked from editing it, but I did go and read Fremer’s review of the BMC, and apparently the input resistance of the BMC is "less than 3 ohms", but not zero. This solves my own conundrum; I couldn’t figure out how one would create a zero ohm input.  Fremer seemed to really like the BMC, but he could not bring himself to say it was the equal of two $30K-ish phono stages (Ypsilon and Boulder), although he admitted it was very close to those two.
hdm
971 posts
 
Pretty sure that balanced  topology is not required with a current mode phono pre; the Nibiru, and I believe other current mode phono preamps in existence before the Aqvox were all single ended. I believe the Aqvox was the first to combine both.There are now others, including the BMC and MR Labs Vera.

My understanding is that you are not really dealing with load with these phono preamps; essentially the preamp is designed with a very low internal impedance current driven input (I believe both the Aqvox and the BMC are around 2-3 ohms-others may be different) which essentially acts as a short circuit to the cartridge and eliminates or greatly reduces noise and ringing that might be associated with a typical voltage input and loading options.

The BMC is essentially a turbo charged Aqvox built to higher standards and designed by the same guy, who now runs BMC. The BMC may well be better but I still like the Aqvox's ability for fully variable gain adjustment within its range. I think Candeias should have implemented that into the BMC.

Below are links to Fremer's reviews on both the BMC and the MR Labs, which might give you a bit more technical insight into the designs; most of it goes right over my head ;)

http://https//www.stereophile.com/content/bmc-phono-mcci-phono-preamplifier

http://https//www.analogplanet.com/content/current-affair-intriguing-mr-labs-vera-20-mc-phono-preamplifier
lewm
6,770 posts
 
I guess you’re saying that the Aqvox provides for continuous adjustment of gain anywhere between some lower and upper limits. The BMC (as you likely know) has internal jumpers that allow for "low", "medium", and "high" gain. Those should be sufficient for most situations; you would choose your setting once, to match the particular cartridge you are mating to it, and then forgeddaboudit. I looked up the Nibiru, too. Although it accepts RCA connections for input and output, implying SE, one cannot be certain that the actual business is not done in the balanced mode; there is just no information provided, and the phono circuit is hidden from view by a copper shield. Anyway, I feel like we might be guilty of hi-jacking the thread, for which I apologize to the OP. Thanks for responding to my question about current drive in the first place. I am tempted to try one out in my system.

I read Fremer's reviews in the course of my recent research.  He certainly did seem sincere in his praise for the MCCI, even in comparison to some very much more expensive units that he is known to adore.  In his summation, it seemed he had to backtrack to come up with some reasons why the $30,000+ units he compared it to are a slight bit better than the MCCI. Surely, the MCCI had better measurements than the megabuck units. (As I recall, the Boulder 2008 is pretty noisy.)  
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