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.
33b07c8b a6d8 4786 bcd8 c42e79bdc595folkfreak
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.
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.)
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

@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 ...
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.
Dear @folkfreak : Agree with @lewm in that you can't diminish an active high gain phonolinepreamp..

If I remember there is other thread that appeared months ago with the same problem as you with ARC electronics ( I can't remember if was you in that thread. ).

Anyway, problem belongs to ARC design, weird as @almarg pointed out. Normally inside a same manufacturer trys that its models can stay matched when used together. here with the HG option the line stage puts in clipping condition.

Everything the same a high gain phonolinepreamp always outperforms the one passive one ( SUT. ), even this SUT's can makes very good job.

The main disadvantages of a SUT are the frequency range limitations at both frequency extremes ( especially in the low bass. ) and noise levels.

I own  an active high gain phonolinepreamp and rigth now I'm using/testing through a SUT and performs good.

I'm speaking of a full function phonolinepreamp that always beats separated phono stage and line stage. It's better a phonolinepreamp it does not matters if use a SUT to cope LOMC needs.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.


So accepting that my caveats with high gain may all be down to shortcomings in the ARC design (although no reviewer of this model line has ever said that) the main point I was trying to make was regarding my experience adding 14dB in my preferred low gain set up. My amps are VTL MB 450 III with a rated input sensitivity of 775mV. Assuming I’m getting the same voltage level output from the preamp with or without the step up why would the performance with the step up be so different? Is it inherent to the step up or is it also a problem of trying to amplify too small a signal at the pre amp (or for that matter at the phono stage). Should manufacturers quote a minimum desirable input sensitivity just as they quote a maximum?

@rauliruegas not sure why you feel the step up limits the frequency extremes? I’m hearing sub bass I never knew was there (eg super low bass drum, door slam like, buried in mix of Krall “Departure Bay”) and much more realistic and extended treble. If you’ve never tried a Stevens & Billington based design maybe you should check one out. I know Jonathan went all out with mine and like I said they’re not so expensive
Dear @folkfreak : What defines the maximum is the phono or line stage overload figure.

Now, the whole subject it's not if your SUT is outstanding ( because it's not. ), it's a good SUT. A SUT is not a rocket science design. I'm not trying to diminish your SUT only to put our foots in the floor. I'm sure your SUT is a good design. I think I tested over 30 different SUT's over my audio life and many of them really good as the Denon 1000  or the top Technics where both has the wider frequency range I seen in any SUT but even that can't even a SS active design in this regards. The Denon is something to listen and to caryy because it weigths 10 fully kilos.

Anyway, What you are experienced tells me that the ARC designs are not up to the task against your new items. I posted " every thing the same " a fullfunction high gain unit beats its counterpart with passive unit as the SUT. EVERY THING THE SAME.

Problem is that you had not a really good design. It's not easy to design it.

Checking levels of quality performance alone needs better electronics designs.

Now, the very important subject is that you are satisfied as never before, good ! !


Regards and enjoy the MUSIC NOT DISTORTIONS,
R.
Assuming I’m getting the same voltage level output from the preamp with or without the step up why would the performance with the step up be so different? Is it inherent to the step up or is it also a problem of trying to amplify too small a signal at the pre amp (or for that matter at the phono stage).
I wouldn’t want to speculate as to the specific technical factors that may be involved, at least without being intimately familiar with the design of the phono stage. I would just note, again, that the two gain choices provided by the phono stage are very atypical, while the output level of your cartridge is well within the bounds of being typical for an LOMC.

I’ll add, also, that the gain of your VTL monoblocks is several db greater than the gain of most power amps, to a greater or lesser extent depending on the mode and feedback settings you are using. So with many other amps the low upstream gain you had prior to the addition of the SUT would probably have resulted in your 70% to 80% settings of the volume control being in the vicinity of 100%.

Best regards,
-- Al


Hello Folkfreak.

I use a pair of Lundahl 1931Ag SUT's. These are bare transformers, and require a box, connectors, etc. About $2000 in parts. The whole is quite simple to fabricate and the sound is quite stellar.

My new DIY phono/pre will have sufficient gain to work without, and I am looking forward to the comparison.
Moving coil cartridges have low voltage but high current. Therefore the best way to achieve higher voltage from a mc cartridge is to use a transformer. This neatly accomplishes the task WITHOUT adding ANY noise to the tiny signal from the mc cartridge! ALL active devices (tubes and transistors) ADD noise to the signal. What you will hear is a CLEARER  musical signal with more of the subtle micro details already present in those vinyl grooves - that were masked by the NOISE of active devices! 
I have been using various transformers with different mc cartridges for 40 years and have always found them preferable to any active device - even the esteemed Levinson JC-1.
I agree with folkfreak, which is why Bob's Devices now has the SKY 10.  The SKY 10 has step up ratios of 1:10 and 1:5.  The 1:5 ratio was selected to meet the needs of those phono stages that have high gain settings of around 60 dB.   
A while ago I realized that for a cart to sound it's best the voltage should be brought as close to 1V as possible. Get that gain from a SUT or from the phono stage. As you found out, getting that extra gain really makes the cart sing. For your cart it looks like 69bdb is the amount of gain you need to have to bring it up to 1V. I agree with your thoughts that having too much gain can be problematic for the system with the extra noise it can bring but that is why some phono stages are better than others. SUTs I think do sound great and almost always make a good phono stage sound great. It's only when you already have a great phono stage that the SUT may or may not sound better than what the internal gain is capable of doing on it's own. Either way get that gain up to closer to 1V for the cart.
roberjerman, I cannot argue with your thesis, because it is after all based on your own opinion, but with all due respect, the Levinson JC1, while it is historically significant, is ancient in design and parts used, compared to the best modern high gain phono stages, like Raul's and Almarg's, and dare I say my own.  (JC1 was a pre-preamplifier or "head amp", as I recall, or was it a phono stage with high gain?  I used to own a Counterpoint SA2 head amp, which is now easily beaten by many top caliber high gain all in one phono stages.)  But I am not taking the position that all high gain phono stages are to be preferred compared to all SUTs plus low gain phono stage combos, even though the former is my decided preference.  I am only saying that either approach can be superb, done right.
I do not recall the technical details as to why, but my reading indicated that matching the proper SUT to the particular cartridge is critical, and is not only a matter of gain.  My experience was also that very small differences in loading were very clearly audible with a highly resolving phono system.  Cabling also becomes critical due to the "multiplication" effects of the SUT.  Since analog is not my primary source (GASP!!!) I have chosen to avoid the multiple variables involved with and SUT, and have gotten very satisfactory results with either a high gain LOMC phono stage or a head amp plus MM phono stage over the years.  I have heard excellent results either way.
Dear @swampwalker : The cartridge it self is no sensible to load impedance. What is sensible to load impedance changes si the phono stage dpending on the quality of that design..


@rauliruegas-  All system integration decisions involve trade-offs. I do not have even a smidgen of the knowledge necessary to parse out the differential effect of load impedance on the various elements of the analog reproduction chain.  I only know that I was able to hear the difference between very small variations in the load resistors placed across a very high quality SUT.  Therefore, I've opted for a solution that eliminates the variables associated with an SUT by going w sufficient gain in the phono stage to allow a 0.25 mV LOMC to perform well.  Maybe if I had some knowledge of electrical engineering and circuit design, I would come to a different decision. 
Raul & Michael (Swampwalker), you’re both right. Regarding the multiplication effect Michael mentioned, the transformer will of course step down the resistive component of the input impedance of the phono stage as it is seen by the cartridge, but at the same time it will step up (multiply) the input capacitance of the phono stage and the capacitance of the cable connecting the SUT to the phono stage, again as seen by the cartridge. That will tend to work in the direction of lowering the frequency and increasing the amplitude of the resonant peak in frequency response resulting from the interaction of cartridge inductance and the load capacitance applied to the cartridge. In the case of LOMCs that peak occurs at RF frequencies, but may affect sonics to the extent that the design of the phono stage allows audible frequencies to be affected by RF energy that is fed into it, and to the extent that the bandwidth of the SUT allows frequencies in the vicinity of that peak to be conveyed to the phono stage.


Also, differences in loading can affect the behavior of the transformer itself, including its susceptibility to effects such as ringing. The concept of an “ideal transformer” is often used in electrical engineering, because modeling a transformer as a theoretically ideal device can be useful for purposes of circuit analysis. But an “ideal transformer” is something that in the real world can only be approximated. So the noise-free gain provided by a SUT, that was referred to earlier, does not necessarily come without tradeoffs.

Atmasphere has made similar points in a number of past threads.

Regards,
-- Al


Don't mean to hijack this thread but this has me wondering. Is it worthwhile to add a SUT just to get about 12 dB of gain? I have a cart that puts out 1mv (Zyx 4D Mono) so it's just in the middle of the range - a bit low output for a MM stage, but high enough that it clips if I run it through the MC on my phono stage (Doshi Alaap). I'm using the MM stage now, but it seems to lack some dynamics (don't notice any extra noise though). I've emailed with Dave Slagle about making a low output SUT, but it seems like a lot to spend for only 10-14 dBs of gain, and maybe no corresponding sonic benefit. Anyone out there encountered this situation? thanks 
I believe it would make sense. Some of the best phono stages use a transformer gain stage first to match the cartridge. Take for example the Tenor phono stage which has a 4 Times (ie 6dB) transformer stage as the first step in amplification
http://www.monoandstereo.com/2016/09/tenor-phono-1-review-by-richard-h-mak.html
Dear @folkfreak : Not really, the best phono stages has not transformers in its design, are active high gain designs.

Tenor is not of the best out there. In its design the transformer is a must to has but this transformer does not means is one of the best because it's not and for very good reasons.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.
@jollytinker while @rauliruegas and I may have different opinions on what makes for a good sounding phono stage my observation was merely to assure you that adding a low gain step up is a valid approach

 You will have to take into account the cost of another interconnect and the issue of dealing with potential hum problems however 

My problem with transformers was the theory ''behind'' them .

As Dertonarm ever explained the inductance and impedance

of both need to correspond. In my case I would need to sell my

home in order to buy as many transformers as the numer of my

MC's with different impedance. BTW my both phono-pres (Basis

Exclusive and Klyne) have 4 amplification stages from 40 dB on

while both user manual's recommend the use of the lowest

amplification for the given MC. This of course imply or assume

that ''the higher the amplification the higher distortions''.

But then I ''discovered'' Denon AU-S1 SUT which covers 2-40

Ohms impedances. Probably based on the ''other theory'' which

assume amplification as the only important issue.

I use this SUT in my second system with Klyne at 40 dB and

SP 10, mk 2 with different Ikeda's tonearms all with removable

headshells. This way I can change carts in 10 min. time.

I am very happy with this SUT + the rest combo.

I don't care for the possible answer to the question if SUT's

ar better than active pres or the other way round.

Dear @folkfreak : Yes, I agree with you that the SUT is a valid option but not necessary thwe best one for the cartridge signal. As you said it needs additional IC cables with additional connectors where the signal must pass.

I'm not against SUTs per sé, as you said is a valid option but there are better options. 

Problem with active high gain stages resides in its design where the designer has to have a real deep knowledge levels and skills to that kind of " problematic " design and that's why exist not many active good designs and when the design comes of tubes the " solution " is a SUT inside that tube design because tubes levels of noise preclude to design an all tube active high gain phono stage. Other designers of tube electronics decided that the first gain stage be handled by SS devices.

The use of external SUTs can produce in the audiophiles several questions and normally came by the cartridge needs of load impedance when the LOMC cartridge is no sensitive to changes in load impedance as I staed here and where @almarg gaves a wider explanation about and where other gentlemans in other threads repeated the same several times.

Of course that always exist people that read but understand nothing, just never learn. Are ignorants for say the least.

Anywa, again SUT is a valid option.



Regards and enjoy the MUSIC NOT DISTORTIONS,
R.


Question for the group. I think we all agree that it's important to choose wisely when matching a SUT to a specific cart (1:10, 1:20, 1:5, all that good stuff). How do the phono stage manufactures handle this with such a one size fits all approach when they build in a SUT to the phono stage?
A very recent experience:  Last week my neighbor emailed me, as he usually does when he has a question about his very expensive audio system.  He recently purchased a new $30K phono stage (39db gain according to mfg spec) which is fed by a SUT suited to his previous phono stage (which had 42db gain, minimum).  Between the phono stage and his amplifiers, he was using a very high quality passive attenuator.  With his new phono, he was noticing that the sonics were occasionally anemic during orchestral crescendos and on similar demanding passages, and he wanted me to help him decide which of two more powerful power amplifiers he should now buy, in order to fix his problem.  

I noted that he currently owns an 8W SET powering a pair of speakers rated at 105db(!), and I told him his problem should hardly be an indication of a need for more amplifier power.  (8W should produce 107db spl at 10 feet from a 105db speaker, based on my calculations.) Instead, I advised him to save his $$$; I guessed that his upstream components were not driving his amps to their full output.  Indeed, if I assumed that his SUT was providing about 5.0mV at its outputs (i.e., the voltage output of a typical HO MM), I calculated that the signal voltage delivered to the amplifiers via the passive attenuators was less than 0.5V, not enough to drive most amplifiers to full output.  To prove the point, I lent him my vintage Quicksilver preamplifier which can provide quite a bit of gain via its active linestage section, although I don't know precisely how much gain in db.  Once we inserted the Q into his signal path in lieu of the passive attenuator, all was well, and I left it with him for a couple of days.  I have no intention of parting with my Q, so he bought an (expensive) active linestage of his own.  Now he's "cooking with gas".  What was striking to both of us is that the active stage made his system sound better in all respects, not just on musical peaks.  And strangely, this applies also to digital, where gain should not have been a problem.
The story has no moral, because he has now also made a deposit on a more powerful amplifier even though I proved to him he did not need it.  I am happy to help him, because, besides the fact that he is a nice guy, listening to his system after each "upgrade" allows me to educate myself on the price/performance ratio prevalent in high end audio, at no cost to me.
I thought I saw an excel based calculator out here somewhere that would answer a lot of these questions for a given cart. Punch in output of cart, SUT, Phono, Pre, ect and it would tell you what you needed to stay in "ideal" ranges.

Anyone have the link?
@robd2- Here is the gain calculator and a good explanation of how the parameters interact: https://www.kabusa.com/pregain.htm.

As far as how phono stage manufacturers handle the issue of SUT:cart matching, they basically don't.  Some have a choice of loading, but I don't if there are any that have a choice of step-up ratio.
Interesting calculator. A few questions, when they give you ideal gain does that mean total as in phono + line stage? Or is that only phono? For example for 0.5mV output cart they say 56db is the optimal gain.
i guess this would be phono only as the line stage only comes into the equation when volumne is at 100%?
@robd2 to your question I've no idea other than to suggest that perhaps they start from a flexible multi-tap arrangement like MFA offer (offering a choice of 1:5, 1:10 or 1:20 and 10K-80K loading)? I'd rather have the step up designed for the specific cartridge however

One other benefit of a step up is the ability to optimize grounding for the cartridge/turntable separate from the phono stage. As I observed here (https://forum.audiogon.com/discussions/a-new-ground-benefits-of-introducing-the-synergistic-research...) I found that grounding my phono stage when it was directly connected to the cartridge made the sound worse. Now I go via a step up I recently added the SR Ground block back to the phono stage and found a noticeable positive improvement in removing a level of hash I was not aware of. 

Maybe the next step is to try something like this (http://www.monoandstereo.com/2017/11/new-akiko-audio-phono-booster.html#more) on the step up itself?
Robd2, the KAB gain calculator addresses only the gain provided by the phono stage. Also, it can be very important to note the following statement that is provided in its introductory text:
The optimum gain is based on acheiving [sic] 325mV rms output at 5 cm/s. For the current crop of CD recorders, 300mV is required for 0dB recording level with the recorder’s level control set at max. Aiming for 325mV gives a little margin.
So the calculator will not necessarily provide valid results in a situation where the phono stage is driving a line stage preamp which in turn is driving a power amp. For example, a reasonable line stage gain of say 10 db would boost that 325 mv to about 1 volt if the volume control is set at max. More often than not, 1 volt will not be enough to drive a power amp to full power. And that's with the volume control at max!  And of course there are many line stages which provide less than 10 db of gain.

So take the results provided by this calculator with several grains of salt, and throw in some additional db as well :-)

Regards,
-- Al

Swampwalker, Among commercial phono stages that do have built-in SUTs, the Allnic H3000 does provide a choice as regards gain, probably by internally selecting primaries or secondaries that afford different turns ratios.  Of course, the H3000 is not a "budget" product.

For manufacturers of MM phono stages that do not incorporate SUTs to facilitate the use of MC phono cartridges, there really is not much more they could do, except possibly allow for a choice of load resistances so as to accommodate a wide range of turns ratios in whatever SUT is chosen by the end user.
Great thread guys and thank you so much for the info. Let me lob one more question into the mix to keep the conversation going. Looks like we also need to watch out for too much gain (overload margin). 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?
One quick question to the amp design gurus. My power amps (VTL MB-450 III) have variable sensitivity as a function of damping factor. I'm guessing (based on what I hear) that lower damping factor (less feedback) generally means more sensitivity? As the range is from 0.75V to 2V sensitivity it would be good just to confirm that the 0.75V is for the lowest DF and the 2V for the highest? That way I know if I'm getting in range with my gain cascade
Folkfreak, yes, that is correct.  Everything else being equal, reducing the amount of feedback will reduce damping factor, increase gain, and make the amp more sensitive.  Meaning that the sensitivity number, i.e., the input voltage required to drive the amp to its maximum power capability, will become lower.

Regards,
-- Al
 
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

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
Great thread @folkfreak these are very important measurements when pairing equipment.
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.  
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.
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?  
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.
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
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.)  
No worries @lewm I think we are all interested in different ways of skinning the cat of phono reproduction. 
Responding to the original post here - re not overloading with excess gain - I have to add that with SUTs, of which I have several and love their resultant sound, the gain of the SUT is linked to its output impedance which when combined with the 47K MM phono stage input creates the load the MC cartridge see. With a x10 SUT the load is 470 ohms and with a x22 SUT the load is 97 ohms. This load applied to the MC cartridge will greatly effect the sound. This is the challenge with SUTs and MC carts. matching the gain and the load as the cart sees it.