Step Up to the Plate - Bent Audio Mu
Bent Audio Mu Step-Up Transformers
This is a long tale of a journey I made to find the best possible unit to go between my Koetsu Black cartridge & Audio Research SP8.
Introduction (or would that be a Pre-pre-amble?)
Step- up transformers are tricky devices to pin down. Dealers rarely stock a wide range for you to audition and the results you get are highly dependant on the cartridge and pre-amp you’re using, making a home loan or a very trusted advisor essential. Your choice of cartridge is likely to change quite often because they have a finite life which may shortened by their incredible fragility. All of this makes the market a bit of a minefield for the hapless buyer.
Background
For those who don’t know, a step-up does the job of boosting the tiny signal from a moving coil cartridge up to a level that a moving magnet phono stage can deal with. It’s a passive component which uses some of the (relatively) abundant current present in the signal from an MC cartridge, converting it into enough voltage (which is not abundant) for the MM stage to work with. This is done by taking advantage of the strange phenomenon of transformers which boosts voltage in proportion to the ratio of the number of turns of wire on its input compared to those on its output.
A step-up is different to a head amp, which uses active (and, therefore, powered) circuitry.
Both approaches have their advantages and disadvantages, and wax and wane in their popularity. I tend to agree with the view that a good step-up can have the ability to portray very fine detail and dynamics better than an active device………but, there are some very good active devices out there too.
Of course, if you have a high output moving coil or a moving magnet, none of this stuff need bother you at all & you can laugh at us poor casualties who need another link in an already long chain (along with all the cables & connectors that go with it).
With step-up transformers, there are more complications involved; not least is that of the number of turns your transformers have should give you enough, but not too much, extra gain to appropriately bridge the gap between the cartridge & phono-stage – too much and you could end up with excessive noise – too little and it’s too quiet. If it’s wrong in either direction, you risk having to change the volume every time you change to or from your phono input and all the problems that can give you (such as low flying speaker cones if you forget).
The next hiccup you face is that of cartridge loading. You see, every cartridge likes to work into a particular resistance (or load). Most MM stages are (or very close to) 47 K ohm resistance and most MM cartridges like to see this load – so that’s easy to arrange. Moving coils are fussy, cantankerous beasts and usually like something completely different from the load that the phono stage shows them. So, to optimise everything, you have to intervene by sticking a resistor of an appropriate value in the way. Cartridge manufacturers often give a recommendation, but requirements are liable to differ from system to system – experimentation often pays off.
My Dilemma
I’m a long-term Koetsu Black user (I’m on my third one) and my pre-amp is an Audio Research SP8 which is an all valve (including power supply) design. It has a very, very good MM phono stage, that even after all these years, sees off many modern designs. The output of the standard SP8 is enormous & I swear it could drive some high sensitivity speakers without a power amp. For an MC cartridge, the Koetsu has quite a healthy output and with the gain turned up a good bit, everything sounded great, with no excess noise – even with an ear right up to a tweeter. The trouble is that I was always terrified that I might switch to CD or tuner without first backing off the gain – I did so, once or twice, & the neighbours weren’t happy. So, I had to find a way of matching the gain of the phono input to the line stages without compromising the fantastic sound I was getting. This was before I got my Accuphase CD player & tuner, which both have variable gain outputs.
The Search
So, I looked at step-up devices & head amps, but the taxman got to me first & I settled for a cheap option – a Hi-Fi News Blackhead, which functionally did the job, but was a bit lacking in the sound quality department. In the meantime I did a lot of research – reading and talking to people about their experiences, getting brief opportunities of listening to various devices. After all this, I still didn’t know which would be the best approach – active or passive. Then, after a while, I got to the point where I felt I could try a few things out.
At first, I tried three different step-up devices: an EAR – The Head, an Audio Innovations Series 1000 and a home made job, assembled from some microphone transformers. I didn’t feel like any of them approached the sound I had been getting from the original high gain set up, so I kept looking and tried God knows how many other transformers – several more EAR’s, Quicksilver, Koetsu, Audio Note, more home brewed mic trannies, home brewed S&B’s etc, etc…….with nothing really being fully up to my expectations for one reason or another. I discovered one thing, though; these things can be a bit tricky to move on if you’re not happy with them. My finances wouldn’t allow me to buy something new till I’d sold the one I’d rejected, so the process was very long winded.
I couldn’t decide on a transformer, so I looked at active devices – an LFD MC1 phono-stage firing into a line input on the SP8 and a Conrad Johnson head amp (I can’t remember the model) which used Nuvistor valves. The CJ was quite good, and even better when I fixed the broken ‘suspension’ (the circuit was designed to be hanging – or bouncing – in mid air, suspended by elastic bands which had snapped), but the LFD was the winner & I snapped it up. I lived with this for years and loved it, in fact, I still own it. I hankered after a better LFD and started saving up the pennies. One night I felt like tinkering, so I put the Koetsu into the SP8’s onboard phono stage again & was reminded just how magical the combination was. So, it was back to the drawing board! I spent some time experimenting with different valves in the SP8 & found a fantastic combination of Mullard CV4004’s & JAN 5751’s. Pretty soon after, there was the inevitable glass of wine induced memory lapse over input switching, causing tense relations with neighbours & we were right back where we started again!!
Bingo! (Finding My Holy Grail).
Then I started hearing things about the Bent Audio Mu step-up, which was made with S&B transformers. Initially, I discounted them because I’d already tried S&B’s, then I heard a set & couldn’t work out why they sounded different – until I discovered they were wound with different wire.
Not only that but, by now, there was the option of copper or silver.
There was an option of 1:5, 1:10 or 1:20 step-up ratios – which give 14, 20 or 26 dB gain.
Another option allowed switching between two of the taps.
There was the cheaper and, potentially, more fun option of buying a kit of parts.
The pound was looking extremely good against the dollar.
…………………..interesting!
I got in touch with John Chapman at Bent Audio & it turned out that the kit was no longer available.
John took the refreshing approach that the system rather than his profit & loss statement defined whether copper or silver was better. He recognises that the silver version could sound better, but only within the context of an appropriate system. So there is no pressure whatsoever to spend more on the pricier option, merely good solid advice on what might be the most suitable option. My experiments with copper, silver plated copper & pure silver interconnects had already told me that too much silver in the system was not good, so I placed an order for a set of 1:20 coppers (26dB gain) without the switching. 1:20 is the same setting as the Koetsu transformer & loading can be adjusted by adding resistors to binding posts on top of the units. I’m not bothered about switching or other fripperies, as I only have one cartridge. If, at some time in the future, I need another gain ratio, then the appropriate tap can be selected and soldered from the spare unconnected ones inside the units.
By now, S&B were producing the Music First unit at several times what John was asking for his model despite the fact that he was shipping from Vancouver!!
At this point, there was a delay, because S&B started making life difficult for John after collaboration in a project to jointly market Bent Audio’s pre-amp fell foul of a difference of opinion in how products should be retailed. Being the true gent that he is, John will speak no ill will of them, despite much of his business being thrown into upheaval. The upshot was that he no longer had access to any more transformers & the price of those that he’d committed to kept ratcheting up & up. Despite all of this, he honoured the price he had quoted me (did I mention he’s a gent of the first order?)
So, after a wait of several months, mainly caused by the trans-continental shenanigans my trannies arrived. John also attaches transformers to his burn-in kit - what he calls ‘the cooker’ for a long period before he ships them. John tells me they were the last pair made with S&B’s (though I’ve heard others claim they own the last pair, not that I really give a stuff!).
Gotcha! - The arrival
The first thing to strike you is their size, weight & the meticulous way that they’re put together. The S&B cans are each swathed in Sorbothane & housed in a three & a half inch diameter, four inch high black anodised cylinder with quarter inch thick sides.
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Units with CD case for scale
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Exiting from the top of each is a captive cable made from shielded Continuous Cast copper with PTFE insulation, directly connected inside to the transformer tap and terminated in an RCA Bullet plug. The cable is short at eight inches in order to keep the signal path as short as possible, so the units sit behind your pre-amp. On top of each unit is a good quality RCA socket, and a knurled earth post.
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There are also two binding posts for clamping loading resistors, should they be required. On the underside are six small feet and also included was a pack of 12 pairs of resistors of varying values to facilitate loading the cartridge.
Top of unit, showing connection hardware & with a cartridge loading resistor attached
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I’d already made up a pair of cables to use for earthing the units to my SP8, so I was ready to go – I found that connection was dead simple, but placement was a bit tricky because of those short captive leads.
Earth cables attached with banana plugs (commoned at pre-amp end), for ease of connection
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Eventually, I settled on an arrangement that worked & I fired everything up with no resistor between the binding posts. I think the album I chose to play was ‘The Greatest’ by Cat Power, I switched the arm cable from the Mu’s & gave a couple of tracks a whizz with the Koetsu into the LFD. It sounded just great as usual, but then I put the new trannies in circuit and………well, shit-me-britches if this wasn’t the best sound I’d ever heard in my own home – maybe something else was at work - the mains or psychology maybe, but it seemed even better than I remembered the sound with it direct into the phono-stage.
Optimisation
After a few days of hi-fi clichés like rediscovering record collections, veils being lifted, untold amounts of new detail on well known records etc, I started playing with loading resistors in an effort to find the optimum value. The results of doing this are well documented elsewhere, but I was surprised at the large differences that this simple change can make to the sound. On measurement, it transpired that my SP8 was not a tiny way off the 47 K standard that I’d been led to expect, so, some calculation was required to find the actual resistance that each value was showing the cartridge – I wanted to try two extremes around the value that Koetsu load their own step-up with (~100 ohm). The actual job is incredibly simple – much more so than with many other devices. Dip switches on the underside or under the bonnet? No thanks! You just slip your chosen resistor between the binding posts & spin them down. It takes literally a couple of seconds for each channel & you can do it while you’re playing a disc, so you can instantly hear the effect. The general rule seems to be that a higher value will bring up the treble a little and give a slightly more open sound – too much, & you get an effect that John Chapman calls “A little zippy”(!!). A value that’s too low removes some life and presence from the sound. These changes are immediately identifiable and obvious to anyone with a pair of ears. Once I’d tried the two extreme values, I moved up & down the range in progressively smaller jumps till I found the value that seemed to give the best balance. I then left these resistors in-line for several days and thoroughly got to know the sound before I did anything else.
The next step was to remove the resistors, using the trannies as supplied & assess whether it was better without them at all. I’m really glad I did this because the sound was clearly better without them in-line. Next up I decided to try several different brands of resistor of that value in order to see whether it was component quality that was making the difference. I assembled a collection of different brands and though there was a clear difference between each, I felt that the sound was always better with none connected, so that’s how it’s stayed ever since.
The search for my ideal was definitely long and pretty expensive all told, but I learnt such a lot that I feel it really was worthwhile. Not to mention the fact that I ended up with a superb solution to my problem. My Mu’s aren’t as flexible as some other options out there, due to their lack of switching, but the sound is improved because of it. After all, most of us only use one cartridge & it’s easy enough to make the changes you may require if you swap for a different cartridge. How do they sound? Well….they don’t. That’s all I can say! They’re just a great piece of hi-fi & one of the best buys I’ve ever made.
Since all this, John’s been testing many different transformers to try to find an alternative & I believe he’s found one now. He said he’d not use anything that sounded worse than his old model – preferring to lose the income than to supply an inferior model. Now that’s a measure of the guy. (Did I mention that he’s also a true gent?)
