An Unexpected Problem with SUTs
As many of you know we produce high-end SUTs (Step Up Transformer) for MC cartridges.
We put a lot of effort in our products, the components we use are all first quality and, first of all, the transformers, made on order from a world acclaimed British company; apart from that the manufacturing process and the quality test is second to none.
Of course, you might not like the sound of the unit, but, under a technical point of view, these SUTs are as good as it gets.
Having said that, I want to share with you a little story about one of our transformers.
Time ago we sold an excellent 1:10 SUT to a very demanding customer from abroad, received the SUT, the customer complained about the sound, dull and lifeless while the sound was ok with an active MC preamplifier.
The Cartridge/SUT combo was well known, and we have happy customers using that cartridge (one of the finest Ortofon on the market) with our units.
Intrigued I asked the customer, very collaborative indeed, to carry on some tests.
The test was straightforward, try to change some of the components (preamp, power amp) and see if anything change.
To our surprise, changing the power amplifier with another one, the sound became dynamic, bright and full of life.
Both the power amplifiers where high-end, the one with dull sound, a Mosfet power amp worth several thousand £, the “good” one an excellent tube amp.
Bear in mind that with the active MC preamplifier both the power amps sounded very well.
So, what’s the heck was going on?
My first suggestion was to change the turntable/tonearm setup.
In my opinion the culprit was a low frequency resonance in the turntable/tonearm/cartridge combo that was cut by the active MC preamp but transmitted down the line by the SUT, the horribly expensive Mosfet power amp, despite the price tag, started to oscillate when received such resonances while the tube amp did not.
Following my suggestion, the customer provided a different setup for the turntable, and the MosFET started to play how it should, very well indeed.
The caveat of this lay in one of the many differences between SUTs and ‘headlamps’ (active MC preamplifiers), the frequency response.
Here is a picture of a frequency response test made on our MC01/20, 1:20 SUT.
The trace is for the range between 0.1 Hz (yes 0.1) till 380 Hz, the signal generator was set to sweep from 1 Hz to 400 Hz.
The Input impedance 15 Ohm (similar to an average MC cartridge), the load at the output the canonical 47 KOhms.
Here is a picture of the setup:
In the tracing graph we can see that the attenuation at 1 Hz (yes, one) is just 5 dB, if the resonance is, say, 4 Hz, the attenuation will be 1.5-2 dB, while with an active MC preamp would be more than 12 dB, sometimes 20 dB. Please note that the 10 Hz mark is just 0.35 dB from the 20 Hz mark that is at 0 dB from the rest of the trace.
Now, there are on the market, a lot of shiny, aesthetically appealing devices, most of them with fancy names, even fancier promises of ‘liquid’ sound, 4D soundstage (whatever it means), micro definition and whatever more come to the fantasy of the marketing department. Advertised and ‘independently tested and reviewed’ on specialised papers and websites, with huge budgets, but at the cost of poor engineering and standard components, despite the very high price tag.
At the end of the day, I must thanks a customer that trusted me and my product and not the hype of some well-advertised producer. I didn’t have to take my SUT back, the customer is enjoying his transformer and gets rid of nasty resonances he was not aware existed. We are all happy.