Passive SUT vs. active MC preamp?

[hifi_dave]

As usual, it depends on the units you are comparing. Some head-amps (active) are very good but some are very bad. Some step-up x'mers are very good and some are very bad.

If you already have a very good phono stage, addding a step-up/head-amp is the way to go. If your phono stage is average, it might be best to consider a better complete phono stage with MC capability.

[Mika K]

Exactly. And also system synergy is very important. The most important interface is the cartridge to SUT/head-amp as that really defines what you can achieve. Then after that also SUT/head-amp needs to work well with the used phonostage. These naturally also include cables etc

I have really good sounding MM tube stage at my Shindo preamp, so it's natural to choose some option that I can use with this and my main cartridges. I have been using few SUTs but still don't have the best pairing here. Is the next solution SUT or head-amp I do not know. And of course can always add other separate phonostage at some point for my decks but still..

[Rowlf]

I have never used a setup. One area I could not understand with step-up though, is that the you cannot change the values of the input impedence, which was set for a MM cartridge. How is that going to work? Or have I got that wrong?

[hifi_dave]

The job of a step-up or a transformer is to load a MC properly and supply the extra gain needed to work into a MM input.

[Rowlf]

Is that to say there will be settings on the step up transformer to set the impedence to 100ohm? And leave the MM phono at default 47k ohm?

[hifi_dave]

Some SS step-ups and x'mers do have a few loading options but not all. The input impedance of the MM stage is usually 47K.

[Chris]

I use a souped up (Salas Shunt regulated) Hagerman Piccolo in front of my souped up (Salas Shunt Regulated) P10 MM stage with teflons and Tantalums all over the place and from my dim memories of my Haufe SUT used with a 103Pro into a Lehmann Black Cube I w2ould agree with the OP´s suspicions. The SUT combination was "nice" - my present set-up is "big and evil". But no idea what´s doing what.

[Barry]

SUTs vis à vis Head Amps


Moving-coil cartridges tend to have a much lower output than that from fixed coil designs. To allow moving coil cartridges to be used with preamplifiers having a standard sensitivity of 2 – 3mV, with a load impedance of 47kΩ, additional voltage gain has to be provided. This can be achieved in one of two ways: through the use of a step-up transformer (SUT) or by using a specialised amplifier, a so-called ‘head amplifier’.

Like many things in audio, where there are alternative ways of doing things, it does not necessarily follow that one method is preferable to the other: each have advantages and disadvantages.

Step-up Transformers (SUT)
The first moving-coil cartridges used a SUT to provide a suitable voltage output. Often these transformers were integral with the cartridge, later to relieve the cartridge weight the two were separated, making the transformer an ‘off-board’, item.

Transformers have much to commend them. The most obvious is that of simplicity, and with it reliability: there is little to go wrong or fail with a transformer. Another advantage is transformers have a very low noise figure. They are passive devices (they achieve voltage gain, but cannot produce power gain) so the Nyquist (thermal) noise generated, being determined by the resistance of the windings, will be low - lower than that of a head-amp.

Transformers can have a sufficiently wide bandwidth: a response 10Hz - 20kHz, flat to within 1dB, is readily achievable with good design. The design of the transformer core will largely determine the low-frequency response, whereas the high frequency response is largely determined by the intra-winding capacitance. The frequency response will depend on the loading of both the windings. Excessive preamplifier capacitance will tend to suppress the treble response, whereas a low cartridge coil resistance will curtail the bass.

The voltage gain provided by a transformer is given by the turns-ratio, that is, the ratio of the number of turns of the secondary winding to that of the primary. Thus if the secondary winding has 10x as many turns as that of the primary winding, then the voltage gain will be 10:1 (≡ 20dB).

Just as the voltage will be increased by the turns-ratio, so the current be reduced by the same amount. This means the impedance is ‘transformed’ as the square of the turns-ratio. So taking the above example, a 10:1 transformer will transform the input impedance of the preamp by a factor of 100, that is, the nominal 47kΩ impedance will be reduced to 470Ω, and this is the impedance that will load the cartridge. In general it is best to provide a load that is at least 3x that of the coil resistance. A figure of 100 - 200Ω is usually recommended, and will be achieved by shunting the secondary transformer winding with a resistor.

However, as far as the preamplifier is concerned, the transformed cartridge impedance provides a poor match to the preamp. A typical preamplifier fixed-coil cartridge input is designed to achieve its best noise and bandwidth when fed from a source of 1kΩ or less. Some moving coil cartridges can have a coil resistance of 30Ω and require a gain of 20x (26dB), so the transformed impedance as seen by the preamplifier will be 12kΩ.


Head amplifiers
Head amplifiers are specialised audio amplifiers designed specifically to load the cartridge with 100Ω or so, provide a gain of up to 30dB with low noise. They are much more versatile than transformers: the loading and gain is virtually independent of frequency; and the frequency response itself is as wide as is necessary.

Being much more complicated than transformers, and involving possibly dozens of components they will of necessity be less reliable. Amplifiers cannot be a low noise as transformers. One way to achieve a low noise figure is to use multiple transistors in parallel, though there is a limit to how many can be used; eight seems to be the limit before problems due to Miller capacitance start to have an effect. A very few head amps use a pair of ‘low noise’ triode valves in parallel. As a guide, a moving-coil cartridge having a coil resistance of 3.3Ω will have a noise voltage of –147dBu, whereas discrete transistors (such as a 2N4403) will have a noise of –135dBu and the best integrated circuits have a noise of no better than –125dBu. These figures are to be compared with a typical cartridge output of say –73dBu. What this means is a typical solid-state head amp can have a signal to noise ratio of 80dB. Being active devices they need a power supply, which can impact on the noise performance - enthusiasts will use batteries to achieve the best performance possible.

Whilst producing a very flat and wide frequency response, the overall distortion performance is also commendably low, though in the mid band transformers can often display lower distortion than their head-amp counterparts.



So to summarise, transformers provide a simple, reliable means of providing voltage gain, with an adequately wide and flat frequency response, provided care is take with the loading on both windings. Despite being the lowest noise devices, the overall system noise performance will not necessarily be optimum. The mid-band distortion via transformers can often be lower than that of head-amps.

Head-amps are more versatile providing adjustable loading and gain that is independent of frequency. The overall system noise performance can be better than those using a transformer, and the distortion will often be lower at the frequency extremes compared with transformers. Not being passive, head-amps require an external power supply.

[Marco]

Missed this thread until now, but as I have considerable experience of experimenting in this particular area with a variety of cartridges, SUTs and head amps, I'll add my thoughts later, which will hopefully be helpful to Mika

One question, which cartridge do you intend to use with the eventual head amp/SUT of your choice?

Marco.

[Mika K]

Cheers Marco, that's great!

I've been playing with the idea of retip my Zu modified Denon DL-103 at some point but at the moment I'm mainly using Transfiguration Phoenix II with my SUT. So the insight for this type of modern MCs from makers like Transfiguration, Lyra, Dynavector, Ortofon etc. are also equally valuable in this case..