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SUT vs. Gain Stage

jonnyglass

jonnyglass

New Member
I'm wondering why dedicated phono stages use gain stages rather than step-up-transformers. From my understanding SUT's are quieter and add less noise/distortion than the extra gain stage.

It is a matter of cost? or because matching a SUT to the cartridge is more complicated than just using a gain stage? Other reasons? Are there advantages to the gain stage that I'm unaware of?
 
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Some phonostages do use step-up transformers, but you're right, there aren't many, and it's largely down to cost. Transistors cost pennies whereas transformers cost tens of pounds (or dollars).
 
You might figure all a SUT does is boost voltage, so if you have a 1:10 turn ratio and you put in 1mV - you end up getting roughly 10mV at the output, et voila! Unfortunately, that's not all a SUT does. A SUT also adjusts the resistive load seen by the phono cartridge. The resistive load is the result of dividing the MM input stage resistor's value (typically 47KΩ) by the square of the turn ratio (in a 1:10 SUT, this becomes 10²=100). So in the example above our cart now sees the resistive load of 470Ω (which is the result of 47000/100).

So where does this lead us? There's just one more mystery to unveil. The resistive load interaction with the internal cart impedance affects the total voltage output. If the resistive load, for example, equals the internal impedance, then the output voltage is halved.

The bottom line is the output voltage is no longer dictated solely by the SUT's turn ratio. In fact, it's a balance act between the internal resistance of the MC cartridge, the resistive load seen by the MC cartridge (affected by the MM input stage resistor), and the output voltage of the MC cartridge.

Now, MM input stages have fixed minimum (input sensitivity) and maximum (overload) thresholds and we have to stick to a range between those. Since it all boils down to the specific parameters of the cart (output voltage and internal impedance) and is affected by the MM input resistance as well as the SUT's turn ratio, you can now see why it's extremely difficult to incorporate a one-SUT-fits-all solution into a phono stage. It's complex, expensive and probably quite restrictive in terms of tweaking to a specific cart.

There are many threads here dealing with just matching a SUT with a single MC cart. Imagine what it takes to match all MC carts...
 
Transformers are costly to build especially with good materials and low quantity makes it worse. Silicon is cheap in comparison.

How come someone sings the national at the ball game?

"Because they're too cheap to hire a band"

Your mileage may vary. My experience is that a good SUT/MC can sound perfect!
 
jonnyglass said:
I'm wondering why dedicated phono stages use gain stages rather than step-up-transformers. From my understanding SUT's are quieter and add less noise/distortion than the extra gain stage.
Click to expand...

I have bounced around on this question over the last couple of years and before I used the preamp @mkane referenced I would have agreed a MM preamp and a SUT was the preferred way to go. I believe proper design and implementation are key to having a MC preamp w/o SUTs perform better. Wyn's MC preamp is quieter than my previous preamp/SUT combos and Wyn's preamp in MM with my SUTs. A trio of other factors also come into play. First, it can often take some trial and error to find the best location for the SUT to avoid hum and that is not a problem with this MC preamp. Second, loading a SUT/MM preamp is a little more difficult and there are varying opinions on how best to do that. With Wyn's MC preamp I just drop in a resistor of the value of the load I want to use. Lastly, it is not quite correct to say Wyn's preamp sounds better because it doesn't really "sound" like anything. Which in the end is the goal for a phono preamp.
 
I think it depends upon the desired gain. For twenty some years, I've owned two different ARC preamps with a high gain traditional two stage approach. My current SP20 has a total of 72 db split 14 db line / 58 db phono. And offers adjustable load impedance on the fly via the remote.

That "goldilocks" approach has worked well with my 1.0 mV mid output moving coils and eliminates the need for a third stage of any type. I'm a believer in the KISS principle. :)
 
Today with very low noise preamps there is no real need for a SUT. Years ago transistors were noisy so large gain needed a SUT, things have moved on.
Chris
 
Transformers are expensive and sensitive to hum pickup from stray AC magnetic fields, such as from power transformers in adjacent equipment. They can work very well if hum doesn't intrude. With an active preamp, hiss can be an issue. Active pre-preamps need a power supply which is a possible avenue for noise and ground loops to sneak in unless batteries are used, whereas xfmrs do not have these problems. It's difficult to get a good design in either implementation. They each have their pros & cons.
 
Amplifying a LOMC cartridge is not dissimilar to amplifying a microphone. It used to be very common to use a transformer at the input of a microphone preamp and large mixing desks with 24 or more inputs would have a transformer on each input. That makes such a desk expensive and adds weight and bulk, so replacing the transformers with transistor circuits was very appealing to manufacturers. By the 1980's all mixing desks at the low end and middle of the price spectrum had transformerless mic inputs and only the most expensive desks still had transformers. These days very few mixing desks have transformer-coupled mic preamps, but you can buy standalone mic preamps (a bit like an outboard phonostage) and the most expensive ones have transformers.

There's no escaping the fact that transformers are expensive and transistors are cheap.
 
I used SUTs for many years with my Ortofons back in the 60's 70's and 80's. That was basically all we had to chose from economically. Today its just the other way around. Plus a good phono stage allows you to tune the load for the cartridge and optimize the gain. Something you can't do with an SUT. I guess Ortofon still makes SUT's for their specific cartridges, but they aren't cheap anymore either. Dynavecttor was making SUT's recently and you can always purchase Jensen SUT's. They make great audio transformers. You also have to remember a SUT isn't a neutral device. It has capacitance and inductive issues which can greatly influence the sound. Unless well shielded they are subject to RF noise switching noise and stray magnetic fields from your electronic components. So they can be a pain in the neck to get installed for best results.
 
twiiii said:
I used SUTs for many years with my Ortofons back in the 60's 70's and 80's.
Click to expand...
Brings back old memories. My first MC cartridge purchased as a teen in the 70s was an SL15E with the STM72 transformer. Used with an SME3009 on a Technics SL110 turntable for a couple of years. :)
 
I think a well chosen SUT can still be the best interface to many LOMC cartridges, especially in comparison to the voltage amplifiers typically used, and especially for the low impedance current generator designs that used to be more prevalent. There are some good current amplifier phono preamps around now too so those would provide another alternative.
 
rothwellaudio said:
Amplifying a LOMC cartridge is not dissimilar to amplifying a microphone. It used to be very common to use a transformer at the input of a microphone preamp and large mixing desks with 24 or more inputs would have a transformer on each input. That makes such a desk expensive and adds weight and bulk, so replacing the transformers with transistor circuits was very appealing to manufacturers. By the 1980's all mixing desks at the low end and middle of the price spectrum had transformerless mic inputs and only the most expensive desks still had transformers. These days very few mixing desks have transformer-coupled mic preamps, but you can buy standalone mic preamps (a bit like an outboard phonostage) and the most expensive ones have transformers.

There's no escaping the fact that transformers are expensive and transistors are cheap.
Click to expand...
It does not make them better though.. There is no free lunch no matter how amplification is done.
Chris
 
twiiii said:
I used SUTs for many years with my Ortofons back in the 60's 70's and 80's. That was basically all we had to chose from economically. Today its just the other way around. Plus a good phono stage allows you to tune the load for the cartridge and optimize the gain. Something you can't do with an SUT. I guess Ortofon still makes SUT's for their specific cartridges, but they aren't cheap anymore either. Dynavecttor was making SUT's recently and you can always purchase Jensen SUT's. They make great audio transformers. You also have to remember a SUT isn't a neutral device. It has capacitance and inductive issues which can greatly influence the sound. Unless well shielded they are subject to RF noise switching noise and stray magnetic fields from your electronic components. So they can be a pain in the neck to get installed for best results.
Click to expand...
Exactly my point re no free lunch relating to a SUT.
Chris
 
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