SUT - electrical theory and practical experience

Dear @mulveling  : SutherlaND 20/20 mk2 IS A VERY GOOD ss PHONO STAGE DESIGN ( LIKE THE pS aUDIO sTELLAR. ) AN YES BEEN an active high gain design just outperforms I think almost any SUT.

 

Here a review from 2011 and read it and please read carefully its measurements including the overload numbers and this Sutherland humble price tag as the PS Audio Stellar always be a challenge for any other phono stage surrounded by a good system:

 

 

 

R.

 

 

MK2 is even better than the one reviewed.

@rauliruegas 

I should be sure you have that RIAA deviation already measured and maybe you can share with us and your customers.

I'm hesitant to give you a measurement which you will certainly turn into a cudgel to use on me in the future but typical units are ± a third of a dB across the audio band.

dave

  

Dear @mulveling : SutherlaND 20/20 mk2 IS A VERY GOOD ss PHONO STAGE DESIGN ( LIKE THE pS aUDIO sTELLAR. ) AN YES BEEN an active high gain design just outperforms I think almost any SUT.

@rauliruegas Hi Raul,

I’m sure the Sutherland is a lovely MC stage. I get that it’s an active trans-impedance amplifier, different from my JFET MC stages or SUTs. At this point of my journey, I have no interest reading published reviews of gear.

What I’m not quite sure of is your angle here. Please let me know which of the following apply (check all that apply lol):

• The JFET MC stages in the Herron VTPH-2A and my ARC Ref 3SE are not "good". If you recall, I prefer selected SUTs over both of these. (I’ve also had other JFET MC stages too, but these two are generally lauded as very good if not exceptional)
• More generally, both voltage amplifiers and SUTs are not "good". Trans-impedance is the way
• It’s actually my system and/or hearing that is not good :)

Dear @mulveling  : For a LOMC cartridge in high gain nactive designs the device that conforms better electrically to MC cartridges are bipolars no JFET. So why almost all that kind of AHG designs use JFET? because bipolars are a " pain " to " handled " in that stage JFET is very easy on the design and is very good in dedicated MM stages.

 

No, transimpedance is not the way. Good current or voñltage designs makes excellent job. You have to think that in the last 40 years 99% of phono stages including the top ones are voltage designs with no compliants about and you don't need to care that the cartridge for a current unit needs to be very low internal impedance.

 

Time ago wpalmer that has true high knowledge levels in electronics due that he laborated for several years in Analog Devises as designer and group leader and posted this about:

 

" 

I'm in the process of having a phono preamp made manufacturable by a well known audio company. The first thing they insisted upon was that the design would be implemented in SMD- purely for manufacturing purposes. This had it's own unique set of difficulties- for example, some of my preferred capacitors were not available on SMD, so a search for suitable items of a different technology (ceramic MLCC C0G vs Polypropylene film) was needed, together with a proof that the distortion characteristics were essentially equivalent.

The other aspect was that the preamp design investigated two possible implementations, either a voltage mode input, or a current mode input, and with the 75uS RIAA TC implemented in the first stage.

I ended up with a voltage mode input, as when designed for equivalent gain and RIAA characteristic there is, generally, no measurable or audible difference.

However, there are indeed differences in operation and implementation.

To put this somewhat in context, I designed a number of "transimpedance" amps while at ADI, most notably the AD846, together with a number of conventional opamps, so I am familiar with the concepts. The AD846 was designed to have almost perfect current conveyance properties and could be operated open loop as a transimpedance amplifier. Most opamps/amplifiers use negative feedback to achieve this goal, or have high distortion levels if operated open loop.

As I don't want to make this into a "white paper" I'll try to be brief.

1. A phono cartridge is a voltage generator (Vs) with an output impedance which is mostly a resistance in series with an inductance (R+Ls). This can be converted into the Thevenin equivalent current source- which is a scaled current (Vs/(R+Ls)) with the output impedance in parallel to ground.

If you take this current and drive it into a virtual ground, which shorts out the shunt components, and convey it to the load resistance (Rl) then the output voltage is Vs*Rl/(R+Ls). If Ls is small then the gain is completely dependent on the series resistance of the cartridge and will vary from cartridge to cartridge, and if LS is large there will be a HF roll off.

Any shunt capacitance will be essentially ignored.

If there is a resistor added in series with the virtual ground, then the current is shared between the equivalent shunt components and the series R, and the gain becomes even more variable.

Voltage mode lacks this complexity. Provided the load impedance is relatively large compared to R the gain is easy to determine.

However, the shunt capacitance is not ignored, but for MC cartridges the inductor is so small that all parasitic capacitance values are irrelevant, assuming the load R is small enough to damp out the LC resonance. 

Current mode does not experience this resonance, but correct loading Rs plus a phono stage with a suitably good ultrasonic overload margin will take care of the potential problem.

MM cartridges into a virtual ground are not rational unless there is a very large resistor in series with the virtual ground. This is because the R is large but L is even larger.

 

2. In voltage mode the input stage is non-inverting and will experience potentially substantial deviations about ground. For many opamps, particularly earlier generations than the most recent ones, this voltage will cause increases in distortion due to the operating conditions of the input transistors being changed. 

This distortion can be highly sensitive to the source impedance and the input signal level.

In the transresistance mode the virtual ground does not move, removing this source of distortion.

Modern audio IC opamps are generally designed with this problem in mind and exhibit negligible changes in distortion when operated in non-inverting mode.

These are the two main aspects of current versus voltage mode inputs.

There are other, more obscure, aspects.

I chose voltage mode, but also chose appropriate opamps to minimize the down side of the choice.

The bottom line is, when properly designed both current and voltage input designs can have equivalent performance from the perspective of distortion, frequency response etc.- but the voltage input design is more predictable and easier to specify. 

The voltage input design produced has very precise gain, extremely precise RIAA compliance and unmeasurably low distortion.  ""

 

From the last 2 years exist a " fever " for the current designs and obviously that the customers with unit that handled in both versions prefer teh current one and I understand it because they bougth that phono stage for the current design. For me is only a today fashion.

 

R.