Voltage mode vs current mode phono stages

@lewm , I looked at the BMC and read Fremer's review. The current price is $4800.00 which is not bad at all. They are built in China.

Comparing the BMC to the Seta + is relatively easy in regard to their physical attributes. The BMC is very nice looking although some would argue that the on-off switch(that huge knob in the center) is a little over-baked. The Seta + is industrial looking in comparison. Inside the units is a different story. The BMC uses through hole phenolic boards and the Seta + SMD construction on fiberglass boards. The BMC uses standard regulated power supplies (good ones) while the Seta + uses  lead acid batteries and a charging system that disconnects from the unit when in use, perfect isolation. The Seta + is made in America and costs over $9K with the optional RIAA board. Made in America covers the price differential. 

Fremer reviewed an older version of the BMC and liked it very much. He also reviewed the the Seta's little brother the Lino C which he also liked very much. I suspect both units are excellent phono stages and would have been SOTA if they were three times the price. I am in the same boat Lew is in. I really like the idea of a current mode phono stage but am not going to spend silly money on the CH Precision unit. I do not need the added complexity and it is a very complex unit and I do not feel like spending money on complexity. But I would bet dollars to donuts that blinded, few of us if any could reliably tell the difference between the three stages. Mr Fremer would argue with that, but he has better hearing than I have. 

Mr Fremer would argue with that, but he has better hearing than I have. 

Really? I spoke to him at Axpona and he could hardly hear me, had to repeat myself like 10 times. 😉

Fremer by and large has better equipment than most of us have, not necessarily better hearing.  He's old, like we are.  Like I said before, what is the big deal about SMD components?  Good for op amps in circuits with very very wide bandwidth but not necessarily an audible upgrade with a phono stage.  That said, the quality of construction of the Seta circuit does appear to be very high, judging only from photos.  I've not seen the innards of a BMC. I'm not buying any time soon, or maybe ever.

Dear @lewm ​@mijostyn   : " what is the big deal about SMD components?  Good for op amps in circuits with very very wide bandwidth but not necessarily an audible upgrade with a phono stage.   "

 

Really? because it's precesily where the SMD can helps especiaLLY IN THE riaa CIRCUIT WHERE NORMALLY IS NOT DEVELOPED HIGH TEMPERATURE. wITH smd parts THE SIGNAL " RUNS " a way shorter path and in a phono circuit that's important.

 

Several months now I told you to change the attenuators in your phonolinepreamp for attenuators with the same Swiss mechanism but with SMD parts. I don't know if you did it and if not then do it and latter on come back with muyjost to comment about. Till you try it and almost as always everything belongs to your " imagination " that's wider than mine because I always try and make tests.

 

Regard and enjoy the MUSIC NOT DISTORTIONS,

R.

IMO, informed by imagination and. some high ( as in orbital and beyond ) end electronics fabrication experience, you want a designer fluent in: point to point, boards, multi-layer boards..and SMD and integrated circuits. Helps if they have ears also. 

Carry on…..

Raul, are you thinking of the Goldpoint series attenuators for your and my Phonolinepreamp? Yes, you did advise me to replace the originals with those Goldpoints that use SMDs. Can you tell me what value, in terms of Kohms? One thing I noted is that Goldpoint claim they no longer need to sell ladder type attenuators (one resistor in circuit per position both in series and in parallel with the signal) because the SMD resistors raise the quality of the series type to that of a ladder type. (For others, series attenuator will have a string of resistors soldered end to end, with resistors added as you go increasingly attenuate the signal. This is usually inferior to a ladder type attenuator, if you are seeking Nirvana. Since there are so many solder joints in a series type attenuator circuit, it is no wonder at all that SMDs will work better.) I probably should look into doing the mod but when I did, I also considered using Khozmo shunt type attenuators and got stuck on the decision between the two. I use the Khozmo stereo balanced shunt attenuator in my MP1 and they are excellent. A shunt type places only one resistor in the circuit in parallel with the signal per setting. What I tried to convey to Mijostyn is that I would not choose between two components based ONLY on the fact that one uses SMDs and one does not or that one uses fiberglass PCBs and one does not.

@lewm , you still have not told me why you gravitate towards the BMC. The quality and type of construction used in a unit are important indicators as is circuit design. As far as sonic performance is concerned, it is much harder to get a bead on it and frankly all great phono stages should sound exactly the same. We usually only get to hear them after the purchase. I will go with the best construction every time for the type of unit I want in this case a Current mode phono stage.  

Dear @mijostyn  : Many years ago I had the opportunity to listen almost the first ( I think? ) BMC current phono design and I can't remember nothing especial about. The SMD subject it's not only that we can have shorter signal paths in the boards but with lower noise and distortion levels.

 

I know you are in deep foundation with the current phono stages that's not a new technology but what is almost new are the latest 3-6 manufacturers with that kind of design including CH that in this case and with out listen it  is more marketing than other real value but I never listen to it.

J.Carr latter cartridge models came with very low internal  resistance and inductance and seems to me that he designed that way not precisely thinking on the " new " current phono designs because to me that could not has " common " sense " when the 99.9% of all phono stages are voltage designs.

 

Today " current " is the histoty of the day but not a consolidated/matured phono stage industry design. 

I think that the " devotes " ( as you ) to that current phono stage designs need  give more time to the designer knowledge deeper down there along the understanding of them with the cartridge role.

 

This is something as in the Cartridge Loading 2 where the OP with not really high knowledge levels about now is " fasinated " in that thread with the dialogue that analize current vs voltage phono stages but with out really know for sure through measures tests and listen tests. I'm sure that his knowledge level about improved " nothing " because that kind ofdialogue is not really something that today can enhance our each one listened sessions that at the end it's what all of us are looking for.

 

Anyway, to each his own.

 

Regards and enjoy the MUSIC NOT DISTPORTIONS,

R.

lewm: More than a year ago I emailed that information, please look for. Btw, the signal in the design unit was designed it pass through only one resistor in each attenuator.

@rauliruegas , I can not disagree. I am taking a risk with the Channel D unit but then I hardly ever get a chance to audition units before I buy them. I do my research and cross my fingers. Rob Robinson is a very bright engineer who is totally bonkers over vinyl. His construction methods are right up to date and you can use his units with digital RIAA correction which again theoretically has its benefits, theoretically. We shall see. If it does not I'll sell it and move on. 

I have this acre of land right in the middle of Central park:-)

Michael Fremer"s latest review of the Channel D Lino C 3.3

@ledoux1238 Thanks! I did not know he had reviewed it. @rauliruegas check out ledoux's link. I know you are into SMD and ultra accurate RIAA circuits. 

Dear @mijostyn  : Thank's.  That only confirm why several top ranked SS designs manufactres/designers are using from some years now thwe SMD. It's not only Channel D but Boulder, CH, and many others. To many real quality advantages to let pass.

 

Today I think that the only advantage of true hole devices is that any gentleman/audiophile/reviewer or whatever when open that SS design at least can see and read inside the circuit boards which devices ( vishat, caddok, bourns, panasonic, tdk, kemet, xicon, etc. ) are using the manufacturers  and the cirduit boards looks really good with. In change with SMD you have not so easy that information.

 

Anyway thank's again.

R.

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.