khfm865

Balanced phono stages - Anyone?

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Hi all,

I recently acquired the BMC MCCI, which is a very interesting design and kind of no holds barred version of the infamous Aqvox - same designer Carlos Candeias. One talented chap as he was also responsible for the famous CEC belt drive and burmester belt drive CD players.

The bmc mcci is a current amplification design and is best with cartridges with low internal impedance hence why I am moving my much loved Benz on. It only accepts a balanced input and outputs fully balanced as you would expect. Anyway, it produces a very quiet soundstage (quietest I ever heard in my system) which I suppose is not surprising given the balanced circuitry. I wondered why more designers and vinyl lovers don't opt for this route of vinyl replay as noise is one of the disadvantages of vinyl IME.

Welcome any thoughts or insights or experiences. Anyone else fully balanced for their vinyl replay?

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Please can you explain; Do you mean floating when you say balanced for the input? AFAIK pretty much all low output m/c stages float their inputs, but am happy to be corrected?

Glad to hear it is performimg for you :^:)

I suppose one reason is cost, another might be that vinyl is not a significant source for pro environments where balanced conections are prevalent, and a further might be that whilst noise is an issue, surface noise should be so much louder than self noise from the electronics, whatever the phono stage configuration, that it becomes arguably unimportant to reduce still further?

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Super Wammer

Had an Ayre p5xe phonostage and it definitely performed way better fully balanced in and out. Quiet, dynamic, detailed but not bright, not sure I've actually used anything better since......

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Please can you explain; Do you mean floating when you say balanced for the input? AFAIK pretty much all low output m/c stages float their inputs, but am happy to be corrected?

Glad to hear it is performimg for you :^:)

I suppose one reason is cost, another might be that vinyl is not a significant source for pro environments where balanced conections are prevalent, and a further might be that whilst noise is an issue, surface noise should be so much louder than self noise from the electronics, whatever the phono stage configuration, that it becomes arguably unimportant to reduce still further?

Allmost all phono stages, whether MM or MC have unbalanced inputs, even if the input phono socket floats from the chassis, it is still connected to audio earth, and so unbalanced. A very few have properly balanced inputs, using XLR connectors, of possibly the smaller Lemo connector. Most MC cartridges are inherently balanced, as the magnetic circuit isn't electrically connected to any of the four cartridge pins, and the coils are symmetrical about earth. Most MM cartridges could be balanced, but most have a mumetal screening can over the coils, and this can is connected to one of the ground pins. Shure do this very obviously on the V15III, and the screening can connection can be removed if one so wishes. Having the can connected to one side unbalances the cartridge, so using one into those rare properly balanced inputs negates the benefits.

In my view, all cartridges should be wired balanced and all phono stages should have balanced inputs,with any screening can on MM cartridge connected to the headshell/arm metalwork, not to audio ground. It's how microphones are wired, and there's little difference between a MC microphone and a MC cartridge.

S

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Thanks Serge for the comprehensive response - just back from work so did not have chance to reply :)

Reading the BMC MCCI manual was very enlightening regarding permissible wiring to enable a balanced connection from cartridge to BMC input. Serge was as ever on the money regarding the earth - in my system I had to be very careful to ensure that the internal wiring in my junction box from the tonearm was correct. VPI make a balanced junction box which makes life easy - converts Lemo to xlr.

There are some severe warning in the manual for dangerous wiring connections for cables and terminations - was an eye opener for me as I thought things were much more straightforward!

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Super Wammer

It's interesting that the bmc is a current amplification design. There aren't many on the markets. I recently tried a Bakoon and now own a 47 lab, both are current amplification. Don't know much about the technicality of it but I like the sound a lot. Dead quiet, dynamic and tonally rich

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I use the Ayre and an Aqvox in fully balanced mode and they're excellent.

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It's interesting that the bmc is a current amplification design. There aren't many on the markets. I recently tried a Bakoon and now own a 47 lab, both are current amplification. Don't know much about the technicality of it but I like the sound a lot. Dead quiet, dynamic and tonally rich

Using the cartridge's current output rather than its voltage has a number of issues. Firstly, all cartridges are designed to be loaded with a certain minimum resistance, not to see a short-circuit, which what a current input is. Very low output cartridges like the IO are almost current devices as their minimum load is very low, so they may wellbe happy with a current input. Other MCs with DC resistances of 20-50 ohms and which expect to see a load of 100-500 ohms may not be happy into a short circuit. The argument made for current input of damping doesn't convince as the electromechanical coupling in a cartridge is weak, so there won't be much mechanical damping from a short circuit.

Furthermore, the cartridge's magnetic circuit isn't designed to drive current, and it may depend on what sort of cores the coils are wound on whether there's any possibility of saturation. Air cored coils, such as Audio Technica use are probably not bothered by seeing a short circuit.

I don't see what advantage using a cartridge's current output really is. A cynic might say that it's being different so that one has a story to tell,i.e. marketing, or possibly because a designer just wanted to be different to see if he could. Technically I fail to see what can be achieved this way that can't be done just as well conventionally.

S

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Hi Serge,

Beyond my expertise but it certainly sounds good as does aqvox. Below is the marketing blurb:

When amplifying a signal from an MC pick-up, the interaction between the MC pick-up and the input is a critical element, as is optimal utilization of the very fine MC signal. Let us compare the traditional voltage input with the CI current input in terms of signal strength, damping, and amplification:

1. Signal Strength:

Moving Magnet (MM) and Moving Coil (MC) Cartridges at Voltage and Current Inputs MM cartridges have the advantage, at the voltage amplifier input, of an output voltage almost ten times higher than MC pick-ups. It is the other way round at the CI input: MC pick-ups can provide up to ten times higher currents than MM systems. Consequently, an MC system is the stronger not the weaker system at the CI current input.

2. Damping an MC Pick-Up as an Oscillating System

A phono pick-up cartridge is an oscillating electro-mechanical system. At a voltage input, the pick-up signal will be read, but the pick-up emits practically no energy to the amplifier circuit. With a high-impedance input, the oscillation energy of an MC pick-up stays undamped and is especially noticeable in the high-frequency range. Cartridge loading resistors are connected parallel to the input to damp these oscillations. Energy is always destroyed in these resistors, so trimming a system is a compromise: The resistance must be low enough to damp system oscillations to a bearable level yet high enough for the system not to sound completely de-energized.

At the other end of the transmission path, with an amplifier driving the speakers, there is good reason not to use an amplifier with a high-impedance output, and to connect parallel resistors to damp the natural oscillation of the speakers. Power amplifiers which simultaneously drive the speakers and, through low-impedance, dampen their oscillations, are a well established standard which makes perfect sense: Driving and damping the speaker as an oscillating, electro-mechanical system go hand in hand.

Making the amplifier input low-impedance is unusual but makes perfect sense with the MC input: Signal processing and damping the oscillating electro- mechanical system through an amplifier input allows the MC system's energy to flow as current into the input. Energy is discharged this way but not destroyed. In fact, it is used for signal amplification and to conjure a vitality not previously heard.

3. Use of the Original Input Signal Current

The fine original current that the MC system feeds into the low-impedance CI input flows right through the amplification stage. At its end is a defined signal voltage through the termination. The termination can be a simple resistor, or with PHONO, a part of the RIAA filter network. At the CI circuit output, you can find the same electrons that are fed into the input. The original signal is used instead of a copy. In this way, the very delicate, finely tuned MC signal experiences the most sensitive method of amplification, and is conserved in all its richness and tonal diversity.

Amplifier Circuit

1. Balanced Current Injection input

2. Common base circuit for highest bandwidth

3. Widely automatic gain adjustment

4. Feedback free circuit with very short signal path

5. Full balanced circuit

6. Balanced LEF single ended class-A output

The Current Injection Circuit

The Current Injection input circuit is a remarkable implementation of the principle of the common-base circuit. It is known for having the widest broadband, the least distortion, the best dynamics, and the most musical sound of the three base circuits, the benefit of not making a copy. Actually this CI is rather an I/V converter than an amplifier in the traditional sense. Dynamic losses and distortions are remarkably lower compared to common voltage amplifiers. But it has a low-impedance input and a high-impedance output. Fortunately, the circuit is a perfect match for MC pick-ups and D/A converters, our best audiophile sources. For decades, some of the best MC amplifiers in the world have used the common-base circuit. However, only unearthed balanced input allows the MC cartridge to be coupled directly with a common-base circuit without risk or comprises. The Current Injection circuit, compared with traditional circuits, significantly reduces dynamic loss, and signal distortion and discoloration.

LEF Output Stage

Because the Current Injection circuit has a high-impedance output, a stable buffer is required to drive the load. Seen dynamically, there is no alternative to single-ended amplifiers, which run in Class-A drive. In a Load-Effect Free drive, the single-ended Class-A transistor, which is decisive for the sound, is freed from the work of driving the load. This is why we say it is Load-Effect Free. The quality goes through the natural lack of distortion up and above the classical Class-A mode, and renders negative feedback needless.

Truly Balanced Circuit

In balanced Class-A mode, the supply voltage is not modulated through the music signal. One must keep Phono in mind. The output of a phono-amplifier produces a few volts at high levels, but the input has a sensitivity in the μVolt area. The balanced Class-A mode leads to a distinct increase in the inner tranquillity of the sound pattern, leaving more room for fine details, dynamic leaps, relaxation of the performance, and three-dimensional spatial illustration. In other words, an intense, in-depth music listening experience.

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Interesting about your comment re: audio note Io. BMC provide guidance on suitable cartridges by a formula that account for output in the numerator and the internal cart resistance * phono input resistance in he denominator. Was calculating current outputs of various cartridges that I am interested in - the Io would be viable without a step up according to the formula but it does not produce as much current as say a Lyra.

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Allmost all phono stages, whether MM or MC have unbalanced inputs, even if the input phono socket floats from the chassis, it is still connected to audio earth, and so unbalanced. A very few have properly balanced inputs, using XLR connectors, of possibly the smaller Lemo connector. Most MC cartridges are inherently balanced, as the magnetic circuit isn't electrically connected to any of the four cartridge pins, and the coils are symmetrical about earth. Most MM cartridges could be balanced, but most have a mumetal screening can over the coils, and this can is connected to one of the ground pins. Shure do this very obviously on the V15III, and the screening can connection can be removed if one so wishes. Having the can connected to one side unbalances the cartridge, so using one into those rare properly balanced inputs negates the benefits.

In my view, all cartridges should be wired balanced and all phono stages should have balanced inputs,with any screening can on MM cartridge connected to the headshell/arm metalwork, not to audio ground. It's how microphones are wired, and there's little difference between a MC microphone and a MC cartridge.

S

Thank you Serge,

I stand corrected and informed :^

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Interesting about your comment re: audio note Io. BMC provide guidance on suitable cartridges by a formula that account for output in the numerator and the internal cart resistance * phono input resistance in he denominator. Was calculating current outputs of various cartridges that I am interested in - the Io would be viable without a step up according to the formula but it does not produce as much current as say a Lyra.

They don't quote the actual input impedance of their amplifier. One can assume it must be essentially zero, but may not be, so the formula they provide isn't terribly useful, except if one assumes that the cartridge's resistance is much greater than the amplifier's input impedance.

An IO is an unusual cartridge in that the coils have very few turns and so generate a very low voltage,but as they also have a very low resistance, it will generate quite a lot of current. That's not to say that other cartridges won't generate more current, as it depends also on the magnetic circuit of the cartridge and what compromises / design choices were made. However, in the specific case of the IO, having a very low resistance, it means that the input impedance of the amplifier, if not actually zero, will have much more of an effect in limiting the current than it would for a cartridge with a higher coil resistance.

S

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They don't quote the actual input impedance of their amplifier. One can assume it must be essentially zero, but may not be, so the formula they provide isn't terribly useful, except if one assumes that the cartridge's resistance is much greater than the amplifier's input impedance.

An IO is an unusual cartridge in that the coils have very few turns and so generate a very low voltage,but as they also have a very low resistance, it will generate quite a lot of current. That's not to say that other cartridges won't generate more current, as it depends also on the magnetic circuit of the cartridge and what compromises / design choices were made. However, in the specific case of the IO, having a very low resistance, it means that the input impedance of the amplifier, if not actually zero, will have much more of an effect in limiting the current than it would for a cartridge with a higher coil resistance.

S

Hi Serge,

They do state the phono amp impedance in the manual where the formula is - it is <3ohms.

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Hi Serge,

They do state the phono amp impedance in the manual where the formula is - it is <3ohms.

Missed that, thanks.

Yes, with 3 ohm impedance, most MC cartridges will see that as a short circuit as it will be a very small proportion of their resistance, whilst for an IO, which if I remember correctly, has itself a 3 ohm resistance, it will limit the IO's current to half what it would be into a real short circuit.

S

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Missed that, thanks.

Yes, with 3 ohm impedance, most MC cartridges will see that as a short circuit as it will be a very small proportion of their resistance, whilst for an IO, which if I remember correctly, has itself a 3 ohm resistance, it will limit the IO's current to half what it would be into a real short circuit.

S

Serge - Io impedance is 1 ohm

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