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Accession for Moving Coil

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Aussie Mick View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Aussie Mick Quote  Post ReplyReply Direct Link To This Post Posted: 29 Aug 2017 at 9:43am
Thanks, Graham,
     On the one hand, sad face. On the other, we all know how good the Elevator is. You're still winning.
All the best,
Michael.
Rega RP8 - Apheta 2 - Accession MC Enigma PS -Solo ULDE (Focal Utopia) - PS Audio M700 - Fical Kanta No2
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ICL1P Quote  Post ReplyReply Direct Link To This Post Posted: 30 Aug 2017 at 7:55pm
Originally posted by Graham Slee Graham Slee wrote:

I hit the 'emergency stop button'.

...

I would rather scrap the thing ...


Does this mean it's not going to materialise or that it's back in the design phase?
Ifor
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Reflex M & ACCESSION M, CuSat50, Majestic DAC, a Proprius pair.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 30 Aug 2017 at 8:33pm
Abandoned
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 30 Aug 2017 at 8:35pm
I am currently enjoying getting my hearing back using a 2M Black and 11 year old Gram Amp 2 (the original Communicator). What a relief!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Drewan77 Quote  Post ReplyReply Direct Link To This Post Posted: 30 Aug 2017 at 9:37pm
This latest confuses me (which isn't difficult!) - if the Accession MC is abandoned I guess this means that it sounds inferior to the MM version + an Elevator?

Or is it that Graham does not believe that any MC cartridge/phono could sound superior to a high end MM/MI?
Older than I once was, younger than I'll be
.............................
Andrew
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 31 Aug 2017 at 1:52am
MC cartridge outputs are in the range 0.1 to 0.7mV and electrically can reach 2 MHz. Because of their constant velocity character the output rises at 20dB per decade, and distortion products reaching 2MHz (no music signal up there) would be 66dB higher which equates to 600mV for an average 0.3mV 1kHz rated output.

There will be distortion products because the vibration system encounters friction as well, and also electro magnetic interference which is not effectively screened by the archaic tone arm electrical characteristics.

At the opposite end of the spectrum the output at 10Hz is a mere 0.03mV, or 30 micro-volts.

The type of electronics required for both low noise to allow 0.03mV a reasonable signal to noise ratio, and input natural slew rate of 11V/uS to cover the ultrasonic artefacts is extremely limited.

There is enhanced slew rate which is better termed forced slew rate which occurs in their output stages, not naturally in their inputs. The input slew rate is nothing like the quoted slew rate. Slew rate equals 0.3 x Ft in a differential amplifier (or an op-amp) and for a 10MHz gain-bandwidth product amp it is 3V/uS. Insufficient for an 11V/uS input possibility.

This can be improved only by emitter degeneration in a BJT amp, or through the use of input FETs. In both cases there is a noise penalty.

There are however two FET input op-amps that exist which should be capable of doing it, but they both feature enhanced (forced) output slew rate. There are also three other BJT op-amps which feature enhanced slew rate which could be used.

The OP37 is a decompensated version of the OP27 and is only unconditionally stable if compensated to be an OP27 and so has a slew rate of just 2.8V/uS. Unsuitable.

The NE5534 with unity gain compensation is 7V/uS. Almost suitable you would think, but (difficult to find) applications data puts its real input slew rate at just 0.5V/uS.

A particular video op-amp is much faster at 230V/uS and uses a different type of enhancement which is cumbersome to stabilize unless most of the enhancement is given up by overcompensation. It will manage the required input slew rate. However its input current is far too high for the size of feedback resistor required, so it is a non-starter in many ways.

This leaves the two FET devices: OPA827 and AD4627. Note they end in 27. It's because they are FET input variants of the OP27.

All five op-amps are three stage devices to obtain the open loop gain they possess. The third stage has to use enhancement techniques to reach the gain bandwidth product such a high open loop gain dictates whilst maintaining their -20dB per decade slope.

In the case of the OPA827 and AD4627 they share the same enhancement as the OP27. Their slew rate is an FET input multiple of 2.8V/uS. But if you take the 8MHz gain bandwidth product of the OP27 and apply the slew rate formula you will note it should read 2.4V/uS. This is indicative of output slew rate manipulation or enhancement.

If the two FET input op-amps did not have this enhancement we would be left with a good natural input slew rate suitable for MC, but it is necessary in a three stage device, and so the result will be distortion. In fact, simulation shows ultrahigh frequency instability if used as suggested by their respective manufacturers.

Through noise gain manipulation these op-amps were made to stabilise by tricking them by increasing their gain without changing the signal gain. However, any increase in gain reduces the bandwidth, and in reality this reduces the true slew rate.

In all cases there is slew induced distortion set off by the many high frequency artefacts including the mixing-in of noise and the ever present radio frequencies introduced since the early days of low output moving coil. This is evident on listening as 'new' or alien sounds are produced especially but not limited to the upper mid band.

We could always reduce the input slew rate presented to the range of possible amps. It could be limited in frequency by adding input capacitance to the stage. But where do you set it?

The cartridges I'm using range from 12uH to 56uH inductance, and from 8 to 43 Ohms equivalent series resistance. Some such as the SPU's will have lower inductance and I'm sure there will be one or two MCs with higher inductance. The range is something like 5:1, whereas with MMs it is less than 2:1.

This smells of cheating. We may as well have tone-controls, but that is a taboo, and so is frequency limiting.

I know it is used in some MC phono stages, and remember a discussion by one well-known designer suggesting the use of a 10nF load capacitor. If it were only that simple. Capacitors come with inductance which competes with cartridge inductance to produce frequency spikes, which if they happen in the wrong place ruin the sound. It would be unfair for one or two MC cartridges to carry the can for this.

So what is the solution for MC users? They could switch to MM where these problems don't exist, or don't exist to such extremes. The alternative could be to use a device which can make use of a particular video op-amp, overcompensated as it has to be, but with much lower gain which makes it viable. So what device is that? The Elevator EXP moving coil step-up amplifier!

PS. Please do not write in with your suggestion of op-amp. I have been there, bought the tee-shirt, and it shrank. And after 40 years of doing this I'll not be very happy about others trying to tell me how to do my job. Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 31 Aug 2017 at 7:18am
Thank you for the detailed technical summary graham.

In summarising I read this as the MC concept is a fundamentally flawed design (or at least has sufficiently big holes in it to render it inferior to the listener than mm) which may be overcome with the MC accession if there was suitable electronic devices to accommodate it which there isn't?

There appeared general agreement from the mule device last year that it sounded better than the accession m & Elevator? Were those initial conclusions erroneous or has something else changed?
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