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Accession for Moving Coil

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Graham Slee View Drop Down
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    Posted: 18 Aug 2016 at 2:15pm
Listening right now to the Accession MC. Yes, as rumoured elsewhere on this forum my aim was to extend the Accession's technology to work directly with moving coil cartridges.

The important function of the Accession phono stage is to EQ the rising response of magnetic cartridges. This isn't something subtle, it is a 45 degree upward slope where the output of the cartridge doubles every octave (every doubling in frequency). Not many know about this, but now you do.

What I didn't want to do is place a pre-pre before the Accession single slope integrator because that kind of defeats the object.

Direct conversion gives rise to much more information being passed through without the ups and downs to frequency and phase conventional phono stages do right at the frequencies we are most sensitive to.

OK, those ups and downs have to take place to apply the right EQ for the record, but right in the middle of such extreme amplification as needed by a phono cartridge, isn't the place to do it - but we all did.

For years I struggled with the awkward interaction of cartridge and record equalisation done in one, which conventional wisdom says should be so. They even split the curve, did it passively, or a combination of active and passive techniques, and in doing so it may improve things, but still we have a serious interaction to smooth over.

I made a number of early Reflex phono stages all with slightly different resistor values to shift the so called 318uS (500Hz to you and me) zero up and down by tiny increments. If anything it caused a 0.1dB difference in tolerance, and in fact with one or two it made it 0.1dB more accurate. The difference it made to the sound was unbelievable - most of it in a negative sense.

It just goes to show how sensitive the adjustment of the 318uS zero is. There is no real way to get it accurate. Audio researcher Stanley P Lipsh*tz proposed a number of equations, but in real life the results are far from ideal (how dare I say that?!...)

It's because the RIAA is entwined in the amplification circuit or circuits in one way or another that discrepancies creep in. They are unavoidable. Amplifier stages of all kinds are not perfect - they don't fit with ideal theory.

And then there's another effect I have found over the years and whenever an integrator's response starts to kick up before falling off again - exactly what an RIAA stage does, the sound suffers.

All amplifiers have an integrator response somewhere when you think of it. Mainly it's beyond the audible frequencies where the response droops off toward its closure frequency. For stability the slope where it passes through unity gain has to be close to 20dB/decade, but below unity gain a circuit can kick up again, and even though it remains safely below unity gain, the effects can be heard.

So when you arrange to have a zero in a 20dB/decade slope within the audible range it is going to exhibit itself in a non too pleasing way.

The Accession does away with the zero in the gain stage and shifts it downstream to where it is less likely to offend. In fact, because of the cartridge EQ it becomes a pole similar to what you get in tone controls (and that's what they originally used for EQ!)

But that doesn't explain what effect having a pre-pre (a preceding stage) causes, and why I think for the Accession it doesn't bring much to the table.

Every stage has its own open-loop frequency response which isn't tremendously wide, so negative feedback which reduces gain spreads it out a bit to get that wide frequency response needed for audio.

For the lower frequencies you therefore get true 180 degree negative feedback, which is good, and at higher frequencies you get around 90 degree negative feedback, which isn't as good because it favours the rising edges and in doing so moves the ear's focus toward the highs, and the lows can appear a little too tight - even though the analyser proves a flat response.

With the Accession integrator nearly all the negative feedback (above 50Hz) is 90 degrees, and based on the above it sounds like I'm talking myself out of a job.

The difference is the incoming signal is rising at 20dB/decade so is plus 90 degrees, and if you now add the integrator's -90 degrees response, it totals 180 degrees. This explains the low frequency improvement the Accession brings - the bass has some meat to it - it sounds real.

Therefore, in my opinion, it would be best to do direct conversion from the cartridge and not have any preceding stage.

This is where I needed a volunteer who has the necessary equipment for comparison between the Elevator with Accession, and the Accession MC prototype. Who? No other than our resident prolific you tube channel creator Bob!

Now, a digital representation of vinyl won't sound as good as playing vinyl direct, but Bob has the requisite items and a wide selection of moving coil cartridges to play with. After uploading my choice of album tracks to some web space we were able to download the FLACs for a good old comparison session.

After several email exchanges discussing all our findings: Bob's, mine, John's and Leo's, we all concluded that the Accession MC alone had the edge.

There are limitations however. The range of input impedances is not as great as it is using the Elevator EXP as MC step-up. The Accession MC can only go to 500 ohms maximum. I've included three options of 100 ohms (pretty standard); 300 ohms which many ask me for; and the maximum 500 ohms which will add some sparkle to such as the DL103 - not quite the 1k tg prefers but close enough I would think.

So there we have it - the Accession MC. And now Bob will hopefully chip in with his own words?

I'm thinking about getting five made and out to the loaner program so members can beta it.

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Richardl60 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2016 at 7:06pm
Very interesting Graham.

I was surprised to read the end conclusion given what had gone before.

Whilst I cannot say it would be a specific issue I do use 1000 ohm on my Dynavector.

Whilst it wouldn't be an issue for me whether combined gain was higher or lower than the elevator/accession combo but presumably this would cater for low output Moving coils mine is 0.26 or 0.28mv from memory.

Sounds like watch this space then!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2016 at 7:10pm
PS as was recently commented on the interconnect link between the elevator/accession will impact on the results and no doubt Graham, Bob etc will have been using your own leads so it is possible this variable could be removed from the equation with the MC stage for other users who may not use these (for better for worse as the saying goes).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaveG Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2016 at 9:30pm
hmmmm....interesting that the presumably cheaper 1 box solution could sound the best. Not cheaper for me personally unfortunately as I already have the 2 box in place. I'd certainly like to listen to one and 100ohms is right on the money for my preferred Ortofons anyway. Although I've experimented with different loadings in the 10 years I've owned an EXP, I always return to 100.
Dave

Michell Gorbe + HR PSU -> Cadenza Bronze -> SME V -> Elevator -> Accession -> Proprius -> B&W CM6 s2 | Cusat 50 & Spatia cables ->
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Suggs Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2016 at 6:39am
...hmmmmm indeed.

This does sound intriguing as it does seem, as stated by DaveG, to conclude in the opposite way to that which sounded best previously i.e. Reflex M + Elevator is undoubtedly better sounding then the one box Reflex C.

My concerns too are over the level of gain compared to the two box set up. In my system I always feel I could do with a tad more overall gain on the phono side of things as the noise level does reach a quite audible level, although easily obscured by music playing. I certainly wouldn't want less gain

The other point raised previously concerning cartridge loading is also a concern...I have found the Dynavector to sound best to my lugs with the loading on the Elevator set to either 840 or 1000 ohms.

Thankfully, as one of Graham's loaner organisers, I'll be in a position to try these comparisons out for myself, hopefully in the not too distant future Wink Anyone else in Europe who would like to compare for themselves, let me know by PM and I'll start a queue

Derek
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2016 at 7:07am
Interesting what you say about loading and gain, which Dynavector do you have?
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2016 at 9:18am
I will never alienate friends and existing customers by producing something better cheaper. An Accession MC, which is in "beta" at present, would sell for similar as the Elevator + Accession MM. Interestingly some low output Dynavectors work into 30 ohms, so it will be good to have feedback on how the limited impedance range fits with these sound wise.
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