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Old 2nd June 2007, 11:01 PM   #1
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Default To tacho or not to tacho?

I'm currently using an Origin Live DC kit on a Pink Triangle PT Too. Excellent sound generally EXCEPT for questionable speed stability.

On 33rpm, DC volts to the motor is about 2.02V. Putting an LP on the TT, then a paper strobe disc, I can set the speed up, but there is a subtle but noticeable drop in speed when putting the stylus on the record. DC volts goes up very slightly to compensate, which it almost (but not quite) manages to do.

That's the first thing. Next, when I bought this Origin Live kit, it came with no instructions, despite repeated calls to their office. The PSU is a fairly standard ac-ac brick with switchable output voltages from 3 - 12V. I tried most of the settings and the 9V one seems to give the best results. Recently the voltage switch has gone resistive, causing all manner of voltage and TT pitch fluctuations, so I have removed the switch altogether and hard-wired it to 9V.

The motor is mounted on the subchassis - there is a marginal increase in background noise but going down this route solved all sorts of instability problems, because while the subchassis is free to move, the motor to platter distance is always fixed.

I've read many of the threads on this site regarding different DC motor controllers, and there seems to be some resistance against using a tacho feedback arrangement. I'm curious to know why this approach is not favoured. Sure, the system is always trying to correct for errors, but if properly designed with the right values for the loop filter and gain, would this not offer a more stable drive system, both over the short and long term periods?
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Old 5th June 2007, 03:30 PM   #2
abril is offline abril  United Kingdom
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Hi
Personally not a fan of servo control as I don't believe they have any hope of coping with transients particularly at the outer edge,as far as I'm concerned the only way is with a motor that generates enough torque to cope.
Fascinated to know that you feel that Origin live motor suffers with questionable speed stability (I was considering buying a PTTOO with an OL conversion) as I would be interested to know what motor they use as I am thinking of DC'ing my LPT.
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Old 6th June 2007, 03:14 AM   #3
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Default Re: To tacho or not to tacho?

Quote:
Originally posted by starfield
Sure, the system is always trying to correct for errors, but if properly designed with the right values for the loop filter and gain, would this not offer a more stable drive system, both over the short and long term periods?
Therein lies the rub - the "right values for the loop filter" are not easily established.

If the tacho is on the platter you have the problem that the belt drive system's transfer function has both frequency dependent and torque dependent components, complicating loop design. The most popular approach is to use a very slow loop but this naturally results in large deviations from the set point value. Another approach is to compensate for short term deviation at the motor itself (using I x R compensation or similar) and then use the loop to trim for long term drift. This unfortunately does not correct for the major torque dependent speed problem in belt drive TTs, namely belt creep.

Running the tacho directly on the motor is the other plan but every arrangement I have seen increases motor noise. Obviously running the tacho on the motor does nothing for belt creep issues either.
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Old 6th June 2007, 09:51 PM   #4
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Well to be honest, the pitch stability on my PTTOO was, quite frankly, less than satisfactory with the original ac motor and outboard PSU. Acoustic guitar notes, for example, wavered audibly. I think some people are more sensitive than others to this effect but I can honestly say it was never a problem with my old Linn LP12 Valhalla for example, which had rock steady timing.

Not only that, but you could see an LF oscillation in the turntable and subchassis assembly. With the stylus on the record, the arm would quiver sometimes.

The PT went back to the factory twice. The nice chaps there upgraded the bearing to an Anniversary one, and changed the subchassis and springs all for free. When it came back it was better but the effect was still there. Then the PSU started playing up - the motor would stop all by itself then restart which made listening to music anything but relaxing.

At that point, I threw in the ac towel and bought an Origin Live Advanced Kit - this would have been in about 2001.

Result - motor speed would not slow down enough to drive the PT on 33rpm. Origin sent me a new PCB (which didn't work at all!), so I drew out the circuit and modified the first one to give me about 2V out. Result was a more stable drive, reduced wow, an increase in detail and a slightly thinner bass.

A while later, after still not being impressed with the overall speed stability, (fixed motor + wobbly subchassis = instability) I hacked the PT subchassis and mounted the motor directly on it. This was a big improvement. Background noise went up a bit, but not significantly. And that's the system I've been running with since.

But I still think the speed control could be better, and I don't like the Origin circuit. One thing it does not do is feed a clean DC supply to the motor.

So I went out and bought a DC Airpax motor with tacho from Mcleenan Servo, and designed a circuit around an LM2917 v to f converter, OP27 op-amp and a darlington transistor output stage. There was some fiddling around with the component values, and I ended up using two sets of loop filters, but the motor is beautifully quiet and the speed stable, with extra torque available on demand, provided by a smooth DC supply, not a PWM system or a 'ripple injection' system as apparently used by Origin.

However this is on a test bench and NOT on a turntable, for the simple reason I don't have a pulley yet. So the theory has yet to be proved, but I'd be happy to upload the circuit for someone else to try.

Also if someone can suggest where I can obtain a suitable pulley, I'd be most grateful. The obvious answer is rip the one of the Origin motor but I don't want to damage it.

(Incidently, I read that Origin are now using plastic pulleys...wtf?).
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Old 12th June 2007, 08:23 AM   #5
Alex M is offline Alex M  United Kingdom
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Default PT Too

I've owned a TOO since 1990, and although I always loved its clarity, presence and delicacy, I found that the speed stability varied from week to week, as well as being critically dependent on the shelf under it. Last year the PSU started to fail - first 45RPM went, and then the speed at 33RPM started to wobble - and I realised that I had to decide whether to get the deck fixed or to start thinking about replacing it with something else.

So last October I sent my deck to Arthur at The Funk Firm, and had it upgraded to full Anniversary Vector status. This involved replacement of the subchassis and top plate, fitting a DC motor with current feedback to the subchassis, and the addition of a couple of pulleys for Vector operation. This immediately cured the speed stability problems I had with the TOO, and also hugely improved the bass and reduced noise levels.

The only drawback with the new setup is that it takes a while to get up to speed (especially if I want to brush the LP). There is also less room inside the plinth for the thicker subchassis, so if I bounce the suspension too hard it knocks against something inside, but that's no problem in practice.

I wrote a review of the upgrade for the Audio Asylum: http://www.audioasylum.com/cgi/m.mpl...97163&review=1

Anyway, the current servo seems to work as designed - in comparisons between LP and CD there there is no obvious pitch instability, and piano or orchestral recordings sound much more "anchored" in space. I haven't much idea how Arthur's "K-drive" controller works, though I gather he has a patent pending on it.

By the way, all three pulleys in the Vector system are plastic...

Alex
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Old 13th June 2007, 12:13 AM   #6
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Default Re: PT Too

Quote:
Originally posted by Alex M
This involved replacement of the subchassis and top plate, fitting a DC motor with current feedback to the subchassis, and the addition of a couple of pulleys for Vector operation.

then

I haven't much idea how Arthur's "K-drive" controller works, though I gather he has a patent pending on it.

Alex
If your first statment is correct (the motor uses current feedback) then the K drive would be using some form of I x R compensation. Since this isn't patentable, it may be that A.K is using something else.

I assume the vector pulleys are passive (eg there is only one motor). I wonder what effect this has. Multiple pulleys with multiple motors appears to be one avenue to reducing belt creep which appears to be the major unsolved problem with belt drive speed stability.
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Old 13th June 2007, 06:48 PM   #7
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Thanks Alex, some good info there. I read your review and realised I'd seen that before.

Agree with many of your comments. I like the thinking behind The Pink Triangle, as a concept it's well thought out, but as an engineering solution it is flawed in terms of pitch stability, even by PT's own admissions. But the overall sound stage is remarkable for its low colouration, there is a depth and perspective to performances that other turnables miss, particularly on live recordings.

I find a useful trick when comparing turntables is to place the stylus on a stationary LP, then gently tap the disc and the top plate and see what comes back. On a PT, a disc tap produces what sounds like, well, someone tapping the disc, while the top plate is almost totally dead.

Try the same on a Linn LP12. There is a pronounced 'boing' in the upper bass as the subchassis/bearing resonate, and this can take several seconds to die away. Top plate isolation is also nowhere near as good as the PT. So in effect, the turntable audibly colours everything you play on it.

Getting back to the motor drive chain, agree about belt creep but this does seem to be drastically reduced by placing the DC motor on the subchassis.

Now, these plastic pulleys - what diameter shaft do they take?
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Old 15th June 2007, 02:20 PM   #8
Alex M is offline Alex M  United Kingdom
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Quote:
I find a useful trick when comparing turntables is to place the stylus on a stationary LP, then gently tap the disc and the top plate and see what comes back. On a PT, a disc tap produces what sounds like, well, someone tapping the disc, while the top plate is almost totally dead.
As I mentioned in my review, the Achromat further improves on the "tap test"!

Quote:
I assume the vector pulleys are passive (eg there is only one motor). I wonder what effect this has. Multiple pulleys with multiple motors appears to be one avenue to reducing belt creep which appears to be the major unsolved problem with belt drive speed stability.
Don't know about that, but in this case - as you surmise - there is one actively driven pulley and two passive ones. The pulleys are laid out on the vertices of a non-equilateral triangle, which Arthur claims is the optimum layout. He has always maintained that with multiple motors the problem is getting all the motors to drive the belt at exactly the same linear speed.

Quote:
Now, these plastic pulleys - what diameter shaft do they take?
Pretty narrow - I would guess 1.5-2mm.

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Old 27th June 2007, 09:43 PM   #9
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Alex,

Given some time has now passed since you reviewed the PT Anniversary Vector, I would be interested to know what your thoughts are 7 months down the line.

Is the pitch stability really that good?

Is it worth the amount expended?

I ask, because I like the overall sound of the PT TOO so much, but just cannot live with the pitch instability any longer.

Your comments are welcome.
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Old 4th July 2007, 01:15 PM   #10
Alex M is offline Alex M  United Kingdom
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Sorry about the delay - I've just got back from holiday.

Yes, I'm still very pleased with my upgraded deck.

Just using my ears I can't say if the pitch stability is perfect, but I certainly haven't noticed any problems at all listening to music since the upgrade. The crucial test for me is to put on an LP after a recording on CD (since you can get used to a turntable's shortcomings quite easily most of the time it that is all you listen to!). With my old TOO certain music could give me the sensation of seasickness (as I mentioned in my review) under these conditions, but with the new deck there is no audible instability. The only thing I should mention is the motor drive gives a low torque, so the deck takes several tens of seconds to get up to speed, and sometimes in the first minute or two I can hear a little pitch shift.

As far as value for money, it is hard to say, as I haven't heard any other recent decks in the price range to make comparisons. I can only say that if you already have the Pink deck the upgrades take you several steps up the ladder and eliminate all the weaknesses (that I am aware of, anyway) of the PT TOO.

You should note that Funk's prices have increased, as they now charge for labour, on top of the quoted parts cost, although these prices aren't listed on the web site.

Alex
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