Turntable Evaluation - what do you need to know and how do you know it?

[tima]

I find this very interesting and relevant to the discussion of not only measurements, but verification of specifications and claims made by mfrs.

Any system with feedback will have a delay as it requires an error component to create a correction if the process variable (speed in this case) is not constant. The error term is usually run through a low pass filter to smooth the response, but even if it isn't, the inertia of the platter creates a time constant that cannot be eliminated, therefore any correction will take a finite amount of time to be effective. The control loop can be tuned to provide the best performance, but there will always be a trade off between fastest settling time and lowest noise. While the control loop can be tuned, it is constrained within a certain bandwidth dictated by the physical make up of the drive components: Platter inertia, motor torque, available power from the supply, encoder resolution and sampling time. To give an example, using a DSP based controller on a belt drive motor running at 600 RPM with a very low inertia rotor, the bandwidth is high and the control loop can eliminate speed variations in a 100µSec (10kHz) time frame, but this is not possible with a DD table with a heavy platter and a power supply with limited current capability.

It would be interesting if mfrs would give specifics about the control loop they use for a DD table ie: Loop bandwidth, settling time, % overshoot, sampling time, resolution etc., as well as the rationale for choosing the particular solution (lowest noise, best speed stability, most dynamic sound, etc.). While these specs would be difficult for the consumer to verify, reviewers could measure and verify and it does give the consumer a basis for comparison.

Some claims made by mfrs are easily verified or refuted. If a mfr says their DD table does not slow down with applied drag, it is easy to test for this by lightly touching the platter while playing a record and listening for a change in pitch. If the servo response time is too slow to compensate for such a slow pace test of drag (low loop BW), it also won't be able to respond to dynamic drag caused by variable groove modulation which occurs much more rapidly by definition. If a mfr is willing to stretch the truth on such an easily verifiable metric, I would be suspicious of other claims that could be more difficult to verify.

As far as lag time goes, it's a trade-off. By definition any feedback mechanism will have it. But, that doesn't mean all such are equal or that stable accuracy is thwarted by response time versus no feedback. You cited a DD with a heavy platter ... okay, use a lighter one. There aren't any inertialess tables, but a design that doesn't make use of inertia or count on it for stability may take a different approach. There are techniques for much finer grained encoder resolution and sampling time. Controller software can run on higher MIPS processors and be predictive based on available data. I disagree that slowing a platter with your finger is equivalent to groove modulation. (Was your example hypothetical or is there really a manufacturer that claims a table will not slow with applied drag?) I agree there are limits - physicality and its laws - but it is possible to shave response time finer and finer and I believe the results of doing so are audible.

I agree that more info on the control loop would be a good thing - it's near non-existent. Some manufacturer's (not necessarily table makers) don't want to talk with their customers or reviewers in technical terms. I've been discouraged from that in certain reviews - which I ignored. Others fear loss of proprietary technique, etc. Fewer technical specs mean less opportunity for confirmation, etc. It's also possible to give up specs as measurements without divulging implementation detail.

 

[Phoenix Engineering]

As far as lag time goes, it's a trade-off. By definition any feedback mechanism will have it. But, that doesn't mean all such are equal or that stable accuracy is thwarted by response time versus no feedback. You cited a DD with a heavy platter ... okay, use a lighter one. There aren't any inertialess tables, but a design that doesn't make use of inertia or count on it for stability may take a different approach. There are techniques for much finer grained encoder resolution and sampling time. Controller software can run on higher MIPS processors and be predictive based on available data.

If I recall, the Japanese mfrs made DD tables in the '70's and '80's with relatively light platters that supposedly corrected many times per revolution and some reviewers complained that it created a "grainy" sound. From what I've seen, the control mechanism was fairly simple with only a proportional error amp. Modern controllers can implement a full PID feedback loop that may provide better results. I think a heavy platter automatically reduces W&F, but it also automatically reduces the loop BW unless you have a massive motor and power supply, which potentially increases noise and vibration. The point is, there are trade offs with almost any design, but it is how you implement the particular choices. FYI, I don't think MIPS is a limiting factor at these speeds.

I disagree that slowing a platter with your finger is equivalent to groove modulation. (Was your example hypothetical or is there really a manufacturer that claims a table will not slow with applied drag?) I agree there are limits - physicality and its laws - but it is possible to shave response time finer and finer and I believe the results of doing so are audible.

Can you explain why finger drag and groove modulation are different? Aside from the obvious difference in amplitude and rate, they should produce the same effect: slowing of the platter. My point was that a servo system is only useful for controlling changes in speed that occur at the upper loop BW or slower; if the changes occur faster than the servo can compensate for, the speed will not remain constant. It was not a hypothetical example; VPI claims the HW40 does not slow with finger drag and they imply it can compensate for changes in groove modulation. The platter does slow with finger drag, which means that even slowly applied drag is outside (above) its upper BW limit; dynamics caused by changes in groove modulation will be even faster and shorter lived, so the servo system will have no affect on those as well.

I agree that more info on the control loop would be a good thing - it's near non-existent. Some manufacturer's (not necessarily table makers) don't want to talk with their customers or reviewers in technical terms. I've been discouraged from that in certain reviews - which I ignored. Others fear loss of proprietary technique, etc. Fewer technical specs mean less opportunity for confirmation, etc. It's also possible to give up specs as measurements without divulging implementation detail.

I agree with you, but I think we are in the minority. When I first looked into marketing the Falcon PSU and RR tach, I spoke with the senior hardware purchaser for one of the large on-line retailers. He had a technical background and I was explaining in technical terms, all the reasons I thought my products were superior to the competition. He stopped me and insisted that no one wants to hear a technical explanation of the product, they only want to hear how good it will make their music sound. I pointed out that I have no control over all the variables that affect a listener's experience (room acoustics, type of music, hearing quality, volume level, etc.), but he was convinced that I should just lay on the purple prose, that it would be more effective. I never agreed with that philosophy, but he obviously understood the market better than I did.

I also agree that a mfr can explain the operation of their design in detail without giving away any secrets. Designs such as this are heavily embedded in the software where details of the implementation can easily be hidden.

 

[tima]

Thanks for your follow-up.

If I recall, the Japanese mfrs made DD tables in the '70's and '80's with relatively light platters that supposedly corrected many times per revolution and some reviewers complained that it created a "grainy" sound. From what I've seen, the control mechanism was fairly simple with only a proportional error amp. Modern controllers can implement a full PID feedback loop that may provide better results. I think a heavy platter automatically reduces W&F, but it also automatically reduces the loop BW unless you have a massive motor and power supply, which potentially increases noise and vibration. The point is, there are trade offs with almost any design, but it is how you implement the particular choices. FYI, I don't think MIPS is a limiting factor at these speeds.

The platter of my Monaco 2 (M2) is magnesium and it can correct many times per second. I know of no review that says its sound is grainy. We agree that implementation is a key. Computer throughput should be relative to the amount of data processed. The M2 reports encoder data to the controller over 166k per second and can emit over 1k corrections per second. It has a 40mips cpu. I don't know if that is overkill.

Can you explain why finger drag and groove modulation are different? Aside from the obvious difference in amplitude and rate, they should produce the same effect: slowing of the platter.

Not sure about the 'why?' but from simple experience I see the controller's lock light flicker when a finger is applied to the platter, and see no change when the stylus hits the record. This is simplistic evidence and why I said the two were not the same in effect. I do find it hard to believe, at least theoretically, that any platter will not slow when a finger is applied.

When I first looked into marketing the Falcon PSU and RR tach, I spoke with the senior hardware purchaser for one of the large on-line retailers. He had a technical background and I was explaining in technical terms, all the reasons I thought my products were superior to the competition. He stopped me and insisted that no one wants to hear a technical explanation of the product, they only want to hear how good it will make their music sound.

Yes, that doesn't surprise me. Off the top I doubt many customers will say they can hear a difference between 0.1% (1000ppm) versus 0.0001% (1ppm) deviation from perfect 33-1/3rpm. That is until they hear it. I probably would also be sceptical. We don't seem to recognize how sensitive are our ears because we're not called upon to use them that way and typically not presented with opportunities of situations to discriminate between.

 

[Phoenix Engineering]

Not sure about the 'why?' but from simple experience I see the controller's lock light flicker when a finger is applied to the platter, and see no change when the stylus hits the record. This is simplistic evidence and why I said the two were not the same in effect. I do find it hard to believe, at least theoretically, that any platter will not slow when a finger is applied.

I should have said "Theoretically, groove modulation will cause slowing of the platter", as I've never been able to measure it either, even with a tach that measures to 3 places right of the decimal. If it exists, it must be infinitesimally small and makes VPI's claim even more suspicious. It would be interesting to hear how they are measuring this.

Another spec that might be relevant in this case is the resolution of the PWM output. The supply voltage for the output is 36VDC; with a 10bit PWM controller, the minimum voltage step would be ~35mV (36/2^10), which could be considered rather coarse, especially since the motor requires ~350mV for 33 RPM. Even at full torque output at platter start up, the voltage never exceeds 4V, so I'm not sure why the supply is so high (36V)?

 

[Dr Vinyl]

The ideal way it to listen to turntables in your own system.
The biggest hurdle is to have a properly set up table, you can have a super turntable and if it's not set up properly you are wasting your money.
Get a Smartractor and the Analog Magik record and spend 2 or 3 hours working on your table, arm, and cartridge and it will be more rewarding than spending 5k on a cartridge that is eyeballed in your arm.

 

[DasguteOhr]

a good dealer who will allow you to listen to the turntable at your place. in deposit against the value of the device. not just for a day, new things always sound good at the beginning.
Forums are good for getting suggestions. In the end, trust your ears and your friends 4 or 6 ears will hear more than two.

 

[spiritofmusic]

No-one is home demoing TTs in the UK, as far as I'm aware.

 

[pcosta]

No dealer I know does that in Canada. You can listen in the shop, if they have inventory. Most often they are ordering it for you to buy unheard.

 

[mtemur]

You can listen in the shop, if they have inventory. Most often they are ordering it for you to buy unheard.

That’s why everybody’s happy with their turntables (including me).

 

[pcosta]

That’s why everybody’s happy with their turntables (including me).

We should be happy and blessed that many of us are able to own and play with some great gear. Some of us have some awesome and unique and uncommon turntables, such as my EMT 927st, but this audiophile disease takes hold and we just can't stop looking for more.

 

[Syntax]

That’s why everybody’s happy with their turntables (including me).

At the beginning of my audiophile journey I discovered very fast, that the sonic descriptions in reviews were ... well you know...., everything was great, always super, always recommended, even ....my choices were rolling a $$$ dice based on that. I stopped burning money and went to dealers, offered them serious money for 2 h in their listening room and they built the system I asked for a few day earlier ... it was a win win stuation.
I did support the Dealer and started to learn ...

 

[mtemur]

We should be happy and blessed that many of us are able to own and play with some great gear. Some of us have some awesome and unique and uncommon turntables, such as my EMT 927st, but this audiophile disease takes hold and we just can't stop looking for more.

Lucky you. All EMT turntables I've listened to were sounding great almost on par with some hi-end, hi-tech tables.
what I'm trying to say that we audiophiles have a tendency to like what we own in our system when we are demoing against a dealers unit without paying. but if we already payed, then most probably we like it when we first listen to it in our system. the same rule applies when your friend brings over his unit or cable to compare with your's. most audiophiles end up liking their own units not his friend's or some dealer's. if the the sound is undeniably better than his own unit than usually there are some other reasons like repairability, technology, appearance, loss of value etc.
By the way I'm not a dealer or never been a dealer/seller or anything like that.

 

[pcosta]

Lucky you. All EMT turntables I've listened to were sounding great almost on par with some hi-end, hi-tech tables.
what I'm trying to say that we audiophiles have a tendency to like what we own in our system when we are demoing against a dealers unit without paying. but if we already payed, then most probably we like it when we first listen to it in our system. the same rule applies when your friend brings over his unit or cable to compare with your's. most audiophiles end up liking their own units not his friend's or some dealer's. if the the sound is undeniably better than his own unit than usually there are some other reasons like repairability, technology, appearance, loss of value etc.
By the way I'm not a dealer or never been a dealer/seller or anything like that.

So true, sometimes we are blinded by our own stubborness to admit there may be something wrong with our sound. Pride or embarrassment to admit we bought a very expensive boutique item and it sounds like crap or its broken.

 

[J.R. Boisclair]

A topic came up In Phoenix Engineering's thread VPI HW40 Direct Drive about a buyer's responsibility to be self-informed prior to making a turntable purchase.

I noted that while we have several resources for test/evaluation information about electronics and speakers (eg. Atkinson, Bascom King, etc ), there is a general absence of independent test results for turntables - meaning confirmation of specs and other technical information relative to what a manufacturer provides.

Imo, this is a gaping hole in the turntable market place. The terminology of turntable evaluation and what counts as important is scant and unclear. Few manufacturers offer specs that customers can use for comparison. Few manufacturers offer information about their table's drive system (motor, controller). I read more turntable buying decisions based on price and looks than anything else. Vienna discusses discrepancies between manufacturer information and actual product.

-- Absent readily available journalistic or consultancy information about turntable design and performance: If a potential customer, typical audiophile or even a reviewer wanted to evaluate turntable speed accuracy, stability, and noise what should he do? I'm talking about someone who is neither an engineer nor owner of specialized diagnostic equipment.

-- What information should the manufacturer provide and how does the buyer assess this?

-- What other aspects of a turntable can and should a potential buyer learn about on their own?

PeterA suggested using a Sutherland Timeline - a laser emitting timer placed on a table's spindle for gauging speed stability and accuracy. Take one of those from dealer to dealer? Fremer publishes graphics from a cell phone app that uses a 3150Hz test tone. Analog Magik gives the results of a playing a test tone record into its computer program. Take your laptop and test record from dealer to dealer? Phoenix Engineering's own RoadRunner tachometer was popular when available. Should a table buyer invest in (or more likely, borrow) test equipment before making a purchase?

And what about assessing turntable noise? In one of my Monaco reviews I used a stethoscope; surely there is something better.

Before making a turntable purchase, what information should a consumer have and how should he get it?

Here's a couple things one can do to assess engineering quality and bearing health with one cheap and one commonly available item:

Turn on the motor and keep the tonearm on the armrest. With a stethoscope applied in various areas of the plinth (particularly around the tonearm and even ON the tonearm) listen for any bearing noise. If you do hear something through the stethoscope when the platter is spinning (compare to sound when it is stationary), you can isolate the any motor noise from bearing noise by removing the drive belt and hand-spinning the platter and listening again.

Second, put your smartphone video on and increase the magnification to ~4x. Stand the phone up on a stable surface aiming at the platter and AT platter height with platter edge in focus. It is important to zoom in on the platter's edge pretty tightly. Turn on motor and film the platter for a few revolutions. Look at video and see if the platter rises and lowers within the video frame as it spins. This assesses perpendicularity of the bearing to the platter and the bearing fit and tolerance. You might be surprised how many tables have the platter rising and dropping with every revolution. This causes a few issues for playback, as you might imagine!

You can check for bearing fit and tolerance by simply trying to rock the platter in your hands (one hand pushing down, the other pulling up and then alternating quickly), being very sensitive to feeling any play. Very few turntables will survive this simple test of engineering quality. The better bearings will (of course) always have SOME play but you won't feel it rocking at all whatsoever.

 

[tima]

You can check for bearing fit and tolerance by simply trying to rock the platter in your hands (one hand pushing down, the other pulling up and then alternating quickly), being very sensitive to feeling any play. Very few turntables will survive this simple test of engineering quality. The better bearings will (of course) always have SOME play but you won't feel it rocking at all whatsoever.

I'd be pretty cautious about doing this with someone else's turntable.

 

[J.R. Boisclair]

I'd be pretty cautious about doing this with someone else's turntable.

Fingertip pressure is all you need. I should have added that point. If you muscle it you will not likely feel the bearing play.

 

[PeterA]

No dealer I know does that in Canada. You can listen in the shop, if they have inventory. Most often they are ordering it for you to buy unheard.

I had the rare opportunity to directly compare turntables in my system for a month and then keep the one I wanted and return the one that I did not like as much for a full refund.

 

[Rt66indierock]

A topic came up In Phoenix Engineering's thread VPI HW40 Direct Drive about a buyer's responsibility to be self-informed prior to making a turntable purchase.

I noted that while we have several resources for test/evaluation information about electronics and speakers (eg. Atkinson, Bascom King, etc ), there is a general absence of independent test results for turntables - meaning confirmation of specs and other technical information relative to what a manufacturer provides.

Imo, this is a gaping hole in the turntable market place. The terminology of turntable evaluation and what counts as important is scant and unclear. Few manufacturers offer specs that customers can use for comparison. Few manufacturers offer information about their table's drive system (motor, controller). I read more turntable buying decisions based on price and looks than anything else. Vienna discusses discrepancies between manufacturer information and actual product.

-- Absent readily available journalistic or consultancy information about turntable design and performance: If a potential customer, typical audiophile or even a reviewer wanted to evaluate turntable speed accuracy, stability, and noise what should he do? I'm talking about someone who is neither an engineer nor owner of specialized diagnostic equipment.

-- What information should the manufacturer provide and how does the buyer assess this?

-- What other aspects of a turntable can and should a potential buyer learn about on their own?

PeterA suggested using a Sutherland Timeline - a laser emitting timer placed on a table's spindle for gauging speed stability and accuracy. Take one of those from dealer to dealer? Fremer publishes graphics from a cell phone app that uses a 3150Hz test tone. Analog Magik gives the results of a playing a test tone record into its computer program. Take your laptop and test record from dealer to dealer? Phoenix Engineering's own RoadRunner tachometer was popular when available. Should a table buyer invest in (or more likely, borrow) test equipment before making a purchase?

And what about assessing turntable noise? In one of my Monaco reviews I used a stethoscope; surely there is something better.

Before making a turntable purchase, what information should a consumer have and how should he get it?

Tim, those of us who can measure audio equipment aren’t interested in measuring turntables. No point, we are near instrument grade measurements now on DACs, and amplifiers if desired. Are we anywhere close to instrument grade measurements on turntables?

We aren’t all that interested in measuring DACs either. Once I measured and listened to the Okto Research Stereo DAC 8 in 2020 I lost interest in measuring DACs . It is far beyond anything we can hear.

 

[bonzo75]

Wow, I was enjoying this thread and just noticed that both of these links lead nowhere. They have been scrubbed. I wanted to read some of Phoenix Engineering's posts for reference and helpful information and I can not search his post history, unlike other former members. Why is that? I suppose I can still do a Google search for his content. Weird.

that’s because his profile is not available, possibly because he deleted it.

to search, you will need to add a search item in the search icon in the top corner, and if you select by member Phoenix engineering, you will still get his posts

 

[Steve Williams]

Wow, I was enjoying this thread and just noticed that both of these links lead nowhere. They have been scrubbed. I wanted to read some of Phoenix Engineering's posts for reference and helpful information and I can not search his post history, unlike other former members. Why is that? I suppose I can still do a Google search for his content. Weird.

I assure you this is not another one of your conspiracy theories.
It has a very simple answer so properly described by Ked.

that’s because his profile is not available, possibly because he deleted it.

to search, you will need to add a search item in the search icon in the top corner, and if you select by member Phoenix engineering, you will still get his posts