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Old 4th May 2004, 02:18 PM   #1
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Default New power supply

Maybe you already know about my power supply for the Thorens TD160, with a signal generator from a dhaen pcb.
After so long time, I looked again at it, made some tests and find out that the main advantage was to get rid of the voltage divider 10K resistor, because it's a great noise maker. But I want to develop another ps.
I'm thinking about the second signal, now created with a simple cap, like in a standard synch motor.
My plan to get rid of it: From the output of the signal generator I take the standard signal, and also take other from a cap. So I have two signal 90 phased. Then apply a volumen control for each signal, I amplify with two GC and two output transformers (I can wind my own trafos here, so I'll adapt the impedances for the GC to be looking at a 8ohm impedance), and the trafo wouldn't be looking at the phasing capacitor. Besides, the second signal can be made as high (or low) in volts as required.
Does this look ok?
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Old 6th May 2004, 10:35 AM   #2
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Hi.
I'm also doing a ps and have done extensive tests measuring vibration and speed in various possible configurations. I don't know about your previous ps, but if I understand you correctly, you want to do the phase shift before the power amps and then feed each motor phase seperatly. If thats so, I think it is a very good move. My tests show that the phase-shifting cap makes the motor more noisy than it needs to be.

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Old 6th May 2004, 11:15 AM   #3
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You can check the thread called "change in frequency mains " or something like that. You can find useful info there.
I'm interested in your tests, could you please detail?
Better than the cap shifting, I will invert the signal with an inverting output amp, so the shift is always 90 (provided resistive loads, or identical impedance loads, of course).
Have you found any suitable AC motor?
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Old 6th May 2004, 11:40 AM   #4
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I'm using the tt's original AC motor. From the brand name it looks like something from the east.

Quote:
I'm interested in your tests, could you please detail?
This is a cut from a post I made at the vinyl engine forum a few days ago:

===============

I have been designing a motor controller 'a la VPI's SDS for myself and now that it is a working prototype I have taken the time to test its efects on the performance of the TT by measuring both motor vibration and platter speed. I'm preparing the data to present it in a website dedicated to the project, but in the mean time here's some thoughts based on those tests (figured this might be useful in this thread).

Regarding motor vibration:
I've found that removing the capacitor and feeding the motor with two separate phases will cause the biggest improvement, reducing vibration by a large factor. Next comes reducing voltage, although, from a certain point, the vibration will increase again. My motor (16V) will vibrate the least when fed with 14V even if it can mantain speed with only 6V. The icing on the cake is phase triming, which provides a smaller but still measurable improvement. My motor literaly feels like its stopped with all those above and a phase trimmed to 92.8 deg.

Regarding mains, I measured 4% THD and looking at the scope the peaks were actualy clipped. This, of course, doesn't help the vibration of the motor. I live in an EEC country, so this will probably be similar in other coutries.

[...] I think you should also consider this: mains also has low freq. "noise" (FM, actually) and that will certaily afect TT performance.
Just last night I measured platter speed and found a peak dev. of 0.02 RPM (0.06%) when using mains feed and 0.005 RPM (0.015%) when using my device (over a period of 5 min). This numbers, are, of course, influenced by the wow introduced by the electromechanical system of the TT itself, but that is the same on both measurements. I think my device's figure is actually being influenced by this and if I had a better TT I would probably get even better results (but this is just a guess).

And then there's the absolute mains freq. and platter/pulley tolerances, which, combined, usually make the platter turn at something else than 33.3 RPM. In my case, I was actually surprised by mains freq. ( I remember it hoovered arround 50.0 Hz. ), but that translated into a platter speed of 33.50 RPM. Again using my device, I was able to get 33.333 RPM (average).

===============

To this I should add that speed measurements were made with a record free platter and that taking the same measurements while playing a record worsened the numbers a bit, suggesting that stylus drag does influence speed stability.

Guilherme.
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Old 6th May 2004, 12:06 PM   #5
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Ah, so you discovered that the cap is the most important thing to avoid... interesting. How did you measure the vibration, just the fingers? I usually use my sthetoscope
I suppose that the deviation from the 90 is caused by the difference in the loads that the not-exact windings represent, was 92.8 the best result by far, or was small difference when compared to 90?

I have a lot of suitable transformers, so I can try to invert a singal easily in a short time.
I also have a 1KVA 220/110V transformer, so I'll have the chance to compare what fdegrove suggested.
BTW, I've rewired my RB250, but my stylus left me and I'm still waiting for the goldring1042 to listen again to records.
I had a RPM from Pro-ject, and I know your tt
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Old 6th May 2004, 12:26 PM   #6
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Quote:
How did you measure the vibration, just the fingers?
Not really.
I used a non-magnetic piezo binded to the side of the motor so it is measuring lateral movements. The signal is amplified on the spot by a discrete in-amp and sent to the scope/fft.

Speed is measured with an optic sensor measuring platter lap times with a 10Mhz clock and sending results to the pc via rs232.

92.8deg is a small improvement when compared to no-cap and low-voltage, but still measurable and one that you can feel in your hand. I don't remember the exact number, but I can check it.
I do remember the improvement in vibration from going stock to using my device with all features running: -19 dB.

Quote:
I had a RPM from Pro-ject, and I know your tt
You should hear it while being driven by my device .

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Old 6th May 2004, 12:56 PM   #7
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Wow, nice equipment , maybe better than my sthetoscope
19dB seems a big improvement, I think that this could be heard.

I don't have your equipment so I have to keep the analog track, and build an inverting stage with another GC. I think in a week, everything will be at test stage.

Are you planning to publish your design?
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Old 7th May 2004, 12:28 PM   #8
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Yes, it will be public.

A few screenshots I've taken for my website...


Measuring vibration - as I described earlier, a piezo is binded to the side of the motor. As it moves, the piezo slightly bends and produces a voltage. That voltage is what you see on these pics.

This one is from using the stock trafo while playing a record:
Click the image to open in full size.

You can see an initial "sine" caused by the power being fed to the motor; the second lower one is the system moving to rest - this would continue to decay if nothing else happened. But the second phase kicks in and so the process repeats itself.

If you look close at the peaks you'll notice a slight amplitude modulation. It's arround 5Hz in frequency. The pulley/platter ratio is 1:10 so a platter moving at 33.3 rpm has a frequency of around 0.5 Hz. So the 5 Hz. would be, more or less, the frequency of the rotating pulley. I think that modulation is caused by the pulley not being perfetly centered; as it moves, it very slightly changes the tension and thus causes the modulation.

Next pic is same record, but now with my device driving the motor:
Click the image to open in full size.

I don't have a pic here, but if I add gain to take a closer look the wave is not perfect anymore but is mixed with spurious repeating noise together with a much more evident 5Hz. modulation. In other words, the tipical buzzing from the AC motors is so down that it is getting at the same level of other types of noises that have nothing to do with the fact of it being AC. I think you get my drift .

Next pic is a bit large, so here's a link instead:
http://www.itonami.com/temp/t3.jpg

The yellow line is platter speed with the stock trafo, the red is using my device. Note that It's not at 33.3 on purpose, so the two would be close. The blue dialog is the software I made to take speed measurements; those stats are for the red line.

The peak stat is actually peak-to-peak, so to be comparable to the standard wow figures shown on datasheets it should really read 0.0034.

By the way, the manufacturer of this tt specs it with a wow/flutter of 0.08% (if memory serves me), that would give a 0.026 peak dev.

The measured frequency coming out of the supply is within +/- 50 ppm. so these deviations are still wider than those of the frequency itself. Therefor, I think they are not caused by the supply but by the rest of system. A better tt should, in principle, give even better numbers. The limit of the speed stability of the tt becames the tt system itself and is not swamped by the crappy mains feed.


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Old 7th May 2004, 01:07 PM   #9
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Thanks for the pictures, they show an interesting work indeed. The difference between the piezo output is impressive.
I'm happy to hear that you design will be public, thanks in advance for sharing
I'd like to hear your comments about the differences between the standard AC and your supply.

BTW, I didn't know you had a website
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Old 7th May 2004, 02:42 PM   #10
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I meant the screenshots are for my *future* website.

Quote:
I'd like to hear your comments about the differences between the standard AC and your supply.
Besides what I already said, I can add this: that ten-minute yellow line on the graph really doesn't say it all. While debugging this project I've done numerous test runs over several diferent days and I've found that mains does drift a bit with some days being nastier than others. In fact, I'm thinking of logging mains freq over a period of, say, one week just to put some numbers on this. Add 4% thd to this and some voltage fluctuations, and well...

The website I'm making really details all of this with several diferent psu configurations and comparisons among them. I'm planning of putting it online even before the psu project is completly finished.

I noticed your comment about trying a large trafo. I've also made some tests with one of those and I'd like to hear your comments once you've tried it so I can compare with my own impressions.

Best Regards,

Guilherme.
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