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Old 27th March 2013, 12:47 PM   #91
msdin is online now msdin  United Kingdom
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Berger Lahr stepper motors these may be better than the Rapid Electronics motor.

http://download.schneider-electric.c...1150300579.pdf
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Old 27th March 2013, 02:59 PM   #92
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I choose the Rapid one as it looked so much like the expensive Airpax in terms of size and power ( Philips etc , Older Linn and Rega ) . The most interesting point is the generic nature of all and the fundamental limitations ( distrotion ) . Did anyone try a motor with smaller stepping angle at 50/60 Hz ?
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Old 28th March 2013, 03:36 PM   #93
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This type of drive has often been used for synchronous motors . It is under biased class B . Linn Valhalla and Thoren's TD125 come to mind . I have often wondered about this so decided to give it a try . I doubt it took 20 minutes to do ( no PCB ) .

The distortion is not too bad at all . It has extended harmonics right up to 1 kHz due to the notching . Some of what is seen would be less with a low distortion oscillator . The circuit is exactly as shown . It should run quite well from a NE5532 as oscillator .

The 10 uf somewhat surprisingly is hopeless . It suggests a different value would be better .

As an addition to a Quadrature oscillator it might be better than many would think ( 4 transistors ) . The un-heatsinked transistors were not even warm .
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Old 28th March 2013, 06:35 PM   #94
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The drive board for the Phonosophie No.3 uses a Quadrature oscillator with a zener agc which then feeds 2 H-bridge amplifiers to supply the 2 motor windings. The transistors in the H-bridge are TO92 packages.
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Old 29th March 2013, 06:05 AM   #95
nezbleu is online now nezbleu  Canada
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For this sort of circuit (and synchronous motors) is there a way t determine the required value of the series capacitor to get 90deg phase shift at frequency F? I know the DC resistance of the coil is a factor, but I assume the effective impedance of the motor winding at F, based on its DCR and its inductance, would both effect the required capacitance to achieve 90deg phase shift.

I ask about this because I am installing an older Thorens 16-pole motor intoa anothe Thorens turntable, which has a different value cap than I expected. Is there a formua whereby I can guesstimate the right value of C, without access to a 'scope?

Thanks. (and sorry for the hijack)
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Old 29th March 2013, 10:52 AM   #96
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I tried to work out the phase shift . The reality says trial and error is best . My reverse engineering solution suggests when voltages are equal it is likely to be approximately correct . As you can see when phase 2 ( orange ) is so distorted as in my last example , it is anyone's guess what " roughly equal " is . Vibration and torque ( finger test ) says plenty , better when the motor is off of the turntable ( be careful if high voltage ) .

Looking back , the TDA 2040 and transformer solution roughly agreed . The class B/C buffer via 10 uF produced a very distorted wave and noticeable reduction in torque . If there ever was an argument for negative feedback this might be it . The brown phase is OK , a bridge circuit seems practical ( 90 degrees bridge , a T bridge almost ) . The idea here is to build something cheap which is much better than average . If possible built on strip-board . No voltage dangers and no real need for an oscilloscope . The class B/C is about 9 dB better than a cheap transformer solution so not dismissed at this stage . Class B/C is noted for toughness / stability . The main focus is that the cheap transformer has considerable squaring of the sine wave . Even if a high grade transformer the mains is often at - 25 dB distortion . That means one starts at best at - 15 to - 25 dB . I did prove the motor to be a major source of distortion . None the less it seems a shame to make it worse .

I had difficulty making a buffered quadrature oscillator work . The idea of a sort of H bridge might work . One problem I had was that the BD135/6 buffer is working in current drive . The input wave looks nasty to the buffer . That wouldn't be the best thing to pass forward to the cosine stage ( the output although notched would be better ) . Using an LM324 might be the answer as that allows very simple buffering ( buffered buffer ) . The LM 324 was said in a forum to be hopeless even at 60 Hz , I was pleasantly surprised how OK it was ( the maths says it should be ) . The other solution is to add simple biasing of 2 x 10 K and 2 x 1N4148 ( 1N40001- 7 ) per buffer . That might allow reasonable performance without instability . Feeding that forward would be OK . The 2 x 10 K can be played with to have better distortion or cool running . NE 5532 for it's ability to provide current and cheapness would be prime . It has a bad reputation for being fussy so LM324 might be better . 5532 might also raise a few more volts ? If I can get the dam thing to fly I will post . Dream would be 5532 in quadrature , 2 x B/C buffer , loop feedback with 47R feed-forward resistor/capacitor ( usually 47 R ) .

Last edited by nigel pearson; 29th March 2013 at 10:55 AM.
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Old 29th March 2013, 06:44 PM   #97
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My beginners luck seems to have dried up with a simple oscillator . This is a very nice ready made design ( PCB files provided ) . That and 2 x TDA 2030 should be ideal . The perfect circle trace looks very promising .
http://www.piclist.com/images/www/ho...eagle38_03.gif
http://www.piclist.com/images/www/ho.../e_eagle38.htm
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Old 31st March 2013, 11:06 AM   #98
MiiB is online now MiiB  Denmark
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Would it make any sense to revert to current drive..??
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Old 31st March 2013, 02:00 PM   #99
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The Thorens TD125 shows how to do the job . I object to the class B/C output stage as I feel a power op amp will do a better job ( TDA 2030 ) .

I had very good initial results with the National Semiconductors circuit . I have adapted here to show TDA 2030 . As yet I have not got it to work properly as drawn . It should be close to working I feel . In H bridge TDA 2030 usually has a Zobel circuit between outputs ( perhaps 4R7 + 0.1 uF ) . In this T bridge I think it should still work ? The output would directly drive the motor with no additional components bar a power supply . Brown to sine and orange to cosine , yellow and black to 0V .
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Old 6th April 2013, 03:21 AM   #100
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Default DIY platter and Bearing

Hi, have been talking to TemaadAudio about their new range of DIY platters and bearing. Bearing with 10mm spindle is around $85 and apparently is the same bearing they use in their $3500 ref T/T.
A 35mm Platter is $188 and they will make any platter up to 100mm thick to special order. They also have an interesting conceipt of using 2 platter with isolation spikes placed between them. Not sure weather to go for a straight 70mm platter or 2 x 35mm platters with isolation spikes.
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