AC Regeneration PSU for Turntable (Universal) - diyAudio
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Old 1st July 2008, 02:07 PM   #1
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Default AC Regeneration PSU for Turntable (Universal)

Hi there

I have searched the forum for similar discussions of a valid design but all has been a bit hazy.

Basic unit specs:

Two vari frequencies (for 33.3 and 45 rpm)
Stable operation
High current capability without strain
Output voltage adjustable
One-box solution (no odd pieces of amps and iPods )

This is what I have come up with. I am no electro guru but this is my best guess for a PSU to fit the above requirments.

Wien-bridge oscillator with two fixed cap banks as per each speed for selection. Fed by a regulated PSU.
TDA7249 IC Op-Amp as driver stage for motor/transformer - at least 5 amps capability.

A super advanced PSU with processor control is not really nessesary (display etc). You want a stable supply to the motor (especially frequency).

Anybidy care to expand on this and comment/make suggestions on the schematic. Like I said - I am no expert but this is my best guess.

Dewald
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Old 2nd July 2008, 12:31 AM   #2
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If you are looking to run a single phase (induction) motor, don't use a class AB power op amp as the power amplifier. You will have a devil of a time compensating it due to the effect of the motor inductance on the feedback loop. T amps and their ilk are a far easier solution.

If you are looking to run a two phase (synchronous) motor splitting the two phases and establishing quadrature correctly is very important to optimal performance.

I once used Wien bridge and then some more complex analogue oscillators (modified Fraser multiple phase shift) but the drift is difficult to control. Yes square waves are difficult to filter but Direct Digital Synthesis gets around that very neatly.

Is the "one box solution" allowed to have multiple components inside?
PS (or battery) > frequency generator > output amplifier(s) > transformer(s)

There are good reasons for keeping the sections separate.
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Old 2nd July 2008, 06:57 AM   #3
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Mark

I did not realize that the amp in the schematic is class A/B - the data sheet doesnt mention it.

What is needed is a class B amp for best efficiency.

Also looking at doing this oscillator solution than the above unit: http://www.maxim-ic.com/appnotes.cfm/an_pk/3846

A dual phase unit is not needed as the phase shift for AC Synch motors is done by a cap (but you knew that).

I want a PSU with a single phase that will enable me to use it on a varety of AC motor turntables. Converting these tables to DC motor is not an option. The PSU must also have Low Voltage (12 - 24vac) and High Voltage (110v - 220v) output, selectable by a toggle switch. (Look at the puny "Speedboxes" from Pro-Ject)

Last night I tried my 12vdc-to-230vac inverter on my turntable and the results were quite impressive! This is what I want - no need for an over-fancy device.

A single box solution is also a must - clutter is not an option. Mains operated also a must.

I think the inclution of an analogue voltage and frequency meter on the front could be a good idea. Maybe a ammeter as well.

Dewald

PS - Any tips for a high power class B amp in Chip form?
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Old 2nd July 2008, 12:09 PM   #4
johnm is offline johnm  United Kingdom
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Would this universal PSU be compatible with the Garrard 301 or 401... on paper?

- John
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Old 2nd July 2008, 12:38 PM   #5
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I repeat: Do not use a class AB (or class B) power op amp for an induction motor, the inductance causes huge problems. Class D gives you better efficiency than Class B anyway.

I didn't say that a two phase supply was necessary, I said it was important to optimal performance. It is of course up to you what compromises in performance you will accept.
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Old 2nd July 2008, 01:08 PM   #6
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Quote:
Originally posted by Mark Kelly
I repeat: Do not use a class AB (or class B) power op amp for an induction motor, the inductance causes huge problems. Class D gives you better efficiency than Class B anyway.

I didn't say that a two phase supply was necessary, I said it was important to optimal performance. It is of course up to you what compromises in performance you will accept.
Okay... any suggestions for a high power class D amp? All I get is a 20w class D chip amp in mono... (RS Comp)

D
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Old 2nd July 2008, 01:11 PM   #7
bigwill is offline bigwill  United Kingdom
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"Fun" solution: How about a tube or two driving a mains transformer (with the tube being fed by the oscillator)
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Old 2nd July 2008, 01:21 PM   #8
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Quote:
Originally posted by bigwill
"Fun" solution: How about a tube or two driving a mains transformer (with the tube being fed by the oscillator)
No please - simplicity and efficiency is very important. It must be a sleek black box that does the job it is made for - to make a turntable run smooth and true.

You may certainly experiment with a tube PSU but I would rather go SS. I think Mark Kelly made a tube PSU for a Garrard 301...?

D
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Old 2nd July 2008, 01:23 PM   #9
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Quote:
Originally posted by johnm
Would this universal PSU be compatible with the Garrard 301 or 401... on paper?

- John
Yes. I want a PSU with variable frequency and voltage that would be able to power any turntable with an AC motor. Garrards, Regas', Linns', AR's etc...

D
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Old 2nd July 2008, 10:11 PM   #10
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Here is a block diagram of the idea I have in my head:

Click the image to open in full size.

R1 & R2 - PSU Regulators
T1 - Mains IN transfo
T2 - AC signal OUT transfo
A1 - Gain stage
A2 - Power Amplifier
WB VFO - Wien-Bridge Vari Frequency Oscillator
UI C - User Interface Controller

D
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