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Homebrew Motor Control Linn LP12 - Circuit Description wanted for Sine Wave Osc.
Homebrew Motor Control Linn LP12 - Circuit Description wanted for Sine Wave Osc.
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Old 26th November 2019, 02:33 AM   #1
tiefbassuebertr is offline tiefbassuebertr  Germany
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Default Homebrew Motor Control Linn LP12 - Circuit Description wanted for Sine Wave Osc.

From a friend I got for repair a diy motor control for Linn's LP-12.
Compared to the X-tal versions with divider from Linn model "Valhalla" and "Lingo" is here a sine wave generator in use - similar to Linn's AXIS motor control - go to
http://www.turntablepsu.com/images/Axis%20Schematic.pdf
In the attachment you will find the schematics, which I have drawn out.
Who can give a description, how the circuit of the sine wave generator works or can give an advice, whether this circuit was based on an existing commercial product.
Thank you very much therefore.

P.S.: The reason for not correct working condition you will find in the fifth image.
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Old 26th November 2019, 03:36 PM   #2
Pyramid is offline Pyramid  United States
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It's hard to tell from your hand drawn schematic, but the original Axis controller was essentially a state variable filter with 2 back to back integrators and feedback to produce oscillations and a 0° and 90° output. In order to be stable and limit the output to a sinewave, the feedback network is either fixed (cheap and cheerful way of doing it) or uses an AGC circuit (better control of Q and output level). The RC time constant formed by the feedback cap in the integrator and the input resistor determine the frequency.

A good description of the circuit can be found Here; page 4 of the PDF breaks down the description of a state variable filter used as an oscillator.
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Old 26th November 2019, 05:25 PM   #3
tiefbassuebertr is offline tiefbassuebertr  Germany
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Thank you for your information.
In the attachment you will find a simplified schematic (oscillator only without output gain stages so as without variable resistor networks for two speeds and without power supply.

After cleaning and and re-soldering of various solder joints on solder site the motor control works now fine without any temporarily occurred unwanted effects (tested only with a single motor as to see under
Plattenspieler Motor (Rega, Linn), Hifi-Klassiker - HIFI-FORUM - first image).

the second and third image from my attachment shows the PCB after my service.

I will report concerning the results by listening test in the next weeks.

P.S.: One of both LED's (fourth image) is drawn in the wrong direction.
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File Type: jpg DSCF5328.jpg (990.9 KB, 36 views)

Last edited by tiefbassuebertr; 26th November 2019 at 05:31 PM.
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Old 2nd December 2019, 10:04 PM   #4
tiefbassuebertr is offline tiefbassuebertr  Germany
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Last weekend I have heard this device against the last version from Linn's Lingo. There were at whole 3 music lovers in this listening test. Record Player was Linn's LP12, Tornarm Morch DP-6 and cartridge Audio Technica's ART-9.
The result was unanimous - sonic quality was better in all respects with the homebrew (diy) version.
The most striking feature was the improved spatial representation of voices and instruments.

This evening I receive a PM. Here it was written that the circuit from my created schematic on this device based on the approach of Dr. Fuß - go to
Linn Lingo vs. Dr. Fuß or Square-Wave vs. Sine Wave Oscillator for Motor Control

Last edited by tiefbassuebertr; 2nd December 2019 at 10:07 PM.
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Old 6th December 2019, 06:17 PM   #5
Jon Lord is offline Jon Lord  Germany
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Many years ago I built a squarewave to sin/cos-converter based on switched capacitor filter IC LTC1065. With a little bit of HC logic it is possible to derive pretty precise Sinewaves from a precise clock with almost no other external circuitry.

The application was pretty similar: A synchronous motor driven by two linear power amplifiers for extremely constant speed and low wow/flutter.

After changing over to a simple little microcontroller (PIC12C508 or the like) to generate the two phase shifted square waves and a 100x as high reference clock for the Switched C-filter, I had a rather cheap two chip (2xPDIP8) solution for a crystal stable SIN/COS generator.
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Last edited by Jon Lord; 6th December 2019 at 06:21 PM.
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Old 6th December 2019, 06:45 PM   #6
Jon Lord is offline Jon Lord  Germany
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State variable oscillators are pretty tricky for stabilizing their amplitude without clipping the sine wave etc.

Here you can find some Theory:

http://ww1.microchip.com/downloads/e...tes/00866a.pdf

Some elaborate designs to cope with stabilization while maintaining low distortion can be found in Linear technology's AN-67 Sine oscillator or in Hameg's HM8037 sine generator service manual.

https://www.google.com/url?sa=t&rct=...Y78pWGmFxlTJLx

With a little tuning of this circuit -120dB THD can be squeezed out of a HM8037.
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It accompanies myself on the grand piano, that's not half bad !

Last edited by Jon Lord; 6th December 2019 at 06:56 PM.
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Old 6th December 2019, 06:52 PM   #7
Jon Lord is offline Jon Lord  Germany
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The classic way to stabilize such oscillators is a small light bulb, as a kind of PTC for reducing gain with amplitude.

I think the BF245 has this function in your circuit (votage controlled resistor for AGC).
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Old 7th December 2019, 01:31 AM   #8
tiefbassuebertr is offline tiefbassuebertr  Germany
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Thank you very much for all hints.
Here the remaining images so as (in the following posting) some images from the genuine motor control device:
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Old 7th December 2019, 01:35 AM   #9
tiefbassuebertr is offline tiefbassuebertr  Germany
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Default Dr. Fuß (Music Components) Genuine Device

the difference should be clearly visible. Other transformer, other PCB, other monolithic power amp IC (LM4700 instead TDA7293) and probably other schematic.
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Last edited by tiefbassuebertr; 7th December 2019 at 01:37 AM.
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