Using isolation transformers with TT motors? - diyAudio
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Old 12th February 2003, 04:23 PM   #1
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Location: Oregon, USA
Default Using isolation transformers with TT motors?


My first post to the analogue (been a while since I've spelt it that way ) forum here. I've been looking for reading material on turntable power supplies, and I found many threads on various forums discussing something known as the "DIY Armageddon". Obviously, this was for the LP12, but some people said that AR and Rega tables use the same kind of motor and should benefit from something like this (I have a P3).

Anyway, I saw many people recommend using an isolation transformer with the TT's existing motor and controller, saying that this improved the sound. An old peice of equipment called the "RCA Isotap" (or "Viz Isotap", I believe it's a rebadged version of the same component) was mentioned severa times. Well, I found one on eBay for $25, so it's currently on its way to me.

My question is, what exactly is the benefit gained from using an isolation transformer? The Isotap allows a drop in voltage level, and I can see how that might make a difference. It apparently reduces torque and therefore increases startup time (which doesn't bother me too much), but apparently also reduces vibrations in the motor.

Also, the controller circuit schematic shown on most pages is a dropping resistor folowed by a capacitor on one of the two "hot" lines going to the motor. Could someone please explain what this circuit does, and how it controls an AC synchronous motor (I'm fairly comfortable with basic electrical/electronics principles). I know it's something to do with putting a phase delay between those two lines, but that's all I know.

Final question - the Rega P9 or P25 has a "special" motor controller that has pots that are used for trimming the motor. The pots are adjusted at the factory for minimum motor vibrations. I'm assuming that this is similar to my question above, and that somehow adjusting the phase delay between those two lines adjusts the vibrations in the motor? If so, then what would a circuit look like for me to adjust phase with a variable resistor (only because those are easier to find than variable capacitors).

Here's a copy-paste from another page so that people know what I'm talking about:


PHP Code:
---/\/\/\-----+------------- motor 1
 |        |   +---| |---+--- 
motor 2
110V     79V  
|         |
 |        |   +---| |---+
motor common 
The resistor is orange, orange, ? i.e. 33x? Ohm. I measured the current of my Mantra motor (same as in LP12) that was 10mA. So I'd guess the dark third ring is red. => 3.3KOhm (31V / 3.3k = 10mA).

The caps are Siemens types MKT. (Siemens MKH) These rectangular green types, where the connecting legs are soldered onto the outside case. Dimensions are 7.5mm leg distance and about 4 mm thick.

So lets guess a little bit. My Mantra had a similar circuit (130V to 75V through serial resistor) and then a 220nF cap in series to one motor leg. There are several capacitors that would match from the dimensions side. There is a 220nF 100V type cap with just the right dimensions, But in the Armageddon has two in parallel. There is also a 100nF 250V type which is the same size so that could be the solution."
I had to use PHP tags to get this to format correctly. The 31V is the drop across the resistor (which shows up as ///), that leaves 79V going out with 110V coming in.

It's near the bottom of this page:

Thanks in advance,

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Old 12th February 2003, 07:04 PM   #2
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I do not see what an isolation transformer should do to the sound of your TT caused by the motor. It is nothing more than a 1:1 transformer that provides safety rated isolation and makes you free from earth by floating your supply.

The resistor-capacitor is used to phase-shift one winding of the motor with respect to the oher winding. This way you get a rotating magnetic field. This field is driving the rotor.
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Old 12th February 2003, 07:13 PM   #3
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Note that the RPM is determined by frequency, and torque by power. Reducing voltage, while keeping the same load on the motor, will draw more current, making a hotter motor. Obviously, a series resistor will limit ultimate current, thus leaving the motor less than synchronous, given the load is adequate.

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Old 12th February 2003, 09:03 PM   #4
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OK, so this is probably not going to do much good, and may do some harm. I'll keep an eye out for the motor heating up.

The resistor-capacitor is used to phase-shift one winding of the motor with respect to the oher winding.
Is it possible to change that to a circuit with a potentiometer that varies the phase between the two windings? In its current form, changing the resistor value will only change the current flow through the motor. I think I would need a resistor in the two sections going to the motor windings - either in series with the capacitor, on the other leg, or across the two legs.
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Old 13th February 2003, 01:14 AM   #5
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Originally posted by Sch3mat1c
Reducing voltage, while keeping the same load on the motor, will draw more current, making a hotter motor. Tim
On a synchronous motor, (big ones at least!) the ideal voltage for a given load is where the supply voltage and current are exactly in phase. This would also be the lowest current point. Vary the voltage with a variac and watch the phase of the current with the second channel of a scope.
Best-ever T/S parameter spreadsheet.
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Old 13th February 2003, 05:16 AM   #6
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OK, that's something to keep in mind for whenever I buy a scope and a variac. I guess it's high time I added these to my toolset.

So it's not the phase between the two winding supplies that is adjusted, but the supply voltage (which changes the phase relationship between the voltage and the current)? In that case, a variable dropping resistor (or experiments with multiple values) would have the same effect, right?
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Old 13th February 2003, 06:16 AM   #7
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Join Date: Oct 2002
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Default HI,Thanks for the great imfo.


Thankyou for all of the members with imfo regarding the Oracle delphi that i have.

The pics look really great, They give me a great idea on what to achieve.

I looked at the sme arms and they do get great reviews,I was wondering what the strengths are with the desighn of them.
I think i have seen some on ebay but they tend to get snapped up pretty quick.

Also has anybody got a solution for making the delphi stop bouncing around whenever anybody walks near the turntable.

I have the deck on a good quality hi fi stand and it still bouces on this.

Thanks very much
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