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-   -   Armageddon clone for LP12 - why such a large transformer? (http://www.diyaudio.com/forums/analogue-source/225733-armageddon-clone-lp12-why-such-large-transformer.html)

sharpi31 15th December 2012 10:18 AM

Armageddon clone for LP12 - why such a large transformer?
 
Very probably missing something basic, so please put me right.

The Armageddon is basically a big transformer with 110v secondary (or dual 55v) with large series R (3.3k) between one motor connection and the transformer secondary. There is also a 0.2uf phase shift cap connected to the motor side of the 3.3k resistor and third motor connection.

It seems to be taken for granted that the transformer needs to be 400VA or so. When I do the sums I can't see how more than 33mA could ever be pulled, which would suggest that the transformer could be 3.6VA and do th job.

It's often stated that the low impedance of the 400VA transformer is the reason it is required, but I can't see how this matters given the large series R.

Ultimately, I have a 65VA transformer with suitable secondaries and want to know if this could be used without significantly compromising the result.

Many thanks in advance.

Armageddon Clone

freax 15th December 2012 10:24 AM

I can think of only one good reason, voltage regulation.

Aside from that, prevention of theft.

I cannot imagine a huge transformer like that being too good on the motor, especially if its hooked up as the schematic is designed, I bet that 3.3k ohm 3 watt resistor limits that.

Quote:

the red letters refer to the photos of my circuit below. This is basically the same as the other circuit, but with a switch to drop the resistance for initial start up of the turntable, and a variable resistor to allow me to increase the resistance to drop the voltage down as much as possible to minimise motor vibration. X and Y are the where the red and blue wires attach - Z is the connection for the two grey wires.
Wow

Be careful or that could actually create hum/vibration by reducing the voltage going to the motor.

sq225917 15th December 2012 02:35 PM

Regulation, regulation, regulation.

6L6 15th December 2012 02:43 PM

Here is an idea - build (breadboard, do a case for it later) the circuit with your transformer. If the basic sonic signature appeals to you then pursue it with the bigger trafo. TT PSU do make a diference and you might find that you don't like it vs. what you have.

I have never regretted breadboarding a project. Even if it works perfectly from the start. I have regretted completing projects whole-hog in the beginning that needed a bunch of modifications and tweaks later to get it working properly.


Quote:

Originally Posted by freax
Be careful or that could actually create hum/vibration by reducing the voltage going to the motor.

One of the design points of the Armageddon is that the motor worked on a reduced voltage, in theory to reduce the strength of the synchronous pulses. That said, your point is very valid.

sq225917 15th December 2012 03:23 PM

The Geddon works on quite a high voltage compared to most other LP12 psu's, the lowered voltage on the 2nd rail is to reduce the effects of cogging. But all is does in reality is alter the frequency of the inherent speed instability.The best way to reduce the noise from cogging is to increase the load on the motor so that the magnetic field in the motor collapses, damping the cogging effects to next to nothing.

Try it for youself, hold the motor in your hand and let it free spin, now lay a finger tip on the top of the pulley to add some drag and feel how much smoother it runs....

Now figure out how to do that in your deck.

sharpi31 15th December 2012 04:15 PM

Thanks all.

As 6L6 recommends I'll use the tx I've got and see whether I like the results.

I appreciate that a bigger tx will have better voltage regulation, but must admit that given the very small loads involved I don't understand why this would be so critical. I probably just don't fully understand :-)

I should have mentioned that I'll be using an EI type rather than toroidal. I thought EI types offered better isolation from HF noise, although I could also see bandwidth limitation skewing the sign wave, so not sure how much of a compromise this will be.

Time for an experiment!

6L6 16th December 2012 05:05 AM

Any (fantastically minute) waveform skewing due to bandwith limits of an EI will be practically unmeasurable after the series resistor. And this is going to be operating at mains frequency, which is exactly what the transformer is made for.

I would actually prefer the EI in this case as it will not couple all the HF crap that lay on the mains these days.

:) :) :)

sq225917 16th December 2012 10:53 PM

http://farm8.staticflickr.com/7176/6...c242e5b0_b.jpg

There's no substitute for having built a few of the damn things.

argibbo 18th December 2012 05:31 PM

To my mind it's not critical, it's just a way of selling you an un-necessarily large transformer. A while back my Valhalla board packed up and I found the original mains frequency locked circuit, dead simple, on a forum. As a temporary measure I built the circuit and the motor ran beautifully. Several years on, my Sondek is still running on the original circuit. The Valhalla board is still awaiting repair, when I get round to it. There's no damn difference at all! If they sell you 400 worth of upgrade, when you've paid 400 then by the simple power of suggestion you'll hear 400's worth of difference. Peter Belt made a living out of this sort of 'psychoacoustics'

Meanwhile, there's a whole shed load of tricks, based on simple engineering logic, you can do to the Sondek to improve it without spending anything more than DIY money.

Best of luck. Andy

sreten 18th December 2012 06:52 PM

Hi,

Its that big just to make sure the box you buy seems heavy
enough to be worth the silly price your paying for what is inside.

rgds, sreten.

The transformer is simply the same size as one of Naims Flatcaps.


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