I may be one of the very few people on this forum powering my turntable with a Geddon Clone ( blatant copy of a Naim Armageddon PSU ) but highly recommend them as a simple but efficient way of getting clean filtered AC power from the mains without resorting to complex electronics.

However two of the undesirable side effects of using such a huge toroidal transformer ( 500va in my case ) is the tendency to blow fuses at switch on, and usually a faint hum.

The former is caused by in-rush current flowing in to saturate the mass of copper wire, and the second is usually attributed to small amounts of DC on the mains.

The main culprit for producing this DC 'pollution' is 1/2 wave rectifiers used in things like hair dryers and light dimmers.

The recommended way to solve the in rush current is a soft start circuit and the DC with a DC blocker filter. Both of these are relatively basic circuits but I'm always looking for ways to keep it simple.

Often a resister is used in some circuits to control in-rush which is then bypassed with a relay after a second or two.

I figured as I was using a 3.3k resister on the secondary windings to reduce the 100v to 75v what about placing that on the primary windings instead.

Sure enough it works brilliantly as I can now use a 2 amp fuse instead of the 5amp I had been forced to use to prevent blowing. As a bonus the transformer is now completely silent. I cant hear any hum even with my ear pressed against the housing.

Total cost $0.

Cheers

Graham

Another good 'Graham' solution, Graham.

The Geddon cct is basically just a big TX?
Feeding the usual split phase cap motor cct?

I'm not an EEngineer, but I guess that the main reason why a R is not often put on the mains primary side is that the R would be subject to 50Hz swinging peak-to-peak voltage of 230 to 240x1.4x2 = 645 to 672V. Also a higher value R is needed to achieve the same % voltage drop, ie....

If a 3ks R is needed to drop the secondary voltage 25% from 100v to 75v, the a much larger R is probably needed to drop the prim voltage by a similar 25%, ie say from 240v to 180vac? In other words, the 3k3 R on the primary would not drop the sec voltage down to the previous 75vac.

All speculation on my part.

DarkLantern blog - darklanternforowen.wordpress.com

Nelson Pass uses thermistors to stop inrush current

www.diyaudio.com/forums/pass-labs/27520-cl-60-inrush-current-limiter.html

www.diyaudio.com/forums/pass-labs/279736-dual-mono-setup-f6.html

Yes Owen, the Geddon is just a big bad toroidal transformer dropping the mains voltage to whatever works best for the synchronous motor and splitting the phase 90 degrees to one winding with a capacitor. Couldn't be simpler but it works just fine.
And yes you are correct, I had to increase the resistance by adding a further 1.5k ohms to achieve the same 75 volts output. I forgot to mention this in my write up. Good spotting Sherlock.
I can understand the resistor limiting current flow to prevent the inrush, but haven't quite figured out how it managed to completely eliminate any hum.

Resilient mounting and a heavily damped/sealed box, may be best solution for the TX hum (EM vibration, airborne + transferred to the box).

Yup, a thermistor is often used in amps, avoids increased PSU impedance after warm up.

DarkLantern blog - darklanternforowen.wordpress.com

Graham - I have a young engineer friend in USA who powers his (DIY) TT using this sine wave generator:



As you can see, AC freq is pretty much continuously variable, for controlling TT speed.
A diyaudio.com project, Kit parts were avail for around US$78 (2026/17) + a little amp to drive the motor (US%10 ebay).
Seems like it could be used on any AC motor TT.
www.diyaudio.com/forums/analogue-source/298018-diy-4-phase-sinewave-generator-turntable-motor-drive.html

FYI/enjoyment.

DarkLantern blog - darklanternforowen.wordpress.com

Apr 25, 2018 16:15:30 GMT 12 Graham said:

However two of the undesirable side effects of using such a huge toroidal transformer ( 500va in my case ) is the tendency to blow fuses at switch on, and usually a faint hum.

The former is caused by in-rush current flowing in to saturate the mass of copper wire, and the second is usually attributed to small amounts of DC on the mains.

The main culprit for producing this DC 'pollution' is 1/2 wave rectifiers used in things like hair dryers and light dimmers.

(my bold emphasis)

First, I apologise that I've only been a very sporadic visitor for quite some time.
Second, that this may not be the appropriate place to put this, or desired - I thought I saw recently a post about sudden unexplained transformer hum, which jogged my memory, and this was not that post - but it seemed the only near relevant one after jumping through the absurd captcha prove you're a human loops to submit a search query on as I recall, "transformer hum".

Anyway, to expand on blocking small amounts of DC on the mains, this, from the very last pages - a sort of appendix, final notes - of
High-end valve amplifiers 2 : new models and applications / Menno van der Veen
might be interesting to think about.
(scanned - apologies for the offset!))

Thoughts:

How much AC current are the capacitors to pass in this circuit?

In filter operations, max ripple current is specified, for any capacitor (for which specifications are available).

Is that related, here?

Anyway, food for thought, perhaps?

And if anyone could guess what the post might have been that jogged my memory on this, perhaps they could re-post this (&or image links) there?

Cheers, and Thanks!

Ross Matheson

The capacitors should be rated for the current rating the circuit uses, which is related to the fuse rating. (As should the diodes.) The ripple current rating of a cap is the maximum current that can safely be passed through it without heating it up too much, and should be rated at more than the current required from the circuit in series with it. Most caps can handle up to double the ripple current rating for very short periods such as a switch on surge. If they are called upon to regularly handle peak current it would be best to have the ripple current rating more than the required circuit current. If necessary, if the ripple current of a cap is lower than what the circuit needs, more caps could be put in parallel to achieve the rating. Most electrolytic caps have the ripple current shown on their data sheet and is different for each voltage and capacity of cap.

AMR-iFi R&D