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Resistance for Gedon

Chris Marshall

Chris Marshall

pfm Member
Hi Guys

I'm finally getting round to building a Gedon for my lp12.

I'm fairly clear on what I need for most of it, but I'd appreciate some advice. I want to use a variable pot to alter the voltage.

My transformer has dual 120v primaries and dual 50v secondaries so I should be getting 95-100v out.

I was planning on just using a pot instead of a resistor, along the lines of a 5k 2w wirewound.

http://www.farnell.com/datasheets/44915.pdf

Is that suitable? Or does anyone know where I can find a better one?

Also does anyone know a dpst switch that will fit into the hole on the Linn?

many thanks

Chris
 
Those 2W pots look just fine.

It may be an idea to use a resistor in series - say 2Kohm, 5W wirewound; that way the (small) power dissipation is split between the parts and if anything goes wrong with the pot wiper, the motor is protected.
 
Chris, CPC stock some switches that fit into the LP12 hole. I keep meaning to get round to doing that bit myself.

Try prod code SW01099 for a near drop in fit, and SW01561/SW01562/SW01563 for something that needs a little more doctoring (needs reducing by 1mm in width - not the bezel though), but looks nicer (and has versions with a red or green dot LED available). Not sure if the LED will illuminate though, as it says you need 230VAC to light them, and if you're planning what I think you're planning, you'll already be down to 85VAC or thereabouts when the power gets to the switch.

If you can wait, I'll be ordering some stuff from CPC soon enough, so I can add a switch for you in there and stick it in the post when it arrives.
 
Hi Chris

I chose this 3W 5K from CPC but it's yet to be deployed. I built the power supply into its box last week, without the pot, and need to put the board together this weekend. I'll play with variable voltage once the main theory has been turned into practice.

Good luck with yours.

Mick
 
I incorporated a 5K 2watt Pot into my 'Geddon it works just fine. I think mine was pretty much the same as the one you propose, although i think i got mine from Maplin . I actually wired mine up as a Variable resistor (rheostat ) rather than a Potentiometer.

I must admit that although i have a fairly wide voltage range with my Geddon i can't recall hearing any night and day differences in the sound, at the two extremes. i ended up pretty much running my LP12 at the lowest voltage where the platter would start/spin 'plus a gnats cock'.
I'm going to try and find a bit of time to have another listen to the geddon and if it doesn't do anything significant over my Linn Basik ( currently in use) or Valhalla PSU's then it will be getting flogged or 'In the loft'.

Sids Geddon adventure
 
im building an armageddon at the moment the same as richard has on his web site but on testing the output voltage cannot get the voltage to drop on the output of the resistors still have the full output from the transformer do i need to connect the motor to measure the voltage drop?
 
Originally posted by brian t
on testing the output voltage cannot get the voltage to drop on the output of the resistors still have the full output from the transformer do i need to connect the motor to measure the voltage drop?
Click to expand...
Hi Brian

The short answer is yes. A longer answer would be that you'll only see a voltage drop if there is a load connected. A resistor across the Geddon output and measuring the voltage across the resistor should do it. To give you an idea of size, I measured my motor windings at 4.83k red/grey and 4.7k blue/grey.

Mick
 
Thanks for the info guys.

Richard

Thanks for the offer of the order, I'm going to order the rest of the stuff anyway so I'll be ok.

I want to do the variable one from scratch be cause it's taken me so long to get round to this that I'd better do it right first time!

I'll go for the resistor and the pot in series as suggested by Martin.

I am planning on using Speakon connectors for the umbilical to the deck, can anyone confirm that the contacts are touchproof?

The plan is to put it in a box made from a frame of ash like the lp12, with the profile to match, aluminium top, bottom and front plates and stained to match the lp12. It sounds good, I'm not sure my skills are as good as some other forum members, let's hope it looks good.

Thanks again for the info.

cheers

Chris
 
Originally posted by Chris Marshall
I am planning on using Speakon connectors for the umbilical to the deck, can anyone confirm that the contacts are touchproof?
Click to expand...
Hi Chris

Which ones?

FYI, I chose a Neutrik 3 pole 20A 250V AC locking mains plug and socket, Rapid Electronics parts 20-0845 and 20-0850 as they do the job well, are finger proof and look cool. I think CPC do them but I don't know the part numbers.

Mick
 
One other thing about having a resistor as well as a pot - although the "recipe" calls for a 3K resistor, both I and others (can't recall who now - one of the Tims I think) have found that the resistor value can be much higher - I got to about 6K plus the 3K pot before I could make the platter stall. So you may want to buy a couple of resistor values while you're at it to make sure you have the right bits in the armoury.

If you're buying from Farnell rather than CPC, you need to look out for a switch with a cutout of around 27x22mm.
 
I did indeed find that a higher resistance could be used to start the motor and higher still to keep it running. I've now got two switches on the Geddon. One for 'start' - the second for 'run'. The first one switches power from the secondaries to two resistors in parallel, the second drops out one of the resistors.

I can't remember the actual values I used - but if you want to know I'll have a look for you.

To do this using the LP12 switch cutout you'd need a fancy two position switch. I just put two pushbuttons on the geddon front panel
 
Thought it was you. 6k8 rings a bell, but it's going to depend on the ACTUAL voltage coming from the individual transfromer.

I did the double switch thing too - as you say Tim, 2 resistors in parallel to start up, and the toggle switch drops one of them out of the circuit when fired up.

I may try and do something so the main "switch on" is from the LP12 top plate, and if I want to twiddle the toggle, then I can do that from the Geddon box. It either means leaving the transformer powered up all the time (and just switching the secondaries' feed off and on to fire up the LP12), or running a lot of cable in and out of the LP12. I'm sure there's a more elegant way somewhere.

Did you ever get that 45RPM valhalla going? Now THERE'S a Holy Grail.
 
I ended up running my primaries all of the time and just switching out the secondaries. I seem to recall that Swiching the whole unit on from cold used to blow my fuse or trip out the breaker in my CU. ( many months ago now so can't remember).

Regarding the switch, RS components sell a switch that fits the LP12 cut out perefectly. One was supplied with my Linn Basik PSU, far nicer than that garish red neon thing i see on some decks. Only thing is though that it 'rocks' sideways rather than front - back. You soon adapt to this complication though ;)
 
I haven't gone back to meddling with the valhalla yet - my electronics knowledge is not sufficiently strong to risk unplanned combustion. For the record I fitted a different freq crystal and different value caps on the output (can't remember values). What happened was that I got voltage onto one leg but nothing on the other.

The geddon was better anyway and I'm not that bothered about 45rpm - it was just worth a try at the time - which I don't have a lot of at the moment
 
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