A twisted tale about a logarithmic relay attenuator

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#261
Okay, works fine with the JT. Each side of the JT trafo secondaries can do 500mA. I simply seperated the two secondaries, one for the JT, one for the remote board.

At 5VDC, then motor draws 30-32mA in normal usage, 40mA when clutching at the ends of the rotation.

Side Note (aka: Don't try this at home):
So, if you apply trafo voltage to the +5VDC and GND pads on the JT, the relays make really cool strobing sounds. Didn't seem to kill it though... this time. 😉
 
#265
Hi Brian,

Thanks for the ultra prompt feedback. I am serious about assembling my Twisted Pear Pot. If you have an extra board available, I would be grateful if I could purchase that from you. Thanks,

Carl

P.S. Do you see a part number on the motorized pot that you bought at Goldmine? I would like to try to research it.
 
#267
Looks like the potentiometer itself is made by Shokai and is 20k ohms with resistance taper pattern B=linear (see plot below). I think all the other numbers on the Mabuchi motor pertain to the motor itself.

taper_ABDW.jpg


I worry that the Shokai unit is too large for the space I have allocated in my enclosure.

Check out this motorized Alpha pot at Mouser.

Nice board.
 
#268
BrianDonegan said:
Okay, works fine with the JT. Each side of the JT trafo secondaries can do 500mA. I simply seperated the two secondaries, one for the JT, one for the remote board.

At 5VDC, then motor draws 30-32mA in normal usage, 40mA when clutching at the ends of the rotation.
Click to expand...

Brian,

I like the idea of supplying the JT and remote controller separately with individual regulation. Although quite unnecessary it seems more elegant.

Are you using a dual secondary 6V transformer? I noticed that my JT became unstable with just a 6V input when driving two relay boards. With a 6V dual secondary transformer and balanced or dual mono JT, one's only option is to feed the JT 12V to keep it operating reliably.

On the downside, the JT's 7805 really cranks out the heat with a 12V input. I think I am going to have to change to a 7 or 8 V dual secondary transformer to put my mind at ease about the heat. Then I can drive the JT with one of the secondaries and the remote controller with the other.

Of course I'll need to change the 7812 in your remote controller's schematic to another 7805--right? Plus, I assume the motor is more "user friendly" at 5V as opposed to 12V.

P.S. Nice controlled board. Hint...
 
#269
I actually first did this for the Kookaburra, and did not have any power supply components on it. Then for the prototype I thought I should add two vregs so I could play with the voltage to the motor, then added the rectifiers to make life easier. Seems to be the best solution, as it leaves the JT and Kook power supplies alone to do their thing.

I ended up with dual 7805's, so in the newer board design there is only one. I toyed with a 317 for customizable motor voltage, but dropped it, cause it was silly.

I tested the JT with the trafo we sell for it (Amveco 7V(500ma) + 7V(500ma) mini torroid). It had no issues with stability.

Again, this was just a quick and dirty approach, but quick can be nice.

I think the Alpha pot is likely an audio taper, which would be wierd, but probably a better quality unit. Looks to be about the same size however.


P.S. Nice controlled board. Hint...
Click to expand...
I don't know what you could possibly mean 😉
 
#270
Would anyone care to hazard a guess as to how far you could simply extend the leads to the control pot, over CAT-5, say?

I'm thinking of having a JT in front of the active XO of each of a pair of powered monitors I'm building, the idea being to drive them directly with the balanced outputs of my CDR-631. Still have to stick the big knob somewhere , and I'm guessing it's easier to extend the pot(s) than the controls from the PIC.

RandyB
 
#271
Hi Z33
10m of cat5 (0.5mm) has about 1r8 for the total run there and back (20m).

Even 50m, to the other end of the barn, would only be 9r.

The control pot is, I think, 25k so only the very lowest resistance would be affected and I suspect that Russ will come back and confirm suitability or otherwise.
 
#272
Hi Andrew, as long as the voltage drop accross the wire to the pot is not more than 20mV you should be fine. The voltage through the wire will be 5V. It should be pretty easy to test with a length of wire and a DMM. I think it would work even with a substantial length of cat5. To test just attatch the pot to the wire, and the wire to the controller, and check that all of the outputs go high when the volume is set to max. If that happens, you will be all set.

Cheers!
Russ
 
#273
Thanks guys! This doesn't sound like much of a practical limitation; I don't imagine the relay control lines could be run quite so far, even over CAT-5. I haven't completely given up on the idea of a wireless remote, but it's good to know there's a viable fallback. I guess there's some value to being able to just point at the volume control on my Thoebe when my wife 'asks' me to turn it down, but ...

I was considering an X-BOSOZ for the new system, with a pair of JT on the balanced outs. Then I got to thinking it made better sense to drive the cables at full output, and put the attenuators in the monitors. About then I realized the CDR631 was already capable of 2V RMS, or ~5.5 V-PP, which I think ought to be enough to make it across the room on low impedance, balanced cables, so I've deferred the X-BOSOZ until I really need a new preamp. Or until I just can't stand not having an X-BOSOZ. :bawling:

RandyB
 
#277
Hello brian ,

I have some trouble with joshua 3 balanced type.
The problem is probably the connection with 5K pot.
The resistor value start at begin point at 2570 first click 2621 second 2657 third 2693 vive or six 2831 seven 2936 ….. at several clicks it goes to 6.38K and then to 2.57K – 2.62K at several clicks it goes to 2.95K with other words its goes up and down.
It suit be started at exampled 2500 and goes down.
Power is stable I’ using 12 - 30va
The problem is with both boards tested alone.

THX
Peter
 
#278
I think your board is OK. Remember this is a ladder atten. not just a series resistor. Try putting a small voltage at the signal input and measuring the voltage at the output. Make sure you got in and out right, it makes a difference. Then short the input of the relay board and measure resistance at the output...it should be a constant ~750r the full travel of the system.

I bet you'll see it's just fine🙂

Edit: The connection to the pot is (i think) quite simple. If you get it backwards the whole thing will work backwards (I think)🙂
 
#279
Stevox said:
Hello brian ,

I have some trouble with joshua 3 balanced type.
The problem is probably the connection with 5K pot.
The resistor value start at begin point at 2570 first click 2621 second 2657 third 2693 vive or six 2831 seven 2936 ….. at several clicks it goes to 6.38K and then to 2.57K – 2.62K at several clicks it goes to 2.95K with other words its goes up and down.
It suit be started at exampled 2500 and goes down.
Power is stable I’ using 12 - 30va
The problem is with both boards tested alone.

THX
Peter
Click to expand...


You are measuring input impedance which in this design varies with the attenuation. It will not be constant like a pot or a series attenuator. The output inpedance is constant, and the voltage attenuation is logarithmic.

Cheers!
Russ
 
#280
Hi,

I was going out for a series resistor, so okay that was wrong.
I measure before resistance at the output and yes it was constant ~750R.
Yet I connected one channel with mine cd player and on the other end mine amp.
And yes its works only there is one step when I degrease the volume its goes harder sometimes a plop and then it goes lower.
The other thing is that it doesn’t start for from zero.
And yes there was a little hum. But okay it was only for test.
So now I want it to build behind the Bozos preamp from kari so I can play balanced with it.
Thanks Guys I let you know how it goes.

THX
Peter.
 
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