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Digital Pots for Tube Amps - solved!

1/1/2005 5:14 PM
Don Symes
Digital Pots for Tube Amps - solved!
After kicking this problem around for going on 6 years, I have FINALLY hit upon a solution that looks workable.  
The problem was that I wanted a reasonable simple, compact circuit I could use to replace any audio-taper pot (linear taper is a little easier). Randall Smith's patent on this concept is pretty limited and kinda large in terms of PCB area and requires careful selection of precision resistors.  
My circuit uses a Maxim 1801 dual log-taper digital pot, an opamp, a 47k resistor and a dual-element LDR, plus a resistor of the pot's target value, and runs from a 5V supply.  
A rheostat-connected pot needs one of these circuits, a potentiometer (voltage divider) requires one MAX1801 and two of everything else. The obvious move is to buid pairs and provide 4 pot terminals to allow one assembly to be used in either configuration.  
The Maxim part is controlled by a 2- or 3-wire serial interface and can be daisy chained.  
I've done some fiddling with placement and sizes - it looks like about 2 inches square.  
Now I'm futzing around with a rotary encoder and arced array of LEDs to replace the physical pots and knobs for the user interface side. It's nifty that the active circuit needn't be anywhere near the control circuit.  
A SuperSpice ( schematic and a .pdf version are available on request - just post a reply to this post with an email address in the header - it will stay hidden from harvesters, but I can still reach you.  
Happy New (Gregorian) Year!
And now, a word from our sponsors:

1/2/2005 11:34 AM

Hi Don,  
I'm interested in this schematic.  
1/2/2005 11:45 AM
Le Basseur

Me too,thanks!
1/2/2005 5:04 PM

Even though it's over my head, it is something I'm interested about dabbling. Count me in too! TIA!
1/2/2005 6:19 PM
Don Symes
I go to the Hamamatsu website to get dimensions and pricing on their dual-LDR optocoupler and they've GONE OUT OF THE BIZ. As of September, 2003, Hamamatsu no longer manufactures optocouplers.  
Can anyone source a photocell with TWO INDEPENDENT photoresistors with a dark resistance of at least 1M and a full-on resistance of less than 5k?  
The resistors can NOT have a common terminal.  
1/3/2005 12:54 AM

try perkins elmer? or silonex? they both make quite a few vactrols. silonex makes the boogie ones, if not mistaken.  
1/3/2005 2:00 AM
My solution
I also thought long and hard about that issue last year and published a little page on my server then forgot about it. It looks like we have come to similar conclusions. I would be intrested in seeing what you've come up with. The work I did can be seen here.

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