As some of you have probably read in another thread, i live in india, and finding good quality parts here in retail quantities is sometimes extremely difficult - e.g., you can NEVER find 1% resistors rated higher than 1/2W.
my big problem currently is the amp i'm building for my dad's 60th - it's based on this build: http://diyaudioprojects.com/Tubes/EL84-Push-Pull/
now, one thing that is seriously lacking locally is the availability of good quality potentiometers - the best i've seen till now are alpha taiwan, and believe me, i've looked. HARD. i need a dual 100k log pot, and i've been toying wth this here idea:
1. take two superbright white LEDs and closely matched LDRs. build DIY optocouplers in the usual stompbox fashion.
2. Put the LDRs in parallel with 100k 1% metal film resistors. the parallel combination gave me exactly 100k (dark).
3. using a 1/2 to 1W wirewound (if necessary) 10k-47k pot, limit the current to the LED. using limiting resistor, of course.
on full brightness, i got 5k in the parallel combination. i was not using a tube, but merely
would this work noiselessly? is there something i've gravely overlooked? will a single 1W 10k pot be sufficient to control 3 LEDs (i want one on the panel that brightens according to the set volume level - practically in tune with the others.
thanks for all the help - i'm working against time here, or i'd have tried it out myself. need to fashion the frontplate and send it offfor chrome plating tomorrow or friday, hich is the reason for my urgency...
Optical volume control for a hi-fi amp?
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analogguru
- Old Solderhand
Information
The best solution for you would be such a potentiometer from alps:
http://www.drtube.nl/products/alpsbvmc.jpg
datasheet here:
http://www.drtube.nl/datasheets/alps-bv.pdf
can be bought here for EUR 30,-- (in different values):
http://www.drtube.nl/tubeshop.htm
I know, I know..... EUR 30,---
but it´s a good quality potentiometer and can be controlled the easy way remotely with the motor (4 transistor bridge-control).
You can buy two of them, built the second in your wah and make the platform of the wah move up and down remotely - what a fun....
There exists also cheaper versions of such potentiometers, but I don´t know the exact part-number.
analogguru
http://www.drtube.nl/products/alpsbvmc.jpg
datasheet here:
http://www.drtube.nl/datasheets/alps-bv.pdf
can be bought here for EUR 30,-- (in different values):
http://www.drtube.nl/tubeshop.htm
I know, I know..... EUR 30,---
but it´s a good quality potentiometer and can be controlled the easy way remotely with the motor (4 transistor bridge-control).
You can buy two of them, built the second in your wah and make the platform of the wah move up and down remotely - what a fun....
There exists also cheaper versions of such potentiometers, but I don´t know the exact part-number.
analogguru
There´s a sucker born every minute - and too many of them end up in the bootweak pedal biz.
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flood
- Resistor Ronker
Information
AG, 30€ + versand ist ganz schön viel, besonders wenn man es in indischen rupees rechnet und zahlt... der euro-kurs ist seit märz ca. 16-20% gestiegen (inkl. bankgebühr) 
i will keep expensive pots as a final resort, but let me tell you this: the sum total of all my parts (excluding the toroidal PT and OTs) until now has been €30, along with wood for the cabinet!
don't want to nitpick, really, but that potentiometer is both out of my budget and reach for right now. one thing that is highly forbidding is the cost of shipping and the nightmare in india called postal service....
but thanks for the tip. will make a note of it and keep it for future purposes... i am still very curious as to whether my LED/LDR idea would work, and if there is something that i am overlooking, e.g. noise, nonlinearity.
i'm just a noob at this, so do forgive the inaneness of the question.
EDIT: it's interesting you posted this - i just remembered i've never seen motorized pots in the local market. should take a look again - but considering that people buy ampliiers there for less than 50€ which CRACKLE ever so nicely after one year, you'd think zackie boy desgined them....
also, i don't owe a wah pedal yet maybe at the end of this year, a weeping demon, cause i tried it and loved it. need that extra bottom range for bass too.
i recall having seeing cheaper versions. will scour the web. i believe the love of german/austrian DIYers' lives - pollin elektronik - had something in "restposten"...
i will keep expensive pots as a final resort, but let me tell you this: the sum total of all my parts (excluding the toroidal PT and OTs) until now has been €30, along with wood for the cabinet!
don't want to nitpick, really, but that potentiometer is both out of my budget and reach for right now. one thing that is highly forbidding is the cost of shipping and the nightmare in india called postal service....
but thanks for the tip. will make a note of it and keep it for future purposes... i am still very curious as to whether my LED/LDR idea would work, and if there is something that i am overlooking, e.g. noise, nonlinearity.
i'm just a noob at this, so do forgive the inaneness of the question.
EDIT: it's interesting you posted this - i just remembered i've never seen motorized pots in the local market. should take a look again - but considering that people buy ampliiers there for less than 50€ which CRACKLE ever so nicely after one year, you'd think zackie boy desgined them....
also, i don't owe a wah pedal yet
maybe at the end of this year, a weeping demon, cause i tried it and loved it. need that extra bottom range for bass too.i recall having seeing cheaper versions. will scour the web. i believe the love of german/austrian DIYers' lives - pollin elektronik - had something in "restposten"...
In the interest of full disclosure, I am Animal Factory Amplification.
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flood
- Resistor Ronker
Information
ok i think i may have answered my own question and discovered a new dimension to my stupidity:
at max darkness the parallel combination will give me 100 kOhm - with no shunt to ground i.e. voltage divider in place, the effects of this "volume control" will be an unattenuated level of the incoming signal
... am i right?
just breadboarded it (without audio signal) keeping n eye on the resistance... it moves decently from 83k (min. brightness) to about 12k, then jumps with the sudden increase in brightness of the LED at very low pot resistance to about 5k. so not very usable at the high current draw settings and also, the jump would be quite nasty, i imagine, in terms of the volume jump (assuming opf course that one uses a preset shunt resistor of about 27k or so).
and i had such hopes for this idea. noob is as noob does
at max darkness the parallel combination will give me 100 kOhm - with no shunt to ground i.e. voltage divider in place, the effects of this "volume control" will be an unattenuated level of the incoming signal
... am i right? just breadboarded it (without audio signal) keeping n eye on the resistance... it moves decently from 83k (min. brightness) to about 12k, then jumps with the sudden increase in brightness of the LED at very low pot resistance to about 5k. so not very usable at the high current draw settings and also, the jump would be quite nasty, i imagine, in terms of the volume jump (assuming opf course that one uses a preset shunt resistor of about 27k or so).
and i had such hopes for this idea. noob is as noob does
In the interest of full disclosure, I am Animal Factory Amplification.
-
flood
- Resistor Ronker
Information
...or maybe not - i found this thread over at diyaudio (i think registration is necessary to view it). simple enough to do without a schematic: two LDRs in a voltage divider configuration, dual pot to drive the two LEDs in a manner that would increase resistance of one pair while decreasing the voltage of the other.
problem: if you don't want to have individual volume controls for left and right, you would need a quad pot - i am trying to work around this by implementing the pot in a see-saw manner (dunno if that'll work, but trying will cost me next to nothing).
an interesting design - the designer touts it to have excellent sonic characteristics too, especially for people who cannot afford good quality pots. will breadboard this soon and post again - i do not have anything to compare my results to, since this is my first foray, but would rather test the circuit for smoothness of travel, reaction time, hysterisis etc.
problem: if you don't want to have individual volume controls for left and right, you would need a quad pot - i am trying to work around this by implementing the pot in a see-saw manner (dunno if that'll work, but trying will cost me next to nothing).
an interesting design - the designer touts it to have excellent sonic characteristics too, especially for people who cannot afford good quality pots. will breadboard this soon and post again - i do not have anything to compare my results to, since this is my first foray, but would rather test the circuit for smoothness of travel, reaction time, hysterisis etc.
In the interest of full disclosure, I am Animal Factory Amplification.