Alright, drawing stuff here. Noticed i had some 100k digital pots, so will try them out while i wait for my LDR order to deliver.

If i want to "emulate" a gain pot for example that is 1M, but the digital pots and ldrs are only up to 100k, and jfets even lower. the pot is located between 2 gain stages, 1 to GND, 2 to stage 2, 3 to stage 1.

The only solution i came up with (and i'm not even sure that it's a usable one) was to connect a buffer in place of 2 with a 1M resistor to GND, and another in place of 3 with a 100k attenuator after it, in order not to mess up the impedance of the circuit. seems a bit clumsy... How would you guys that actually know what you're doing go about this?

Thanks Roly, great answer as always

I will look into the LDR solution, it seems to be the most viable solution to achieve a somewhat wide range of variable resistance.

I got the most experince with Pic Mcu's, and got loads of them laying around here, so i will probably use one of those. I did design a "multi effect control" way back that had a bunch of send/returns to patch in different pedals in presets based on a pic with a 4*20 alphanumeric display. I never built it, but had it running in sim, i might base everything off that one.

I was also experimenting a little today with running the signal through 3 parallel high pass/low pass filters, and then attenuate each one separately and mixing them back together. But i dont know. felt a bit akward, and not sure if it would even work.

Hey.

Preparing for a few gigs and the very late album release, i've noticed that even though i'm down with a 4 button pedal i've built (distortion, gain boost, delay and volume boost), there's just too much stomping. I would like to mod my pedal to be programmable with presets instead. using an mcu, display and momentary switches instead to switch on/off multiple effects, but also change tone, delay and volume.

I can get the digital part off easily, and i already got the analog part. Now interfacing them in a convenient way is where i've hit the brick wall.

I know about digital pots, but i've also heard lots of arguments against them. i dont really need insane resolution, but it wouldnt hurt to get atleast a few possible variations, which kinda rules out the string resistor solution i experimented with, as i need 1 resistor per setting. i also found a forum thread about using LDR's, the way mesa boogie does in the triaxis amp.

But i'm really not sure what route to go here, something that wont enlarge the casing by a factor of 10 would be nice.

i did connect a jfet shunt to GND with a R2R ladder to set the level for the gain and volume, and well, it seems to work. But not tested enough yet. The tone stack though is where i'm entirely lost. Any suggestions are welcome

Thanx guys, will tell him to drop by with the amp and i'll get going with some testing.

Hey guys, my preamp project still on hold. :/ too much work for me with stuff for the band and the upcoming album release, a friend of mine though is in need of help. His Rocktron rampage SS amp is acting a little wierd. It takes numerous power cyclings to get running, the power led is on every time, but the sound is not. occasionally volume starts fading and the amp gets silent during play aswell.

I inspected the board, and everything looks fine, power resistor and all, entire board looks unheated in every way, electrolyte caps looks fine too, no bulging, although, i have no meter to check them. :/

Any hints on what to look for? there's not really much to the circuits in it, so i might just try and replace some stuff if somebody got a hunch here.

Hey. Looks like a nice app

Joe:

Delete the openal32.dll from the app directory, and check if you already have openAL installed, in which case deleting the openal dll should resolve the issue. Otherwise, try installing openAL (and still delete the openAL dll's from the dir). It's known to behave a bit dodgy sometimes when you have a different version installed in your system, but running another version dll.

Hey.

Currently working on som high priority stuff for the band, so going a little slow with the amp project. I had to make a better backing track solution than our current mp3 variant, the new backing tracks from our producer really need to be stereo (or preferrably 4-8 channels), so i've made a small pic-controlled remote control with a display and buttons that controls a computer through the serial port running a minimalist microcore linux (6 mb iso) to play multichannel Flac-files.

The reason i did not go with +-15 for the opamp in the cab sim is that the one i found, and liked was made for single supply. The tone stack will be tweaked later on. The cab sim + tonestack i posted here is something that i use as a "plugin module" to test the actual preamp circuit while designing and experimenting without having to make a huge breadboard circuit just to test a couple of gain stages etc. There will be +-12 for the jfets though in the preamp itself. I've been looking quite alot at the Rod elliot project, but i really like the sound i get from jfet circuits i've been building and want to do a PROPER design with those. No trimpots, doing correct biasing etc.

That fan control looks Nice! will try it out if i can fit the board in the case. Space is getting tight! :S Note to self: next psu i build will be atleast twice the width of an ATX psu Case.

Was soldering my prototype board, but i really need to start over, i dont like having the 4700uF caps that close to the heatsink. So will either have to try and move them some, or raise the regulator board a little so that they wont get more heat than they have to. Completed the LCD Daughter board and shot a photo with them in the front panel my GF is working on, It will be polished and engraved though, only rough cut so far. And i need way smaller knobs.

Going a bit slower than i anticipated, but getting forward. Installed the transformer, did all the wiring up to the bridge rectifier and figured out where on the heatsink i want to mount it later on,a used LOTS of heatshrink tube. Love that stuff. Now i'm going away over the weekend, so probably wont get much done. But i need some mounting materials anyway, so might aswell go away while waiting for the delivery.

By the way, Roly. what's up with electronics and Australia? Is everyone a pro engineer over there?

Stumbled across this guy (Dave Jones) a few days ago.
http://www.eevblog.com/episodes/
That stuff is excellent! i've learnt so much just watching those teardowns and of course his PSU design project, and on top of that, you also got Rod Elliot, and you seem to know quite alot yourself.

Yeah, i plan to upload both pic code and schematics when done.

Waiting for my goldsmith Girlfriend to make the front panel for the PSU as she offered to. Will be a brass plate.

Before i start soldering the boards i did some measurements while moving around the guts of the supply to make sure my board wont get too big. I also put in a small DC fan, not sure if i will need to use it, but, hey. when did you last hear of a power supply running too cool? ^^ Also got my 2000mA transformer, so i figure the 150 mA used by the fan (well actually i think i'll just pwm it and get it down some more) wont be a problem. Now the fancy way to do this would be to let the pic read a temp sensor and adjust the fan accordingly, but that means i will have to redesign a little, and pick a bigger Pic, no spare pins atm. I think i'll probably just run this one of a potted 555 if at all.

Also looking at fabricating a small enclosure that will seal the input jack + switch. i dont want any mains points touchable at all, especially with the small hole array at the back panel of the ATX case. inserting a guitar string or wire by mistake would be bad. So i will make some sort of bracket to make sure it cant get nasty.

Attached pictures of the "mockup" placement and the LCD, Terminals + knobs, still in bags.

Oh, and the plate which the transformer secondary wires are resting at on the picture will be the front. Gonna cut it into a frame for attaching the front panel when i get it.

Well, assembler is a great way to gain maximum control, but so inconvenient to work with. So much simpler to reuse my old LCD functions i wrote ages ago in C. Going for dual displays, one for each rail, as i noticed i didnt have any bigger displays than 16*2 with backlight. all my 20*2 and 20*4 are without.

so it kinda looks like this right now:

| Rail -      xxxxmA     |   Rail +     xxxxmA  |
| -xx.xxV [xxxxmA]    |   xx-xxV  [xxxxmA] |

top line shows actual current, bottom line shows set voltage and current limit.

1 pot for each rail voltage, and another for setting the current limit. Those are actually analog, which might be odd when there's an MCU involved, but i decided to go analog for the PSU itself and Digital for the monitoring and limiting.

I got the firmware done in C now with 60 % of the code space used instead of the flowcode variant that hit the ceiling, and am working on the PCB layout for the supply. So hopefully it will be done in a couple of days, decided to order a bigger transformer too, if this works out the way i hope i might upgrade it to a 2A 12-0-12 instead of the 500mA one i'm using later on if i want to begin looking at power amps too. (although i will need an even bigger one if i wanna do a 100W amp, there's still a long way there. Preamp is next.

Tried a different approach writing the firmware. Tried out FlowCode, microcontroller programming without actual programming. Really fun to work with, but terribly optmized code. My program got 250 words too big at first. managed to optimize by hand down to 3 words+, but realized this is a no-go as i need to add offset voltage compensation and so on.

So i guess i will make a rewrite from scratch in C, I do work as a programmer, so it's my cup of tea. But for anyone wanting to experiment with MCU's, and dont care about using a bigger chip with the benefit of no coding, i highly recommend flowcode.

That sounds like a great collection. i pick up most stuff that's going to the scrapheap at work, mostly printers, computers and such, but there are always parts to scavenge, or fix. gotten some quite nice heat sinks from laser printer power supplies, and using the casing from an ATX supply for my lab supply. a colleague plugged it in a 230V outlet without checking the 115/230 switch in the back. it went boom, quite a few scorched components. but the case and heat sinks are fine.

My back is getting alot better, i can get out of bed instantly when i wake up. A few days ago i had to crawl out of bed. I definately need to stay away from heavy lifting for a while, fortunately we are finishing up our album at the moment, a couple of months left to release party, tour etc.

Almost done with my lab supply design now. Been plowing through electrical safety rules for a couple of days to make sure it's as safe as possible, i want to go through every possible measure.

Will fine tune, simulate and code some more and then post schematics.

i'm mostly basing the supply itself on Rod Elliots +/- 25 lab supply, http://sound.westhost.com/project44.htm, but with a 12-0-12 1 A transformer instead.

The addition i've made is digital monitoring and current control.
There's a TIP122 darlington on each rail driven by Bc547/557's with led indicators that light up when current is limited and the transistors choke the regulator inputs. the limits are set by pots connected to the Adc of a pic16F690, there's also an overload LED that lights when the total current gets above what the transformer can safely deliver, will set this value in software. it's easy enough to change if i make a bigger supply with the same control logic.

On the output side there's a 0.1 ohm shunt resistor on each rail. each side of the resistor goes through a 1:10 voltage divider, buffered (positive side)/inverted (negative side) by 2 lm358 opamps, and differential amplified by a gain of 100 by another lm358. the outputs on the diff amps goes to pic Adc, so does the voltages from the buffer after the shunt resistors for voltage indication.

Now, i'm aware that i will lose some precision with this.
1. the lm358 is not rail to rail, and will have an offset voltage. i will have to adjust for that in software
2. the voltage dividers will lower my precision some.

I'm setting the PIC's Vref at 2.5 volts, the Adc should never go above 1.2 volts in though. but this gives me about 2.44 mV resolution, which i feel is sufficient for my needs.

i know i'm a bit off the original project, but i feel that proper tools will improve the testing and design, so i rather spend more time getting to the finish line.

That's great Phatt! Thanks!

I've been laying a bit low the past week, literally actually. Lumbago..

Waiting for a bunch of 4700uF capacitors now in order to complete my supply.