Stormbringer Preamp - A beginners project log/diary

[Roly]

"electronic recycling plant"    You wanna see my whole house mate!  I'll just mention seven console organs and about 30(?) computers; the synths, guitars, amps and cabs you will just have to imagine - then there's my electronics stuff...  bush heaven.   

"Lumbago"  Bugger.  Had a back injury when I was a teen, quickly learned to "lift from the knees".  I got arthritis in my hands so badly I switched from guitar back to keys - cured!  So play more, but get a good strap, wide.  Take care.  :tu:

[stormbringer]

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.

[Roly]

A PIC!  "Software"?  Egad, I stop when the smoke gets too thick!

This is very good to hear.  Back when I was a teenager it was a matter of build my test gear or go without, even my first multimeter, a habit that has stuck.  As I'm sure you are aware, building test equipment is rather more demanding than just amps &c; your results are only as good as your test gear, and to deliver results you can depend on it has to be an order of magnitude better than the amps (or whatever) under test, and this can be very demanding.  Oscillators in particular can be quite an adventure.

A long while ago I bought a kit for a Noise and Distortion meter after a constructional article in a reputable electronics magazine.  Well!  First thing, if you ever want to build any sort of instrument with aspirations to precision, start with a metal case.

It took a deal of redesign and much added shielding to get it a) stable, and b) satisfactory.  It now resolves down to -85dB, but in it's original incarnation was lucky to make -40dB and very prone to simply taking off under its own steam.  I never saw a letter of complaint and have to think that all the hapless kit builders thought it must be all their fault.

I hope you intend to fully document your build and put it up on the net.  If so I'd be happy to give it space on ozvalveamps.org.   :dbtu:

[stormbringer]

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.

[Roly]

Assembler Roolz!   :dbtu:   :lmao:

[stormbringer]

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.

[Roly]

I'm reminded of the saying that "it's hard to remember you came to drain the swamp when you are up to your arse in alligators".   :cheesy:



In very slightly related news; I bought a bunch of coloured screw terminals a couple of weeks ago intending to fit them to a computer power supply for bench use.  Well I had to buy some more yesterday because the first lot slipped through the same wormhole in space that pens and socks use to go to a parallel (or perhaps divergent) universe.  I've turned my workshop upside down three times last week, discovering many long forgotten things along the way, but of terminals, not a trace.   :grr

[stormbringer]

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.

[joecool85]

Exciting project.  So since you are doing this with a PIC, will you share your code?

[stormbringer]

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

[stormbringer]

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.

[Roly]

Murphy's Law of projects - everything takes twice as long and costs twice as much as expected (except when it takes longer or costs more  ).

Fan control;

http://www.ssguitar.com/index.php?topic=2591.0

Heatshrinking all the mains stuff helps to prevent "errant guitar string" moments.

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

:lmao:

I've only seen a bit of Dave's stuff but he sure likes to rip into things.  Rod is very good value and sets a cracking standard on his website that we try to live up to.  I'm (supposedly) retired after a long and varied career in electronics including consumer, industrial, bio-med, programmer and of course theater/music/soundie, and been a musician, keys and guitar, since I was a kid (but not a very good one which is why my career was in electronics and not show business  8| ).

[stormbringer]

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.

[Roly]

Very steampunk.   :dbtu:

I've found that I can bolt up to three gutted 'puter PSU cases together for valve amp chassis'.  I'm nothing if not cheap.   

Smaller knobs?  Bigger panel!

[ghoshsubha444@gmail.com]

Hello Stormbringer,

I am currently building my amp too. recently finished power amp module and cabinet speaker assembly. Stuck on preamp. My requirement is quite same as yours. can you please elaborate your plans on adding several components. since you have already completed tonestack+cabsim. is it common for all the channels(tone stack).? Or each individual channel will have a separate but same tone stack? what is your plan on different channel. Mr Fahey recommends 1.Elliot project 27/27b 2. Marshall Lead 12 preamp.

dont know but curious: why dont you use higher voltages like +-15v for opamp biasing which is very common in commercial amps?

you can check my thread recently started:
http://www.ssguitar.com/index.php?topic=2759.0

thanks