Stormbringer Preamp - A beginners project log/diary

[Roly]

Pictures (and circuits) or it didn't happen.   :cheesy:

[stormbringer]

Well, here's a sound clip. Nothing special. just hooked up my homebuilt OD808 Clone before the tonestack and recorded some random riff through the sound card. first without cab sim, then with cab sim.

http://soundcloud.com/gmprog/cabsimtest-1

The cab sim schematic


Attached the tone stack aswell, it does have an added presence knob through a switch to turn it on/off + volume knob.

Using this in front of the tone stack.
Simple Jfet Buffer

[polo16mi]

 :dbtu: Cool sound, dude!

What´s your axe? Pickup that she load?

[stormbringer]

It's an ESP LTD FX260SM with passive EMG's

[stormbringer]

Looking at various power supply solutions, and decided to do some experiments. got this AC brick that i dont use for anything. 12v, 1A AC output. More than enough current for my preamp, and i dont have to dabble with mains either until i feel more comfortable with it. Got transformer, fuses/holders and all in stock, but there's something about 230v that gives me chills down my spine, even though i have actually installed mains outlets and stuff (under the supervision of my uncle though, a certified electrician). I rather wait for that until it's time to install the entire thing in a case.

Now, according to Rod Elliot's page http://sound.westhost.com/project05.htm, a 16v AC should be able to give +/-15v using a full wave voltage doubler. and 12 VAC +/-12.

Quite interested i decided to try this out in both ltspice and Proteus. but both sims gives me around +/-11.2 VDC. Guess i will breadboard this next and see what the results are. i'm just curious to why i get these results.

Another question that i have been unable to find the answer to, is how i calculate maximum current with this type of circuit? It can't be as simple as just halving the AC adaptor's avaliable current?

Edit: regarding the current i found out through ltspice simulation that the capacitor values are linked to the current/ripple. so that answers my question partially, but i'm still curious about how the current draw is defined from the adaptor/transformer itself.
Attaching the ltspice circuit below

[J M Fahey]

There must be something wrong.
Those caps will charge to the peak of 12VAC, in practice you'll have +/-16Vdc or a little more unloaded (because real world transformers are slightly overwound) or with the small consumption of any preamp.
Now if you pull 1/2A from each rail, yes, a cheap transformer will have losses, although going down to less than 12V seems too much.
I'm talking about raw voltages, of course.
Rodd Elliott refers to regulated rails, and regulators will , of course, give you the exact value indicated by they name, but need *at least* 3 or 4 V above it for themselves.

[Roly]

there's something about 230v that gives me chills down my spine

And so it damn well should!  I'm retired now but part of my gig included industrial installations and repairs, and sometimes live board work, variable speed drives up to 100HP.  Shaking hands with beef is only part of it; the domestic supply can deliver an awesome amount of power into a short for a few hundred milliseconds before the protection pops, sending a spray of white hot metal in all directions.  When you get casual about dealing with the power mains it's time to quit while you are still alive.  I like your plan and would do the same myself.

{I'm in Australia, 240V 50Hz, where are you?}

Yeah, there's something wrong there.  12VAC is root(2) more peak, and unloaded the caps should both charge to the +ve and -ve peak values (less diode drops);

12 * 1.414 = 16.968 or +/-17 volts, but as JM says a real tranny is "soggier" and will start out higher no load and may end up lower at full load current.

the output voltage is given by

Voffset+Vamp*sin(p*Phi/180)

that is (ignoring Voffset = 0);

e = E_max * Sin(Theta)

i.e. "Vamp" = E_max = the maximum or peak voltage, not V_rms, so E_max (i.e V_amp) has to be multiplied by root(2) = 1.4142 to be the same as "12 volts"(rms).

12 * 1.4142 = 16.9704, call it 17 volts.


Just a couple of things to remember if you are going to follow this up with 7812/7912 three pin regulators; a) the pin order is different between the +ve and -ve regs (and the -ve tab has to be insulated), and b) stick small caps right on the reg pins, input to ground and output to ground (0.01-0.1uF) to prevent any instability.

[stormbringer]

Thanx for the replies guys.

I will make a test circuit for this and try it out. Oh, and i'm planning lm317/lm337 regulators. I really like those.

i'm in sweden 230v/50hz. Yeah, i got alot of respect for high currents and voltages. i always take precautions when working with electricity, even if it's not in the lethal range ie down at 5v and low current, but gaining that extra few seconds from not discharging capacitors etc just isnt worth it. Only shock i have ever had was an electric fence for horses, it was terrible. i can only imagine what (actually i dont even want to) the mains outlet would feel like.

A janitor at work got his shoulder against a rail in the roof that shouldnt even have current that caused his muscles to contract enough to break both his shoulders. No, i'm staying careful.

[J M Fahey]

Well, unless I'm mistaken, Scandinavian Countries (including Sweden) don't even have grounded outlets because high resistance soil can't be trusted for grounding purposes, go figure.
That's why their Electrical Safety rules are the toughest in the World.

[Roly]

Worst boot I can remember getting was in a factory working on a press guard installation, standing on a plastic milk crate I somehow managed to get each hand between two phases, 415V - worst case you could imagine with the current path through my chest cavity.  Felt like I had been kicked in the chest by a horse for a few days afterward (and a hell of a lot more careful forever thereafter). 

When probing live valve/tube amps I keep my wrist resting on the grounded chassis, and the other hand in my back pocket - an old trick that is a leading cause of old technicians.  Or as a boss of mine was fond of saying, "there are careless techs, and old techs".  :dbtu:

[stormbringer]

Yeah, outlets in kitchens and bathrooms are always grounded. And if you install new outlets anywhere nowadays they have to be, but for example in older apartments (like mine), there are no grounded outlets in the other rooms. so i usually ground myself to the radiator (they are water heated in my apartment) before handling semiconductors. Maybe it's not needed, but i havent broken anything due to ESD so far.

[phatt]

Hi stormbringer,

Some of my schematics might interest you? Give you ideas to ponder :tu:
http://www.ssguitar.com/index.php?topic=2013.0.

In my experience you still may need post cab sim tweaking.
Give me a week or 2 and I'll be posting my latest adventures,, which has many more tricks.
Phil.

[stormbringer]

Thanx Phatt, will keep an eye out.

Right now i'll just go with the cabsim + tone stack i built while working on the gain stages and experimenting, later on however i will explore those parts a little deeper, right now i just needed something that worked while working on the basic parts of the preamp.

Project update:

Built a voltage doubler and hooked up a dual gang Pot to let me tune voltage between 1.5-12v from some scrap parts (well, the pot + capacitors atleast). Waiting for a LM337 to arrive so that i can complete and test it, Also found yet another AC-AC adaptor (15 VA) while searching my electronic recycling plant (Or as my girlfriend calls it, the basement, good backup in the case my 12 VA adaptor doesnt pull it off. Unless i get the awaited LM337 tomorrow i'll probably make a small voltage monitor using a pic + lcd display for the circuit. Easier tuning in voltage in a visual way than using a manual voltmeter.

[phatt]

Another question that i have been unable to find the answer to, is how i calculate maximum current with this type of circuit? It can't be as simple as just halving the AC adaptor's avaliable current?

Yep that one messed with my brain some years back.
The current rating of the rectified secondary voltage is dependant on what type of rectification is used.
This little gem helped me get my head around it.

Take note of the *IDC over IAC*
Phil.

[stormbringer]

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.