Intro & Lead 12 Preamp Design

[Burt Cocain]

Hi folks, I'm returning to building pedals after about ten years. Recently changed careers, so need something technical but cheaper than cars or bikes to play with in my free time whilst I'm re-training at work (and on a severe budget...).

I've got tools, but am still collecting parts for my first builds - I intend to make a batch of Electra distortions, from the most basic, through some of the Lovepedal variations as my first foray back into fuzz guts.

As a wee project in the mean time, I really fancy making up a tagboard layout for a Marshall Lead 12 preamp - I'm quite fond of them, and would like to have a little preamp-in-a-box. There doesn't seem to be any layout available, and I'm looking forward to the challenge.

I reckon I can find my way around the schematic, but I've hit my first stumbling block... I see the two chips require a +18v, and a -18v (it's actually 19v on the marshall schematic, but i figured it would be OK to go in a shade low...) I have no experience with bipolar power supplies, but from my digging around here, I suspect I'd be able to use the voltage doubler to get 18v from a battery, and the -18v circuit snippet from the Rusty Box layout on here?

If anyone can advise, I'd really appreciate some input. Planning on starting later tonight as a reward for studying *of all things* an electronics module all day...

Finally, I must add, I really love this site - it was browsing on here that has given me the bug again, thanks for all the great work guys!

Burt

[Burt Cocain]

Bugger, never saw the introductions thread!

[IvIark]

Hi and welcome to the site.  If you use this layout it will give you +/- around 17V from a 9V supply and so should be fine for your project assuming the circuit doesn't require more current than can be provided by the IC.  



Good luck with the build and let us know how you get on.

[Burt Cocain]

Hi Mark, that cicuit looks ideal, thanks. I made a start, getting my head around the schematic last night and I'll be doing a bit more on my study breaks today. Cheers!

[Burt Cocain]

Right, I've had a crack at this. First attempt at stripboard layout, first attempt at interpreting a schematic this way, first dabble with Layout Creator.

I've noticed already that I can do a bit of tidying up, and save a few rows - also, this is hot off the press, and I haven't checked it for complete and utter nonsense yet. I'll do that tomorrow when I'm fresh...

I'm not entirely sure what to do with the bipolar power. I'm assuming that +18v goes to pin 4 on the 1458 chip, and -18v is effectively ground? If anyone could eleborate I'd really appreciate it.

Please, by all means rip this to shreds. Now that I've dabbled, I'm keen to get involved again.

Cheers,

Burt

[glasspak73]

Is the bipolar + and - 18 volt dc mostly for the power amp stage of the amplifier? The schematic i read shows the 16 volt + goes to pin 8 of IC-1 and -16 volt goes to pin 4 of IC-1.But won't this preamp distortion part of this circuit run at regular 9volts instead of using the bipolar converter? Sorry if i am wrong about this, I assumed the bipolar power is better for the power amp stage to create the higher output for a speaker.I'm still learning so thought i should ask.

[Burt Cocain]

I think you're right - the full schematic shows the +/-18v going off the power amp section towards "IC1" and "IC2" - I'd assumed those were the chips on the preamp section, the they're not - they're "ICA" and "ICB"

So I'm not sure what voltage the preamp runs at. I'm sure there must be a clue on the schem somewhere...

*EDIT* Yes, of course there is a clue. 1458 datasheet shows operating voltage between 18 & 22v. So, 9v seems a bit low - but when I look at the datasheets of things I know work well in pedals like 4558, TL072 - the supply voltageis the same, and I know those work at 9v... *

[glasspak73]

I always try to use 9 volts first then work out any buggs.Once i have the circuit sounding good to my ears then i try different voltages.This way always helps me if i have to debug any problems and i don't damage any charge pump ICs usually lol.I'm still learning every day on here and hopefully try to help out when and if i can.I'm looking forward to seeing how this circuit turns out.The amp sounds awsome on youtube videos

[Burt Cocain]

Thanks for the advice. Yeah, they certainly do soon good. I think everyone's early years of playing guitar puts them off small solid state amps.

I got my lead 12 a few weeks ago, and its as good as the youtube videos make it sound. Tubescreamer copy in front, and it sounds brilliant, with loads of scope to play with the guitar volume control.

I'm so fond of the amp that I'd like to getthis project together so I'm never without one. Would like to try it into a wee ebay power amp - but I'll see how hard it is to clone the Marshall power section and find the trannies first...

If anyone has any comments on my layout, I'd really appreaciate them - will tidy it up a bit tonight if time allows, spotted a few revisions already.

Cheers