Four new works in progress - help needed

[Guitar Gear Reviews]

Hi again, Gear here with some more new stuff and some questions.

As the title says, I am working on 4 different projects right now - a passive pedal, modifying some old loudspeakers, adding phase and series switching to a Strat, and a new amp that is something of a cross between the Noisy Cricket and the Tufnel Distortion. http://www.beavisaudio.com/projects/tufnel/index.htm

Continued from my website:
http://electricguitargear.blogspot.com/2014/01/works-in-progress.html


          The final and most exciting of these projects is to build a new amp circuit loosely based on both the Tufnel Distortion and Noisy Cricket into a steel military ammo box.  I decided to name the circuit "The Trooper" because of both the enclosure's military nature and because Iron Maiden is awesome 

          Aside from the fact that the enclosure is perfect, it should be really neat because it consists of two LM386s in series - the output of one runs into the next, which should generate both a lot of sound and a lot of distortion.  The controls will consist of:

Switches:
A power switch that toggles between battery or wall adapter
A switch that toggles between the built-in speaker and an external cab
A grit switch a la Noisy Cricket
A tone (bass/treble) switch

Pots:
Two gain pots (drive and distortion)
A Big Muff Pi tone knob
A volume control

          I already started the enclosure by insulating it and installing the speaker (same one from the experimental amp).  I drilled holes for the sound and bolted the speaker onto the side.

          If you're familiar with circuits I'd really appreciate for you to take a look over the schematic and let me know what you think about it - opinions, predictions, flaws, etc.  Please let me know if anything looks like it will be a problem in the design, especially part values.  Much appreciated, check back soon for updates on how these projects go!

Full version with other projects for schematic http://electricguitargear.blogspot.com/2014/01/works-in-progress.html

I couldn't find out how to downsize the picture so I just linked it.

What I really need is some feedback and also some proofreading of the schematic.  Tell me what you think 

[Guitar Gear Reviews]

Alright, I realized I was a little vague, so here's what I need to know:


Will a 1K ohm pot work for volume?  The standard for Little Gem and similar is 25 ohms - which I can't easily get.  On the flip side, the Tufnel Distortion this is loosely based on uses a 100K ohm pot, but also has higher output.  Should it work okay?

In a similar fashion, will a 100 ohm trimpot work in between the two LM386's to reduce the signal strength going into the second? (I still want to be able to get relatively clean sounds)

Does it matter if the output capacitor comes before or after the output volume control?

This one's out of curiosity - why is it usually suggested to have both a large and small capacitor decoupling the power supply?  From my understanding, the capacitance would just be added on anyway and be hardly any different from just the large one.   

This is more opinionated - is out of phase switching or series switching more useful on a Strat?  Series seems more useful but out of phase is... intriguing.


Replies greatly appreciated!

[Guitar Gear Reviews]

Bump.  If I seem impatient it's because I'd like to order the parts today.

[dwmorrin]

Will a 1K ohm pot work for volume?  The standard for Little Gem and similar is 25 ohms - which I can't easily get.  On the flip side, the Tufnel Distortion this is loosely based on uses a 100K ohm pot, but also has higher output.  Should it work okay?

The Little Gem is running into speaker loads (4-16Ω, usually) whereas the Tufnel is running into "guitar" inputs (100k-10M, usually).
The 25Ω pot only makes sense if running into a speaker load.  Although it isn't much wattage, you may want a pot with a higher wattage rating than usual.
A 1k is 40x larger than 25Ω.  I'm going to guess that you'll see very little reduction in output until the last little bit of rotation.  Not to mention that you're adding about 1k of series resistance to the output, which may or may not have some kind of effect.
You'll have to treat your output jack like a "line out" and you'll want the low ohm control to only affect the speaker load, and the higher ohm control for the jack output.

In a similar fashion, will a 100 ohm trimpot work in between the two LM386's to reduce the signal strength going into the second? (I still want to be able to get relatively clean sounds)

You've got a 1M output resistor going into a 100Ω pot.  Yes, that will reduce the signal.  Google "voltage division" or "voltage divider" for further understanding.

Does it matter if the output capacitor comes before or after the output volume control?

Leave the output control after the output capacitor.

This one's out of curiosity - why is it usually suggested to have both a large and small capacitor decoupling the power supply?  From my understanding, the capacitance would just be added on anyway and be hardly any different from just the large one.   

I believe the small cap is for high frequencies, radio and up.  The total capacitance on paper is just added together, but I think the real world tests show that there is something to having both small and large together.

This is more opinionated - is out of phase switching or series switching more useful on a Strat?  Series seems more useful but out of phase is... intriguing.

In a quick A/B test, series in-phase gives a noticeable boost, like a humbucker (except bucking the hum requires the right coil wind as well), whereas out-of-phase wirings, whether series or parallel tend to sound weak and thin in general.
So, I would vote for series in-phase as the more useful mod.

[Guitar Gear Reviews]

Thanks a lot man!  Bad news about the volume pot, but I was kind of expecting that.  I saw somewhere where someone made a volume knob for a similar amp with a 1k ohm pot and a resistor but I can't find it again. 

And about the trimpot, yes I was already using it as a voltage divider, not a straight up resistor.

I'm pretty sure radio waves are low frequencies and that's what the big cap is for, but you're right there must be something to it...

I'll probably go with the series wiring because I'm also doing the "Black Ice Overdrive" aka Schottky diode clipping mod and higher output would help with that.

Cool beans.

[B Tremblay]

The handle on an ammo box will rattle quite easily. You may want to dampen it somehow or replace it. I put a Ruby in one and replaced the handle with a drawer pull.

[Guitar Gear Reviews]

Thanks, hadn't thought of that.  I didn't know whether anyone else had used an ammo box for an amp before, but its too good an opportunity to pass up.  

I also wanted to mention that for the ground I'm using a washer for a singular ground point.  I just wrapped the wires around the edge and boom - even without solder I measured under 1 ohm resistance across the entire ground. 

The only thing that concerns me a little is that it looks a little too much like an electromagnet for my tastes... but no worries right?

[dwmorrin]

I'm pretty sure radio waves are low frequencies and that's what the big cap is for, but you're right there must be something to it...

All is relative, but when discussing audio circuits, I think of RF to be stuff above 20kHz.  Wikipedia says radio frequencies are anything below 300GHz (http://en.wikipedia.org/wiki/Radio_spectrum).
The large cap should be smoothing out slow changes, while the little cap is sensitive to quick changes.

I'm just noticing the battery switching.  Did you know that most power jacks are switched?  That is, you can wire them so that inserting the power plug defeats the battery?  Possibly you choose this scheme for another reason, but it seems like you (or a friend, enemy, etc) could throw the power switch to the battery position by accident when using the wall wart, and there would be confusion when the battery failed.

It would also be a good idea to use a stereo input jack and switch the battery on and off via the input jack.  See the beavis site or almost any pedal schematic to get the idea.  This, in conjunction with a power switch, would be good for battery life.

[Guitar Gear Reviews]

I'm just noticing the battery switching.  Did you know that most power jacks are switched?  That is, you can wire them so that inserting the power plug defeats the battery?  Possibly you choose this scheme for another reason, but it seems like you (or a friend, enemy, etc) could throw the power switch to the battery position by accident when using the wall wart, and there would be confusion when the battery failed.

These are the power jacks I am planning on getting.

https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10001&langId=-1&catalogId=10001&productId=2151136

I don't believe they're switching, they have 2 terminals.

What do you mean about the battery failing when the wall wart is switched on?  Turning that on disconnects the battery, and I chose a 3PST switch so there would be an in between setting so both can't be on at once.  It's good to be aware of such issues (exploding batteries etc.)

Muchos gracias.


Btw, I found the schematic for a more convenient volume knob.

http://tech-tut.com/wp-content/uploads/2010/06/modified-amp-schematic.jpg

Since I already have a 1k pot and no way of getting 25 ohm ones that should be perfect.

[dwmorrin]

Try something like this: https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_151590_-1
I just clicked around from your link... I think it's the same pin size.  I leave it to you to check the details.

The battery won't explode.  The scene is like this: you plug into the wall, accidentally switch to battery power, forget or don't care about turning it off (because you don't realize it's on battery mode), come back a few hours later and the battery is dead.
Just a "worst case" scenario.  Your switching scheme does work.  I would prefer the simplicity of a basic ON/OFF switch, and then let the fact that I've either got a power supply plugged in or not determine whether it's on battery or wall power.

[GGBB]

I'm pretty sure radio waves are low frequencies and that's what the big cap is for, but you're right there must be something to it...

All is relative, but when discussing audio circuits, I think of RF to be stuff above 20kHz.  Wikipedia says radio frequencies are anything below 300GHz (http://en.wikipedia.org/wiki/Radio_spectrum).
The large cap should be smoothing out slow changes, while the little cap is sensitive to quick changes.

The ESR of aluminum electrolytics goes up as frequency goes up, so the additional small cap - preferably NC0(C0G) - provides the low resistance path to ground for higher frequencies that the big cap can't provide.

[pinkjimiphoton]

my advice?
get a breadboard.

i am gonna go out on a limb, and say a tufnel into a ruby probably won't do much more than oscillate. you'll have to tame the gain some, and be careful with caps or it's just gonna be a motorboating nitemare (don't ask me how i know this)

it can be done... but i don't think you'll be real happy with it. good luck tho, sounds like a cool idea!

[Guitar Gear Reviews]

The battery won't explode.  The scene is like this: you plug into the wall, accidentally switch to battery power, forget or don't care about turning it off (because you don't realize it's on battery mode), come back a few hours later and the battery is dead.
Just a "worst case" scenario.  Your switching scheme does work.  I would prefer the simplicity of a basic ON/OFF switch, and then let the fact that I've either got a power supply plugged in or not determine whether it's on battery or wall power.

Okay, glad that I can read electrical circuits then

I'm going to get one of each type of input jack since I need another for a different project as well and I'll see what I like.

i am gonna go out on a limb, and say a tufnel into a ruby probably won't do much more than oscillate. you'll have to tame the gain some, and be careful with caps or it's just gonna be a motorboating nitemare (don't ask me how i know this)

It's not a Tufnel into a Ruby, more a Ruby into a Cricket.  As for taming the gain, that will take place in between the two transistors.  I have a Big Muff Pi tone control that will cut down some signal strength and also at least one 1M ohm resistor in between.

The Tufnel originally used just a 10K ohm resistor in between, but I plan to be able to make relatively clean sounds as well.  That in mind, I'll just add 1M resistors, in series and/or parallel until I'm happy with the amount of signal going into the second stage.

Here's a pretty basic distortion pedal I found that uses two 1M ohm resistors before the output.

http://www.instructables.com/files/deriv/FB6/2CLT/H5408HMH/FB62CLTH5408HMH.LARGE.jpg

And here's pretty much the same thing by another person without any resistors.

http://www.instructables.com/file/F0V3X4VFINO74G5

And Aron's Smash Drive has a 500K ohm resistor.

http://www.diystompboxes.com/pedals/schems/smashdrive.gif

So I should be in the right range here.   

[Guitar Gear Reviews]

The updated schemat with different part values.  Thanks for all the help. 

[PRR]

> why is it usually suggested to have both a large and small capacitor decoupling the power supply?

220uFd is physically large. Radio waves are small. Certainly older electrolytics didn't bypass radio waves well. Like a bear-trap isn't much good for mice.

And the LM386 does have response into the radio band. And you have two sucking on the same supply.

That said.... in the last decade, electrolytics have improved their high-frequency performance a lot. I wouldn't expect a single LM386 to really need the small bypass cap on the power supply. With two LM386, it may get hairy.

But do what you like. Omitting this, or the 10r+47n on the output, won't blow-up the chips. They may scream radio waves. You won't hear this directly. It may broadcast hash into nearby AM radios. Notably, while screaming 1MHz the chip can't *also* be handling your precious sound with delicacy and precision. But maybe that's not what you want anyway.

[pinkjimiphoton]

if it works, it's gonna be loud. i like that.

but you may not like the 386 distortion so much .... the bmp tone control should help a lot.

why not mimic a guitar amp, and use a 1m pot for one of them resistors? then you can use it as a master volume to control how much dirt  you generate with the first stage... a tone changer. rubys sound different pegged than quiet, so you could have the choice of just preamp distortion, or the preamp driving the power amp into distortion, or anywhere in between. yes, it's a pot, so ya need one more chassis hole, but it will offer a lot more versatility i'd think to the circuit you're going for.

i mean, you could even put a crude effects loop right there too... stick a rebote in there or something.

i'm digging your ideas. simple and elegant. have ya breadboarded it yet, or are you one of them wack jobs like me (reformed slightly) that builds things as proof of concepts?

i built a 386 chip amp for dick wagner.  gave myself two weeks to get it up and running for when he came to visit last year.

simple idea, 386 based ruby power amp, treble booster and overdriver into a simple 1 jfet preamp. simple bmp-ish tone control.

each stage worked. each stage (cuz everything was on switches and i'd test each part as i went as i built it) but when combined, i found i kept "losing" gain. what i thought shoulda been screamin' freakin' loud wasn't.

what i DID get was motorboating.. a lot. lotta creative snubber networks to cure that. more gain gone.

some circuits just didn't wanna work. i had even made a proper, amp style power supply so each stage was decoupled from the others with it's own power filters etc... more gain gone.

in the end, it sounded kinda like a fender by the time i got all the oscillations and crap out.

my little ruby can drive a 4x12 to an appreciable volume.

the one i built for dick sounded ok, but it could barely drive the 4" speaker i'd installed in the lunchbox i put the whole mess in.

so i re-imagined, and re-built the circuit several times. ended up being a ruby driving the speaker,
and i gave him a dano overdrive. it was the 11th hour, i'd been up for two weeks, many all-nighters trying to make this guy something worthy.

and fail'd most epically, even with some stellar support from other forumites. i found 386 based things to be prone to a lot of problems.. motorboating, oscillation, hiss...

not trying to be a buzzkill, just a caveat from my tales of woe, hoping to save another musicians days of bleak abject codpiece scratching with a dull,
glazed look in their eyes...

don't get me wrong,, I WANT THIS THING TO WORK!!! so good mojo sent!!