Wooden "Stompbox"

[newey]

Yeah, I have a few of those lying around, although the ends seem to be somewhat blackened with soot . . .

Actually, rather than using that technique (which will probably work), I had thought of a non-solder connection using the conductive-glue copper tape, since I have to use that to shield this anyway.

My thought was sort of a "sandwich" of layers of conductive copper tape. First lay down a bottom layer for shielding. Then, cover the end of that where the contacts will be with electrical tape to prevent the contacts from hitting the shielding. Then, 2 narrow strips of copper tape on top of that, one for each contact, and separated from each other. The strips run out far enough from the component to allow easy soldering to each strip. Next, the component is taped down to the strips so that each contact connects to its strip, using electrical tape. Then, another layer of copper tape over the top for shielding.

Ought to work, think I'll give it a try next week when I get the chance.

[cynical1]

May 15, 2010 12:00:28 GMT -5 sumgai said:

c1,

No, that's what we professionals call an ancillary usage. I'll leave it to your superior sleuthing abilities to determine the primary usage, as designed and intended.

...actually, years ago in a previous life, we called the hemostats... Working as an orderly we ran across them all the time...although, we employed them for a much more nefarious purpose...

Sorry, but the picture just induced a flashback...

Happy Trails

Cynical One

[newey]

we employed them for a much more nefarious purpose...

Hopefully, you ran them through the autoclave before re-use- you know, blood products and all that . . .

[ashcatlt]

I don't think anybody who can see me will sell me hemostats. Not without a snicker anyway.

Alligator clips work for that too...

[newey]

Back on topic here. I tried to wire the 2nd PVDF piezo film pickup as per my previous suggestion, simply taping it down to the contacts. And still no signal. I'll have to recheck all the connections just to be sure, but nonetheless another failure with these things.

And Contact Gold now has these backordered until June. They also have a prewired and preshielded version, but it's $42 vs. $5, so I'm not going there.

I may just scrap the idea and use a second disc piezo for the second pickup, see what I get that way.

But I'm getting pretty good sound as it is, with just the one disc, so plan B may be more appropriate. This would be to remove the blend pot and just use a regular pot, wired as a tone control instead of as a volume. And just stick with the one disc.

[newey]

I got the 2 piezo discs connected and working, but it was no improvement in the sound- and in fact started to feedback at a lower volume than with just one.

Sumgai posted a link about not over-thinking this, and I've perhaps been guilty of that. Simple seems to sound best, particularly with the foam dampening.

So, it'll be a single tone pot (per Ash's excellent suggestion, the volume pot isn't much use) and a single piezo disc, without any active circuitry.

While I await arrival of long-barrel jacks and pots, I'm wondering about the appropriate cap value and pot value for this- a passively-wired piezo?

The blend pot I was using was 250K. Would going to, say, 100K, make it more low-end? I'm not worried about the potential "muddy factor" too much in something I'm tapping on with my foot.

And, cap value? I'm not really interested in experimenting too much with this, a good compromise value would be fine, I just have no idea what a piezo wants in a tone control.

[JohnH]

Hi newey - piezos respond differently to being loaded by tone controls than do guitar pups. wheras a smaller value pot on a guitar will reduce treble, the same with a piezo will reduce bass.

So your piezo, is it one of those buzzer discs and what diameter? and is it the intention that the tone control will reduce treble, but keep bass intact? and is it going into some kind of guitar amp type input, ie resonably high impedance? With those info it might be modellable and we can pick some values for a passive set up.
John

[newey]

The piezo disc is one of these:



The second-largest one, which is 35mm. The specs are found here.

I'm using my Traynor bass amp, which has both high and low inputs- I've been using the high one, figuring since it's passive . . .

I've found it also sounds good with a bit of echo added- I'm running it through a Dano "Slap Echo" pedal (which is true bypass, I believe, and so no buffering when it's off.)

[JohnH]

Thats a good meaty piezo, and with a capacitance of 32nF, it has plenty of grunt to drive circuits, unlike more feeble piezos usually used on instruments.

I know the main issue is to get a good bass thump, so how about this:



C1 is the piezo capacitance, and R1 is the amp input. There is an extra resistor R3, which helps to separate the tone control from the piezo so that treble is cut without losing bass.

Here's a response, at 10% increments of turning the 100k tone pot:



The other thing you can add if you need to actually cut bass is to put a resistor or pot directly across the piezo.

This circuit causes about a 3db loss of volume overall, which is hopefully not too much

John

[newey]

That was quick! Good work, John! +1!

6.8nF on the cap comports with my thinking. Of course, what I have on hand are 2.2 and 4.7. Hmm, how about one of each of those, in series?

Have to pick up some 47K resistors . . . . And, 100K for the pot? That seems low in light of what you said about losing bass.

[JohnH]

Those two caps in parallel will make about the right value, or you might find the 4.7 or even the 2.2 on its own sounds better (higher frequency at which treble roll off will start) . You can use any tone pot, but I think 100k gives you a nice even spread, and your piezo is so large that it is not really affecting bass. Also its affect on bass is protected by the tone cap and the 47k.

On my guitar piezo, my disc is a bit smaller, and I still needed 150k across it to reduce sub-sonic thumps.

If you nave any spare pots lying around, you could use one for the 47k resistor, and so have the ability to tweak it, even if it is hidden inside the box.

John

[newey]

So, translating John's sim schematic into a wiring diagram, I get this:



Good?

[JohnH]

newey, the tone pot will operate like a guitar tone control, so I suggest flipping the lugs to mirror them left to right to get normal operation where clockwise rotation increases treble. (assuming we are looking from the back of the pot as usual).

This will also give the most even spacing of tones with a log pot. If you look at the frequency response graph, the upper traces with max treble are bunched up, corresponding to 10% increments of a linear pot. Thats OK, but a log pot will be better.

Also, the 47k to go to the centre of the piezo please!

(and a wire to connect to the hot output)

John

[ashcatlt]

John, I'm noticing that you didn't include the cable capacitance in that model there. Is there something special about the piezo that makes it so we can ignore it? Is this because it's too small to be significant in this case?

[JohnH]

ash - thats a fair cop! probably should have had some. A rerun with about 600pF across the output, for a 15 to 20' cable adds an overall rolloff from about 8kHz upwards

[newey]

Aha! A wire to the output indeed!



Other fixes made as well.

[roosterman]

Im a new guy here (hi!) so I hope you wont mind me jumping on this thread.

Iv made loads of stomp boxes, and usually use piezos encased in hardwood for the pickup. The rest of the space I pack out with foam padding.

Lately Iv been experimenting with mag pups and speaker cones too. Speaker cones work really well as pickups, and give a good loud 'thump', though Im yet to get a good noise from a mag (plenty folk do, just not me).

Im now on with a stomper with tone and volume pots, though its causing me a headache. Iv never had much success with piezos and tone pots, and this is no exception. I figured Id have to use a resistor to reduce the signal bewteen the vol and tone pot, but it doesnt seem to work.

Im using a standard wiring pattern as per mag pups, but with the above mod. I tried a .47 cap too, but to no avail - is this because I needed a .0047 instead? I will of course now try a .0047 and .0022 in parallel as per the diagram... heres hoping.
Pots on mine are 500k by the way.
Cheers

[JohnH]

Hi RM, and welcome to GN2. If you have a look at reply 38 on page 3 of this thread, theres an idea for newey's box. The best values would depend on your piezo though

J

[roosterman]

Thanks John

Yep I had a read right through but (bad news), Iv tried the above circuit this morning and I get no audible tone differential. I tried swapping the 47k too - the thinking being that the ratio between the pot and resistor is the important bit. I tried a 33, 22, 15 etc, but still no joy.

Im sure there is a way, but with my limited knowledge and limited time on this one, Im going plan B. 2 piezos, one directly on the stomper lid, the other at the back end packed in foam, each with its own volume - they should be noticably different but with the independant vols they it should be possible to mix the sound fairly succesfully.

Il be keeping an eye on the thread - lets hope someone cracks it )

[JohnH]

OK, well thanks for trying it. With that circuit, if it is not noticeable as drawn, I think my next step would be to increase the size of the capacitor from 6.8nF up to say 22nF (ie, a common guitar tone cap). At some point, that cap, plus the 47k resistor must start making an effective treble cut filter.

But I also think your idea of adding a second, more muffled piezo and mixing the sounds is a good one

cheers

John

[newey]

Don't know how I missed this thread yesterday. But I, too, had no luck wiring the tone control as shown above. I suspected the cap to be bad- I had culled the parts box so I don't really know if it was a viable one. After some troubleshooting failed to produce results, I went back to just a simple volume control.

As I noted earlier, I tried 2 piezos together (tried both series and parallel). I didn't think the 2 were any better than the single one. Although I didn't try separate controls, that may be worthwhile to try.

I, too, pack the box with foam, that was the best dampener I found after much experimentation.

For practice use, the pots aren't really needed on this, but might be useful in a stage environment. I'm using mine through a 40 watt SS bass amp, with the treble and mid EQs at 0, and the bass at about 5-6. That seems to give the best kick-drum like thump- as you add mid or treble, you get more of a knock-at-the-door sound (which is a useful sound for more acoustic, folky-type stuff.)

[sumgai]

r-man,

Hi, and to the NutzHouse!


Let me set you on the right track here:

Jul 1, 2010 13:01:07 GMT -5 roosterman said:

I figured Id have to use a resistor to reduce the signal bewteen the vol and tone pot, but it doesnt seem to work.

In the most simple terms, a single resistor in series with another device is a good way to reduce the current flowing round the circuit. Voltage is affected, thanks to Mr. Ohm, but not to the extent that you're looking for, so......

I tried swapping the 47k too - the thinking being that the ratio between the pot and resistor is the important bit. I tried a 33, 22, 15 etc, but still no joy.

You've got the right idea, but you went the wrong way. Yes, it is a ratio thing, meaning that when two resistors are in series, and a tap is taken at the connecting point, you now have a voltage divider. What you did was to decrease the amount of voltage reduction at the tapped point - you should have increased the resistor value, which would then reduce the apparent signal at the tapped point. Going upwards from 47KΩ, such as 82KΩ or 100KΩ would have given you a more noticible drop in signal level.

The tone control seems to be worthless......

That would be explained by the fact that the piezo is a capacitive device, and just as it presents a neglible load to an amplifier when used as a tweeter, so too is the impedance as a signal source extremely high. In other words, you have a source that has a very high output impedance that wants to see a matching or near-matching impedance as a load, and that's not happening here. Two things happen at this point - going high impedance into low is the better way to transfer current, whereas this is a voltage source, and the impedances should be reversed; and as it happens, this mismatch also has the dubious distinction of damping lower frequencies, sometimes by a lot. Which goes to explain why a small pre-amp of some sort is needed - it's meant to get the impedance down to something useful, which in turn will seemingly "recover" the lower frequencies that the piezo sensed but couldn't pass on to the outside world. Refer to any of JohnH's pre-amp schematics, found in various places on this Forum. (For this kind of amp, J-FET's are much better than regular junction transistors.)

Im using a standard wiring pattern as per mag pups, but with the above mod. I tried a .47 cap too, but to no avail - is this because I needed a .0047 instead? I will of course now try a .0047 and .0022 in parallel as per the diagram... heres hoping.
Pots on mine are 500k by the way.

Try fixing the above problems first, then I think this issue will also be resolved. The hint contained herein is that the piezo is not an inductor, and shouldn't be treated as such.

HTH



sumgai

[roosterman]

Brilliant! Thanks for all the info - Il definately try again, as soon as my patience returns....

I have used preamps and Eqs with piezos in the past with great success, but I kinda prefer having knobs to twiddle Not that I ever twiddle them mind, but they look better too.

For the stomper I just built, I ended up using a piezo (with its own volume) and a speaker cone (again with its own volume). The speaker on its own gave a really bassy kick, while the piezo gave the aformentioned 'knock-at-the-door'.

Thanks again chaps - will keep you posted

[ashcatlt]

Just wanted to say that I think the speaker thing roosterman's talking about is probably the ultimate answer here.

[ashcatlt]

Hey newey, I found your thread!

Doesn't look like we ever came to this particular idea. Considering what we do around here, I think we really dropped the ball.

[newey]

That's got it! Exactly the desired tone, and the guy's playing is perfect for this sort of thing- and mean a$$ to boot. Wish my slide work was 0.10 as good as his.

This thread started with Seasick Steve, with his "Mississippi Drum Machine" (on British TV no less . . .), and this guy's using a MS license plate from Clarksdale's county- so I guess it's come full circle in a way.

This is a good deal more woodworking than I did, but the results speak for themselves, and it'd be sturdy enough for a touring situation, which I doubt mine would stand up to.

I have had a company car for the past 10 years, and so I haven't bought my own plates in a while- but I kept my last set from my 2003 Honda. As a die-hard racing fan, I had the Ohio vanity plate "GRN FLAG"- which is still hanging on my garage wall and would be perfect for this. I also have some extra P-bass pickups from my bass build . . .

[ashcatlt]

I'd imagine it would resist feedback a bit better than a wooden box, too. Maybe I'm wrong on that, but it just doesn't seem like the license plate is as likely to find a single resonant frequency. Of course, any pickup will work in this. In fact, I'm thinking if you want maximum thump without a whole lot of top end, a real overwound HB would be the best you could get. You can get as Nutz as you want with this, really. Any and all wiring schemes for guitar would work in this thing, though the results might not be quite as predictable.