Faking the mini

[DarwinStrings]

A guy who bought one of my guitars("Rose Cliche") asked me to install a K & K Pure Mini, He bought it from our one and only music shop at about $240.

I thought it would be a good opportunity to make a faker pure mini and put it in the guitar I play which is similar to his and compare the El Cheapo to the real McCoy through the same pre amp and amp.

I bought 20 discs for three bucks and some potting epoxy for eleven so it looks right but am chomping at the bit to be able to do a side by side comparison. Will reply here with a opinion once I am done.

fake pure mini.jpg (64.73 KiB) Viewed 20546 times

[garryalb]

This should be an interesting test, I have wondered if there is anything special in the discs. May I suggest that shielded cable will be mandatory, a break in the shielding between a piezo and pre amp / amp will introduce heaps of noise.

Garry.

[DarwinStrings]

I did a bit of reading up on them but it was some time ago. Now I may have this wrong cause memory isn't so reliable but some of the stuff I read seemed to indicate that a piezo disk is a piezo disk and as a simple non moving parts device it is hard to improve on. I also read that shielding on these wasn't needed like it is with a coil/magnet PU and although I have never cut the wire on a K & K it doesn't look like they are shielded, like I said I could have it wrong so if anyone has any info I'd like to hear it.

[DarwinStrings]

I should add that I had assumed that shielding would be needed and I do have shielded wire so maybe I should re-make it with shielding.

[Trevor Gore]

...although I have never cut the wire on a K & K it doesn't look like they are shielded, like I said I could have it wrong so if anyone has any info I'd like to hear it.

The K&Ks are shielded.

I also read that shielding on these wasn't needed like it is with a coil/magnet PU...

The traditional piezo is a charge developing device, i.e. it generates voltage rather than current, so typically needs to run into a very high input impedance or else the charge is lost before enough signal is generated to drive anything. High input impedance devices with wires hanging off the input tend to act like a radio receiver and pick up ambient electromagnetic signals/noise.

New piezo materials (like whatever the K&Ks are made from) are very high output, to the extent that I need to reduce the input impedance to balance the sound when they're fitted to my guitars. They have enough output to drive the line input of my audio amp and produce a really good sound (that's the NAD amp and the RLH speakers). The line input impedance is ~ 10k ohm iirc.

So I guess the message is that there's a variety of piezo materials out there. They can probably all be made to work, but they will need different treatments.

These guys are doing a K&K knock-off. They pot them in a different way to K&K which may help reduce the bass response a bit which is normally too strong when they're fitted to my guitars. More here. I haven't used them (yet).

[kiwigeo]

Like any product I factor in after sales service and warranty when evaluating the price. I had reason to ask a few questions via email to K and K on one occasion...the emails were answered within a day and the service was excellent.

[DarwinStrings]

You have a way with words Trevor.

So make it again with shielded wire, trouble is the shielded wire I have(the smallest I could find) is heavier than what I used and I had considered that. The wire is thin but the flex is thick giving it the extra weight.

Okay so you reckon that when it come to these discs the ceramic may differ from disc to disc making for higher or lower output.

The last time I used cheap discs I used wire without a shield and when plugged straight into a electric amp the noise was low, certainly tolerable.

So thanks for the heads up guys I will go borrow that guitar(with the cheap un-shielded discs) back and see how it sounds using the K & K Pure XLR preamp with the same amp and decide if I need to further buggerise around to get a reasonable plugged in sound.

Oh and Trevor I had wondered why they needed to be potted for any other reason other than to slow or stop corrosion?

[DarwinStrings]

Yep Martin, I recommend to customers that they buy the K & K thought as far as the forums are concerned the JJB work as well but for myself I like to muck about and see what I get or I would just buy the K & K.

[Trevor Gore]

Trevor I had wondered why they needed to be potted for any other reason other than to slow or stop corrosion?

I'm speculating here, because I've never tried to design a piezo pickup. But like the sound is affected by how you glue the things on (double tape vs. super glue, for example) the potting is likely to have an effect on the stiffness of the disc (and therefore its sensitivity as per the link in my previous post), and the stiffness together with the mass will have an effect on the frequency response and no doubt there will be an effect on the damping, too. So basically much like a guitar top in miniature. Like all electromechanical devices the designer has to consider both the mechanical characteristics and the electrical characteristics. For me, I'd like a pickup with less bass and more high trebles (>10kHz) that also avoids the picking transients etc, (which get worse as the frequency response goes up). It's all a trade-off, which is why some pickups sound a lot better in some guitars than others.

[DarwinStrings]

Ah yes and not only the potting, that you also CA it to the bridge plate so now you have it sandwiched between the potting epoxy and the board.

Even without the shielding these things are not very noisy. I just went and borrowed that guitar, it was my first falcate and just has 3 x 15mm unpotted discs with shield less wiring and there ain't much noise. So i plugged that PU in the photo in and it doesn't have much noise either though when I temporarily shielded it did cut the little bit of noise that was there.

[kiwigeo]

In Darwin "potting" means something a bit different:

cheech_marijuana_car.jpg (30.86 KiB) Viewed 20451 times

[Trevor Gore]

How much noise gets picked up also depends on how much there is around. On stage, with a bunch of thyristor dimmers not far away, might be a different story.

[DarwinStrings]

Hey I recognise that guy in the photo from my last trip to the Adelaide hills Martin, is he a mate of yours?

So I've just a about 20 minutes playing this guitar plugged with the two bob special PU and the K & K Pure XLR preamp. As a acoustic electric it's not too bad, a bit of knob tweaking as well as spring reverb and it sounds as good as many of the plugged in acoustics I have heard lately at a local open mic session (Cole Clarks, Matons, Lags and a variety of Asian made stuff)

[DarwinStrings]

True Trevor, won't take long to re-make it with a shield. I potted 12 discs so will play it safe.

[DarwinStrings]

Hmmm should I just shield the hot with the ground or shield both hot and ground with another ground?

[Trevor Gore]

K&K use a single core coax, so the shield is the ground/return. Probably keeps the cable capacitance (as well as the mass) down.

[DarwinStrings]

Thanks Trevor. I have some single core coax that looks a bit thicker than the K&K stuff(from memory). The flex I have is 3mm Dia but does anyone know if I can get thinner than that? My guess is the K&K is about 2.5mm maybe less so someone must have some somewhere unless it is like the rubber case seals and they have it made to suit.

[DarwinStrings]

I am curious about why you've been cutting up K&K leads Trevor. Don't you like the jacks they use?

[Trevor Gore]

The sockets are fine, but I rewire them to get two separate output channels. The standard socket is a stereo one, which is handy.

[DarwinStrings]

I like this quote from the JJB mob about their pure mini cloneish PU

"Our new line of pickups are constructed with the finest components available today. In fact, they are the same components that are used by other companies that sell their systems for more than twice as much. This system is the same exact configuration that is sold for $130.00 by a very well known company"

[Trevor Gore]

...except if you look really closely at the specs they say they're wired to a mono socket...

[garryalb]

Hi Jim,

I have this coax, it was only available from the big guys in 100M rolls. The core is 10 x .12, 1.6mm outside diameter. The other is Jaycar dual core for comparison. Don't know if it suits but will be lower mass, makes me wonder why K&K don't use something smaller.

Happy to send you some if you like,

Garry.

[DarwinStrings]

Yep they are a bit cheeky Trevor, like we know who is the "very well known company" but because they don't say K & K their components could be anything.

Cheers Gary, a couple of metres would go a long way if you can spare it. The 3mm stuff I have is the Jaycar single core and I could only find the equivalent on the net.

[jeffhigh]

Cheap RCA stereo leads work fine as a source of lightweight shielded cable in my experience

[DarwinStrings]

Good thinking Jeff, I went through the old PS2 box and found some at about 2.4mm so that's a little better than the Jaycar stuff.