musical thoughts

This was originally posted on the SuperSite forum (but I am the author and it’s my mod). It originally took the form of several posts talking about the idea and then the execution of the idea. I’ve tried to present it here in more of an article format.

I’m intrigued by dnarkosis’s writeup on the bright kit mod.

It sure looks like a couple inexpensive piezo tweeters wired in parallel to the big drivers. I just blundered into a dead cheap source for the piezo tweeters on the popular auction site. It strikes me that with a couple vampire taps you could inexpensively replicate the bright kit mod.

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I have ordered the Motorola Piezos, four pieces cost around 30 bucks at my door. I did some more reading and have made some design decisions.

One question is whether to adopt the simple crossover-free strategy the original bright kit mod uses, or try to cross them over. I decided to keep it simple and not cross them over for two reasons. First, I’d like it if the mod could be repeated, so keeping it easy serves that goal. Second, I invested in the better piezos so i expect them to exhibit better power handling than cheap knockoffs would.

Here’s my diagram, and I think it’s identical to the bright kit mod. Mod additions are in the blue box.

Note: I changed to parallel piezos rather than series after installing them.

It seems like a cutout switch would be handy to restore the original tone profile. Just a DPST switch would do it, and it would be cool to put it down by the pedal so you could kick the bright mod in.

I’m also exploring whether I can mount the piezos coaxially with the 12″ drivers, and avoid drilling new holes. That could make this a potentially entirely reversible mod. My Rhodes is all original, so I’m at least acting like I don’t want to mod it for now (I previously did major mods on a Stage 73, say 20 years ago).

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Added: I was able to dig up a very worthwhile app note about these drivers.

This thing explains why you can cross over if you want, but you don’t really have to.

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Okay, here’s the walkthrough with pics.

Bill of Materials:
4 Motorola KSN1005 piezo tweeters ($30 including shipping, ebay)
4 3M Quick Connects ($4 for 16pack, found at Walmart)
2 power resistors ($2 average, I used ones on hand 15W 12ohm)
18 gauge hookup wire ($4.50 for a spool of more than I needed)
solder

Tools Needed:
Soldering Iron
pliers
Screwdriver to open up the suitcase
3″ hole saw (could use a router, rotozip, or jigsaw)

I chose to proceed with wiring mine as shown in the diagram:

Note: I changed to parallel piezos rather than series after installing them.

1. I arranged my piezos into two pairs, and soldered the resistor to the left terminal of the left on in each pair. The resistor can go anywhere in the circuit and have the same effect. I used a 12 ohm, 15W power resistor, but you can use any power resistor rated 2W or above with a value from 10-50 ohms. (See the note at the end about why you want a resistor.) I soldered the resistor connections.

2. My hookup wire came as 2-conductor “zip cord” color-coded red and black. I cut two two-foot lengths, separated about 6 inches of one end and stripped the insulation off the last half inch or so of each wire. The other end of these leads was left alone–you want the full insulation on to use the quick connects. Taking care to maintain consistent polarity, I soldered these hookup wire pairs to the outer set of terminals for each pair of piezos.

3. I cut another two-foot length of hookup wire and separated it fully into two single wires. I stripped the ends, and soldered it to the inner terminals of each pair of piezos.

At this point I had two completed assemblies, each of which is two piezos in series with a resistor, as depicted in the schematic above.

4. I opened up the suitcase from the back side (opposite the keys) and gently lowered the panel to the floor. There’s enough slack in the speaker wires to allow this. Then I made the hookups using the quick connects. Each assembly takes two quick connects, one per wire. The 12″ speakers in the suitcase are wired in parallel, two per amp channel (left and right) I placed a quick connect onto the black wire of the left pair, so that the black wire passed through the connector. I attached the black wire from the piezo assembly so the wire entered the opening in the quick connect. With pliers I crimped the metal blade down, making the connection (instructions are on the package, but test this on some extra wire if you have doubts). I then engaged the other three quick connects in a similar manner, taking care to observe polarity (black wires to black, red wires to white). you can tap either pair of wires, everything is in parallel across the amp outputs.

(Sorry, no pic here, I was too excited to hear my mod in action!)

I powered up the amp and played a little bit, and heard some sound coming from the piezos. The electrical connections were working properly.

5. Using the hole saw, I drilled four, three-inch holes, placing each piezo in a vertical line with the 12″ driver nearest. One assembly was the left, and one the right side. Each assembly had a piezo in the front and back of the suitcase. I used 1/2″ wood screws to attach the piezos to the panel.

I was worried about shredding my grille cloth with the hole saw. It turns out there’s a gap behind the cloth, so if you are cautious, you can drill your holes perfectly round with no damage to the grille cloth. (this was an unexpected bonus–I was resigned to replace my grille cloth which smelled of smoke anyway)

There’s lots of slack in my wires, I figured better to leave them long. You can trim yours if you choose. I got really lucky on clearing the amp–you should probably offset the right piezo in the back panel so you avoid a collision with the power amp.

At this point I put everything back together, cleaned up, and rocked out for a while. Total time to install was under two hours, including time spent on photos. Total cost just over 40 bucks, I owned all the necessary tools already. Conservatively figure 45 dollars in parts.

***tech note: what the hell is the resistor for?
It is not there to affect the sound, I can quantitatively determine that the piezos look roughly like a 500 ohm load over the frequency range of interest, so tacking another few tens of ohms on is not changing the output level. So can you omit the resistor? No. A piezo looks like a capacitor to the amp output, and some amplifiers (I have no opinion on whether the Suitcase amp is one of these) will go into oscillation (this is bad) if the load is too capacitive. So the resistor is there as cheap insurance against hosing your amp, not for any reason to do with the sound. The value is non-critical.
Reference: http://linear1.org/i/piezo.pdf

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So, how does it sound?

It’s a definite improvement. It is actually a fairly subtle change though. I was expecting face-melting treble, but what I got was improved clarity in the high end. I have a concentric knob preamp, and the effect of turning treble up to 3 o’clock exceeds the effect of the mod.

However…

There’s a qualitative difference between the treble boost you get from the piezo and from the tone control. The piezo enhancement is in no way harsh. It’s actually only getting a very small amount of signal energy from the amp, because the low end of its frequency response is 4kHz, and there is really very little content at that band and above. To convince myself of that, I did a spectrum analysis of a bit of playing recorded direct off the satellite outs:

By the time you get to 4kHz, the signal is -64dB (again, this is an attempt to characterize the source signal driving the suitcase amp, not the speaker system’s response). There’s just not a lot of content above 4kHz. The piezos just add presence. They definitely sharpened the stereo tremolo effect to my ear. They did nothing to alleviate the murky midrange that typifies a suitcase.

For me the sweet spot for the treble knob after the mod was about 1 o’clock, where I had a tendency previously to set it to the maximum (like the rest of suitcase owners, as dnarkosis’s page trenchantly observes). With the treble set to 12 o’clock, I had a hard time perceiving any effect of the piezos.

I may rewire these all in parallel to attempt to coax more sound out. It’s worth noting, you can cram more than four in there with no ill effects at all. These do not load the amp the way a moving coil does.

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I converted to parallel tonight. Each piezo is now in parallel, exactly like the kit. As expected, it raised the treble appreciably.

The tone controls can be left flat and you get a pretty pleasing tone. I still prefer a taste of treble boost. I don’t favor a dyno tone, but I do like it bright.