DIY Bright mod

[Rob A]

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 (here). It strikes me that with a couple vampire taps you could replicate the bright kit mod.

I'm not sure what the rationale is for the resistors in the bright kit. If anyone wants to enlighten me, I'd like to hear it. I can't tell from the photo if they are in series or parallel with the driver. I don't understand why two would have it and two would not. Is it two drivers + 1 resistor in series, parallel tapped into each side of the amp? What value resistor?

Added:
http://members.misty.com/don/pzfix.html
It looks like this is the rationale for a series resistor. this page also lends some credence to adding them with no crossover. I like it; this looks doable. I may even have the resistors on hand already.
Additional piezo+resistor info here:
http://colomar.com/Shavano/crossover6db.html

Added 2: The tweeters would likely work even better if they weren't aimed at your kneecaps. Maybe an outboard enclosure makes more sense than mounting them in the suitcase bottom.

Added 3: Even if you insist on gen-u-ine Motorola tweeters, you can make out okay on ebay. If I do this I'll probably spend the little bit extra for the good piezos.

[Rob A]

Okay, I talked myself into proceeding.

I have ordered the Motorola Piezos, 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.


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).

[bjammerz]

I'm looking at a way to brighten up the ol suitcase as well, keep us posted!

[dnarkosis]

Just off the top of my head (I don't remember exactly), I think the Speakeasy tweeters are essentially the same ones as Radio Shack carries (or at least did carry). I checked out the ones from Radio Shack because I, too, was thinking about doing it myself, but I'm not really an electronics guy, didn't understand the resistors, and just generally didn't feel up to it. I was willing to pay someone else to have done the homework and wiring etc.

So I, too, will be intrigued by what you come up with. If you'd like, we can add your experiences to the tweeter blog.

[Rob A]

I'll keep this thread as a worklog.

I scrutinized your pictures intensely to come up with my diagram, dnarkosis. I'm also pretty sure that the Speakeasy kit is using "high-quality" piezos rather than knockoffs.

I refuse to buy anything at Radio S*ack. For whatever that's worth.

[dnarkosis]

I'm not sure what the rationale is for the resistors in the bright kit. If anyone wants to enlighten me, I'd like to hear it. I can't tell from the photo if they are in series or parallel with the driver

Would it help if I sent you the original high-rez version of the installation in which you can see both speakers and both tweeters after the wiring has been hooked up? Just let me know.

I don't understand why two would have it and two would not. Is it two drivers + 1 resistor in series, parallel tapped into each side of the amp?

The only thing Stever at Speakeasy told me was (I think I mention this in the blog) that it "balances the load."

[Rob A]

I wouldn't mind looking at the sheet, thanks.

Added: I was able to dig up a very worthwhile app note about these drivers.
http://linear1.org/i/piezo.pdf

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

Independent of how Speakeasy designed their kit, I think I want the configuration in my diagram for the reasons previously stated.

[dnarkosis]

I wouldn't mind looking at the sheet, thanks

I meant the hi-rez versions of the jpg photos on my blog -

If you're thinking of diagrams from Speakeasy: there weren't any, just a rough outline of how the hookup went and a rather confusing set of instructions (does it matter whether the resistors go on the audience side or the player's side? difficult to determine from their instructions; I had to call them).

The original jpgs are probably too large to post here, but I'd be glad to send one (or all) to you by email.

[Rob A]

I'm with you now--I would like to take a peek, I think they used Motorola piezos.

[Rob A]

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:


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

[Rob A]

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.

[bjammerz]

fantastic rob, thanks for all the exploration. Thinking about doing it to mine now!

[dnarkosis]

Yes, excellent explanation, excellent pix, excellent evaluation --

Honest question for us non-electronics types:

I may rewire these all in parallel to attempt to coax more sound out.

Why might the parallel wiring make a difference? And what is that difference?

When you say "coax more sound out," do you mean more gain or better frequency response or both?

[Rob A]

Great question.

I deliberately chose the series combination of two piezos in order to lower the output a little bit. I was under the impression that I would make a large increase in the treble response with this mod and I wanted to moderate that somewhat. My perceptions may have been influenced by using cheap monitors with piezo tweeters in the past, and getting obnoxious amounts of treble.

If you look at the signal in terms of power, almost all of it is going to the 12" driver. A tiny amount of power is going to get delivered to the piezos, no matter how they are connected. Another way to say that is their impedance is very high. And another way to say that is that they are a small load on the amplifier.

So recalling that all circuit elements connected in parallel have the same voltage across them (by definition), my series combination of two tweeters will divide that driving voltage equally. A setup with parallel piezos, each would get the full drive voltage.

[dnarkosis]

Rob:

Many thanks. Makes sense.

I notice that the new Rhodes has 2 Horns + 4x12" Cones + Crossovers -- I thought I saw tweeters in some of hte NAMM pix, but maybe I'm mistaken.

I'm wondering about the crossovers -- how much difference that will make in this whole frequency and voltage issue.