More Cockpit Speakers

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Back to Home ... Projects ... Stereo Page ... s/v Stella Blue home

Years ago I added these Bose speakers in the cockpit. It isn't the best location,
but the holes were already there.

They've been great, but when sailing they don't work too well. The problem is that the mid range and higher frequencies are down by the helmsperson's feet, and don't bounce up to the ears. So if you're at the helm, all you can hear is bass. If you're forward, under the dodger, they sound perfect. The low frequencies come across great everywhere, of course.

So I decided to add small Polyplanar speakers under the dodger, so I can
blast tunes when singlehanding.

The challenge was to filter the bass out, so that I don't blow the speakers
when the stereo's cranked up.
Sometimes it's fun to crank the stereo.
Especially when single handing.

This led to a bunch of reading.
I learned about Impedence,
and high/low pass filters.
It turns out that if you connect speakers in parallel, the total impedence is cut in half. So if you have two sets of 4 Ohm speakers, and connect them in parallel, it will put a 2 Ohm load on the amplifier, which for some reason doubles the effective wattage. This will cause the amplifier to overheat and stress components and eventually blow up.

It is not intuitive to me, but I don't want to become an audio engineer,
so I'm taking it all on faith.

Using Ancor step-down butt connectors, I spliced a second speaker wire run into the main speaker wires, setting up a parallel connection. This seemed easier than running additional wires all the way back to the stereo.
It's all covered in good heat shrink. (Yes, it looks really funny.)

If I have a problem, I'll have to run new wire all the way back and add a second small amplifier to the stereo. I'm trying to avoid that.
On a sailboat I really don't want to create a stereo system
that sucks a lot of DC power. So I'll try this first.

There are different kinds of capacitors, but "Electrolytic" is the kind that works for stereos.
At least, that's what it said on the front of the drawer at Radio Shack. They're so cheap, I bought a pair of many different values to try them out at the boat, to see what sounded best.

I started out with 470pF.
According to the table, this should have filtered out everything out
from about 200Hz down.

Instead, it made them sound like little tweeters. On to the next pair.
This doesn't match the table, and I quickly decided to just try them all
and see what sounded right.

I kept working my way down to smaller and smaller capacitors.

Someone sent me an email stating that speaker capacitors should be unpolarized. Bummer. Radio Shack carries those, and maybe I'll need to go back
and get unpolarized ones.

Not that I know the difference between polarized and unpolarized,
or why one is better for speakers.

All I want to do is crank the stereo, without sucking major amps
from my house bank
or blowing out the speakers..

My Sony stereo says it can handle speakers between 4-8 Ohm impedence.
The new little Poly Planar speakers have 4 Ohm impedence.
The Bose speakers have "4-8" Ohm impedence.
That doesn't make sense to me, as I thought speaker load would be a fixed value.

I also found a copyrighted article and a table that someone scanned in showing the effect of various capacitors when used as a high-pass filter. This seemed to make sense to me, but again I don't truly understand. Looking at the table, it seems to me that a higher number on the capacitor will set the crossover frequency lower.
But it didn't seem to work out that way when I got into it.

The whole impedence question has me totally confused. How can my Bose speakers have an impedence of 4 to 8 Ohms?
It seems like it should be 4 or 8, but not both. Perhaps that means that they can handle an amp designed for that range. Or, perhaps what it means is that they'll sound great with a total system impedence within that range.
Well, I think my total system impedence is 2.6, and the Bose speakers sound great.
But... it would be helpful to have more information.

I found a really cool impedence calculator at http://www.duncanamps.com/technical/impedance.html
That link is a spreadsheet that calculates impedence for various configurations. According to the calculator, I'm screwing up and stressing my amplifier. Oh Well. I don't want to install a separate power amp in my boat.
It would be dumb to have a stereo that sucks up DC amps from my house bank.
If I blow up the stereo, I'll get another one. Shucks, they're cheaper than cat food these days.

After working through eight sets of capacitors,
it seemed like the tiniest ones worked best.

Gee, when I went to a nearby car stereo place
(which has been there for 20 years) and talked to the owner,
he said he doesn't do this stuff any more and that
using capacitors to build DIY filters is "old school."
But he did grin and tell me to get a 10pF capacitor
from Radio Shack. Lo and behold, the 10pF capacitor
was the one with the best sound. Go figure.
You know, it's hard to find someone who actually knows what he or she is talking about, and I should have trusted this guy. But I had become jaded, and didn't trust him, so spent $20 on capacitors that didn't work, just to test. ( Oh, gosh.)

I soldered a 10pF and 22pF set onto a small breadboard,
and then soldered the speaker wires on. At the moment, I'm using the 10pF caps, but put the larger size on there in case I change my mind later.I'm still debating whether I should remake the thing with unpolarized capacitors.
Maybe next year.

I used a Blue Seas clam fitting to get the wires through the deck. Here, the hole is drilled and the balsa core reamed back. The entire void is filled with epoxy and colloidal silica, and the screws have been dipped in oil and set into the epoxy.

After it cures, I'll clean it up and drill a new hole for the wires. These clam fittings are great, but I still don't want to risk a leak into the deck.

The speakers are tightly mounted to the
steel handrails,
and touch the dodger supports. That should prevent lines getting tangled up behind them.

It took about three days
to run the wires under the headliner. Outside the boat, they're protected by 3/8" split black sheathing, so the wires don't
get ripped up or
destroyed by UV.

With these little speakers handling high and mid ranges, and the Bose speakers doing full range, the sound is great.
I'm not going to replace the capacitors with "unpolarized" ones, unless I can find a really
good reason.

As long as I was pulling wire, I also pulled a circuit up for a cockpit light.
It's inside the black plastic sheath, and sealed with heat shrink for now. One of these days I'll find a nice little cockpit light.

Okay! Ready for summertime tunes!