Boat Stereo, high pass filter

[nathanlee]

Hi Folks,

I was going to post this to the lounge, but then I thought it's both practical, and could be of use to other boaties, so decided it should go here.

I'm in the process of building a stereo system for my new boat. Sound quality is important, so standard marine speakers cut in to a panel is a no go.

So, I have bought a pair of Cambridge Audio Min11's (they're tiny, and good), that have a designed frequency of 120hz upwards. I have also bought a tiny under seat active car sub (Pioneer TS-WX110A), which runs directly off 12v and will live in a locker. The problem is my plan has fallen flat on it's bum trying to power the Min's. I was going to use a 2ch car amp with a 120hz high pass filter. Trouble is, I can't find one, and neither can the car audio shop. Best he could do had a 130hz filter, but that would mean there's a 5hz "hole" between the sub and the min's - would that even be noticeable?

Failing that, does anybody know where I can get a (n ideally passive) 120hz high pass filter?

Sorry it's not ever so boaty, but it is a bit, and hopefully useful for anybody else looking to get some half decent noise in their boat.

Nathan

 

[lw395]

I am inclined to suggest not bothering.
The speakers will reject low frequencies fairly harmlessly, unless you are putting huge amounts of power it.

Filters are not a 'brick wall' response, a '120Hz filter' will typically have a 1dB loss at 120Hz, then progressively increase in attenuation at lower frequencies. The problem is, it will usually cause some phase shift around the transition, which may not sound right. Particularly if the two channels are not exactly matched.
Filter design is a big, dull subject, I have several books on it.
You quickly get into personal taste and subjective perceptions of sound. In general I am a fan of keeping systems simple.
If your main speakers are about 2 or 3 dB down where the sub-woofer is starting to work, that's probably a reasonable starting point IMHO.
Bearing in mind your boat is not an anechoic chamber or Opera hall.

I will refrain from making a comment about ultrasonic antifoul at this point.

 

[bedouin]

I don't think you will notice the "hole". A first order filter is going to roll off a 6 dB/octave
so you will barely hear the attenuation at 120Hz.

Anyway what matters is not the bottom of the frequency response of the Min11s but how you roll off the frequency response of the sub

 

[nathanlee]

I will refrain from making a comment about ultrasonic antifoul at this point.

I'll remember that excuse when I'm having a 2am rum and dub session, and the neighbours start banging on the hatch.

"Loud mushic? No, thasch my anti foul. Hic"

Thanks for the advice. If I can get away with no filter and no distortion, I'll be a happy (and less skint) man. Just a bit concerned about the TINY speakers trying to handle rumbling explosions on a film or similar.

 

[nathanlee]

Ooh, crikey. No, it's going to need a filter. I've just plugged one in to the head unit. They will accept full range (and in fairness do a bloody decent job to a point), but with any moderate volume they start beating themselves up.

 

[mikefleetwood]

Do you know if the "full range" speakers are 8 Ohm or 4 Ohm?

Easiest high-pass filter is just to put a capacitor in series with one wire to the speaker. If 8 Ohm, then a capacitor of 150uF in series will give you -3dB at 130Hz, which is close enough to what you want.

There are, of course, a couple of problems with doing this. First, no one makes 150uF capacitors, second, the capacitor needs to be non-polarised. However, that's easy to fix - fit two 330uF capacitors in series, wired back-to-back and that should work. Suggest RS 526-1878 at 36p each in a pack of 5 as a suitable place to start.

if the speakers are 4 Ohm you need to double the capacitance, say, 2 x 680uF in back-to-back series instead. Working voltage isn't very critical, just as long as it's more than 12V

 

[lw395]

Do you have a separate power amp to the small speakers?
If so, putting the filter before the power amp might be better.
The caps suggested only have a current rating of 1.5 amps or so, which is about 18W into 8ohm speakers.
That's fairly loud in a small space.
Some caps intended for power supply use have very wide tolerances, often allowed to have 200% of nominal capacitance, even the +/-20% of the one quoted might be an issue in theory, but in practice, a batch is likely to be closer than that.

 

[nathanlee]

Do you know if the "full range" speakers are 8 Ohm or 4 Ohm?

Speakers are 8ohm, amp will be 4ohm, because I can't find a decent 8ohm 12v amp. Does that change things ever so much. Great suggestion, by the way, thank you.


I suppose I could run the proper Cambridge amp on an inverter, but it's a terribly inefficient way to go about things, especially as I've just forked out for a full LED replacement in an effort to save energy.

 

[nathanlee]

Do you have a separate power amp to the small speakers?
If so, putting the filter before the power amp might be better.

Probably. Most likely, in fact. I haven't bought the amp yet. I'll be happy if a trip to maplin, a project box, and a sheet of breadboard is all I need to solve this.

 

[bedouin]

I have to say I don't think they are a very good choice to drive from car audio.

They have a sensitivity of 85dB and a nominal impedance of 8 ohms - that means that any car amp is really going to struggle to drive them.

 

[mikefleetwood]

Speakers are 8ohm, amp will be 4ohm, because I can't find a decent 8ohm 12v amp..

8 ohm speakers is fine. "4 ohm" output of amp is not particularly important - it just means it is designed to survive driving 4 ohm speakers (most car amps do), which would give you about twice the power output.

Domestic "hi-fi" speakers are usually lower efficency than car (or PA - like I'm listening to at the moment, with 6kW available if I turn it up!) speakers, in other words, lower dB sound level per Watt power. However, in balance to that, car speakers have to still sound good over the sound of road/air/engine noise - I assume you plan to listen under smewhat quieter conditions! Even if you fancy a proper blast, it will probably be much lower power than you expect before it gets uncomfortable - especiially in a relatively cramped space liveaboard.

 

[nathanlee]

They have a sensitivity of 85dB and a nominal impedance of 8 ohms - that means that any car amp is really going to struggle to drive them.

Not this one. http://www.caraudiocentre.co.uk/product_m-focal-foc-fpdual-monitor_p-24073.htm

Phwoa!

If only I was rich enough to waste that amount of money... I would.

After some digging, I've found a 600w amp with and adjustable hpf for £69. If it's adjustable, I can tweak it until it sounds right. Might be the easiest solution.

 

[nathanlee]

Even if you fancy a proper blast, it will probably be much lower power than you expect before it gets uncomfortable - especiially in a relatively cramped space liveaboard.

Yeah, I suspect so. I'm not one for being anti social, but there are the odd moments when, as you say, one fancies going to eleven.

totally OT, but I once went to a drum and bass night with Dillinja and Lemon D's Valve system. 96kw at the fridge in Brixton. I have never experienced anything so insanely bassy.

If you're in to PA's... here's a great 8min documentary about it. http://www.youtube.com/watch?v=zeuRN1B5T5w How they managed to build it is pretty impressive.

 

[bedouin]

Not this one. http://www.caraudiocentre.co.uk/product_m-focal-foc-fpdual-monitor_p-24073.htm

Phwoa!

If only I was rich enough to waste that amount of money... I would.

After some digging, I've found a 600w amp with and adjustable hpf for £69. If it's adjustable, I can tweak it until it sounds right. Might be the easiest solution.

In order to get that sort of power it must have internal power supplies that transform the 12V input to something significantly higher.

Without that you are pretty much limited to 30-40 W into 4 ohms or 18W into 8 ohms.

So a pair of 85dB 8 ohms speakers will give you something like 94dB maximum output - which is probably fine not over generous

 

[Richard10002]

Nathan,

Am I missing something here? Why not get something like a second hand JVC AVX44 off eBay for about £120. This will drive speakers and subwoofer, (the specs say 5.1).

I had an AVX1 on Rogue, driving 4 Kef Egg speakers, and it was awesome in the cabin, even without a subwoofer. The DVD and surround sound facility would be an added bonus.

 

[nathanlee]

In order to get that sort of power it must have internal power supplies that transform the 12V input to something significantly higher.

Without that you are pretty much limited to 30-40 W into 4 ohms or 18W into 8 ohms.

You can definitely get more than 40w out of 12v. http://www.youtube.com/watch?v=CewacsP6TSY < 13 seconds in. Rather silly, and bad music, but I love the fact that there are nutters out there that do this sort of thing.

Somebody once explained to me about the swing on 12v not allowing for higher power, but they must indeed step it up because you can definitely get some high powered 12v amps.

 

[bedouin]

You can definitely get more than 40w out of 12v. http://www.youtube.com/watch?v=CewacsP6TSY < 13 seconds in. Rather silly, and bad music, but I love the fact that there are nutters out there that do this sort of thing.

Somebody once explained to me about the swing on 12v not allowing for higher power, but they must indeed step it up because you can definitely get some high powered 12v amps.

Yes - you certainly can do it is complex/expensive and most moderately priced car audio systems don't.

In fact given reasonably sensitive speakers 12V is plenty - speakers designed for car use are 4 ohm with sensitivities normally 90dB or more. It is just that speakers designed for domestic HiFi are sometimes very inefficient - and particularly when trying to get a lot of sound out of a small driver.

 

[Ruffles]

In my experience, using normal music rather than a test signal, if you're hearing distortion at low frequencies due insufficient travel of the low frequency element then you will be overloading the speakers at higher frequencies. In other words you may find the speaker coils overheat if you filter out the bass!

In my (vague and befuddled) memory, small speakers tend to boost bass response to counter the natural 6db/octave roll off as the wavelength approaches the diameter of the element. This is either a reflex port (hole with a loo roll glued in it) or a series capacitor.

You could either insert a sock in the port or bridge the capacitor with a paperclip.

But I bet you blow them up!

 

[mikefleetwood]

Unfortunately, a bit more complicated than that.

The lowest frequency a speaker will reproduce is (mostly) a combination of two factors - the resonant frequency of the cabinet and the mechanical resonant frequency of the speaker cone. Usually the two are optimised so that the cabinet frequency is a little lower than the cone frequency, but, of course there are exceptions.

With a sealed box type cabinet, the lower limit is set by the cabinet resonance - this usually results in smoother response, so usually found in "hi-fi" speakers, but lower efficiency. Here, the power limit is set by the ability of the speaker to dissipate the heat - thermal limiting.

With base reflex cabinet (and horns) the cabinet resonance is set lower than the speaker resonance, to get improved low frequency output. In order to get a smooth response, the speaker and cabinet need to be carefully matched. They usually offer slightly higher efficiency, but care must be taken in regard to the cone travel at lower frequencies - giving mechancal limiting below resonance and thermal limiting above.

Either way, putting a sock in the port won't help much and there won't be a capacitor in series to short out - if there is, it's there to limit the low frequencies, so shorting it out would be counter productive!

Depending on your budget, it could be worth shopping around for small hi-fi speakers, but it will probably have to be a trade-off between efficiency and low frequency performance.