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mihalis 19th May 2008 04:05 AM

Putting a chipamp inside a speaker
Hi all,
I'm hoping to get some basic advice on a project I'm about to start. I have a speaker that I built in high school, but haven't really been able to use since then because, well, it's just one speaker. To make it more useful I wanted to get or make an amplifier that is compact enough to either go inside it and make it an active speaker, or fit in a box on top of it. I intend to then use this as a keyboard or laptop amp for rehearsals and performances. Given the application, my main requirements are that it be compact, relatively inexpensive, and reasonably powerful. Audiophile-level sound quality is less of a concern.

So far I think the best way to meet these requirements is the Velleman K8060 kit, along with the transformer and heatsink sold by Velleman to go with it. I also chose this in part because while I've built a couple of guitar pedals before, I haven't done anything with amplifiers or mains power, so I need my hand held to some extent.

In case no one is horrified at what I plan to do so far, I have a few major areas of curiosity or concern:

1. I've read about keeping power components and the associated wires away from amplifier stuff to avoid hum and other issues. If I put them on opposite sides of the speaker interior, they could be about 10 inches apart, or 14 if I put one on the top and one on the bottom. Does this seem sufficient? Also, how much of this is really about distance as opposed to shielding? Would I be better off putting each part inside an internal metal enclosure?

2. Sort of related to the last question, are there likely to be heat or interference issues due to putting this in an unventilated wooden box, as opposed to a somewhat ventilated metal one?

3. I'm probably going to rout out part of the wood and put in a metal panel to which I can more easily attach input jacks, volume control, etc. Does it make sense to use this as my 'chassis ground?'

I assume there must be solutions to these issues since commercial active speakers are quite common, but I've never taken any apart to see how it's handled. By the way, I'm ashamed to say I don't remember anything about the speaker, like the impedance, but it's a fairly typical home stereo style speaker, with an 8" woofer and a tweeter. Your thoughts are much appreciated.

Bone 19th May 2008 10:11 AM

As this is a high powered amplifier you'll need to have the heatsink on the outside of the speaker case. which means you'll have to cut a hole in the case to mount the transistors on the heatsink!
The Power supply for the amplifier would be better put in the bottom of the case as the transformer will be heavy.

Regards Tony

bm0rg 20th May 2008 10:24 AM

2 Attachment(s)
This any inspiration to you?

Colin 20th May 2008 08:59 PM

Have a look at the commercial gainclone building thread for some ideas on construction. PD's gainclones only seem to need an aluminium bar for heatsinking, which may make things easier. You could probably assemble the whole thing - transformer and all - in a diecast box and mount onto the back of the enclosure.

Alternatively one of the class D amps may make the requirements simpler as they seem to require little heatsinking, being so efficient. If you Google Flying Mole you'll see they do a small mono amp specifically designed to mount on the back of a speaker for active use.

Hope this gives some inspiration.

mihalis 21st May 2008 04:17 AM

Thanks everyone. It is possible to mount the heatsink as described, but probably not ideal, as it might require removing as much as 25-30% of the back face of the speaker enclosure. I haven't looked into Class D in detail, but right now the PD gainclone is looking like the way to go. I had previously been intimidated by trying to figure out the choice of transformer and the heatsink/enclosure, but the thread you linked to has good advice on both.

The main area I'm still confused about is what I can get away with in terms of placement of the transformer and the circuit boards. Can you put both inside a small enclosure? Or is it just something one needs to experiment with?

Colin 21st May 2008 10:54 AM

PMC Monitors put the amp on the outside (Flying Mole in this case) - download the brochure for pics. This gives you more flexibility, daves hacking the enclosure about and keeps potentially vibration-sensitive components out of the enclosure. If you pick the right metal box (a low, wide diecast Hammond box would be ideal) you may be able to use the box as the heatsink, bolt everything inside (inc the transformer - see PD's integrated on his website) and you'd have a robust amp enclosure which wasn't too obtrusive.

mihalis 22nd May 2008 04:10 AM

Thanks Colin. I agree that putting the amp on the outside is looking like a sensible approach. I hadn't thought of how vibration might affect the components. I tried to investigate some of the Class D possibilities, but I'm left with a couple of questions.

[1] Does Class D offer a significant advantage over PD's LM3875 kit in terms of space and heat? His integrated amplifier after all is using an LM3875 and has a pretty small heatsink.

[2] Do you have suggestions on how to obtain a reasonably priced Class D amplifier or kit? The ones I have found prices on so far are quite high-powered and high-priced--I was hoping to spend $150 or less on this, since I only need about 60W of mono and I'm not aiming for pristine quality. And I haven't found anyone selling Flying Mole's OEM products, presumably because they are marketing them to manufacturers and not retail consumers.

Thanks again!

Colin 22nd May 2008 09:28 AM

The class D forum will lead you to links for kits. Flying Mole only do commercial units, as far as I know, but I only mention them as an example. There are several Class D kits out there - this one seems to get a lot of mention. I believe some of the class D kits use surface mount components which my soldering skills would certainly struggle to complete.

The advantage of class D is efficiency, which means they don't use much power when left on all the time. But I think the gainclone is much the same and I've read a few posts (Peter Daniel for one) to say they prefer the sound of the GC. I've used a t-amp and am currently playing with a Teac tripath amp. They're both very clean sounding but perhaps a little cold. Given the simplicity of the GC, that's the route I'd take.

If you search Peter's massive thread he does show an example of a tiny Gainclone chassis he built for fitting into a loudspeaker, complete with a pic.

mihalis 29th May 2008 08:33 AM

Thanks a lot for your advice Colin. I don't think I could handle any surface mount soldering either, so I went with Peter's LM3875 kit. It does indeed seem like the simplest option, and there's so much support for it in this community. My plan is to get the kit assembled first, and then figure out the chassis. As a novice there are too many variables to try to get it completely sorted out ahead of time. I'll keep you posted on my progress.

Colin 29th May 2008 11:32 AM

Here is another one for inspiration. Canford are professional broadcast so there are a couple of amps on this site designed to be bolted onto the back of a speaker.

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