Hello and some Questions.

[westers151]

Firstly I'm new here, so Hi. Secondly, I'm very new to Diy HiFi and electronics in general. However, I can follow instructions and How To's so whilst I might not understand why a circuit and component is used, I can follow an online build/instruction about what part goes where.

Anyway, a few weeks ago I stumbled across the chipamp design and started to realise that here is a good first build of a DIY amp that is both simple and cheap (compared to buying commercial). And from all accounts it sounds considerably better than a lot of commercial amps. Will it really take on and match amps costing £1000's?

Now for some more serious questions: Heat and getting rid of it. I understand the concept of heat sinks, but have no idea as to how big a heatsink to use for th Gainclone, nor how hot it will run. I've read a lot about it under normal loads it's not too bad, but what is "normal load" and what is "not too bad"?

I've also seen a lot of people use the metal casing as an extra conduit for the heat, but I'm not a fan of the normal casings you can get - unless you get something purpose built out of billet aluminium (very costly), the metal enclosures are ugly.

So that got me thinking about how I could make a case that for me is more attractive, and I've arrived at the possibility of cases out of fibreglass. I can make a mould and then lay up the glass so it's thick/strong, but I'm now concerned that the fibreglass will act too much as an insulator and trap the heat that the heatsink is giving off and so make things worse. To counter that I've come up with a plan to ventilate the top of the case via a system of "chimneys" (open tubes from the top of the case to the heat sink - I could have just drilled holes, but I wanted to make them look a bit more attractive), but again I'm not sure if this will get rid of enough heat? Finally I could drill holes into the backplate of the case (it would be hidden from view) to allow a little more air to circulate.

So, does anyone have any views on this? Ideally I want a rectangular case that is about 10 * 6 * 3.5 inches in dimension. I'd like to get the transformer into that as well, as well as a pot, but is that going to make it too crammed for a decent sized heatsink (whatever decent sized means - some help would be good here).

If I have to I'll build a seperate case for the transformer (I take it these don't get too hot as I don't see much in the way of cooling for them).

First of many questions I think. Now off to start reading my introduction to electronics book !!

Thanks for the help.

Ian

[Nuuk]

Welcome to the forum, and to the hobby.

Tell us what speakers you intend to drive and we can have a stab at estimating heatsink size for you. If the speakers are an easy load, you can get away with a smallish heatsink and you don't need to worry too much about the fibreglass case retaining a lot of heat. That said could you not work in some sort of ventilation grilles?

If you haven't already found it, may I suggest that you have a good look at the Gainclone pages at Decibel Dungeon, including the Gallery where you may pick up some inspiration.

[fooboo]

Hi

Ive only been at this a week or so. Decibel Dungeon is highly recommended as a place to learn plenty Theres a good bunch of chaps here as well of course who have been very patient with my questions and of course the knowledge base herein is pretty extensive! Good luck and above all have fun

regards

Fooboo

[wakibaki]

Originally posted by westers151 Will it really take on and match amps costing £1000's?

Yes. Active loudspeakers designed by one of the foremost speaker experts use LM3886s. See http://www.linkwitzlab.com/ Look at the Pluto design.

Amplifiers are so good now that they're interesting only from the point of view of their cheapness or efficiency. Lots of people like designing (or buying) amps, but it's about as meaningful an exercise now as onanism.

Originally posted by westers151 have no idea as to how big a heatsink to use for th Gainclone, nor how hot it will run ... that got me thinking about how I could make a case that for me is more attractive, and I've arrived at the possibility of cases out of fibreglass

Ah, now you're talking engineering. You can get the anticipated dissipation of your chosen chip from the datasheet, and calculate how many Watts/Degree C your heatsink needs to have given the ambient and the maximum temperature rise you want to see. Most people in amateur applications prefer to overestimate, since it's simpler and less stressful on the brain and other components.

Steer clear of unconventional materials on your first build. There's no reason you can't use fibreglass, but how many fibreglass amplifiers do you see being sold? Stick with aluminium, with wood and steel (in appropriate places) being second choices. With an aluminium chassis you can often make your heatsink integral with the box, this is a sensible use of materials.

w

[john blackburn]

Fibreglass could get a bit aromatic when warm and it's horrible to work with. If you want an illustration, try doing a clutch on a Reliant Robin, you will be scratching your arms to blood for days.

I've got LM3875s on a pair of these heatsinks in a poorly ventilated steel chassis......

http://www.madaboutsound.com/prod_de...7&sa=cf&ctid=7

With the volume at 9 to 10 o'clock on a 22v transformer into CSS FR125s it gets faintly warm after a couple of hours, and stays at that temperature for days. They should be well up to the job if you have suitable speakers.

Here's a non metal case design using similar heatsinks.

http://www.vikash.info/audio/tatca/images.asp

John

[danielwritesbac]

Perhaps directly underneath the heatsink is a good spot for air inlet?

[Nuuk]

Originally posted by danielwritesbac
Perhaps directly underneath the heatsink is a good spot for air inlet?

Yes making a chimney to encourage convection is always a good idea. Even though the fins of a CPU heatsink are too close together to be ideal, even that will work with some added convection.

Here's an example showing the apertures in the base of the case, and, in the second picture, in the top of the case with the heatsink visible.

[zebra100]

Hi Ian,

I have a dual mono LM3875 running on a pair of 18-0-18v transformers. The chips are mounted on the rear panel (aluminium) with a couple of small (1"x 1.5") heat sinks. I play the amp through 8 ohm speakers at moderate listening levels. The amp never gets hot, and it sound great. I got the case from here http://www.hifi2000.it/default.asp?Langid=1. They were quite cheap, until sterling crashed against the euro. Very good quality, and they don't look too bad either.

[westers151]

All

Many thanks for your welcome and response. I'm reading this in work at the moment so it's only a quick response. I'll spend more time later to look more closely at the suggestions.

Speaker wise I'll be connecting into 8 ohm Tanny F1 Customs which are only 87db efficient. I've heard that the chip amps prefer more efficient speakers so I'm guessing the Tannoys will make it work a little harder and so be hotter?

DD is a site I've already looked at many times (thanks) and I've been reading PD's thread on his chip amp build as well - I've been lurking here for the last two weeks reading as much as I can understand, but as in all things new a lot of the jargon is a bit jibberish to me, particularly all the codes for various components !!

I'll see how the Tannoy's sound, but I also have plans in my head to build some custom speakers - I've been reading up on speaker designs and came across a USA company called Audio Nirvana who make efficient drives, as well as their own speaker plans. They seem to have got some good reviews from various other forums I've been on.

I might have to rethink my design and go back to an aluminium case.

[zebra100]

Ian'

I am using Tannoy Mercury M1's which are also 87db/w/m. The LM3875's seem to cope well with this load. The lack of heat may be down to the 18-0-18v, 120VA transformers though. The amps still sounds awesome.

Chris.