Chipamp Lm3886 project, need help...

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Good afternoon to all,

I am preparing a lm3886 stereo amplifier with single toroidal transformer for my Brother and would like to do it as integrated as I can.

My plan was originally to just make a nice case, a pot for the volume and be happy with it. Now I understand I might want to add a passive premap and a device to protect the speakers.

Step 1, I would like to find a simple passive preamp which does not require an external power supply. I did find a schematic/kits for a selector switch and a stepped attenuator for the volume. Is this interesting? I saw some website where DIYer's were commenting that such a setup would not be sufficient as you would need a buffer, why a buffer anyway?

Step 2, for DC and starter protection of speakers I found a velleman kit K4700 Is this a good kit? It has an onboard power supply, from what I have read that could be an issue as it would make me run mains cabling close to the signal cabling which seems to not be ideal.

There are alternative kits which need a transformer, is it possible to connect them to the toroidal transformer used for the amplifier? if yes do I need to oversize the transformer?

Any idea about the above (or simple circuit schematic I could follow instead of getting kits) would be very very helpful..............

And finally the real important question: Do I need to build all this to have a safe/stable good sounding amp?

Oups post quite long!

Thanks for your future comments!!!!

Regards

Geoff
 
Step 1. A passive preamp is nothing else, but a potentiometer and a source selector. A stepped attenuator is better than a potentiometer in that it has a better channel match, especially at low settings. The price difference is significant. I recommend to try a potentiometer first and then decide, if it lacks anything you expect.

A buffer can help to keep the input impedance constant, so that the output filter of the source sees a constant load. A passive preamplifier is not so constant and may lead to slight changes in the frequency response. Again try the passive version first. If it does not work, you can use the potentiometer and selector to build an active preamp.

Step 2. DC protection is always recommendable. Most people don't use it, however. They rely on the ICs built-in protection system, and hope that the IC does not blow up, which would lead to high DC at the output.

It depends on the voltage, whether you can use the same transformer for amplifier and protection circuits. Usually the latter use little power, so there is no need to oversize the amplifier transformer for them, if you connect them to the same transformer.

The amplifier will be good-sounding and stable with only the passive preamp and source selector, if you get the grounding right. It will be 95 % safe without the DC protection, and 99 % safe with the DC protection, provided all other safety precautions are taken. I. e. sufficient heatsink, correctly sized and placed fuses, correct choice of supply voltages, earthing, etc.
 
Thank Pacific!

Thank you very much for your advices, it really clarified things a bit. In terms of DC protection, I would prefer to add some as this will used by my brother and hence I would not want to fry his speakers...:mad: Which he will have to buy anyway :headshot:

I found this on ebay at a decent price, 10 seconds delay at startup and DC protection for speakers etc:

Top quality Speaker Protection module for Audio DIY - eBay (item 300348517006 end time Oct-16-09 13:27:46 PDT)

As far as the preamp is concern I will still start with stepped attenuator straight away and a selector switch so that my brother can select between his pc and a cd player. Will the difference in source power be a problem? Maybe I should plan to add a resistor to the PC input? I guess it would be relevant only if he uses his headphone jack as a source (which would be a waste on the amp maybe :confused:

I found that kit on ebay seems nice I will use all but the case I guess........... 10k 50K 100K Passive preamp step attenuator Kit Alps - eBay (item 220474085063 end time Oct-30-09 07:17:29 PDT)

Well, last step is to order all and see how I can design a case to fit it all!

Thx again.

Regards

Geoff
 
Better to spend your energy and money on designing a good heatsink than on speaker protection. Incidents with 3886s destroying speakers are rare. A stepped attenuator is overkill until you've got the basics worked out. You can add these frills later.

It's always the way with electronics projects. Focus on getting the core functions up and don't get run away with big ideas and you'll waste a lot less effort because the less there is to debug the easier it is.

w
 
Good afternoon,

Thanks for the advice Wakibaki However I am not worried about the basic assembly for the amplifier, I did build a stereo lm1875 using the chipamp pcb and it worked straight away. It just took some time to be confident what I was doing would work. My only potential problem seems the power supply as I am using a +/- 20 V toroid which might be a bit high. You are right that the basics need to be covered first.

I checked the PSU needs for lm3886 and I will use an airlink toroid of 28V for the stereo lm3886, seems the heatsink will need around 1.2 c/w thermal resistance, I remember some threads where some forum members did advise to halve the value given by the National pdf. I will therefore go for 0.6 C/W if not too big.

As for the preamp, passive seems a very simple design and I'd like to add a selector switch for ar least a pair of RCA's and a 3.5 mm jack. The only thing I do not know is if I need to adapt the gain from the amplifier with respect to the attenuator and how.

Well end of lunch break :eek:)

Thx for the posts!

Regards

Geoffroy
 
Indeed, the heatsink I would need for (provided I go for 0.6 C/W resistance) are basically as long as the case I am thinking about and as high (about 200 x 100 x 60high fins). That's serious stuff :eek:

The heatsink requirements for a LM3886 aren't very high IME. It's not a class A amp after all and I don't think you'll be running the thing at full throttle 24/7. If however that's the case I pity your neighbours... :)

WRT your toroid, it's possible to measure the output voltage but that value doesn't account for transformer regulation. Buying a new one for your project ensures you get a toroid with the correct specs, twin secondaries and ample VA rating.

These are my LM3886 monoblock amps:
An externally hosted image should be here but it was not working when we last tested it.

The aluminium cases are approx. 100x110x170mm. They never get too hot to touch. The LM3886 has thermal protection built-in and I haven't been able to trigger that. Yet. :)
 
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Nice cases :eek:) Simple but nice.... Did you cut the hole for the electric plug yourself?

My biggest problem is the lack of tools for cutting metal hence creating a case.... I thought about perspex but too flexible really.

I have a case from an Arcam alpha 7 amp but the heatsink is very small.

Thx for the pics, they showed me there has to be a middle ground between the 1 ton heatsinks and not heatsinks......

Cheers

Geoff
 
The front and rear panels are 1.5 mm aluminium and hence are quite easy to drill, saw and file. For mounting the IEC connector: draw rectangle, dril 4 holes in each corner, use jig saw and saw from hole to hole. File edges straight & smooth.

When you have a case with a 1.5 mm steel back, well that's a different story as I recently discovered...

The entire case acts as a heatsink and it cools surprisingly well. The LM3886 doesn't require a huge heatsink in normal playback situations. It can't hurt to use a large heatsink though. Decent heatsinks and cases are expensive. You could go the surplus route but that may result in a less appealing looking amp according to the missus.
 
I agree, surplus route is not the nicest looking one. you can, however, have nice surprises like shop giving away items which avoid buying spares or even a perfect case :D (one even gave me a working brand new condition cd player thinking it was damaged......I don t feel like cutting it to pieces though :().

I have been looking for a place to pick up junk industrial electronics and the like in Central London mainly for the heatsinks (one can cost up to 40£ each new :eek:) but to no avail, you case example now makes my brain work a bit harder :D
 
max transformer voltage

be careful with a 240:28Vac transformer for a chipamp.

On maximum suppliers voltage tolerance (in the UK <=254Vac), the 28Vac with 7% regulation will give
254/240 * 1.07 * 28 = 31.7Vac
and this after rectifying and smoothing gives >44Vdc at the amplifier.

A 230:28vac 7% transformer could give 46Vdc in the same circumstances.
 
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