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Chip Amps Amplifiers based on integrated circuits

Chipamp Lm3886 project, need help...
Chipamp Lm3886 project, need help...
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Old 13th October 2009, 11:52 AM   #1
Geoffroy is offline Geoffroy  United Kingdom
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Default Chipamp Lm3886 project, need help...

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
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Old 13th October 2009, 03:32 PM   #2
pacificblue is offline pacificblue  Germany
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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.
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Old 13th October 2009, 06:57 PM   #3
Geoffroy is offline Geoffroy  United Kingdom
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Default 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... Which he will have to buy anyway

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

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
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Old 13th October 2009, 07:58 PM   #4
wakibaki is offline wakibaki  United Kingdom
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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
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Old 13th October 2009, 08:01 PM   #5
cjkpkg is offline cjkpkg  United States
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This preamp looks interesting and was discussed here on a previous thread HERE

Seems like folks were happy with the results.
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Old 14th October 2009, 01:28 PM   #6
Geoffroy is offline Geoffroy  United Kingdom
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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 )

Thx for the posts!

Regards

Geoffroy
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Old 16th October 2009, 02:31 AM   #7
Redshift187 is offline Redshift187  Canada
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A 28V transformer is a bit too high for an LM3886. The maximum voltage for that chip is +-42V DC. A 28V transformer will give you almost 40V at full load, and will likely exceed 42V at idle.
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Old 16th October 2009, 04:38 PM   #8
pacificblue is offline pacificblue  Germany
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The LM3886 withstands 47 V at idle. A 2x28 V transformer is still okay, yet limits the LM3886 to 8 Ohm speakers.
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Old 17th October 2009, 11:00 AM   #9
Geoffroy is offline Geoffroy  United Kingdom
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8 ohms only, would it be the effect of the low speaker load on the amplifier? Meaning that at lower impedances it would overheat or something?
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Old 17th October 2009, 03:32 PM   #10
pacificblue is offline pacificblue  Germany
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Exactly. With 4 Ohm speakers transformers with more than 2x18 V for the TF or 2x22 V for the T package are not recommendable. And that already requires big heatsinks.
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