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Old 11th August 2009, 06:12 PM   #1
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Default Limiting power to 25W?

I want to buy one of these amplifier boards for a amp I am building for my stepdad, but the speakers that he intends to use on them are 6 ohm 25W Sony SS-D170 speakers. Since the amp board output is 68W peak, and the power input requirement is 28-0-28 (which I wouldn't want to mess with in an attempt to lower the output power) what should I do to keep this power down and prevent the speakers from blowing if someone unknowingly cranks the amp up too high?

The amp board on ebay:

http://cgi.ebay.ca/2-x-68W-Watt-LM38...d=p3286.c0.m14

Any help with this dilemma would be appreciated. Thanks.
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Old 11th August 2009, 06:27 PM   #2
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The answer is you probably don't need to do anything except perhaps limit the gain (which I'll say more about). You might want to read up on speaker power, speaker failure, etc. Speaker power ratings are only a very rough guideline. For example, it's not uncommon to run clean peaks of 200+ watts into a "50 watt" speaker without any problems or harm.

Speakers are more often damaged by amplifier clipping, which tends to happen more when the amp is too small, rather than by amps that are too big. I know that's not intuitive, but it's true. Tweeters especially hate clipping.

You can simply drop the power supply rails and the amp will probably work OK as a 25 watt amp. Rails of around 22 or 23 volts should give you around 25 watts out. But, honestly, that just means it won't play as loud before it clips which means someone is more likely to drive it into clipping.

If you're really worried about someone being crazy with the volume control, a better solution might be to limit the gain somewhere (i.e. at the input of the amp) so even when turned all the way up, the amp will just barely clip. The downside to this method is some source material is recorded at a really low level and you don't have any extra gain to boost it up. But, these days, that's not usually a problem with most source material.
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Old 11th August 2009, 07:05 PM   #3
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Okay, I get what you're saying. Sounds like its safe to just leave it the way it is then, since the people using it aren't completely ignorant, and since the clipping sounds terrible, they wouldn't keep it turned up to a horrible volume like that anyway.

If the speakers blow, how likely is it that the amp will be damaged? I know OPTs get damaged when shorted or near-shorted out, but I'm not sure how susceptible op-amps are.

Also, would the secondary on my transformer be a 28-0-28, basically a 56V with a center tap, using the CT as the 0V? Or is there something actually different with it. If I got a toroid that had two seperate 28V windings, I should be able to put them in series and put the connecting wires to the 0V on the amp board, right?

If I have two 28V windings on the transformer, do both need to be rated for 5.5A or does their total current rating need to equal 5.5A? For example, 2x 28V 3A windings in series = 6A, assuming the primary winding is 6A capable too.
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Old 11th August 2009, 09:55 PM   #4
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It's not that likely the speakers will damage the amp. That's one nice thing about chip amps, they have their own protection circuits. Transformer sizing runs a wide range so it depends on who you ask. Some go totally overkill and some want to save money.

You want somewhere in the range of +/- 28 to +/- 35 volts DC (not AC). So that means a transformer that's rated at roughly 0.7 times that for each winding (so 20-0-20 to 25-0-25 aka 50 VCT). The DC voltage is 1.4 times the AC voltage under load and will be even higher unloaded depending on the quality of the transformer. But figure 1.5 to 1.6 times the rated AC voltage when the amp is just idling.

Given you have 6 ohm speakers, I'd probably go with a 40 VCT transformer. That will give +/- 28 volts DC under load and probably around +/- 32 volts idling. So you'll get around 50 watts continuous per channel into 6 ohms and have dynamic headroom close to 70 watts per channel. A 25-0-25 (50 VCT) would give you more power but stress the LM3886 more.

If you go with 40 VCT I'd get a transformer rated for at least 100 VA but 120 VA or 160 VA would be better. So that's 2.5 amps to 4 amps per winding (or in "series" configuration). 5.5 amps is bit of overkill but would be fine if you can get a good deal on the transformer. But, given the application, smaller would probably be fine. And those numbers are for a single transformer for a stereo amp (2 channels). If you go with the 50 VCT, then figure 3 to 5 amps (150 - 250 VA).
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Old 12th August 2009, 10:20 AM   #5
AndrewT is offline AndrewT  Scotland
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I agree with Rocket.
Build the chipamp to give 60W to 70W into 6r0.
Use the volume control sensibly.

The more the overhead for very short term transients the cleaner the sound is that comes from the speakers.

Is the speaker really 6ohm, or could it be 4 to 8ohm? By using a 4ohm/8ohm crossover to feed a 4ohm bass/mid and an 8ohm treble driver.
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Old 12th August 2009, 02:07 PM   #6
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Quote:
Originally posted by AndrewT
I agree with Rocket.
Build the chipamp to give 60W to 70W into 6r0.
Use the volume control sensibly.

The more the overhead for very short term transients the cleaner the sound is that comes from the speakers.

Is the speaker really 6ohm, or could it be 4 to 8ohm? By using a 4ohm/8ohm crossover to feed a 4ohm bass/mid and an 8ohm treble driver.
Well the speakers aren't mine, and I haven't seen them in person. This was just what I was told about them, and their datasheet is incredibly thin. In fact, it didn't even mention power or impedance ratings. To be honest, I don't think that it matters greatly if they die and need to be replaced.

I bought some transformers yesterday from a surplus store in Toronto, they're 60V CT which is a little high, and certainly high for a 6 ohm speaker, but I think it should work alright as long as people are easy on the volume. Not limiting the amp is good for expandability so I think it will be good.
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Old 12th August 2009, 02:21 PM   #7
AndrewT is offline AndrewT  Scotland
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60Vct is too high for any 84V chipamp.

54Vct or possibly 56Vct is about the limit for an 84V chipamp.
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Old 12th August 2009, 05:31 PM   #8
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if you use that than regulate to +-30v or so.

or get a 18-0-18 to 25-0-25 trafo.

your 60v ct (30-0-30) will give dc: 42.3-0-42.3. mains fluctuation, or the fact that sometimes transformers give a volt or two more, will fry chips.

if driving 4 ohm loads, use lower supply voltage, or huge heat sinks.

there are many formulas in the datasheets for these things

if you want 25w output power you need

17.32V peak output
2.8A peak output

*for a 6 ohm load

the trafo voltage will sag a bit under load,
so you need +- 25VDC rails and a transformer with enough current.

DC volts = AC volts * 1.41

when looking at trafos it will say a "va" rating. if a trafo is 60vct and 300va it can supply 5 amps (300/60)

use should use a transformer rated above the current you need, for headroom.
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