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Old 28th March 2003, 10:00 PM   #1
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Default Power output into small loads

I'm new to DIY audio and I have a question regarding amplifier design. I've never designed or made an amplifier (I'm beginning an electronics degree later this year, so I'll keep you posted), so the answer may be obvious. If this is the case, I apolagise. My question is this:
You see Krell (and other) power amps whose output into 2 ohms is double its output into a 4 ohm load, which is again double its output into an 8 Ohm load. Is this quality, a) worth achieving, and b) difficult to achieve, or c) just a bragging right and of no consequence to amplifier performance?

Sorry if this question gets asked a lot around here. It's just that some of the amplifier kits I've looked at a little (though not understanding very much), such as the Leach double-barrelled amplifier, the Seal Electronics kits, and the AKSA units do not exhibit this. Is it really that difficult to do?

Thanks,

Matt
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Old 31st March 2003, 09:08 PM   #2
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Any thoughts? Anyone??

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Old 1st April 2003, 10:53 AM   #3
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quote

You see Krell (and other) power amps whose output into 2 ohms is double its output into a 4 ohm load, which is again double its output into an 8 Ohm load. Is this quality, a) worth achieving, and b) difficult to achieve, or c) just a bragging right and of no consequence to amplifier performance?

a) absolutely worth achieving. Lots of speakers (wilson for
example) have crossover/driver combinations that have serious
dips in impedance in the low frequency area. The krell MRA
amplifier has 216 output transistors per channel. Plus power
supply regulators.

b)difficult to achieve, you bet, you need lots and lots of output
transistors in parallel to supply the huge currents necessary.
You also need a very large power supply (typically regulated)
to supply the output section with enough stable power. The
SOA of the output transistors come into play heavily as
unregulated power supplys under light load will typically put
the voltage on the rails too high for the output transistors.

c) in my opinion the higher the damping factor the better
the sound. Krell and Levinson excell at low damping factors.
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Old 1st April 2003, 11:13 AM   #4
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Quote:
Originally posted by kevin gilmore
c) in my opinion the higher the damping factor the better
the sound. Krell and Levinson excell at low damping factors.
Kevin, isn't C a little bit of bragging? Aren't mixing the parameters a little bit? Damping factor has to do with the output stage and the feedback.

Weak PS (cheap not good for the sound), normal PS, huge PS (costs money).

I think "weak PS" is nothing to go for. I think "sufficient" or "over dimensioned" is better, but "huge" is cool only if you think it's cool and also if you have the parts and/or can afford to buy them.

You have also different types of usage: home, disco, PA etc.

As a rule, 100 W amp needs a 150-300 VA transformer for each channel. Over 500 VA is "huge".
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Old 1st April 2003, 03:05 PM   #5
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Hi,

I was just passing by and I think everyone should read this article:

http://www.tnt-audio.com/clinica/ssps1_e.html

Very good one.
Print it, and read it all.
Wingfeather, I think this may answer to most of your questions.
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Old 1st April 2003, 03:25 PM   #6
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Default Re: Power output into small loads

Quote:
Originally posted by Wingfeather
Is this quality, a) worth achieving, and b) difficult to achieve, or c) just a bragging right and of no consequence to amplifier performance?
To be exact here, it's virtually impossible to achieve in real life. What manufactures do is play a game with the specs. They actually lower the 8 and 4 ohm ratings well below what the amp can really do so that is appears to double its power into 4 and 2 ohms.

In other words, they build the amp, measure it, and it produces say 420 watts into 2 ohms, they will rate it at 400 watts into 2 ohms, 200 watts into 4 ohms and 100 watts into 8 ohms. In reality, it will probably produce say 250 - 300 watts into 4 ohms and 140 - 170 watts into 8 ohms.

The only way to make an amp really double its power would be to artificially limit the power into the higher impedances by various means. But even having a "perfect" regulated power supply won't let a conventional amp naturally double it's real world power into lower impedances due to additional Vcesat drops and other losses.

But as others have pointed out, the cost to make an amp that approaches doubling its power even down to 2 ohms is very high. So unless you have an unlimited budget, it usually makes more sense to accept a more reasonable compromise.
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Old 3rd April 2003, 07:55 PM   #7
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Thanks to everyone who has replied - that's some interesting reading there, though it may take a bit of pondering for me to understand it all.
I also have another question.

http://www.transformersareus.co.uk/toroidals.htm

Are these power supplies suitable for audio? They sell 330VA units for £10 and 530VA units for £18, plus others up to 2kVA as custom orders - sounds like a steal.
There isn't too much on the site about the quality of the units per se, and if there was I probably wouldn't understand it anyway. I was wondering if these are cheap n cheerful designs unsuitable for powering audio gear, or if they really are a good deal. If they aren't suitable, what do I need to look for, besides power rating? Plitron has been brought up many times (Randy Slone uses them in his kits), and their PSUs are quite a bit more expensive than the Gold Seal ones (they're also not in the UK). How do their products rate in the general scheme of things?

Thanks for your time!
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Old 5th April 2003, 09:40 AM   #8
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Default Power output into small loads

Yes, it is very desirable for the amp to be able to deliver high output power into small loads. I can give you following example: I have operated my PM-A1 amp with Wilson Audio MAXX speakers. Great experience and great sonic impression. The speaker has nominal impedance of 8 Ohms, but mostly it is 4 Ohms and there is an impedance minimum of 2.2 Ohms near 250Hz. So my amp specified as 33W/8 Ohms and 60W/4 Ohms with output voltage swing +/-23V had to deliver up to some 10 Amps to Wilson. And it did, it gives about 200W of peak output power to small impedance. So you can see that it is very important to give greater power into small loads.
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Old 6th April 2003, 04:16 PM   #9
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"Are these power supplies suitable for audio? They sell 330VA units for £10 and 530VA units for £18, plus others up to 2kVA as custom orders - sounds like a steal."

Do you need 100 units?

That's the minimum qty quoted.

Don' forget to add carriage from HK or China, customs, and VAT.

If they were willing to sell one for 2X the price it would still be a fair deal.
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Old 6th April 2003, 06:30 PM   #10
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*feels like an idiot*

Thanks for bringing that to my attention.
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