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Old 11th June 2006, 07:43 AM   #1
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Default Do I really need/want a Gainclone?

Hi all,

I'm new to this higher power amplifier stuff, having only built headphone amps and a couple of DACs before - so please bear with me whilst I ask what might be some rather obvious questions.

I think(??) I want to build a Gainclone as they seem to be simple to build, reasonably inexpensive, of a good audio quality and reasonably compact.

I was looking at building the LM3886 stereo kit from chipamps.com to power a pair of these older Infinity speakers (use what you got right?) - but it would be nice to keep using it in the future.

I am GUESSING that 40W into 6ohms will be enough, my current amp is 25W and seems to be enough for most situations. Let me know if I'm wrong. I haven't really found what I should be aiming for in terms of amp power vs. speaker power without destroying anything.

In my digging I think I've figured out that I'd like to use a 225VA toroidal transformer with 2x0-18V secondaries which rectified would give just on +/- 25V. If I'm reading page 14 of the LM3886 Datasheet properly I'm figuring this would give me about 40W output power at 6ohms?

The 225VA transformer gives just over 4A on each of the secondaries, where the datasheet tells me that 40W output into 6ohms requires 3.65A = sqrt((2x40W)/6Ohms).
Hrm, I think I just realised that I'm building two of these (stereo) off one transformer so perhaps I need a higher VA rated unit?

I want to avoid the cost of a custom wound transformer and RSwww.co.nz had a Nuvotem unit I mention above for $65 NZ + tax - so it would be great to know if it is truly suitable. The next step up is a 225VA or 300VA unit with 2x25V secondaries - which recitfies to +/- 35V - this appears to be good for 8ohms, but would mean I'd need a big heatsink for 6ohms.

I am planning on heatsinking the LM3886 chips to the side of a custom chassis built from aluminium, I envisaged something between 5mm-10mm thick, 75mm high and 200mm long on that side. If that stays too hot I will move to a finned heatsink.

I had planned to add a volume pot - something like a Dual 50K ALPS RK27 - found on eBay an elsewhere. This is AFAIK a pretty standard value - let me know if I'm wrong

And grounding? I take it the signal grounds are NOT tied to the chassis ground (which I assume would be tied to Earth?)

I think that's it - thanks in advance
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Old 11th June 2006, 10:15 AM   #2
lndm is offline lndm  Australia
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Hi raromachine

Quote:
Originally posted by raromachine
I am GUESSING that 40W into 6ohms will be enough, my current amp is 25W and seems to be enough for most situations... ...without destroying anything.
There is really not much difference between 40W and 25W, but the extra may be welcome all the same. Generally, a higher power amplifier will be better for your speakers provided you use it sensibly. A small amp pushed hard may be harmful to your tweeters.

Quote:
The 225VA transformer gives just over 4A on each of the secondaries, where the datasheet tells me that 40W output into 6ohms requires 3.65A = sqrt((2x40W)/6Ohms).
Hrm, I think I just realised that I'm building two of these (stereo) off one transformer so perhaps I need a higher VA rated unit?
I think the 225VA will be enough for stereo. You may find a benefit of a larger xfmr will be that your amp will reproduce peaks more convincingly, and recover from them a little better. Recommended

Quote:
I had planned to add a volume pot - something like a Dual 50K ALPS RK27 - found on eBay an elsewhere. This is AFAIK a pretty standard value - let me know if I'm wrong
Yes, it is. I use Alps, no complaints.

Good luck.
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Old 12th June 2006, 06:54 AM   #3
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Hiya lndm - thanks for your reply!

Quote:
[B]There is really not much difference between 40W and 25W, but the extra may be welcome all the same. Generally, a higher power amplifier will be better for your speakers provided you use it sensibly. A small amp pushed hard may be harmful to your tweeters.

I think the 225VA will be enough for stereo. You may find a benefit of a larger xfmr will be that your amp will reproduce peaks more convincingly, and recover from them a little better...[B]
So I might be better off to go up to a 300VA 2x0-25VAC (-/+ 35V DC rectified) transformer and up the output power.

This would mean I'd be able to keep using it if I went to larger speakers in the future, the quality of the sound might go up in some instances and

And downside being that I'd need to invest in more serious heatsinking?
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Old 12th June 2006, 07:51 AM   #4
lndm is offline lndm  Australia
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Some might say that with the higher supply rails, the sound quality would be almost as good as with the lower rails. I think it is a fine point and a fair compromise, whatever you choose will be good.

Quote:
This would mean I'd be able to keep using it if I went to larger speakers in the future
Larger speakers are not necessarily harder to drive. In fact, larger speakers are sometimes easier to drive and further, some larger speakers are more efficient requiring less power for the same loudness.
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Old 12th June 2006, 08:12 AM   #5
Duo is offline Duo  Canada
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I should add here that your calculated current draw per channel of 3.65A is much higher than the average current for almost any type of music programming even during operation into the clipping region.

A power supply with sufficient filter capacity will integrate the current transients over time so that the integral, which the transformer sees, is a less transient looking level that defines the average of the audio current.

To put that simply: A transformer rated at 4A should really do it unless you expect to crank out a steady wave at clipping level all day long.

To add: Anything bigger than required is nice to have anyway, I love to over-do my designs a bit to allow for reliability, stability, and solidity. (And just to say it is -capable- of more than it's really rated for).
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Old 12th June 2006, 01:09 PM   #6
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Thanks lndm and Dou - some interesting points and good information

Is there any way to estimate what the load on the transformer might be? Do I need to worry about regulation ratings (and what exactly do they mean - the further you are from the VA rating the higher Vout?) on higher VA rated transformers?

Also - does anyone have a primer on understanding speaker Wattage and sensitivity ratings? I clearly need to read up on this.

Getting there - raro.
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Old 12th June 2006, 04:29 PM   #7
Duo is offline Duo  Canada
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hm, there's a lot of good info on the web about this, though it can, like anything on the web, be difficult to find.

A good place to start is the Elliot Sound Products site. There's more good information and ideas crammed inta that site than I've seen in a lot of things. He has an excellent projects section and articless section on his site:

Rod Elliot's Artilces Rod Elliot's Projects


As far as load on the transformer, this will depend mostly on what is being amplified. For example, if you set the amplifier to amplify a pure sine wave at its rated power output, you'll be placing a full RMS load on the transformer which easily exceeds your transformer rating in this case.

The key here is that you are more likely going to be listening to music. The average power of most music is very low compared to the peak or transient power that occurs on beats or crescendos. This depends a bit on the dynamic range of the music, ie; if the dynamic range of the music is very wide, you'll find the peaks sometimes rise many times higher than the average power level. In highly compressed music, as on FM radio, you'll find the power is distributed much more evenly between loud and soft. This leads to a really lifeless sound.

What it means is that you could play a compressed recording really loud and not worry about going into clipping because of the compressed nature of the transients. This means more average power and thus for listening at high volume, a relatively steady load on the amplifier.

Luckily, you're still not going to find much that's compressed so far that you have to worry.

As far as regulation ratings go in transformers, they tell the designer how well the output of the transformer holds its voltage level during load. All transformers exhibit this 'droop,' and some a lot more than others.

At the first few percent of the power supply load, you'll see some droop occur. This is with the rectification and filtering in place of course. That's because the capacitors become overcharged due to overshoot, pumping, AC line peaks, etc, and the voltage drops to its normal value under just a small load such as that required to bias the amplifier.

Then when you start drawing power, the transformer output will droop just a small bit. For example; I have a 2000VA transformer in one amp I designed and it normally sits at 126V rail-to-rail when idle. At full output on 4ohms (about 500W per channel), the voltage will drop considerably to 117V at times depending on AC line conditions.

I don't have any reading material handy on transformer regulation but someone else on here should be able to fill you in on specifics.

When I get back to the computer, I'll fill you in on speaker info if no one has beat me to it.
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Old 12th June 2006, 10:25 PM   #8
lndm is offline lndm  Australia
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Quote:
Originally posted by Duo
I don't have any reading material handy on transformer regulation but someone else on here should be able to fill you in on specifics.
The output voltage of a transformer is typically given as under full load. To obtain the numbers, take the difference between the full load voltage and the no load voltage, then divide by the full load voltage. Multiply by 100 to convert to a percentage.

A person might assume that all xfmrs will have a load regulation figure based on quality of manufacturer, however, it often has to do with size. A transformer of the size you are looking at usually has a load regulation of better than 10%, and higher than about 500VA is often better than 5%.

Combined with the safety margin you've already considered, therefore, there are two reasons that a larger xfmr will give better load regulation.

This is really more important in an amp that draws a varying amount of current, as Dou covered. A large filter can partially compensate for a small xfmr. Some would say it's not as good, some would say it's good enough. I feel that if size and budget say yes, that I will say yes too, but there is the law of diminishing returns that puts an upper limit on the benefit.

Also, I prefer to use smaller filter capacitors for their sonic qualities, though they can be doubled up. This is not necessary though, just a preference.
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Old 13th June 2006, 12:15 PM   #9
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The 225VA trannsformer should do fine for your proposed amp. Any unregulated supply is going to float high some and then droop some (relative to the floating voltage if not below the rated (at full load) voltage). The speakers, transformer and chassis/sinking seem a good match, you could do it all differently but then that's the reason to do ANOTHER gainclone. IMO, no matter how you did the first one you will want to try something new on the next.
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Old 14th June 2006, 08:10 AM   #10
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Thanks ! and thanks again Dou and lndm

Looks like all I have to do now is wait for Chipamp.com to get some kits back in stock (I've priced PCBs and similar quality parts and can't get it anywhere near as good a price shipped to NZ).

Quote:
you could do it all differently but then that's the reason to do ANOTHER gainclone. IMO, no matter how you did the first one you will want to try something new on the next.
Hah - that's always how it starts: 2 soldering irons, 3 multimeters, a tonne of components, 5 headphone amps and 2 DACs later...
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