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Old 6th October 2008, 03:04 PM   #1
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Default Very noob question

Hi i'm building myself a gainclone and i'm wondering about the transformer that i have.
It is 300Va 12-0-12 and i'm starting to think that it ain't gonna give me enough power.

I have it in the amp, wired up as should be, and it gives me 12-13v on each secondary but then i hook up 35a rectifiers on the end of it and it only gives out about 3.6v.
I bought this transformer on the recommendation from the guy in the store, telling me that he'd built one before and this is the one i must use (one of their most expensive transformers too by the way).
I've done some googling and it appears nobody is using anything under 18v minimum. Do i need a bigger one?
And why would there be such a major drop after the rectifier??

Thanks
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Old 6th October 2008, 03:35 PM   #2
pilli is offline pilli  France
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Default Re: Very noob question

Quote:
Originally posted by snails07
... then i hook up 35a rectifiers on the end of it
If you attach a schematic how you hook it up, somebody will check if everything's right, or give further suggestions.

Cheers,
pilli

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Old 6th October 2008, 05:31 PM   #3
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Your power output is affected by your transformer voltage as well as your speaker impedance. 12V is a bit low. Check out this post:
http://www.diyaudio.com/forums/showt...55#post1532655
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Old 6th October 2008, 05:56 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
12Vac should give at least 16Vdc and probably nearer 18Vdc.
This is good enough for almost 20W into 4r0.

If you need more then take that 12Vac transformer back and ask for the correct one.
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Old 7th October 2008, 12:35 AM   #5
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Hi, thanks for the replies.
Well i worked out what the problem was. I had cut down the secondary leads coming out of the transformer and had crimped the terminals on the end without filing the enamel off the ends of the wires.
Yes, i'm very new to all this stuff!

But the good news is, after i had done all that, it actually works.
I nearly fell over when i heard music coming out of it. I'm very happy.
But there's a few little problems, such as when i turn the volume up, it goes down and vice versa. I'm guessing it's wired back to front so i'll have to re-wire that.

Also, how can i get more volume out of it? Is that to do with the transformer? I'm assuming input power would relate to the output power of it..
And how do i measure such things as signal to noise ratio and distortion etc... Does anybody actually measure such things as this?

Thanks alot
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Old 7th October 2008, 02:01 AM   #6
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The transformer limits maximum power - as AndrewT has pointed out, at the moment, you're limited to about 20 watts per side by the low voltage. You haven't mentioned what chip you're using - if you're using LM3886 or LM4780, you can get that to nearly 60 watts per side by changing to a 25-0-25 transformer.

However, at the moment, it is probably your source limiting maximum volume, since, assuming typical consumer loudspeakers with an 86dB / watt efficiency, you can reach 102dB from two speakers at 20 watts per side. To reach higher volumes, then, you need a stronger source - since the gain of your amplifier is fixed, you do indeed need a certain amount of input voltage to reach a certain amount of output power. Usually this involves adding a preamp, which can be very simple, or very complex - I recommend a simple design based around a dual opamp such as OPA2132, or, for better fidelity, LM4562, OPA2227, OPA2107, or any of the other more expensive suggestions you can find on these forums. A good place to start is http://sound.westhost.com/project88.htm.

Builders of chipamps in the datasheet configuration or similar don't typically bother to measure distortion, since it doesn't change much from one implementation of a given design to the next.
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Old 7th October 2008, 02:34 AM   #7
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Thanks for the info, i've checked out the ESP site a few times, and to be honest most of what he says is going over my head.

So if i stick a pair of opamps before my LM3875 chips that will increase the voltage a little before being amplified again? Would it be much of an improvement?

Sorry for all the dumb questions
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Old 7th October 2008, 06:23 AM   #8
Nuuk is offline Nuuk  United Kingdom
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I would suggest re-wiring your pot correctly before you assess how much power you may or may not need.

Some knowledge of your speakers would also allow us to suggest the correct transformer for your chip amp.

If you haven't done so yet, please go here and do some reading. (And it will mostly go over your head the first couple of times you read it)
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Old 7th October 2008, 06:24 AM   #9
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EDIT: Yes, Nuuk is quite right that you need to fix your pot before taking a look at this, but assuming that you do, and then find that your sources still can't drive the amplifier to the desired volume level (and assuming that said volume level is before the amp starts clipping due to the low supply voltage), take a look at what I've written below.


http://sound.westhost.com/p88-f2.gif

OK, take a look at that diagram - that stage on its own will suffice as a preamp, the only change necessary is to change the output resistor (R5) to something lower - 100 ohms would be a good value, or even a little lower. C1 should also probably be a bit larger - 2.2uF or 3.3uF.....follow the same guidelines as selecting input caps for your gainclone. Technically a preamp should have output capacitors too, but since you know that it will be used with your gainclone, which should have input capacitors, you can omit these from the preamp.

Gain is calculated by:

1 + (R3 / R4)

so the given values on that diagram will give a total gain of 2x, or 3dB. This might still be a bit small....changing R4 to 4K3 will bring total gain to 3.3x (10.4dB), which should be enough for all sources.

You can put the pot before or after the preamp, depending, mostly, on how much effort it takes to remove from where it is. There is some merit to the argument saying it should be before the preamp to prevent overloading its input stage.
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Old 7th October 2008, 10:14 PM   #10
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Ok, i've re-wired the pot and sat back and listened to it for a while and i probably don't really need anymore power.

The speakers are just little crappy pioneer ones that came with the old hi-fi so they won't be helping too much.
I'm planning on building a nice set of speakers which is what i planned to do before i stumbled upon this whole amp building thing, because i'm more woodworker than electronics guy (as you might be able to tell electronics ain't my strong point).

But would adding a preamp make it sound any better at all or is it just purely to increase the gain?

Thanks again guys
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