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Old 9th August 2005, 09:03 PM   #1
dome406 is offline dome406  United Kingdom
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Default First Gainclone

I have just ordered the LM3875 kit from audiosector and sorry that i am asking so many questions, I just want my first amp to work and to avoid as many possible mistakes if I can.

I am in the UK so 230V power, I was thinking about getting two 18v 225va transfomers for monoblocks. Is 18v sufficient to power my 4ohm adire 281 speakers.

http://uk.farnell.com/jsp/endecaSear...4335314&N=401l

And there should be a picture attached of the type of transformer. Is this the correct type to use with kit. I was a bit confused about the new revision 3 boards as it says 'ac1 ac1(with a line above) ac2 ac2 (with a line above)' compared to the rev2 boards which have 'AC1H, AC1N' sorry if this is a stupid question.

Also does the revision 3 board come with the zobel network etc. so will I have to order 10,000uf capacitors?

Are the capacitors dangerous and need discharging when working on after it has been powered up? What is the best way to do this if so?

Thanks very much for your help and sorry for all the questions.
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Old 10th August 2005, 03:11 AM   #2
digi01 is offline digi01  China
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Default Re: First Gainclone

Quote:
I have just ordered the LM3875 kit from audiosector and sorry that i am asking so many questions, I just want my first amp to work and to avoid as many possible mistakes if I can.
I am in the UK so 230V power, I was thinking about getting two 18v 225va transfomers for monoblocks. Is 18v sufficient to power my 4ohm adire 281 speakers.
http://uk.farnell.com/jsp/endecaSear...4335314&N=401l
I think the tr is ok for mono block.at 4ohm load will get 24W output per amp.
Pdmax =|VCC|*|VCC|/2*3.14*31.4*RL = 22*22/2*3.14*3.14*4 = 24W

Quote:
Also does the revision 3 board come with the zobel network etc. so will I have to order 10,000uf capacitors?
Are the capacitors dangerous and need discharging when working on after it has been powered up? What is the best way to do this if so?
I think the kit from briangt has include all parts.the discharging resistors is surely required for big caps.

good luck on your amp
Z'
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Old 10th August 2005, 03:12 AM   #3
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You should be fine with that transformer. Looks like the next step up would be 2x25V, which might be a little hot for your 4ohm speakers, unless you don't plan on running it too hard.

You could use either the 230V primary that you selected, or a dual 115v primary, wiring the two in series to achieve a single 230v primary.

Not sure of that particular kit on the Zobel.

The caps will be able to zap you, but they will only be around 25V. Not something I would be overly concerned about. If you want to, you can short the terminals with a resistor held with pliers. By no means life threatening. May not even feel it, but I wouldn't find out purposely.
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Old 10th August 2005, 04:48 AM   #4
homer09 is offline homer09  Canada
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Default Re: Re: First Gainclone

Quote:
Originally posted by digi01


I think the tr is ok for mono block.at 4ohm load will get 24W output per amp.
Pdmax =|VCC|*|VCC|/2*3.14*31.4*RL = 22*22/2*3.14*3.14*4 = 24W

Z'

hey digi01, can you please explain this formula? where did 22 come from if he has 18VAC secondaries or ~25VDC? and what are the 3.14 and 31.4? thanks
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Old 10th August 2005, 05:04 AM   #5
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Here's the graph representing output power vs supply voltage for different loads. With 4 ohm speakers and 25V DC (what you be getting from 18V AC transformer) the power into 4 ohms will be around 45W. The 225VA transformer although not that big, should be still enough for two channels.

The classic kit comes with Zobel components ( 2R7 and 0.01u). The Premium kit does not include Zobel components as I perceive it as degradation (to the sound).

10,000uf capacitors are needed if you decide to implement a snubber. You might like it or not, depends on your system and taste. Those caps can be ordered separately, snubber components will be included if asked for.
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Old 10th August 2005, 05:28 AM   #6
dome406 is offline dome406  United Kingdom
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Thanks very much, I would be happy with even 24watts from the amplifier because what they are running on now is truly terrible 30watt cobined amp which is really old and it doesn't like 4ohm speakers and is all corroded inside.

I only askedabout the caps because I was asking about the caps becase I got zapped by a fully charged camera capcitor and it wasn't too pleasent.

Can't wait to finally start building .

Thanks again for all your help and a fast reply.
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Old 10th August 2005, 06:38 AM   #7
digi01 is offline digi01  China
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Default Re: Re: Re: First Gainclone

Quote:
Originally posted by homer09
hey digi01, can you please explain this formula? where did 22 come from if he has 18VAC secondaries or ~25VDC? and what are the 3.14 and 31.4? thanks

yeah,when the secondaries is no loading,it is 18*1.414 ~=25VDC.However with loading,it should be 18*1.2~=22VDC

the 3.14 is circular constant.sorry I write a wrong dot.
this formula is the single ended amp Pdmax equation:
Pdmax =(|VCC|*|VCC|)/(2*3.14*3.14*RL)
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