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Old 22nd November 2013, 11:32 AM   #1
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Default first DIY chip Amp

Hello all and congrats on a wonderful site. First I will introduce myself im 28 and live in the USA and work as an automotive tech. I Have some electronic experience.
With that said on to the topic. I have some samples from TI 3xlm3886 and 3xlm4780 thanks TI. Now I have been reading over all the info on these gainclones and the like.
The more I dive in the more questions arise. So here is, I am after a 2 chanel Amp with 2 bridged lm4780s, my questions are.

How would I go about adding gain control to the standard bridged lm4780 stereo gainclone
As i undersrand how you would on inverting or non but on the bridged is both?

Second im not 100% on my PSU needs my mains voltage is 119 from the wall how big of a xformer is needed?

Then there's the heat sink would 2 standerd PC CPU coolers work

Or if there is a beter option with the opamps I have on hand

This amp is intended to be used with 8ohm speakers an as a home theater amp

Thanks in advance for any help
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Old 22nd November 2013, 04:18 PM   #2
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Most designs for power amplifiers are for a fixed gain where the amp will work best. Typically gain control is done at the preamp. Almost everything is set for the preamp to output 1-1.5V and the amp to accept this to drive it to maximum power output. If you are going to run it from a fixed preamp, a 10k variable resistor/pot between the preamp and the power amp is about right.

I'm guessing you want about 120 Watts per channel or 240 Watts total from your description. Good guess would be transformer that will be ~70% of its max at full power output or 340 Watts to keep things from burning up. Coffee hasn't taken effect yet this morning, you sometimes can't find the exact transformer you need. You could get something in the range of 30VAC at 12 amps to 45VAC at 8 amps. Looking at the specs, you are not to exceed 84V<94?> for the device. To do the conversion from AC to DC for a rectified power supple you just multiply times 1.4 so it implies your drop dead burn up the device number would be a 60VAC transformer. Of course if all you can find are 34VAC at 5 amps, you can use two, one for each channel.

Heat sink, biggest piece of finned aluminum you can get your hands on. Yes a fan would help. Mild nag here, ~5 Watts per channel is a comfortable listening level. George Thorougood and the Destroyers have been known to use 600 Watts for some venues. You are building a couple of hundred Watts of amp. If you miss on a few parts, for example transformers that only have 16VAC, it will limit the output such that it would be hard to burn anything out with 8 Ohm speakers and still produce a lot of sound.
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Old 22nd November 2013, 04:32 PM   #3
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72 views no replies am i thrown to the wolves.
this is the diagram i am looking at A 120 Watt Bridged

if i have read correctly it wants +/-26v, to my understanding the transformers output depends on the voltage input, plus losses in rectifier and caps
how would one calculate for that?

second the gain is a sum of the resistors used on the input signal and feedback like explained here Shavano Music Online - Introduction to Op-Amps - Part 1

so on the bridged lm4780 would i put a liner pot at r2 and r7 of the diagram linked above

as for the heat sink well lets tackle one thing at a time

ok again thanks for any help (pls dont throw this audio noob to the wolves)
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Old 22nd November 2013, 04:49 PM   #4
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you slipped that reply in there on me now!

any how, so its not to important to get the power dead on and as long as its not over or under specs for the opamp i will be ok?

would something along the lines of this work to power 2 lm4780's Avel Y236652 250VA 25V+25V Toroidal Transformer | 122-625

as for the gain i do not have a preamp not in the budget at the moment to buy/build one
the plan was to build the amp with 3-5 selectable inputs and control the gain of the amp with a liner pot or maybe a stepped atun.
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Old 22nd November 2013, 05:41 PM   #5
AndrewT is online now AndrewT  Scotland
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use one 3886 and build a temporary single channel amplifier.
Add a potentiometer volume control.
Find out about your build, why it works, what bits don't work as expected, how much room it takes up, how you solved the hum and buzz issues.

Then you are ready to look at what is required to build a stereo amplifier.
Those two builds may just about give you enough information to decide if a bridged amplifier is within your capabilities and resources.
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Old 22nd November 2013, 06:41 PM   #6
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That is a good idea and I would still have parts to build another
I may just have to do that but it would still bring me to the question of how to
Ajust the gain on a bridged lm4780 that one half is inverting and the other is non inverting
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Old 23rd November 2013, 07:30 AM   #7
AndrewT is online now AndrewT  Scotland
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Don't adjust the gain of a build that is already successful and recommended.
The stability margins are tied very closely with the gain.
Change the gain and you could end up with a far worse performing amplifier and at worst an oscillator.

Select equipment with just sufficient output that at maximum volume control the very loudest (0dBfs in digital) just reach clipping.

Then your volume control is all you need to adjust to your listening mood.

Have you read:
What is Gain Structure?
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Old 23rd November 2013, 01:08 PM   #8
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Wow Andrew that was a very good read thank you
Can you recommend any more good reading
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Old 24th November 2013, 07:55 AM   #9
AndrewT is online now AndrewT  Scotland
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what about the other articles?
We had a Wiki here and some very good guides among that lot, but it seems to have disappeared.
DD site and ESP site are both very good.
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Old 24th November 2013, 05:20 PM   #10
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Check out Peter's site: DIY Chip Amplifier Kits, PCB's, Components and Information.

I built one of his LM3875 premium kits in dual mono and could not be happier.

Best

Michael
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