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Old 16th March 2007, 06:19 AM   #1
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Default How much to build 300-400 watt amp?

I am looking to build an amp of hifi quality that does 300 to 400 watts rms stereo at 8 ohms and can take a 4 ohm load (if possible 2ohm as long as its safe for the amp and doesn't cost an extra million). How much would something like this cost minus the case? It seems all the pieces are fairly cheap, but there are alot of little pieces. This will be my first amp build so I may need some help on finding parts. This forum seems to be full of audio guru so I am sure that I am in good hands. A newbie question: what is the difference between a chip amp and solid state?

Thanks guys
Dan
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Old 16th March 2007, 03:38 PM   #2
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Default 300 to 400 watts??

First a solid state descrete amplifier uses all separate transistors op-amps resistors, capacitors ect...... and and usually much more complicated then a monolithic chip amp.
A chip amp is an all in one deal, take the LM3875 add +volts -volts two resistors and away you go.
I would suggest starting slow a 300 to 400 watt design descrete or a bridged, parellel chip amp are all pretty complex monsters or maybe take a look in the class D forum.
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Old 16th March 2007, 06:29 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
each chipamp can give about 60W of reliable power.
If you need 300W then you must use at least 5 chipamps.
If you want 8ohm and 4ohm ability then you must double the chipamp count to 10 minimum.
If you omit the 4ohm ability and allow for 8ohm or 2ohm then you can bridge or parallel but you need an even number of chipamps so 6 becomes the minimum required number.

3chipamps in parallel can push upto 180W into 2.7ohms (8/3).
Bridge these so that you have a bridged/parallel arrangement and you get 360W into 5.4ohms or 240W into 8ohms. Since 240W from 6chipamps is just 40W average from each you could increase the supply voltage slightly to get increased power into the 8ohm load.

Parallel all 6chipamps and you get 360W into 1.4ohms and upto 240W into 2ohms. Again the high voltage option is available to extract more power into the 2ohm load

BTW,
bridging and paralleling is very wasteful of smoothing capacitance. It will cost many times more for the capacitance than it will to buy and assemble the chipamps and PCBs.


It would be MUCH simpler to build a discrete amplifier that can be designed for any sensible load and power that you need.
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Old 16th March 2007, 08:56 PM   #4
eris is offline eris  United States
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Default Wasteful of Smoothing Capacitance?

Andrew,

Could you explain the comment about wasting smoothing capacitance? I was under the impression that chip amps have very high rectifier noise rejection. Won't that make up for it?

thanks,

eris
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Old 17th March 2007, 12:03 AM   #5
ttan98 is offline ttan98  Australia
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There so many people here building high powered amp based on Chipamp, you can get the boards from them. with the chipamp comparable in price or cheaper with output transistors building a high powered chipamp would be cheaper than discrete besides you use less components.

Furthermore using discrete components you need to hunt around for a suitable design and in addition test it extensively before you would be confortable to use it. Chipamp is so easy to make you cannot get it wrong esp using the PCB board.

Smoothing capacitance there is no difference in the total amount of capcitance use in eithe chipamp or discrete transistors for the same amount of power output.
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Old 17th March 2007, 11:19 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
I and many others have used two criteria for determining smoothing capacitance built into an amplifier PSU.

1. the RC time constant of output load (Rload) and the smoothing capacitance on each supply rail (C) must be double the high pass RC filter on the input to the amplifier.
2. the capacitance must supply the peak current sent to the load, this is 2 to 3mF/Apk.

A single channel amplifier of 400W into 8ohm is putting 80Vpk and 10Apk into an 8ohm load. This requires +-20mF to meet criteria 1, if input RC=80mS. It also requires +-20mF to +-30mF to meet criteria 2.
It happens in this 400W case that both requirements are similar.
A discrete 400W amplifier needs about +-20mF/channel.

Now consider a bridged arrangement.
To send 400W to an 8ohm load, each half of the bridge must be able to send 200W into 4ohm.
Each half bridge needs +-40mF to meet criteria 1 if the bass response is to match the RC=80mS chosen above. The peak current into the 4ohm load is still 10Apk and so criteria 2 requires +-20mF to +-30mF.
This time the capacitance requirements do not closely match. I suggest that the higher value should be chosen to give equal performance. We have +-40mF/half bridge.

The whole bridged amplfier therefore requires +-80mF/channel to meet that 400W into 8ohms specification. This applies to any bridged arrangement irrespective of whether it is based on discrete or chipamp.

A bridged amplifier requires four times as much smoothing capacitance as a single amplifier. The biggest saving is that the voltage rating of the capacitors is very different. caps above 75Vdc tend to be expensive, but are they four times as expensive?

Do not fall into the trap of accepting lower performance without taking steps to ensure your decisions are fully informed.

There is another capacitance criteria that I do not use:- +-100uF/W, This would result in +-40mF for the 400W amp and +-20mF for each half of the bridged amp. If you decide that this is more appropriate then the same capacitance will be required for either arrangement.
I suggest that amps based on this do not result in the same bass performance.
I wonder if the complaints about poor bass from some chipamp builders are the result of insufficient smoothing capacitance, particularly, when using bridged amps to obtain more power for sub-bass use?
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Old 17th March 2007, 01:17 PM   #7
ttan98 is offline ttan98  Australia
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Default AndrewT

you are everywhere!!!!!

I cannot catch up with you?
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Old 17th March 2007, 01:46 PM   #8
paulb is offline paulb  Canada
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Default Re: How much to build 300-400 watt amp?

Quote:
Originally posted by saabracer23
I am looking to build an amp of hifi quality that does 300 to 400 watts rms stereo at 8 ohms and can take a 4 ohm load (if possible 2ohm as long as its safe for the amp and doesn't cost an extra million). How much would something like this cost minus the case? It seems all the pieces are fairly cheap, but there are alot of little pieces. This will be my first amp build so I may need some help on finding parts. This forum seems to be full of audio guru so I am sure that I am in good hands. A newbie question: what is the difference between a chip amp and solid state?

Thanks guys
Dan
An amp with this power level is not recommended for your first amp. Can you tell us where you power requirements come from?
If you're sure you need it, you should build a small amplifier first, for practice. A power supply capable of powering the larger amp is no small undertaking.
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Old 17th March 2007, 02:05 PM   #9
AndrewT is offline AndrewT  Scotland
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Default Re: AndrewT

Quote:
Originally posted by ttan98
you are everywhere!!!!!

I cannot catch up with you?
more importantly, do you understand my argument?

Do you agree with any of my arguments?
Quote:
Smoothing capacitance there is no difference in the total amount of capcitance use in eithe chipamp or discrete transistors for the same amount of power output.
do you see there can be differences of opinion on this statement?

I side with the view that: bridged amps NEED more capacitance to match the performance of a single amp doing the same duty.
You choose to not believe that, or are unaware of what affects bass performance.
Another chipamp thread even claimed that a PSU consisting of just +-1mF on the chipamp did not compromise the bass performance. I tried explaining, but left them to dream (=consider mid & treble performance above all other balanced audio considerations) on their own.
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Old 17th March 2007, 03:00 PM   #10
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Hi. My first project was Rod Elliot's 300 watt sub amp which I later upgraded to 500 watts. I ignored all the warnings about not attempting an amp this large as your first project, I didn't want anything smaller. I didn't even use a proper pc board, but stripped all the componets off an old integrated reciever board, sanded off the traces and used that wired p2p.
The final cost of this amp was around $600.00 just for parts ( bought batches of this, of that) I have gone on to use some of these other componets in other projects, therefore this waters down that price a bit.
Working with a power supply that runs 140 volts DC is damgerous, but that point will not be driven home by working with a 40 volt supply, one must be extremely careful when handling electricity.
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