3-Ch LM3875 w/ Tone Control and Active Crossover - diyAudio
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Old 9th January 2005, 11:50 PM   #1
StalfoS is offline StalfoS  Canada
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Default 3-Ch LM3875 w/ Tone Control and Active Crossover

This will be my first construction of a power amp, and I would like to overview my design and get some feedback as to whether I am an the right track.

Overview:
-The unit will consist of three power amplifiers based on the LM3875 chip. One for the left chanel, one for the right chanel, and one for a subwoofer.
-The pre-processing stage will consist of a fairly simple 3-knob tone control.
-The tone control will be fed into an active crossover. There should be the option of bypassing this crossover so that the amp can be operated without a subwoofer.

Power Amp Considerations:
-I was planning on using the basic non-inverting setup for the LM3875 chip, as detailed in the kit on chip ampwebsite http://www.chipamp.com/lm3875.shtml.

Tone Control:
-I have attached the schematic. Any Op-amp recomendations?

Sub crossover:
-I have attached the schematic.
-Would it be desirable to incorperate some sort of high pass filter to the left and right chanels at the same crossover frequency as the sub filter?

Power Supply considerations:
-This is the area that I am most unfamiliar with, as I have never constructed anything that didnt run on a 9v adapers/battery.
-Will it be sufficient to use one transformer, one rectifying circuit for all three amps, and then to use a rail-powered dual polarity zener-regulated powersupply for the signal processing components?
-What type of transformer am I looking at here? I was hoping to salvage one off of an old bass amp that I have kicking around, however I dont know if it will be powerfull enough, and Im not sure how to tell...

Is there anything else I should be considering? Please comment on my design!

cheers,
Nathan
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Old 10th January 2005, 03:53 AM   #2
maxw is offline maxw  United Kingdom
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Default Re: 3-Ch LM3875 w/ Tone Control and Active Crossover

Quote:
Originally posted by StalfoS
Power Supply considerations:
-This is the area that I am most unfamiliar with, as I have never constructed anything that didnt run on a 9v adapers/battery.
-Will it be sufficient to use one transformer, one rectifying circuit for all three amps, and then to use a rail-powered dual polarity zener-regulated powersupply for the signal processing components?
-What type of transformer am I looking at here? I was hoping to salvage one off of an old bass amp that I have kicking around, however I dont know if it will be powerfull enough, and Im not sure how to tell...

Well to power 3x LM3875 you would need about 100VA per chip...so a 300VA or bigger transformer would be suitable...about 2x26V secondaries or under.

Then to power the pre power parts you would want about 2x 6-12V depending on what power your opamps need or can use. You might find a big transformer for your Power amp ICs that has a small output for this part but if you dont you could make what I did here:
Help with regulated +/- 15V PSU schematic
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Old 10th January 2005, 07:00 AM   #3
StalfoS is offline StalfoS  Canada
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And here is the subwoofer filter...
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Old 10th January 2005, 07:31 AM   #4
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Default Re: 3-Ch LM3875 w/ Tone Control and Active Crossover

Quote:
Originally posted by StalfoS
...
-The pre-processing stage will consist of a fairly simple 3-knob tone control.
...
[/B]

I am not sure the "middle" control is worth having. Multiple tone controls (filters) are used in recording mastering to create specific effects - to emphasize certain sounds, for example. In that case the turnover frequencies and bandwidth are usually also adjustable. In a playback system, you mostly want to balance your speakers in-room response. This is properly the job of the active crossover anyhow, I think, so if you have a sufficiently controllable crossover, the tone controls will be less important.

If it were me, I'd eliminate the tone controls and put in a flexible crossover with controllable turnover frequencies and gain.

Just a thought...

tim
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Old 10th January 2005, 10:42 AM   #5
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I see you took the pages right out of Randy Slone's book.

You can't go wrong except for the tone control that timbarnes has pointed out.

Good luck!
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Old 10th January 2005, 07:41 PM   #6
StalfoS is offline StalfoS  Canada
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Yes.. randy stones book is fantastic...

I definately see your point about the tone control..

I could just have seperate volume knobs for the sub and the left and right channel, and scrap the tone control. Thant might make more sense.

In a left/right/sub set up, do you guys think that it would be important to design the crossover so that the low frequencies only go to the sub and the higher frequencies only go to the left/right, or would t be more desireable to have no filtering for the left/right chanels? It may depend on what kind of speakers I am using, but in general I would want it to be as flexible as possible.
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Old 11th January 2005, 12:54 AM   #7
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imo one 3875 is not enough for an average sub.

One or two 3886 would be better. Also it should have a seperate supply with a bit more capacitance, or I guess you could use the snubber setup.
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