Help a Newbie: 5 Channel Inverted LM3875TF Amp on the Cheap - diyAudio
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Old 27th August 2006, 04:05 PM   #1
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Default Help a Newbie: 5 Channel Inverted LM3875TF Amp on the Cheap

I am building a 5 channel amp for home theater, and would like some decent power out of these chips as I will probably have fairly inefficient 8 ohm speakers in the mid 80db range that will need to be driven to loud volumes within a small room (12'x12'). (Adire Extremis drivers.)

In addition, the primary goal of this project (besides having fun!) is to save money and create something that has a large bang for the buck factor. Otherwise I would just buy a discounted Panasonic XR series receiver.

What I need help with is primarily the power supply component selection and, secondarily, the amp board component selection with an eye toward keeping costs down, yet not compromising sound quality much.

Aside from the LM chip, here is what I am thinking of using for each of the 5 amp boards, and their respective costs from Digikey:

qty | part# | Desc | cost ea. x qty (total)
------------------------------------------------
2 P5586-ND CAP 1000UF 63V ELECT NHG RADIAL 3.56
2 BC1114CT-ND CAP .10UF 100V CERAMIC X7R 10% .38
1 478-3383-ND CAP POLYFILM BOX .1UF 63V 10% .11
1 495-1168-ND CAP 1.0UF 100V METAL POLY .70
1 10W-2-ND RES 10 OHM 2W 5% METAL OXIDE .18
1 10.0KXBK-ND RES 10.0K OHM 1/4W 1% METAL FILM .10
2 221KXBK-ND RES 221K OHM 1/4W 1% METAL FILM .20
1 TC86P-104-ND TRIMPOT 100K OHM 6MM CERMET .23
1 Amp PCB board from tech diy 6.67
---------
$12.13/channel+LM chip

Please take a peek at the Digikey part numbers and let me know if any of the parts are inadequate for the high sound quality goal without breaking the bank. If there is a much better sounding part for a few cents more (say input capacitors, for example), please tell me, and I will be most grateful.

OK, onward to the most puzzling part of the project, power supply selection.

Two options, first is an array of SMPS. I am considering a model with the following specifications, but am concerned with possible low wattage output to the loudspeakers (10 watts per channel?):

Output Voltage: 12V/15V
Adjustment Range: 11.0V to 16.0V
Current: 9.2/7.3A (12V/15V, respectively)
Line Regulation: 0.1%
Load Regulation: 0.5%
Ripple and Noise: 1%

......Is it correct that I would need ten of these supplies? Could I get away with fewer?

Second option: Standard toroid supply. I am looking at this model as it has a favorable price:

35V - 0 - 35V 500Va

I am guessing that means it would output both the + and - 35V channels and feature a total amperage of about 7.14 amps. Would that be sufficient to supply all 5 channels? I suppose the concern with such a supply would be its suitability for a 4 ohm load. But the power output into 8 ohms should be around 59 watts.

Other toroids to consider (more than one of these would break the bank):

16Vac x 2 300VA
18Vac x 2 400VA
20Vac x 2 400VA
23Vac x 2 400VA
25Vac x 2 400VA
28Vac x 2 500VA
31Vac x 2 500VA
31Vac x 2 800VA (expensive)
35Vac x 2 600VA (very expensive)
39Vac x 2 800VA (very expensive)

Would any of those be ideal for powering 5 chipamps? I worry the VA rating is too low for 5 boards.

Or, perhaps a pair of any of these:

28.4Vac x 2 150VA
25Vac x 2 100VA
42Vct 21V- 0 - 21V (VA rating unknown)

Thanks everyone!
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Old 27th August 2006, 04:28 PM   #2
Bazukaz is offline Bazukaz  Lithuania
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Hi,
The DIY amplifier probably won't cost you much less than a discount receiver. However , i think it is advantage to DIY because you learn things .
For a power supply , i'd recommend you to use a toroid transformer and bridge rectifier.
500W is plenty for 5 LM3875. However , these chips will hardly drive 4 ohm loads , especially on +-35V supplies. So , if you plan to use 4 ohm loads as well , i'd recommend to use LM3886 - it has higher output current capability.
The power supply voltage depends on what loads do you plan to use.
+-35V is OK to drive 8 ohms , but may be too high to drive 4 ohm speakers.
The power supply voltage can be approximated by U = (U1-1.2) / 1.41 , where U1 - transformer's AC voltage. So , with 23 volt trannie , we have around +-30 volts rectified.
I highly recommend you to download Overture Design Guide excel spreadsheet from www.national.com.

Regards,
Lukas.
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Old 28th August 2006, 10:44 PM   #3
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Thank you Lukas.

Although a toroid might make the most sense, the lighter weight and potentially lower price still attracts me to SMPS.

Will +/- 48Vdc @ 1A per rail work? (I understand this supply is a bit saggy so it would actually be about +/- 43Vdc/rail.) I'd use one of these SMPS per channel, so 5 total.

The LM3875 data sheet says it works with up to a 84Vdc supply, and won't start hard clipping until 70Vdc or so..... can I trust that?
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Old 1st September 2006, 01:52 PM   #4
traw is offline traw  United States
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hmmm, specs of the smps sound familiar to me as i've used a module that has those parameters. If it is (a lamba module), it can be easily modified to provide 39v rails. can typically run 2 per the 200w, stock, module. just make sure to have good sinking as when running the chip amps > 35, they really start to produce some heat. I've also used a simple emitter follower secondary regulator to drop the voltage down to 36 - can either mod unit to 39 or use stock, 43-48 but burning off more in the regulator needing solid heatsinking again. I prefer the secondary regulator despite being more effort involved. Again if same module, there is also a tweak changing a cap and resistor to tighten that sag to 1%. I've tried that one too... not sure i heard difference though.
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