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Old 25th January 2011, 03:03 AM   #1
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Default Subwoofer Amp Design Suggestions

Hey guys!

Ive got a crappy home theatre sub hooked up to an LM3886 amp that I built awhile ago, and its still pounding along great! However, Im looking into upgrading, and thus I need advice.

Ive settled on 2 10" Alpine Type-R's. They have dual 2-ohm voice coils and are rated at 500w RMS. This means I can have the following impedance configurations:
- two 1 ohm loads
+ one 0.5 ohm load
+ one 2 ohm load

- two 4 ohm loads
+ one 2 ohm load
+ one 8 ohm load

One issue thats popped into my mind this far is how I am going to drive the subs. Will I have one big amp and hook them both together? Or should I drive them separately? Common sense tells me that if I drive them separately, they need to be in separate chambers, whereas if I drive them as one I can have them both in the same chamber. Ideally this is what I would like to do.

Now, I know Im not going to build a 1kW amp to power these subs - I have no intention of doing that. I was hoping for around ~200-300w per sub. Seems reasonable?

Ive looked into the following:
- bridged/parallel'd LM3886's - I would need quite a few of them
- parallel'd TDA7293's - 2 would give me ~250w into 2 ohms, are they 0.5-ohm stable?
- a more complex driver, the LME49810/30 - I have no experience with this chip at all

What do you guys recommend? Any and all ideas are welcome!

Last edited by SpikedCola; 25th January 2011 at 03:28 AM.
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Old 25th January 2011, 09:57 AM   #2
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cheapest solution that will probably be loud enough:

set up each speaker to 4 ohms.

get one 220va 22 0 22v( 18 0 18 should be safer) toroidal , a nice large heatsink and around 22000uf per voltage rail. and use one lm3886 per speaker. (2 lm3886 in total)



More expensive solutions that will give more headroom:

Set up each speaker as 4 ohms.
Get one 400va 25 0 25 transformer and at the very least 22000uf per voltage rail and use 2 x lm3886 in parallel per speaker. (4 in total)

lm3886's in parallel is tricky. You need to get expensive 0.1% resistors.

good luck
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Old 25th January 2011, 11:26 AM   #3
AndrewT is offline AndrewT  Scotland
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I agree with Tang:
set up each driver as a 4ohm with the two 2ohm voice coils in series.
Choose an amplifier that will run from a 40Vac+40Vac or 45Vac+45Vac or 50Vac+50Vac transformer.

The lme498xx driver chips come in 3 versions. Download the 3 datasheets and compare what each is better at doing.
This is the easiest way, by far, to get 200W to 500W into a speaker.
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Old 25th January 2011, 07:32 PM   #4
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From my research:

LME49811 - little bit more output current, output devices are bjt's
LME49810 - built in baker clamp (why would I want this?), output devices are bjt's
LME49830 - output devices are fets

What would be better for my situation, fets or bjt's? Which parts would you guys recommend? I have no idea where to begin with output drivers. However ON Semi has a pretty good sample program so Ill likely order my drivers from there, whatever they may be. Im quite familiar with Eagle so Ill be designing my own PCBs aswell.
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Old 26th January 2011, 08:50 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
look at ONsemi MJ21193/4 and MJL21193/4
The To3 versions are reputed to be very rugged.
For drivers MJE15034/5, only 4A but massive SOA for a To220

For a bass amp I'd go for the 810 using BJTs.
For a really good wideband amp I'd use the 811 with an output triple, (the pre-drivers are already in the 810).

Now go and find all the threads on the LME series and read read read.
Come back with questions in the appropriate threads.
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Old 15th February 2011, 10:35 PM   #6
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Hey guys, Im back!

After lots of reading Ive settled on a design, and Ive gotten started on it. Ive attached to this post 3 images:

- PSU schematic
- PSU board design
- Amp schematic

The design is based off Panson Audio's driver & power boards. Im holding off on doing the board layout for the amp until people more knowledgeable than me can have a look at it.

My load will be either 0.5/2/8 ohms, depending on the voice coil configuration. Which one is the most efficient? I would assume the smallest load resistance, but Im sure there are practical problems that get in the way. Im sure this obvious, but how do I calculate the approximate output power of the amp?

Also, this is the transformer Im using.

The boards will be etched by me (the first run anyways), hence the lack of silk on the PSU board. If Ive forgotten anything, please let me know!
Attached Images
File Type: png psu-schem-v1.0.png (29.2 KB, 342 views)
File Type: png amp-schem-v1.0.png (134.4 KB, 346 views)
File Type: png psu-board-v1.0.png (61.8 KB, 327 views)

Last edited by SpikedCola; 15th February 2011 at 10:51 PM.
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Old 15th February 2011, 11:18 PM   #7
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Nice but one thing: get rid of those 'boutique' TO-220 diodes and use bridge rectifiers like this: Digi-Key - GBJ2506-FDI-ND (Manufacturer - GBJ2506-F)
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Old 16th February 2011, 01:13 AM   #8
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For next time I will - I have 10 mur860's from a previous sample order to use up. May as well use them!

On the positive side, I was bored after uploading and decided to do up a board for the amp. It's attached to this post. Again, comments are welcome, Im by no means a "professional" at this stuff (just an EE student who's always after cleaner fuller bass). Ive slightly modified the schematic so Ive reattached it aswell.
Attached Images
File Type: png amp-board-v1.0.png (100.8 KB, 313 views)
File Type: png amp-schem-v1.0.png (130.7 KB, 308 views)

Last edited by SpikedCola; 16th February 2011 at 01:25 AM.
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Old 16th February 2011, 08:29 AM   #9
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That 10 ohm resistor is going to be dissipating over twice the energy you are putting into your speakers. It will smoke instantly.
Methinks your are missing an inductor.
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Old 16th February 2011, 10:25 AM   #10
AndrewT is offline AndrewT  Scotland
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A 40-0-40 transformer using a 4pr output stage cannot drive half an ohm of reactive speaker.

It might just be able to drive a 2ohm reactive speaker but you would have to ensure it can never run hot. Cool to warm will require big heatsinks.

8ohm ohms speaker will pull a maximum output power of between 160W and 180W per speaker.
4ohms or parallel 8//8ohm would seem ideal for your amp proposal.

No output Zobel.
DC coupled.
NFB at opposite end from speaker feed.
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Last edited by AndrewT; 16th February 2011 at 10:29 AM.
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