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Old 26th March 2011, 01:48 AM   #1
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Default Need help with LM4780 Sub Amp please!

Hello,

I just got two boards in from China today. An Lm3886 x2 board and a LM4780 board.

Here's my dillema:

I bought this subwoofer preamp board which preamps the signal, and then combines the signal into a single OUT/GROUND.

Subwoofer (12HZ-48HZ)+(28HZ-112HZ) preamplifier board - eBay (item 200588911747 end time Mar-27-11 08:31:34 PDT)


This is my LM4780 board I bought: LM4780 stereo/parallel power amplifier PCB ! - eBay (item 320407281063 end time Apr-24-11 04:20:54 PDT)


What I am confused about is how to properly wire this. The preamp board has one mono output, while this LM4780 board has two inputs. According to the eBay seller, this can be used in parallel which would mean ONE mono input and ONE mono output.

So to which terminals do I have to wire this board? And do I exclude any components on the board because I'm only using "one channel"?

What I mean is, do I fill the entire board up with components and where do I have to wire my input/output?

There are good pictures of the top and bottom of the board on the eBay listing.

Thanks a bunch.
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Old 26th March 2011, 03:31 AM   #2
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Do you have a schematic? The LM4780 will be used in bridged mode, so 1 terminal will go to the inverting input and 1 terminal to the noninverting input.
IMO Unless you have some sort of plan you'll continue to be frustrated.
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Old 26th March 2011, 03:42 AM   #3
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It's going to be in parallel mode not bridged, and the seller did not supply a schematic. I think I will try to request one.

But they were advertising the board as if it was simple to use either parallel/stereo...

Thanks for the advice.
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Old 26th March 2011, 03:45 AM   #4
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Here's what it looks like:

Click the image to open in full size.

Click the image to open in full size.
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Old 26th March 2011, 04:44 AM   #5
godfrey is offline godfrey  South Africa
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Depends what you want to do...
If you want to drive two subwoofers with the LM4780 then it's easy: Build everything as per plan, then connect the preamp output to both inputs on the power amp, and connect one sub to each output of the power amp.

If you only want to drive one sub, there's two choices:

Parallel output
This is similar to the above, except that you need to replace the two 7uH coils with low value, high power resistors. Then you can connect both outputs together to the same sub. This is the best plan for low impedance subs e.g. 4 ohms, but can also be used with others.

Bridged output
This is good for getting higher power into higher impedance subs e.g. 8 ohms+. Unfortunately, this requires more substantial changes to the circuit. Totally do-able, just needs some more thought.

btw, The ideal power supply voltage will depend on which scheme you use, and your speaker's impedance. So... best to figure that out before buying the transformer (unless you already did).

As an aside, it's nice to see a chipamp kit that allows for sensibly sized power supply capacitors, and has sensibly organized layout for the power supply wiring and PCB tracks. On the downside, they seem to have slightly cocked up the input RF filter, but nevermind, it will still work fine. Actually, they get bonus points just for including an input filter and output coil, which is more than some do. I like this board

p.s. For reference, LM4780 datasheet is here.

edit: Oops, missed a few posts while I was looking stuff up...
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Old 26th March 2011, 05:04 AM   #6
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Quote:
Originally Posted by godfrey View Post
Depends what you want to do...
If you want to drive two subwoofers with the LM4780 then it's easy: Build everything as per plan, then connect the preamp output to both inputs on the power amp, and connect one sub to each output of the power amp.

If you only want to drive one sub, there's two choices:

Parallel output
This is similar to the above, except that you need to replace the two 7uH coils with low value, high power resistors. Then you can connect both outputs together to the same sub. This is the best plan for low impedance subs e.g. 4 ohms, but can also be used with others.

Bridged output
This is good for getting higher power into higher impedance subs e.g. 8 ohms+. Unfortunately, this requires more substantial changes to the circuit. Totally do-able, just needs some more thought.

btw, The ideal power supply voltage will depend on which scheme you use, and your speaker's impedance. So... best to figure that out before buying the transformer (unless you already did).

As an aside, it's nice to see a chipamp kit that allows for sensibly sized power supply capacitors, and has sensibly organized layout for the power supply wiring and PCB tracks. On the downside, they seem to have slightly cocked up the input RF filter, but nevermind, it will still work fine. Actually, they get bonus points just for including an input filter and output coil, which is more than some do. I like this board

p.s. For reference, LM4780 datasheet is here.

edit: Oops, missed a few posts while I was looking stuff up...
I LOVE YOU.

I will be using a single 4-ohm sub.

Yes, I have found that the Jim's Audio boards are superb overall. Not without their faults, but very good.

As per the low resistance, high power resistors that replace the coils...is there a way to calculate the exact value? Well, I'm sure there is...but does anyone know?

In the meantime, I'll check out the datasheet again.

Much thanks.
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Old 26th March 2011, 05:25 AM   #7
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I haven't bought the transformer(s) yet.

I'm going to combine the LM3886 x2 board which will be powering the "satellites" and the LM4780 board which will power the sub. It's going to be a hi-end 2.1 system. I know 2.1 is generically a cheap way to get thud, but hey it works most of the time.

In which case, I think I'll use two separate transformers. One for the satellite amp and one for the sub. Each channel on the satellite amp is roughly 8ohm. (8 ohm midrange + 5ohm tweeter), but it's still nominally 8 ohm. The subwoofer is 4ohm. I'm thinking maybe a 22VAC for the satellites and a 18VAC for the subwoofer if I can find it.

And this thing isn't cheap....Between the crossovers/boards/components/speakers/cabinets/wires....sheeessh. I should go back to building cMoys.
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Old 26th March 2011, 06:48 AM   #8
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by TheLaw117 View Post
I will be using a single 4-ohm sub.
OK, parallel it is then.
Quote:
Originally Posted by TheLaw117 View Post
As per the low resistance, high power resistors that replace the coils...is there a way to calculate the exact value? Well, I'm sure there is...but does anyone know?
Short answer: the datasheet recommends 0.1 ohms but I'd recommend higher, maybe 0.47 ohms 10W or so.

Long answer: Unless you go bananas with component matching, there's going to be some difference between the output voltages of the two amp channels. e.g. If you use 1% resistors, there could be up to 4% difference, so when the output voltage is supposed to be 25 V, there could be a difference of up to 1V between channels. (That's worst case. Statistically, probably much less, but still...).

Problem is, when you connect the two outputs together, the two amps will promptly get into an argument about exactly what the output voltage should be. High currents will flow in an attempt to resolve the issue, temperatures will rise, and before you know it the protection circuitry will kick in and shut one (or both) of them down in a desperate attempt to prevent permanent damage to your silicon. The result of all this might politely be referred to as "distortion".

Point is that the two amps are supposed to be working together to supply power to the speaker, not wasting power fighting with each other.

Now if you put a resistor in series with each output, then they can agree to differ. The two amps will still have different output voltages, but now that just means they won't share the speaker current equally. Higher resistance means more equal sharing, but it also means lower damping factor for the speaker, and more power wasted as heat in the resistors.

With a 4 ohm sub and 0.4R resistors, you still have a damping factor of about 20, and only 5% wasted power. Seems like a good compromise to me.

btw: If this is going to be a dedicated mono amp, you can save a couple of bucks by leaving out some of the parts. e.g. You don't need two input coupling caps (the expensive ones), when there's only one input.
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Old 27th March 2011, 06:03 PM   #9
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Quote:
Originally Posted by godfrey View Post
OK, parallel it is then.

Short answer: the datasheet recommends 0.1 ohms but I'd recommend higher, maybe 0.47 ohms 10W or so.

Long answer: Unless you go bananas with component matching, there's going to be some difference between the output voltages of the two amp channels. e.g. If you use 1% resistors, there could be up to 4% difference, so when the output voltage is supposed to be 25 V, there could be a difference of up to 1V between channels. (That's worst case. Statistically, probably much less, but still...).

Problem is, when you connect the two outputs together, the two amps will promptly get into an argument about exactly what the output voltage should be. High currents will flow in an attempt to resolve the issue, temperatures will rise, and before you know it the protection circuitry will kick in and shut one (or both) of them down in a desperate attempt to prevent permanent damage to your silicon. The result of all this might politely be referred to as "distortion".

Point is that the two amps are supposed to be working together to supply power to the speaker, not wasting power fighting with each other.

Now if you put a resistor in series with each output, then they can agree to differ. The two amps will still have different output voltages, but now that just means they won't share the speaker current equally. Higher resistance means more equal sharing, but it also means lower damping factor for the speaker, and more power wasted as heat in the resistors.

With a 4 ohm sub and 0.4R resistors, you still have a damping factor of about 20, and only 5% wasted power. Seems like a good compromise to me.

btw: If this is going to be a dedicated mono amp, you can save a couple of bucks by leaving out some of the parts. e.g. You don't need two input coupling caps (the expensive ones), when there's only one input.
Thanks for the reply. Sorry for not getting back soon enough. I'll try some high power 0.47Ohm.

In regards to the coupling caps. Those are the huge WIMA type caps on the right of the board, right?

I'm confused now. Do I only input into one channel? I mean. The subwoofer preamp board takes two channels and combines them into one. Then, I have to split this mono signal into two channels and connect each channel (L + GND) (R + GND). Then, on the output, L + R get combined and GND + GND get connected to each other again? So the final is one mono ouput?

And in regards to the coupling caps, how would I only need one. I'm still technically driving two channels though, right?

Is that right?
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Old 30th March 2011, 03:51 AM   #10
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Just got the subwoofer preamp board. =D. Still unsure about wiring the entire thing up. If you know, I'd appreciate it.
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