LM4780 PCB

[akira]

Hi guys,

I am in the process of gathering the parts for a bridged LM4780. I would like to use two LM4780 per chanell, bridged with a DRV134.

I am looking for PCBs for this project and can only find companies in the US (chipamp, audiosector). Would you know any in Europe to save some on shipping and custom taxes ?

Thanks for your help,

Cheers

Patrick

[jackand08]

I can't really find any boards (with a decent layout) that are for the LM4780 in bridged mode either-it might be that you have to use an external bridge adapter and connect the in phase and out of phase outputs of this to the two inputs of the LM4780 board.

One thing you must remember is that the lowest impedance speaker you will be able to use with this amp realistically would be an 8 ohm driver (presenting 4 ohms to each half of the amplifier). Even then, you would have to run the power rails at a lower voltage because of the high thermal resistance of the junction to case. You could put as big a heatsink as you want on it but it simply would not be able to transfer the heat from the output transistors (junction) to the case quick enough (to be transferred to the heatsink) and would lead to overheating internally.

Another place you could try would be eBay however, the quality from some sellers is very variable and in some cases you might not even get a board at all.

Maybe this is a gap that the DIY community could fill? A group collaboration to produce freely available LM4780 board layouts that are well optimised. Just a suggestion.

Edit: There is quite a good layout on the National Semiconductor datasheet but this is double sided and would be difficult to make yourself unless you have the equipment. Sending it off for production would also require you to copy the layout onto an appropriate PCB layout program and send it as one of their files.

[pinkmouse]

Here's my old version, been running well as the PC sound system for the past five years. For bridging the sub channel, I just used a balanced line driver IC made up on a bit of perfboard. About half a dozen components, if that.

[akira]

Originally Posted by jackand08

I can't really find any boards (with a decent layout) that are for the LM4780 in bridged mode either-it might be that you have to use an external bridge adapter and connect the in phase and out of phase outputs of this to the two inputs of the LM4780 board.

Hi Jack,

I was planning to go in this direction (bridge/parallel):
Build Power Amplifier LM4780 with BPA-200 Amplifier - Electronics Now! schematics, Electronic circuit, PCB or project for Electronic.

One thing you must remember is that the lowest impedance speaker you will be able to use with this amp realistically would be an 8 ohm driver (presenting 4 ohms to each half of the amplifier). Even then, you would have to run the power rails at a lower voltage because of the high thermal resistance of the junction to case. You could put as big a heatsink as you want on it but it simply would not be able to transfer the heat from the output transistors (junction) to the case quick enough (to be transferred to the heatsink) and would lead to overheating internally.

If I run this BLT setup, I should be quite OK with an 8 Ohm, no ? What do you think would be the rail Voltage I could use ? I am planning to use 2*25VAC Toroid ... I could go for a CRCC or even CRCRC to get it down a bit. Or even get some secondary "loops" out of the toroid if needed.

Thanks for your help anyway

[akira]

Originally Posted by pinkmouse

Here's my old version, been running well as the PC sound system for the past five years. For bridging the sub channel, I just used a balanced line driver IC made up on a bit of perfboard. About half a dozen components, if that.

Hi Pinkmouse,

Thanks for the design !! Is it running bridge or parallel ? I am looking for parallel. I will then bridge two LM4780 with a DRV134.

[pacificblue]

There is a dealer in Germany that offers chipamp PCBs and kits from several sources. Amp4780

[pinkmouse]

Originally Posted by akira

Hi Pinkmouse,

Thanks for the design !! Is it running bridge or parallel ? I am looking for parallel. I will then bridge two LM4780 with a DRV134.

It's parallel - I built three boards, two to do bi-amped satellites, and one with a separate bridge driver, (come to think of it, I think I used the DRV134) for the sub.

Nothing fancy about the layout, but it works well, and as I said, has been in nearly constant use for five years. I'll have a root around and see if I can find a pic.

[jackand08]

Originally Posted by akira

If I run this BLT setup, I should be quite OK with an 8 Ohm, no ? What do you think would be the rail Voltage I could use ? I am planning to use 2*25VAC Toroid ... I could go for a CRCC or even CRCRC to get it down a bit. Or even get some secondary "loops" out of the toroid if needed.

I should think that an 8 ohm speaker will be fine for use with this amplifier circuit but be sure to use the 0.1% tolerance gain setting resistors in the feedback network otherwise large DC offsets could result, triggering any protection circuit in place or damaging loudspeakers if there isn't one.

Regarding the power supply using a 25VAC transformer will give you around 35V unloaded after rectification and filtering. The power dissipation from the chip's into an 8 ohm load shouldn't exceed their maximum.

One more question-is it a BTL (bridge-tied load, basically a bridged) amplifier you want to make or a BPA (bridged parallel amplifier) as the circuit in the link you sent is a BPA circuit. These are both quite different.

Just to confirm, bridged amplifiers give you more voltage swing at the expense of higher power dissipation in each chip. On the other hand, parallel amplifiers give you the same output power as one amplifier but more output current capability without exceeding the chip's limits (the current to the load is spread across two or three chips, which dissipate less power individually), allowing you to drive a lower impedance load. A BPA gives you both advantages: higher power output (more voltage swing), but also the ability that a parallel amp gives you to drive lower impedance loads.

[pinkmouse]

Originally Posted by jackand08

...A BPA gives you both advantages: higher power output (more voltage swing), but also the ability that a parallel amp gives you to drive lower impedance loads.

And lots and lots of heat!

[akira]

Originally Posted by jackand08

One more question-is it a BTL (bridge-tied load, basically a bridged) amplifier you want to make or a BPA (bridged parallel amplifier) as the circuit in the link you sent is a BPA circuit. These are both quite different.

Sorry ... BPA ... you are absolutely right.
I think I should be carefull with the heat. My heatsinks are not that big ... 30cm*8cm*4cm ...