A good single sided PCB for bridged LM4780?

[Ulew]

lots of good tips

Well, to me bridged=twice the power. But maybe that's wrong?
And the speakers can handle the power, but I think the more power you got, the less distortion you will get because you don't have to push the amp to it's limits.
I'm doing this for a special project in school so I have a teacher to help me, but I want to find out the most myself, so that's why I'm asking. I will also build two amps with 1x16W in 4 ohm (or something around that).

 

[wakibaki]

Well, to me bridged=twice the power. But maybe that's wrong?

Maybe.

Why not do a google search 'bridge amplifier' and read up on the basics? It took me all of ten seconds to find the relevant passage.

I know it's tempting to push on and get something built, but electronics is not like lego. The advice you have already been given is good.

w

 

[okapi]

if you do decided to parallel or bridge/parallel chips make sure to use (as National recommends) 0.1% resistors in the feedback loop. It is important to have matched gain so that the load is distributed evenly across chips. The alternative is one chip doing all or most of the work and your amp is not parallel at all.

I think for most applications parallel chips are not necessary. LM3886 chips can output quite a bit of current.

 

[gmphadte]

Here is a 3in1 design for all 3 configurations.

Please check the distances for your components, especially at the inputs.

Enjoy

 

[pacificblue]

Well, to me bridged=twice the power.

P=(U^2)/R
If you double the voltage (bridging!) you get four times the output power on the same speaker. Ohms law is the absolute basic for anything you do with regards to electricity and electronics.

And the speakers can handle the power

They can probably handle the power thermally. Read up on the mechanical limits.

think the more power you got, the less distortion you will get

Have a look at the datasheet. There are curves in it showing how THD is distributed in relation to output power. You will find that it is lowest near the nominal output power.

I will also build two amps with 1x16W in 4 ohm (or something around that).

So, why do you want to bridge? LM1875 or TDA2030 can deliver that amount of power. The LM4780 from your first post is designed for 2x60W, that means 1x120W bridged.

Did you already consider a budget for your amps? That might set you on the right track. 😉

 

[Ulew]

P=(U^2)/R
If you double the voltage (bridging!) you get four times the output power on the same speaker. Ohms law is the absolute basic for anything you do with regards to electricity and electronics.

Yeah, I didn't think that well because I was tired while I wrote it! I was wrong!

They can probably handle the power thermally. Read up on the mechanical limits.

Not to sound angry, but you don't even know what speakers I'm using!

Have a look at the datasheet. There are curves in it showing how THD is distributed in relation to output power. You will find that it is lowest near the nominal output power.

Of course, but I mean that there will be no distortion because you are not using the full output, so the amp isn't pushed so hard.

So, why do you want to bridge? LM1875 or TDA2030 can deliver that amount of power. The LM4780 from your first post is designed for 2x60W, that means 1x120W bridged.

The small amp is for a different pair of speakers which doesn't need that much power. I'm going to use the LM4780 to a pair of speakers that aren't so sensitive.

Did you already consider a budget for your amps? That might set you on the right track. 😉

I don't really have a budget because the school will give me the most components. 😉

 

[Ulew]

(is there an edit-button? anyway I didn't find one)

Okay. Now I've done some planing.

• 2xLM3875 for (56+56W) for two (sub)woofers
  2xLM3885 (38+38W for two front speakers
  2xBD139 (12W) for two back/computer speakers

What does everybody (*cough*pacificblue 😉) think of this?

 

[Redshift187]

It depends on the impedance of your speakers, but:

LM3875: 56W @ 8 ohms @ 1% THD
LM3886: 63W @ 8 ohms @ 1% THD

IMO, you would be better off using the LM3886 for the subs and the LM3875 for the front speakers. You can tone down the power output by using a lower supply voltage, as long as you're above the minimum.

 

[MJL21193]

2xBD139 (12W) for two back/computer speakers

BD139 a chipamp?

IMO, you would be better off using the LM3886 for the subs and the LM3875 for the front speakers. You can tone down the power output by using a lower supply voltage, as long as you're above the minimum.

Save money and buy the LM3886 in quantity and use it for everything. No audible difference when built correctly.

 

[Ulew]

BD139 maybe isn't a chipamp, but it's an amplifier anyway!

And I could use the LM3886 everywhere but then again, I need to bridge, and that wasn't apparently the easiest task.

 

[pacificblue]

Not to sound angry, but you don't even know what speakers I'm using!

If you read up on mechanical limitations, you would find out that I don't need to know that.

I don't really have a budget because the school will give me the most components. 😉

Which components do you get for free? Can you keep them afterwards? And what will you spend on the remaining components?

(is there an edit-button? anyway I didn't find one)

Yes, below your post on the right.

• 2xLM3875 for (56+56W) for two (sub)woofers
  2xLM3885 (38+38W for two front speakers
  2xBD139 (12W) for two back/computer speakers

Ambitious project. The 3885 should read 3875, right?
So you want to go to the limit with the subwoofer amps, but not with the front speakers. And the back/computer speakers get a totally different amp. That means you need at least two amplifier layouts, three different power supplies, and probably three different cases.

What does everybody (*cough*pacificblue 😉) think of this?

Well, since you ask. 😉

• - Use one power supply for all channels. Whether your school pays for it or you, three power supplies will make the project more expensive. They also cause three times as much work.
  - Use the same IC for all channels. Designing and building two circuits is less efficient than copying one circuit. You will probably also have limited time for this project, and a first time project almost always runs across unforeseen obstacles and delays. That is a good reason to keep the workload reasonable from the start.
  - Don't plan on pushing the amplifier for the subs to its limits. You will run into heatsinking issues. Also take into account that you or somebody else might use the amp with different speakers. If they happen to be 4 Ohm, your amp will get into serious trouble. Try a transformer with 2 x 18..20 V. That will give you something between 20 and 36 W into 8 Ohm depending on your power supply design, and will still work fine with 4 Ohm speakers, adequate heatsinking provided.

Here is a nice tool to calculate your SPL gain versus power. If you don't know it already, you need to increase the power ten times to perceive SPL as doubled. And that is only, if your speakers don't run into their mechanical limits before.

 

[MJL21193]

BD139 maybe isn't a chipamp, but it's an amplifier anyway!

And I could use the LM3886 everywhere but then again, I need to bridge, and that wasn't apparently the easiest task.

The Bd139 is a transistor, so yes technically an amplifier.

You can bridge the LM3886's. No problem. Easier than a single LM4780.

 

[pacificblue]

And I could use the LM3886 everywhere but then again, I need to bridge, and that wasn't apparently the easiest task.

The difficulty level remains the same, whether you use the LM3875 or the LM3886. The LM3886 has a mute function that needs additional components, making the LM3875 slightly easier to implement. 68 W against 56 W means less than one dB of SPL difference. You would hardly be able to notice the difference.

So you still want to bridge? Download the "Overture Design Guide" from National's homepage. It will help you to determine the heatsink size.

 

[Ulew]

Pretty much everything is for free, I guess. Maybe I have to pay for somethings, like 50€ or something. We haven't really talked about the details but ofc I want it as cheap as possible. Everything is mine when the project is over!

It turns out I didn't know that much about chipamps as I thought, so it's very nice of you all (*cough*pacificblue 😉) that you help me!

The bridging isn't something that's going to be used if it isn't really necessary. And I guess the LM3875 will be the chip in this setup. But the BD139 amp is already fixed so we don't have to think about that one.

So now this thread is about how I'll get 2x40 W to the subs and 2x30 W to the front speakers. Those values are the max necessary input. Isn't the LM4780 the best chip here? If you feed it with 30-40V there won't be much distortion and it got two channels so there will only be two boards. Or is it better to go with one LM3875/channel? How much current does those need?

 

[pacificblue]

So now this thread is about how I'll get 2x40 W to the subs and 2x30 W to the front speakers. Those values are the max necessary input. Isn't the LM4780 the best chip here? If you feed it with 30-40V there won't be much distortion and it got two channels so there will only be two boards. Or is it better to go with one LM3875/channel? How much current does those need?

38 V is the limit for the LM4780, and that already requires a fan on top of the heatsink. With 30 V you get 44 W into 8 Ohm. 28,7 V give you ~ 40 W and 25,1 V give ~ 30 W into 8 Ohm. Ohms law will help you to find the necessary current. Make your teacher proud and show that you know how to apply it.

Do you know how to calculate the transformer voltage you need for a specific DC rail voltage?

• - LM4780: advantages - two amps in one case; disadvantages - narrow pin spacing makes soldering demanding and shorts likely, 20 pins have to be connected per IC, heatsinking capability is limited, no isolated version is available.
  - LM3875: advantages - only 5 pins need to be connected, wider pin spacing, unused pins can be cut off, improving the pin spacing further, isolated version is available; disadvantages - need double as many ICs, lower output current limit.

The LM3886 may be a compromise. 8 pins have to be connected, an isolated version is available, pin spacing is wide as on LM3875, and it has the same high output current as the LM4780.

 

[Ulew]

Make your teacher proud and show that you know how to apply it.

I just looked at the diagram in the spreadsheet! I didn't keep in mind to use Ohms law! That will make it a lot easier!

Do you know how to calculate the transformer voltage you need for a specific DC rail voltage?

Isn't that AC*(square root)2=DC?

The LM3886 it is! There's lots of good PCBs of that one here, so I will hunt one down tomorrow! Now it's time for the bed! 😉

 

[pacificblue]

Isn't that AC*(square root)2=DC?

It is (AC-(voltage drop at diodes))*SQR(2)*(efficiency under load).
Voltage drop at diodes depends on your topology, whether you use a center tapped transformer with only one rectifier bridge(~ 0,7 V) or a dual secondary transformer with two rectifier bridges (~ 1,4 V). Efficiency under load will be bigger than or equal to 90 % and lower than 100 %.

You will get something above 1,41 times the nominal transformer voltage with no load, depending on the transformers regulation factor. This is important to determine the voltage rating for all components especially the capacitors.
Under full load the DC voltage will drop. By how much depends on the transformer's size and regulation factor and on the amount of capacity in the power supply. Difficult to calculate, and you can only measure it, when the amp is finished. So it is wise to assume 100 % efficiency to determine the heat sink size, voltage and current ratings and 90 % efficiency to estimate the achievable output power.

 

[kuldeepsingh]

mmm .. too much mathematics..

 

[Ulew]

So a toroidal transformer with 2x24VAC out will give me 29-32VDC in to the amp? Does that sound OK?
If I need around 80-100VA/channel, can I use a 400VA to the four boards (there will be four boards, right? 4xLM3886?) ? I mean, there's only two outputs, but can I just split the two cables to four?

 

[pacificblue]

So a toroidal transformer with 2x24VAC out will give me 29-32VDC in to the amp? Does that sound OK?

Yes. Did you check the heatsinking you need?

If I need around 80-100VA/channel, can I use a 400VA to the four boards (there will be four boards, right? 4xLM3886?) ?

Yes, to both. Where do the 80-100VA/channel come from? 32 V per rail give you ~50 W into 8 Ohm/channel. Transformers between 200 and 600 VA are fine, where the bigger rating may improve the acoustic performance.

I mean, there's only two outputs, but can I just split the two cables to four?

Depends on your source and the input resistance you use. It will work that way, if the combined impedance is not too low for the driving source. An input buffer in between is of course a more elegant solution.