LM1875 bridge config

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Gents,

I'd like to build a 2.1 audio system for my PC.
I'm planning to use some LM1875 from an old project.
Could anybody suggest me any bridge circuit for the sub-woofer using LM1875 ?? :confused:

So far I've found several bridge circuits but for another ICs : NSC BPA-100 and LM1876 / TDA2030 bridge config suggested in datasheet .

Thanks in advance


Tincho
 
gootee, paulb,

Thanks for your posts.

I've already read National app note 1132 but was not sure if that circuits could be build using LM1875.


gootee,

Can you please send me more circuit details? Why two inputs? Do I have to connect the speakers betweem LM1875 outputs? Do you know which is the benfits of adding compared to the simplest 1132 app note diagram?

Thanks in advance.
Tincho
 

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Tincho said:
gootee, paulb,

Thanks for your posts.

I've already read National app note 1132 but was not sure if that circuits could be build using LM1875.


gootee,

Can you please send me more circuit details? Why two inputs? Do I have to connect the speakers betweem LM1875 outputs? Do you know which is the benfits of adding compared to the simplest 1132 app note diagram?

Thanks in advance.
Tincho

Hi Tincho,

I'm not sure what you mean by "two inputs". I see only one input, in the schematic from figure 3, on Page 6 of AN-1192. (If you meant that the input goes to two amplifiers' inputs, see the next paragraph.)

A bridged amplifier setup IS, by definition, when a speaker is connected between two amplifier outputs, with one amplifier configured as an inverting amp and the other as non-inverting, with the same input signal. That way, when one amp pushes, the other pulls, and you get two times as much voltage-swing across the speaker, compared to just one amp that pushes and pulls the speaker against ground.

And, since power = voltage squared divided by resistance, bridging results in FOUR times as much power to the speaker, compared to only one amp.

As AndrewT noted, the Zobel networks and RF input filter have been omitted, in that schematic.

See the LM1875 datasheet, for the schematic that includes the Zobel components. It's typically something like 1 Ohms in series with 0.22 uF, from the amplifier output to ground. (Make sure that the Zobels have their own ground return conductor, all the way back to your power supply. i.e. Do not let them share a ground-return wire or pcb trace with the other grounds shown in that schematic.)

Using an RF lowpass filter at the input would be wise. Just after the input, you should have a small-ish series resistor, with a small capacitor to ground just after it. The filter's corner (-3 dB) frequency will be 1 / (2 x Pi x R x C). You could probably use something like 220 Ohms and 2200 pF (= .0022 uF or 2.2 nF), which would give you a lowpass filter with a corner frequency of about 330 kHz. (Technically, you should then reduce Ri2's value by 220 Ohms, to maintain the proper gain, and also the matching of the impedances seen by the inputs of U2.)

ALSO omitted in that schematic are the power supply bypass capacitors, that should be as close as possible to the chipamps' power supply pins. I would forget the 100uF values shown in the first schematic in the LM1875 datasheet and go with the 1000 uF shown in the circuit board layouts near the end of the datasheet. The 0.1uF capacitors should be connected right AT the power pins of the chips, if possible (and certainly not, for example, two centimeters away from the pins). It would probably also be wise to put a 0.1 uF or 0.22 uF capacitor directly between the V+ and V- power supply pins of each chipamp.
 
Tom,Andrew,

Thanks a lot for your clarifications about the circuit.

Actually I've made a mistake because I wrote gootee instead of paulb when asking regarding 1875composite circuit made by palub.

Anyway it looks like AN-1132 circuit is much simpler.

Time to put hands on the hardware now.

Thanks again!!

Best
Tincho
 
Tincho,

I know you have already got some 1875s, and by all means test them in bridged mode to see if it is alright, but they are completely the wrong chip to to use for any sort of sub, especially bridged. They simply don't have the current capability. You save yourself £3 by using them, but a single 3886 will outdo what they can do in a bridged configuration. You might just about get away with what you want in a simple parallel setup, where the current is halved for each, but I would not set too much store by just what you have to hand in front of you.
 
What is the speaker load?
If 8 ohms, one pair of 2-chip parallel LM1875's might do for bridging.
If 4 ohms, one pair of 3-chip parallel LM3886's might do for bridging.

P.S.
Paralleled LM1875 into a 4 ohm load = 55 watts. Solo LM3886 into a 4 ohm load = ~45 watts due to on-chip limiter activation.
If you want more power than these, make BPA300 with 6 LM3886 and a small op-amp for phase inverter, and you can add a Stereo buffer (at subwoofer amp input) so you can downmix to mono without smashing the imaging of your main speakers.
 
they are completely the wrong chip to to use for any sort of sub, especially bridged.

It depends. For a top notch subwoofer that is supposed to deliver the lowest audible notes at original concert listening levels, any chip amp is the wrong choice.

If you choose the woofer and enclosure wisely, you can end up with a surprisingly decent sub for civilised listening levels even with an LM1875.

And in this case, where the subs only needs to improve the performance of PC speakers with a probably rather small woofer, an LM1875 is likely to be more than sufficient, whether bridged or not.
 
For the simplest method, go for Project 14 at ESP

Bridging Adapter For Power Amps

i've being using this method for my LM1875 DIY amp ... and it works nicely ....

Good luck


Hi, I am trying to build a stereo power amp using four TDA2030A or (better) LM1875 in bridge mode. So two ICs bridged for each channel, I am hoping to get up to 35W per channel, but less is OK too.

Can you please help me formulate my design? The link does not work anymore, so I will need a schematic, plus any tips on how to do (exactly what you said!) a DIY LM1875 amp.

Thank you very much!
 
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