LM1875 bridge config - diyAudio
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Old 10th January 2008, 08:33 PM   #1
Tincho is offline Tincho  Argentina
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Default LM1875 bridge config

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 ??

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
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Old 10th January 2008, 11:42 PM   #2
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Hi tincho, try this, maybe is usefull to you

http://www.diyaudio.com/forums/showt...38#post1393838

Cheers
Iván
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Old 11th January 2008, 02:37 PM   #3
Tincho is offline Tincho  Argentina
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Gracias Ivan,

Actually I was looking for someone that already tested the config. On the other hand the other config looks simpler.

Regards,
Tincho
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Old 15th January 2008, 01:48 AM   #4
paulb is offline paulb  Canada
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I have an amplifier with the schematic Ivan pointed to. It works.
There's no magic about bridging. See this:
http://sound.westhost.com/bridging.htm
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Old 15th January 2008, 05:55 AM   #5
gootee is offline gootee  United States
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Here is a well-known National Semi appnote that has examples of bridging and paralleling, with sample power calculations, etc:

http://www.national.com/an/AN/AN-1192.pdf
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Old 16th January 2008, 06:02 PM   #6
Tincho is offline Tincho  Argentina
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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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File Type: jpg bridge an-1132.jpg (44.3 KB, 1031 views)
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Old 16th January 2008, 06:02 PM   #7
Tincho is offline Tincho  Argentina
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paulb,

Please disregard my last question, ESP article has the answer.

Thanks,
Tincho
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Old 16th January 2008, 06:11 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi Tincho,
note that the input impedance (Zin) of the bridged circuit shown is 4k3 (4k7//47k).
This requires a low source impedance that can feed this input.
The RF input filter and the output Zobels are omitted from this schematic.
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Old 17th January 2008, 07:05 AM   #9
gootee is offline gootee  United States
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Quote:
Originally posted by 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
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.
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Old 17th January 2008, 11:29 AM   #10
Tincho is offline Tincho  Argentina
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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
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