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Old 9th February 2004, 01:48 AM   #1
JhonDoe is offline JhonDoe  Philippines
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Default Bridge LM1875 HS

I desided to bridge LM1875 to much more power than a single chip my question is..... Can I mount the 2 chips in one heatsink(w/out mica insulator) I mean the chip is direct mount in HS?

How much the size of the HS for this chips(the Ideal size)?
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Old 9th February 2004, 11:32 AM   #2
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Yes you can. The 1875's tab is connected to the - supply. You can mount them without an insulator on a single heatsink.

They don't need a huge heatsink. I used 1 about 4 inches wide, 1 inch deep and 1.5 inches tall with 14 fins. It barely got warm at all, but I was just running the chips in stereo.

But why bridge the 1875? Just get a chip such as the 3875 (56W) or the 3886 (68W) and use it in single mode. You'll get more power. Also, the lm1875 (just like almost every other chip amp can only drive an 8-ohm speaker at 40W in bridge mode (no 4-ohm). 3875 is better for 8-ohm loads also with 56W. The 3886 is designed to produce 68W into a 4-ohm.

Which chip to choose depends on the space you have (for heatsink size), the power you want and the load you are driving.
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Old 9th February 2004, 06:44 PM   #3
macboy is offline macboy  Canada
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Quote:
But why bridge the 1875?
Bridging two lower-power chips is desirable if:
  • You have a only a relatively low power supply voltage available, but want to drive a higher impedance load.
  • It is what you happen to have in your parts box
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Old 10th February 2004, 01:02 AM   #4
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Oh wait, I read the other 1875 post by JhonDoe. Thats the only chip he can get.

I just suggested the 3875/3886 so he didn't have to go through the hassle of bridging chips and stuff.

What impedence are you driving? if it is less than 8-ohm, use 2 lm1875 in parallel. 8-ohm or more use bridge.
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Old 10th February 2004, 01:48 AM   #5
JhonDoe is offline JhonDoe  Philippines
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Quote:
Originally posted by soundNERD
Oh wait, I read the other 1875 post by JhonDoe. Thats the only chip he can get.

I just suggested the 3875/3886 so he didn't have to go through the hassle of bridging chips and stuff.

What impedence are you driving? if it is less than 8-ohm, use 2 lm1875 in parallel. 8-ohm or more use bridge.

I got 4 ohm speaker, now i'm ready to test my bridge LM1875 but i got some problem.... When i test the output (only supply is present) I get some DC reading around 20V and one of the chip is disipates to much heat..

I used the schematic proded by the wegsite of sam electronics
can someone examine it? and give me some thought
Attached Images
File Type: png lm1875schem.png (2.5 KB, 1707 views)
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Old 10th February 2004, 11:18 AM   #6
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When an amp is in bridged mode, each sees the speaker as half its real impedence. Anotherwords, they each see it as 2-ohm which is too low to drive with a bridged configuration of the 1875.

I'm working on some schematics for single, bridge and parallel on my website if you don't mind waiting a week or so.

When you tested the chip. did you have anything connected besides the power? No speaker or input, right?

Did you check for shorts and stuff?

And, are you using a split supply (+ , gnd , and -)?
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Old 10th February 2004, 01:22 PM   #7
macboy is offline macboy  Canada
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Quote:
got 4 ohm speaker, now i'm ready to test my bridge LM1875 but i got some problem.... When i test the output (only supply is present) I get some DC reading around 20V and one of the chip is disipates to much heat..
First of all, bridging into 4 ohms is a no-no. You should build a parallel amp. The results will be much better.

The heat is being generated because one of your chips is oscillating. With no load connected, they should run cool.

The problem with this schematic is that it is not designed to be used with the LM1875. It may work fine for some other chip amps, but it will not work with the National ones. The circuit is not fundamentally flawed, but it makes some assumptions about the op-amps (chip amps) used that are not necessarily true.

The most erroneous of these assumptions is that the op-amp are stable at unity gain (gain = 1 or -1). The national chip amps are generally stable only with gain of 10 (or -10) and higher. If you look at the circuit, you will see that the second opamp, IC2, gets its input from the output of IC1 (via R5), and it operates as an inverting amp with a gain of -1.

Trying to fix the circuit is a moot point since you should not use a bridge amp with a 4 ohm load anyway. There are a couple of active threads here dealing with parallel amps. Check them out.
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Old 10th February 2004, 01:52 PM   #8
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Default Hi All

I think what macboy said is true.

Please look to "bridging the STK4048XI is the best" thread,you may find an answer to your questions JhonDoe.

The unity gain is used with SGS TDA2040 and so,I have seen such bridging methods alot there.

BR
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Old 12th February 2004, 01:41 AM   #9
JhonDoe is offline JhonDoe  Philippines
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Quote:
Originally posted by soundNERD

I'm working on some schematics for single, bridge and parallel on my website if you don't mind waiting a week or so.
I'm willing to wait soundnerd..thanks

Quote:
When you tested the chip. did you have anything connected besides the power? No speaker or input, right?
right!..

Quote:
Did you check for shorts and stuff?
I did, several times...

Quote:
And, are you using a split supply (+ , gnd , and -)?
Yup!... i got 25-0-25 PSU

I really wish i could find other chips, such as LM3875 or LM3876
so that i dont have to bridge them to gain power coz i only have 50 watts speaker..

Quote:
Originally posted by metal

Please look to "bridging the STK4048XI is the best" thread,you may find an answer to your questions JhonDoe.
Before i begun to buid an OP amp i did check the abailability of the chip in local store, yes.. and STK40XX series is abailable but it is very expensive. SKT40XX is raging 100w higher output, i got only 50w monitor.
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Old 12th February 2004, 01:49 AM   #10
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About the 100W chip, it will have no problem with the 50W speaker. As long as you don't crank it you will have no problems.

And I have the schematic and pcb for the single operation in eagle format here:

http://electronet.5u.com/schematics/eagle/

The bridge, parallel and the actual html pages are coming soon.

-Mike
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