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Old 30th July 2011, 10:06 PM   #1
Havoxx is offline Havoxx  United States
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Default LM1875 Ground issue

Ok, so I've finally had the time to try to build an 1875T chipamp for a PC subwoofer, and I'm having a bit of an issue, I think it may be ground related, I'm still pretty new to this, but it's distorted quite a bit, and I can't seem to figure out what it could be. I'm using this schematic, I can't seem to figure it out, I'm using a 15v supply, as I don't want too much bass(in an apartment).
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Old 30th July 2011, 10:19 PM   #2
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make sure you did not swap polarity on input cable...
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Old 30th July 2011, 10:35 PM   #3
Bone is offline Bone  United Kingdom
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Is that 15 - 0 - 15v or only 15v between + & - ?
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Old 30th July 2011, 10:40 PM   #4
gootee is offline gootee  United States
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It might be clipping.

What is your maximum peak input voltage? Your gain is about 33.4 [(R2 + R3) / R2], which seems a little high. Even if the amp could swing to the +/-15v rails (which it can't), your max input level could only be +/- 0.45 Volts.

What is your load (speaker) impedance?

With +/-15v supply, you can only expect to get a maximum of about 9.5 Watts output. Watts = V squared over R. So, for an 8 Ohm speaker, 10 Watts would mean 8.71 V RMS max output which for a sine wave would be +/- 12.33 V peak, so your gain of 33.4 would imply a max input peak of roughly +/- 0.37 Volts.

If your speaker is 4 Ohms, 9.5 Watts would imply 6.16 V RMS max output or roughly +/- 0.26 Volts max input peak.

If you can't measure your input level's peak, you could raise R2 to 2.2k in order to lower the gain to 11, which is near the minimum stable value of 10, and see if that helps. Alternatively, you could put a pot or voltage divider on the input, to lower the input voltage level.

Check my arithmetic.

After that is all sorted out, you should probably also add the Zobel network, as shown in the datasheet at national.com, which is a 1 Ohm resistor in series with a 0.22 uF capacitor, from output to ground.

Tom

Last edited by gootee; 30th July 2011 at 11:09 PM.
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Old 30th July 2011, 10:56 PM   #5
gootee is offline gootee  United States
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Are you going to add a low-pass filter, so you don't also send all of the treble to the subwoofer speaker? Or is that already being done?

Last edited by gootee; 30th July 2011 at 11:10 PM.
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Old 30th July 2011, 10:59 PM   #6
gootee is offline gootee  United States
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Quote:
Originally Posted by Bone View Post
Is that 15 - 0 - 15v or only 15v between + & - ?
Good point. If he's using a single supply, 0-15 V, then he's using the wrong schematic.
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Old 30th July 2011, 11:00 PM   #7
Havoxx is offline Havoxx  United States
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Not sure how to test if it's -15, do I just check the leads in opposite order? That gives -14.8 same as the positive side, now as far as a filter, it's for a PC surround sound, already has a subwoofer out, so I shouldn't need a filter?
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Old 30th July 2011, 11:20 PM   #8
gootee is offline gootee  United States
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Quote:
Originally Posted by Havoxx View Post
Not sure how to test if it's -15, do I just check the leads in opposite order? That gives -14.8 same as the positive side, now as far as a filter, it's for a PC surround sound, already has a subwoofer out, so I shouldn't need a filter?
Correct. No filter needed.

What about your input signal voltage level? Sounds like you have a multimeter. See if you can tell what the highest peak voltage is. You could also measure as AC volts which would probably be in RMS Volts. You can't quite calculate the real peak voltage from RMS unless the signal is a pure sine wave, in which case you could just multiply the RMS value by the square root of two, i.e. 1.414. But you can multiply by that anyway and probably see if you're in the ballpark. OR, if you can change the input signal level, just try turning it down and see if the distortion goes away. But I still think your gain looks a little high.

If you have the specs for your system, just try to find out what the signal level range is, for that subwoofer output.
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Old 30th July 2011, 11:29 PM   #9
gootee is offline gootee  United States
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Quote:
Originally Posted by Havoxx View Post
Not sure how to test if it's -15, do I just check the leads in opposite order? That gives -14.8 same as the positive side, now as far as a filter, it's for a PC surround sound, already has a subwoofer out, so I shouldn't need a filter?
To be clear, do you have two wires or three wires for DC power to the chipamp circuit?

You should have three wires, if you have +15 V, Gnd, and -15 V.

Last edited by gootee; 30th July 2011 at 11:33 PM.
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Old 31st July 2011, 02:03 AM   #10
Havoxx is offline Havoxx  United States
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Ah ok, then I have two wires.
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