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Troubleshooting LM1875
Troubleshooting LM1875
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Old 28th August 2018, 12:27 AM   #11
johnr66 is offline johnr66  United States
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I measured the maximum current of the LM1875 at 2.8 amps rms with a non-inductive 2 ohm load. Above that, output doesn't increase much due to current limiting. With a +/- 20v supply, the ic is at 2.8a with a 4 ohm load and this doesn't permit any headroom for a reactive load such as a speaker. I'd back off to +/-16v for some headroom or even lower for more headroom.

The nice thing about the 1875 is that the current limit is unobtrusive when it operates with reactive loads, whereas ICs like the TDA2050 or 2040 make loud popping sounds.

If the ic is oscillating under heavier loads (higher OP currents), you have a supply decoupling issue. The film decoupling caps should be close to the IC and the traces must be as short as possible to ground. Small signal ground traces should not have any heavy currents flowing in it. Properly set up, I could not make the ic oscillate with a 4 ohm load and a .1uf film capacitor in parallel across the output.
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Old 29th August 2018, 06:54 PM   #12
leRooie is offline leRooie  Netherlands
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Quote:
Originally Posted by FauxFrench View Post
For a start, replace the 100nF for C4 with at least 47uF.

The problems I observed are described here in posting #66: LM1875 Need help
I got a couple of very kind responses from someone more experienced than I. The responses were in short "don't use the LM1875 with more than 25V and if you use 4 ohm loads reduce the supply voltage".

The self-oscillation I observed was worse the more I increased the supply voltage and decreased the load impedance. I was careful with my power line decoupling but with 8 Ohm load and good 24V supply the self-oscillation would start. With a 4 Ohm load it would start around 18V.

When you are in your studio tomorrow, try to put a 10 Ohm power resistor in series with the 4 Ohm speaker and see if it still stutters. You can disconnect the speaker for the good channel meanwhile.
Funny enough, when I checked with my testspeakers in my studio yesterday, all was fine. Perfect signal, no weird drops. And yes, their impedance was 6 ohms, so higher than my home speakers. I'll check out the link you posted, thanks a bunch!

Quote:
Originally Posted by johnr66 View Post
I measured the maximum current of the LM1875 at 2.8 amps rms with a non-inductive 2 ohm load. Above that, output doesn't increase much due to current limiting. With a +/- 20v supply, the ic is at 2.8a with a 4 ohm load and this doesn't permit any headroom for a reactive load such as a speaker. I'd back off to +/-16v for some headroom or even lower for more headroom.
I think I understand, but isn't it weird that only the left channel seemed to be problematic? The right was fine, even at higher output levels.

Last edited by leRooie; 29th August 2018 at 07:10 PM.
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Old 22nd September 2018, 07:00 PM   #13
leRooie is offline leRooie  Netherlands
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Join Date: Dec 2015
Location: Haarlem
Finally had some time to continue testing the amp, the problems I previously had are gone, and all the line inputs work perfectly. Only the Phono amp seems to act weird, but I'll get into that later.

A much weirder and more annoying problem I have is the following: whenever I connect the power cable to the amp without flipping the switch on the amp itself, there's a loud humm coming out of the speakers. I'm measuring about -180 mV between the speaker terminal and ground on the left channel. The right channel is a bit less, about -120 mV. As soon as I flip the on switch, it's gone and everything works normal, but as soon as I turn the amp back off, it starts again after a while. Any idea what this might be? I followed the Chipamp.com building guide, but sourced the parts myself. Any change there's a certain (wrong) part that can cause this?

Thanks!
Sietse
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Old 22nd September 2018, 09:38 PM   #14
FauxFrench is offline FauxFrench  France
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When you turn the amplifier ON, the speakers are controlled by the very low output impedance of the amplifiers. When you turn the amplifiers OFF, the amplifiers become high impedance and the speakers are "floating". The low impedance of the speakers should not allow for hum-fields to be heard in the speakers.
If you disconnect the speakers from the amplifier output but leave the speaker wires close to the amplifier, is the hum then gone?
Do you have Thiele-elements (coil in parallel with a resistor) in the output of your amplifier?
Do you switch OFF the amplifiers on the secondary side of the transformer?
Could we eventually have a photo of the inside of your amplifier?

Last edited by FauxFrench; 22nd September 2018 at 09:41 PM.
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Old 23rd September 2018, 03:47 PM   #15
leRooie is offline leRooie  Netherlands
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Thanks FauxFrench for helping out. It seems I have found the problem. The power switch is on the primary side of the transformer. However, I read about placing a X1 rated cap between the poles of the switch and placed one earlier. As soon as I removed it the problem was gone.

It seems the cap is letting some current through and actually operates the whole thing under a low current causing the noise. All is working fine now apart from a click when powering down the amp, but I'll live with that.

Still got some noise issues with my phono amp though, but I'm gonna go ahead and ask them in the VSPS thread that's here on the board somewhere. Thanks for all the help guys!

Cheers,
Sietse
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Old 23rd September 2018, 03:56 PM   #16
FauxFrench is offline FauxFrench  France
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Yes, even small capacitors conduct AC current! Well investigated.
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