Hi guys,
I recently finished building my first chipamp, with LM1875 boards from chipamp.com from when they were still around (must be almost 3 years ago now). I'm very pleased with the result, but I'm running into some problems after a first test. Both channels worked before, but after re-soldering some internal wiring and starting everything up again, the left channel seems to be bad.
The symptoms are: there's audio coming out, but very regularly, the signal drops. When playing on low volumes, it's less often, but at higher volumes, the drop out frequency increases and becomes a stutter. The right channel works fine, all inputs behave the same.
Some details: single bridge rectifier, stereo setup, I included a phono amp (the VSPS) on one of the inputs. I sourced the parts myself.
My knowledge of circuits is limited and I'm hoping one of you recognises the problem and can maybe point me in the right direction. Thanks a million!
Sietse
I recently finished building my first chipamp, with LM1875 boards from chipamp.com from when they were still around (must be almost 3 years ago now). I'm very pleased with the result, but I'm running into some problems after a first test. Both channels worked before, but after re-soldering some internal wiring and starting everything up again, the left channel seems to be bad.
The symptoms are: there's audio coming out, but very regularly, the signal drops. When playing on low volumes, it's less often, but at higher volumes, the drop out frequency increases and becomes a stutter. The right channel works fine, all inputs behave the same.
Some details: single bridge rectifier, stereo setup, I included a phono amp (the VSPS) on one of the inputs. I sourced the parts myself.
My knowledge of circuits is limited and I'm hoping one of you recognises the problem and can maybe point me in the right direction. Thanks a million!
Sietse
Can you post the complete schematic? Sounds like either an instability, or a lack
of DC offset voltage (if it's a single supply circuit).
Would you happen to have an oscilloscope? My experience with LM1875, apart from that it sounds very good when working correctly, is that it has a bad habit of self-oscillating (around 1MHz) above a certain output signal level. You won't hear the self-oscillation as such only the side effects. It can still provide an output signal but it tends to get very warm. Does your LM1875 become hot when it stutters?
Sure, but I used the standard schematic for the boards, same values etc. The right channel works fine.
Unfortunately, I don't. The left channel, which has the problem, does run significantly hotter than the right channel, which sounds fine.
Thanks for the reply's guys!
As it runs hotter than the other channel it is likely to be such high frequency oscillation. It is not something trivial to get rid of.
What are your supply voltages?
What is your speaker impedance?
On that faulty channel, do you have a DC voltage on the output when the LM1875 becomes hot? (you can measure with a voltmeter in DC setting).
What are your supply voltages?
What is your speaker impedance?
On that faulty channel, do you have a DC voltage on the output when the LM1875 becomes hot? (you can measure with a voltmeter in DC setting).
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Supply is 25V, speaker impedance is 4 ohm. I'm back in my studio tomorrow and will check the DC offset.
Yes!
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.
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.
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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.
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.
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!
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.
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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
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
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?
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?
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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
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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