Franz G said:
No.
I mounted the resistors in a way that I can quickly remove them, so I will listen to it in both ways.
Great, Carlos!
I am interested in the difference, soundwise!
Please go back to my last posting, I edited it and attached a picture from my actual workbench.
Sorry, I am an exhibitionist, regarding my amps, and I have some (many?) beers inside me, right now.
😀
I am interested in the difference, soundwise!
Please go back to my last posting, I edited it and attached a picture from my actual workbench.
Sorry, I am an exhibitionist, regarding my amps, and I have some (many?) beers inside me, right now.
😀
Puzzling problem, experts needed
Ok, here's a puzzle for the experts. It's regarding the 4780 amp described a couple pages back - http://www.diyaudio.com/forums/showthread.php?postid=502637#post502637
I changed the buffer to a low value resistor for the LP (110R, 30nf, 20k input) and was doing some listening tests. I noticed that the right channel was a little louder, so I started doing some checking. I disconnected the speakers and checked the offset, which was about -30mv on the right (relative to ground) and about +30mv on the left. But if I turn the amp off and then turn it back on, I get -21v (yes volts) on the right, but 30mv on the left (same as before). This doesn't happen with a load connected, and if I turn it on without a load and briefly short the output, the offset drops back to -30mv and stays there. If I let it run at -21v (no load), the amp heats as if I were driving it at fairly hard.
It gets more odd. If I ground the output of the buffer and then turn the amp on, the offset is -30mv. If I turn the amp on, then ground the buffer, the offset starts at -21v, then drops to -30mv when I ground. And here's the really odd part: If I switch the buffer outputs left to right, I get the low offset on both channels. So, only when the right buffer is connected to the right channel do I get a no load -21v offset. BTW, the supply voltage is about +/- 24.
Some sort of oscillation?
Sheldon
Ok, here's a puzzle for the experts. It's regarding the 4780 amp described a couple pages back - http://www.diyaudio.com/forums/showthread.php?postid=502637#post502637
I changed the buffer to a low value resistor for the LP (110R, 30nf, 20k input) and was doing some listening tests. I noticed that the right channel was a little louder, so I started doing some checking. I disconnected the speakers and checked the offset, which was about -30mv on the right (relative to ground) and about +30mv on the left. But if I turn the amp off and then turn it back on, I get -21v (yes volts) on the right, but 30mv on the left (same as before). This doesn't happen with a load connected, and if I turn it on without a load and briefly short the output, the offset drops back to -30mv and stays there. If I let it run at -21v (no load), the amp heats as if I were driving it at fairly hard.
It gets more odd. If I ground the output of the buffer and then turn the amp on, the offset is -30mv. If I turn the amp on, then ground the buffer, the offset starts at -21v, then drops to -30mv when I ground. And here's the really odd part: If I switch the buffer outputs left to right, I get the low offset on both channels. So, only when the right buffer is connected to the right channel do I get a no load -21v offset. BTW, the supply voltage is about +/- 24.
Some sort of oscillation?
Sheldon
Noise from volume????
Yesterday I finished the T-network chip amplifier but some noise sound came from my speaker like "chi" sound. I thought it may be occurred from the volume because last time I applied this kind of volume (Alps 50K) to my old noninverting gainclone and also heard the chi sound from speaker same as this time. Could you suggest me other reason which may generate this kind of noise out? Thank you.
Yesterday I finished the T-network chip amplifier but some noise sound came from my speaker like "chi" sound. I thought it may be occurred from the volume because last time I applied this kind of volume (Alps 50K) to my old noninverting gainclone and also heard the chi sound from speaker same as this time. Could you suggest me other reason which may generate this kind of noise out? Thank you.
Not enough data...........
Hi Gengcard,
You have hardly given any information to work on.
Do you get this sound when operating the volume control or when it is stationary ?
If it is while rotating the volume control shaft , it could be due to dc on it and a dirty or faulty slider inside the volume pot.
That is just a guess .. a lot of other things could be the cause .
Why dont you give more info.
Is the noise continuous or short duration , is the heat sink warm or hot etc. What about dc at the output and maybe the input ?
Cheers.
Do you have a scope or access to one ?
Hi Gengcard,
You have hardly given any information to work on.
Do you get this sound when operating the volume control or when it is stationary ?
If it is while rotating the volume control shaft , it could be due to dc on it and a dirty or faulty slider inside the volume pot.
That is just a guess .. a lot of other things could be the cause .
Why dont you give more info.
Is the noise continuous or short duration , is the heat sink warm or hot etc. What about dc at the output and maybe the input ?
Cheers.
Do you have a scope or access to one ?
Thank you for the suggestion.
It's not cause from heat sink certainly because it's not hot.
For volume pot, It's a general volume, I will take it photo and post in this thread next time. I connected the PCB board to the volume via wire harness, this may be a reason of this noise 😕
That noise is continuous sound, not short duration.
It's not cause from heat sink certainly because it's not hot.
For volume pot, It's a general volume, I will take it photo and post in this thread next time. I connected the PCB board to the volume via wire harness, this may be a reason of this noise 😕
That noise is continuous sound, not short duration.
Attachments
gengcard said:
R1 should be 10K or adjusted for minimum offset.
R5 should be smaller too, like 22K or so.
R6 looks a bit small perhaps, but that should depend on how it sounds. How did you get to that value?
Carlos
I just finished another LM3875T VBTNIGC, 22K input-R, 10k/100R/10K and 10K from input to ground with the following results:
-2.7 mV and -20 mV DC bias at output.
Franz
-2.7 mV and -20 mV DC bias at output.
Franz
Good result Franz - must be those beers that you had last night! 😉
VBTNIGC is a little confusing. I thought at first that you had built a non-inverted GC but I take it that what you are describing is a valve-buffered, T-Network inverted GainClone!
VBTNIGC is a little confusing. I thought at first that you had built a non-inverted GC but I take it that what you are describing is a valve-buffered, T-Network inverted GainClone!
Franz, if you can to get lowest distortion, impedances on both inputs of amp must be approximately the same 😉 .
Upupa Epops said:
Yes.
Franz, you can try 22k from NI to ground.
I decided not to change the input resistor, I use 10k on inverting input, so I will remove the multi-trim pots I used for test and use 10k from NI to ground.
I prefer to use smaller value resistors from NI to ground, that's why I've chosen these values.
Feedback is 4.7k/56R/4.7k.
carlmart said:
You are absolutely right.
The cause of the noise on gencard's amp is that 220k resistor from NI to ground.
EDIT:
Also, 4.7uf is too high a value as input cap.
It won't do much to protect eventual DC from the source getting in and being amplified 21x, the gain of gencard's amp.
Are you sure?
I am speaking about an inverting amp, so, the signal should not be influenced by the impedance of the noninverting input, imho.
Franz
Franz G said:
Yes, it's an inverting amp.
The higher the resistor you user from NI to ground, the more noise you have.
Try 220k there and you'll see/listen to the noise.😀
You would have to use a small cap across it.
Matched impedances on both inputs are also important.
22k probably is not be ideal value to have the lower DC (that resistor value should be Ri//Rf, but on T-Network maby it's not as simple...).
Try it.
I prefered to mantain 10k as input impedance, you can try it too, but you would have to also change the feedback resistors.
Sheldon:
Are you sure, the supply voltage is on both rails O.K, while you have this strange situation?
Carlos:
Noise and distortion are not the same thing, as you surely know!
Upupa:
Why should I not believe you? But it would be nice, when you could explain, why the distortion is lower in this case.
It seems absolutely logical to me, in the noninverting config, as in this case every difference between the noninverting input and the inverting input will cause distortion.
But in the inverting topology?
Franz
Edit: I am not sure, if lower distortion is on the wishlist, as it is already much lower than the distortion of every speaker!
Are you sure, the supply voltage is on both rails O.K, while you have this strange situation?
Carlos:
Noise and distortion are not the same thing, as you surely know!
Upupa:
Why should I not believe you? But it would be nice, when you could explain, why the distortion is lower in this case.
It seems absolutely logical to me, in the noninverting config, as in this case every difference between the noninverting input and the inverting input will cause distortion.
But in the inverting topology?
Franz
Edit: I am not sure, if lower distortion is on the wishlist, as it is already much lower than the distortion of every speaker!
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