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Old 10th July 2014, 04:07 PM   #1
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Default Line Stage Preamp: need help eliminating nasty hum!

This is a dual IT LME49720 op amp line-level preamp. Started out with a single (dual channel) op amp, no hum, but hardly any gain. A second op amp was needed, so it got it.

Now it has a nasty DC hum at the output along with the correct level gain. The hum level follows along with the volume control level. Only clue is that adding the second op amp introduces the hum.

Can you folks provide me a clue as to what is missing in the attached schemas?

TIA!
Attached Images
File Type: jpg DSC00084.jpg (282.9 KB, 308 views)
File Type: png schema.png (128.2 KB, 292 views)
File Type: png POWER SCHEMA.png (126.3 KB, 283 views)
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Last edited by whaleman; 10th July 2014 at 04:16 PM.
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Old 10th July 2014, 04:20 PM   #2
Mooly is offline Mooly  United Kingdom
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Its very neatly built but unfortunately I can't tell from a picture.

Start with basics. Does it hum with shorting plugs fitted to the input selected, and with NO other leads attached apart from the output to the amp ?

And is the hum pure and deep or has it a harsh raspy buzz quality to it ?
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Old 10th July 2014, 04:23 PM   #3
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when I first looked at your post, there was a partial schematic, now its gone and only the photo remains. Why did you add a second stage? Changing the resistor values in the feedback network would give you more gain. Why did you put the input selector sw next to the power on off sw? Most folks would have chosen a non inverting to an inverting setup, maybe you needed that. Is there a chasis ground somewhere? Did you use voltage regulators?
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Old 10th July 2014, 04:38 PM   #4
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Originally Posted by multisync View Post
when I first looked at your post, there was a partial schematic, now its gone and only the photo remains. Why did you add a second stage? Changing the resistor values in the feedback network would give you more gain. Why did you put the input selector sw next to the power on off sw? Most folks would have chosen a non inverting to an inverting setup, maybe you needed that. Is there a chasis ground somewhere? Did you use voltage regulators?
The schematics are there, I had to update one of the two. The pre sounded a bit shy with the one stage. And as you see, I did increase the size of the feedback resistor on the second op amp. I've always done the input selector on the left side w/o problems and is not as close to the AC switch as the picture leads one to believe. The chassis is grounded with earth ground. There is a switch between chassis ground and signal and op amp grounds. You can see it on the back plate with a blue wire. It does not help. Maybe I need to remove earth ground...no voltage regulators used since the op amp can take a wide range from the 9 VDC provided.
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Old 10th July 2014, 04:52 PM   #5
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Hi waleman,
why all the high-end components? With the bad building you're doing everything to nothing. The signal cables are too long and too sloppy. I see everywhere loops ...
The cheap transformer is also a nice station.
And from the bad component dimensioning I do not want to talk.

Look at the construction of commercial devices, and be wise
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Old 10th July 2014, 05:05 PM   #6
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Originally Posted by moschfet View Post
Hi waleman,
why all the high-end components? With the bad building you're doing everything to nothing. The signal cables are too long and too sloppy. I see everywhere loops ...
The cheap transformer is also a nice station.
And from the bad component dimensioning I do not want to talk.

Look at the construction of commercial devices, and be wise
Ever hear of leftover soup?
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Old 10th July 2014, 05:35 PM   #7
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Default Some thoughts

Quote:
Originally Posted by whaleman View Post
This is a dual IT LME49720 op amp line-level preamp. Started out with a single (dual channel) op amp, no hum, but hardly any gain. A second op amp was needed, so it got it.

Now it has a nasty DC hum at the output along with the correct level gain. The hum level follows along with the volume control level. Only clue is that adding the second op amp introduces the hum.

Can you folks provide me a clue as to what is missing in the attached schemas?

TIA!
Hi Waleman, couple of overall comments...

Second op amp does not make sense. You've set the first stage gain to 1.5, second one - 10, so total gain = 15. You can get exactly the same 15, putting 150K resistor instead of 15K at the first op amp and removing the second one. Less active (and passive) elements - less distortion - better sound. I mean, it's fine to use more elements when there is some purpose for it, but not in this particular case.

I would use regulated supply for a line amp - simple 7809 / 7909 regulator chips (cost nothing, don't require any additional parts) will dramatically improve supply quality.

Wiring needs complete redesign. In such builds I usually use shielded co-axial cables for internal signal connections. You also need to pay attention to grounding scheme - there is a lot of material here on this matter.

Attached is just an example of my 7-channel filter build - dead silent when idle.

Hope this helps

Cheers,
Valery
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Old 10th July 2014, 06:50 PM   #8
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An example of a good construction
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Old 10th July 2014, 06:58 PM   #9
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Quote:
Originally Posted by moschfet View Post
An example of a good construction
Yep
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Old 10th July 2014, 08:09 PM   #10
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Originally Posted by vzaichenko View Post
Yep
Thanks for the replies, I am finally in front of the unit.

Agree, that is a good layout, but this is just a spare chassis that started out as a different pre and spare op amps (leftover meal sort of). Once I have it figured out, I will clean up the wires, that at this time, are not causing the hum. I like twisted pair CAT6 over shielded coax, but is is prone to some x-talk within the inputs...though the cable makers claim not.

I may just try a 150K resistor for feedback on op amp 1 and eliminate op amp 2. When it was one single op amp, it sounded very three dimensional and analog like.

Will then proceed to clean things up.
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