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Old 8th March 2015, 11:44 AM   #1
PsychoM is offline PsychoM  Belgium
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Default 100Hz Hum in Lehmann Linear clone

I have a 100Hz hum in my Lehmann linear clone.
It is constant, the volume knob does not affect it. I am certain it is NOT a 50Hz hum.
The hum stops when i pull the plug (the music goes on for a few seconds because of the large supply caps). So i am certain it is rectifier-related.

It is barely audible with my most efficient headphones. But it should not be audible at all.

Does somebody have any experience with this problem? Especially Lehmann Linear clone owners (Lovely Cube etc)?



The board is a cheap board from aliexpress, but I have replaced all the semiconductors in case they were counterfeit. I used output transistors with about the same hFe (BD-139-16 and BD140-16, the 16 indicates the hFe group). The driver transistors were upgraded to their low-noise variants.
I also upgraded all but the largest electrolytics to Panasonic FC.
Input caps were changed too. All other components were measured and tested OK.

The diodes (1N4007) were replaced with something I had laying around: BYV-27. Those are ultra-fast recovery diodes (for SMPS), not really meant for audio applications. Can those be a problem?

I measured about -20mV DC offset on both outputs. Isn't that a little too much for a headphone amplifier? Inbalance in the output stage might explain the hum. How can I correct for this?

I cannot measure 100Hz hum on the output with my scope, it is not sensitive enough. The only signal I can measure is the DC offset, and a 10mV RMS 100Mhz signal (Radio interference?).

Any ideas? I included the schematic.
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Old 8th March 2015, 12:16 PM   #2
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Hi
dissconnect the transformer centre tap P2 from ground and connect it directly to the junction of C205 and C206 so that the charging pulses go straight back to the transformer and not through ground .
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Old 8th March 2015, 12:48 PM   #3
PsychoM is offline PsychoM  Belgium
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Quote:
Originally Posted by epicyclic View Post
Hi
dissconnect the transformer centre tap P2 from ground and connect it directly to the junction of C205 and C206 so that the charging pulses go straight back to the transformer and not through ground .
Thanks for the advice.
However, I tried it and it does not help :/. Signal ground and power ground is seperate, btw.
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Old 8th March 2015, 01:41 PM   #4
Stixx is offline Stixx  Germany
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Quote:
Signal ground and power ground is seperate, btw.
What do you mean by that... two connections to the chassis? Then you have a ground loop. All ground connections eventually go to one point... safety earth.
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Old 8th March 2015, 01:44 PM   #5
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dos the 100Mhz go if you short circuit the input .
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Old 8th March 2015, 02:42 PM   #6
PsychoM is offline PsychoM  Belgium
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Originally Posted by Stixx View Post
What do you mean by that... two connections to the chassis? Then you have a ground loop. All ground connections eventually go to one point... safety earth.
No no, only one connection to the chassis, but it's not a direct connection, there is a resistor in between (power ground =/= signal ground).
It's not a ground loop (that would be 50Hz i think).

I found out something else: Changing the gain setting on the preamp makes the hum louder or softer. So the opamp in the preamp reacts to this.

I tried replacing the opamp (an OPA2134) with a spare NE5532 i had lying around, but unfortunately, on of the pins was bent and I didn't see it. All I heard was a loud 'pop' and now my headphones are dead, so probably big DC on the output.
I can't test it anymore, because I don't have a spare pair of crappy headphones lying around...

By the way, maybe the preamp reacts to ripple on the power supply or large current spikes from the big power caps charging. This might be because I replaced the preamp's 470uF capacitors by 1500uF Panasonic FC (I only had these lying around). They also have very small ESR so big charging currents...

EDIT: i found another pair of crappy headphones. Unfortunately, these cans are much less efficient so the noise is almost impossible to hear (if you didn't know it was there, you wouldn't hear it). But it is still there, with a NE5532 or a OPA2134.

(by the way, the amp itself sounds brilliant and volume-dependent input noise is almost non-existent.)

Last edited by PsychoM; 8th March 2015 at 02:55 PM.
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Old 10th March 2015, 09:24 AM   #7
wwenze is offline wwenze  Singapore
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100Hz or double the mains frequency points to issues with your power supply at the secondary, mainly your caps but can be a bad regulator too.

Double the frequency due to AC to DC retification.

Or if it's 100Hz fundamental but with higher order components (e.g. a chirp that happens 100 times a second) I would suspect rectifier. A scope with FFT comes in useful, or a sound card and you can download either ARTA or Audacity to do the FFT.

Does your rectifier have all sorts of fancy snubber caps attached and are you using some fancy fast or low dropout diodes. How is your ground routing? And how symmetric are your + and - rails and what is the impedance of +/- and ground? All to consider when troubleshooting.

Last edited by wwenze; 10th March 2015 at 09:33 AM.
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Old 10th March 2015, 12:13 PM   #8
PsychoM is offline PsychoM  Belgium
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Originally Posted by wwenze View Post
100Hz or double the mains frequency points to issues with your power supply at the secondary, mainly your caps but can be a bad regulator too.
The main supply caps are cheap and chinese 'Nover' caps. I have tested them for capacitance (they test ok) but I can't test them for ESR.

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Originally Posted by wwenze View Post
Or if it's 100Hz fundamental but with higher order components (e.g. a chirp that happens 100 times a second) I would suspect rectifier. A scope with FFT comes in useful, or a sound card and you can download either ARTA or Audacity to do the FFT.
I have a scope, but it does not have FFT capabilities. The signal is so weak I cannot see any form of 100Hz signal in it on my scope. I can see mainly the DC offset, some 100Mhz interference and some other noise.

Quote:
Originally Posted by wwenze View Post
Does your rectifier have all sorts of fancy snubber caps attached and are you using some fancy fast or low dropout diodes.
There are just some small caps over the diodes. The diodes are indeed fancy fast diodes, I can swap them for 1N4007's and see the problem is still there.

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Originally Posted by wwenze View Post

How is your ground routing?

And how symmetric are your + and - rails and what is the impedance of +/- and ground? All to consider when troubleshooting.
I'm gonna check on that.
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Old 10th March 2015, 12:48 PM   #9
wwenze is offline wwenze  Singapore
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FFT is useful in that you're unlikely to visually see a component of a signal that is 100x smaller than the main signal (or noise), but with FFT it will show up as a -40dB which is noticeably high.

If you suspect high ESR, spam more CRC filtering. Especially the R part, since it sounds like you have lots of spare capacitance.
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Old 11th March 2015, 07:39 PM   #10
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First of all I would check whether the regulators are doing their job properly. Did you replace those as well?
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