Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Headphone Systems Everything to do with Headphones

Hum issues- Lehmann audio black cube linear
Hum issues- Lehmann audio black cube linear
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 22nd May 2020, 10:22 AM   #1
spierdolony is offline spierdolony
diyAudio Member
 
Join Date: Nov 2016
Default Hum issues- Lehmann audio black cube linear

I own an original Lehmann audio black cube linear that has recently started a hum issue.
I know this headphone amp has been cloned many times so am hoping someone familiar could help me troubleshoot.

The symptoms:

When I touch the volume knob I get hum. Both in the headphones and the line out

The hum is louder at high volume and worse when set to high gain settings

When I touch both the volume knob and a grounded part of the chassis such as the RCA connector the hum reduces or goes away.
It does not matter what amps/dacs are connected. The problem is persistent and only if I touch the volume knob.


Where do I start? What could cause this to start occurring after so many years?

Thank you for the assistance.
  Reply With Quote
Old 22nd May 2020, 10:27 PM   #2
Mark Tillotson is offline Mark Tillotson
diyAudio Member
 
Join Date: May 2018
Location: Cambridge UK
The grounding is failing somewhere, perhaps between chassis and front panel? Probably
oxidation over the years. The volume pot itself should be firmly grounded to the front panel,
that might be coming loose too (although usually the control would feel loose).
  Reply With Quote
Old 22nd May 2020, 11:53 PM   #3
spierdolony is offline spierdolony
diyAudio Member
 
Join Date: Nov 2016
Thanks Mark,

The washer that secures the pot to the front panel was loose.
I tightened it and it fixed the hum straight away. I exaggerated it in the pics.

I'm surprised because I've never taken this amp apart before.

Now the next question......

Since I have it open do I change the OPA2134 for a LM4562 that I have spare?
Attached Images
File Type: jpg IMG_20200523_094030.jpg (433.0 KB, 50 views)
File Type: jpg IMG_20200523_094035.jpg (626.7 KB, 50 views)
File Type: jpg IMG_20200523_094300.jpg (547.8 KB, 49 views)
  Reply With Quote
Old 23rd May 2020, 04:24 AM   #4
sgrossklass is offline sgrossklass  Germany
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
The opamp is not really a bottleneck in this design, unless you find that you have audible output noise (would need very sensitive headphones). The limiting factor is the diamond buffer output stage being outside the feedback loop entirely.
  Reply With Quote
Old 23rd May 2020, 06:57 AM   #5
spierdolony is offline spierdolony
diyAudio Member
 
Join Date: Nov 2016
Quote:
Originally Posted by sgrossklass View Post
The opamp is not really a bottleneck in this design, unless you find that you have audible output noise (would need very sensitive headphones). The limiting factor is the diamond buffer output stage being outside the feedback loop entirely.
Sounds you like you are familiar with the circuit.

What are the pros and cons of the buffer stage outside of the feedback loop vs inside?
  Reply With Quote
Old 23rd May 2020, 02:38 PM   #6
evonimos is offline evonimos
diyAudio Member
 
Join Date: May 2019
The LM4562 is a very good opamp but some consider it to be a bit dry and clinical.
Too accurate you'd might say or even slightly 'thin' or without too much 'weight'.
This is in contrast to the more 'warm' and tube-like sound of the OPA2134.

Still, try the National opamp and you might like the change.

It all depends on personal preference.



However, I think the LM4562 will benefit from a better bypass capacitor scheme.
Another thing is to measure the DC ofsett before changing the opamps.
Meassure the DC ofseet with the OPA2134 and write it down.

Then do the same with the LM4562 and compare the figures.
You wan them to stay as low as possible.
  Reply With Quote
Old 28th May 2020, 09:08 AM   #7
Mark Tillotson is offline Mark Tillotson
diyAudio Member
 
Join Date: May 2018
Location: Cambridge UK
Anyone who uses the phrase too accurate as a criticism is not able to think straight! I mean really...
  Reply With Quote
Old 28th May 2020, 11:21 AM   #8
BamboszeK is offline BamboszeK  Poland
diyAudio Member
 
BamboszeK's Avatar
 
Join Date: Jun 2012
Quote:
Originally Posted by spierdolony View Post
Sounds you like you are familiar with the circuit.

What are the pros and cons of the buffer stage outside of the feedback loop vs inside?
In very simple words (maybe even oversimplified): Input stage which is opamp in this case has usually very low distortion but doesn't provide much output current. Output stage is opposite - high power but high distortion. By putting output stage in negative feedback loop of input stage you are getting best of two worlds. Feedback is correcting errors of output stage. Basically this means you are getting amplifier with performance of input stage and high power of output stage.
There are of course some trade offs. Nested feedback amplifiers are much harder to design and stabilize.
Similar idea to BCL but with nested feedback - Headphone Amplifier or even more reading Composite amplifiers - what's the story?
  Reply With Quote
Old Today, 08:25 AM   #9
evonimos is offline evonimos
diyAudio Member
 
Join Date: May 2019
Quote:
Originally Posted by Mark Tillotson View Post
Anyone who uses the phrase too accurate as a criticism is not able to think straight! I mean really...
It's all in the ears of the beholder..
I like the sound of it.
To be prescise, the sound of LME47920HA which is a metal can equivalent.

I like the LME47920HA enough, to use it in place of either the more expensive Muses01 or the very nice sounding OPA2134.


But as i wrote, it all depends on who is listening.
  Reply With Quote

Reply


Hum issues- Lehmann audio black cube linearHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
100Hz Hum in Lehmann Linear clone PsychoM Headphone Systems 53 8th July 2019 03:48 PM
Lehmann Black Cube Linear Clone FS cadaverdog Swap Meet 4 3rd May 2019 09:31 PM
Lehmann Audio Black Cube SE II question mdelrossi Analogue Source 15 27th November 2018 10:55 AM
FS: Lehmann Audio Black Cube MM/MC Phonostage enjoythemusic1 Swap Meet 0 27th August 2010 04:27 PM
Lehmann Black Cube Phono Stage mg16 Swap Meet 1 14th April 2008 04:22 PM


New To Site? Need Help?

All times are GMT. The time now is 03:01 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki