The "Lovely Cube" Headphone Amp (Lehmann Black Cube Linear Clone)

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  1. francisdemarte
    Just logged in to check and its good to see this thread is still going strong after all these years!
  2. vrajcevski
    assembled this one:
    the parts were really wellmatched - apart from 2 resistors that were wrong ( 68 Ohms supplied instead of 680) but the rest was pretty much spot on - the components supplied were actually wthin 0.1% from each other.

    So - Listened frost with the supplied OPA2134 - I did notice a bit of edginess at larger volumes - both on HD650 and AKG K7xx, AND the case was getting fairly hot. Swapped for LM4562 - better sound overall - not much BUT the edginess was still there - and the case was getting pretty warm - Not Hot BUT warmer than I had liked it. DC offset was really small hovering around 1mv ( probably within the tolerances of my meter).

    So decided to go with an LM 4562HA in a metal can, mounted on a stand and with a heatsink . so - Edginess gone, and after about 1 hour of pretty loud listening, the case is not even luke warm - so it seems possible that the typical OpAmps that I had used - all in a plastic case had some high frequency instability - and the Metal can one is Stable - hence the heat. I guess I will not know till I get and osciloscope.

    In terms of modifying - I think I will hold back on any component swaps - and focus on MAYBE adding a shunt regulator.

    But so far - sounds fantastic I'll just listen for a while till I get itchy fingers and figure out a way how to splice a regulator between the 317/3317 and the rest in an attempt to get the ripple further down.

    Must thank everybody for all the details in this thread
  3. noobDiyAudio
    Hi I just bought a lehmann clone / lovely cube equivalent (board assembled) from eBay. I am currently having an issue where if I plugged my headphone all the way into either of the front 2 headphone jacks, only the right channel works. If I plug it almost in, then both channel works fine but lacks the same sound quality as if I plugged it all the way in (even though it is only 1 channel when all the way in, I can tell the difference). I tried the output at the back and only the left channel works.

    Any help would be much appreciated, I can replace components if need be. Thanks.
  4. penmarker
    Report to the seller and request for replacement. All assembled boards are supposed to be tested before shipped out.
    noobDiyAudio likes this.
  5. noobDiyAudio
    That is one of the option I am considering. I was just hoping if there's a known fix that exist to avoid going through the whole process of sending it back and waiting for a replacement.
  6. mikaelmark
    A thinkable failure reason to try first is if it´s dirt or oxide at either the phono jack or the socket, you can just clean the connectors metal surface with some alcohol (Isopronanol or similar).

    If that will not solve it, you can try this step: As I´ve had a similar problem with mine; there was silence at one channel when touching the headphone cables 6,35 mm (1/4 inch) jack, and it was the phono jacks three metal connectors pressing not firm enough. The solution was to open up the enclosures lid and simply pushing gentle at all the three metal connectors at the female phone socket with a plastic tool.

    The last step you can try, if the problem still are there; visually check/measure all solderings to be sure they are good and measure all resistors they have correct values.

    Santa heasdphones.jpg
    Have a Merry Christmas everbody!
    Last edited: Dec 25, 2017
  7. vrajcevski
    So Assuming you have had a chance to read through the full thread - as much as it is very long, you need to be prepared to open the case- and if you are- when you do and turn the amplifier on- check that BOTh internal LEDs are on. that would establish that at least to a point you have the power going to both channels. There is no standard published troubleshooting method so first things first - before you get to unscrewing and removing the board to check for cold solder joints or checking for shorts or pressing and squeezing, first vicula inspection that at least you have the voltages in check the internal LEDs. They SHOULD be on, even if you have removed the OpAmp from the socket. In fact witht he AMPunplugged from Power, you could remove the OpAmp, and the 2 LEDs should be on. if they are on, then you can start looking for other things.

    I think earlier in this thread -and i don;t know the post there was a diagram with voltages that can be checked. Do You have at least a universl meter? they are not expensive and are necessary for any kind of troubleshooting.

    Also - be mindful that the primary of the transfomer is at Grid (High) voltage, and if the safety case grounding is not properly done it can be lethal. even if grounded properly, I would feel a more comfortable with an ELCB protected power point or at least extension cable.
  8. noobDiyAudio
    Thanks for the feedback, I measured all resistors, all have the correct values.

    Thanks for the feedback. Both internal LEDs are on. I have a multi-meter, output from the voltage regulator seems fine. I can't seem to find the diagrams with voltages that you refer to. Could you please link it to me if you manage to track it down.

    I guess more information I can give is that when I have the headphone plugged in at the front, 2/3 of the way in (with 2 connection touching the headphone jack), I get sound on both side of the headphone. With this setup, I change the balance to LEFT or RIGHT balance and in both case, there is still sound coming from both side of the headphone. It seems that both side of the headphone is linked to either 1 of the channel when plugged in 2/3 of the way.

    Again the problem still persist that if I plugged the headphone all the way in, I only get the right channel. It seems that the last connection (the inner most one out of the 3) for the front jacks is not working properly.

    Would anyone know what sort of voltages I should be expecting from the transistors? Or any other advice on how to move further?
  9. penmarker
    Or it could be the connectors? I have this problem if I use my TRRS earphones due to the microphone need extra terminal on the jack. Do you have a different headphone with a 1/4" termination? Are you using an adapter for mini to 1/4" jack? Does the line out only one channel happens all the time or only when you plug in a headphone?
  10. vrajcevski
    I am with Penmraker on this one- Considering LEDs are on - cheap phones WITHOUT a microphone would do fine to test.
  11. noobDiyAudio
    I have done the test with cheap phones WITHOUT a microphone. I still only get the RIGHT channel on the front head phone jack and only the LEFT from the pre-amp output at the back (without anything plugged in at the front). I suspect that the different in right and left from the front to the back is just me connecting the RCA at the back incorrect.
  12. vrajcevski
    Ok, So I Don;t know how to link directly to a post - but post 389 by Flukell has the voltages as measured on a Lehman amplifier with the diagram for one channel. I'd say if you also measure those voltages on the working channel you will have direct reference from you r version of the board as well as component. but they are a good reference and start. at least it will point you to which area to focus on.

    See if this link works well:

    If not you will have to search for Post 389
  13. noobDiyAudio
    Thank you for finding the post. I just checked and all the voltages is ok.
  14. mikaelmark
    As I already stated in my earlier post, I have experienced problem with my 6,35 mm socket - and the solution was to carefully bend the three connectors down with no headphone jack inserted.

    To verify this is case also for you, please try to cut the cable on a old/damaged headphones or use a upgrade cable with no headphones to it.

    Insert the jack into the amp´s socket, and with power turned OFF, open the enclosures lid and measure each at a time of the three connectors at the socket and the other end of the cable. You should have no more than a few Ohms. With the probes still left, gently push the jack in all directions and see if the resistans are changing. Repeat this for all three connectors. As you are aware your right channel are working fine, you can use that as a guideline (right channel using the middle connector and the inner (tip at the jack) are for the left channel and the connector closest to the inserting hole/nearest eclosure are ground).

    If the inner connector measure different to the other, then you can try to do like me and carefully push it gently downwards.[​IMG]
  15. noobDiyAudio
    Thank you for the instruction. Just checked the connector and they both read the same resistance.
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