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

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  1. nanchangbob
    Using a test tone and oscilloscope would of course be nice, but since you probably don't have an o-scope you can select AC on your DVM and place the ground lead on the ground pin " of the 6.35 headphone jack. Play a test tone and then
    place the red lead on left and then the right channel pins. You will read an AC voltage when playing the tone, both channels should be almost equal. If you have voltage on the input side of the socket when the headphones are plugged in
    and nothing on channel of output side of the 6.35mm socket your problem is with the socket. If you don't have an AC voltage on either the input or output side of socket on one channel you issue is before the output socket.

    I don't have one of these jacks but if memory serves me correctly I believe the ground is the pin farthest from the front panel. "verify" These jack's are switched, so when nothing is plugged in you will not read continuity across the pins when you plug in a jack you will read close to zero ohms across the jack. "no power applied of course".
  2. vrajcevski
    Do you have a spare Op Amptoswap? even ne 5532 would do to test. Ultimately any of the ones mentioned Dual OpAmps will be OK to test. cold solder could also be the issueso that one is probably best done to refresh all solder joints instead fo trying to figure out which one is bad. also - we have not really looked at the input signal coming through ? so check if any of the wires from the input actually connect to the board.

    We know you don;t get output, but not sure if you have looked if you get input at all. if you for example- as the previous poster mentioned- runa a 2 channel 50 Hz test tone, any digital meter will be able to at least show a value on the AC range.
  3. noobDiyAudio
    So I did more digging and replaced the input capacitor for the side that is not working, the sound did not work but at least the AC voltage measured at the input and output of the opamp for that side is no longer 0. I also replaced the capacitor near the OPAMP for the channel that's faulty. Still no luck on getting both channel to work.

    Please see the attached image.

    It seems that the working side has a voltage (AC) of 0.492V for both input and output channel at 0 gain. While the non working side has 0.014V at 0 gain. It seems like something is restricting the voltage going into the OPAMP. I check the circuit diagram on the first page of this thread and the only thing that is before the OPAMP are the capacitors (which I have replaced) and the poteniometer (which I desoldered and check externally).

    To go further, I have also checked the DC voltages of the transistors and they are all the same as each other (since 1 channel works the transistor for the faulty channel should show different values).

    Anyone got any ideas on what's going on. I'm stuck on what to do next.

  4. precsmo
    ddepo by dfewo precsaseomo
    Last edited: Jan 5, 2018
  5. vrajcevski
    Swapping components is not the best way forward- you say you swapped the capacitor at the input on the non working channel . Do you get 0.492 V AC after it? just before the ALPS POT or not ?

    My suggestion would be to remove the opamp from the socket. and then measure if you are getting the same voltage on the inputs of each half. If you follow the diagram - once the Op Amp is removed, you should have nearly identical AC voltage to ground on the sockets for Pin 3 and pin 5. If you don;t then the only options would be shorted or flaky capacitors connected between the input pins and ground OR some kind of short to ground from the socket itself. - Or dead channel on the chip.

    With 0DB setting, the OpAmp has amplification of 1 - ( o Db) which means the input voltage should be the same as the output voltage on the opamp - which you have on the working channel. on the non Working channel the voltages are very small so - it is possible that the number you have is within the margin of error of the voltmeter - but they are essentially 0 ( 14 mV is all but zero).
  6. noobDiyAudio
    Hi so I took out the OpAmp and the input voltage I got on Pin 5 is 0.495 V (AC) while Pin 3 is 0.033 V (AC). I checked for short circuit and even replaced both the caps near the OpAmp, still same result. I did noticed something strange though, if I plugged in the amp, leave it on for a few minute then turn it off, when I check the continuity between the Right side and the middle ground (at the input socket) my multimeter buzz not beep but buzz. Really odd, the same behavior can be exhibit when I check between ground and the resistor next to the right gain switch (from the photo I previously attached). When I check continuity elsewhere the multimeter beeps as normal, only when I do the steps mentioned will it buzz.
  7. vrajcevski
    ok, so could be anything - related to cold solder - I would simply resolder all the spots in the path from the input to the opamp - i don;t think you have a [roblem further back, but may be worth checking anyway. it sounds like a cold solder that is intermitently connecting, and it sounds like the 0.033vAC you are getting is actually caused by a ground loop, rather than any input coming throough.

    Also , when you take the board out, check for anything at the bottom touching the case and shorting. While you are at it, just check all the traces and solder points for shorts. the fact that your input to the OpAmp is so low simply limits the space to look for. there are probably about 6- 7 solder points to check.
  8. nanchangbob
    OK, I think you need to reset and look at the more analytically. If your going to start messing with electronics you should really do some reading or take a course as it can hurt or even kill you.

    That said, look at what you have, one channel not operational. If you have one channel working you can assume that your voltages on your op-amp are not your problem. Op-amps have positive and negative supplies and since you are using a dual op-amp than your supplies are good since one channel is working. As I mentioned before you can download a test tone and check the output using a DVM on AC this will only let you know that the signal is reaching your jack. Here is a place you can download the test tones.

    If you are going to troubleshoot this thing you really need an o-scope, I have a cheap Hantek that I use sometimes and I have uploaded a few pictures of the signals.

    First is the input and output of the op-amp, I normally keep the switches both down for 0db gain, but I switched one up so that you could see the signal gain. I clipped the channel one lead on the 10K resistor going into the op-amp and the channel two probe was connected to the 47R on the output. Signal-in "good" signal out with gain = good op-amp. You would then jump to the output if both channels check good. Checking AC voltage around the op-amp will show you little to nothing. Remember an op-amp amplifies the difference of the inputs, If the chips have the +VCC and -VCC it will amplify the difference of the inputs.

    If you have actually downloaded and played the test tones and check at the output jack and one channel has readable voltage and one doesn't the first place to start if your not willing or able to get an o-scope replace the op-amp. You can buy a OPA2134 from eBay for $3.99 including shipping. Do not do any soldering and don't pull your amp apart. When you place the op-amp in the socket be careful with the pins. I hold it in my fingers and bend the pins on a desk to make sure the align properly with the holes. Also make sure the notch on the top of the op-amp is aligned correctly. Good luck!
  9. noobDiyAudio
    Thanks for the advice I have since cleaned all the solder joints and resoldered all the points on the input side of both channels (just to be sure)

    Hi thanks for the concern, I am experienced with electronics. I have spare OPAMPs, unfortunately I do not have the o-scope. I have taken the OPAMP out of the circuit and the problem seem to be from the input. There's not much to the input circuit, but after triple checking everything , including for shorts the problem still persists. This is a really odd issue.
  10. noobDiyAudio
    Thanks for those who have assisted me during this issue.

    Attached are more pictures of the voltage measured at the specified given test conditions, I measured both the working Right Channel and the non-working Left Channel.

    I have checked the continuity of the circuit and for shorts to ground. I am really stumped as there are not much components between the signal and the input to the OPAMP and I have checked all the components (even replacing some of them). I also replaced the terminal at the input end, it's really odd how at the point where the signal enters the circuit (the terminal) the voltage is already incorrect. I also tried using different input jacks and cables and the problem is still there. left_channel_voltage_circuit.PNG right_channel_voltage_circuit.PNG
  11. nanchangbob
    What is the resistance reading to ground from R201 and R205?

    What is the resistance reading from both sides of the input capacitor to pin 5 of the op-amp?
  12. noobDiyAudio
    The resistance reading from R201 and R205 to ground are:
    R201 and R205 to ground = 48.7k (pot end)
    R201 and R205 to ground = 58.3k (input end pin 3 and 5)
    input caps to pin 3 and 5 = 58.3k
  13. nanchangbob
    To make sure there is no confusion!

    1. Op-amp is in placed and not pulled.
    2. Measure pin 7 to ground that is R205 according to the schematic you provided.
    3. Measure pin 3 to ground that is R201.
    4. Put one tip of your dvm tip on the RCA input center pin and put the other dvm prob tip on pin 3 of your op-amp. Just a simple continuity test. test resistance. Mine measure around 26K ohms, it will depend on the capacitor, but it should be something near that value.
    5. Repeat the above steps on your right channel. You should then be able to determine which side of the op-amp you are having a problem. You mentioned you have a lot of op-amps? Did you try replacing the one in your unit with another dual op-amp?

    With my op-amp in place I read over 13.5 M ohms on pin's 1 & 7 of my op-amp.
  14. vrajcevski
    asmuch as I can agree with what you said, the whole thread started as support to somebody who has no or minimal knowledge, and minimal tools. AND no titme or possibly inclination to go take a course and buy tools.

    the topology of the amplifier is quite simple.

    if you remove the OpAmp, you basicaly have the input signal terminating on the Opamp socket input pins. this then goes Nowhere UNLESS the OpAmp is in the socket and working .

    if you feed the same signal on each channel ( rca) you should expect to get the same value on each channel input pin on the socket - and if you don;t then you have a problem in the path to to OpAmp socket input pin.

    if you do get the same vaolue , then it is likely that there is a dead opamp channel.

    the whole discussion has gone the way it has simply because of the assumption of minimal knowledge of electronics and no measurement instruments apart from universal meter - tape and gum so to speak. I still think eliminating cold solder or even something as trivial as short under the board across the positive and negative inputs on one channel is simple enough to do .

    But yes, a course and instruments would be great if the interest is there - otherwise it will be much cheaper to simply go to the seller and get a replacement or refund -with the risk that the same problem may happen again.
  15. vrajcevski
    based on the No OPamp measurements, it looks like you get o.o3 at the very RCA- I assume you have checked for no shorts on the RCA itself? if you just sum the voltages on the path from the very input to the input pins, you get almost no voltage drops, meaning minimlal current flow .

    What is the input AC voltage of the signal before you plug the RCA cable into the input? I assume it should be 0.485V or close? If that is true then the only thing to explain drop to 0.031v there is a short to ground somewhere. to prove- or disprove - what is the resistance between Signal and ground you get on the RCA socket itself? for DC it should be open circuits after the input capacitors and c318 charge completely- this would be no signal and no power on measurement.
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