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The "Lovely Cube" Headphone Amp (Lehmann Black Cube Linear Clone) - Page 72

post #1066 of 1171

Hi!

 

You need a DMM - "Digital Multimeter". Use the mode for 200mV or 20mV and use the red and black "chopsticks" to read the... let me call it failure rating (DC Offset) of the Output.

Here is a good "how to" on this website, but for speaker amps ->  http://www.wikihow.com/Measure-DC-Offset

For Headphone amps is nearly the same:

 

*Open the case for measurements and put one test lead to the phone output. In case you don't know the connection of a phone connector ->

 

Tip (close to the heatsinks)

Ring 1

Ring 2 (close to the front)

Left Channel Right Channel GND

 

So measure this:

between ring 2 and ring 1

between ring 2 and tip

 

0mV is very good < less than 50mV is fair but ok < more than 51mV is bad < when there is a offset bigger than 100mV, do yourself a favor and don't connect such expensive phones like the audio technica. They may take damage.

You did not solder the LC by yourself, do you?

 

* if you never opened an electronic device before, please be careful or ask someone who has experience. Inside the LC are some voltages, that can hurt and close to the transformer (the round thing) is enough voltage for very serious injuries! Every device powered by a wall outlet can be dangerous, when you open it!!!

Also don't start to put test leads everywhere just for fun, i did that when i started in DIY. Some components may take damage if you create bridges for the voltage with your DMM.

 

kind regards!

 

PS: The quality of components... that is a very large topic on a headphone amp. Best way to solve a problem is to find the reason. Check the DC offset, then we will go further ;)

 

(edit: it was late and my mind spoke german to me ;) )


Edited by nippon - 9/19/13 at 5:17am
post #1067 of 1171

Hi,

Thank you very much, nippon !

I checked the DC off set , it's less than 50mV

I bought the kit on ebay, components originating from China, that may be the problem ?

although I do not use more than 50% volume, but if i use the gain, the hum destroy everything :angry_face:

 

kind regards!

post #1068 of 1171

Hi,

you're welcome!

I'm glad that i maybe can help someone to achieve a better sound quality and i know your situation :)

So you bought a kit, that means you solder it yourself?

 

You should try to imagine how this circuit is working.

There is a power supply -> delivering 2x 15V via 1xLM317/1xLM337

First the audio signal reaches the OPA, and from there it is amplified at the NPN & PNP transistors (BC550/560 & BD139/BD140)

When we find the spot which makes the hum noises we can possibly improve that area.

Search for the datasheets of all that components (LM317,LM...,BC550,...BD140)

 

Well, let's measure some more points:

Power Supply:

The voltage of the powersupply should be nearly the same. Are there really around 15V getting to the OPA and the transistors?

(in my opinion the LM317/LM337 is a good voltage regulator but not the quietest)

 

Audio section:

First check the OPA. Identify the model and find the datasheet.

Is he getting enough current?

Is there a DC offset on it's 2x outputs? (measure between "out A" and " -In A" , 20mV on the DMM)

Is it very hot after the amp is running for quite a while?

 

Afterwards you can compare the measurements of the transistor pairs. Just let me say, there is a large space between a working and a not working transistor. High quality audio products got matched transistors to prevent the signal from getting interfered. (i hope there are some experienced guys reading my reply & correct me if i'm wrong ;)  )

We're searching not for some differences of 4%, we're looking for a big imbalance.

 

If so far the voltage is ok, the OPA is ok and the transistors are working correctly, you can try to add a feedback modification.

 

Keep in mind, that is a big circle of components and the audio signal is influenced by all of them.

When you bought a very cheap kit from ebay, than you will get parts that work together but there is no internal harmony.

IMO that is one of the reasons why the Matrix M-Stage and the Lovely Cube are more expensive.

And of course maybe you got super sensitive ears :P

 

 

kind regards

nippon

post #1069 of 1171

linhhonlangdu,

 

when you switch the power off on your cube do both the leds go off at the same rate, as I had the dreaded hum on my cube, but mine is built with quality parts, one led fades in less than 2 seconds and the other more than 5, I have checked the soldering under the board over and over,shaking the board, proding all the components.

  I was checking the offset for the first time Today, and rather miraculously the hum disappeared,  now both leds also fade at exactly the same rate.

    hopefully the hum will never return.

post #1070 of 1171
Quote:
Originally Posted by forumfan View Post
 

linhhonlangdu,

 

when you switch the power off on your cube do both the leds go off at the same rate, as I had the dreaded hum on my cube, but mine is built with quality parts, one led fades in less than 2 seconds and the other more than 5, I have checked the soldering under the board over and over,shaking the board, proding all the components.

  I was checking the offset for the first time Today, and rather miraculously the hum disappeared,  now both leds also fade at exactly the same rate.

    hopefully the hum will never return.

 

Hi forumfan,

 

the leds are also a little mysterious on my LC clone. When i switch my power on, they are starting to glow in balance, but if i'm power off the amp one led glows maybe 1,5 seconds longer. I asked in my thread (hifi-forum) about it and the people were assuming that there is nothing to worry about it. Both leds are getting the same amount of current and it should only be a matter of how fast the power fades away (depending on how slow the LM317/LM337 are reacting on the "power off").

But there is something i would like to ask you guys:

 

I measured this results in my power supply (picture) -> can you please measure these points on your LCs?

The point is that there is a difference between the DC 20,7V and the 17,8V - using a 2x15V AC transformer.

(My DC voltage is now equal because i installed 2x resistor trimmers (not in the picture) so that i got now DC 2x 15V instead of 14,9V/14,7V for the circuit)

 

kind regards

nippon

post #1071 of 1171

Hi everyone,

When i switch the power off on my cube do both the leds go off at the same rate

I have measured all points in the direction of nippon ( again, thank you very much ) everything is at an acceptable level, so I've replaced a few componen ( I have replaced the capacitor 470uF )

My LC was bettter !

After this time, my experience is: quality of components is very important for sound quality

Thank you very very much !

post #1072 of 1171

Hi Nippon,

  I am using a 2 x 15 volt transformer and I get  AC values of 18.5 for both and DC 23.3 for both at the points you specify.

 

                                                                          Regards,

 

                                                                                             Forumfan

post #1073 of 1171

If anyone has a technical eye, can they tell me which of the following clones has the better components? My friend in the south east says he can get these models for me but they vary in cost. Obviously I don't have the technical know how to see which models are better value. Any help would be appreciated.

 

Just for comparison here is the original laser collection lovely cube:

 

lovelycubenichicon03.jpg

 

Option A:

T2E7uvXmVXXXXXXXXX_!!82949822.jpg

 

Option B:

 

T2YyAwXiFXXXXXXXXX_!!82949822.jpg

 

Option C:

 

T23bhAXlFNXXXXXXXX_!!78617301.jpg

 

Option D:

 

T2qxkWXc0aXXXXXXXX_!!647667811.jpg

post #1074 of 1171

Hi akms,

  the problem is whether or not the parts are actually genuine, but I would discount B as I don't like the look of those large silver capacitors, they are certainly not a desirable brand.  It really is worthwhile buying the board and going through this thread to select your own parts, the extra cost in my opinion is really worth it.

post #1075 of 1171

Yes, i agree with forumfan.

Buying the PCB or a kit that contains it, gives you the option to check and change all parts. If you are not interested in DIY, just spend more money on the Lovely Cube or the Matrix M-Stage.

 

Every picture of your A - D variants has some good and some bad things on it. Once i bought a kit for 30€ and thought that would be a great deal. 60 days later i invested about 70€ more to get a result i like today.

post #1076 of 1171
Thanks for the replies. I saw that the lovely cube is around $230, but these models are all below $150 fully assembled. I'm not technical enough to choose and swap components so I'll guess I'll read a bit deeper into it before making a decision.

Also does anyone know what these are like paired with Beyedynamic DT880 600ohm and Fostex T50RP?
post #1077 of 1171

My right channel recently started having problems. The volume is low and sound is distorted/weak. I tested the DAC and headphones without the amp and they work fine. Anyone know which amp component might be failing?


Edited by taiyoyuden - 9/29/13 at 4:02am
post #1078 of 1171

If your Op amp plugs into a socket you could try giving that a little poke as I have had a similar problem when my Op amp was not pushed home in the socket fully, it could just be a bad connection elsewhere, the input cable connections are another likely possibility. 

post #1079 of 1171

Thanks! I pushed the opamp down some and it's working now.

post #1080 of 1171

Hi Patrick,

 

I like the thought of using an alternative  to the 317 and 337 regulators.  I think the LT1083 is a positive regulator?  Wouldn't you need one side to be a LT3015?

 

 I will be heading to the U.S. next month to visit family and I have a set of Salas BIB low voltage shunt regulator boards from DIY audio waiting for me.  I might try the same thing with those.  Anyway if I'm wrong about the 1083 let me know.

 

Thanks,

Bob 

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