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The "Lovely Cube" Headphone Amp (Lehmann Black Cube Linear Clone) - Page 66

post #976 of 1171
Quote:
Originally Posted by Big Sky Audio View Post

I have owned a Lehmann Audio BCL for over a year. This may not be the correct place to post regarding this amp but the upgrade mod that I have performed will apply equally to the Lovely Cube clone. I use Sennheiser HD-650s but have also used HD-800s and it is already well known that the Sennheisers match up extremely well with the BCL as they do with the Musical Fidelity X-CAN v3. In both cases, the synergy between the amp and phones is remarkable.

 

First, get rid of the input capacitors as they are polyester type (notoriously bad for good sound) and replace them with polypropylene. They are expensive but well worth changing. The larger ones are 1.5uF in the LC but 2.2uF in the BCL. Next, I would replace the Op-Amp as the OPA2134 is good but the sound is lacking in the very lowest bass registers. After trying many different Op-Amps, my personal favorite is the OPA2604 and it does not matter whether it is the original BB or the TI/BB version. If you have any acoustic music with low bass in it you will notice right away that the very lowest bass is produced well with the OPA2604 but somewhat absent with the original OPA2134. It is my absolute favorite chip and prefer it over all others including a set of OPA627Bs I have on hand. I enjoy everything about it and the amp has more detail, is plenty fast, and much better on the low frequencies while the highs remain clear and unstressed. If you do not want to solder (best way) in your choice of Op-Amp then use a really high quality dip socket. I use the Swiss made Augat brand as they are high retention type with 4-fingered contacts for reliability. It is probably the next best thing to soldering it in place.

 

Next, I would replace the six 470uF/35V electrolytics with Panasonic FC series and I took it one step further and replaced the Op-Amp decoupling caps with .1uF polyesters (they were .022uF originally). The main power supply capacitors were changed to Nichicon KG series Gold Tune Type II. The value was left at 4700uF each as that is more than sufficient capacity given the power level of the amp. From this point on, returns on investment with further parts changing produces diminishing returns. But I did replace resistors R13, R14, R21~R24 with PRP PR9372 metal film. The resistors used in local feedback for the Op-Amp (R21& R22) can be important and they can have a direct effect on the overall sound. Vishay Dale are very good with a neutral sound but I wanted the PRPs as they are non-magnetic and designed for audio purposes. I hope this has helped!

 

 

BCL board v4


Can you tell me how does each mod improve the sound?

 

post #977 of 1171
Quote:
Originally Posted by Big Sky Audio View Post

I have owned a Lehmann Audio BCL for over a year. This may not be the correct place to post regarding this amp but the upgrade mod that I have performed will apply equally to the Lovely Cube clone [...]

<snip>

 

Thank you so much for taking the time to post this.  I have a LC kit still in the box and I intend to implement your recommendations.

 

post #978 of 1171

If you want change capacitors, avoid WIMA FKP2 at all costs. It is high quality capacitor but it is not made for audio. Especially near OPAMP... Every polystyrene (even 50 years old) kicks its ass.

 

Only capacitor that is good for signal circuit is Wima MKP10. That capacitor behaves almost like ERO.

Wima MKP4 I use only in places where I need more low frequencies.

Wima MKS2 I use in power supplies.


If OPA604 (or OPA2604) is placed in good circuit, it will blow away OPA2132/2134 and lots more expensive opamps. But OPA604/2604 is like king. It needs the best treatment and it is very picky about capacitors, resistors and circuit topology. But when it really sings - then you see the difference.

 

OPA2132/2134 opamps are "good for everything" because they don't change character with circuit topology. You can put them in a switch power supply, geiger counter, audio device, car... You can put them even in toaster - if there is any that needs opamp. It will behave in the same way. It is easy to make circuit  (also cheaper for manufacturers) when you have opamp that likes all types of circuit. And that is what manufacturers like the most.  less expenses = more money. Today all opamps are good enough for average listener and his demands.

 

As far as I know (and tried) for output resistors you can use Allen Bradley or (if you can find it) Riken Ohm resistors. They have something that fills music with magic.

Other option is Kiwame/KOA Speer. Superior details and slightly aggressive. Not so magically like Riken and Bradley, but jump ahead from stock carbon composition Dale. And many many miles away from every stock metal film/karbon film resistor.


Edited by BlaBlaBla - 2/11/12 at 10:08am
post #979 of 1171
Quote:
Originally Posted by BlaBlaBla View Post

 

You can find matched BC550/560 transistors but it is not worth because soldering and de-soldering them is tricky process. The best way to low down DC offset in your amplifier is to replace 1k1 resistors with trimmer pot and 1K resistor in serial connection (that will make usable range from 1K to 1K2 which is enough for voltage tuning between 14V to 16V. Also you can put there one 2K trimmer pot but that I would not recommend because voltage tuning will be less precise AND trimmer pot has deviation (usually 2%) that can make your voltages different - every time when you turn on your amplifier. You can use vertical or horizontal trimmer pot. 100R resistor leave as it is.

 

From my experience with LC topology I know that below 14V regulated DC voltage after regulators, DC offset on headphone out is negligible and you do not need matched transistors (BC and BD) and deviation in manufacturing process is enough tight. But when you try to use +/-18V DC and more, then matched transistors are of big importance. Unfortunately "matching transistors right on the table" is only the first part of story because when you match something that means that you need exactly the same environment for what you have matched. That means that matched transistors must always have the same temperature (transistor characteristic varies with temperature). So, it is not easy to do without custom made PCB and heat sinks. In old amplifiers that was done with 3 pin sockets that were soldered on the PCB and in there different transistors were placed and tried. When the lowest DC (or the best characteristic on the scope) was achieved transistors were soldered.


 

Hi BlaBlaBla,

 

I rebuilt my LC, following your instructions. Now it is another amp!

 

But I have other doubts:
If I set the same voltage  15.3 V and -15.3V with the pot, I measure a DC offset of 27mV  in a channel and - 6mV in the other one.

There Is another way for adjust the dc offset without set  the power section, ? For example, could I put a potentiometer for each channel before of opamp or transistors in order to have indipendent setting?

 

P.S. I can't  match the transistors. I haven't got enough components


 

 

post #980 of 1171
Quote:
Originally Posted by supertrichi View Post


 

Hi BlaBlaBla,

 

I rebuilt my LC, following your instructions. Now it is another amp!

 

But I have other doubts:
If I set the same voltage  15.3 V and -15.3V with the pot, I measure a DC offset of 27mV  in a channel and - 6mV in the other one.

There Is another way for adjust the dc offset without set  the power section, ? For example, could I put a potentiometer for each channel before of opamp or transistors in order to have indipendent setting?

 

P.S. I can't  match the transistors. I haven't got enough components


 

 

 

You should try lowering voltages with trimpot. The lower the input voltages are - the lower DC offset will be. So try lowering your input voltages to +/-15V or even lower (around 14.5V).

Try lowering down both voltages. But that you will be able to do only if you have two trimpots on input (one for positive and one for negative rail).

 

For example, lower your positive rail to 15V and then - tune the negative one. If offset in each channel is still above 10mV try the same but with 14.9V. Until you get satisfied DC voltages (lower than 10mV is good, and lower than 5mV is safe for all kinds of headphones).

I think that somewhere around 14.8-15V you will be able to equalize your DC offset. Do not worry even if you find a hot spot below 14.8V (lets say around 14.5V). In real world it doesn't matter if power stage of headphone amplifier is driven by 30V (+/-15) or by 28V (+/-14V).

 


Edited by BlaBlaBla - 2/12/12 at 3:02am
post #981 of 1171

I put a new pot instead of 1.1k resistence and I can adjust the voltage indipendent.
But not the DC offset!

The DC offset changes in both channel and remain near 17 mV of difference between ones.
Indipendent if I set the positive or negative  voltage.

 

Now I choice +15 V -14.5V and offset -8.5mV and 8 mV

post #982 of 1171
Quote:
Originally Posted by supertrichi View Post

I put a new pot instead of 1.1k resistence and I can adjust the voltage indipendent.
But not the DC offset!

The DC offset changes in both channel and remain near 17 mV of difference between ones.
Indipendent if I set the positive or negative  voltage.

 

Now I choice +15 V -14.5V and offset -8.5mV and 8 mV


It is strange...  With pots I am able to tune offset down to 1 mV. Probably transistors are from different series.

How good is sound from the amplifier? Do you hear something unusual?

 

post #983 of 1171
Quote:
Originally Posted by proid View Post


Can you tell me how does each mod improve the sound?

 


Whatever you do, try polystyrene as opamp coupling (10nF 63V). In combination with OPA2604 and Grado SR225 deep bass really kicks butt. Almost like sub woofer.

With OPA2107 bass is higher but more wide, and I prefer this combination for watching movies.

post #984 of 1171
Quote:
Originally Posted by BlaBlaBla View Post


It is strange...  With pots I am able to tune offset down to 1 mV. Probably transistors are from different series.

How good is sound from the amplifier? Do you hear something unusual?

 



 

No, nothing strange.
After changing the capacitors, following your instructions, it plays better.
I changed also the transistors with OnSemi, unfortunately they don't match.

it just Remains a sensation of less headstage, particularly with K701, but certainly, the basses are a little bit presence

 

post #985 of 1171
How are these made? Doesn't Lehmann have a patent on the Cube?
post #986 of 1171

 I read through the thread.  A lot of good information.  In the past I was more of a two channel stereo guy.  But I do have some HD600's and a self modified Creek OBH-11.  Now I took a new job and I will move to a new city.  So I will become a dedicated head-fi guy.  I need a new amp and this looks like a great project.  I'm working and living in China and I managed to work my way through Taobao and I did come across the boards used in the lovely cube. But I came across the board below and it had great feedback from the people who bought it.  So I bought it and I'm now populating the boards.

 

  The LC boards were about $4.00 and these were around $11.00, It is a little bigger so I will by a bigger case for it.  I already have a 15v *2 50 VA transformer and more than half the parts I need.  I will get the op-amps and capacitors next month when I'm visiting my family in the U.S.  The eyelets look dark in the pictures but they nice copper pads.  It even has the correct holes to install the trimmer pot.

 

 

.IMG_0020a.jpg

 

 

.IMG_0008a.jpg
 

post #987 of 1171

I just recieved my LC and have to say that i was very surprised about the excellent job.

 

Did anybody ever tried to use orange drops 715 as replacement for the yellow coupling capacitors?

As the values are the same for the DC blocking could I use them also in that area?

Could I use these orange drops in parallel with MKP4 2,2uF (Original was MKT 1,5uF)

 

I replaced the low pass filter with Polystyrene caps same value.

I saw however that Stephen used in mine ERO1838 V instead of the blue MKT.

 

Appreciate your opinion.

post #988 of 1171

Two questions - if anyone knows.

 

Are the output RCA's volume controlled? or is it a pass through? and if I just want pass through, can I just bridge the wiring between the sets of RCAs or does that cause issues? 

 

Second - could I use an external power supply instead of one of their Toroids if I were trying to save space? If so, what are the requirements?  

post #989 of 1171
Quote:
Originally Posted by liamstrain View Post

Two questions - if anyone knows.

 

Are the output RCA's volume controlled? or is it a pass through? and if I just want pass through, can I just bridge the wiring between the sets of RCAs or does that cause issues? 

 

Second - could I use an external power supply instead of one of their Toroids if I were trying to save space? If so, what are the requirements?  

 



Output RCA are volume controlled because everything goes through volume pot.

post #990 of 1171

Any thoughts on this question?

 

 

Quote:

 

Second - could I use an external power supply instead of one of their Toroids if I were trying to save space? If so, what are the requirements?  

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