[Headamp] upgrading a Lehmann BCL clone - diyAudio
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Old 16th January 2016, 01:20 PM   #1
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Default [Headamp] upgrading a Lehmann BCL clone

Dear DIYers,

I decided to throw myself on a DIY Lehmann BCL clone. I am a beginner and you should treat me as a fresh one.

I know my soldering and how to work clean and I'm a perfectionist but next to that I have barely the proper knowledge to make right decisions on circuitry.

Lately I have been reading a lot on this forum and other forums to gather more knowledge about different parts and what they are supposed to do. Got me a little wiser but not anything near my goal. That's why I want to share my project here. I hope to get some good tips here about my next few steps.

Since I am currently located in China, I can get my hands on a lot of parts easily and cheap and I ordered a clone PCB and a package of parts that was supposed to be almost identical to the latest version of the original BCL. The components are genuine from RS-components. The tranformer used was a german made BLOCK 40VA instead of the original custom made 30VA.

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After I put it all together and put it up for the smoke test all worked fine and I was amazed what this machine was capable of even with stock components.

Click the image to open in full size. Click the image to open in full size.

I decided to upgrade:
  • Replaced transformer. Original 40VA BLOCK to 60VA BLOCK (just fits. Maximum size without any concessions or modifications to case) Also braided the cables for less interference like the original Lehmanns.
  • Replaced input capacitors EPCOS MKP 654 1.5uf 250V with Mundorf MCap MKP 2.2uf 250V (Like Lehmann BCL SE version).
  • Replaced all Wima FKP2 22nf 63V caps for ERO MKP 1830 22nf 63V caps.
  • Replaced 2 Wima FKP2 100pf 100v 630V caps with LCR 100pf 150V .polystyrene caps. (als have some ERO MKP 1830 for these to try).
  • Replaced 4 Nippon LZX 470uf 25V caps for Panasonic FR 470uf 35V caps.
  • Replaced 2 other (opamp) Nippon LZX 470uf 25V caps with Panasonic Pureism 470uf 25V caps.
  • Replaced cabling with Mogami Neglex 2549 cables. Soldered to the PCB.
  • Replaced opamp with dual discrete opamp OPA111VM with class A mod with 2 x 2.7K resistors. (Still working on a better adapter with less components and soldered.)

Click the image to open in full size. Click the image to open in full size.

A shielded Schurter powerswitch with fuse is on the way to light this baby up again.

Now I am looking to upgrade a few last things.

When looking at the original Lehmann BCL SE I see that the 22nf WIMA bypass caps are removed.

Click the image to open in full size.

I replaced these with ERO's but I have learned that mostly the best cap is no cap. Capacitors are a necessary to block noise and stuff. My question would be: what can I do with these? Can I just remove them and check if that improves the sound while not creating noise or a big DC offset. I read some people bypassed these with copper wire? I am not sure why. I posted a PCB overview above.

Can someone with a bit of technical knowledge help me out on how and why the caps are there and what to do with them?

I am also doubting if I should upgrade the Vishay BC's for the SE's Mlytics. The Mlytics will cost me about $50 for a pair. Will this investment be returned in significantly better soundquality? I upgraded the transformer to 60VA will this have any influence on my choice for capacitor capacity? Also for the 470uf caps?

And maybe any other weaknesses visible in the circuit and component choice I could look into?

I hope I can get some guidance here. I am open to learning and your advice on these matters

Last edited by Cidious; 16th January 2016 at 01:41 PM.
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Old 16th January 2016, 01:41 PM   #2
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something is going wrong with the pictures. I will try another host.
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Old 16th January 2016, 01:59 PM   #3
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You have linked to the thumbnails rather than the original, big images. Which, btw, you could also add as attachments here so they don't get lost. (Once added, you can copy the link and insert a new image using it.)

Please check whether the PCB is the exact same one as mentioned here, and hence whether it may require the same grounding fixes.
100Hz Hum in Lehmann Linear clone

The original parts that you list as replaced all would have been plenty good enough. You won't see me complaining about Nippon Chemicon und WIMA caps. It's not like you've got some Chinese off-brand stuff there.

The Lehmann's Achilles heel is the diamond buffer output stage outside the feedback loop anyway. They probably couldn't get things stable otherwise (at least RJM's Sapphire v1 with similar circuitry apparently often exhibited oscillation issues in GNFB mode) and hence left it at that. Best results are hence expected into mid-high impedance loads.

What sort of headphones do you use anyway?

EDIT: Errr, oops....?! Where'd the first post go?!
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Old 16th January 2016, 04:58 PM   #4
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I will upload all the pics here.

The PCB is different than the one from your link. This is a 1:1 clone from the Lehmann and revision 4.0. I am in close contact with the shop that cloned this PCB and they are quite serious about that they wanted it to get as close as possible to the original design.

I have no humming problems whatsoever. Everything is stable. Just looking to upgrade it further. Not out of necessity but out of interest and for the learning experience. Trying to understand this little machine.

The WIMA FKP2 caps where supposed to be not very well suited for the signal path. Polystyrene caps would be a better pick for this use? please enlighten me. Thats why I swapped them for ERO's.

The Nichicon's are not bad caps but the Panasonic FR's are better and I had them lying around.

I run the MrSpeakers Mad Dog Pros and DT1770s on it.
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Old 16th January 2016, 05:09 PM   #5
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I can not post any pictures here yet.
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Old 16th January 2016, 11:52 PM   #6
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Looks like a fun project, on my headphone amp, I have used a snubber on the transformer secondaries, in a CRC arrangement. The values that worked well were a .01uf 600v fkp1 across the leads, and a .1uf 160v polystyrene in series with an 18 ohm R across the leads.
This added a lower noise floor, and better timbre, more bass punch.
Also used some Philips soft recovery diodes that resulted in a similar effect.
The smaller transformers tend to be a bit more noisy it seems, or benefit from doing this.
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Old 17th January 2016, 12:02 AM   #7
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I looked up the schematic of this and found its almost identical to the headamp I bought from Taobao last year - this one : https://item.taobao.com/item.htm?spm...8-a0e4a92c4918

A major upgrade can be had from increasing the impedance of the feedback network around the opamp - mine was using a TL072 which has fairly lousy output drive.
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Old 17th January 2016, 12:59 AM   #8
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Lehmann clones are pretty common, yeah (to the great chagrin of Mr. Lehmann, I might add).
Quote:
Originally Posted by abraxalito View Post
A major upgrade can be had from increasing the impedance of the feedback network around the opamp - mine was using a TL072 which has fairly lousy output drive.
Looks like that was an el cheapo clone then. I'd rather swap out the opamp. The original uses an OPA2134, which should have little difficulty driving 4k7 + 680 ohms worst-case.

That circuit looks like a case for OPA1642s on adapter boards (FET input, good input impedance linearity). Some further accomodation would be needed for bipolar input types like NE5532 in order to avoid scratchy pot syndrome, though LM4562 may work as-is.

While I'm looking at that part of the circuit, the 10k series resistor between the volume pot and opamp input sure seems excessive. There goes your impedance balance. In high gain, unbalance between the inverting and noninverting input terminals may end up being as high as 12.5k + 10k vs. 680R||4k7 worst-case, which is almost 22k. The OPA2134 datasheet recommends no more than 2k - it's a classic JFET input opamp with the associated input capacitance nonlinearity over common mode swing. I'd swap that 10k for more like 1-2.2k and find some other means of providing RF filtering if need be.
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Old 17th January 2016, 03:19 AM   #9
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Quote:
Originally Posted by sgrossklass View Post
Looks like that was an el cheapo clone then.
Not entirely as the case is a thing of beauty and the opamp was in a ceramic package. But it was only 368RMB fully boxed and ready to roll (not a kit) so yeah it was definitely cheap.

Quote:
I'd rather swap out the opamp. The original uses an OPA2134, which should have little difficulty driving 4k7 + 680 ohms worst-case.
Each to their own. Dynamics I've found improve no matter what the opamp so I nowadays choose the cheapest that gives the sound I'm looking for.
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Old 17th January 2016, 06:35 AM   #10
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So the thread took a lift off. I am a bit confused about if the tips are pointed towards my project or the other one.

I will try to get pictures up as soon as possible.
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