[Build] CAPI ROCLEM MEC31, Mic / Line Preamp, Official Support Thread

[jsteiger]

Hey folks!

Since our new ROCLEM MEC31 preamp kits are shipping, I figured its time to start the official support thread.

Full details and automated checkout can be found here:
https://capi-gear.com/catalog/index.php?cPath=240_242

The build guide is following our Pictorial Build Guide format. All BOMs, support docs and the build pictorial are located here:
https://capi-gear.com/catalog/Build_ROCLEM_MEC31_Rev_A.php

Since this is the official support thread, please post all questions, components and/or build problems here. I will do my best to help you get things sorted. This is the place to handle issues, not with a direct email. Thanks for understanding.

And heres a pic of the beauty ;D


Cheers, Jeff

 

[woofy]

Getting started on mine now. Hopefully i will not have to return here lol. Very excited to compare them to the vp312, which is currently my favorite.

 

[jschobben]

Hello, all!

Also excited to start recording with the MEC31 and compare to the vp28 and other preamps if people are interested.
@woofy would love to hear your comparison as well if possible!

I do also have a question about a problem I'm having with the Preflight Testing 8.0

Readings

-v +v 247K
-v c 27.3K
+v c 287K
c o 228K
+v o 835K
-v o 435K

-V C is not reading over 200k Ω

It was not easy to measure with a small manual multi-meter.
The readings took a long time to settle. (the last 2 numbers probably didn't settle fully)

I double checked the polarity and placement of the components and those should be all in the right spot.
The solder job isn't the cleanest probably, but there are no bridges.

Potential problems

1. There is only one spot I tried to clean up a bridge that's not looking so pretty..
2. There was one Grayhill switch that arrived with a damaged casing, but it does work (rotates clean) it was like this when I took it out of the bubble wrap.
3. Buy a better multi-meter, something like "suggestion".

I added photos that are hopefully relevant. o/

 

[jsteiger]

I just made a few little updates to the expected results in section 8. Yours seem very fine to me. No need to wait until they fully settle.

I like a Fluke meter myself. I recall a few threads around here discussing DMM's.

 

[jschobben]

Thank you Jeff!

Will check the threads, but firsts... Preamp time!

[Conclusion]
It works!
Will try to find some time this weekend to do a comparison. o/

 

[zzzzz]

@jsteiger so glad you came out with this to make the AML-27 attainable to everyone!

What is the TL081 IC doing?

Edit: ah, I see the “signal” led feature now. I assume it’s buffer for that.

 

[jsteiger]

@jsteiger so glad you came out with this to make the AML-27 attainable to everyone!

What is the TL081 IC doing?

Edit: ah, I see the “signal” led feature now. I assume it’s buffer for that.

Yes indeed. Signal present circuit.

 

[Ooooh_ImAboutToJazz]

I have found that the preamp gain knob slides over the stepped gain selections because there isn't enough torque/grip from the greyhill smooth switch arm. The screws in the Roclem knob are very easy to strip if trying to tighten for better grip. It's frustrating to make it this far in the build and be stopped by a problem like this. Has anyone else found this to be a problem too? If so, are there any recommendations around this?

PS. I found I had a similar result to Jschobben when doing my pre flight test. -V and C registered ~31k ohms and not over 200k. I'm assuming this should also be alright based on the response to his post.

 

[Jim50hertz]

I have found that the preamp gain knob slides over the stepped gain selections because there isn't enough torque/grip from the greyhill smooth switch arm. The screws in the Roclem knob are very easy to strip if trying to tighten for better grip. It's frustrating to make it this far in the build and be stopped by a problem like this. Has anyone else found this to be a problem too? If so, are there any recommendations around this?

PS. I found I had a similar result to Jschobben when doing my pre flight test. -V and C registered ~31k ohms and not over 200k. I'm assuming this should also be alright based on the response to his post.

No problem with the knobs here but I'll be mindful of this.

Re the -v and c resistance measurement, mine was also in the kHz region. They work great and sound great.