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-   -   JA Pre and MyRef - FE Integration (http://www.diyaudio.com/forums/analog-line-level/214380-ja-pre-myref-fe-integration.html)

bcmbob 12th June 2012 11:03 PM

JA Pre and MyRef - FE Integration
 
While participating in the beta process for the new LM3886 based MyRef Fremen Edition power amp, I have tested several approaches to volume control. The Parasound JC-2 has been described as a “Straight Wire with Gain” solution. There are already a couple of threads dealing with this design; however the goal here is to focus on a specific offering by an established eBay supplier.

Symmetric Complementary J-FET preamplifier JC-2 kit ! | eBay

A similar product is available but I have not had any experience with the supplier.

JC-2 Preamplifier Class A Dual differential FET input PER-AMP Kit DIY | eBay

For some background on developments here at diyAudio:

http://www.diyaudio.com/forums/solid...-2-preamp.html

http://www.diyaudio.com/forums/solid...ml#post1468164

The primary reason to focus on a kit is simply the availability of a common starting point. Some forum members may prefer a more open approach. A close integration between this preamp and the Fremen Edition power amps could produce a world class combination.


Some other interesting material:
Audio Advisor Insight - Parasound JC-1 and JC-2 Back Story - YouTube
Parasound Halo JC-2 Preamp Review - YouTube

NOTE: These two links are included as general public information only. I contacted Parasound in California and received notice that the kits above are not authorized JC-2 versions, nor do they in any way represent the products and/or designs of that company. Their representative has stated there is no desire to encumber the DIY community on their part, but appreciate a clear distinction between their circuits and those being developed without official sanction. The title of this thread was to be “JA JC-2 Optimization”, but has been changed to “JA Pre and MyRef Integration” accordingly.

In any event, I think it is a killer preamp package and hope it will attract much interest.

The Floor Is Open.

bcmbob 13th June 2012 01:13 AM

7 Attachment(s)
Here are some photos of my current build process. Still needs pot shaft extension, clean-up and painting.

erin 13th June 2012 02:01 PM

Is there any dc offset on the output when you turn it on?

erin 13th June 2012 02:04 PM

How would you describe the sound?

SoIL4x4 13th June 2012 02:11 PM

Great work so far Bob. I am very interested in this project, it may be a good alternate for me if the LN does not work out. Mating it to the MyRef FE project is a great idea too. I think having more FE RC users with a similar setup will help the group agree on the improvements of various component swaps. The price is hard to beat too. A savvy builder could probably build a finished stock unit for around $100.

Thanks Bob

bcmbob 13th June 2012 02:12 PM

One channel is showing 9.1 mV and the other is 5.4 mV. That indicates I need to adjust the on-board trimmers again. The process is clearly explained in the documentation.

erin 13th June 2012 02:28 PM

Sounds worrying to me. Does it drift with heat, or time?
I would not want any DC going to my amp.

bcmbob 13th June 2012 02:41 PM

erin,

From my experience using two types of LDR attenuation, a Dynaco PAT-4A, dbx 234, Carver C-1, the pre on a Lexicon usb module - this unit and the bare pot have been the cleanest and least intrusive. If I had to describe "the sound" I could only suggest a bit of "sweetening" across the entire spectrum.

EDIT: The LDRs don't introduce artifacts to my ears, but there is a bit of horizontal stage drift at the extremes of the controlling volume pot. Both the LS and LN have a balance pot to address this.

Because my eBay pot is now on the pre, I ordered these to pieces to try/compare as both a bare pot and a component of this build. Never had the highly rated Alps (also used by Parasound) so it will be interesting - assuming they are not knock-offs

ALPS 100KA LOG Stereo Audio RK27 Potentiometer Pot 1pc | eBay

1pc PCB for ALPS RK27 RK16 Potentiometer Tin-Spraying | eBay

bcmbob 13th June 2012 02:46 PM

Your DC concerns are certainly valid. I can only tell you that with several sessions of continued play ranging from four to six hours, the pre has been stable and caused no problems with several MyRefs and the Fremen Eddition betas.

bcmbob 13th June 2012 03:20 PM

Typo: The value of the alps pot ordered in post #8 is 10K not 100K


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