Variation on the JC-2 preamplifier

[radule]

John,

i would like to put JC2 i same box with First Watt F5 power amp.
Wich kind of voltage stabilisation for JC2 you recommending if I use same power supply
for preamp and amp.
Power supply is +/-23V , toroid, double brigde and 2x68000uF, with less then 0.5v voltage ripple.

PS i will try JC2 with all Toshiba semi, k170bl, j74BL, 2sc4793, 2sa1837.

 

[john curl]

A complementary (2) cap multiplier might do the job.

 

[MartinV56]

2016 New JC 2 V2 Preamplfifer Mini Class A FET Preamp DIY Kit Alps Potentiometer | eBay

Assembeld JC 2 V2 Preamplfifer Mini Class A FET Preamp Board Alps Potentiometer | eBay

I opted to go for a JC2 preamp version JC 2.2 at ebay.
Someone has worked this version?, thanks

 

[radule]

PS i will try JC2 with all Toshiba semi, k170bl, j74BL, 2sc4793, 2sa1837.

These days I have a time for playing so I exploring this simple topology.

On the bench I tried this topology with matched 2sc4793/2sa1837, and there is nothing to see distortion related. To see some harmonics output must be heavy loaded. - 1kohms or so...

I will need it only to drive 50kohms load and 15cm of twisted pair with max output of 3Vrms, so I decided to try mosfet version (couriosity ).

I measured mosfet circuit in open loop with 4k7 load (inverting input 1k to gnd) and OL bandwith is about 40kHz, gain is about 55dB.
THD is around 0.003% with 15dB of gain, when loaded only with feedback network.

I dont know how it sound but I think that is good result from simple circuit,
without to much feedback.
Any suggestion circuit related?

Circuit and few pictures is in attach.

 

[Max Headroom]

Been using the JC-2 Pre-amp for a few months now, still like the sound, but it has just started giving an intermittent pop, with some cracking and static noises came from JC. Does anyone have any idea what could be happening, or how I could stop it.

Sounds like you have a defective/leaky transistor.
Heating the board and then cooling (freeze spray) individual transistors should highlight the culprit.

Dan.

 

[Max Headroom]

Hi max, thanks for the reply, As i am an electronics Luddite, I am not sure how to do that.
a) Are you taking about the Jfets.
b) if so it my be easier for me just to replace all on them. Cheers

Yes, it is likely that the fault is with one of the Jfets, though any of the transistors could be at fault.
Try heating individual transistors with a soldering iron, this will usually encourage the noise fault, and then cool the transistor with a cotton tip soaked in alcohol etc.....you should be able to make the noise fault come and go.
Try this and get back to me.

This design relies on matched fets, you will need a set if you have established that the fault is with a Fet.


Dan.

 

[PB2]

By the way, it is not necessary to build a JC-2 line amp from scratch. Just get a completed unit E-Bay online from China. I paid $126 + shipping. The case alone is worth that much.

Hi John, Any updates on this preamp?

 

[PB2]

Youse are all deaf and wasting youse time....youse needs one of these .

Assembled MARZ Preamplifier board with ALPS potentiometer (support 4 ways input)-in Amplifier from Consumer Electronics on Aliexpress.com | Alibaba Group
View attachment 462229

View attachment 462228

Only joking, feel free to discuss.

Dan.

Anyone have a link to a full schematic of this "classic Marantz" preamp?

 

[carlmart]

Found a review of that preamp on Audiokarma, and the comments were it had some distortion as is and with some ccs mods sounded better, but noisy.

 

[PB2]

Found a review of that preamp on Audiokarma, and the comments were it had some distortion as is and with some ccs mods sounded better, but noisy.

Several pages back in this thread someone pointed out hash in the measured FFT plot. JC noted that the compensation caps were missing on the output transistors, installing them fixes the hash.
Not sure if that was the problem that you are talking about. I would worry about fake or poor quality FETS. If I buy one, I am going to be ready to rebuild the board with good parts from a reputable supplier. I bought matched FETs for the front end several years ago.

 

[audiohed]

JC-2 Preamplifier

My JC-2 pre-amp just finished last month. It is a simply but very good sounding pre-amp which I highly recommend for diyer.

I just finished my JC-2. Went thru the basic tests, and she's sounding beautiful, despite the fact I hastily hooked it up to a cheesy cd player and computer speakers.

 

[audiohed]

JC-2 Preamplifier

Just finished my JC-2. I purchased bare board from Jim's Audio and bought my own components. Wasn't sure to expect from this...smoke maybe , lol. Sort of got thru the test process (only a multimeter), and hooked it up to a cheesy portable cd player, and the laptop, driving some little computer speakers. Sounds really good! Can't wait to finish a power amp for it.

 

[lineup]

I recently purchased the JC-2 preamp kit that tubeshunter is selling on eBay. I made some changes to the stock component values to improve performance and reliablility. I also increased the gain to a little over 20 dB.

Congratulations to you to be a genuine JC-2 preamplifier owner.
John Curl is the MAN

 

[carlmart]

One thing John Curl states on the first page of this thread is that matching is important.

What control or expectation can we have for the first stage FETs being matched?

 

[carlmart]

The JC-2 line-amp circuit which is usually around does not include important parts, like the gain-network, input and output parts.

As I want to simulate the line-amp in LTSpice, I added the missing parts.

Please tell me if something is missing.

 

[latrade24]

Hello,

I think that the realization of this preamp presents two major difficulties:
the transistors matched and the cooling of the two output transistors.
I realized according to the scheme of John Curl two modules lines to repair the JC2 of a friend. I made a circuit board to the dimensions of the modules. I bought the FET transistors at this address. For both output transistors, I used the carefully paired BD139 / BD140 (I had bought about 60). The advantage of these boxes is to be able to fix a small aluminum cooler.
I set the output quiescent current at 40mA and the chiller is hot, but not too much hot.
My friend also has a ML1, we detect absolutely no difference in hearing between the ML1 and the JC2 (speakers JBL 4344 + Macintosh 250).
(Sorry for my bad English )

 

[Bill poster]

Bumped this thread after looking at a s/h pre amp (YS Audio) for sale and not realising it was a John Curl clone until I googled.
I read up on the Mark Levinson brand history, ie pretty messed up, with all the back stabbing and ownership changes over the yrs.

 

[keilau]

Bumped this thread after looking at a s/h pre amp (YS Audio) for sale and not realising it was a John Curl clone until I googled.
I read up on the Mark Levinson brand history, ie pretty messed up, with all the back stabbing and ownership changes over the yrs.

The YS Audio offer,
YS-Audio imitation JC-2 preamplifier HIFI EXQUIS class A JC2 pre-amp | eBay


This JC-2 pre has an output buffer stage, making it different from the Mark Levinson one that John Curl discussed earlier. Do you know where it comes from?

John made it crystal clear that this circuit needs well matched JFET for the IPS, which I doubt the e/bay version (either one) had.

 

[redjr]

I built my JC-2 based on the original JC schematic:

I know this is a very old thread, but..... Is there a PCB somewhere out there in the wild for this schematic? Is it in the public domain?

 

[keilau]

I know this is a very old thread, but..... Is there a PCB somewhere out there in the wild for this schematic? Is it in the public domain?

John Curl has been very gracious in sharing his design here. But many of his work were done for clients with commercial interest. Most of the JC2 clone on e-bay are not far off in topology and layout.
Class A HIFI Preamplifier Fully symmetric FET Input Based on JC-2 DIY PAMP Kits | eBay

The trick in getting good result is in matching the input devices.