I just found the JC-80 PCB from Jim's audio, the same ebay seller of this PCB! That will be my next pre project. After the tube buffer project...
Looking forward to it and thanks for the tip. This one sounds great BTW.
It is almost Halloween here in the US, so I don't feel too bad about reviving this zombie/old thread
Here is a JC-2 pre I just built with balanced XLR and RCA outputs. I call it the Cathedral 2-Eleven (I am not sure it will be a good idea to get to 7).
Nice build.
I will go up on the artikkel one day and take a picture of my old box. I think I built it around 1981/81. First I built one in1980, or just after the Audio Amateur published the crucial diagram.
If you search ebay for e.g. "Preamplifier PCB" you will find PCBs for several of Erno Borbely designs (Erno has closed down his web page, so this is a source, but probably illegal). You will also find John Curl's designs, John maybe does not care if JC-2 and JC-80 are very old designs now, and he has advanced with Parasound and the Blowtorch. But surely, PCBs for Nelson Pass' B-1 are illegal clones.
We DIYers should take care buying legal PCBs etc. After all, these designers make their living from this.
The JC-2 has both +In and -In. Below is a schematic that I took a couple minutes to create based on the earlier one in this thread. Unfortunately, I am unable to successfully acquire anything that resembles useful audio at the output (20k load) until I understand the purpose of the -In and how to utilize it. Perhaps the generic models for the devices may be the cause, but I am not familiar enough with spice to know. Thank you in advance to John or anyone who can update me on this methodology behind the JC-2's design.
JC-2 feedback loop
If you study the attached picture taken from The Audio Amateur in the late 70ties (I think) you can see the feedback ground leg of 2 kOhms and the feedback loop consisting of the 4.53 kOhm resistor and a switched resistor array to set the gain at 11 discrete positions. Leftmost is OFF, the clockwise from -4 dB, -3 dB, -2 dB, -1 dB, 0 dB at twelve o'clock, and then +1 dB, +2 dB, +3 dB, +4 dB and finally +5 dB.
Please also note the MONO/STEREO switch and the LOW/HIGH switch, both at the front and at the circuit diagram at the input of the line operational amplifier. There is also a "COUPLING SWITCH" at the output shorting a 2 uF capacitor. This capacitor is there to block DC from entering your power amplifier and ultimately your loudspeakers.
I have once read some John Curl initiated modification, but I do not have these handy at all. Sorry.
I have one ML-1 myself, and I have built a few in the past. I would propose to lower the gain of the line amplifier. When using my ML-1 I usually toggle the LO/HI switch to low and run the volume knob at 9 o'clock.
Ït is still a decent preamplifier albeit it is probably possible to build a preamplifier at least as good with present day's better 8 pin IC operational amplifiers.
One last word, the referred Audio Amateur page contains an error, but this is corrected in your LT spice circuit I can see.
If you study the attached picture taken from The Audio Amateur in the late 70ties (I think) you can see the feedback ground leg of 2 kOhms and the feedback loop consisting of the 4.53 kOhm resistor and a switched resistor array to set the gain at 11 discrete positions. Leftmost is OFF, the clockwise from -4 dB, -3 dB, -2 dB, -1 dB, 0 dB at twelve o'clock, and then +1 dB, +2 dB, +3 dB, +4 dB and finally +5 dB.
An externally hosted image should be here but it was not working when we last tested it.
Please also note the MONO/STEREO switch and the LOW/HIGH switch, both at the front and at the circuit diagram at the input of the line operational amplifier. There is also a "COUPLING SWITCH" at the output shorting a 2 uF capacitor. This capacitor is there to block DC from entering your power amplifier and ultimately your loudspeakers.
An externally hosted image should be here but it was not working when we last tested it.
I have once read some John Curl initiated modification, but I do not have these handy at all. Sorry.
I have one ML-1 myself, and I have built a few in the past. I would propose to lower the gain of the line amplifier. When using my ML-1 I usually toggle the LO/HI switch to low and run the volume knob at 9 o'clock.
Ït is still a decent preamplifier albeit it is probably possible to build a preamplifier at least as good with present day's better 8 pin IC operational amplifiers.
One last word, the referred Audio Amateur page contains an error, but this is corrected in your LT spice circuit I can see.
Probably you have to put in some decent JFET spice models. The bipolars are already in the library if I am not mistaken.
2N4401 spice model
.model Q2N4401 NPN(Is=26.03f Xti=3 Eg=1.11 Vaf=90.7 Bf=4.292K Ne=1.244
+ Ise=26.03f Ikf=.2061 Xtb=1.5 Br=1.01 Nc=2 Isc=0 Ikr=0 Rc=.5
+ Cjc=11.01p Mjc=.3763 Vjc=.75 Fc=.5 Cje=24.07p Mje=.3641 Vje=.75
+ Tr=233.7n Tf=466.5p Itf=0 Vtf=0 Xtf=0 Rb=10)
* Motorola pid=2N4400 case=TO92
* 88-09-13 bam creation
.model Q2N4401 NPN(Is=26.03f Xti=3 Eg=1.11 Vaf=90.7 Bf=4.292K Ne=1.244
+ Ise=26.03f Ikf=.2061 Xtb=1.5 Br=1.01 Nc=2 Isc=0 Ikr=0 Rc=.5
+ Cjc=11.01p Mjc=.3763 Vjc=.75 Fc=.5 Cje=24.07p Mje=.3641 Vje=.75
+ Tr=233.7n Tf=466.5p Itf=0 Vtf=0 Xtf=0 Rb=10)
* Motorola pid=2N4400 case=TO92
* 88-09-13 bam creation
2N4403 spice model
.model Q2N4403 PNP(Is=650.6E-18 Xti=3 Eg=1.11 Vaf=115.7 Bf=216.2 Ne=1.829
+ Ise=58.72f Ikf=1.079 Xtb=1.5 Br=3.578 Nc=2 Isc=0 Ikr=0 Rc=.715
+ Cjc=14.76p Mjc=.5383 Vjc=.75 Fc=.5 Cje=19.82p Mje=.3357 Vje=.75
+ Tr=111.6n Tf=603.7p Itf=.65 Vtf=5 Xtf=1.7 Rb=10)
* National pid=63 case=TO92
* 88-09-09 bam creation
.model Q2N4403 PNP(Is=650.6E-18 Xti=3 Eg=1.11 Vaf=115.7 Bf=216.2 Ne=1.829
+ Ise=58.72f Ikf=1.079 Xtb=1.5 Br=3.578 Nc=2 Isc=0 Ikr=0 Rc=.715
+ Cjc=14.76p Mjc=.5383 Vjc=.75 Fc=.5 Cje=19.82p Mje=.3357 Vje=.75
+ Tr=111.6n Tf=603.7p Itf=.65 Vtf=5 Xtf=1.7 Rb=10)
* National pid=63 case=TO92
* 88-09-09 bam creation
Oh bother...this still hasn't changed the gain of 1 and offset issues. I can take another screenshot to substantiate the problem. I am not pulling your leg, this has me concerned. It almost has to be the default generic models that are doing it. How do I use your info above to create new components? Might want to PM that one. Thank you greatly.
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Yeah, the problem with this design is everyone has a different schematic. I used the on in this thread as a reference but another one the web showed 100k resistors. I tried both values and also different resistances for the 500 ohms, all to no avail. I have been up and down it front to back and I can't get it to work. It has to be the generic spice models for npn/pnp. I'll need a tutorial on installing your models' data. I managed to install a full library of vacuum models in 2 minutes with step by step instructions, so I believe someone could instruct me on creating models in a few messages. At that point, I can get an idea of how the JC-2 operates.
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