Need to build JC 2 preamp

[Woadl]

Hello I have just discovered this thread. I have a JC-2 and I had a faulty tape monitor switch and in trying to take it apart and cleaning it it fell apart. Can someone recommend a replacement that would just fit back in to the chassi ?

I live in Australia

 

[gk7]

The switches used in my ML-1 (later JC-2) are still available at Mouser.

 

[Woadl]

Thanks Georg I had a look there but I was not too sure which one
any chance of a part number ?
do you think it will fit straight into the chassi ?

 

[erez1012]

Hi
just buy the jc-2 kit from eBay:
the recommends are:
• Input stage : 2.5-3MA
• Voltage amplifier stage : 20-25 MA

i suppose that VR2 change the currents input stage? and 3mA*R17=0.18v ??
how i change the amplifier stage currents ?

the gain according to eBay is 3.7, can i change it? how?
thanks

 

[carlmart]

If I remember well, there's another part of the circuit that is not included in the schematic, involving an output capacitor.

 

[erez1012]

If I remember well, there's another part of the circuit that is not included in the schematic, involving an output capacitor.

you talk about the feedback? i think the big cap its the feedback.
the input, its balance or un balance?
thanks

 

[gk7]

If I remember well, there's another part of the circuit that is not included in the schematic, involving an output capacitor.

The output capacitor is on the mainboard, I´ve posted the schematic already here:

http://www.diyaudio.com/forums/solid-state/10160-need-build-jc-2-preamp-8.html#post3407627

 

[erez1012]

gK7:
maybe you can help me with my questions?
thanks

 

[gk7]

Well, not really. Most of the ebay clones are only loosely based on the original.
Without information about the specific implementation its impossible to give any
support. Actually the seller of this kit should answer the questions of his customers.

 

[erez1012]

hi gK7
my question are very general, i tray to ask the seller.

 

[erez1012]

can i input balance signal from DAC to the jc-2?
thanks

 

[AndrewT]

No.
The input is unbalanced.

You will need a bal to unbal convertor circuit. There are lots on this Forum.

Bal Dac > bal connector > bal to unbal convertor > unbal connector > unbal input

 

[gk7]

I have played with LT Spice today and did a simulation of the
line stage.
Thanks to Bob Cordell (for the models) and dado (models and
spice parameters).

Now this is just a simulation, but at least from this theoretical
point of view it is perfect:
gain of 10, ca. 1V out, Total Harmonic Distortion: 0.000022%

 

[gk7]

Same thing with load on the output (10k parallel to 500p),
astonishing is that now the distortion is even lower (0.000015%).

 

[john65b]

No.
The input is unbalanced.

Sorry if I am a little dense, but I had same question - what is -IN for if not balanced input (missing the R15 // C6 to ground)?

 

[gk7]

For connecting the feedback. Of course you could build something with a symmetrical input with this circuit the same way you would do with a single opamp.

 

[AndrewT]

change R15 to 4M7 and the impedances becomes balanced.

 

[koldby]

Is this so Andrew?
In no signal situation this is true, but what about when signal appears at input
R15 is connected to ground(no AC) but R19 is connected to output and a lot of AC.
To me it looks like a dynamically unbalanced input , impedance wise.
Koldby

 

[john curl]

The JC-2 is NOT a balanced input preamplifier. IT is single input.

 

[AndrewT]

JC,
post455 shows two inputs
+IN and -IN.

I take from your reply that it is not a JC-2 schematic.