Amp Camp Pre+Headphone Amp - ACP+

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It looks like you have the pins reversed:

Check the On Semiconductor website for the latest Rev 3 version of the J113 data sheet. It shows different pin order and is the same pin order that the illustration from Mouser shows. It appears the data sheet at the On Semiconductor site is not correct?

Dneu2011 - where did you buy your J113s? The color of the ink on the identification markings looks different than on the ones I have. I bought mine from Mouser a couple of months ago and the markings on mine are a faint white color. The color of the markings on yours looks much darker. Could just be the lighting in the photo though. Just wondering if yours are genuine.
I used the same circuit as you did to match mine without frying any J113s.
 
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The info that I posted was from Mouser for a Fairchild part. Apparently Fairchild is now part of ON Semiconductor.
 

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The info that I posted was from Mouser for a Fairchild part. Apparently Fairchild is now part of ON Semiconductor.

Good news. I flipped the J113 and my breadboard was able to correctly measure. I had enough to get two perfectly matched pairs, but I was hoping to get some help determining the best resistor for 10ma.

With a 125 ohm resistor, I was not able to get any of my Jfets to read as high as 1.25V. Instead, I had a range from .600V to .800V.
 
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Good news. I flipped the J113 and my breadboard was able to correctly measure. I had enough to get two perfectly matched pairs, but I was hoping to get some help determining the best resistor for 10ma.

With a 125 ohm resistor, I was not able to get any of my Jfets to read as high as 1.25V. Instead, I had a range from .600V to .800V.

An ideal approach could be to install the closest J113 pair you have, and then choose a resistor value that gets you just shy of the Idss of your J74’s. For my specific J113 and J74s, I ended up using a 60R at R4
 
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Joined 2012
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Good news. I flipped the J113 and my breadboard was able to correctly measure. I had enough to get two perfectly matched pairs, but I was hoping to get some help determining the best resistor for 10ma.

With a 125 ohm resistor, I was not able to get any of my Jfets to read as high as 1.25V. Instead, I had a range from .600V to .800V.

Use Ohm's Law to determine the current:

I=V/R, so I=0.800V/125R= 0.0064A or 6.4mA

Determining the resistor value will be a bit of trial and error. You need to change the resistor, measure the voltage drop across it, and then use that voltage and resistor value to re-calculate the current. Adjust the resistor value until the calculated current is approximately 10mA. It doesn't have to be exactly 10mA. My ACP+ has a 125R resistor and it measures 1.36V = 10.9mA.

Since your current is less than 10mA, try a lower R, say 100R, measure the voltage, calculate the current, and see where it's at. Then adjust the R accordingly until you get 10mA.
 
music soothes the savage beast
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Hello Adason,

I think you will have ear-pleasing fun! :D

Enjoy the good sound! :violin::eguitar::sax:

Cheers
Dirk :cheers:

I do! I like the sound a lot. I need to put front and back panel on the board in order to move it downstairs and test it against a dozen other headphone amps.
Should be fun.

What I like about this circuit is that all the parts are available (i use linear systems jfets) and the board is large and easy to work with.

Week ago I finished a20 preamp/headphone amp, which is great sounding too, and I will place it in nice (bamboo) box. But the board is too tiny to enjoy working on it.
It will be interesting to compare these two for sound quality.
 
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Ben Mah, so you are doing this in the complete circuit, and not just with a temporary set up with a voltage source, FET, and resistor?

I did not do it myself as 125R worked in my ACP+, but Dneu2011 did the test on a bread board to determine the resistor value. The J113 works as a current source in the ACP+ circuit with Vds at 12V or so. The test replicates the ACP+ current source scheme, also using 12V, so the resistor value determined from the test should also work in the circuit.
 
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It's only a matter of time until someone releases their own redesign entitled "ACP+ Using Premium Parts for Premium Sound". You can bet there will be PCB footprints for lots of different electrolytic capacitor form factors, including several parallel film "bypass" units. I myself did this to the output cap of the Noir HPA and to the bias spreader AC bypass cap of the M2x power amp (both of these are sold in the diyAudio Store) -- just to placate (read: shut them up) builders afflicted with Audiophilia Nervosa, who desperately wish there was a way to spend $125 for each component on the board.