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Old 24th April 2011, 03:51 AM   #11
Fenris is offline Fenris  United States
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the 2SK170s are BL grade for the source follower (the 5 in parallel). I don't remember which type are in the gain stage, but it's low current anyway so it doesn't matter. Just make sure that the JFET acting as a constant current device is lower in IDSS than the other. For a 300 ohm load, you'll need more voltage gain and less current. A voltage gain of 6 would be about right. I'd go with a 330K resistor for the feedback and a 47K for the series input. I'd also run at least +/-12v rails and change the resistor in the LED string to about 1K. Ideally, the voltage taken at the top of the resistor should be about -50mv. The value can be calculated by knowing the IDSS of the constant current JFET. Take your negative power supply voltage, subtract the voltage drop across the LEDs, and size the resistor according to the current.

For 300 ohm headphones you will need more voltage and less current. But the power handling ability of the JFETS is limited. You may want to reduce the current through the JFETS acting as current sources for the source follower. This will keep them within their dissipation limits.
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Old 24th April 2011, 05:07 PM   #12
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thankU
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Old 24th April 2011, 06:37 PM   #13
Fenris is offline Fenris  United States
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I'll actuall ammed what I said earlier. It might be better to just go with the standard BOZ style resistor biased front end similar to this:
Jfet BOZ
and eliminate the feedback. Overcomplicating this circuit might not be best, even if it has less theoretical distortion.

I'm going to install RMAA and test things out in the next few days.
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Old 26th April 2011, 06:10 PM   #14
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How much did it cost?

Would it be possible to power AKG K271 MKII with this?
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Old 27th April 2011, 12:55 PM   #15
Fenris is offline Fenris  United States
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How much did it cost?

Would it be possible to power AKG K271 MKII with this?
Easily. The output impedance is below 10 ohms and depending on gain and load can output over 100mw. The K271s are more efficient than most and have a nominal impedace of around 55 ohms. I'd recommend a bit more gain (maybe about 4). I'm even thinking about a different version with a MOSFET and LM317 CCS as the source follower (to reduce parts count). I'm also looking at a DC servo to eliminate the output capacitors.

Cost isn't that high, about $15 for the JFETs, $6 for the DC-DC converters, $20 or so for the rest of the passives, and about $15 for the PCB. Cost could be cut $10-20 by going with MOSFETs and 317s and a DC servo (eliminating the bigger OSCONs). I'm still playing with this on my days off and am trying to get some decent measurements.
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Old 27th April 2011, 03:25 PM   #16
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Intresting. But for a newbie like me, would it be possible to make a nice list of components needed and step-to-step guide and a good map of building it?
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Old 27th April 2011, 04:58 PM   #17
Fenris is offline Fenris  United States
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At the moment, this isn't for a novice. The resistors are all 0805 surface mount, which isn't the easiest to solder. You more or less need a double sided PCB made since everything is so compact. If you want to use perfboard and through-hole components this could be done but it would take up more space. It also depends on what kind of power supply you want to use - the DC-DC converters I used might be a bit expensive to ship to Sweden.

I've got to work for the next 7 days straight. After that I'll get some time to work up a perfboard layout and try to use commonly available components.
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Old 27th April 2011, 05:47 PM   #18
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Okay, surface mount is difficult i've heard.
Taking up more space is not really a problem. Will be stationary on my desktop so...
I'd like a powersupply which is not batteries, since i use my headphones very often.
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Old 7th May 2011, 02:54 PM   #19
Fenris is offline Fenris  United States
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I decided to do a minimalist version of this schematic with the idea of a low cost, easy to source design for the first-timer. Here's the design I came up with. It has a JFET configured for a gain of 2. The design assumes an Idss of the JFET of approximately 10ma. It is designed to be run at approximately 60% of that in order to stay in the most linear region. That would be 6ma which would equate to 3.35v across the 560 ohm resistor. The adjustable pot that defines the virtual ground is adjusted so that the output at the virtual ground is .1v lower (this is the Vgs drop in the datasheet). The output of the JFET feeds a directly coupled MOSFET (IRF610) source follower with a 80ma constant current source. The output is capacitively coupled. I built a prototype this week using "bugsplat" construction in an old external hard drive case. It's powered by a laptop power supply. I've still got some white noise and humming issues to solve, but the power output is perfect for headphones (very, very loud at maximum with 32 ohm headphones, but power is limited to 0.2 watts).

I've got pictures I'll post in a little while. If anyone has any advice on how to get rid of the hum and static (which may be related to my construction technique or power supply) it would be greatly appreciated.
Attached Images
File Type: gif headphone.gif (15.6 KB, 908 views)
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Old 8th May 2011, 03:39 AM   #20
Bigun is offline Bigun  Canada
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I've got pictures I'll post in a little while. If anyone has any advice on how to get rid of the hum and static (which may be related to my construction technique or power supply) it would be greatly appreciated.
I would guess it's the power supply being noisy and the amplifier having low PSRR. If so, put something between the supply and the amp to clean up the power rail. Something like CRC filtering or common mode choke on both V+ and Gnd.
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