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Old 21st April 2011, 02:59 AM   #1
Fenris is offline Fenris  United States
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Default Nelson Pass inspired headphone amp

I just finished a headphone amp inspired by many of Nelson Pass's designs. It's basically a JFET son of Zen front end with a paralleled B1 buffer like the Beast with 1000 JFETs, but with only 5. It runs class A with global feedback and a total gain of 2. It's targeted for use with 32 ohm headphones which don't need alot of voltage gain, but can use the current. It's running at about 50ma per channel, so it dissipates about 2 to three watts. The JFETS do get a bit toasty - about 65 degrees C (155 degrees F). It does sound very nice however - I'm hearing details I've never heard before and now I think I'm held back by my source.

I used two 5v to 9v DC-DC converters to provide power. They operate at 1MHz, so it's well above audibility. The 5v is input through a mini USB port, so the power source can be a wall wart or computer. Electrolytic caps are 270uf OSCONs on the power supply and on the output. There are also 2.2uf film caps on the output and 4.7uF tants on the power supply. All JFETs are matched 2SK170.
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Old 21st April 2011, 03:55 AM   #2
prakit is offline prakit  Thailand
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Nice built!
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Old 21st April 2011, 03:57 AM   #3
Bonsai is offline Bonsai  Taiwan
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Looks good.

Are your diodes the right way around though in your circuit?
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Old 21st April 2011, 06:02 AM   #4
agdr is offline agdr  United States
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Interesting circuit - and great build job getting it all into the Altoid can!

Idea - maybe re-spinning it to use DN2540 depletion mosfets would help with the hot jfet issue. The N3 version (TO-92s) are rated at 1W and up to 120mA. The two jfet current sources would be replaced by depletion mosfet sources with a 100R gate resistor and somewhere around a 43R source resistor. The source resistor would set the current to 50mA, 5x the 10-12mA the 2SK170 jfet's datasheet curves seem to show at Vgs=0. So a single depletion mosfet would do what the 5 jfets in parallel are doing, saving a bit of board space. You could even go with the (TO-220) N5 version of the DN2540 for the two output FETs which would dissipate better. Those are rated at 15W and 500mA but otherwise have all the same specs as the N3 version. Mouser has the Supertex stuff.

http://www.supertex.com/pdf/datasheets/DN2540.pdf

An introduction to depletion-mode MOSFETs (current source in fig 7, minus the 100R gate resistor that should be there)

Just a thought.

Last edited by agdr; 21st April 2011 at 06:30 AM.
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Old 21st April 2011, 09:57 AM   #5
Fenris is offline Fenris  United States
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Quote:
Originally Posted by Bonsai View Post
Looks good.

Are your diodes the right way around though in your circuit?
They're LEDs and I may have drawn them the wrong way around in the circuit, I'm always forgetting which direction they go in schematics. They are the right way in the physical circuit. They're old 3mm low brightness LEDs, those seem to have the lowest AC impedance. Plus they provide a nice visual indicator that the power is on.

I thought about other devices for the CCS, but the JFETS just seemed the most logical. A single TO-92 device of any sort may have dissipated too much on its own - spreading it across 5 devices increases surface area and reliability. The heat dissipation will always be the same since it's running Class A. Putting some space between the devices might be nice for air movement, but 65 C is still a fairly good temperature by discrete component standards. Plus it seemed more in line with the whole "bunch of JFETS" theme.
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Old 21st April 2011, 10:16 AM   #6
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Fenris,

That's a nice circuit! Having held the actual 1000- beast board, I have a weak spot for similar implementations!
I see that the gain of the circuit depends on the pot setting - whatever the equivalent pot impedance is, it's in series with the 10k feedback resistor.
That does modify the actual volume law when turning the pot.
What value of pot did you use?

jan didden
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Old 21st April 2011, 07:22 PM   #7
agdr is offline agdr  United States
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Quote:
Originally Posted by Fenris View Post
The heat dissipation will always be the same since it's running Class A. Putting some space between the devices might be nice for air movement, but 65 C is still a fairly good temperature by discrete component standards. Plus it seemed more in line with the whole "bunch of JFETS" theme.
You are right - that eventually hit me later on. Total dissipation would be the same. And probably no practical way to mount the TO-220 versions on the back of the board to heat sink out to the can.

Well congrats again on the build!
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Old 22nd April 2011, 12:02 AM   #8
Fenris is offline Fenris  United States
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Originally Posted by janneman View Post
Fenris,

That's a nice circuit! Having held the actual 1000- beast board, I have a weak spot for similar implementations!
I see that the gain of the circuit depends on the pot setting - whatever the equivalent pot impedance is, it's in series with the 10k feedback resistor.
That does modify the actual volume law when turning the pot.
What value of pot did you use?

jan didden
It's an Alps linear 20K (3B curve), which is high enough for almost every source (CD player, DAC, sound card, iPod, etc). In practice, the overall volume control is fairly linear in the middle, although it does lose some sensitivity at the end of its travel. Higher values of feedback resistors would help, since that's the lowest value of pot in that model line.
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Old 23rd April 2011, 12:06 AM   #9
Fenris is offline Fenris  United States
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Doing the math, the high value of the pot is really limiting my overall gain. I'm probably getting only about a third of the gain (and 1/10 the power) I could. My breadboard prototype used a 2K pot which had minimal impact on the feedback/gain relationship. I'm going to do a quick fix by changing the 1M input grounding resistor to 4.7k, that should give me about twice the voltage gain and four times the power output. Looking at the curves, this will greatly linearize the voltage output and render the power output a nice, shallow exponential increase. As it stands now, the voltage output levels off and the power output only increases linearly (which doesn't correspond to how our hearing works). If I were to do it over again I'd use 47K for the feedback resistor, 20K for the input series resistor, and a 4.7K for the grounding resistor
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Old 23rd April 2011, 08:58 AM   #10
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Quote:
Originally Posted by Fenris View Post
Doing the math, the high value of the pot is really limiting my overall gain. I'm probably getting only about a third of the gain (and 1/10 the power) I could. My breadboard prototype used a 2K pot which had minimal impact on the feedback/gain relationship. I'm going to do a quick fix by changing the 1M input grounding resistor to 4.7k, that should give me about twice the voltage gain and four times the power output. Looking at the curves, this will greatly linearize the voltage output and render the power output a nice, shallow exponential increase. As it stands now, the voltage output levels off and the power output only increases linearly (which doesn't correspond to how our hearing works). If I were to do it over again I'd use 47K for the feedback resistor, 20K for the input series resistor, and a 4.7K for the grounding resistor
With this mod: "47K for the feedback resistor, 20K for the input series resistor, and a 4.7K for the grounding resistor & 10K log pot" can drive 300 ohms loads like Sennheisers HD600-650? wich Idss values for matcthed SK170, are BL grade?

Last edited by merlin el mago; 23rd April 2011 at 09:00 AM.
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