A Pass inspired Sk170 Headphone Amp - is this stupid - diyAudio
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Old 29th January 2015, 10:03 PM   #1
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Default A Pass inspired Sk170 Headphone Amp - is this stupid

Howy Diynosaurs,

i need your help. After a long abscence i am back in the Hifi Diy. Since i am writing my master thesis i needed something to clear my head... so why not build a headphone amplifier. My Yamaha C2a (great preamp for my F5) has no headphone output. Great lets do this.

Looking through various circuits on the net i decided it had to be simple and parts must come from the parts bin. I found a lot of 2K170 in the basemnet. Great lets use these, if papa can drive speakers with them then it must e possible to drive a headphone with the too.

So here are the design goal:
Battery Operated (12v) because i have 12V Battery left over
2Sk170 as output device
Class A
little to no Phase shift
Must be able to Drive low Impedance Headphones like the Shure SR840 to 110 db with little distorsion and some headroom left.

So here is the first prototype i came up with in simulation. A Jfet common drain current buffer. Distorsion seems ok in simulation, but testing a prototype channel with some cheapo earphones distorsion seemd a little highher. But in general, great sound even with the elko.


So where to go from here? I could bias the Jfets even higher als long as i stay probably up to 3000mw dissapation per jefet, add more jfets maybe up to ten, but i will need more bc550 in the current sink because the single one is already at its limits.

And there is one problem i have. When powering the unit or connecting the headphones i get a quite loud pop in the headphones, i guess the elko cap is discharging through the headphones, what can i do about this?
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Old 29th January 2015, 11:23 PM   #2
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A few Pass designs over here as B1 and few others are a text book example circuits, so you have nothing to be ashamed for. My advice is to insert say about 4.7 Ohm source resistors at each of the jfet sources so you could measure how`s the current sharing between jfets It could be important if they`re not matched. Also you could try lower value R3, let`s say 100-220 ohms because of paralleling jfet`s Cgd-s and you could also lower input coupling cap, for a 500K input impedance (run the AC analysis in MCAP, All that things you could check in simulator with the good accuracy.) but It` won`t change the sound if the capacitor type is the same.
I guess you ment 300mW per jfet, that`s ok but don`t go over that, 200 - 250mW is a better choice. BC550 should be thermaly coupled to LED etc and you could change it to BD139 to be on the safe side. . Everything will work even the wat you draw but better to do it right and it`s not so complicated.. Cheers! Try bypassing output elco with film cap for sound tuning etc...
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Old 30th January 2015, 12:07 AM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by Razorblade View Post
When powering the unit or connecting the headphones i get a quite loud pop in the headphones,
i guess the elko cap is discharging through the headphones, what can i do about this?
Good luck. Just add a resistor at the output after the coupling capacitor of around 1k Ohms to stop the popping.
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Old 30th January 2015, 12:52 AM   #4
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Looking at your current source isn't it set fore about 130 ma ?
I don't think 4 of the 2sk170 will do anything any where near
that. Now what might be neat is the Ld1014d power jfet instead
of the 2sk170's.
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Old 30th January 2015, 09:13 AM   #5
juma is offline juma  Germany
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Razor,
That won't work very well. To prevent oscillations you need a gate stopper on every JFET's gate, and to prevent current hogging you need a resistor in every JFET's Source. You need a resistor to discharge the output and input cap. Also, you need a fillter on voltage divider at JFET's gates - battery is not so clean as one might think. Also, the CCS (Q5) to supply the LED Vref should be used. Five instead of four Source Followers JFETs is better (10 is even better).

You don't write what is the grade of your JFETs (GR, BL or V) but use the highest Idss parts you got and set the P1 so that you get about 80-90% of Idss through every Source Follower JFET (measure the voltage drop on 4R7 Source resistors).
If you got, for example, JFETs with Idss=10mA you'll wan't to run about 9mA through each of them, so the Q6 CCS should supply about 40-45mA (if you use 5 JFETs as in my sch.). The LED used should be nothing fancy, ultra bright etc., just plain old, 3mm red one. If Q6 gets hot, use the bigger one - BD139 or similar.
Viel Spass !

Edit: there's an error - the resistor in Q7's Drain shouldn't be there, that Drain should connect straight to +12V
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Last edited by juma; 30th January 2015 at 09:35 AM.
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Old 30th January 2015, 09:20 AM   #6
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Quote:
Originally Posted by woody View Post
Looking at your current source isn't it set fore about 130 ma ?
I don't think 4 of the 2sk170 will do anything any where near
that. Now what might be neat is the Ld1014d power jfet instead
of the 2sk170's.
Yes I did`nt noticed that and low load (44 ohms) so 4 jfets won`t do it...
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Last edited by bogdan_borko; 30th January 2015 at 09:29 AM.
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Old 30th January 2015, 09:52 AM   #7
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Howdy,

guys could you please enlighten me whats exactly wrong with the circuit, it simulated fine and also the Prototype did work very well. I am still learning, in simulation all parts were within their specs and the prototype didnt explode.
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Old 30th January 2015, 10:09 AM   #8
juma is offline juma  Germany
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Quote:
Originally Posted by Razorblade View Post
... it simulated fine...
...I am still learning...
Sims are not exact picture of the reality - there are no ideally matched parts, no absolutely predictable parasistics, etc.

Google for "Borbely JFET new frontier", read those two PDFs and you'll know much more than you do now...
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Old 30th January 2015, 12:09 PM   #9
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I'm quite interested in such a design. I have a stash of 100 2sk170bl and just recently had the same preamp/headphone amp idea. I don't have any EE knowledge apart from following schematics and solder PCB's I'll follow this thread with great interest!
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Old 30th January 2015, 12:16 PM   #10
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How about: When VGS reaches 0 then your stupid little Jfet will saturate and will only deliver IDss?

Is that a starting point?
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