Inverted Aleph J headphone amp with 2SK715 and VP2106 - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 31st January 2015, 08:19 PM   #1
Ralph G is offline Ralph G  Netherlands
diyAudio Member
 
Join Date: Dec 2008
Default Inverted Aleph J headphone amp with 2SK170 and FQP3P50

Hello,

First a short introduction of myself. I'm the kind of silent member. I've been reading a lot of posts for a lot of years, but never made a new post (till now).

I've done a few DIY audio projects in the past. In the beginning with opamps and other semiconductors, but since 15 years or so only tubes. I really like the airy, lifelike sound of good tube stuff, but I would like to give solid state another try. Because of the philosophy of Nelson Pass' stuff (little feedback, single ended) it has to be something based on his designs.

My current pre and power amps are fine, so no need to replace them. My current headphone amp however is not perfect: a SRPP OTL amp is nice for high impedance headphones, but not for low impedance ones. So a Nelson Pass based headphone amp it has to be.

I like the simple designs of the Zen series, but I don't like the output cap. To make a long story short: it has to be an Aleph J amp.

Of course I don't like any of the existing designs so I would like to make a few changes.

First the design goals:

* 5V RMS (to drive old Sennheisers and Beyers)
* gain around 20 dB
* 100 mA RMS (to drive Hifimans or Grados)
* J fet FE
* small signal Mosfet for the current

To reach these goals I decided to go for:

* 2 x 12 volt for the power supply (about 20V tt, so 7V RMS)
* 120 mA iddle (with a bit of air because of the Aleph CS)

To find a decent J fet is tricky. All the really nice P channel devices are very difficult to get. The only through hole parts left are the J175 or J176. The transconductance of these parts is not great, so I decided to go for the inverted option: use N channel Jfets and P channel power fets.

As a N channel Jfet I thought of the following devices:

* PF5102 (very cheap, but not a very high transconductance)
* BF862 or BF861 (nice Dutch design, but only as SMD)
* 2SK170 (the original, but expensive)
* 2SK117 (a bit cheaper, but lower transconductance)
* 2SK715 (a little gem I think)

This 2SK715 looks very nice. Very high transconductance (but needs a bit of current), very low noise. Through hole package and cheap. Also low deviation (4 grades available). I think it is a Sanyo design. Mouser has the W version on stock. The biggest issue with this device is the maximum voltage (15V), but that's no problem in this case.

For the power end everybody uses the same IRF510 or 610. Boring
The small Zetex fets get good reviews, so why not use a couple of these. The VP2106 version of Supertex should be capable of 1 Watt. Using 30 mA per fet at 12 volts is 360 mW. Should be fine. Four pairs or even more. Why not. They are cheap.

I lowered the zener values a bit because of the low operating voltage of the amplifier. The CS zener is running at a high current. The BC639 and BC640 were chosen because I'm Dutch (they are cheaper). And they are also very robust (needed because of the high FE CS current).

I'm not an electrical Engineer and I don't have nice simulation tools so I hope somebody likes to assist.

I made a drawing with a nice open source design tool, but this tool is a bit limited in its possibilities (or I am). I can't get it to flip transistors, so the schematic looks a bit funny.

It's basically an Aleph H headphone amp (Metalman), inverted (based on the BF862 mini Aleph) with a Aleph J front end (NP).

The parts that I don't understand fully are marked in red (or orange, or pink).

In the Aleph CS are 2 1k resistors. In some schematics these are equal (Aleph H both 750, in Babelfish J headphone both 820), but in Nelsons original Aleph J these are not equal. Why?

The Aleph CS driving resistor is also 1k. This is taken from the Babelfish JH (and the original Aleph J is nearby at 1.2k). In the Aleph H it is much lower. Why? And is this related to the output resistor (0.12 ohm in this case).

If I understand correctly the feedback part are the 221k and 22k resistor. This would result in a 20 dB gain (correct???).

Please comment.

Update: I changed the design to 2SK170 as input and FQP3P50 as output. The schematics are posted on page 3.
Attached Files
File Type: pdf Aleph headphone schematics.pdf (27.8 KB, 265 views)

Last edited by Ralph G; 29th April 2015 at 08:25 PM. Reason: New schematic
  Reply With Quote
Old 31st January 2015, 09:31 PM   #2
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
can you get J271 in Lalaland ?
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 31st January 2015, 09:54 PM   #3
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
Natuurlijk.

MMBFJ271 Fairchild Semiconductor | Mouser
J271 InterFET | Mouser

But Ralph wasn't asking for help to source P-JFETs.


Patrick
__________________
xen-audio.com
  Reply With Quote
Old 31st January 2015, 10:23 PM   #4
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
then help him , if you have time to type , in this very moment
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 1st February 2015, 05:57 AM   #5
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
Actually I agree with ZM this time.
If 2SJ74 is the problem then you should try to use J271 and leave the rest as the original.

If your goal is however to have a go at designing circuits, then you at least need to learn to simulate.
In amplifier design, you are not only interested in THD, but also frequency response and stability.
LTSpice is free and easy enough to learn.

And 360mW is about the absolute maximum I would push through a TO92 device without extra heat sinking.
But that is just my own preference.


Patrick
__________________
xen-audio.com
  Reply With Quote
Old 1st February 2015, 08:49 AM   #6
Ralph G is offline Ralph G  Netherlands
diyAudio Member
 
Join Date: Dec 2008
Thank you guys for the answers.

I know the J271. This Jfet has a bit lower transconductance than the original Toshiba parts and it needs more current (so a lower source resistor). This is not good for the gain. Also this Jfet is expensive or you have to buy a SMD. The J175/J176 are the same parts, but less expensive and with even bigger spread of specs (so you have to buy more).

I think the 2SK715 has far better specs. Very high transconductance, very low noise, very low capacity. And cheap in a SPA package.

You read about people looking for a decent P-channel Jfet. Why not use a N-channel Jfet and a P-channel current amp? The only reason I could find is the midrange transconductance issue of the P-channel IR parts. That is the reason I want to use the VP2106 in stead of the IRF9610.

I know a lot of people like to look at huge heatsinks in a power amp. I like them too. But for this amp I would like to use TO92 powerfets, because of the nice specs, especially as a P-channel part

I've to apologize for not simulating. I didn't realize simulation programs are free nowadays. They weren't last time I checked (some 20 years ago )

I'll try to run a simulation and find out what every part does. The most important of this exercise of this amp is the learning. If the amp sounds very nice in the end:

If not: I still have my tube amp.
  Reply With Quote
Old 1st February 2015, 09:54 AM   #7
EUVL is offline EUVL  Europe
diyAudio Member
 
Join Date: Oct 2003
In the Aleph J the bias of the front end JFET diff pair has no relationship to their Idss.
And N/P Fets has totally different capacitances.

The rest I leave you to find out from your simulation, as part of the DIY fun.




Patrick
__________________
xen-audio.com
  Reply With Quote
Old 1st February 2015, 10:58 AM   #8
Ralph G is offline Ralph G  Netherlands
diyAudio Member
 
Join Date: Dec 2008
I thought most Jfets are biased around half their Idss?
  Reply With Quote
Old 1st February 2015, 07:12 PM   #9
Ralph G is offline Ralph G  Netherlands
diyAudio Member
 
Join Date: Dec 2008
I've installed LTspice and made the schematic. For the input JFET I took the BF862, because I couldn't find a 2SK715 spice model.

At this moment I've got the DC values OK (zero volt at the output, 30 mA idle current per output FET).

Now I have to figure out how to make nice frequency plots. Untill now everything looks OK.

I don't know if DIYAUDIO accepts .wmf files. Lets see.
  Reply With Quote
Old 1st February 2015, 08:57 PM   #10
Ralph G is offline Ralph G  Netherlands
diyAudio Member
 
Join Date: Dec 2008
No, the .wmf is not here. Let's try .pdf again.

I've changed a few resistor values so that the amp shows symetrical waveforms at full input and a low impedance output.

The Front End CS delivers a current of 21 mA.
All the output mosfets are running at 30 mA.
The Aleph resistor R8 is 470 ohm. If I increase this value, the amp delivers less current at low impedance headphones (makes sense).
I increased R17 to 1 ohm. This too increases the maximum current.

At a 30 ohm load, everything looks OK uptill 12Vtt (a little over 4Vrms). The little fets are generating 400 mAtt combined.
At a 300 ohm load, the max is 21Vtt (7.5Vrms). This should be enough.

I don't know exactly what R17 does (the 1 ohm output resistor). In most schematics this is much lower, but I don't need a very low output impedance. This "high" value gives less distortion (just looking at the waves).

I'll try to get more info from LTspice. Nice program

If anyone has tips and tricks, please let me know.
Attached Files
File Type: pdf Aleph J headphone.pdf (14.2 KB, 123 views)
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
jwb's ultimate aleph headphone amp jwb Headphone Systems 76 9th April 2013 11:01 PM
Inverted composite headphone amp schem shark70 Chip Amps 5 26th March 2010 11:33 PM
Nice way to add a headphone amp/plug to Aleph-30? h_a Headphone Systems 10 8th February 2007 01:28 PM
Aleph headphone amp jwb Headphone Systems 25 21st September 2003 02:17 AM
Circuit questions - Inverted vs non-inverted (again) alvinlim Chip Amps 4 1st July 2003 11:27 AM


New To Site? Need Help?

All times are GMT. The time now is 06:19 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2