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 :D 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 :rolleyes:
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.
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 :D 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 :rolleyes:
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.
