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BF862 based SE Class A Headamp without the HEAT
BF862 based SE Class A Headamp without the HEAT
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Old 20th December 2016, 07:32 AM   #21
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
I had to back off voltage to 18v after Cx (22v upstream). It was getting too warm.
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Old 20th December 2016, 06:33 PM   #22
needtubes is offline needtubes  United States
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I think it might be interesting to do a version of this with the OnSemi 2SK715 JFETs, which have recently been discontinued but are still available in quantity from the usual suppliers. They feature around the same transconductance as the BF862 and are through-hole TO-92, which is more user-friendly. Also, at <2mA (as the input stage runs), these parts feature triode-like curves for a SIT-ish characteristic.
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Old 20th December 2016, 07:18 PM   #23
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Quote:
Originally Posted by needtubes View Post
I think it might be interesting to do a version of this with the OnSemi 2SK715 JFETs, which have recently been discontinued but are still available in quantity from the usual suppliers. They feature around the same transconductance as the BF862 and are through-hole TO-92, which is more user-friendly. Also, at <2mA (as the input stage runs), these parts feature triode-like curves for a SIT-ish characteristic.
I'll give it a try. They are a little more expensive though. A 40 piece roll of 862's cost me $8. These are $10 for 20. But might have an interesting SIT sound.

Thanks for suggestion.
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Old 21st December 2016, 03:03 AM   #24
didiet78 is offline didiet78  Indonesia
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If you make anoter PCB, add some copper for Gate pin to help dissipate heat
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Old 21st December 2016, 03:32 AM   #25
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
That's why I used a lot of solder on parts to help conduct heat away.
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Old 21st December 2016, 08:34 AM   #26
Calvin is offline Calvin  Germany
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Hi,

using a different JFET isnīt such a bad idea.
The BF862 certainly has a high gm, but that is not required for the input stage SRPP.
To achieve the required gain the source resistors must be chosen large and hence the Id becomes very small.
Fortunately the low Id also corresponds with a comparatively large Vgs figure.
Still though Vgs may be small, at only a couple of dozens or a few hundred mV.
You have to take care that Your input signal peak voltage doesnīt exceed the Vgs as this cuts off the Id completely.
Hence a voltage divider or volume pot is required.
A high Idss/high Vgs JFET on the other hand may allow for several V of input signal before current cuts off.
Where a BF862 allows for example only about 350mVp (@Id~3.5mA) input for undistorted output, a SST4392 allows 2V(@Id~15mA).
For the Buffer-part the Vgs restriction doesnīt apply, but to the (here modulated) current-source it applies again.
Also keep in mind, that a high Vgs part requires accordingly higher rail voltages (Vds), especially for the current source part.
As rule of thumb: the JFETs Vds should be >= 2xVgs (and watch the max. allowed Vdg, with Vdg=Vds-Vgs).
As usual You need to optimize the design to Your needs and choose the devices accordingly.

jauu
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Old 21st December 2016, 09:35 AM   #27
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Quote:
Originally Posted by Calvin View Post
Hi,

using a different JFET isnīt such a bad idea.
The BF862 certainly has a high gm, but that is not required for the input stage SRPP.
To achieve the required gain the source resistors must be chosen large and hence the Id becomes very small.
Fortunately the low Id also corresponds with a comparatively large Vgs figure.
Still though Vgs may be small, at only a couple of dozens or a few hundred mV.
You have to take care that Your input signal peak voltage doesnīt exceed the Vgs as this cuts off the Id completely.
Hence a voltage divider or volume pot is required.
A high Idss/high Vgs JFET on the other hand may allow for several V of input signal before current cuts off.
Where a BF862 allows for example only about 350mVp (@Id~3.5mA) input for undistorted output, a SST4392 allows 2V(@Id~15mA).
For the Buffer-part the Vgs restriction doesnīt apply, but to the (here modulated) current-source it applies again.
Also keep in mind, that a high Vgs part requires accordingly higher rail voltages (Vds), especially for the current source part.
As rule of thumb: the JFETs Vds should be >= 2xVgs (and watch the max. allowed Vdg, with Vdg=Vds-Vgs).
As usual You need to optimize the design to Your needs and choose the devices accordingly.

jauu
Calvin
Would a pair of 2sk170bl on input be better?
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Old 21st December 2016, 10:21 AM   #28
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by xrk971 View Post
That's why I used a lot of solder on parts to help conduct heat away.
You need more radiating area, not more mass (mass only changes the time until final temp is reached).
Large copper areas at the gate pins, and make sure it is *not* reflective (that is, not smooth and not shiny). Make the copper surface as rough as you can, easily done by etching the whole thing again for a short period after removal of the etching resist. This breaks the reflection and increases surface area.
Thick copper and/or a layer of solder near the pins (but not covering the whole copper pours) of course also helps because of its higher thermal conductivity to spread the heat with mimimal temperature drop along its way into the radiating surface.
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Old 21st December 2016, 10:50 AM   #29
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Current design is in a small sealed aluminum box and pretty much everything heats up but seems ok as burn in over several days seems ok. I also touch parts when in and they are warm but not burning hot. 12mA current per output device x average 9v drop (18v supply) is 108mW ea - well below max rating. I suppose I could pot it with thermal conducting comping bonded to bottom? So far it's running well heat wise.
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Old 21st December 2016, 11:36 AM   #30
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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You can also solder some short pieces of copper wire at the FET's pin to help removing heat prom parts. Some spades terminals, as an example. Maintaining them vertical, increases convection of the heat.
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