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Old 15th April 2011, 06:31 PM   #1101
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Quote:
Originally Posted by Sheldon View Post
Look up the symasym schematic. Replace the FC-100 diode string and pot with the symasym's BD135 (or similar) and the resistive divider to its base.

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Thanks for reply.
Isn't it possible just to replace the diode string with NPN only base to emitter junctions and make it like this: RMI-FC100 MKII, single ended folded cascode VAS regarding pot?
Alas BD135, BD139 have TO225 Style1 package (emitter-collector-base). I think better will fit TO225 Style3 (base-collector-emitter), but I only found MJE13003. Do they need to be thermally matched?
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Last edited by AndriyOL; 15th April 2011 at 06:54 PM.
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Old 15th April 2011, 09:07 PM   #1102
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by AndriyOL View Post
Thanks for reply.
Isn't it possible just to replace the diode string with NPN only base to emitter junctions and make it like this: RMI-FC100 MKII, single ended folded cascode VAS regarding pot?
Alas BD135, BD139 have TO225 Style1 package (emitter-collector-base). I think better will fit TO225 Style3 (base-collector-emitter), but I only found MJE13003. Do they need to be thermally matched?

You could use most any diode, if you want to do it that way. Ideally, they will match the temco of the thermaltrak diode. Perfect match is not strictly necessary, but should be reasonably close. You could use the holes in the PCB for that purpose, but I don't see how you are going to get them fastened to the heat sink. Easier to mount them on top of the output devices, and just run wires to the transistor/diode terminals, in which case it doesn't matter what the pin orientation is.


There is no pot in the link you show. But I assume that the bias was set with a pot, the value determined, and a resistor substituted for the pot. You can do that with the standard version if you like, and if you don't intend to change the bias point.


Sheldon
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Old 16th April 2011, 04:13 AM   #1103
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Quote:
temco of the thermaltrak diode
= MUR120 (ON) diode.

OS
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Old 17th April 2011, 10:20 AM   #1104
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Quote:
Originally Posted by Sheldon View Post
You could use most any diode, if you want to do it that way. Ideally, they will match the temco of the thermaltrak diode. Perfect match is not strictly necessary, but should be reasonably close. You could use the holes in the PCB for that purpose, but I don't see how you are going to get them fastened to the heat sink. Easier to mount them on top of the output devices, and just run wires to the transistor/diode terminals, in which case it doesn't matter what the pin orientation is.
I've remade a layout to fit TO225 Style 3 on the heatsink. But if it's not necessary, can I use mur120 or any other equivalent diode is the spare holes of the PCB without fastening them? Are you refer that I can change the pin orientation by wires or it doesn't matter what the diode string orientation is in the schematic?
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Old 17th April 2011, 02:18 PM   #1105
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by AndriyOL View Post
I've remade a layout to fit TO225 Style 3 on the heatsink. But if it's not necessary, can I use mur120 or any other equivalent diode is the spare holes of the PCB without fastening them? Are you refer that I can change the pin orientation by wires or it doesn't matter what the diode string orientation is in the schematic?
Regards.
I was assuming that you are using Mihai's layout, which would make it mechanically difficult to solder transistors or diodes to the board and also attach them to the heatsink. But if you have to change the layout to account for the pinouts of the new output devices, then it should be possible to make the necessary changes to accommodate a diode.

The diode electronic orientation, of course, matters. It must be as shown on the schematic. But if you are not soldering the diode directly onto the board, and instead connect them with wires, then the mechanical orientation is flexible and you can use any suitable form of diode. If you are redoing the layout, then I would suggest that you arrange it so that the diodes sit directly on top of the output devices (except for one, as you need only 5 diodes).

Sheldon

Last edited by Sheldon; 17th April 2011 at 02:21 PM.
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Old 17th April 2011, 04:55 PM   #1106
Sheldon is offline Sheldon  United States
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Originally Posted by Sheldon View Post
If you are redoing the layout, then I would suggest that you arrange it so that the diodes sit directly on top of the output devices
See here: Based on Hugh Dean's AKSA 55
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Old 17th April 2011, 05:24 PM   #1107
roender is offline roender  Romania
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Hi guys!

There is no reason to use an external diode string for the Vbe thermal compensation. It is complicated to mount and there is no advantage from thermal tracking point of view. Instead, use a classical transistor based Vbe multiplier mounted on top of one of the power transistors, like in the AKSA design.
How is life now, here in DIY community?

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Mihai
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Old 17th April 2011, 07:43 PM   #1108
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Originally Posted by Sheldon View Post
But if you have to change the layout to account for the pinouts of the new output devices, then it should be possible to make the necessary changes to accommodate a diode.
That's what I did. I made the layout to attach diodes to the heatsink.

Quote:
Originally Posted by Sheldon View Post
If you are redoing the layout, then I would suggest that you arrange it so that the diodes sit directly on top of the output devices (except for one, as you need only 5 diodes).
What is preferable, attach diodes to the heatsink or place them on top of the output devices?

Quote:
Originally Posted by roender View Post
There is no reason to use an external diode string for the Vbe thermal compensation. It is complicated to mount and there is no advantage from thermal tracking point of view. Instead, use a classical transistor based Vbe multiplier mounted on top of one of the power transistors, like in the AKSA design.
How is life now, here in DIY community?
Hi Mihai. As you see this thread of RMI-FC100 amp cannot sustain without it's designer.
No advantage even if I can attach diodes (base to emitter junctions) to the heatsink (I made it as MKII layout)?
Vbe multiplier might work hot.
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Last edited by AndriyOL; 17th April 2011 at 07:52 PM.
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Old 18th April 2011, 01:26 AM   #1109
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Old 18th April 2011, 04:37 AM   #1110
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by AndriyOL View Post
What is preferable, attach diodes to the heatsink or place them on top of the output devices?
The advantage of the Thermaltrak devices, is that the diode is on the same die as the transistor. Therefore it closely tracks the transistor temperature. The further away it is, the more it lags behind the transistor temperature. So if it can't be on the same die, then mounting it (or a Vbe multiplier) on the output transistor is next best.

Quote:
Originally Posted by AndriyOL View Post
No advantage even if I can attach diodes (base to emitter junctions) to the heatsink (I made it as MKII layout)?
Isn't that exactly what the designer just said, or is there some misunderstanding?

Quote:
Originally Posted by AndriyOL View Post
Vbe multiplier might work hot.
Why would it work hot? It's not dissipating any significant power. It's simply working against temperature related bias change.

Sheldon

Last edited by Sheldon; 18th April 2011 at 04:50 AM.
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