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Old 27th April 2012, 08:50 AM   #321
sonnya is online now sonnya  Denmark
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I will change the fets to J112 and J175.

They are easier to get and lower nV/SQ(Hz)
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Old 27th April 2012, 08:59 AM   #322
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Quote:
Originally Posted by sonnya View Post
Yesterday evening i made a comparison of mosfet versus NJW1302G/3281G with MJF15030/15031 in darlington configuration with an emitter resistor of 0.22R

idle current is 1.8Amp.

In the simulation the THD for the exicons is 0.014% at 2.8Vp into 8R
for the darlington it is 0.004% at 2.8Vp into 8R

The thing is, they are easier to get, and cost 2/3 of the exicons.

At 1.8A and +/-32VDC we will dissipate 57Watt in each output device.

In comparison the exicons can handle 125Watt and the NJW's 200Watt.

With and heatsink of 0.4W/K the heatsink temperature will rise with 45.6K.

With 57Watt in the exicons it will rise to 102.6K leaving only 47Kelvin to the roomtemperature.

With the NJW you will have 35.625K leaving 68Kelvin to the room temperature.

I do not think it is safe to use the exicons or other devices rated lower than 200W for +/-32VDC supply voltage.

There is a lot talking against mosfets here.

Let me know what you think.

- Sonny
Is it not the same power dissipation at the same bias/rails for both types of devices, resulting in a same temperature increase. Using darlingtons would demand thermal compensation of some sort, laterals can handle without.

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I will change the fets to J112 and J175.

They are easier to get and lower nV/SQ(Hz)
Good choice.
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Old 27th April 2012, 09:18 AM   #323
sonnya is online now sonnya  Denmark
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Fine. Then we need to lower the rails.

The layout will not be that different for both types.

So i am going to make two layouts.

TSSA with EXICONS and TSSA with NJW BJT's
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Old 27th April 2012, 09:20 AM   #324
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Originally Posted by Joachim Gerhard View Post
The input stage can be improved much, i do not think that it is just the synergy...
Yes, agree about input stage. Synergy, am I missing somehing here hehe
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Old 27th April 2012, 09:25 AM   #325
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Originally Posted by sonnya View Post
Fine. Then we need to lower the rails.

The layout will not be that different for both types.

So i am going to make two layouts.

TSSA with EXICONS and TSSA with NJW BJT's
What if leave +/-32V and lower the bias, it would be fine to have more output swing headroom here. Distortion profile wouldn't change dramatically by lowering bias to let's say 1,5 A?
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Old 27th April 2012, 09:28 AM   #326
tinitus is offline tinitus  Europe
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and use BIG heatsinks
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Old 27th April 2012, 09:29 AM   #327
sonnya is online now sonnya  Denmark
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No that is not a problem.
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Old 27th April 2012, 09:29 AM   #328
sonnya is online now sonnya  Denmark
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Originally Posted by tinitus View Post
and use BIG heatsinks
0.4W/K IS A BIG HEATSINK!
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Old 27th April 2012, 09:39 AM   #329
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No that is not a problem.
Than stay on +/-32 V
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Old 27th April 2012, 10:19 AM   #330
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Originally Posted by sonnya View Post
This way the current through the input BJT will be the same and symmetrical regardless of bias current and offset adjustment. => Every amp will sound the same way.
What about cascoded inputs? Maybe in this case you wouldn't need extra CCS for bias/offset, as sound would stay pretty much the same regardless input biasing.
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