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Old 16th April 2012, 05:15 PM   #111
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Quote:
Originally Posted by MiiB View Post
what if you reverse the positions.. so you place the resistors at the rails and the current sources where they belong. then you get both....reduced dissipation and no signal induced current modulation.
Changing the position of the serial part inside a current loop doesn't change the voltage drop on that part. I think CCS has to be connected to rail potential otherwise it would loose its reference.

Quote:
Originally Posted by MiiB View Post
Think you should rearrange the driven VAS cascode... make the connection inside the Rail resistors for the input pair.
Can you be more specific or can you please draw it.

Thanks
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Old 16th April 2012, 05:29 PM   #112
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MiiB, maybe you think about something like this?

In this way VAS gain transistor has input-differential current included in its local feedback. That can be helpful in extra reducing the distortions, since it doesn't influence back, 680 ohm resistor can be practically connected to any positive potential.
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Last edited by Lazy Cat; 16th April 2012 at 05:35 PM.
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Old 16th April 2012, 05:33 PM   #113
MiiB is offline MiiB  Denmark
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Sorry I was looking wrongly at the cascode.. connection is as it should be....

Since you have constant current you also have constant voltage drop.. so it creates it own voltage reference.. you can tie it to gnd with at good size cap...that way you make something similar to the zener set bridge you have in the SSA but with fewer parts...and lees loss.

Last edited by MiiB; 16th April 2012 at 05:36 PM.
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Old 16th April 2012, 05:36 PM   #114
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Quote:
Originally Posted by MiiB View Post
Sorry I was looking wrongly at the cascode.. connection is as it should be....
What about the post #112, comment?
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Old 16th April 2012, 06:13 PM   #115
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I don't know what that would do....need to look a little longer at that...
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Old 16th April 2012, 08:27 PM   #116
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Latest evolution that intégrated latest LC modification with value indicated by LC above for +/-50vdc rail. Thanks a lot LC
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Last edited by Idefixes; 16th April 2012 at 08:30 PM.
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Old 17th April 2012, 08:34 AM   #117
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Hi LC, i don't remember me, and didn't catch info again on ssa topic, what is the tolerance range in Vgs for IRF610-IRF9610 matching?

Marc
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Old 17th April 2012, 08:56 AM   #118
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Vgs from both types can be quite different in absolute values, lets say Vgs=4,2 V at Id=100 mA for IRF610 and Vgs=4,5V for IRF9610 at the same conditions. That is not a problem, offset zero setting equals that, more important is transconductance dI/dV ratio (step increase) curve, this has to be equal (very close) for both types. Also NPN/PNP types of one BIGBT has to have hFE within +/-5%.

Regarding TSSA BIGBT HP PCB; is it possible to sueeze PCB more to eliminate empty spaces - quite large empty areas all over the PCB? PCB will be much smaller, more HF acceptable, shorter tracks, etc. What about VAS cascodes heatsinks?
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Old 17th April 2012, 09:11 AM   #119
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For BIGBT i have selected 4 MJL2381/1302 couple from a batch of 100u each after mesuring Vbe and HFE. I was just searching what conditions / mesure i do apply to select go mosfet candidat for BIGBT knowing I had good ones for output bjt. See attached document (pair 3/44; 94/19; 93/20; 84/92)
I take a look at NJW3281G/1302G datatsheet. They seems to good candidat for job as NPN/PNP gain matching is giving in 10%, that let good matching previsions between NPN and PNP after HFE/Vbe mesuring session.
For layout I need to rework to see if i can gain space et reducing traks length, but i am in single layer....that not easy. Pearhaps i can do something with TSSA core to bring it back, more closely to BIGBT module.

For VAS cascode i have let room around them, but i have any precise idea of the heatsink power requierement.

Marc
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Last edited by Idefixes; 17th April 2012 at 09:33 AM.
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Old 17th April 2012, 06:15 PM   #120
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I build the super simple version of post one. It worked right away. I made some additions and changes. I run it on plus-minus 35V with 500 Ohm resistors instead of trimmers. I runs quite hot that way but i can stilll toutch the cooling profile so it must be around 55° Celsius.
I added a low pass filter at 3MHz -3dB. This amp is very fast and with the filter the square wave is clean. I have cap multipliers in the PSU. That way the amp is dead quiet on my 95dB sensitive speakers.
The sound is a bit lighter then with my power buffer ( see MPP ). By adjusting the gain in the preamp and using a warmer sounding cable i got it going. I have to listen more but the amp is very transparent.
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