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Old 27th March 2012, 04:29 PM   #831
ra7 is offline ra7  United States
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People experienced in reading Papa's schematics and PCB layouts will note that he likes to put some filtering capacitance near the main circuit. It probably also helps with transient demands. Of course, this could be wrong and the cap might be serving some other purpose in the SIT circuit.
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Old 28th March 2012, 07:36 AM   #832
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Drat! My first signal tests failed.
I am using neg bias supply for a Vds of around 18V and my lab 30V ps configured as a 2amp ccs. The bias (around 5v) Vds 18v and everything seems to be functioning as expected in steady state. However I was only able to get something like .4V AC rms on a test speaker at 1khz before gross distortion. I have input and output cap as expected and I tested even with music which sounds normal on the speaker at a low level of course. I have not put the scope in yet to actually check exact level of clipping but still it is obvious something is off.
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Old 28th March 2012, 07:50 AM   #833
generg is offline generg  Germany
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Quote:
Originally Posted by ra7 View Post
People experienced in reading Papa's schematics and PCB layouts will note that he likes to put some filtering capacitance near the main circuit. It probably also helps with transient demands. Of course, this could be wrong and the cap might be serving some other purpose in the SIT circuit.

Thank you all for your reflections!

In the SIT user manual papa says that 8 resistors bias the circuit, so I suppose these 8 resistors (each 100W) form the drain resistor. 800W for the drain resistor, not bad, really overkill, he keeps them in a safe area....

Not strained resistors seem to sound better....
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Old 28th March 2012, 10:28 AM   #834
generg is offline generg  Germany
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my last try in the CCS....

you see in the picture that I am a bit old.....

I do not why, but with the 500R pot I manage to get a good mixture of the K2 and k3. Either in series with the cap or parallel to the resistors like now...

without the pot it was much more difficult to get the right mix only by the SIT bias pot and the SS bias pot at the recommended values....

I hope I write no nonsense.....

Of course I would be happy if anybody on his journey could test the 500R pot idea too and possibly confirm the use of it.
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Old 28th March 2012, 10:31 AM   #835
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perfect
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Old 28th March 2012, 10:39 AM   #836
generg is offline generg  Germany
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the idea to handle the pot in this way was the idea of young German Ralph.....
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Old 28th March 2012, 11:07 AM   #837
Salas is online now Salas  Greece
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Quote:
Originally Posted by protos View Post
Drat! My first signal tests failed.
I am using neg bias supply for a Vds of around 18V and my lab 30V ps configured as a 2amp ccs. The bias (around 5v) Vds 18v and everything seems to be functioning as expected in steady state. However I was only able to get something like .4V AC rms on a test speaker at 1khz before gross distortion. I have input and output cap as expected and I tested even with music which sounds normal on the speaker at a low level of course. I have not put the scope in yet to actually check exact level of clipping but still it is obvious something is off.
Is it a real ccs or they just mean the adj. current limit on the lab supply?
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Old 28th March 2012, 11:23 AM   #838
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Originally Posted by generg View Post
the idea to handle the pot in this way was the idea of young German Ralph.....
previous variation (10k pot/variable resistor along with 10k resistor to bias string) gave you ~6db less of max modulation , and softer drive

this variation is giving you broader range , and firmer drive of Mu gate

what's more to you liking , let ears decide
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Old 28th March 2012, 11:28 AM   #839
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Thanks for explanation Zen Mod....
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Old 28th March 2012, 11:28 AM   #840
protos is offline protos  Greece
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Yes it is not a "real" ccs. But for simplicity's sake I am using the bench PS in place of the ccs in the schematic. Basically it is a 30v 2amp ps with adj voltage and current. I max the voltage adjustment so it can swing to 30V and of course limit the current to just under 2amps. At steady state it is settling at about 18v which is the Vds of the sit. So theoretically it behaves like a ccs keeping current constant but swinging the voltage as needed to keep the current at 2A.
I should get at least 7v ac rms swing with this set up I think.
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Last edited by protos; 28th March 2012 at 11:33 AM.
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