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Old 11th March 2010, 01:03 AM   #1101
Salas is offline Salas  Greece
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Quote:
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Is the B1 output in danger at all if it's shorted to gnd for 5 to 10 seconds? I may add a hard-wired mute switch or a 2nd relay...
Don't know, never tried. The 220R should limit current. Be my guest crash testing.
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Old 11th March 2010, 07:56 AM   #1102
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I'm just wondering the same. Does any DIY'er has any experience in shorting the output of a 2SK170? At what output impedance? Any guess about what could happen?
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Old 11th March 2010, 02:08 PM   #1103
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Nothing will happen. It will just mute.
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Old 11th March 2010, 02:10 PM   #1104
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If source out Is 250 Ohm Z IN amp Is 100 Kohm (like F5) and between there Is 20 K shunt volume pot - Do I need B1
have you experimented using just passive volume pot between cd source and 100 K input amp and using B1
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Old 11th March 2010, 04:08 PM   #1105
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Nothing will happen. It will just mute.
Yes, of course it'll mute, but the transistor will be pushing current thru the series output resistor. I want to make sure when that happens, it doesn't go past the resistor's and the transistor's safe limits. I guess the only way to make sure is to measure the V across the resistor while the output is shorted.
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Old 11th March 2010, 04:23 PM   #1106
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It can not pass more than its CCS fet feeds it so I am not very worried, but it remains to be done.
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Old 11th March 2010, 04:25 PM   #1107
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Quote:
Originally Posted by samoloko View Post
If source out Is 250 Ohm Z IN amp Is 100 Kohm (like F5) and between there Is 20 K shunt volume pot - Do I need B1
have you experimented using just passive volume pot between cd source and 100 K input amp and using B1
The people here have done just that. Had been driving amps with a pot before. Its isolation paying some benefit.
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Old 11th March 2010, 04:37 PM   #1108
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It can not pass more than its CCS fet feeds it so I am not very worried, but it remains to be done.
So I suppose that if you have raised the default CCS to, lets say 250mA, that would be the max current?
Wouldn't the maximum current be the IDSS?
What mostly concerns me is burning the output 2SK170 by too much current through it.

As can be seen, I am a noob in transistor matters
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Old 11th March 2010, 04:56 PM   #1109
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The fet under the fet in the audio signal part is the current setting element for B1's bias. So its limit is its IDSS. The part you talked about is the PSU capability.
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Old 12th March 2010, 12:41 AM   #1110
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Unhappy shorted output DCB1

assuming that you have 220R gate stopper and 220R output resistor, and the relay shorts at the RCA output jack (so that output 220R is still in the circuit between the FET and the shorting relay), with a signal of say 2Vrms input and volume pot all the way up, then G-S junction of upper FET can become forward biased and conduct. peak current would be approx 2V * 1.414 / (220R +220R) = 6.4mA. The spec for 2sk170 is 10mA max gate current, so i'd recommend to make the gate and or output resistors bigger (say 470R for gate stopper and 330R on output res) to limit gate current to less than 3mA. still might let the magic smoke out of the parts though

lower CCS fet should be OK (as others have eluded to), as its normally seeing about 10V DS anyway and should be dissipating about 100mW, assuming about +-10V symmetric supply. regarding the upper FET's drain current and power dissipation, i'd have to think about it some more...

Last edited by tincanear; 12th March 2010 at 12:45 AM.
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