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Old 24th October 2011, 04:42 AM   #2281
Salas is offline Salas  Greece
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Has 25K with its pot, 1Meg without.
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Old 24th October 2011, 12:50 PM   #2282
udailey is offline udailey  United States
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Build two. Sandwich the pot between them both. Now you have a full fledged preamp that even those wimpy 14k tube output WE DACs can drive.
Probably wouldnt even have to build two. Just stick some 2SK170's on perfboard and power them with the DCB1 and stick the pot between those two boards.
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Old 24th October 2011, 01:17 PM   #2283
AndrewT is online now AndrewT  Scotland
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The active part of the DCB1 buffer is just two jFETs per channel.
Yes, adding 4jFETs to a perfboard plus a few resistors would be an effective way of buffering the input and output.

I would still be tempted to consider adding the buffer at the output of the source.
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Old 16th November 2011, 09:26 PM   #2284
Geofd is offline Geofd  Switzerland
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Default Shunt Reg output voltage

Spent a lot of time reading many thousands of posts but this is my first post as I want to make sure I have this running as it should for this project (also finishing an F5 amp)
I have just about finished my B1 mezmerize, just waiting for my 22K pot to arrive - All powered up perfectly ( tested before putting in the matched 2SK170 ( Idss 9.55 for the quad)
I matached the LEDS using an Peak ATLAS DCA55 - component analysier - all were matched ( I think at 1.71 V red leds) , but I have slight difference in he shunt output voltage +9.453 and -9.196 , is this OK, or do I need to match the LEDS better ?

Also the voltage seems to drop slightly as the system gets warm is this as it should be

Output offset - no signal Left 0.0004V right 0.0012V - is this acceptable ?

Sorry if these are easy questions but its been a hile since I worked on this stuff this level - I am used to just puting kits together from Jaycar ( recently built the 20W class A amp)- no matching etc
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Old 16th November 2011, 09:59 PM   #2285
AndrewT is online now AndrewT  Scotland
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Your output offsets of -0.4mVdc and -1.6mVdc, are good, better than acceptable.
Your regulator output voltages are as expected for a DCB1.
Yes, there is s light fall in output voltage as the DCB1 warms up.
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Old 19th November 2011, 08:49 PM   #2286
CR500 is offline CR500  United States
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Got a basic question, is the resistor from pot wiper to gnd there to change the law of the pot or does it tweak the sound?

I think I'm ready to start a point-to-point build this week. I powered up 30 jfets for about an hour before measuring their currents. Took them a while to stabilize and had to keep them away from any air flow or the current would float around.

Thanks!
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Old 19th November 2011, 09:19 PM   #2287
Salas is offline Salas  Greece
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Its safety. If the wiper loses contact when a pot craps out eventually you get 4VDC on output. Res 10-20times bigger than the pot itself it even balances its tracks out better. Smaller do the fake trick on linear pots, another story. Not here. You should get a log pot.
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Old 19th November 2011, 11:06 PM   #2288
CR500 is offline CR500  United States
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Ah, that wouldn't be good! My mid / tweeter amp has caps on the input so it would be safe. The woofer amp does not. I plan to come off the pot wiper and into to the B1 (mids / tweeters) as well as an opamp (OPA627) for the woofer section (need about 6 dB gain to make up for the difference in amps). I'll put a cap on the output of the opamp. I'll try it all out on a proto-board first.

Thanks!
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Old 23rd November 2011, 08:51 PM   #2289
CR500 is offline CR500  United States
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Damn, turns out my pot is a 20k linear (motorized). If I drop the wiper resistor to 3.3k, will I still have the open pot factor of safety (I'm thinking it will)? Any other adjustments recommended after that change?

Any suggestions for the dc blocking cap value? I have some half decent 1mfd and 10mfd poly caps.

Thanks!
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Old 23rd November 2011, 08:56 PM   #2290
Salas is offline Salas  Greece
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Leave the 220K input resistor of DCB1 in place and it will be safe even if the wiper ever becomes intermittent. Use 10uF to have good phase towards the bottom of the spectrum.
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