Switchable Hi-Z input impedance, how ? - Page 52 - diyAudio
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Old 18th April 2013, 11:20 PM   #511
tinitus is offline tinitus  Europe
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or slightly simpler
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Old 19th April 2013, 10:42 AM   #512
tinitus is offline tinitus  Europe
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managed to 'arrange' caps between att pot and buffer

and it works...humm is now gone
only a modest snerring coming from instrument remains yet to be fixed

oh, btw, completely forgot about it
was hoping it would also be a good home for simple 12db line out crossover

thanks for your patience
cheers
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Old 20th April 2013, 07:27 AM   #513
tinitus is offline tinitus  Europe
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removing collector resistor....oh horror
endless distortion....no matter what I did
spent half a day changing resistors, with no result

remounting collector resistor....no other changes, worked right away
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Old 20th April 2013, 08:23 AM   #514
AndrewT is offline AndrewT  Scotland
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Originally Posted by tinitus View Post
or slightly simpler
why does "slightly simpler" need three attenuators?
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Old 20th April 2013, 11:08 AM   #515
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Originally Posted by AndrewT View Post
why does "slightly simpler" need three attenuators?
Each with un-needed extra resistors all round them?.

Although I think the idea of the multiple pots is a master volume, and separate output volumes on the two output channels.

But the main problem is trying to design a circuit knowing next to nothing about electronics, and essentially just 'guessing' configurations and values.

FET, master volume, single buffer transistor, two output pots (4.7K off that buffer) - job done).
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Old 20th April 2013, 11:47 AM   #516
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by AndrewT View Post
why does "slightly simpler" need three attenuators?
one controls gain
one controls a woofer
one controls mid/tweeter

buying active xo would automaticly give me the two output attenuators
or a 2-channel PA power amp
one single jfet, and done.....

and I will probably try that too
just to experience how it works
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Old 21st April 2013, 12:56 PM   #517
tinitus is offline tinitus  Europe
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measured voltage drop(s)
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Old 21st April 2013, 12:59 PM   #518
shanx is offline shanx  Canada
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Quote:
changing resistors, with no result
Hi Tinitus, I figure that with 2 jfet stages preceding instead of one, that you had quite a bit of signal gain going on. On the emitter follower buffer did you stay with 830 ohm when making changes? Nigel also suggested putting the two attenuators after the emitter follower, with just master volume at the preamp. That is the ideal spot. As I mentioned earlier, the e-follower has or should have enough current drive to handle your loading of filter circuits, attenuators etc..
Build active xo: just suggestion, but it's only two way xover so not too hard with opamps. you already have power supply, box hardware and buffered preamp, maybe $10 of parts for a simple one.
If you are thinking of running your preamplifier to a mixer board or any professional PA power amp, strongly suggest balanced line out XLR. Use DRV line driver chip to do this. Major advantage is you greatly diminish chances of introducing ground loop hum from equipment on different ac power circuits.
Hope you have fun!
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Old 21st April 2013, 01:16 PM   #519
tinitus is offline tinitus  Europe
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Originally Posted by shanx View Post
active xo..... maybe $10 of parts for a simple one.

....balanced line out XLR. Use DRV line driver chip to do this.
Hope you have fun!
balanced line out would be good for PA poweramp

it was my hope to try build a chip amp
just for a tweeter, with integrated active xo

Quote:
Originally Posted by shanx View Post
Hope you have fun!
when it works thanks

btw, I have a single jfet Hi-Z preamp, with no attenuator
even driving my poweramp(s) directly, it had way too much gain
might work ok, now that I have a buffer with output control
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Old 22nd April 2013, 04:17 AM   #520
shanx is offline shanx  Canada
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There have been many people posting the instrument/amps section, looking to do the preamp/chipamp combination. If you are also looking at that option (for home practice etc), go for it. IMHO most of the characteristic sound of that combination should come from a really well built preamp section, and the chipamp role being to give a neutral amplification of the preamp sound. There are a few parameters that could be manipulated at the chip amp section, but if you are also sending the preamp signal to a mixing board or PA, I presume you will want to have similar sound there as well.
A lot of commercial small practice amps and even larger SS amps are now using chipamps as the power section, but I find they are completely lacking a lot of potential in the preamp section, so I hope working on the pre you get that sound you are after.
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