Switchable Hi-Z input impedance, how ? - Page 49 - diyAudio
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Old 15th April 2013, 08:29 PM   #481
tinitus is offline tinitus  Europe
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1ohm across the inductor took the worst of the deep humm

also tried various adjustments to the buffer
all seemed to work ok, and sounding a bit different
some gave more humm

this one gives a voltage drop of 2.3V and 1.4V on collector and emitter
if I'm not mistaken it gives a current draw of 2mA on both sides
dont know if its good, but I like it
sound is responsive, crisp and clean
and most importantly, easily adjusted finding the right tone
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Old 16th April 2013, 09:34 AM   #482
tinitus is offline tinitus  Europe
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found what caused the excessive humm ('that wasnt there before')

'turn on' resistor from supply/collector to signal input
with the orginally 60k mounted humm is almost gone
and most of the other snerring seem to come from instrument

regarding the power supply noise
now the inductor appears to do nothing good at all
only the 1ohm series resistor seem to works
and the actual sound also appear better with no inductor

will try make a small snubbered CRCRC board
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Old 16th April 2013, 10:48 AM   #483
AndrewT is offline AndrewT  Scotland
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you need a DC block between the attenuator and the biasing resistors.
You will probably need a smoothing cap across the 22k to attenuate 13Vdc ripple.
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Old 16th April 2013, 10:53 AM   #484
AndrewT is offline AndrewT  Scotland
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the 22k+60k bias resistors will apply ~2.8V across the Vbe+680r.
Bias current will end up @ ~4.2mA.
That current will drop ~4.2V across the collector resistor leaving Vce @ ~[13 -4.2 -2.9] =5.9V
Your multimeter should be able to confirm the actual voltages and let you calculate the actual currents.
Your multimeter may also be able to measure the ripple on the 13V supply and if it's bad then also at the 22k.
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Last edited by AndrewT; 16th April 2013 at 10:57 AM.
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Old 16th April 2013, 01:58 PM   #485
tinitus is offline tinitus  Europe
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ac ripple is 7mv(0.007V)

voltage drop have changed after increasing 60k bias resistor
collector voltage drop is now 1.52V, and emitter 1.57V
(measured across resistors, and with measured 13.45Vdc supply voltage)

btw, voltage drop on 60k bias res was measured 1.67V
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Old 16th April 2013, 02:59 PM   #486
AndrewT is offline AndrewT  Scotland
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Some of your bias voltage is leaking out into the attenuator.
You must DC block the input between the attenuator and the bias resistors.
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Old 16th April 2013, 03:08 PM   #487
AndrewT is offline AndrewT  Scotland
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that 7mV will be reduced to about 2mV at the bias point.
That 2mV will appear on the output.
Signal to hum ratio will be terrible.
Fit a cap to smooth the bias voltage.
Possible reduce hum by 20dB and that will show as an improvement of signal to hum ratio of 20dB, try 100uF 25V.
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Old 16th April 2013, 07:24 PM   #488
tinitus is offline tinitus  Europe
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when I changed the att pots from 10k to 100k suspected this
I cut the connection, so its now easy to mount dc blocking caps

will try it now
(but feels a bit stupid since there are caps before the att pots)
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Old 17th April 2013, 09:50 AM   #489
tinitus is offline tinitus  Europe
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reduced emitter resistor from 680ohm to 470

wow...with no other changes, ac ripple just dropped from 7mv to 1mv (or less)
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Old 17th April 2013, 12:14 PM   #490
shanx is offline shanx  Canada
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Hi tinitus, was just checking in on your BJT progress. You mentioned that you wanted to build it as a buffer, so I was wondering what the 1k on the collector is there for. Had assumed you were going originally for an emitter-follower arrangement? Maybe you are going for something else, but the input impedance on the circuit you have could be made a lot higher..
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