Switchable Hi-Z input impedance, how ? - Page 55 - diyAudio
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Old 29th April 2013, 07:11 PM   #541
tinitus is offline tinitus  Europe
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and better

it had become a bit dark, lost clarity, and very 'heavy'
which tend to muffle my ears in the long run

not sure how to describe it
but now it feels like having more string tension
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Old 29th April 2013, 07:55 PM   #542
shanx is offline shanx  Canada
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Hi Tinitus, I think the effect your describing dark, muffled heavy...that is low frequency emphasis. Changing resistor values is playing with the cutoff frequency (relates to the DC blocking capacitor value and resistance network after it). So if you change R so much as to get into the range of the bass' frequency response you can get a brighter sound as the filter will roll off low frequency.
Normally I choose C value after figuring out the load Z, and I usually select an Fcutoff pretty low for full freq response. Then decide if it needs a brighter tone I'll choose a higher roll off frequency with a smaller C. (that part is not exact, depends what is in the parts bin!) Anyhow most caps are pretty widely toleranced so not too critical..just get a rough value, and pick closest you have.
Have fun!
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Old 29th April 2013, 08:42 PM   #543
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by shanx View Post
Changing resistor values is playing with the cutoff frequency (relates to the DC blocking capacitor value and resistance network after it). So if you change R so much as to get into the range of the bass' frequency response you can get a brighter sound as the filter will roll off low frequency.
the changes
new values shown in red, and previous in black

but from what I understand, input and output impedances are not so obvious but should be calculated using bias setting
I think it was something like that

I thought the changing sound I experience would be related to load lines
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Old 29th April 2013, 09:13 PM   #544
shanx is offline shanx  Canada
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Changing sound is related to changing load line, and a few other things including the Fcutoff, harmonic distortion etc. Like choosing the capacitor, I suggest starting with a fairly neutral bias, not running the device close to max dissipation. Just focus on getting a circuit with clean signal with moderate gain and no clipping /cutoff. Once you get there, then you can play around with the biasing and load lines. It doesn't take much change in values to really change that, when you are working with13V supply with 2 gain stages. Hope your having fun.
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Old 29th April 2013, 09:18 PM   #545
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by shanx View Post
Hope your having fun.
when it works, absolutely
and Im satisfied with the buffer now
or at least it will do for experimenting

time to fool around with some tone controls
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Old 30th April 2013, 10:03 AM   #546
AndrewT is online now AndrewT  Scotland
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Tin,
you have repeated the same biasing mistake you had originally.
I told you to use DC blocking capacitors. You continue to ignore this.
As a result you have the 2k of the previous stage and any other parasitic resistance loading the input of the BJT buffer. This completely upsets the biasing of the buffer.

You have either 100k//2k//? in the post541 layout
and
you have 60k//2k//? in the post543 layout
as the effective lower resistor in the biasing string.
This gives the buffer an input impedance ~ 22k//2k//100k/? (about Zin = 1k8). Is your previous stage designed to drive this load? 1uF into 1k8 gives F-3dB ~ 88Hz

You must DC block both sides of the buffer to get consistent results from the circuit.
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Last edited by AndrewT; 30th April 2013 at 10:06 AM.
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Old 30th April 2013, 12:37 PM   #547
shanx is offline shanx  Canada
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Hi Andrew, thanks for pointing out the F-3dB calc, and at 88 Hz is cutting down the fundamentals thoughout the open string range of a standard tuned bass, and the Low E string would be cut about 6 dB. Not necessarily a bad thing for bright sound, but as you said, the input Z on the buffer is too just too low, should be at least 10 K or more. Normally a bass preamp stage should pass as much bandwidth as possible (F- 3db around 5 to 10 Hz) and use a tone stack to do it's job. DC Blocking a must here.
Have fun
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Old 30th April 2013, 01:16 PM   #548
AndrewT is online now AndrewT  Scotland
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Tin just does not have a clue and yet he asks and ignores advice and/or guidance.
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Old 30th April 2013, 01:30 PM   #549
tinitus is offline tinitus  Europe
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glad Im still able to entertain you

no, output and input impedances, which is which, it remains a mystery to me .... and Im probably not alone
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Old 30th April 2013, 01:43 PM   #550
AndrewT is online now AndrewT  Scotland
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It is not funny.
We are here trying to help, but you ignore what is being said.
If something is confusing, then ask another question.

We are expending TIME to help you.
Would you rather that all those answering posts were not made and we left you with all your posts alone while you talk to yourself?
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