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1st post - what kind of filters?

M

Max Fazio

Jan 1, 1970
0
Hi all, first post here.

I'm analyzing a circuit coming from a synthesizer but I wasn't able to
determine a thing: if you look at the PH1 and PH2 sections in the image
linked below

http://img140.imageshack.us/img140/9611/exp5zz.jpg

there is a filtering stage but I wasn't able to understand what kind of
filters they are: my guess is that they are all-pass filters.
Am I wrong?
Can anybody help me identify the kind of filtering showed here?
Thx in advance for any reply.
Max
 
J

John Popelish

Jan 1, 1970
0
Max said:
Hi all, first post here.

I'm analyzing a circuit coming from a synthesizer but I wasn't able to
determine a thing: if you look at the PH1 and PH2 sections in the image
linked below

http://img140.imageshack.us/img140/9611/exp5zz.jpg

there is a filtering stage but I wasn't able to understand what kind of
filters they are: my guess is that they are all-pass filters.
Am I wrong?
Can anybody help me identify the kind of filtering showed here?
Thx in advance for any reply.
Max
I haven't done any analysis, but at first glance, it appears to be a
current controlled (via the LED to photo resistor isolators) variable
low pass filter. The higher the resistance of the photo resistor, the
more treble cut the filter produces. If I have time, tonight, I will
simulate the circuit with a stepped set of values for the photo
resistor and graph the resulting family of frequency response curves,
for you.
 
B

Ban

Jan 1, 1970
0
Max said:
Hi all, first post here.

I'm analyzing a circuit coming from a synthesizer but I wasn't able to
determine a thing: if you look at the PH1 and PH2 sections in the
image linked below

http://img140.imageshack.us/img140/9611/exp5zz.jpg

there is a filtering stage but I wasn't able to understand what kind
of filters they are: my guess is that they are all-pass filters.
Am I wrong?
Can anybody help me identify the kind of filtering showed here?
Thx in advance for any reply.
Max

It's certainly not an allpass filter, which would require an opamp or
transformer.
This is a variable 1st order lowpass with the LDR setting the -3dB frequency
between 40k and 1k. The pot will give a lower slope(around -3dB/oct.)
between 10Hz and 100Hz additionally and thus between 0 and -26dB at 1k.
Important is also the input impedance of the following stage, that is not
shown. generally said there is a lot of interaction but the character is to
filter out the treble without big phase changes.
 
M

Max Fazio

Jan 1, 1970
0
Hi ban
Sounds strange but with a graphic plot of the waveforms the wave changes
really like there was a phasing around 2Khz: the lower the pitch the
strangest looking the wave: at high pitches a given square waveform looks
like a plain, comparator-generated square but when the pitch begins to go
down the waveform phase has something like a rotation on a given point, just
like the waveform was passing through a 2 pole allpass....any clearing about
it?
M
 
B

Ban

Jan 1, 1970
0
Max said:
Hi ban
Sounds strange but with a graphic plot of the waveforms the wave
changes really like there was a phasing around 2Khz: the lower the
pitch the strangest looking the wave: at high pitches a given square
waveform looks like a plain, comparator-generated square but when the
pitch begins to go down the waveform phase has something like a
rotation on a given point, just like the waveform was passing through
a 2 pole allpass....any clearing about it?
M

A 2-pole allpass has a *much* longer delay for low frequencies. This filter
will in a 12h setting of the pot mainly attenuate around 12dB at bass
frequencies, compensating the input capacitor, so the lower end is much
stronger when turning up the gain of the following amp. This is less
pronounced with the LED getting more current.
We cannot hear a phaseshift, how it looks is pretty unimportant. A phaser
creates comb filter effects by delaying the signal and then mixing it with
the undelayed version. this is not happening here.
The phase goes from 0 to maybe -45deg at 500Hz, rises again to -20 and then
goes slowly to -90 like any 1st order lowpass. When there is a continuous
change by fast modulation of the LED, something like a doppler effect is
created. Our ears are sensitive for that. But most of the effect is volume
and lowpass. Since I do not know what the modulation signal of the LED is,
everything will be only speculation. You will have to include this info too.
 
J

John Popelish

Jan 1, 1970
0
Max said:
Hi all, first post here.

I'm analyzing a circuit coming from a synthesizer but I wasn't able to
determine a thing: if you look at the PH1 and PH2 sections in the image
linked below

http://img140.imageshack.us/img140/9611/exp5zz.jpg

there is a filtering stage but I wasn't able to understand what kind of
filters they are: my guess is that they are all-pass filters.
Am I wrong?
Can anybody help me identify the kind of filtering showed here?

I have emailed you several frequency and phase plots to and will post
copies to alt.binaries.schematics.electronic
 
M

Max Fazio

Jan 1, 1970
0
Hi all
I just wanted to thank John and Ban for their unvaluable help , the plots
are very useful to me to study that kind of circuit.
Guys you're great!
M
 
J

John Popelish

Jan 1, 1970
0
Max said:
Hi all
I just wanted to thank John and Ban for their unvaluable help , the plots
are very useful to me to study that kind of circuit.
Guys you're great!

Since this sort of thing is useful to you, you might want to download
the free LTspice circuit simulator I used to produce them. It is
available at:
http://www.linear.com/company/software.jsp

There is also a very good user's group dedicated to the use of this
simulator at :
http://groups.yahoo.com/group/LTspice/
 
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