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
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.
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.
-- ciao Ban Apricale, Italy
frequency
to
Max Fazio wrote:
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.
-- ciao Ban Apricale, Italy
I have emailed you several frequency and phase plots to and will post copies to alt.binaries.schematics.electronic
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
"John P> > Hi all, first post here.
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:
There is also a very good user's group dedicated to the use of this simulator at :
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