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ETI Audio Phaser Project 447

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RickL:
I was looking through my box of finished but not working projects this weekend (you all have a box like this don't you?) and found two(!) copies of the ETI six-stage audio phaser Project 447. I haven't a clue where I downloaded it from so I can't point you to it.

I am wondering if anyone has successfully completed this phaser. There is at least one error on the PCB layout with the bias trim pot connected to the wrong place. I've fixed this on both boards but I continue to get loud static no matter where the trimpot is adjusted.

The clock appears to be working and I get a faint (compared to the static) dry sound through it

This project uses a 4049 hex inverter as the variable resistance element. The auther claims that the inverters are complementary FETs and that the resistance can be controled by applying a voltage its gate. Can someone explain how this works? In the schematic all the inputs to the inverters are attached to the clock signal, the outputs go to each of the allpass stages and both Vcc and Vdd (and pin 16 which is marked "no connection" on the data sheet) are connected to Vref (which in this case isn't exactly half of 9 volts, it uses a 10k/15k reference). Does this make sense?

Rob Strand:

--- Quote ---I haven't a clue where I downloaded it from so I can't point you to it.

--- End quote ---

I think it was from the CAG (Cloned Analogue Gear) site.


--- Quote ---I am wondering if anyone has successfully completed this phaser. There is at least one error on the PCB layout with the bias trim pot connected to the wrong place.
--- End quote ---

I've never build the whole thing on the PCB but I've build my own verions with similar blocks - the idea does work.  I seem to remember the article, or a errata note from the magazine, mentioning the pot isn't as shown on the schematic but it should still work (I think the pot on the PCB went between +V and gnd, instead of Vref and gnd).



--- Quote ---I've fixed this on both boards but I continue to get loud static no matter where the trimpot is adjusted.
--- End quote ---


So you aren't even getting clean signal through?  It sounds like the whole show if off the air.  You should at least be getting clean signal.  Try temprarily lifting the resistor between the all-pass stages and the transistor stage - I think it's 5.6k.  If don't get clean signal now it's time to debug the clean path.  Measure the voltages on the all the opamp outputs and the transistor pins.  From that you should be a able to see where things are pooping out.


--- Quote ---Can someone explain how this works?
--- End quote ---

It's clearer to say the inverters are being used as individual N-channel MOSFETS - the weird connections are all about disabling the P-channel devices and pilfering the N-channels out fo the inverter.  Once you see that yu will find it isn't much different to a JFET design.  JFETs and MOSFETs can both operate as variable resistances - basically the gate voltage controls the resistance of the channel.


--- Quote --- Vref (which in this case isn't exactly half of 9 volts, it uses a 10k/15k reference)
--- End quote ---


Yes that's OK, it should still work.

Mike I.:
Hi Rick,
Take a look at:

http://www5b.biglobe.ne.jp/~houshu/synth/mike_irwin

There is some explanatory info there plus a highpass filter design that uses the 4049 as a bank of identical voltage-controlled resistors. The same  applies to the ETI phaser design (but with a much simpler control circuit).
Regards, Mike

RickL:
I've explained myself badly. I am getting clean sound out of it but it's overlayed by either steady static or pulsed static depending on the adjustment of the trim pot. This is at a *way* louder volume than the clean signal. The clean sound is about as loud as the bypassed sound. The speed pot appears to be working because the  pulses or static bursts change speed with the position of the pot. I think if I can get the inverters to actually work as variable resisters it will work.

I'll try removing the 4049 to see if that removes the noise. If so, I guess that narrows it down to the 4049 circuit as causing the problem.

Thanks for the replies.[/i]

RickL:
It is indeed at CAG. Here:

http://omega.tellus.vallentuna.se/anders/pdf/eti447.pdf

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