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Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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BMS1971
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Post by BMS1971 »

Hello, I recently made a copy of it following old analogguru schematic:
https://web.archive.org/web/20110715173 ... Phazex.gif
but the result is really poor comparing to the original sound found on youtube...
1 thing, schematic have a square wave LFO, and clearly on records the sound is made with a triangular wave FLO. Anyone had the chance to serve one and trace it?
Regards
Benoit
PS: web archive have all the original analogguru schematics if someone is interested
https://web.archive.org/web/20080524102 ... .ohost.de/

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george giblet
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Post by george giblet »

As drawn the LFO should produce an exponential waveform on the 10uF cap. That's same type of waveform the MXR units produce. It's similar to a triangle wave.

Would you say the LFO rising up time is about equal with the falling down time? If that's way off it might sound odd.

What voltage are you getting on the zener. What about Bias and Depth trimpots do they help improve the sound. Those adjustments are very important to the sound.

Have you measured/matched the JFETs? Bad matching can make it sound weak.

Do you know the Vgs(off) (or VP) of the JFETs ?

If your JFETs have particularly low or high VP you might need to make some circuit tweaks. Usually the Bias and Depth trimpots are enough. Another thing you can changes is the zener voltage and R23 - small changes in either direction to see if it pushes the sound towards good.

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Post by jimilee »

I found a layout on diysb and built it. I ended up using j102s for the fets. I also ended up using a 1M bias resistor. I’m not sure what the difference is and why I had to change things, but it sounds great.

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BMS1971
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Post by BMS1971 »

"Would you say the LFO rising up time is about equal with the falling down time?" Here it's definitively quite square to my ears and equal
Zener voltage 4.56V
Trimpot helps a little but not miracle cure
Jfets 2N5457 matched to 5% range
Do you know the Vgs(off) (or VP) of the JFETs ? nope
Cheers
Benoit

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BMS1971
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Post by BMS1971 »

jimilee wrote: 21 Dec 2022, 04:35 I found a layout on diysb and built it. I ended up using j102s for the fets. I also ended up using a 1M bias resistor. I’m not sure what the difference is and why I had to change things, but it sounds great.
I will see if I didn't left another "stupid mistake"
Benoit

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BMS1971
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Post by BMS1971 »

another detail, IF I put the speed very slow, there is absolutely no phasing, need to turn pot about 1/4 to make it work. So I'm suspecting a Jfet problem more than an LFO one...

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Post by lasermonkey »

The not phasing at the slowest speeds issue is something I have encountered on several phasers and it has always been temporary. I think the LFO timing cap needs to charge properly if the phaser has been sitting around unused for some time.

In my experience, letting the phaser run for a while at faster settings clears the fault.

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Post by george giblet »

another detail, IF I put the speed very slow, there is absolutely no phasing, need to turn pot about 1/4 to make it work. So I'm suspecting a Jfet problem more than an LFO one...
When you set the LFO to slow the resistor between the opamp and the 10uF cap is large. If there is a low impedance load connected to the cap (ie. the circuit from the Depth trimmer on beyond) it will prevent the LFO from oscillating. The reason for that is the oscillator works by making the voltage across the cap swing between two voltages, when you load down the cap the loading prevents the voltage reaching the target voltage. If you measure the opamp output voltage when the LFO is stuck you can work out if the loading is to ground (highly likely) or to a positive voltage.

Your zener voltage looks OK. So perhaps you should try putting 1M to 2.2M resistor (TP2) in place of the depth pot then raise R20 until you get enough depth. Each time you play with either TP2 or R20 you will have to tweak the bias pot TP1 for optimal results.

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george giblet
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Post by george giblet »

If you got a layout off the web and it's for BF244's check the pinout against the 2N5457 which has a different pinout.

Try this:
1) Put a 2.7M resistor in series with the 1M Depth trimmer TP2.
2) To start, set TP2 midway.
3) Adjust the Bias trimmer TP1 for best sound.
4) Adjust TP2 for best sound.
5) Repeat (3) and (4) a couple of times.

Then you will find:
- It all works.
- If you find the Depth trimmer TP2 is fully open (1M), try changing R20 from 1M5 to 1M2. If still same situation
then try R20 =1M0 if you get the same problem with 1M5.
- If you find the Depth trimmer TP2 is fully closed (0 ohm), try changing R20 to 2M2, 2M7 etc.

If you have ebay JFETs then you might find R20 needs to be reduced more than increased.

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Post by BMS1971 »

Hello George
Jfets where from USA, I remember the voltage was very consistent but quite low (2.5V If I remember correctly)
I will try your ideas
Benoit

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