Ampeg Phazzer (1974) traced - schematic, looking for advice on modifications

[PRR]

I probably misunderstand "Depth control". But seems to me it wants a fader from straight DC bias (no wobble) to DC plus full wobble.

[pinkjimiphoton]

heheheheh on "the other forum".
yeah, finally got around to scanning the factory service dox...
glad ya built it, i'm just waiting on some vero to do the same.
thanks for the heads up, brother paul!
this awesome

[Rob Strand]

I probably misunderstand "Depth control". But seems to me it wants a fader from straight DC bias (no wobble) to DC plus full wobble.

If you look at the schem at the top of the thread it's got a 1M from the LFO to the gates then another 1M from the gates to the trimpot.  You probably want to keep the 1M from the LFO to the gates and use replace the second 1M with a depth pot.
Having said that the 1M from the LFO to gates could be tweaked to get maximum depth.

(I posted some examples in reply #3.   Then there's this one,
https://www.diystompboxes.com/smfforum/index.php?topic=126876.msg1215253#msg1215253)

[Paul Marossy]

Two other things I forgot to mention:

1 - Looked at the LFO waveform on o'scope with 10x probe. It's not a triangle wave, it's a square wave with a good tilt on the flats.

2 - There's a 0.047uF cap from Pin 3 of CD4069 to "ground" per the factory schematic, but adding to the confusing way they drew and notated it, that cap is actually connected to the "VB" supply on the Aion schematic above. So I guess they meant virtual ground? 

I have a verified reproduction of the factory PCB if anyone is interested (PDF). If you're interested in having that, send me a PM with your email and I can send it to you.

Still am debating how I should try to add a depth control. I have a few ideas, just need to sit down and try them when I get a chance.

[Paul Marossy]

I probably misunderstand "Depth control". But seems to me it wants a fader from straight DC bias (no wobble) to DC plus full wobble.

If you look at the schem at the top of the thread it's got a 1M from the LFO to the gates then another 1M from the gates to the trimpot.  You probably want to keep the 1M from the LFO to the gates and use replace the second 1M with a depth pot.
Having said that the 1M from the LFO to gates could be tweaked to get maximum depth.

I had a vaguely similar thought. So you mean "R17" on the Aion schematic? I don't have much experience with LFOs... would I want to have something in series with that resistor? Would I be correct that the resistance value would have to be larger to decrease the overall depth of the effect?

[rankot]

Two other things I forgot to mention:

1 - Looked at the LFO waveform on o'scope with 10x probe. It's not a triangle wave, it's a square wave with a good tilt on the flats.

2 - There's a 0.047uF cap from Pin 3 of CD4069 to "ground" per the factory schematic, but adding to the confusing way they drew and notated it, that cap is actually connected to the "VB" supply on the Aion schematic above. So I guess they meant virtual ground? 

I have a verified reproduction of the factory PCB if anyone is interested (PDF). If you're interested in having that, send me a PM with your email and I can send it to you.

Still am debating how I should try to add a depth control. I have a few ideas, just need to sit down and try them when I get a chance.

You are right, ground on original schematic is the same as Vb on Aion's. They don't have that capacitor, but maybe it is OK to have it, to smooth LFO a little? If you have a PCB, please send it to me, I'd love to have a look at it, since all those schematics are so blurred.

[Paul Marossy]

Two other things I forgot to mention:

1 - Looked at the LFO waveform on o'scope with 10x probe. It's not a triangle wave, it's a square wave with a good tilt on the flats.

2 - There's a 0.047uF cap from Pin 3 of CD4069 to "ground" per the factory schematic, but adding to the confusing way they drew and notated it, that cap is actually connected to the "VB" supply on the Aion schematic above. So I guess they meant virtual ground? 

I have a verified reproduction of the factory PCB if anyone is interested (PDF). If you're interested in having that, send me a PM with your email and I can send it to you.

Still am debating how I should try to add a depth control. I have a few ideas, just need to sit down and try them when I get a chance.

You are right, ground on original schematic is the same as Vb on Aion's. They don't have that capacitor, but maybe it is OK to have it, to smooth LFO a little? If you have a PCB, please send it to me, I'd love to have a look at it, since all those schematics are so blurred.

PM me your email address and I'll send it you. I think where they show "-" on the schematic is what we normally think of as ground and where they show the symbol we normally think of as ground is the virtual ground.

[pinkjimiphoton]

Two other things I forgot to mention:

1 - Looked at the LFO waveform on o'scope with 10x probe. It's not a triangle wave, it's a square wave with a good tilt on the flats.

2 - There's a 0.047uF cap from Pin 3 of CD4069 to "ground" per the factory schematic, but adding to the confusing way they drew and notated it, that cap is actually connected to the "VB" supply on the Aion schematic above. So I guess they meant virtual ground? 

I have a verified reproduction of the factory PCB if anyone is interested (PDF). If you're interested in having that, send me a PM with your email and I can send it to you.

Still am debating how I should try to add a depth control. I have a few ideas, just need to sit down and try them when I get a chance.

You are right, ground on original schematic is the same as Vb on Aion's. They don't have that capacitor, but maybe it is OK to have it, to smooth LFO a little? If you have a PCB, please send it to me, I'd love to have a look at it, since all those schematics are so blurred.

ranko,
you want me to fill in all the values from the service sheet?
sorry, hard to get a decent scan

[rankot]

Ranko,
you want me to fill in all the values from the service sheet?
sorry, hard to get a decent scan

Thanks a lot, but don't bother yourself - I can decipher that, just was curious to see how to power CD4069 - shall it's "ground" connect to ground (Vb on Aion schematic) or to -9V battery pole (ground on Aion schematic)? Also, how to connect two unused buffers?

[Paul Marossy]

Ranko,
you want me to fill in all the values from the service sheet?
sorry, hard to get a decent scan

Thanks a lot, but don't bother yourself - I can decipher that, just was curious to see how to power CD4069 - shall it's "ground" connect to ground (Vb on Aion schematic) or to -9V battery pole (ground on Aion schematic)? Also, how to connect two unused buffers?

No power to Pin 14 of  CD4069. They only connect Pin 7 to virtual ground.

[Rob Strand]

I had a vaguely similar thought. So you mean "R17" on the Aion schematic? I don't have much experience with LFOs... would I want to have something in series with that resistor? Would I be correct that the resistance value would have to be larger to decrease the overall depth of the effect?

Yes, so replace R17 with a 1M depth pot.    I'd say this would be a good place to start.

Next perhaps tweak R18.

With this type of depth pot it's simple circuit-wise to just have the depth-pot wire back to the bias voltage at the minimum depth setting, as per what PRR has drawn.    The depth is the gates will just sit at a constant DC value.   In reality what turns out better is to wire the depth pot to some slightly different DC voltage than the trimpot setting.  This is a little messier circuit wise.  You can see on the how the DC voltage at minimum depth can be set to some other voltage by looking at the Boss BF2.

1 - Looked at the LFO waveform on o'scope with 10x probe. It's not a triangle wave, it's a square wave with a good tilt on the flats.

Looking at the circuit you wouldn't expect that to be the case.  You would expect something triangular.   (Perhaps try DC coupling the oscilloscope input.)

[Paul Marossy]

I had a vaguely similar thought. So you mean "R17" on the Aion schematic? I don't have much experience with LFOs... would I want to have something in series with that resistor? Would I be correct that the resistance value would have to be larger to decrease the overall depth of the effect?

Yes, so replace R17 with a 1M depth pot.    I'd say this would be a good place to start.

Next perhaps tweak R18.

With this type of depth pot it's simple circuit-wise to just have the depth-pot wire back to the bias voltage at the minimum depth setting, as per what PRR has drawn.    The depth is the gates will just sit at a constant DC value.   In reality what turns out better is to wire the depth pot to some slightly different DC voltage than the trimpot setting.  This is a little messier circuit wise.  You can see on the how the DC voltage at minimum depth can be set to some other voltage by looking at the Boss BF2.

Got it. I can give that a try. That factory schematic is also wrong with respect to R17 & R18. "VC" is coming from the junction of those two resistors. Factory schematic shows only one 1M resistor and there's actually two on the circuit board! Maybe that was a "Friday schematic"?   

[Rob Strand]

Got it. I can give that a try. That factory schematic is also wrong with respect to R17 & R18. "VC" is coming from the junction of those two resistors. Factory schematic shows only one 1M resistor and there's actually two on the circuit board! Maybe that was a "Friday schematic"?   

Well it needs some form of R18 in there otherwise the DC from the trimpot and 1M is going to be totally obliterated by the output of the opamp.  aion says he traced the unit so I'd say that's the one to believe for R18; in fact aion posted his trace before jimi posted the factory schematic.   So yeah, definitely another Friday schematic from the manufacturers.

[Paul Marossy]

1 - Looked at the LFO waveform on o'scope with 10x probe. It's not a triangle wave, it's a square wave with a good tilt on the flats.

Looking at the circuit you wouldn't expect that to be the case.  You would expect something triangular.   (Perhaps try DC coupling the oscilloscope input.)

Oops, you are correct. My mistake. I was looking at the wrong place in the circuit. It starts out as a square wave at 10K resistor at the output of first opamp (one side of speed pot) and gets transformed to a triangle wave at C9 (10uF at the other connection to the speed pot). I didn't catch that until I modeled it in LTspice (first time using it) and saw that the output was a triangle wave. Went and looked at the actual circuit with the o'scope and it looked very close to what was in LTspice.

I tried messing around with R17/R18 but didn't arrive at anything that I liked or thought sounded good. That 250K trimpot is itself kind of a depth control, but it's really meant to be a set and forget control. My intuition for a "depth control" from the start, in my mind, kind of revolved around playing around with 150K at the output of the filter section (R5 150K). I guess really I was thinking more of a blend/mix control, as opposed to an actual LFO depth control, similar to the mod I did to my Phase 45 which consisted of making the 10K resistor in the feedback loop an adjustable control. That worked out pretty well, so I thought maybe I could do something along similar lines except that this Phazzer circuit doesn't have a feedback path to mess with, which is when I started asking questions. So this last week I experimented with adding a pot in series with R5 until I found something that works pretty well, for the most part. My idea was to basically have a quasi-interstage volume control for the effected side so I could essentially dial back the phaser sound and have the clean guitar part be a little more dominant, but I didn't want to connect it directly to ground so I thought I do it with a capacitor. In the end, I wound up with a 100K-C pot in series with R5, with lug 2 connected to junction of R2/R3, lug 3 connected to Pin 1 IC-3A thru R5 and lug 1 connected to a 3.3uF cap to ground. I'm not sure why it works but it is effective and doesn't mess with the sound from what I can tell, or see on the o'scope. The only quirk with that is that when I go full CCW it brings the intensity back up a little bit, not sure why that happens but probably can be fixed. Anyway, I was just flying by the seat of my pants on this and going with my intuition. So now I am a happy camper. I can now go from a warbly kind of tremolo Oz Noy sound in high speed setting and min "depth" to a really nice phaser sound with "depth" on max., and get nice sounds in between. The addition of the "depth control" does not seem to affect the bypass vs effect on level. I can't tell any drop in volume. If it were an issue you could probably compensate for that with the addition of a small value resistor in feedback loop of the 741.

I think I forgot earlier to mention that I used 741 and 1458s for this build, just to see how it sounded with noise level, etc. It's a pretty quiet circuit, I can really only hear the normal swirling whooshing sound that you get with no input on phasers & flangers. I remember reading some comments that were mentioning hiss but I don't have that problem.

I did have one other thought as an experiment but haven't tried it. What would happen if you took the connection from Pin 7 CD4069 and had a pot wired as variable resistor there instead of it being connected directly to "VB"? Would that mess with the depth of the filter section or would that just cause squealing or the chip to have a meltdown?

[Rob Strand]

I tried messing around with R17/R18 but didn't arrive at anything that I liked or thought sounded good.

The will be a "correct" set of values for R17, R18 and the the trimpot.    That's true for any phaser, for example the MXR phase.  You can play with the 3M3/3M9 to tune the LFO level.

At the LFO there is an implied DC level.  There is a DC level at the trimpot.  With R17/R18 fixed there is a "correct" setting of the trimpot which make the gates sit at the correct DC voltage, a DC voltage between the LFO's DC voltage and the trimpot DC voltage.     This is all normal for Phasers.

However, when you start using R17/R18 as a depth control it's not guaranteed to work.   As soon as you use R17/R18 as a depth control it stuffs the DC level.   Sometime the Boss BF2 method helps here.  (Actually there's a bit more to it since you might need to DC shift the LFO.)

I did have one other thought as an experiment but haven't tried it. What would happen if you took the connection from Pin 7 CD4069 and had a pot wired as variable resistor there instead of it being connected directly to "VB"? Would that mess with the depth of the filter section or would that just cause squealing or the chip to have a meltdown?

It might be possible but only if the drains are AC coupled to the all-pass filters.   You don't want DC across the drains and source.  By just DC shifting pin 7 it will create a DC level across the drains and sources.   The Pearl phaser had a different kind of trickery going on, it's not that easy to understand.

[deadastronaut]

following, ordered up some 4069's to play with this....

[Paul Marossy]

I made video about my Phazzer clone which gives idea of how it sounds, my tweak to circuit and how I made enclosure out of a $5 aluminum cookie sheet purchased at thrift store.