A/DA Final Phase drive pot

[armdnrdy]

I've been working on a project for the Final Phase, among other things. I've gathered up all of the usual schematics and layouts that were available on ADA Depot. (site seems to be gone)

I've run across one thing that I can use some input on. The drive pot symbol in the schematic looks like a center tapped pot, but there is no mention of a C.T. pot in the parts list or schematic.

A C.T. pot will have the "tap" lug either on the body opposite the three solder lugs or as an added fourth lug. The component layout and a bad picture of the trace side don't depict this which leads me to believe this is a standard pot.

It looks as if this is a confusing way to show the juction of R11 and R13 connecting to the wiper of P1 and R10.

Any thought on this?

http://www.aronnelson.com/gallery/main.php/v/diyuser/ADA+Final+Phase+Drive+Section.jpg.html

[armdnrdy]

Okay,
Time to revisit an old post.

I was never able to decipher if the ADA Final Phase uses a center tapped pot or if the factory drawing is just misleading.
I've found no new information, no new gut shots, etc., so I thought that I would seek info in a different manner.

There are quite a few talented individuals who frequent this site that are well versed in the workings of a circuit such as this so......
Which configuration would actually work?

Factory drawing: The center tap would be "tapped" approximately 50%, mid way between lugs 1 & 3.


My interpretation if pot is not center tapped:

[jmasciswannabe]

Sent you a gut shot, all the revision files I have and a novice opinion on why I assume it is a standard pot. Hope this project sees the light of day.

Best of luck, Armdnrdy!!!

[armdnrdy]

Got it.

Thanks Ian.

Still in the same boat. Nothing definitive.

I just had a thought! Why didn't I think of this before 

I guess I could always bread board the drive section. I have a few dual B50K center tapped pots on the way for the Multi Flanger.
I can connect one half of the pot and try it both ways to see which one is correct. 

[armdnrdy]

Well...

Ian (jmasciswannabe) was nice enough to send an email to A/DA inquiring about the type of pot used for the drive control.

The reply wasn't exactly definitive.

"That was 30 years ago and no one here knows about such things. I would think your assumption is correct: the Final Phase did not have center-tapped pots. It is just the way the schematic drawing was made."

So...back to the breadboard!  

When I get a few moments I'll breadboard the front end of the circuit including the drive section to see/hear what it does.

[Scruffie]

I was considering a fabbed project of this with Haberdasher and had always assumed it was how you'd drawn it, even if it is drawn a bit odd, I can't see why it would need a centre tapped pot.

[bool]

At first glance, electrically, it looks to me that it (a) it shouldn't be center tapped, and (b) the P1 along with R11, R12, R13 makes for a sort of "auto level compensation" network that works in tandem with the use of P1 as a "drive gain" control.

Simply, it looks like the turning of the P1 would both set the overdrive "drive" and at the same time attenuate what goes to IC2C accordingly.

A "clever circuit".

[Mark Hammer]

Have any of us ever played a real one?  Perhaps listening to what the pot does might give away what the correct drawing ought to be.  Incidentally, Larry, there are several drawings and issues of the Final Phase.  The drawing subsection you posted is one of those issues (version 9), but another factory schem for a different issue (version 5, with the JFET input buffer instead of an op-amp as in 9) has the pot (also drawn with a seeming centre tap) drawn underneath the op-amp, rather than immediately adjacent to the clipping diodes.  That earlier version also uses a DPDT switch to simultaneously bypass the overdrive stage and adjust level.

Sometimes the way that something is drawn - simply orienting the components a little differently - can befuddle our understanding of a circuit.  In the Version 5 drawing, we see  R10 and R12 tied together, and R11 goes from their junction to what looks like the centre tap.  In the version you posted, the link is shown as going from the junction of R10 and R11 to a presumed centre tap.

[armdnrdy]

Thank you for all of the replies and welcome to Electronic forensics 101. 


Mark, I've been working with the whole PDF Final Phase "package" originally sourced from the now defunct A/DA Depot.

I chose to work up the schematic #1087 because the package included other pages of info referencing that drawing. There is a component overlay, resistor list, and a capacitor list that includes the pots and ICs.

I gave version 5 (the vactrol version) another look this morning and I have to admit...I'm even more unsure as to the pot type used than before!




If you look at the way the 680K resistor is tied into the wiper of the intensity pot, you will see that an effort was made to maneuver around the pot, drawing a wire cross over symbol, before connecting to the wiper. That section could have been drawn like the following modified image to emulate the drawing style of the drive section.



I just find it odd that all three drawings of the Final Phase that are available depict the Drive pot using the known center tapped pot symbol. I have found only two bad images of the trace side of the Final Phase. The original version and the reissue. Both versions show only three solder pads for the board mounted drive pot but......board mounted center tapped pots are also available with a solder lug for the tap as the following image shows. The "tap" connection could be made on the component side of the board.

One thing to take into consideration is that Dave Tarnowski often played outside of the usual design rules and added what he believed to be the best options regardless of cost. Did the ADA Flanger really need the "Threshold" gate circuit? Not really. With the Threshold control CCW the ADA flanger displays no more "flange noise" at idle than other flangers of the period....and maybe even less!

Since looking at the drive section and being able to "hear" what it sounds like is way above my meager talents, I think the best thing to do is to A/B, with and without, a center tapped pot...just to be sure.

I inadvertently stumbled across these (following image) when searching for a different pot. This is the 50KB, dual gang, center tapped pot needed for the Japanese effector book Multiflanger project. I have a few on the way. I can bread board and test the drive section using one half of this pot.

[Mark Hammer]

You're taking on the Multiflanger?  I salute your diligence, my friend.  It seems like a worthy project.  Between the pots and finding an MN3009, I also salute your skill I tracking down parts.  I printed out a couple toner transfer sheets for that build......and they just sit there. 

[armdnrdy]

You're taking on the Multiflanger?  I salute your diligence, my friend.  It seems like a worthy project.  Between the pots and finding an MN3009, I also salute your skill I tracking down parts.  I printed out a couple toner transfer sheets for that build......and they just sit there.  

About all I have is perseverance! but I'm learning.  

I have a few MN3009s, and a few 2SA798s, (which I now know could have been subbed with a couple of matched 2SA733s or 2SA1015s)

I'm going to route a whole new board layout. I'm not a big fan of how far apart the BBD and the clock ICs are on the original. The ICs are on opposite sides of the board, and the clock signals are connected with long jumpers.  
I know enough to smell trouble in that layout!

[armdnrdy]

I received the dual 50KB center tapped pots in the mail and found a moment to bread board the Final Phase drive section.

Here is the circuit that I tested:



My findings are: When the point between R11 (36K) and R13 (150K) is connected to the 50% center tap, the signal is attenuated compared to when the same point is connected to the wiper of the Drive pot.

Other than that difference, the Drive pot reacts the same along its entire travel.

Conclusion: Results inconclusive  I think that the only real way to tell is with the drive circuit incorporated with the rest of the phaser circuit. But....it doesn't make sense to use a center tapped pot if the only thing gained is volume of the drive section...there are other ways to accomplish that. So....the original probably used a regular 50KB pot.