Help understanding a schematic and transistor question

[jfrabat]

I tried to find another copy of the article with clearer photos, clearer layout and the PCB artwork but didn't get anywhere.

I know.... I did exactly the same thing, but came up empty handed as well!

OK a new set of questions; I have been looking online for a +/- 15V DC + GND power supply.  The MEAN WELL RT65C is the only one I could find, and it is not external.  I ordered one along with the other stuff I got for this project.  My issue is that I have a Hammond 1402DV left over that I wanted to use for this project (it is almost exactly the same size as my DAC, so it would go nicely on top of it). 

The issue I have is space if I put the power source in there.  Here is what I mean:



I only got 10mm between the rear of the power source and the tip of the jacks on the back, and 17mm from the lower front lip of the power source to the front panel (this is where the pots will go).  I chose to put the power source like that and have the PCB on the right side because the right side of the power source is solid metal, which would protect the PCB from electrical noise.  I also left the left side of the back panel empty for the AC power chord to run out (my intention is to run it down the left side, to keep it as far away from the electronics as possible).

PCB would be around 50mm X 135mm in the space to the right of the power source (I think that should be enough to get all the components in there).

Now, I COULD just go with a 15VAC brick and get +/-15V DC (like I did with 12V in my EQ), but I like the idea of using the power source, as power SHOULD be cleaner (plus, I do not have enough capacitors and would need to place a new order to build that circuit).  To be honest, I miscalculated the size completely, but I still would like to use it.

My question is, do you think the power source will create much noise?  Or you think I will be OK to work it like this?  I tried understanding the subject (even read this white paper on it) but it is way too far over my head...

[Slowpoke101]

This link may provide a clearer (slightly ) version of the article. It also has images of the circuit board artworks - double sided.

https://web.archive.org/web/20150408034353/http://www.spontis.se/pdf/eti_dicompgate.pdf

Someone has probably already noticed this;
There is a bit of a problem with the component to board overlay. The component layer does not match up with the board image so none of the components seem to connect to the correct circuit board traces. The board image needs to be flipped vertically and then it will match up to the component overlay...Anyone good with Photoshop?

[Rob Strand]

My question is, do you think the power source will create much noise?  Or you think I will be OK to work it like this?  I tried understanding the subject (even read this white paper on it) but it is way too far over my head...

There's no simple way to know.  When you put a circuit close to commercial switch mode the best you can do is expect trouble.   You would really need to do a test.    If you get noise problems they won't be easy to solve and you will need a back-up plan to fall back on.  I'd even be a little hesitant about drilling holes related to the mains power supply.

This link may provide a clearer (slightly ) version of the article. It also has images of the circuit board artworks - double sided.

https://web.archive.org/web/20150408034353/http://www.spontis.se/pdf/eti_dicompgate.pdf

Someone has probably already noticed this;

Excellent.  Thanks a lot for that.

Honestly I saw a lot pins spaced at 0.1" which looked like they went to DIP packages but I assume there we some sort of historical left overs from the PCB layout.    I was assuming if we had the other layers it would all make sense.

So yes, the overlay is correct but the tracks underneath are completely wrong.    Some significant Photo-shopping is required.

That made me review the whole RV6 and SW2 thing.

The first assumption that there's no pads for RV6 is wrong.    After flipping the PCB, there are pads for RV6.   I traced the tracks for RV6 and they look like they are hard-wired to the parts on the PCB;  R70 and R70.   There's no intervening switch SW2 between RV6 and the circuit.

So that made me review the switch SW2.    The text implies SW2 bypasses the noise gate.   Early on I was looking for tracks which gave a hint that the compressor could bypass the noise-gate via the switch SW2 but I couldn't see anything.

I tried to trace the circuit from the PCB overlay in fig 9.   Too many tracks go to the underside of the board so I cannot work it out.    The idea was to see if R27, C14, C15 are all wired via PCB tracks back to IC3b pin 9 + R26.

After a lot of mental twisting and track tracing,  I can make out the 'a' 'b' 'c' SW2 connections are in fact going to the output stage R27, C14, C15.    SW2 does completely bypass the noise gate.

So the bottom line is conclusions we made before are all wrong!

I don't have much doubt the RV6 wiring matches the schematic.

Due to a brain overload swapping between three pics I didn't work out the precise details of how SW2 is wired.    What is clear is the tracks on the PCB and 'a' 'b' 'c' SW2 connections all imply SW2 is bypassing the noise-gate.

[Slowpoke101]

Thanks to having the PCB artworks available I've now worked out how to connect SW2. SW2 is a SPDT switch on the back of RV6 (Trig Level ). SW2 connects to the board at the connections points shown as "a, b and c ". Also note that the schematic is not accurate. The connection between R26 and R27 does not exist on the circuit board.

Point "a" connects to IC2b pin 7. This is the SW2 "off" position.
Point "b" connects to the junction of R27, C14 and C15. "b" connects to the toggle element of the switch.
Point "c" connects to IC3b pin 9 along with R26. This is the SW2 "on" position.

All SW2 does is to bypass IC2b when RV6 is set fully CCW (minimum ) - off.

Now that I've got the artworks I may as well build it and see what else may be wrong. Could be fun.

Ahh..I see that Rob has beat me to it. Good fun.

[Rob Strand]

Thanks to having the PCB artworks available I've now worked out how to connect SW2. SW2 is a SPDT switch on the back of RV6 (Trig Level ). SW2 connects to the board at the connections points shown as "a, b and c ". Also note that the schematic is not accurate. The connection between R26 and R27 does not exist on the circuit board.

Point "a" connects to IC2b pin 7. This is the SW2 "off" position.
Point "b" connects to the junction of R27, C14 and C15. "b" connects to the toggle element of the switch.
Point "c" connects to IC3b pin 9 along with R26. This is the SW2 "on" position.

All SW2 does is to bypass IC2b when RV6 is set fully CCW (minimum ) - off.

Now that I've got the artworks I may as well build it and see what else may be wrong. Could be fun.

Ahh..I see that Rob has beat me to it. Good fun.

Great stuff.    You filled in all the details.

The R26, R27 thing is a little odd.   I had trouble reading some parts on the overlay.   On your clearer scans I can see an R26 but not R27.   R27 (on the schematic) is required for the thing to work.  Parts list matches the schematic.     There seems to be two R29's so maybe the R29 near IC4 is actually R27?

[Slowpoke101]

You got it right Rob.
R27 does not exist on the overlay and R29 appears twice - so far.
The R29 close to IC4 is R27. When I was comparing the artworks, overlay and schematic I did notice that but compensated for it. Now having fooled myself regarding R27 I forgot that I had done so.
Brain-twisting can be difficult to untwist at times  .

[Rob Strand]

R27 does not exist on the overlay and R29 appears twice - so far.
The R29 close to IC4 is R27. When I was comparing the artworks, overlay and schematic I did notice that but compensated for it. Now having fooled myself regarding R27 I forgot that I had done so.
Brain-twisting can be difficult to untwist at times  .

Thanks for checking.   Yes it really twists your mind.   It's amazing how everything tumbles into a heap with only a few errors.     No
official errata for the project either.

I've tried to compile the bugs.    I hope I haven't made any mistakes in the bug list 
---------------------------------------------------------------------------------------
DI Compression Gate, ETI UK Dec 1985
Errata V1.0 (17 Aug 2020)

No official errata for project but there are quite a few errors.
(expected to be listed in the project index of April 1987 issue.)


Errata:

- The PCB tracks on the overlay (fig 9) do not match the parts.
  The PCB tracks need to be flipped top to bottom.

- Q3 (near R48, R49) designator not marked on schematic

- D9 (near R89, Q9)  designator not marked on schematic

- IC4c and IC6c.  Pin number *labels* are incorrect:
  opamp - input should be pin 9  (not pin 10)
  opamp + input should be pin 10 (not pin 9)

-SW2 (Noise-gate bypass) not shown on schematic:
  - Point "a" connects to IC2b pin 7. This is the SW2 "off" position.
  - Point "b" connects to the junction of R27, C14 and C15. "b" connects to the toggle element of the switch.
  - Point "c" connects to IC3b pin 9 along with R26. This is the SW2 "on" position.

- R27 missing from overlay.  R29 (near R28, C15, IC4 on overlay) should be R27.

- Panel Fig. 11 should be Fig. 10 to match the text
  Panel Fig. 12 should be Fig. 11 to match the text

- The pots are not marked with clockwise symbols on the schematic.

[jfrabat]

This link may provide a clearer (slightly ) version of the article. It also has images of the circuit board artworks - double sided.

https://web.archive.org/web/20150408034353/http://www.spontis.se/pdf/eti_dicompgate.pdf

Someone has probably already noticed this;
There is a bit of a problem with the component to board overlay. The component layer does not match up with the board image so none of the components seem to connect to the correct circuit board traces. The board image needs to be flipped vertically and then it will match up to the component overlay...Anyone good with Photoshop?

That is A LOT mode readable.  And no, I had not noticed that; I thought it was the lack of resolution that was not letting me see clearly...

I have reworked the schematic in EAGLE, since I will be making my own board design (I have to squeeze it into a 50mm by 135mm board for it to fit with the power supply in there).  Schematics are ready (well, at least it was until I read the new posts below!), board design is not done yet (not even started).  Is anyone interested in checking my work?  I have reviewed it about 3 times, and seems correct, but the more the schematic is checked, the better!

I also played around with the case design; regardless of it having the power supply inside or not, the case is certainly the right size for this project because of all the inputs / outputs and because of all the knobs and switches.  I think I will follow Rob's advise and tested first before drilling any holes for the power supply.  If push comes to shove, I can make another PCB to split 15VAC into +/- 15V DC.

This is where I am so far in the enclosure design (ignore the colors; those are just layers in AutoCAD):

Front Panel:


Back Panel:

After a lot of mental twisting and track tracing,  I can make out the 'a' 'b' 'c' SW2 connections are in fact going to the output stage R27, C14, C15.    SW2 does completely bypass the noise gate.

So the bottom line is conclusions we made before are all wrong!

I don't have much doubt the RV6 wiring matches the schematic.

Due to a brain overload swapping between three pics I didn't work out the precise details of how SW2 is wired.    What is clear is the tracks on the PCB and 'a' 'b' 'c' SW2 connections all imply SW2 is bypassing the noise-gate.

-SW2 (Noise-gate bypass) not shown on schematic:
  - Point "a" connects to IC2b pin 7. This is the SW2 "off" position.
  - Point "b" connects to the junction of R27, C14 and C15. "b" connects to the toggle element of the switch.
  - Point "c" connects to IC3b pin 9 along with R26. This is the SW2 "on" position.

You lost me there; so how do I include this in the reworked schematics?  Keep in mind I am working with a separate switch rather than the push/pull pot...  If I am understanding correctly, I can use a SPDT switch and connect the pins in the order listed.  Is that correct?  Or do I need a DPDT switch and match one side to the pot wires? 

Thanks to having the PCB artworks available I've now worked out how to connect SW2. SW2 is a SPDT switch on the back of RV6 (Trig Level ). SW2 connects to the board at the connections points shown as "a, b and c ". Also note that the schematic is not accurate. The connection between R26 and R27 does not exist on the circuit board.

Point "a" connects to IC2b pin 7. This is the SW2 "off" position.
Point "b" connects to the junction of R27, C14 and C15. "b" connects to the toggle element of the switch.
Point "c" connects to IC3b pin 9 along with R26. This is the SW2 "on" position.

All SW2 does is to bypass IC2b when RV6 is set fully CCW (minimum ) - off.

Ah, that clears up SW2, thanks!  So many replies!!!  LOL!   So, just to be clear, are R26 and R27 connected or not?

[jfrabat]

By the way, here is my schematics; I included SW2 and broke the link between R26 and R27.  I am also using a combo jack for the mic input, and changed transistors to 2N3906.  Also, I know SW1 should be SPST, but did not have any of those in my Eagle libraries.  I also added 0R to the GND (at the power source) to be able to separate Control Ground (CG in the schematic) from Audio Ground (AG in the schematic), as otherwise, for Eagle, GND is GND (and I want to keep true to the original design which keeps them as separate as possible).  Those will be links when I build the board.

Please let me know if you see anything out of whack!  If the schematics are correct, I will proceed to make the board, and make it public in https://oshpark.com/ for anyone that is interested in making one of these (all switches, pots and jacks will be off board).  I'd even be willing to pay for a batch of 3 and send it to Rob, Druid, or Slowpoke and then you guys can mail the PCB's to the other 2 (I live in Central America, so shipping gets expensive down here!).  You guys always help me when I bite off more than I can chew (which is EVERY time I make a pedal!  LOL!).

SCHEMATIC UPDATED IN POST BELOW

Re-edited: Schematic has been updated to correct a few errors found.
And yes, I should be consistent with the cap symbols, but those I know are the correct size...

[jfrabat]

I am revising the schematics to make the part number match.  I am also finding some errors along the way in the original schematic, so I will sum them up here with the previous errors:

Errata:

- The PCB tracks on the overlay (fig 9) do not match the parts.
  The PCB tracks need to be flipped top to bottom.

- Q3 (near R48, R49) designator not marked on schematic

- D9 (near R89, Q9)  designator not marked on schematic

- IC4c and IC6c.  Pin number *labels* are incorrect:
  opamp - input should be pin 9  (not pin 10)
  opamp + input should be pin 10 (not pin 9)

-SW2 (Noise-gate bypass) not shown on schematic:
  - Point "a" connects to IC2b pin 7. This is the SW2 "off" position.
  - Point "b" connects to the junction of R27, C14 and C15. "b" connects to the toggle element of the switch.
  - Point "c" connects to IC3b pin 9 along with R26. This is the SW2 "on" position.

- R27 missing from overlay.  R29 (near R28, C15, IC4 on overlay) should be R27.

- Panel Fig. 11 should be Fig. 10 to match the text
  Panel Fig. 12 should be Fig. 11 to match the text

- The pots are not marked with clockwise symbols on the schematic.

- IC1a and IC1B are backwards (pins are labeled correctly, but the one with pins 1, 2 and 3 should be AC1a, not AC1b).  Does not make much difference in anything, but to be accurate...

- The depth potentiometer is listed as PR9 in the schematics and layout but as RV9 in the Parts List

- R82 is missing from the Parts List

Here is the schematic with the parts renumbered to match the original schematic (and adding SW2, as well as R91 & R92, which, as mentioned, are for traces naming purposes in Eagle to keep Audio Ground and Control Ground separate).  I will delete the previous schematic to avoid issues.  I have checked this schematic over 5 times, and cannot find any errors, so please shout if you see one!

[Slowpoke101]

Check C22. It may not be polarised correctly.
Other than that I cannot see any other errors yet. Looks good so far.

[jfrabat]

Check C22. It may not be polarised correctly.
Other than that I cannot see any other errors yet. Looks good so far.

You are absolutely right! Corrected it in Eagle.  Is SW2 wired correctly above?  Did I understand that right?

[Slowpoke101]

You have SW2 wired correctly.
So far all looks good.
Hopefully some other people will be able to have a look too and see if they can see anything else that may be a problem. The best thing to do is not look at it for a few days and then come back to it. The break away helps clear your head and you look at it it fresh eyes. Much easier to see any problems.

[jfrabat]

By the way, looking at the pics in the magazine article, the traces of the original board seem HUGE (as in thick)!  I imagine they do not need to be that wide, right?  I can use regular traces, like any of the other pedals I have made in the past, right?  I intend to use 0.4064 mm (16 mil) as default and 1.143 mm (45 mil) for the power rails (+VE, -VE, GND, AG and CG).

[Slowpoke101]

The tracks are rather wide. They don't need to be. Your standard default Eagle track widths should be fine to use. I do advise making the power and ground tracks slightly thicker.

The main reason for the original project to have such wide tracks would be one of ease of production. The magazine article had board artworks included from which the reader could produce their own circuit board. Using a UV positive photo-resist sensitised board and the magazine artwork pages (cut from the magazine ) as photo-tools, the reader could make a board. The reverse pages of the artwork pages were printed with a blue ink that was somewhat transparent to UV light and therefore would not print through onto the sensitised board. To have some degree of success wide tracks were common to these projects but with a good setup you could get good results and finer track widths.

The other reason for the tracks being wide was the actual production of the original artworks - before reasonably priced PCB CAD software and computers that were powerful enough to run the software without costing an arm and a leg. So artworks would be made by using pre-printed DIP pad patterns, pad donuts and a fine black crepe tape all stuck down to a sheet of plastic with 0.05" blue grid line printed on it. Patience was vital. The most common supplier of these torture products was a firm known as Bishop Graphics. Be grateful for software CAD programs..I am.

Edit: The track widths that you noted in your last edit seem to be a good choice.

[Rob Strand]

You guys have been busy!

I bite off more than I can chew (which is EVERY time I make a pedal!  LOL!).

It's not your fault the project has bugs.    The good thing is you are building something you will use.

[jfrabat]

Well....  Scratch that idea about using the power supply...  Turns out, my eagle will only let me do boards 100mm in length.  And since there is NO WAY to fit all those components in an area 50 X 100 mm, I will needed to rethink my strategy. 

My idea now is to make the board itself much bigger (maybe 100 x 100 mm, since I got the space!), and allow for more room to work all the components in.  Taking my learning from my equalizer project, I intended to use a 15VAC brick, and get my +/-15VDC from that.  So I got rid of the power supply part at the bottom right of the schematic (C20, C21, C22 and C23) and replaced that with the separate part at the top left.

I still have the original (with the revisions already discussed) schematic, in case anyone is interested, by the way...



By the way, that 0R resistor in the power supply is once again because of naming issues in Eagle.  It is not necessary at all (same as the grounds above)

And using this brick to power the whole thing.  What do you guys think?

You guys have been busy!

I bite off more than I can chew (which is EVERY time I make a pedal!  LOL!).

It's not your fault the project has bugs.    The good thing is you are building something you will use.

So far, I use everything I have made.  Some more than others, but everything (that works!) get used!  My kid is learning to play bass, so his pedal board has all my non-commonly used pedals!  LOL!

[Rob Strand]

nd using this brick to power the whole thing.  What do you guys think?

It looks like it will do the job.

However, this project is much more demanding than the parametric EQ you built.   The use of resistors in the power rails and all that won't work because the voltage drops are too high.  You will need  a set-up with two regulators, one positive and one negative.

The problem now is how to deal with the turn-on "bang" due to the voltage doubler.

One option is to use bigger supply caps before the regulator, say 2200uF.  In total that's not much more capacitance than you had before.  You can add a low-value resistor in series with the power switch to help reduce bang.   It will need delicate tuning to make it big as possible to reduce the band yet not introduce too much voltage drop.   You might even be able to use a PTC thermistor.

You probably want to keep the power rails at +/-15V DC, so that means 7815 and 7915 regulators.     I'm not sure what unregulated DC voltage you will end-up with under load, perhaps more than 20V, so that might give you some slack in adding a series resistor to "de-bang" the power.

Beyond that things start to get more complicated.

[jfrabat]

Rob, you think something like this may work?  I could even put a fiberglass divider between the power supply and the main board (and I could use some copper tape to shield it somewhat).  That power supply would leave me a 86 X 100mm board, which makes it much more possible to fit all components in...

[Rob Strand]

Rob, you think something like this may work?  I could even put a fiberglass divider between the power supply and the main board (and I could use some copper tape to shield it somewhat).  That power supply would leave me a 86 X 100mm board, which makes it much more possible to fit all components in...

For that one you might need to add a dummy load as the minimum current is 0.1A.    It still has the same problem as the other switchmode.   That one is 25W so it might be less angry than the 65W.   With any switchmode near an audio circuits there's a risk of noise problems.    Even mains wiring too close to audio circuits can cause noise problems.   Copper shielding would help but it doesn't shield magnetic fields very much at all.  Separation distance helps but if you have to move it 300mm away then that's not practical.