question about boost schematic...

[mistermikev]

so jacking around tonight working on my own push/pull version of the pa2.  There is a well known schematic and layout link below.  Here's me admitting what I know and what I don't know about the schematic...

so the thing I'm unsure of is the sv1 which is an emg quick connect.  I have no use for that so don't want it in my layout.  looking at that pin header here's how I understand it:

sv1-5 appears to be the output which is selecting on s1 between a amp that is boosted via R4 resistance, and another amp that has no boost factor between pins 6 and 7 on the TL062.  So that amp is functioning as a unity buffer.

on sv1-4 and 3 are obviously to ground.

sv1-2 is on the back side of a polarity protection diode that would feed any further circuitry 9 volts. 

sv1-1 is our input as it seems to have a pull down resistor of 499k going to ground, then a hi pass filter followed by vb... then feeding into both the buffer and the booster. 

so my question is:

do I have those connections right because according to the commercial version website that quick connector on the below schematic is wired wrong.  Perhaps they were wired dif in the past?

 

below is my version setup to work off of a 500k push pull pot.

EDIT: corrected layout here:

EDIT: doh... doesn't count if no one else caught it... my two 1uf electro caps on the buffer side of things were backwards...

[curtisa]

1 hour ago, mistermikev said:

http://freestompboxes.org/download/file.php?id=15790&mode=view

"... You are not authorised to download this attachment. ..."

 

1 hour ago, mistermikev said:

below is my version setup to work off of a 500k push pull pot.

Ow. Your layout hurts my brain 

The EMG documentation for the PA2 practically gives away the circuit layout for free minus the fine details on exactly what component values to use, but you can probably guess them based on common practice for opamp buffering used in pedals.

From your layout, the first half of the TL062 does variable clean boost from 0 to 20dB, as per the block diagram in the EMG product manual.

Second half of the TL062 doesn't appear to do anything in your layout, whereas according to the EMG literature it's used to provide a plainjane 0dB buffered output for active or passive pickups whenever the boost switch is disengaged. In your layout I can see what appears to be 2x BI connections (BI = buffer input?, supposed to be jumpered together perhaps?), but then BO (buffer output?) doesn't go anywhere. The top BI pin also appears to be jumpered to ground; mistake perhaps?

Your push/pull switch seems to simply bypass the whole lot and short out the output of the 20dB boost stage. I suppose it achieves a similar function to the original, but you lose the low impedance buffering action whenever the boost is disabled.

[Prostheta]

[Prostheta]

Pretty much as @curtisa states. The output is switched between the outputs of IC1A and IC1B. IC1B is operating in non-inverting unity gain mode whereas IC1A is operating as an inverting amp as seen by the negative feedback. It seems a bit redundant to my mind, and I am sure that there is a smarter way of making this work. That being said, it would reduce component count by a couple of SMD parts at best I think, so it would be an exercise in futility regardless of how clever the re-design was.

R2 and R3 hold the point where they meet at half the supply voltage, pushed/pulled by the negative plate of C2. C1 is similar to C2, a DC blocking cap. R1 supplies input impedance at 500k (I actually think this could be raised given TL series op amps input current is in the order of picoamps). This is likely to provide basic compatibility with non-EMG pickups such as standard passives so they sound like they're in a 500k sorta circuit. This is a simplification for brevity.

C3/R6 and C4/R10 provide basic high pass filtering to pull any DC and LF junk from 3.2Hz down. R8 and R9 provide output impedance at a low 2k. C6 is a supply buffering cap, D1 is a reverse supply protection diode.

All the pot version is doing is to pull out R4 into what I'd call VR1 and combine that with a pull pot switch for S1.

This is first year A level electronics, which even I almost remember....! I'd be tempted to use a better op-amp than a TL062 though. They're nice low power parts, but an old design. I'd like to say 80s, but I have nothing other than remembering them being available '90/'91 as an common RS/Rapid catalogue part. I like Ti's OPA chips. Not as low power by any means, but a magnitude better in audio performance....not that it means much for our use....but hey! More better is more more better.

I may have misunderstood or made basic errors somewhere along the line. It's a straight interpretation.

[Prostheta]

Goddamn, I want an old analogue scope now.

[curtisa]

Betterer. Now I can see what's going on a bit clearer.

 

20 minutes ago, Prostheta said:

IC1B is operating in non-inverting unity gain mode whereas IC1A is operating as an inverting amp as seen by the negative feedback

Both opamps are non-inverting (input signal is being applied to both non-inverting inputs simultaneously).

Both opamps have negative feedback, otherwise you'd have one opamp doing it's opamp-y thing while the other was a nice squarewave generator.

 

25 minutes ago, Prostheta said:

I actually think this could be raised given TL series op amps input current is in the order of picoamps

Probably no real value in increasing the input impedance given the application. Worst case source impedance for a super-hot humbucker is, what? 25k? 500k input impedance will never scare such a pickup.

 

28 minutes ago, Prostheta said:

C3/R6 and C4/R10 provide basic high pass filtering to pull any DC and LF junk from 3.2Hz down

They do have that effect, but C3/C4's primary purpose is to prevent the quiescent DC voltage on the output of each of the opamps (nominally half the 9V supply) from upsetting any downstream equipment. R6/R10 also 'pin' the floaty side of the C3/C4 to ground whenever the switch is not selecting that opamp's output, which minimises any popping noises that may occur when selecting boost/no boost.

[Prostheta]

Hahaha! Exactly. This is why I relied on my tutors to fix my errors. Then my work was perfect

Some errors were laziness and some were oversimplification. Both then and now!

[curtisa]

Hey, it's the end of 2019. Lets have a wee dram and forgeddaboudit...

 

[curtisa]

Back to @mistermikev's original question (thread derailment strikes again!):

6 hours ago, mistermikev said:

do I have those connections right

Well, your labelling of each point on SV1 appears to line up with the schematic that @Prostheta linked to (assuming it's the same one you had in your original post), so I'd say you've got it right.

 

Quote

because according to the commercial version website that quick connector on the below schematic is wired wrong.  Perhaps they were wired dif in the past?

Or the schematic has it wrong and EMG have it right? Given it appears to be a unofficial reverse engineering of the original product, it's entirely possible the author mislabelled some of the pins.

I guess you just need to decide what's more important to you - a working circuit or compatibility with other EMG products.

[mistermikev]

4 hours ago, curtisa said:

"... You are not authorised to download this attachment. ..."

 

Ow. Your layout hurts my brain 

The EMG documentation for the PA2 practically gives away the circuit layout for free minus the fine details on exactly what component values to use, but you can probably guess them based on common practice for opamp buffering used in pedals.

From your layout, the first half of the TL062 does variable clean boost from 0 to 20dB, as per the block diagram in the EMG product manual.

Second half of the TL062 doesn't appear to do anything in your layout, whereas according to the EMG literature it's used to provide a plainjane 0dB buffered output for active or passive pickups whenever the boost switch is disengaged. In your layout I can see what appears to be 2x BI connections (BI = buffer input?, supposed to be jumpered together perhaps?), but then BO (buffer output?) doesn't go anywhere. The top BI pin also appears to be jumpered to ground; mistake perhaps?

Your push/pull switch seems to simply bypass the whole lot and short out the output of the 20dB boost stage. I suppose it achieves a similar function to the original, but you lose the low impedance buffering action whenever the boost is disabled.

thank you very much for the reply and detailed info.  The reason I won't use their layout is A) it's SMD and gosh I hate working with that stuff with my sausage fingers! and B ) a layout is protected as artwork by copyright.  - Don't want to go there.  C) I don't want to use a toggle, nor do I want to have their buffer on when not in operation... I freq use fuzz pedals and want to be able to completely hardwire bypass the circuit and maintain the passive sound... plus wanted to have access to the crazy amount of boost via a vol knob (push pull)

wow, you actually digested my layout... cripes... that makes my head spin!  Bravo.  Yes, I'm completely bypassing the buffered part and would send pin 5 to ground (prob not even nec)... but who knows, perhaps in some circuit I want to add a toggle and put in three wires - replacing the jumpers, and be able to go to buffered.

[mistermikev]

1 hour ago, Prostheta said:

Pretty much as @curtisa states. The output is switched between the outputs of IC1A and IC1B. IC1B is operating in non-inverting unity gain mode whereas IC1A is operating as an inverting amp as seen by the negative feedback. It seems a bit redundant to my mind, and I am sure that there is a smarter way of making this work. That being said, it would reduce component count by a couple of SMD parts at best I think, so it would be an exercise in futility regardless of how clever the re-design was.

R2 and R3 hold the point where they meet at half the supply voltage, pushed/pulled by the negative plate of C2. C1 is similar to C2, a DC blocking cap. R1 supplies input impedance at 500k (I actually think this could be raised given TL series op amps input current is in the order of picoamps). This is likely to provide basic compatibility with non-EMG pickups such as standard passives so they sound like they're in a 500k sorta circuit. This is a simplification for brevity.

C3/R6 and C4/R10 provide basic high pass filtering to pull any DC and LF junk from 3.2Hz down. R8 and R9 provide output impedance at a low 2k. C6 is a supply buffering cap, D1 is a reverse supply protection diode.

All the pot version is doing is to pull out R4 into what I'd call VR1 and combine that with a pull pot switch for S1.

This is first year A level electronics, which even I almost remember....! I'd be tempted to use a better op-amp than a TL062 though. They're nice low power parts, but an old design. I'd like to say 80s, but I have nothing other than remembering them being available '90/'91 as an common RS/Rapid catalogue part. I like Ti's OPA chips. Not as low power by any means, but a magnitude better in audio performance....not that it means much for our use....but hey! More better is more more better.

I may have misunderstood or made basic errors somewhere along the line. It's a straight interpretation.

for the record I would toss likes on all other replies here but I'm out again.  originally my link did display and once I actually posted it dissappeared... I just assumed for some reason it couldn't display it - sorry - missed that one.

Hehe, you are a smarty pants.  I actually did the entire layout and had omitted the second half(buffer) and it's parts but decided I'd add it back in since who knows... maybe I start playing arenas and have 50ft cables to endure!  so stupid... I'll actually populate those components and never use them altho i could just skip that section.

input resistance... could be raised to 1m - good call.

that's a pretty detailed interpretation.  thank you for that. 

opa chip... would have to check the pinout but it's pretty std.  I think many would work... 2134?  but may opt for the lower draw as in a tl062 a battery will last years and I oft forget to remove the cable... and I have a gaggle of them.

 

thank you again for the reply!

[mistermikev]

53 minutes ago, curtisa said:

 

They do have that effect, but C3/C4's primary purpose is to prevent the quiescent DC voltage on the output of each of the opamps (nominally half the 9V supply) from upsetting any downstream equipment. R6/R10 also 'pin' the floaty side of the C3/C4 to ground whenever the switch is not selecting that opamp's output, which minimises any popping noises that may occur when selecting boost/no boost.

hehe, in laymens: prevents poppy.

[mistermikev]

31 minutes ago, curtisa said:

Back to @mistermikev's original question (thread derailment strikes again!):

Well, your labelling of each point on SV1 appears to line up with the schematic that @Prostheta linked to (assuming it's the same one you had in your original post), so I'd say you've got it right.

 

Or the schematic has it wrong and EMG have it right? Given it appears to be a unofficial reverse engineering of the original product, it's entirely possible the author mislabelled some of the pins.

I guess you just need to decide what's more important to you - a working circuit or compatibility with other EMG products.

https://www.emgpickups.com/pa2.html

well... you can see it is physically different on rcustom's layout... and this was posted in a place where it got hoops and hurrays by some folks whom I KNOW know what their doing... so it gave me pause, then mentalpause, then self doubt.  Usually someone would catch something like that so I wondered if perhaps they changed their quick connect connections at some point.

oh, I would not use quick connect... it's not even on my board... it was more me being unsure of myself and not wanting to go thru an entire etch and assembly to find out I had something wrong... which I totally anticipated doing... but am very thankful for the feedback/confirmation.

[mistermikev]

thank you both again... thank you very much for the replies!

[curtisa]

9 hours ago, mistermikev said:

well... you can see it is physically different on rcustom's layout... so it gave me pause, then mentalpause, then self doubt.  Usually someone would catch something like that so I wondered if perhaps they changed their quick connect connections at some point.

It's possible that EMG changed their pin arrangement too. I have a feeling they didn't always use their quick connect system, and if you look at the way they arrange each of the connectors on the in/out cables it wouldn't work any other way with their system as it currently stands:

Each cable is signal + shield, so the only way you can make it work is if the corresponding pin arrangement on the PA2 is <in> - <ground> - <ground> - <out> with the input connector flipped as per EMGs recommendation above, or <in> - <ground> - <out> - <ground>.

 

Quote

and this was posted in a place where it got hoops and hurrays by some folks whom I KNOW know what their doing...

That gives me a bit of a chuckle. I hope no-one was too disappointed that the mystical tonal qualities of the PA2 turned out to be nothing more than a textbook application of a 20dB opamp. When EMG say the PA2 is an opamp 20dB clean boost system with variable gain, they weren't mincing their words.

[mistermikev]

16 minutes ago, curtisa said:

It's possible that EMG changed their pin arrangement too. I have a feeling they didn't always use their quick connect system, and if you look at the way they arrange each of the connectors on the in/out cables it wouldn't work any other way with their system as it currently stands:

Each cable is signal + shield, so the only way you can make it work is if the corresponding pin arrangement on the PA2 is <in> - <ground> - <ground> - <out> with the input connector flipped as per EMGs recommendation above, or <in> - <ground> - <out> - <ground>.

 

That gives me a bit of a chuckle. I hope no-one was too disappointed that the mystical tonal qualities of the PA2 turned out to be nothing more than a textbook application of a 20dB opamp. When EMG say the PA2 is an opamp 20dB clean boost system with variable gain, they weren't mincing their words.

well fsb members generally don't regard this circuit so much as good... but more the idea of posting something new and interesting (back 6-7 years ago?) gets hoops and hurrays.  I think part of it is they just love to debunk that anything with widespread commercial success is anything more than "magic beans".  I know there were a few well known members that chimed in on the thread at the time, but no one even mentioned this so... suggests it may have been right then. 

then again... those guys abso hate the clapton mud boost... yet a lot of great guitar players love it so... sometimes I think they are the reverse of folks who think "it's expensive and well recieved so it must be good" in that they think "it's expensive and well received so it must be bad!".

 

[Prostheta]

It's a mentality, whatever it is. Good or bad. Doesn't matter much. If you boil down something enough, the constituents are usually simpler than the black box hype suggests, and the configuration less than magical. PA2s weren't buying anybody Mercs and country club memberships. They do pad out a product range though.