Could someone clever please double check my schematics

[Otwa05]

Hey all, this is a circuit I've drawn up for a phase selectable boost. I've made it up on a bread board and tried testing it with an oscilloscope but I can't for the life of me get that to work haha. It may be entirely possible that I've just set up the bread board or oscilloscope wrong, but I'm starting to think that maybe something is wrong with then schematic? Cause when I tried to simulate it in LtSpice it didn't work either. I think I'm going to go straight to Vero and see if that works but I figured that I'd double check if my layout will work first. Cheers 🙌🏻

[antonis]

IMHO, U1 non-inverting input switch can't work in either setting..
(but I'm not sure  about particular configuration..)

Additionally, a 470R - 1k resistor is needed between C6 and diode pair upper junction as well as cap in series with R10..
(the former for U2 output current limiting wgere the later for rolling-off DC to unity..)

[moid]

I am by no means any kind of expert, but on the vero layout have you just connected the power 9V straight to ground with a jumper cable? Ignore me if this makes the circuit work, I've just not seen that done before.

[anotherjim]

It looks like an "attenuverter" circuit to me, without the ability to reduce the gain. As drawn, a unity inverter and flip the switch for unity non-inverting (both inputs fed same signal). However, input impedance changes between the two.

You could arrange it in reverse. Make U2 a standard variable attenuverter (+1 to -1 AV) and pre-boost non-inverting in U1. The input impedance can then be fixed at whatever you want.
As said, the clip diodes need a series resistor to limit current. Also, if you return them to Vref instead of 0v, need fewer DC blocking capacitors.

A pile of Attenuverters here. The pot ground will be Vref for you.
https://github.com/genghistron84/attenuverter/blob/main/schematic.pdf

[ElectricDruid]

+1 what Antonis said. The switch is in the wrong place.

Only the +ve input of an op-amp needs a bias resistor like that 1M, so that should be on the +ve input only, not connected to the -ve input. Then the switch would have to be arranged to send the signal either to the positive input while grounding the input end of R2, or feeding the signal to the input end of R2, and leaving the +ve input alone.

Even then it's not perfect because you get an inverting gain of x1, but a non-inverting gain of x2.

Given that you have a second op-amp stage, I might be inclined to do the boost in one stage, and then an optional phase-flip in the second. Separating the two tasks makes the design simpler to do.

Finally, if it doesn't work in the Sim, and it doesn't work on the breadboard, what makes you think putting it on vero would fix it?!? That's the victory of hope over experience!! The sim and the breadboard are both telling you there's something wrong with the design, so you need to stick to a solid debugging process and work out what's wrong and why, so you can fix it. In this case, that'd be drawing a simplified schematic for the circuit with the switch in each of the two positions, and then comparing those with the standard inverting and non-inverting op-amp circuits. If you do that and spot the differences, you'll see straight away what's wrong.

[duck_arse]

I don't know anything abouts simming, nor about opamp quirks, nor even the veroboarding. I do know that when you use m instead of M when you want Megohms, you get milliohms, at least in real life. only a slight difference, I grant you [1 x 10⁹ if my thumbs count right], but I've been told sims are picky about this stuff.

[ElectricDruid]

Good spot, Duck!

[anotherjim]

No, I see an attenuverter with the switch acting as the pot at extremes.

Explained...
https://kassu2000.blogspot.com/2018/04/precision-attenuverter-mixer.html

[ElectricDruid]

No, I see an attenuverter with the switch acting as the pot at extremes.

Explained...
https://kassu2000.blogspot.com/2018/04/precision-attenuverter-mixer.html

Gagh! Yeah, you're right!! How come I didn't spot that?!

I take back what I said earlier. It *should* work, especially if you change the 1milliohm resistor to 1Meg, like Duck suggested.

[PRR]

......didn't work......

That's not a useful problem description.

[Otwa05]

No, I see an attenuverter with the switch acting as the pot at extremes.

Explained...
https://kassu2000.blogspot.com/2018/04/precision-attenuverter-mixer.html

Gagh! Yeah, you're right!! How come I didn't spot that?!

I take back what I said earlier. It *should* work, especially if you change the 1milliohm resistor to 1Meg, like Duck suggested.

Hey thanks all for the help so far! I've added the capacitor to ground at R10 and a resistor in series before the diodes, thanks to duck the sim now works! Altough the schematic looks a little messy since when i try to type M it automatically changes to m





The only issue im noticing now is that while the singals are inverted 180deg from each other, they are both ever so slightly out phase relative to the input signal. I'm not sure if this will even be a problem but any ideas as to why thats happening?

I also recognise that I should have included my reference schematic in my original post, so here it is - see topmost ic (resistor values are identical to what i used)



As you can see it just uses the switch, like anotherjim mentioned, as the pot at it's extremes.

[Rob Strand]

Hey thanks all for the help so far! I've added the capacitor to ground at R10 and a resistor in series before the diodes, thanks to duck the sim now works! Altough the schematic looks a little messy since when i try to type M it automatically changes to m

In spice simulators you have to type "Meg" or MEG
and both m and M mean milli (1x10^-3).

You are best getting used to it.

FWIW, the phase flipper idea looks OK.  It's a common ckt.

[anotherjim]

The relative phase shift is probably due to the phase flipper having different AC performance with the 2 different input resistances. C6 (the Paramix scheme) into either R6 or (R7//R6). Any RC combo has some degree of phase shift at some frequency. You could stop it from happening if the flipper had a preceding opamp stage that defines a fixed input impedance and can DC couple to the flipper input so removing any RC phase shift as there is then no C. Also, you can scale down the flipper resistors to 10k since it no longer defines the input impedance from the loop input.

I don't think many designers consider small frequency-dependant phase shifts much in loop control. Even if the loop controller handles it perfectly, what do the pedals in the loop do? It can be mitigated by choosing RC high pass filter turn over well outside the signal bandwidth. Big capacitors basically.

[Otwa05]

The relative phase shift is probably due to the phase flipper having different AC performance with the 2 different input resistances. C6 (the Paramix scheme) into either R6 or (R7//R6). Any RC combo has some degree of phase shift at some frequency. You could stop it from happening if the flipper had a preceding opamp stage that defines a fixed input impedance and can DC couple to the flipper input so removing any RC phase shift as there is then no C. Also, you can scale down the flipper resistors to 10k since it no longer defines the input impedance from the loop input.

So I should swap the order of stages? Boost first then phase flip. You mentioned if I do this I can remove any need for a capacitor in between, I'm assuming that is neglecting the diodes? If I had the boost into diodes into phase flip then I would need some sort of capacitor between then diodes and phase flip right?

Cheers again

[anotherjim]

Not if you have the clip diodes connect to Vref instead of ground, the whole path will be at Vref with diodes in or out of circuit so you don't absolutely need any coupling capacitors. Also, if you go back to using a pot (centre detent W taper 16mm Alpha are available) instead of a switch in the flipper, it will also act as the output level control for an input stage providing a variable boost to drive the diodes or not.

[antonis]

I've added a resistor in series before the diodes,

Although the 1k resistor (R7 in your revised schematic) serves OK for diode pair current limiting, it should be more practical to place it right after C6 (in series with signal axis) to also prevent U2 from heavy loading (next stage low impedance and/or high capacitance..)

[Otwa05]

Not if you have the clip diodes connect to Vref instead of ground, the whole path will be at Vref with diodes in or out of circuit so you don't absolutely need any coupling capacitors. Also, if you go back to using a pot (centre detent W taper 16mm Alpha are available) instead of a switch in the flipper, it will also act as the output level control for an input stage providing a variable boost to drive the diodes or not.

Ok that I've swapped the order now and thats fixed the sligth phasing problem. I also swapped the resistor in series with the diodes to before the switch (which isnt on the schematic right now since i havent got around to programming a switch into Lt) like antonis suggested. I believe my only question now is where do I put the gain pot? I know it has to be between the two op amps, can I just chuck it in before the R7?

[antonis]

I believe my only question now is where do I put the gain pot? I know it has to be between the two op amps, can I just chuck it in before the R7?

Of course you can but it should act as a Level pot rather than a Gain one..
(attenuation only..)

You can use R9 as gain pot (50k should be fine, I guess..) for a gain margin of +1 to +51..

[Otwa05]

You can use R9 as gain pot (50k should be fine, I guess..) for a gain margin of +1 to +51..

Awesome! I think that this is the final design, now i just need to redraw the vero layout - which I'll do tomorrow and post it up to have it double checked.



Thanks everyone for all the help 

[antonis]

Just another (minor) suggestion..

Make C1 about 150pF for more efficient HF filtering..
(at least, for R9 at high gain settings..)