Intersound IVP gyrator circuit output

[jimladladlooklike]

Hi folks,

I have another project on the go for which I require your expertise. It's a Marshall Guv'nor followed by the EQ section from the Intersound IVP then into a boost/distortion circuit which I haven't decided on yet. The Guv'nor part will have trim pots to control bass, mids and treble, leaving the gyrators for the user to dial in frequencies with a bit more versatility.

I have a question about the 4558 at the end of the signal chain (U4A):



I'm guessing this half of the op amp "sum" (may be using that term incorrectly) the signals from each outer pin of the previous potentiometer. The resulting signal then leaves the output, which is where I plan to use the 2nd half of the same op amp to create the boost mentioned above. However, I'm confused as to the purpose of R102 and C48. Can I eliminate these and simply connect the outer pin of the pot to pin 2 of the op amp? Or is this part of the circuit something to do with biasing? I notice this pin is connected to ground (4.5V in the case of this project) via a couple more components:



Thanks in advance for your help!

[FSFX]

R102 is essential. I provides the feedback that in conjunction with U4A and the 50k potentiometers create the summing amp to combine the signals from the gyrators.
You could just connect R102 between pin 1 and pin 2 of the op amp if you are not using the extra transistor power stage that follows it.
C48 just acts as a Miller capacitor to limit the high end response. It is always a good idea to cut out any high end noise and to improve stability.

[PRR]

The fundamental circuit has R102 AND another equal resistor you have cut-off.

[jimladladlooklike]

Ah I see, thank you for the explanation. Have adjusted my layout accordingly.

[jimladladlooklike]

I've finally got round to building this. The Guv'nor and MXR Dist + work great. However, the gyrator circuit not so much. There's a huge drop in volume and none of the 8 pots seem to do very much to change the sound. I suspect from the voltages below that it's something to do with the summing amp section. Please also see the schematic I used for reference. Also note that IC3B is used for the Dist + circuit. At this point I've taken the circuit apart multiple times, measured switches and pots to make sure they're the right value and not faulty, cleaned up the underside of the board, reflowed the majority of the joints. Appreciate your help in advance. Thanks!

IC1
1 - 4.67
2 - 4.67
3 - 4.59
4 - 0.003
5 - 4.59
6 - 4.67
7 - 4.67
8 - 9.31

IC2
1 - 4.67
2 - 4.67
3 - 4.59
4 - 0.003
5 - 4.59
6 - 4.67
7 - 4.67
8 - 9.31

IC3
1 - 2.132
2 - 1.305
3 - 0.816
4 - 0.003
5 - 4.25
6 - 4.69
7 - 4.66
8 - 9.31

[PRR]

Why is IC3 pin 3 so low? Since it comes from off-the-image, we need more clue.

[ElectricDruid]

Do these gyrators work with the resistances tied to ground? I've seen it done that way, but I think in those circuits we might have been looking at a dual supply.

Since this is a single supply, shouldn't those go to Vbias/4.5V instead? I think that ground connection is going to screw it up.

[PRR]

> Since this is a single supply

You are right, Tom. I already expressed my opinion about 'plans' with key details cut-off.
https://www.diystompboxes.com/smfforum/index.php?topic=130027.msg1259476#msg1259476
Draw the WHOLE circuit. None of it is unimportant.

[jimladladlooklike]

Hey guys, apologies, I meant to erase those ground symbols and replace with Vbias, that's how those resistors are connected on my build currently.

Power supply wise, pin 8 of all ICs are connected directly to V+ and pin 4 to ground. That's all that's missing from this diagram (will try and do better and include everything next time).

Also, the op amp symbol second from the right should be labelled IC2B.

I'm gonna investigate IC3A

[antonis]

Could you plz post a complete schematic of your particular build..?? 

[jimladladlooklike]

Yep, of course, as soon as I get round to it

[jimladladlooklike]

Hi, folks. Here's the schematic for what I've built. Look forward to your comments.

[ElectricDruid]

R7 and R8 (both 10K) on the input to the gyrator EQ section could be combined.

More importantly, how/where is the DC bias for U4A?

For style points, I'd also redraw the R36/RV11/D5/D6/C33 section. Physically, it doesn't matter, but conceptually, it makes more sense if it goes "RC Filter -> Clipping -> Volume control". So R36/C33/D5/D6/RV11.

That's all I've spotted for now.

[antonis]

I presume you mean something like this: 

[ElectricDruid]

I now notice on Antonis' nice close-up that C8 and C12 could be combined. One or the other is unnecessary. I'd remove C12.

Also, what's going on with U4B? There's no input signal to it!

[antonis]

Also, what's going on with U4B? There's no input signal to it!

No signal - no cry..

@jimladladlooklike: Your schematic needs a generous revisement..

[ElectricDruid]

No signal - no cry..

A song Bob Marley never quite got around to writing through lack of electrical engineering background, you think?!

[antonis]

Ganja makes no discrimination between majority and minority carriers..
(if it flows, it flows..)

[jimladladlooklike]

Thanks for your help so far everyone.

Here's a revised version... I'm unsure of how I'm meant to bias U4A to be honest.

[antonis]

I'm unsure of how I'm meant to bias U4A to be honest.

R41 is just fine..

(I'd use lower value bias resistors for R2, R34 & R41 due to input bias current but let them be ..)