Intersound IVP layout

[jimladladlooklike]

So I just breadboarded a version of the schematic I posted, except I didn't have an LM741 handy, so opted for an LM386 instead.

It's powered with +9v DC. (The LM386 doesn't can't handle more than 18V, so will have to use a suitable replacement eventually.)

I used two 560r resistors for the voltage divider, hooked up to the two outer pins of a trim pot, the centre pin of which is connected to the non-inverting input of the op amp.

I replaced the transistors with a 2N5088 and a 2N5087.

After measuring, the reading I get are: C1 + terminal = +9v, "ground" terminal = +4.5v and C2 - terminal = 0V. So basically what I have is a voltage divider... I then disconnected the two above capacitor legs and measured there, having noticed that they were connected to +9V and ground, but I don't know what that achieved as the voltages were all over the place. Is there something I'm missing? are the parts I used not the right kind?

EDIT: I was measuring wrong. I was taking the ground reading from the bottom left point of the schematic, not the junction of the two caps.

It works fine! Now I just need to test with an LM741 when I can get some. Thanks for all of your help on this guys, I feel like I'm learning a lot and it's much appreciated.

[iainpunk]

The LM386 isn't a (normal) opamp, its a power amp, with some odd quirks when it comes to biasing, to make it work with less external parts. try any other real opamp, that would probably work better.
if you want to simplify and use the 386, you can leave out the transistors, and put both inputs to ground, this provides a strong, stable Vbias

cheers

[jimladladlooklike]

Oh okay, thanks for the pointer. I was planning on using an LM741 because of it's max input voltage anyway, just wanted to make something similar with what I had to hand. Note to self, always read the datasheet.

[iainpunk]

Note to self, always read the datasheet.

that shouldn't be just a note, that should be your RULE #4, the other rules will come to you along the way.

cheers

[aion]

If you really want it, it's possible to maintain the exact same behavior of the 2MC pots using 1MC pots.
All you have to do is halve *all* the resistances in the EQ section and double the caps.   The idea is called scaling.
Technically you should change the boost/cut pots to 25k as well.

A couple months late here, but does the scaling also apply to the 20k resistor on the op-amp that comes afterward, and/or the 10k resistors that come before? Or is it just starting with the 50k pots (reduced to 25k) and everything beneath them?

[iainpunk]

If you really want it, it's possible to maintain the exact same behavior of the 2MC pots using 1MC pots.
All you have to do is halve *all* the resistances in the EQ section and double the caps.   The idea is called scaling.
Technically you should change the boost/cut pots to 25k as well.

A couple months late here, but does the scaling also apply to the 20k resistor on the op-amp that comes afterward, and/or the 10k resistors that come before? Or is it just starting with the 50k pots (reduced to 25k) and everything beneath them?

this only aplies to the Gyrator circuits themselves, the 50k pots and the 20k resistances stay the same.
you can just use the Gyrator Calculator to calculate the Q and freq. of the Gyrator.

by the way, note how the circuit resembles a Tube Screamer tone control, but the Capacitor-Resistor combo is replaced with gyrators.

cheers

[jimladladlooklike]

Hey again guys.

I've finally gotten around to assembling a rough and ready version of this thing. As you can see it's sort of chopped up into sections. I'm hoping this will make the inevitable de-bugging process easier...



I do have another question though: Should I ground the input jacks via the ground lug on the DC input, or should I tap it from somewhere in the circuit post converting the +32VDC into +/-16VDC? Does it matter? The same question applies to the two preamp gain pots.

Thanks

[jimladladlooklike]

Hi guys,

So I realised I didn't upload the layout I've been working from so here it is:



I'm running it at +9v from my normal power supply and have swapped out the LF356 ICs in the preamp section with 2 x TL071 to compensate for this. Did this just for testings sake until I get my hands on a 32V adapter.

However, I'm having a few issues. there's quite a large voltage drop across the power supply decoupling resistors. It goes from approx +/-4.5V to +/-3V. I think for this reason there is no sound. When I jumper across both resistors with crocodile clips I get some sound when plugged into the low impedance input but nothing when plugged into the high impedance one. Along with this, when plugged into the latter and plugged into neither there is a high pitched sound (oscillation???). It's particularly loud on the output pin (#6) of the top preamp IC on the layout.

At the moment I have connected one jack to the ground lug on the DC socket and so all jacks are connected to that via the chassis, but I'm confused about this. In the schematic there is only one symbol for ground or 0V. obviously a bunch of components are connected to 0V in the circuit, including the input and output jacks. However, when I connected the jacks to 0V, there was 4.5V on the chassis in relation to the DC socket ground lug.

How do I connect the jacks and 0V in the circuit without having this 4.5V floating on the chassis?

[jimladladlooklike]

Okay so I fixed this problem with a simple solution: plastic jacks.

Now I'm still having trouble powering the circuit. I built a charge pump that creates 33V and shows approximately that voltage when measured separate from the effect. However, when it's used to power said load the voltage at the same point in the charge pump circuit drops to around 7V. Is this due to the load current? Like, how much current the effect part of the circuit is drawing? I'm assuming so. When I connected my multimeter in line with the effect and charge pump it drew 150mA of current. Datasheet says max supply current is 160mA so I though this would work.

Basically, I've clearly bitten off more than I can chew here!

[mdcmdcmdc]

I'm sure it's far beyond the point of being at all useful, but if you need to reference anything against an original IVP (voltages etc) I have one here.

[iainpunk]

the max current of a SMPS / boost converter is what it can deliver before going dead, not what it can deliver at full power.
there are dozens of relatively cheap 30v power supplies on the market, you might want to add some filter caps to those, maybe even a regulator chip, but most work quite alright in my experience.

i honestly doubt that you need that much headroom when working with a guitar signal, i bet this circuit would work on + and - 6v as well.

cheers

[jimladladlooklike]

I'm sure it's far beyond the point of being at all useful, but if you need to reference anything against an original IVP (voltages etc) I have one here.

That could come in handy, if you have them. Thanks.

[jimladladlooklike]

So, I think I know where the problem lies. The two preamp chips (tl071) are working fine when I use my 9v wall watt. However the signal cuts out at the junction between the two 20k resistors and pin 2 of the first rc4558. Same thing happens with a tl072 in its place. However, when the chip is removed I get sound at this point. Looking at the schematic shows that pins 3 and 5 are grounded. Is this something that is normal for non-inverting inputs? I've connected them to 0v on the power section of the circuit as pins 8 + 4 are connected to V+ and V-. Have checked and redone some of the solder connections in case of shorts etc. I'm just v confused at this point.

[iainpunk]

Looking at the schematic shows that pins 3 and 5 are grounded. Is this something that is normal for non-inverting inputs?

yes, very normal, mundane even.
this is an inverting opamp stage, which means that the inverting pin should be silent too, but the output should have signal again.
what an opamp does is trying to match the voltage at both pins via the negative feedback loop, if the (+) pin is 0v, the opamp will try and keep the (-) pin at 0v as well, keep this in mind.

if we have an inverting opamp like this:

where both resistors are the same, and we apply 1v to the input, there will be a current flowing through the resistor Rin. to keep the voltage of the (-) pin at 0v the opamp has to take as much current out of that node as the input puts in, to do this it has to generate -1v at the output/the other side of Rf. changing Rf or Rin changes the voltage the opamp needs to generate to keep the (-) input at 0v, this is how the gain is set.
also keep in mind that the inputs of the OpAmp have near-infinite resistance (for all intents and purposes)

cheers

[jimladladlooklike]

Thanks for explaining that to me, much appreciated. I've obviously seen an inverting op amp stage before but I guess I'm second guessing everything at this point!

I have made some corrections to the layout: changed position of wires connected to the 1k2 - 9k6 filter pot. Also made the connection from pin 4 of the 4th 4558 to -16v which wasn't there before.



It seems I've getting signal passing through the circuit now, which is better, but it's strength diminishes at each op amp stage, then at the output it's super weak and fizzy sounding. I'm going to wait and put this to one side until I get paid and can buy a 30v adapter.

Thanks again for all of the help

[jimladladlooklike]

So an update with this project:
- Bought an adapter with 32VDC output, realised it was centre positive so swapped wires round on DC jack.
- Found a couple of not-quite-cut traces after realising one of the op amps was getting a bit hot and sorted them out accordingly.

Now the circuit is starting to sound like its working, sort of. Signal is passing through and all the controls seem to work. There's one issue; a loud hum, about as loud as the guitar signal. Could this be a DC filtering issue?

When I take a measurement of the DC at the + and - inputs of each of the first two op amps relative to the power supply 0V I get 0V, but when I measure the same two pins of the same two op amps relative to the ground lug on the DC input jack I get around 14V, is this the problem?

[antonis]

A rough schematic should help us to understand the difference between ground lug and PS 0V point..

[PRR]

.....relative to the ground lug on the DC input jack I get around 14V, is this the problem?

Why are you saying "ground lug"?

[jimladladlooklike]

Okay, so, a further update. But firstly, thanks again for all of your help folks, I would have lost all hope without your guidance!

I abandoned this for a day or two and when I revisited I decided to try with my 9V Voodoo Labs power supply and wadayaknow? The hum went away. So now I have an almost functioning preamp which works running off of 9V. The EQ section is so fun and versatile, albeit with the wrong taper pots which I can live with. That into the Tube voice section does exactly what I want. (clang clang).

.....relative to the ground lug on the DC input jack I get around 14V, is this the problem?

Why are you saying "ground lug"?

With regards to your question: I mean the ground connection on the DC input jack

A rough schematic should help us to understand the difference between ground lug and PS 0V point..

I'm planning on redoing the schematic once I work out all of the kinks (there's PS filtering caps which I've overlooked and I think there may be an issue with the PS - input pre decoupling network.) but it turns out these parts of the circuit function as I had them connected originally.

Now there are two main issues (besides correcting which wires go to which solder lug on a pot or two) -

It's quiet and, whilst it will most likely be an always on kind of thing, I'd like to have some more volume on tap to drive whatever amp I'm using if necessary. Might this be solved with a higher voltage? I tried a charge pump (tested when disconnected and worked fine) but the output voltage dropped to below 9V when hooked up to the circuit (load??) I can always try a new wall wart I suppose.

When I turn the "lo level" knob up past about 8 o'clock and I hold a note or chord and let it naturally fade out, the notes will waver and falter in and out. They sort of warble.

Cheers again

Jim

[jimladladlooklike]

Hey guys,

I tried the preamp with an 18VDC, 500mA power supply (centre negative this time) and I got this repeated beeping... from the actual power supply plug! Quite alarming

Someone on reddit told me I need to "dereference" the power supply to get proper 0V ground? I've made a rough schem to show how things are grounded.

Cheers