Switch to Swap Op-Amps

[Kesh]

I was thinking of making a Tube Screamer that had pretty much all the usual mods as switchable. But I'm not sure how to handle the op-amp switching.

Is it okay just to switch between outputs, perhaps sending the unused op-amp to a dummy load? This would mean the circuit was still driving the unused op omp, but as the input impedance is super high, I'm not sure it matters.

Or do I have to switch all three connections? There is then the issue that unused op amps shouldn't just be left unconnected.

[Mark Hammer]

This is, to my mind, not a great idea. 

First, the role of the op-amp in a Tube Screamer, or rather the role of the particular one you've chosen, has to do with how it interacts with the diodes in its feedback loop.  It may come in a dual op-amp package, but the half used for the tone circuit could be just about anything and have precious little impact, other than maybe noise level or current drain. 

The chips you want to select between only come in dual versions.  So ultimately, what you want is to swap between two entire chips, since you'll have to have both sections of each chip on the board anyway.  Although I suppose you could leave one to handle the tone control and tie the unused pins of the "spare" sections appropriately.  Still, that's 3 pins to switch, plus the chips would all continue to draw current even when you're not using them.

Seriously, stick a socket in, use one chip at a time and leave it at that.  The differences produced by varying the chip will be audible if you pay really close attention, but the audible differences won't be nearly as great as simply changing from a 1+1 to 2+1 diode complement (uses a SPST switch to do), or changing the .047uf cap to something larger (again, a SPST toggle or SPDT on-off-on if you want more variety).

[Kesh]

I don't mind them drawing current from the rails, I'm more concerned whether the unused op amp, if not connected at its output - but still connected at its inputs - will mess with the rest of the circuit. I think it's only through feedback that an op amp output can influence the inputs, otherwise you'll just get saturation. This may cause crossover noise in the other half of the dual op amp.

That's my hunch anyway.

As for the difference, I'm told the only op amp mod that is audible above the other more noticeable mods, is with a mosfet/bjt op amp swapped for the usual 4558s.

[Mark Hammer]

.Generally, you don't want any op-amps to be powered AND "floating" with their inputs not connected to anything.  Which means that on top of switching which chip the external components go to, you'd need to simultaneously make connections to the unused chip section.

I'm telling you, it's just not practical.  Far easier to built twoTS9s, complete with duplicate controls, and switch between them.  Everything stays connected where it should, you get to switch chps, AND  you get presets

[armdnrdy]

Kesh,

Let me tell you a little story about the way I started building stompboxes.

The owner of the boutique amplifier company, Ugly Amps, gave me a TS-5 plastic cased Tubescreamer. He had modded it with a couple of components and changed out the IC to the fabled 4558!!! It sounded good.

Not too long after that I built a very nice pedal board. One day I was looking at this plastic Tubescreamer and the thought of putting that on my totally cool pedal board was just about too much for me to bear.

How can I put this relic, reminiscent of a seventies German vehicle into a cooler looking enclosure more befitting of my pedal board?

I took to the internet!!! (By the way, that's when I found this place) Once on the internet, I saw that you could do all sorts of things to these gizmos.

That's when I thought.....wouldn't it be nice to have a footswitch that would allow you to switch in between two different drive control settings? I soon learned that it had already been done. Can you say Fulltone? I wasn't very stompbox savvy. (I have been a singer since 1977)  Anyway, I plodded along, rehoused the Tube-N-Stein and added a switched drive pot. Now I can switch between two different drive settings! All is great! Or is it?

I started thinking; wouldn't it be nice to switch between different tones so that when you play a solo it will cut through a little better? In all of the time I spent in the studio watching various guitar players getting their sound and putting down their tracks, I never once witnessed a solo and a rhythm track played with the same tone.

After much thought...I had the ground breaking idea of combining two Tubescreamers in one box! Okay, so that had been done already too!!! I told you...I was a singer!!

Anyway, since then, I have combined many different combinations of overdrive pedals in one box. It saves real estate on your pedal board and they just look way cooler than a plastic TS-5!

So the moral of my story is...build a dual overdrive unit! that way instead of just switching between something that's only going to subtly change the sound, you can make something that's versatile.  

[armdnrdy]

Hey Mark just posted as I was writing my novel. Yes, as you can tell from my short story, I agree!

[Kesh]

Obviously it's my duty to ignore naysayers and plough on anyway.

Here's what I think will work. The big triangle just shows where this would fit in the original circuit.

[Seljer]

you could probably leave both + inputs connected, and have the opamps seperate (i.e. not sharing the same circuitry in the feedback loop), then only switch between the outputs

[Kesh]

That was another thought, but as I will also have lots of switching of diodes etc, I wanted them to share the same feedback loop so such switching would apply to both op-amp options.

A 6P2T would work, and both op-amps, used or not, would be tied to something to keep them sweet. But 6PDT is a weird component so I'm just hoping to use a simpler switch.

[rockhorst]

Hey Kesh, I'm working on basically the same problem for the same purpose (using MadBean's Green Bean as the basic board). Not too long ago Pakrat posted his TS with switchable opamp in the pictures thread. He switched the outputs, seemed to work. So I'm going to try the same thing, but with 4 to 6 opamps. I don't want to use stacked opamps, but I've heard that they don't like a lot of wiring. So I'm going to design a child PCB that holds the opamps and clicks into the opamps socket on the MadBean board, figured out by now how to make that on a single sided PCB without any jumpers. I've found this pedal by Farndurk, doing something similar:

He doesn't seem to have any trouble about adding some wiring to an opamp circuit.

[Jazznoise]

I was gonna suggest switching the outputs! Just connect it to a 10K resistor going to ground when it's not used and you should be fine.

That said, I think the tonal difference will be in the category of "Subtle as f**k". Certainly not relevant during a live show, and swamped by other variables during recording.

[Kesh]

RG has suggested changing the op-amp has the single biggest effect on the sound.

http://www.geofex.com/article_folders/tstech/tsxtech.htm

[danielzink]

I do not remember where I got this from - it's been on my HD forever. Not mine - just posting.

Stacking OpAmps in a TubeScreamer


I really really wanted to be able to toggle opamps to increase my tonal ranges. At first I wanted 3 opamps, but now that I've tested and HEARD the differences between 3 (two at a time), I found I am quite happy with just two selections for actual live playing. The RC4558 gives me that traditional piercing tube screamer, and another chip (burr brown or tlc2272acp) give me another very different tone....my preference is the burr brown. For most playing, for my style, most of the time I'll be on the burr brown with the fat boost engaged.

DISCLAIMER: This works on an OVERDRIVE that uses dual opamps such as the JRC4558D, RC4558P, OPA2134PA, TLC2272ACP....the pinouts for other amps and other circuits might be different. But here is how to do it:

I wanted the opamp switch in the first version I gave my friend but failed at how to do it. Over at Beavis he shows how to stack opamps but says it didn't work, and postings in the forums here weren't helpful. Finally a week or two ago Keith posted that He accomplished it with multiple opamps and a rotary switch (in the past) for whatever circuit he was working on, and that the key was pins 1 and 7 on the opamp.

I did not want to "stack" (solder) them together, I wanted to socket them so I could swap and have flexibility. So, I chose two at a time. They key is still pins 1 and 7 on a switch, and tie the rest of the pins together. I used a piece of stripboard. The solder flowing was much easier for me than perf (and there is allot of tedious soldering).

1) Socket the opamp on the pcb so you can stick wires into the socket

2) Get stripboard and solder two sockets, strips horizontal, sockets verticle. Cut the traces down the middle between the two sides of the sockets.

3) On the stripboard, tie pin 2 to pin 2, 3 to 3, 4 to 4, 5 to 5, 6 to 6, 8 to 8.

4) On the stripboard, run both pin 1 to a DPDT toggle top and bottom left side

5) On the stripboard, run both pin 7 to the DPDT toggle top and bottom right side

6) Run the center lug left side of DPDT to pin 1 in the socket on the PCB

7) Run the center lug right side of DPDT to pin 7 in the socket on the PCB

Run the tied pins from your strip board to each corresponding pin in the socket on the PCB.

I don't have software picture skills like Stephen, so I hand drew this for you guys:

The concept is brilliant...you toggle the input and output and the rest of the pins all receive the same signal from the PCB, but only one truly receives input to "process" and output to "send".

I was confused at first (pre-Keith help) because half of the opamp is dealing with the tone circuitry (if memory serves), but he said all I had to worry about was just the two pins. I suspect that what happens is both chips actually have stuff going on in the other pins, but since only one inputs/outputs at a time it doesn't impact the circuit (or...doesn't have signal to process at all).

Hope this is useful to someone.

Dan

[armdnrdy]

Basically RG is saying that the 4558 and others work well in this circuit and that some later Tubescreamers that were made with the 72558A op amp sound abysmal.

He also went on to say;

"The best thing to do, I believe, if you're hunting the Tube Screamer Holy Grail is to stick in an 8 pin socket on the board, noting the orientation of the original chip carefully, and then just plug in various dual op amps until you find one that conjures up images of your own personal guitar deity."

I think that it would be a good idea to swap op amps in a TS, listen to the difference and then decide if what you hear warrants trying to switch between op amps.

I'm all for trying something different but, I've already been through the whole making the "holy grail" of the overdrive pedals thing! I know from experience that some op amps sound better than others in this circuit, but not different enough to switch between them.

Mark and I suggested the dual overdrive setup because it works! You can vary the drive, tone, and volume of the two separate circuits. You can build two entirely different overdrive circuits and switch between them! This is worth doing! And is very usable!

As Mark, Jazznoise, and I have been trying to tell you, switching between two op amps would be extremely subtle!

Good luck with it.

[Kesh]

But (do I really need to repeat this?) I want a switch, not something I have to open up on my workbench.

And he did say it made a difference, whatever else was said about how you might swap chips. I want to A/B Bi-Mos with 4558s maybe with some others.

But thanks to those that actually contributed with something useful.

[Mark Hammer]

It makes a difference if nothing else is changed.  Or in comparison to something like changing one of the .22uf caps from tantalum to something else.  But you,ll get a more audible change from adding one more diode than from an op-amp change.

[rockhorst]

Building two separate drives from the same basic circuit design: cool DIY project
Figuring out if you can switch op amps in a Screamer Lab kinda way: cool DIY project
Doing something in between the two of those: cool DIY project
Buying two stock TS9s and modding them: cool DIY project...

Kesh, there's at least three good reasons to do this:
-switching opamps in sockets takes time, enough time to forget the details about the previous opamp...with a flick of a switch, you can instantaneously hear and judge for yourself if it's a big difference. Have a looper play the exact same thing twice and play around.
-you'll learn something new
-you'll get rid of that voice in the back of your head saying 'if only...'

So go for it and share!

[Jazznoise]

http://www.youtube.com/watch?v=bpTv2jAree8

'nuff said.

[armdnrdy]

Good find Jazznoise!

Kesh, your comment "But thanks to those that actually contributed with something useful." is a bit off base. You had several people on here that were trying to help you.

Just imagine if someone were to ask you for directions to a road to get to the other side of the river. Imagine that you recently traveled that road and saw that the bridge was washed out. Would you help by offering up your knowledge? Or would you let them drive to the end of the road to find out themselves?

We were offering advice from experience so you wouldn't be disappointed after you wasted your time!
I guess some people just have to find out for themselves!

Have a nice drive!

[rockhorst]

@jazznoise:
Good video, and it's a well known statement by RG that any and all 4558 chips sound basically the same in a TS, and indeed chip switching isn't earth shaking. But there is difference. It is a good video, informative certainly, but there's a few problems with it, mainly that the guy is just bashing along on a G chord. No subtle lines, no attention to note decay.