Switch to Swap Op-Amps

[Jazznoise]

I agree that the video is not exactly good science. But then very little in the audio realm is!

I never denied a difference, simply said it's subtle. And in the world of fuzzboxes, the difference is probably as subtle as they get!

[pakrat]

@Kesh I did build a tube screamer with 2 opamps in the stack format just like the screamerlab on the beavisaudio site. It does work, although I wish now that I made a seperate board for the opamps instead of actually stacking them. There is definitely a difference between certain opamps to my ears. I agree that you should try it and judge for yourself.

[rockhorst]

When I get it done, I'll share my board layout for the switching. It would only be switching the outputs (like Pakrat did) but can contain more than just the two opamp design that was previously posted.

[The Tone God]

The Acute Modestscream we make is a TS that has four switchable opamps. The outputs are the only thing switched. The power and inputs are wired in parallel. As long as you don't use some odd ball opamp with weird loading characteristics on the inputs it should be fine to wire the opamps this way. I don't know how much popping there will be if you switch while powered on in your design.

As to the effect the reviewer at Premier Guitar seem to think it had an impact on the sound:

http://www.premierguitar.com/Magazine/Issue/2012/Mar/The_Tone_God_Acute_ModestScream_Pedal_Review.aspx

but if you want to compare check out our video where we switch the opamps on the fly around 3:10:

Andrew

[rockhorst]

@Tonegod: I stumbled upon that thing a while back, quite the ultimate TS is there ever was one...Amazing!

By the way, I was wondering if it would make a difference if you use a TS input buffer before the opamps or not...I would guess that it minimizes loading effects. Usually I leave it out and build a more Son of Screamer type layout, but I think I'll stick it in for this.

[The Tone God]

@Tonegod: I stumbled upon that thing a while back, quite the ultimate TS is there ever was one...Amazing!

By the way, I was wondering if it would make a difference if you use a TS input buffer before the opamps or not...I would guess that it minimizes loading effects. Usually I leave it out and build a more Son of Screamer type layout, but I think I'll stick it in for this.

Thanks!

I would use a buffer if you plan on using the "FAT" mod that connects the diode stage's gain directly to the bias voltage without the cap. If you connect to the bias voltage with an unbuffered input the output will sound weird and miss biased. If you are using a cap on the diode stage gain then you can get away without the buffer stage. Thats been my experience anyways. We do have a buffer on the input of the Acute Modestscream.

Andrew

[brett]

Hi
I'm no expert, I suggest you don't take my comments too seriously. Have your own fun.

First, I don't see why you'd do all of that electonics stuff of having two op-amps and then putting a clumsy old-fashioned mechanical switch in there. Why not switch some CMOS or JFETs between conduction and non-conducting. Shunt-to-ground is fine, but maybe add 1k of (extra) resistance to reduce battery drain.

Second, if you put the best op-amp in, why do you want a less good one too? Of the common ones the NE5532 is the quietest, the TL072 is ok for noise. Old 4558s sound like waterfalls, but newer ones are quieter (that's right kids, the old ones are way different to the new ones. Which have got the mojo? Ask someone who cares.

My blind tests indicate that the only audible differences are in noise. Which means the NE5532 wins. But if you live in the city, you might not even notice hear the hiss of a noisey old piece of junk like a 4558 anyway. (not sure, but I believe that they might have started out way back when as a pair of 741s stuck together. Hsssssss)
just my 2c..

[brett]

Hi again
I was just thinking (dangerous) - the feedback cap in a tubescreamer has a big effect on the sound, especially when the gain is turned up.
At 350k in the feedback loop a 51pF cap has a roll-off point of 8.9 KHz.
If you use the more common 47pF cap (and assuming it's a true value), the roll-off is at 9.7 kHz.

But caps vary all over the place. A 47pF cap might be 60% greater than the label value (they are commonly rated +80/-20 %). That makes it a 75pF cap, with a roll-off point of 6.3 kHz. That would have a lot less 'buzz' and 'attack' than a 'real' 47 pF cap.

So unseen variation in the feedback capacitor (value alone - excluding type etc) changes the tone a lot between tubescreamers. You might want to rig something where the same cap is used for all of the op-amps. Otherwise you are comparing caps, resistors, and a whole bunch of stuff that is probably much more important than the chip. Years ago I built a tubescreamer with a 220pF cap and it gave an interesting mellow/deadish tone, so I can recommend switching between caps (and probably feedback and bias resistors) for fun.
cheers

[cys]

Apologies in advance for posting to a really ancient thread...

I recently successfully added a rotary switch that selects between three different op-amps to a GGG Tube Screamer clone. The op-amp switch is an 8 pole, 3 throw switch that I picked up on ebay for less than $10. The op-amps sit on veroboard, and there are lots of wires... It's a brute force solution, but it works.

[moweryelectronics]

Would this work?
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[slacker]

Fixed the link for you.



No that won't work, you can't connect the outputs of the opamps together, the easiest thing is to leave the inputs connected together and switch between the outputs.

[MrStab]

if people insist on using this approach, as with anything that would otherwise involve 20 million wires in a high-gain circuit, then i would really recommend FET switching instead. more complicated? maybe, but you only need a coupla signal-free control wires hanging off the board.

with your typical overdrive, you'll probably get away with passive switching. i just think you can clear up a lot of spaghetti and minimise oscillation & popping risk by going active.

[Gus]

The only time I "think" I might hear a different with different opamps(beside noise as Brett posted about) in this type circuit is with tone set to max treble and the gain at max
Anyway as soon as it is in a mix or live in a band setting I don't think you can hear anything change with the IC

The high pass and tone EQ setup seems to matter more

[amptramp]

Get yourself an old turret tuner from a television from the 1950's to the end of the tube era:



The tuner has an array of coil pieces on removable sections that plug into the turret.  Replace the coils with op amps and you can switch between them breaking all connections with the ones not in use.  A 5-pole rotary switch may do the same thing - switch Vcc, Vss, -in, +in and output all at once and know that the unselected op amps are not powered when another op amp is running.