OpAmps for DIY Bass & Guitar Pedals

[Hink]

Dean

[khanyz]

@Dean - I ordered some samples, thanks. They shipped faster than most orders I place.

@insonicbloom - Thanks for the post, vey informative. However, there is a difference in OpAmps and in Capacitors. In terms of manufacturing tolerances and materials used alone, the final products will differ in how they work. As part of my degree (Physics, specialising in Condensed Matter/Solid State & Electronics), I studied the design and manufacture of ICs and other Components. Referring to Sprague or Burr Brown, I am specifing the type and quality of the components used, whether they are actually made by Vishay, TI or they are OEM. In the case of ICs, the capacitance limitations does lead to some similarity in design but the quality and tolerance of manufacture is absolutlely key to the specs of the finished product. Whether the human ear can hear all this is, of course, subjective but for me, I can.

The difference between Silicon and Germanium is more marked because of the voltage drop, mainly. The models used for then usually only differ by that, but the chracteristic semiconductor potential wells are diffent, it's just the maths gets sticky really quickly.

As for discrete, I've designed quite a few amps, but not really for audio use. I've just tried one, and it seems to work OK but will need some tweaking as it's designed for higher frequency use and I just did a blanket reduction to bring it's response down. Whilst Capacitance and Inductance are the main key frequency components, the tolerance of the resistors and the switching speed of components are also factors. Whilst the switching speed is usually well outside the frequency range of human hearing, cumulative effects can creep in, but they can usually be ignored. Resistor tolerance is a more noticable effect.

Anyway, thanks for all the info and is the cable thing about crystal and magnetic alignment at lower temps? Sounds feasible but the environment needs to be pretty specific.

Thanks for the advice,
Nigel

[insonicbloom]

haha no I was being sarcastic about the frozen cables,watch the Aussie video on that.

but as I said with capacitors it is the value that matters not the material, the materials used are for stability issues and not how they colour the sound, capacitors were never made specifically for sound.
you talk about the IC's being subjective but my point is this, if something takes a "trained ear" to spot then what is the point in having it?
I really do think it is a placebo effect because when you're talking about something as primitive as a overdrive pedal all it is really doing is clipping the signal and pushing it through a filter.

anyway I'm going to do a video on this swapping out op-amp business in a controlled environment. I have designed a 10v precision voltage/current source so there are no fluctuations there and I am going to gut the TS9 that Dean mistakenly left at my house. I shall do controlled frequency sweeps and measure the signal on a high bandwidth scope and then for completeness I'll play guitar through it, which will be pre-recorded clean so as there will be no variation in playing velocity etc.

[khanyz]

^
Oh, I see. I'm not just using OpAmps for overdrive/distortion but for clean boosts and tones. This is also for Bass and Acoustic. Things like noise etc. are noticeable. I understand that is your just going to drive the hell out of it, there's less need to be picky.

As for the trained ear, I don't know if mine's trained but it's pretty good. Since I'm making stuff for me, and I can tell the difference then that's the point. It's my standard I'm judging by, and that's why the personality behind the effect is important. Even if I did start making stuff for commercial production, I would still use the same standard and not assume an inferior one for an end user. It's probably why it would fail.

As for capacitors, as with all other components, tolerance is something you need to factor in, and compensate for. The material used for a component is key in the tolerance spec. Also the charging/discharging characteristics of different materials aren't the same, that's why the frequency characteristics aren't the same. Maybe for the limited range of audio this is minimal but I would never assume that it's negligible or can be disregarded. I've done far too many studies on noise for that.

It's quite nice having a reasonable discussion, with different views on KVR. I've never seen it before .

Cheers,
Nigel

[xtp]

If you are buying & trying here's 2 opamps I came across while doing some research on one of my projects. I have never used them , they are very low noise & high Gbw but expensive as far as opamps go.

LT1115 & AD797.

...discrete might actually be cheaper than these little suckers.

[khanyz]

^

Thanks, I have an LT1115 on order and I'll check out the AD797. I haven't been impressed by the Linear ones so far, but the Analogs are nice.

Cheers,
Nigel

[jonnyG]

It's been a long time since I was deeply into building pedals but I remember that the TL07x opamps that I used pretty exclusively were prone under certain circumstances to phase reversal. This caused distortion that was harmonically unrelated to the input tone. I remember the first time I heard it when I was testing some high gain distortion topologies - I hooked up the oscilloscope and what I saw looked like some very random PWM!

That was about 20 years ago. Recently I got bitten by the hardware bug again and decided to do some research on the old problem - and bingo! Good old internet...

http://www.eetimes.com/electronics-news ... rsal-story

Any fule kno that one opamp should sound exactly like another under normal circumstances, but they might find themselves being subject to circumstances that are anything but normal - like exceeding common mode voltage, rail to rail clipping and whatnot.

The moral of the story for me is to use your ears and your test gear! Read the manufacturer's data sheet too. Oh, and try to avoid subjective gobshite like "darker" or "more open". Leave that to the audiophiles as it just perpetuates the old "it's an art, not a science" dogma.

Anyway Nigel, I'm 20 years out of date so I'm a bit behind re. the latest opamps, but some of my considerations may be (in no particular order):

1) low noise,

2) low power consumption (for battery powered pedals),

3) stability not dependent on physical layout (for stripboard layouts etc),

4) low offset current and high input impedance (for high gain, high impedance circuits)

5) rail to rail output swing (dependent on application a bit more overhead may be welcome).

Use chip sockets then you can try different opamps. Use, abuse, listen and view the results via a scope and software spectrum analyser (I use Voxengo Span).

Hope this rambling helps a bit!

[khanyz]

^
Thanks for the advice. I know about the phase issue with OpAmps as I've used some uncompensated ones, but the phase reversal thing is new to me.

Cheers,
Nigel

[jonnyG]

Oh, and +1 on the diy stompboxes forum - it's a treasure trove of circuit ideas, useful links and all. Plenty of very clever folks there too.

I guess that what I was trying to say in my previous post (albeit rather poorly) is that there's no universal opamp. An LM833 is a very high quality item that is very useful for eg driving a low impedance load, but it might not be optimal for another application where say a fet opamp (or simply a cheaper one) would be more appropriate.

One other tip I'd like to share is to check out as many schematics related to your application as you can. Try to get into the designer's head a little. If a commercial design uses an apparently over specced (and therefore more costly) device then ask yourself why. Perhaps the designer had to overcome a problem that is not immediately apparent.

[Hink]

this has been a great thread, thanx everyone...I dont want to go sticky crazy but there is so much info here I might make this sticky...what do you folks think?

[Dean Aka Nekro]

Well considering that if anyone decides to get into build thier own or modding what they already have, Then just having one clear thread regarding this topic would to me make sense instead of the possiblitiy of future splinter threads where someone would most likely link back to this one John, Nigel, xtp, insonicbloom, Eric and johhnyG. So this one can grow as and when

So my £0.02p FWIW is yes

Nice one and all the best to all

Dean

[Uncle E]

^Oh, I see. I'm not just using OpAmps for overdrive/distortion but for clean boosts and tones. This is also for Bass and Acoustic. Things like noise etc. are noticeable. I understand that is your just going to drive the hell out of it, there's less need to be picky.

The impact of the op amp is quite evident in the TS9 and OCD, too.

As for the trained ear, I don't know if mine's trained but it's pretty good.

The common listener won't hear the difference between a mid-90's TS9 and a Fulldrive but the distinction between the two should be clear to any good guitarist.

[Uncle E]

AD797

If it helps at all, the Mad Professor Mighty Red Distortion uses it and sounds fantastic.

[khanyz]

A sticky seems like a sensible idea, with the content level, as other forums seems to have too many OpAmp threads.

A quick question of sourcing though. Has anyone used www.futurlec.com ? They seems to have the best prices on low quantities of AD797s (as they are not available as samples).

In the UK, a great source is www.doctortweek.co.uk .

As for my OpAmp choice, of the ones I've tried so far, the OP27GPZ has been very good. I would recommend trying it in boost and mild overdrive (blues) pedals.

Cheers,
Nigel

[Hink]

okay Nigel, you've started this thread and contributed so much as has my new friend insonicbloom I think you're right...we should not let such good info slip away