OpAmps for DIY Bass & Guitar Pedals

...and how to do so...
Jim Y
KVRist
402 posts since 29 Jun, 2008 from Mid Wales, UK.

Post Mon Nov 11, 2013 3:07 am

Special low-noise for mic pre-amps maybe, but the rest?
Anything.
I don't see the need for fast amps in audio unless you really want them to pick up mobile phones (a source of interference that wasn't around when many classic circuits were designed).
For instruments - is a little noise a bad thing? - the classic tracks from the past must contain a lot of noise. I have a half-brained theory that a little noise makes a bed for the tone to sit on.

One of my favorite guitar sounds belongs to one Wilko Johnson (of Dr Feelgood) - a Telecaster straight into a H|H IC100 combo amp. This amp exclusively used uA741 chips.
Of course, these 741's were the original 14pin DIL packaged ones, and we all know they sound both warmer and brighter than the 8pin DIL or even the metal can ones.
;)

User avatar
khanyz
KVRAF
1556 posts since 16 Jul, 2004 from Deepest Yorkshire

Post Mon Nov 11, 2013 4:16 am

^ IMHO you are kind of right in that noise is an important component in Music but unless you're the J&MC, you need to control/tune it to your needs.

You can take a noisy circuit, filter out parts and make a great sound. This can also lead to unexpected results, both good and bad.

However, you can also take a very clean circuit, add noise/distortion (phase and amplitude) by adding components (such as diodes, capacitors etc.) and also get a great sound, but usually with some added flexibility. For this you need to start with a clean/linear circuit and this requires components which can operate well outside of the audio range, to provide headroom.

As far as interference goes, you always need to shield the circuits from external sources, but also to shield external sources from the cicuit. Distortion circuits, in particular, can operate at frequencies well above audio. This is done with a metal case (Faraday Cage) or similar, which is connected to ground or another potential reservior.

Confucios says there are many roads to the same destination, some have been trodden before, some are dirty but all are rock.

Cheers.
I miss MindPrint. My TRIO needs a big brother.

Jim Y
KVRist
402 posts since 29 Jun, 2008 from Mid Wales, UK.

Post Wed Nov 13, 2013 7:56 am

Well of course you can screen your build - but you can't always control whatever else is going to be connected. I've seen schematics of modern commercial designs with ferrite beads and suppression modules at every connector. The cell-phone can find it's way in anywhere. That said, your circuit doesn't have to be fast to demod any RF that get's in.
On top of that, with high speed parts you run the risk of parasitic oscillations which you won't see without a very high speed scope. This issue alone stops me wanting to to use fast slew-rate or high GBW parts. I usually only do instrument electronics myself and there really isn't much over 8Khz to consider.

chroma
KVRian
793 posts since 10 Jan, 2010

Post Wed Nov 13, 2013 8:08 am

can you still get 2604s? i thought they were out of production.

and... the tle2072 is another opamp that sounds better than an 072, is about half the price of the 2604/2227/etc. and is usually safe to replace in tl072 circuits... worth a shot at least...

Jim Y
KVRist
402 posts since 29 Jun, 2008 from Mid Wales, UK.

Re: OpAmps for DIY Bass & Guitar Pedals

Post Fri Jan 10, 2014 11:46 am

I return to this thread since reading up on the subject of fast op-amps in audio.
Seems that a fast amp can actually improve RF suppression. What happens is that at the top end where gain runs out then the output impedance of the amp goes sky-high. This lets RF in via the output, then the feedback network back to the input, getting demodulated by any non-linearity and output as audible noise.

I have some Event monitors where the chip-amps do exactly this if a cell phone is anywhere near. (Must get some ferrites for the speaker wires which is the cure for this apparently)
Source is here...
http://sound.westhost.com/highspeed.htm#a1
An interesting site. Don't go near it if you believe green caps sound better than orange ones (or vice versa). ;)

tannoyman
KVRer
2 posts since 24 May, 2014

Re: OpAmps for DIY Bass & Guitar Pedals

Post Sat May 24, 2014 10:37 am

How about soldering in 8 pin IC sockets to accept a variety of op-amp models, so you can experiment with different ones in circuit without the hassle of de-soldering each time?

pilgrim_heart
KVRist
184 posts since 20 Jul, 2004

Re: OpAmps for DIY Bass & Guitar Pedals

Post Fri Apr 17, 2015 6:41 am

Note too that speed pretty much always have to be traded for power consumption so if you could actually redesign the OP you'd probably want to reduce the speed to whatever you actually need. Of course that's not possible when buying them ready-made but it makes sense to not go for more GBW than needed. You can make a quick calculation of this by simply dividing the GBW by the max gain (linear, so if you need dB values just round up to the nearest multiple of 20dB, then 20dB->10x, 40dB->100x etc.) you want to get from the OP in question, the result is the bandwidth of the finished amplifier circuit. Video OPs etc. shouldn't seem very appealing when doing this unless you want massive amounts of gain.

A quick guess about stuff like tube screamers where one upsets the feedback loop all the time is that slew rate might make a noticeable difference. When an OP loop is upset it often results in slewing, if it does in this case the speed at which it happens might affect how harmonics are produced etc. Could possibly be at least part of the explanation for these differences in sound when using different OPs (assuming that they are not placebo).

PurpleCatfishBettie
KVRAF
3273 posts since 22 Jul, 2009

Re: OpAmps for DIY Bass & Guitar Pedals

Post Sun Mar 05, 2017 1:24 am

on some opamp circuits, is it a given that +- are bridged with a capacitor and resistor? i saw a short video about building a 'fuzz face', and one of the last things the guy went over was putting in a cap and resistor (or was it 2 caps?) bridging +- on the breadboard. turns out it were 1 electrolytic cap, and 1 'regular' cap:

https://www.youtube.com/watch?v=Vwsoe8RUJmo

a number of mistakes in this quick video (the narration calls it out), but that part near the end with the caps bridging -+ had me wondering about circuits with opamps in general...

PurpleCatfishBettie
KVRAF
3273 posts since 22 Jul, 2009

Re: OpAmps for DIY Bass & Guitar Pedals

Post Mon Mar 06, 2017 3:24 am

this is probably a bad example. it kind of looks like he's hooking 2 schematics together, first a fuzz face, then something with the op amp. i was just taken aback by the part about 'be sure to bridge power and ground with these caps', and wondered if that were 'a thing' with other op amp circuits.

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