Component Considerations 1 (Op-Amps)

Op-Amps

The original question was specifically about op-amps, but if this proves useful I may write a bit about other components too

(All of this is my current opinion, there is more detail, but maybe not just yet).

So, what's an op-amp? At its heart, it's a very high gain amplifier which, with some surrounding resistors, can be tamed to make it a _useful_ amplifier. Wikipedia has to say this (for those that are interested) or, if you have a bit of time, try this video

So what can you do with an op-amp? Many things, including:

• amplifiers;
• mixers;
• filters;
• oscillators;
• …

I'll write about these a bit later.

In the context of AE Modular, there are some things which must be considered:

• Single Supply (+5v):- Typical op-amp circuits expect split supplies, which is what you'll find in Eurorack for example (+12v and -12v), designing for single supplies means that some extra considerations might be needed.
• Low voltage (+5v):- 5v does not give us very much to play with. To keep sonic performance as good as possible, we want to make our signal as large as possible (ideally signals would swing all the way from 0 to +5v).
• Low speed (audio):- op-amps can be used over a wide range of frequencies, but since we're mainly dealing with audio, we don't need to use anything with too wide a bandwidth (which means we can use cheaper devices!)

From a DiY perspective, the pinouts of op-amps are generally consistent, so a circuit can usually be designed and op-amps swapped in and out without having to redesign the circuit layout (although the performance of the circuit may of course change!)

Op-amps usually exist in families - where there will be variations typically consisting of one, two or four op-amps in a single package. One example of this is the TL07x (or TL08x) family, where x can be replaced by 1, 2, or 4 (giving us part numbers TL071, TL072 and TL074). The single and dual parts are usually contained in a single 8-pin package, whilst the quad parts are usually in 14 pin packages.

So, which op-amps should I buy for my experiments? Well, given the restrictions listed above we need something which will operate on a low voltage and will allow the inputs and outputs to swing to close to the supply rails.

General purpose op-amps such as the LM358 (datasheet), and the TL07x/TL08x (datasheet) series are not perfect choices (although they are cheap and plentiful) for the following reasons:

LM358 plus points: can operate on voltages between 3 and 32v; cheap; plentiful.
LM358 minus points: output voltage is limited to between 0 and around 70% of the supply voltage (for a 5v supply); 1MHz bandwidth

TL07x plus points: cheap; plentiful; 3MHz bandwidth
TL07x minus points: Needs 10v minimum supply; output voltage is limited to between 0 and around 75% of the supply voltage (for a 5v supply)

These are both pretty old designs (TL07x datasheet dates from 1978), so we can do better with more modern designs built for low voltage applications such as battery-powered audio. Here are a couple that I have played with:

TLV237x (datasheet) -- Like the TL07x series, there are single, dual and quad versions of this op-amp.

TLV237x plus points: widely available; can operate as low as 2.7v; inputs and outputs can go within 0.05v of the supply; 3MHz bandwidth
TLV237x minus points: not as cheap as some op-amps

MCP600x (Datasheet) -- These are another family which I use (only because they were slightly cheaper than the TLV237x series!)

MCP600x plus points -- widely available; can operate from as low as 1.8v; inputs and outputs can go within 0.03v of the supply
MCP600x minus points -- 1MHz bandwidth

(note that there is also the MCP60x series (datasheet) which is possibly slightly better than the MCP600x series, but for audio there's not much between them!)

If you're interested in single-supply considerations when designing circuits, these PDFs have maths and graphs in them

www.ti.com/lit/an/sloa030a/sloa030a.pdf?ts=1591126834928 (2001)
www.ti.com/lit/an/sboa059/sboa059.pdf?ts=1591126849060 (1986)

So there's a start. What do you want to build?

Sure!

The thing about op-amps is that their gain actually slopes off at higher frequencies; the bandwidth quoted is actually the 'unity gain bandwidth', where the op-amp is configured so that it is not amplifying at all. This translates into a thing called the 'gain-bandwidth product' (it turns out that if you multiply the gain of your op-amp circuit by the measured bandwidth you end up with a constant for each type of op-amp).

So, in summary, at higher gains, the bandwidth is less, but you can work it out simply (gain * bandwidth = unity gain bandwidth).

As you pointed out, our hearing only goes up to 20kHz, so given a theoretical op-amp with a bandwidth of 2MHz, we could configure the circuit with a gain of up to 100 (because 100*20000 = 2000000) without hearing any roll off of higher frequencies. If we were amplifying really quiet sounds and wanted a gain of 1000, then we would be limited to frequencies up to 2kHz, so an op-amp with a larger bandwidth would then be needed.

Of course this is theoretical, so practical circuits may give slightly different results…

great thread! I'm new to a lot of the DIY side of things too, always nice having a read through to try and get my head around more of this. 

For sure - no point in having lots of op-amps which may or may not work in a given situation


You're completely right, this is unlikely to affect audio circuits, perhaps until oscillators are being built and you start wondering why are my square waves not quite square enough But that's probably a later topic!!


Yes, I think you are probably right there

I have plenty of pedals too; I may be able to take a look at what it would take to feed audio in and out to them — I suspect not much is needed to be honest. I suppose it depends upon how 'quick and dirty' a solution is needed