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How To Choose An Op Amp For Your Audio Project

op amp circuit symbol basics audio project guitar pedal how to choose select
circuit symbol

Selecting the right operational amplifier (short: op amp) for your DIY-project can be a hard task. The variety of op amps is comparable to the number of stars in the sky and the number of specifications in the datasheet is sometimes overwhelming, so how do you choose the right op amp for a specific project? In the following I want to give you an overview of the most important datasheet specifications that you should look after. This little guide is addressed to electronics beginners who want to dive a bit deeper into the topic of operational amplifiers.

What Is An Operational Amplifier?

An operational amplifier is basically a differential amplifier with a very high amplification (ideally infinite). It has a non-inverting and an inverting input, an output and of course pins for the power supply. By using feedback from the output to one of the inputs you can realize many different circuits, for example inverting and non-inverting amplifiers (with finite amplification), integrators, differentiators, low-pass and high-pass filters and many more. For further reading on the different circuits check out Texas Instruments’ “AN-20 An Application Guide For Op Amps”.

 

The Most Important Datasheet Specifications:

 

1. Supply Voltage

The one most important spec you should check in the datasheet is the Supply Voltage Range. You find it at Recommended Operating Conditions. If your Supply Voltage is too low your op amp won’t work properly, if it is too high you may even destroy it.

 

2. Input Common Mode Voltage Range

This specification is sometimes overlooked. The Input Common Mode Voltage Range defines the allowed voltage range of your inputs without malfunction of the op amp. Op amps with rail to rail inputs have an Input Common Mode Voltage Range of V- < Vin < V+, others may include only the positive or the negative rail or neither.

Example:
The typical Input Common Mode Voltage Range of the TL072 includes the positive rail and needs 3 volts of margin to the negative rail:
V- + 3V < Vin < V+                             at 10V supply voltage your input voltage range is 3V to 9V

In the Recommended Operating Conditions TI adds a big safety margin:
V- + 4V < Vin < V+4V                    at 10V supply voltage your input voltage range is only 4V to 6V.

If you want to design your circuit properly you have to use the voltage range stated in the Recommended Operating Conditions (minimum values), however, if you are developing an individual item you can get by with using the typical values by screening the op amps.

 

3. Input Impedance

The Input Impedance of op amps is of special interest when the signal source impedance is high, for example when using passive guitar pickups. If so, you may want to choose an op amp with JFET-inputs (Input Impedance goes up to 1000GΩ). Op amps with bipolar inputs typically have Input Impedances of about 10MΩ.

 

4. Equivalent Input Noise and Typical Harmonic Distortion

Op amps do not only amplify your source signal, they also add harmonics and noise. The Equivalent Input Noise and Typical Harmonic Distortion are either stated separately or mixed together as THD+N.
For audio applications it is essential to choose an op amp with low noise and distortion.

 

5. Supply Current

If your device runs on batteries it is important to keep an eye on the Supply Current. The Supply Current or Quiescent Current provides information on the energy consumption of the op amp without load.
Unfortunately the rule applies that low supply currents result in high noise. You see, choosing the right component is always a trade-off – welcome to electronics 😉

 

6. Output Impedance

The Output Impedance is important if the load impedance is low, for example if you want to drive headphones or an LED. The Output Impedance has to be much lower than the load impedance, otherwise there would be a big voltage drop on the output (voltage divider between Output Impedance and load impedance). If you don’t find the Output Impedance in your datasheet the Output Source Current gives you similar information.

 

For detailed information on the above stated specifications and more I recommend you to read TI’s “Understanding Operational Amplifier Specifications“.

 

How Do I Find The Right Op Amp?

I personally like to use the website of Texas Instruments, because I can type in the specifications that are important to me and the website then shows me suitable op amps including an approximate price. If you want to include other manufacturers too, a great way to do this is using the search function of a big distributor, e.g. Mouser.

 

I hope this little guide is useful to you. If you are a guitar player, please check out the products on this website: My company Songbird FX is a manufacturer of high quality micro-USB rechargeable guitar pedals.

 

Keep On Rocking!
Dieter

Bluebird Drive – World’s First Micro-USB Rechargeable Analog Guitar Effect Is Out Now

Today the sale of the ‘Bluebird Drive’ guitar overdrive pedal started. You can purchase it directly from us via https://www.songbirdfx.com/products.

The Bluebird Drive is a simple but effective overdrive that keeps the original timbre of the guitar and takes it one step further – no tone knob needed. It is the world’s first analog guitar effect, that is rechargeable via the included Micro-USB cable. Just plug it into your Smartphone wall charger or your computer and in less than two hours it will be fully charged. The integrated Li-Ion battery lasts for more than 100 hours of playing time. Highest build quality makes the Bluebird Drive the perfect choice for professional and semi-professional musicians as well as demanding hobby guitarists.

 

Songbird FX - Bluebird Drive Si and Ge - Silicon Germanium Clipping Stage Diodes Overdrive Pedal Rechargeable Micro-USB

‘Bluebird Drive Si’ (silicon clipping stage) and ‘Bluebird Drive Ge’ (germanium clipping stage)

 

The Bluebird Drive is available in two versions:

  • Bluebird Drive Si:

    Legendary 1N4148 silicon clipping diodes deliver a great range of sounds, going from clean boost to singing leads.

  • Bluebird Drive Ge:

    Rare soviet D9D germanium clipping diodes (NOS) give you a sound that goes from warm overdrive to fuzzy leads. The asymmetrical clipping stage combines the characteristical low clipping threshold of germanium diodes on the positive half wave with higher output level on the negative half wave (achieved by connecting a silicon 1N4148 in series to the D9D).

 

Keep on rocking!

Dieter

The Production Of The Bluebird Drive Has Started

Hello my friends!

 

Last week the Alpha Potentiometers I ordered arrived.

Alpha Potentiometers Guitar Effects Pedal
Alpha potentiometers

 

Now that all components are available the series production of the revolutionary Bluebird Drive pedals has begun. The Bluebird Drive is the world’s first analog guitar effect, that is rechargeable via Micro-USB.

guitar effects pedal production hand crafted
assembly of the Bluebird Drive

Soon the first pedals will be available for purchase, so stay tuned!

 

Rock On \m/

Dieter

A New Guitar Effects Company Arises

songbirdfx songbird fx logo
Songbird FX logo

Hello my friends!

This is my first post, so I’m going to introduce myself and my company. My name is Dieter and I’m the founder of Songbird FX. My profession is Electronics Engineering and I love to play Rock ‘N’ Roll guitar. Songbird FX is a new guitar effects manufacturer that is based in Molln, Austria. Our first pedal, the Bluebird Drive is the world’s first analog guitar effect, that is rechargeable via Micro-USB.

dieter songbird fx
a picture of me

 

The story of Songbird FX

Did you ever want to play guitar through your favourite guitar effect but the 9V battery was empty and you didn’t have an AC adaptor at hand? That’s what happened to me in December 2015 when I decided to develop a Micro-USB charging circuit for lithium batteries which I then combined with a vintage overdrive circuit – the Bluebird Drive was born. It has a battery life of over 100 hours and is fully charged in less than 2 hours.

Then the real work began: finding manufacturers for high quality components and enclosures, making a PCB layout and creating mechanical drawings, creating a brand, founding a company…

Currently I’m waiting for the last components to arrive so I can start production. In the next weeks the first pedals should be available for purchase, so stay tuned!

 

Rock On \m/

Dieter