# DIYstompboxes.com

## DIY Stompboxes => Building your own stompbox => Topic started by: TimWaldvogel on November 24, 2010, 06:02:06 PM

Title: trying to understand differential op amp circuits
Post by: TimWaldvogel on November 24, 2010, 06:02:06 PM
i am designing\building a mic preamp and i am trying to understand how the whole thing works.

i want to do a balanced op amp mic input out the op amp the 1st stage of gain be adjustable as a pre gain and then there will be a 3 band eq and a adjustable post gain stage with a transformer balanced output. with a +18 volt power supply

i just dont quite understand how you go about biasing it from the power supply. on guitar circuits there is not a hot and a cold so i know i could do like a 10k resistor coming from the 18v and bias it to +9 and -9 or whatever before the non inverting input.

how would you bias something that goes into both. do you ignore the cold? or do i do it to hot and cold? any urls you could send me with information on better understanding this would be great
Title: Re: trying to understand differential op amp circuits
Post by: brett on November 24, 2010, 06:54:24 PM
Hi
you need to avoid any DC offset between + and -.  It's not difficult.
Rather than give you a fish, here's a fishing lesson: search the www and look at how others have done it.
cheers
Title: Re: trying to understand differential op amp circuits
Post by: Gurner on November 24, 2010, 06:55:06 PM
i am designing\building a mic preamp and i am trying to understand how the whole thing works.

i want to do a balanced op amp mic input out the op amp the 1st stage of gain be adjustable as a pre gain and then there will be a 3 band eq and a adjustable post gain stage with a transformer balanced output. with a +18 volt power supply

i just dont quite understand how you go about biasing it from the power supply. on guitar circuits there is not a hot and a cold so i know i could do like a 10k resistor coming from the 18v and bias it to +9 and -9 or whatever before the non inverting input.

how would you bias something that goes into both. do you ignore the cold? or do i do it to hot and cold? any urls you could send me with information on better understanding this would be great

So you're using a single supply of 18V? Therefore you'll be setting you opamp 'bias' at about 9V? (ie a virtual ground arrangement)

In one arrangment, if the 9V bias can go onto the common wire of your 3 wire balanced cable (any noise on this 9V rail becomes common and largely cancelled).

Remember for optimum cancellation, you need 1% resistors - better still just buy an instrumentation amplifier IC (it's essentially a differential amplifier without the hassle!) - it'll have all the laser trimmed close tolerance resistors needed integrated  internally ......leaving you only needing to choose one external resistor (the gain set resistor)

Edit: a decent circuit here - he's made it for an (unusual), balanced guitar pickup - but you could easily use it for your mic - he's using two supply rails +/- ...therefore he references ground throughout the preamp, but if you're using a virtual ground, replace those ground connections with your virtual ground....

http://www.joebrown.org.uk/wp/?p=1846 (Schematic For Balanced Magnetic Pickup Pre-amplifier)
Title: Re: trying to understand differential op amp circuits
Post by: R.G. on November 24, 2010, 06:55:46 PM
http://www.media.mit.edu/resenv/classes/MAS836/bias.pdf (http://www.media.mit.edu/resenv/classes/MAS836/bias.pdf)

Third item down when you google "biasing differential amplifiers".
Title: Re: trying to understand differential op amp circuits
Post by: TimWaldvogel on November 24, 2010, 08:06:24 PM
Thanks guys I will attempt to wrap my head around that. My other question would be, when doing a transformerless input, how it the input impedance set? With identical resistors at the inverting and non inverting inputs?