replacing AD821 transistor pair (electronotes VCO)

[carlhudson83]

Hi all..

I'm looking at building myself a Thru-Zero VCO, as per the Electronotes 1981 edition schematic - http://electronotes.netfirms.com/EN129.pdf

And I've come across a problem that seems quite common amongst the DIY-ers here - the obsolete AD821 matched PNP transistor pair.

Over here in the UK, I see that RS Electronics sell a MAT 03 - http://uk.rs-online.com/web/p/bipolar-transistors/7098521/?searchTerm=MAT03&relevancy-data=636F3D3226696E3D4931384E4B6E6F776E41734D504E266C753D656E266D6D3D6D61746368616C6C7061727469616C26706D3D5E285C772B5B2D5C2E2F252C5D2A292B2426706F3D313326736E3D592673743D4D414E5F504152545F4E554D424552267573743D4D415430332677633D424F544826 (although it's a little pricey!)


Is it just a simple case of replacing the AD821 with this component (allowing of course for a slight variation of tone/noise/etc) - or will some of the resistor / capacitor values need changing?

Also, there's some diodes lurking in the circuit that aren't specified in the document.. Would it be OK to put in something standard like a 1N4148?


Best,

Carl Hudson

[andrewF]

There is the ssm2220, a lot cheaper -
http://uk.rs-online.com/web/p/bipolar-transistors/5228809/

or look for 2SA798 or A798, plenty on ebay for approx $4 (ssm2220 would be better)

You can also buy a batch of general purpose pnp transistors and hand-match them, various circuits around, I like the one Ian Fritz has shared.
you >probably< won't need to change any other component values.

Any general purpose diode, like the 1N4148, is fine
Enjoy

[carlhudson83]

Great stuff - thanks!

Carl