The PY2OHH pages contain a useful link to a TI paper on active filters. In my experience, virtually any audio or RF transistor (e.g. 2N3904 NPN) can replace the op-amps with little effect. Some prefer "perfect" op-amps whereas I find transistors easier to work with and place. The performance difference is very very small.
2N3906 is a ubiquitous PNP equivalent. The 2N3904 and 2N3906 devices are available from a range of suppliers for just a few pence each only. A few 2N3904s in parallel with small emitter resistors make a good HF/LF QRP PA replacement for a 2N3866 too. My recommendation is buy 100 of each (2N3904 and 2N3906) and stock your junk-box. Less than £10 and it will be a long time before you run short! Think of all those projects.
See http://py2ohh.w2c.com.br/trx/filtroativo/filter%20design%20in%20thirty%20seconds.pdf for the active AF filter paper.
14 Apr 2014
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