CW filter circuit help

Can anyone help me with this filter circuit. Confused by the 2 capacitors designated 8200.


This looks a bit weird. As Andy says the caps are 8.2nF.

Take a look at the ‘Bandpass’ section of

for something a bit more usual.

73, Colin

If stuck, you could also use 8200pF.

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This is a high pass / low pass filter pair with gain provided by the 709. Together they form a band pass filter which has a variable centre frequency. These days a TL071 would be a better low noise option for the op amp, (cost about 10p UK).
The capacitors marked 8200 are 8200pF = 8.2nF. To get best performance each should be within 5% of the same value. Please do not expect too much of this little circuit.


I have an unused uA709 in a TO-5 can somewhere with a 1970 date code ISTR. Tell me it’s worth a fortune as a collectors item nowadays? 'coz it’s certainly not worth anything as an opamp!

Thanks for the answers. I was thinking 8.2nF but the symbols through me. I’ll look up the TL071 also and see what its about.


The use of two dual pots, plus the balanced +/- power supply probably required for the circuit you show, is enough to scare me off.

However, if you have the dual pots and balanced supply already, the circuit looks so simple that it might be fun to try! I’ve never seen it before - thanks for putting it up here.

Thanks for the wonderful long S2S on Wednesday! I was sitting at 9770 feet in the snow, and your signal was so easy to copy on 20M from so far away!



It was great to hear you George! I was sitting in snow also at a staggering 426 ft LOL. PEI is pretty much flat. Your sigs was good to.

I built one of the audio filter kits sold by New England QRP club and called the NESCAF, they use a switched capacitor type filter chip. It has more components but more performance than the simple op amp circuits. A control for centre frequency and another for bandwidth. Works like a charm. Look up NESCAF kits on the web, you’ll find them.

External filters have a slight disadvantage over internal filters, obviously. But when the mech filter is no longer available, Digital filtering like those sold by Sotabeams, or SCAF type filters will be the way to go.

Andrew VK1DA/2UH

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The performance of filters is related to the number of poles that they have. Think of it as being like the number of cascaded tuned circuits. Your simple opamp circuit has one pole, the NESCAF filters have four poles. Our DSP modules have hundreds of poles.

Here’s a pic showing my built NESCAF filter under test on my daughter’s kitchen bench (she was out that night). Testing the Nescaf audio filter | The NESCAF audio filter us… | Flickr

Hi Andrew, I was looking at the NESCAF filter but they are sold out and have been for a while now. I’m on the list for when they are produced again.

Hi Richard, I have been looking at the Laserbeam and it is quite impressive. Considering it too.

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That was the situation when I enquired a few years ago. Eventually they had new stocks and emailed me to say it was available. Re the laserbeam dsp filters, vk3ye has a review on his website. I think if you have an ft817 the option to install the filter internally is very attractive.
73 Andrew VK1DA/VK2UH