Peach with two band tone control

[Rob Strand]

> it is good safe practice to include it.

But... but... buttt... that adds 30 cents to the cost of my build!!! You are going to bankrupt me! (Like the Silicon Valley Bank run today.)

I gave-up preaching Applications Note AN69, LM380, which is on my bedside shelf. Nobody believes stuff written on paper anymore. (Also for some years AN69 was missing from the WWW, so existed only in the so-called memory of old geeks.)

I don't think it helps anyone   The datasheets come from that base-info and they haven't been updated.   I'm sure the designs have been revised but they still target the old specs.

[amptramp]

Going further back into Jurassic electronic design, the General Electric Transistor Manual showed a number of discrete amplifier designs and all versions used a 10 ohm / 0.1 µF Zobel network.

The network is sized for the speaker it is to be used with or the effective impedance of the crossovers + speakers.  A speaker has a resonance at a low frequency, maybe 70 Hz for a guitar speaker, which gives a high impedance at 70 Hz followed by a minimum impedance at around 300 Hz and an impedance that rises with frequency at higher frequencies, in other words, it looks like an inductor above 300 Hz.  The Zobel network is in parallel with the speaker and the combined circuit of speaker and Zobel network is flat at higher frequencies.  This avoids instability in the amplifier caused by feedback issues.  The R in the network is usually chosen to be 1.25 times the speaker impedance.  The C is chosen to be Speaker L / Zobel R².

I would leave space on the board for a Zobel network but not populate it.  If high-frequency oscillation becomes a problem, you can add the network.

[Rob Strand]

The R in the network is usually chosen to be 1.25 times the speaker impedance.  The C is chosen to be Speaker L / Zobel R2.

That rule of thumb comes from speaker design so the crossover behaves more like the theory with a real speaker.   For amps you might see nominal 100nF + 10R  or 100nF + 4R7.  That's kind of the amplifier rule of thumb, it's better than nothing.  It's not uncommon for amplifier Zobel networks to use resistances different to the target speaker load.  That's because it's determined by amplifier stability.   The LM380 values of 2R7 and 50nF are a good example.   Some power supplies have funky valued Zobel networks to solve odd-ball cases of instability.

[soggybag]

Here's my completed build.

http://www.super-freq.com/peach-pit/