[sdiy] Transistor Sallen-Key lowpass filter question
tom at electricdruid.net
Mon Nov 19 11:33:26 CET 2018
Thanks both. That’s it exactly. Adding a 6V source in series with the input magically sorts everything out:
Synth & Stompbox DIY
> On 19 Nov 2018, at 02:36, Ben Bradley <ben.pi.bradley at gmail.com> wrote:
> Yes, it's pure AC, so the transistor has 0V bias. Easiest thing to do
> is add a 6V DC voltage source in series with V1.
> On Sun, Nov 18, 2018 at 7:40 PM Tristan <tu at alphalink.com.au> wrote:
>> Is V1 a pure AC source? V1 should also include a positive DC bias of around 6V for the transistor circuit to
>> work. Otherwise you could just be getting half wave rectified pulses coming out on the positive peaks of
>> the AC input waveform.
>> The opamp example probably assumes the opamp is powered by bipolar supply rails so it works with no
>> DC bias on the input.
>> On Mon, Nov 19th, 2018 at 10:59 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
>>> Can anyone tell me why a transistor-based Sallen-Key filter like this one
>>> would have such a heavy roll-off at the bass end?
>>> According to the sim (LTSpice), the response looks like this:
>>> Thereâ€™s a significant drop with a -3dB cutoff of about 200Hz. Whatâ€™s
>>> causing that?
>>> Using the same passive components, but with a op-amp in place of the tranny,
>>> with the tools at..
>>> ..doesnâ€™t give the same effect. Am I seeing a simulation effect or a real
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