[sdiy] Transistor Sallen-Key lowpass filter question

[Tom Wiltshire]

Thanks both. That’s it exactly. Adding a 6V source in series with the input magically sorts everything out:

https://electricdruid.net/wp-content/uploads/2018/11/DelayFilterFixed.png
https://electricdruid.net/wp-content/uploads/2018/11/DelayFilterResponse.png

==================
       Electric Druid
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.
>> 
>> /Tristan
>> 
>> 
>> 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?
>>> 
>>>      https://electricdruid.net/wp-content/uploads/2018/11/DelayFilterSchematic.png
>>> 
>>> According to the sim (LTSpice), the response looks like this:
>>> 
>>>      https://electricdruid.net/wp-content/uploads/2018/11/DelayFilter.png
>>> 
>>> There’s a significant drop with a -3dB cutoff of about 200Hz. What’s
>>> causing that?
>>> 
>>> Using the same passive components, but with a op-amp in place of the tranny,
>>> with the tools at..
>>> 
>>>      http://sim.okawa-denshi.jp/en/Sallen3tool.php
>>> 
>>> ..doesn’t give the same effect. Am I seeing a simulation effect or a real
>>> phenomena?
>>> 
>>> Thanks,
>>> Tom
>>> 
>>> 
>>> 
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>> 
>> 
>> 
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