tirsdag 1. januar 2019

A closer look at the responses of the jp6 filter after fixing CV and bug

To make a good state variable filter with 12/24dB HP, LP and 12dB BP I simulated the JP6 filter and tapped it at various places to look at the responses.

For my simulated circuit, setting cutoff CV to 1.35 to get results comparable to my previous erroneous circuit. Resonance CV control is still a bit wonky but now it uses an expo converter. I have set reso to 0.5V (but there is a missing current limiting resistor in the circuit) to get 13dB resonance.

24dB LP:
- Cutoff: 500Hz
- Resonance: 13dB at 1.24kHz

24dB HP:
- Cutoff 3.2kHz
- Resonance: 13.2dB at 1.4kHz

12dB LP + 12dB HP,
12dB HP + 12 dB LP
- LP cutoff: 2.6kHz
- HP cutoff: 694Hz
- Center: 1.33kHz
- 13.4dB attenuation on top
- Resonance: Top has 1.1dB amplification

12dB LP with connection to second stage
- Cutoff: 772Hz
- Base attenuation 1.44dB
- Resonance: Top has 6.6dB amplification over base at 1.25kHz, base is -13dB

12dB LP without connection to second stage
- Cutoff: 769Hz
- Base amplification 1.4dB
- Resonance: Top has 6.6dB amplification over base at 1.25Hz, base is -10dB

12dB HP with connection to second stage
- Cutoff: 2.1kHz
- Base attenuation 1.7dB
- Resonance: Top has 6.4dB amplification over base at 1.4kHz, base is -13dB

12dB HP without connection to second stage
- Cutoff: 2.1kHz
- Base amplification 1.1dB
- Resonance: Top has 6.65dB amplification over base at 1.4kHz, base is -10dB


BTW: I have found another serious issue. The cap on the output amp prevents the filter from getting a higher LP cutoff than 14k. removing it gives a cutoff of about 20kHz at most. This does not seem right compared to the promised 5-50kHz in the datasheet.

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