LPF (6th order) with TL072C - Page 1
Forum section: electronics / projects

author: JimWest
author_rank: Newbie
timestamp: November 23, 2019, 06:03:51 pm
content: Hi Guys,I have a tricky job today. I hope you can help me.At the moment I'm recreating an old sound module from 1992.Unfortunately, I only have pictures of the board. But I was able to trace the circuit.However, I miss the values ​​for the capacitors. These are SMD capacitors (0805) except C3/C4 these are 1206.I think the circuit should be something like a LPF (6th order). At least according to the data sheet of the following surround sound ICs.Does anyone have an idea whether the circuit makes sense so far and which values ​​are suitable for the capacitors?Thank you for your help!

author: iMo
author_rank: Super Contributor
timestamp: November 23, 2019, 06:40:37 pm
content: Download the LTspice and put it there..

author: JimWest
author_rank: Newbie
timestamp: November 23, 2019, 10:16:05 pm
content: Oh cool. I did not know that there is something like that. I will try it. Thanks for the tip.

author: Yansi
author_rank: Super Contributor
timestamp: November 23, 2019, 10:24:42 pm
content: U2.2 is a textbook sallen-key third order lowpass. Canbe readily calculated by using filter tables, or just find almost any online web application for that. (preceding amplifier is just a textbook inverting amplififer, with a small compensation/bandwidth limiting cap. Input coupling cap sets the low frequency cutoff (C1, R1).Not sure how you came up with sixth order. No way would that be needed.PS: Okawa-Denshi has a very good set of tools for filters:http://sim.okawa-denshi.jp/en/Fkeisan.htm//EDIT: C3, C4 = 100nF. Decoupling.

author: iMo
author_rank: Super Contributor
timestamp: November 24, 2019, 11:14:13 am
content: Below attached the .asc for LTspice. Doubleclick and it opens in LTspice. Click the runner to simulate the AC sweep.You may select the opamps and R,C values and simulate.

author: JimWest
author_rank: Newbie
timestamp: November 27, 2019, 10:07:46 am
content: imo, Yansi, thank you for your help! You helped me a lot.I will continue to research whether I can learn more about the characteristics of the LPFs with the help of the data sheet of the subsequent sound chip.It may then be possible to determine the values ​​of the capacitors.

author: Yansi
author_rank: Super Contributor
timestamp: November 27, 2019, 06:26:53 pm
content: Shall be pretty straightforward to determine the values, once you know the purpose of the circuit.Input amplifier gain is -1, input impedance 30k. Output impedance is low. (always a mandatory requirement to feed the sallen-key filter).The LP filter has a gain of 1 in the passband. Once cutoff frequency is known (the purpose of the filter there is to do what? What part of what circuit does it serve?), be easy enough.Y.

author: JimWest
author_rank: Newbie
timestamp: November 28, 2019, 10:03:08 am
content: The LPF gets as input an audio signal.The filtered signal from the LPF is then routed to the DSP. In between is a 10uF capacitor.I have attached a link to the datasheet of the DSP:http://www.dtech.lv/files_ym/ym7128.pdfIs the sampling frequency fixed or is it variable depending on the clock input of the microcontroller interface SCI?How is the cut-off frequency determined when the DSP has a sampling frequency of 23.6kHz?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 28, 2019, 12:25:24 pm
content: Hi,Because of the Nyquist sampling theory, the maximum cut-off frequency would be 1/2 the sampling frequency, 23.6kHz / 2 = 11.8kHz.I would expect the LPF to be lower than this, 6 to 8kHz.Do you have a working one that you can measure?Regards,Jay_Diddy_B

author: Yansi
author_rank: Super Contributor
timestamp: November 28, 2019, 12:57:57 pm
content: Quote from: Jay_Diddy_B on November 28, 2019, 12:25:24 pmProbably no, because the ADC is already oversampled, 2x.

author: JimWest
author_rank: Newbie
timestamp: November 28, 2019, 01:41:59 pm
content: Quote from: Jay_Diddy_B on November 28, 2019, 12:25:24 pmNo, I only have a picture of the board.Based on the picture I have, I was able to identify two capacitors. One is marked A3 (1nF), the other A2(100pF).The one with A2 is used in the output LPF of the sound IC. The other A3 in the input LPF as C5 in the above circuit.If I interpret the capacitor values only on their optical appearance, the resulting cut-off frequency is 21685Hz.I was looking around the internet and found a service manual from Sony. They used the same IC for Dolby Surround.The cut-off frequency of the Sony circuit is something about 9773Hz.

author: Yansi
author_rank: Super Contributor
timestamp: November 28, 2019, 06:06:33 pm
content: Those 10kHz cutoff is probably right. 20 is not.You can't simulate the filter without the first input RC stage. This way you are being missguided about the result. The sallen-key filter is always designed for a zero output impedance source at the input of the filter. Hence omitting the first RC lowpass filter pole is a mistake (I mean R3 C5 on your original schematic).Also, resulting filter Q shall not be much above 0.7, the filter is not likely to be designed resonant (peaking on high frequencies).Y.

author: iMo
author_rank: Super Contributor
timestamp: November 28, 2019, 06:32:37 pm
content: Antialiasing low pass for the DSP?

author: Yansi
author_rank: Super Contributor
timestamp: November 28, 2019, 06:37:46 pm
content: it is an antialias filter.But unlikely at 20kHZ with a 23kHz sampling rate, even though the ADC is 2x oversampled.