LM324N - Active Filter Audio Bandpass Filter - Need Help Changing B/W & Fc  - Page 1
Forum section: electronics / rf-microwave

author: AJ3G
author_rank: Regular Contributor
timestamp: August 06, 2025, 07:56:32 pm
content: All:I am trying to modify an Active Audio Filter utilizing a LM324N. Attached is a copy of the schematic in question. According to the Circuit Description the following was given:- Fc = 700Hz- B/W = 450Hz (-3dB)- 6dB BW = 500Hz- Bandpass- Purpose = Active CW FilterIt is my goal to modify this circuit in a way to shift up the Fc and increase the B/W so that I can utilize it for Single Sideband Reception (SSB) and Possibly AM. The approach I have taken (Right or Wrong) was to attempt using some of the modeling software for Active RC Filters to simulate this change. To date I have tried TI FilterPro, and Ti Workbench to simulate. When I enter the specifications listed above, I am not getting R/C values anywhere close to what is seen in this schematic. As a matter of fact, all the simulations seem to spit out asymmetrical designs, where this appears to be symmetrical. It appears most of the modeling software I have used seems to favor Dual RAIL Devices, where this schematic is single ended, and uses pseudo ground.Wondering if anybody here can point me in the right direction as to why I am seeing such a discrepancy, and possibly recommend another approach to modeling this active filter so I can begin the process of changing its design parameters?Many Thanks,Rich

author: LM21
author_rank: Regular Contributor
timestamp: August 07, 2025, 04:26:35 pm
content: LM324 is an ancient OPAMP. A modern chip will work better. LTSpice is a good simulation tool. Your circuit is in a radio, but it is more of analog electronics. You could find some help in analog electronics forum.

author: AJ3G
author_rank: Regular Contributor
timestamp: August 07, 2025, 04:42:05 pm
content: LM21:Yes, this is an ancient device. I am building a Heathkit HW-99, which was a CW only Novice Amateur Radio Transceiver from the mid to late 80's. I have opted to receive SSB signals, so I am making modifications to the BFO to accommodate SSB reception. This is the reason, I must increase the B/W of the Active Audio Filter. I was not sure if this was the most appropriate forum to mention my issue. I will repost over on the Analog Design Forum.Thanks for pointing me in the right direction!Rich

author: G0HZU
author_rank: Super Contributor
timestamp: August 07, 2025, 06:41:29 pm
content: Could you make a little piggy PCB for the SSB active filter and have both CW and SSB via a switch?The reason I ask is that an SSB filter is almost a decade wide and it's asking a lot of just two opamps to implement this. I think it would end up being a high pass filter with very gentle rolloff above 2.7 kHz.With four opamps it might achieve something like the response below. Much nicer and it could be done on a small piggy PCB.Also, is the first IF filter wide enough for SSB?

author: AJ3G
author_rank: Regular Contributor
timestamp: August 07, 2025, 06:59:01 pm
content: I think I could add a daughter or “Piggy PCB” as you suggest. The first IF Stock Filter is 3KHz wide (at the 3dB point), so the bottle neck would be the Active Filter. I could use the Mode Switch (LSB/USB/CW) to do the switching, I would just utilize a double ganged switch, or I could do the switching of the two Active Filters when switching the XTAL Filters I will be adding to the 8.83MHz IF Strip.One Idea I did have was to allow the selectivity of the XTAL IF Filters do all the heavy lifting and simply bypass the first two op-amps in the Active Filter by cutting the input and output pins thus leaving the final op amp (buffer amp) in the circuit. I was going to breadboard that idea tomorrow, when the spare LM324N’s I ordered come in.

author: AJ3G
author_rank: Regular Contributor
timestamp: August 07, 2025, 08:55:56 pm
content: All:I have moved this post over to the Projects, Design, and Technical Forum, as I agree with another member this may not be the best forum. I have tried to delete it here, but it keeps coming back. I will work on consolidating this and apologize for the double post. I assumed when I hit delete it would kill the post, but apparently not.Rich