How do I fix the output of my analog sound equalizer? - Page 1
Forum section: electronics / beginners

author: moriavia
author_rank: Newbie
timestamp: June 26, 2023, 07:04:59 pm
content: I'm new to electronics and I'm trying to build an analog sound equalizer with an TL071 as an amplifier. I have three different circuits for three different ranges (low frequencies, high frequencies and medium frequencies). Each circuit for each range seems to be working fine, but the output (pin 6) doesn't change as expected. It should filter the signal according to the positions of each potentiometer.The ampOp is in an adder configuration.I tried messing with the resistors values, but to no effect. I know I can adjust each gain individually, but it didn't work.Here's the full circuit (The last resistor is for measuring the output)Circuit made with MultisimFor more information: the input is the audio output of a phone (2V) and the output should also be around that value, because I want to use it in a regular sound boxHow can I fix it?

author: Benta
author_rank: Super Contributor
timestamp: June 26, 2023, 07:30:51 pm
content: The opamp output is connected to ground through a 100 megaohms resistor. I don't see how that should work. I don't even see where the output is, or where it's supposed to be.

author: moriavia
author_rank: Newbie
timestamp: June 26, 2023, 07:38:54 pm
content: The output is between pin 6 and gnd. I put the resistor to act as the load. I'm measuring the output on pin 6

author: pqass
author_rank: Super Contributor
timestamp: June 26, 2023, 07:49:33 pm
content: The V2 symbol should be vertically flipped.As is, pin 4 and 7 are connected to +12V so the chip is effectively unpowered.

author: moriavia
author_rank: Newbie
timestamp: June 26, 2023, 07:58:29 pm
content: I think it's not visible in the image, but pin for is connected to -12V. The symbol is incorrect, but the output is -12V (I've measured it)

author: pqass
author_rank: Super Contributor
timestamp: June 26, 2023, 08:22:04 pm
content: Quote from: moriavia on June 26, 2023, 07:58:29 pmThen there should be +24 between pin 7 (red lead) and pin 4 (black lead of MM). Right?

author: moriavia
author_rank: Newbie
timestamp: June 26, 2023, 08:26:33 pm
content: Precisely. There is. My output is not zero, but it's not being filtered correctly. It's not "equalizing"

author: pqass
author_rank: Super Contributor
timestamp: June 26, 2023, 08:38:01 pm
content: You should readthis.

author: moriavia
author_rank: Newbie
timestamp: June 26, 2023, 08:52:55 pm
content: Thank you! I will read it, but it looks like all the filters featured in this are active. My problem is with the AmpOp as the adder. Thanks regardless

author: Benta
author_rank: Super Contributor
timestamp: June 26, 2023, 08:53:33 pm
content: Quote from: moriavia on June 26, 2023, 08:26:33 pmI'm not surprised. To begin with, you're using first-order filters that do not really filter a lot.But the major problem is that you think you can just add the amplitudes of the filters. You can't. Phase relation needs to be included, and that will give you a different picture.If you're using Multisim, please show the simulation plots.

author: moriavia
author_rank: Newbie
timestamp: June 26, 2023, 09:05:09 pm
content: What would be more helpful? AC sweeps with one pot at 100% and 2 at 0%?

author: pqass
author_rank: Super Contributor
timestamp: June 26, 2023, 09:05:59 pm
content: I don't think the summing amp is your problem. You need to buffer the filter outputs before the pots.You could adapt Figure 18 fromhere.

author: Benta
author_rank: Super Contributor
timestamp: June 26, 2023, 09:51:20 pm
content: Quote from: moriavia on June 26, 2023, 09:05:09 pmYep, try showing that.From my pocket calculator I see a LP at around 580 Hz if R13 and R14 are at zero (with 75% amplitude). And a 4.6 kHz HP when R14 and R15 are at zero.Set R8 to 10%.Try plotting those two situations.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 26, 2023, 10:20:04 pm
content: Quote from: Benta on June 26, 2023, 09:51:20 pmAdjusting R8 has no effect on frequency responses. Only on gain.

author: pqass
author_rank: Super Contributor
timestamp: June 26, 2023, 10:40:19 pm
content: If you buffer your filters, it seems to work. See attached screenshots.Each screenshot shows one pot (LPF, MID, HPF) in the max. position, the rest on min.The sweep is from 100Hz to 15kHz and back; upper showing the source, lower showing the output (100M load).The ALL.png shows all pots to the max which produces a bit of a lumpy output; not flat.Actually, given the Vrms differences in the outputs shown, you're probably going to have to trim/scale each stage for a normalized output.Play with ithere; see right margin for LPF,HPF,MID pot sliders, max is to the left, min to the right.