Can Someone Tell Me if This EQ Circuit Looks Okay? - Page 1
Forum section: electronics / projects

author: hummusdude
author_rank: Regular Contributor
timestamp: July 07, 2020, 01:56:46 pm
content: I'm working on adapting this 5 band active EQ circuit that I found online for stereo and I'm wondering about a node that seems wrong. It uses NE5532 op amps with a single supply and the author shows a connection from the output of the input buffer stage to virtual ground. The design uses multiple-feedback band-pass filters but I thought the multiple part referred to the inverting input.Also, they seem to have gone out of their way to separate signal ground and power ground. I know this is important, at least for power amps, because of ground loop hum issues. Is that also true for low-gain circuits like this?

author: blackdog
author_rank: Frequent Contributor
timestamp: July 07, 2020, 02:08:50 pm
content: Hi hummusdude,All opamps are set to half of the power supply, thats OK, but the capacitor in the inverting input off the last opamp to ground is wrong...C24 of 0,1uFThe resistors for making the virtual ground are a bit on the high side with their value of 100K, I think more like 22K for bipolar opamps and decouple the node of this 2x22K with 220uF.I don't know if all your opamps are properly set for your equalizing frequencies.Kind regards,Bram

author: hummusdude
author_rank: Regular Contributor
timestamp: July 07, 2020, 03:21:45 pm
content: Thanks Bram. I did the calculations on the cutoff freqs and they seem good. You mentioned C24 is wrong...is that because it will ground the ac output?Also, Do you think it's okay if the output of the input buffer (and hence all of the filter inputs as well) is tied to virtual ground (circled in red on the schematic)? That seems odd to my inexperienced eyes.thanks for the help!!

author: Benta
author_rank: Super Contributor
timestamp: July 07, 2020, 04:13:56 pm
content: Agree with blackdog, but I see more issues.- R1 and R2 to are way too large, reduce to 10 kohms.- The purpose of R3, R4, R6 evades me. They'll skew the DC level of the signal.- C2 should have a helper cap for high frequencies in parallel, I suggest 100 nF .- No reason for R8 and C3, all amps are at the same DC level, provided you redesign the R3, R4, R6 stage.- C24 makes absolutely no sense.- The connection you've circled is a schematic error, the dot shouldn't be there.

author: Yansi
author_rank: Super Contributor
timestamp: July 07, 2020, 04:18:28 pm
content: This kind of bandpass-slaptogether topology will have absolutely horrific response, that can't be even properly zeroed out.Do not build.Find a different topology, that uses gyrators.//Such as this one - but verify correctness of the schematic first! I haven't checked, just providing an example of a correct approach:http://www.hobby-circuits.com/circuits/audio/tone-and-volume-control/352/6-band-graphic-equalizer

author: Labrat101
author_rank: Unknown
timestamp: July 07, 2020, 04:24:53 pm
content: Hi I have added the schematic of the TEAC EQ5 which is old skool and works correctlyif you require i can upload the full pdf . its a 10 band . Stereothere are lots of schematics of Teac EQ which are tried and test as good design .This should put you on the right track . Your schematic is wrong as mentioned by the others.RNS

author: Benta
author_rank: Super Contributor
timestamp: July 07, 2020, 04:32:53 pm
content: @Yansi, yes the response won't be nice, I agree. Whether gyrators are better: I don't know.@Labrat101: Nice! One should perhaps add that the complete right side of the schematic is an orgy of flashing LEDs

author: Labrat101
author_rank: Unknown
timestamp: July 07, 2020, 04:45:40 pm
content: Quote from: Benta on July 07, 2020, 04:32:53 pmSorry the top left is one channel and the bottom is the other channel . The right side is the led Matrix indicators . its the entire circuit .Its really a very simple circuit not much in it all op amps and a few transistors GP.The PDF is a little too large to load its 5.2m and 5m is the limit I will break it down or PM

author: Yansi
author_rank: Super Contributor
timestamp: July 07, 2020, 05:07:34 pm
content: Quote from: Benta on July 07, 2020, 04:32:53 pmSure they have to be tuned (Q set) correctly too, but at least, achieving flat response with that topology with zeroed pots is dead easy. If the response with maxed out bands is not exactly flat (it won't be) is not that much of an issue for typical audio work, unlike the flat response which is mandatory when the pots are all in the mid position. Otherwise the EQ is unusable for anything past a guitar amp.

author: hummusdude
author_rank: Regular Contributor
timestamp: July 08, 2020, 03:47:37 am
content: wow... so much great info...thanks to all for your replies! I'm dropping this one no question.Labrat101...might you have a schematic for a 5 or 6 band eq? I know ten bands are considered minimum for actually balancing the response of the room but it's a bit beyond what I had in mind for this stage.

author: Labrat101
author_rank: Unknown
timestamp: July 08, 2020, 02:51:56 pm
content: Quote from: hummusdude on July 08, 2020, 03:47:37 amjust remove the unwanted bands from the schematic each slider has the frequency above it just take your pick . Seven is normally the bareminimum . maybe remove the two higher frequency if your hearing is going . any less it would not be a good EQ ..Also changing the scale on EQ the whole range shifts every frequency will have a reaction to the next .If you want to go mad you could replace all the sliders with Digi pots and have it controlled via arduino and softwareBut that would spoil the idea of a EQThat is for you to do I am Hardware only Man .Have fun its a really simple project with no rocket science involved . all the transistors are General purpose and any good Op amp as its Audio .RNS

author: hummusdude
author_rank: Regular Contributor
timestamp: July 10, 2020, 12:23:33 am
content: That makes sense. I was thinking about using a Dpot for a volume on a pre-amp but I probably won't do it on an eq since I tend to set it and leave it.Thanks again for you advice!

author: Labrat101
author_rank: Unknown
timestamp: July 10, 2020, 09:27:51 am
content: Quote from: hummusdude on July 10, 2020, 12:23:33 amI have loaded the parts list and the circuit diagram as pdf .. Box Not included ...Its from the original papers .. Page 3 has all the op amps and pinouts ..will give you all you need .. Have Fun its a nice weekend project . and keep out of trouble in the lock-downRNSPs Digi Pots can be set and locked via the program pin so they return to that set point .. its on the manufactures Literature.The LED display can be replaced with a VU meter .The unwanted controls put pre sets set at 50% so they can be upgrade in the future .

author: hummusdude
author_rank: Regular Contributor
timestamp: July 10, 2020, 01:38:32 pm
content: Thanks Labrat...this is most appreciated!