Loudness Contour Help - Page 1
Forum section: electronics / beginners

author: chippieboy
author_rank: Newbie
timestamp: September 16, 2025, 12:28:22 am
content: I am first time posting here asking for help explaining how this works (even though that question is forbidden, I’m not sure how else to ask) and what components to change. I need more effect than it currently provides. It follows the typical Fletcher Munson curves. Current curves are depicted using Fuzz Measure software, just not quite enough effect.Now the back story if at all interested:Back in the 80’s when I was in my 20’s the market was phasing out the Loudness option so I built this external Loudness Control to go between a Preamp and Power amp (or tape monitor loop). I felt the existing On button was too much compensation and the Off was nothing so I did a variable. I love this thing and still use it today. Since I am nearing my 70’s I need to modify it to compensate for my hearing deficits. For the life of me I can’t remember much about it and can’t locate any of my documentation.

author: Analog Kid
author_rank: Super Contributor
timestamp: September 16, 2025, 12:50:33 am
content: Interesting. I'm like you here, not sure exactly how this works, although now that I turn that pot in my head I can see that you end up with a low-pass filter combined with a high-pass one.My guess®™ on how to get more effect? Reduce the value of the potentiometer, to 20K or even 10K. That should make the low-pass filter more effective.Others here will be able to give you more definitive answers.

author: golden_labels
author_rank: Super Contributor
timestamp: September 16, 2025, 03:41:52 pm
content: Welcome to the forum! I hope that the utterance about “a question being banned” is just some kind of a misunderstanding and not any kind of hostility.If there is such a possibility, first I would try to ditch the filter altogether and replace it with a graphic equalizer at the source. If you play music from a computer, your system or audio software likely has one. If that’s a different kind of a player, even the built-in 3-range equalizer (bass/mid/tremble) suffices. With these you can set whatever curve your imagination demands. I also believe most will offer better audio fidelity.⁂The circuit may be hard to understand, because the border separating two functional blocks goes straight through one of the elements. Through the potentiometer to be specific. Let’s start with rearranging the schematic a bit to make it clearer what is what.VR1 is split into the upper and bottom parts, R4U and R4B respectively. R3 is moved over C3. Changes nothing electrically, but groups all resistances together for better clarity. Finally, cluster things into their functional groups:Now it should be much clearer, what the circuit is: a notch filter. R1, C1, R4U form the high-pass part, with R4U acting as a controlled bypass. R2, R3, C2, R4B form the low-pass part, with R4B controlling series resistance. Since R4U and R4B are coupled, increasing R4U makes more signal go through the high-pass path, but at the same time R4B is decreased, making more signal go to the low-pass path too. Effectively balancing both “just right.”It should now be clear, why I shifted R3 over C3. This makes replacing the R2, R3, R4B combo with the equivalent resistance an obvious operation.To confirm this is the same circuit, here are calculated gains for R4U and R4B being set to represent the tap being at the maximum (top red line), center (middle magenta line), and minimum (bottom cyan line). They roughly match your measurements:I assume that in the real physical system the bottom frequencies get some attenuation from some setup stage blocking DC. The circuit is also not loaded with 1–50 kΩ resistance it would normally see, if you feed it to the power amp.⁂I believe you can’t do much to make this circuit(1)to produce a more pronounced effect. Both the high-pass and low-pass segments are first-order filters. That means the valley can’t go down faster than 20 dB/decade and it already is close to that. Note that the chart goes from 2 Hz to 200 kHz, one decade more than hearing, to better show filters’ shape.You may increase the potentiometer value. That will shift the cut-off frequency of the low-pass filter to lower values. The gap between filters will widen and, in consequence, make the valley go deeper. But this way you reduce low frequencies by approximately the same amount as what you win in valley’s depth. The 20 dB/decade slope still holds. You also increase series resistance of the whole path, which means the entire signal goes way lower after a load is connected and will require more amplification later. Here are simulated charts for 50 kΩ, 100 kΩ, 200 kΩ, and 500 kΩ (from top to bottom, red, magenta, cyan, white):(1)I assume that by “modify” you mean tweaking a part or two, while keeping the entire design basically the same.

author: jwet
author_rank: Frequent Contributor
timestamp: September 16, 2025, 03:52:38 pm
content: I'll give this a try.First, since its a passive network, it has no gain, this means that it can't "boost" anything. It gives you its effect attenuating everywhere else and then making up its loss with a bit more flat amplifer gain.\The top path is a high pass network. Its set at a high frequency (15Khz). This give you a bit of brightness. You might consider taking this cap value down a bit and making the break point at something more like 10 Khz, this would make C1 about .0015 uF.The other path is actually a mid cut that comes on at about 3Khz. The pot can sum this in to the high pass path to give more or less mid range at your taste.This all adds up to enhanced brightness and more or less attenuated mid range- enhancing low frequencies once a bit of flat gain is added.You can play with frequency breakpoints a bit to tailor it to your aging hearing but you can't do much with passive filters. The next step would be some kind active parametric or graphic equalizer if you want more control.I see that golden_labels posted as I was posting. It looks like a very good answer and I probably wouldn't have answered if it was there when I started. My alternate explanation might help since I've already invested the time.Take care.

author: Analog Kid
author_rank: Super Contributor
timestamp: September 16, 2025, 07:40:54 pm
content: It would be really nice if at least sometimes people responding here would actually answer the question that was asked, instead of immediately morphing the discussion by introducing alternative solutions.The OP wroteQuoteSo with that in mind, how could they modifythe existing circuitto accomplish their goal? (If that's possible.)Yes, we all know that this is a passive filter that will attenuate the signal. I'm sure the OP is aware of this as well.They probably also know about graphic equalizers and such, but perhaps don't want to go that route.Later on y'all can pile on with all the alternatives you care to come up with. But first let's try to answer the question posed, 'k?

author: golden_labels
author_rank: Super Contributor
timestamp: September 16, 2025, 07:55:12 pm
content: Thanks for the advice. Now I have one of my own: read posts to which you reply. It appears you didn’t read a single one in this thread.

author: jwet
author_rank: Frequent Contributor
timestamp: September 16, 2025, 07:56:40 pm
content: Agree- I also gave guidance for mods but you can't do much. Before I post, I try to ask myself, am I bringing any value?

author: Analog Kid
author_rank: Super Contributor
timestamp: September 16, 2025, 08:04:34 pm
content: Quote from: golden_labels on September 16, 2025, 07:55:12 pmNo, I read your pet project there.Nice circuit, well explained.But not what the OP asked for.

author: golden_labels
author_rank: Super Contributor
timestamp: September 16, 2025, 10:02:56 pm
content: In the past 24 hours it’s a second thread in which you show some nonsensical reaction, accusing other users of what they haven’t done, and clearly merely glancing over just a few words of their posts. If you’re drunk, consider taking a nap and not using the keyboard for a day. If you are not drunk, wait a for a day, come back tomorrow, and read carefully what all users involved wrote. You have 5 posts from 4 users to analyze. Including OP in this thread. 🤦

author: themadhippy
author_rank: Super Contributor
timestamp: September 16, 2025, 10:17:52 pm
content: Quotehave you lost the abilaty to read ?or do you just like jumping in feet first? the op askedQuoteBoth golden_labels and jwet gave an explanation of what the circuit is and how it works along with suggestion of components and values to change and how they would affect the circuit,not just some vague guess work about reducing the pot value,maybe.

author: Analog Kid
author_rank: Super Contributor
timestamp: September 16, 2025, 11:45:56 pm
content: Quote from: golden_labels on September 16, 2025, 10:02:56 pmFuck off, OK?

author: chippieboy
author_rank: Newbie
timestamp: September 17, 2025, 03:25:01 am
content: 1stThis was quoted from “NEWBIES PLEASE READ BEFORE POSTING”"How does this work questions are not proper forum etiquette.”That is where I thought my posting might be banned. Looks like my question might be OK after all based on all the replies the first day or so.Thank you all for that.2ndI do not want to use an Equalizer. I have played with “REW” and have a “Mini DSP” and have an 1970’s Analog EQ. For me, an EQ shapes the sound all levels. At higher listening levels the Treble is a bit too bright and the bass is way too heavy. The Loudness Device works best at lower volumes (as in night time listening) as the volume raises the Loudness effect get’s less. I’m not sure if that is by design or something else. Even with the OEM equipment that is how it behaves. I used Yamaha preamps for years just for that option but hard to repair now a days. Last one I sent to the shop took 14 months (and I am in the heart of the Silicon Valley) so I went down a modern path. Now I long more for the Loudness Device.3rdI love the re-draw by “golden_labels” makes more sense to me. I sill do not recall why I did the 15 KHZ bypass, must have been a valid reason but I will keep it for now and play with adjusting the cap. And yes, I assume that by “modify” you mean tweaking a part or two, while keeping the entire design basically the same. That statement is true.I assume that in the real physical system the bottom frequencies get some attenuation from some setup stage blocking DC. This device now goes between the Pre-Amp output and the Main Amp input. I will guess I do not need any DC Blocking and/or some resistance for impeadance matching. Please clarify.4thI have a 100 K pot I will play with. BTW, any suggestions on where to purchase decent pots, caps and resistors? I got the other pots from DigiKey. Not the best quality. I destroyed my existing pot removeing the knob.5thI am aware that is passive and will attenuate everything. I have have an active unit that I remove from a bad Yamaha pre-amp. The active option takes away some of the musics magic so I don’t use it.Please, please, pleses keep the ideas coming !!!

author: golden_labels
author_rank: Super Contributor
timestamp: September 17, 2025, 05:32:31 am
content: Quote from: chippieboy on September 17, 2025, 03:25:01 amI can’t say that with any authority, but I feel like Dave meant the kind of questions where a person wants a circuit to be explained with zero effort on their end. You did tests, you know and understand what is circuit’s observable behavior, described your own modifications. I don’t think the post falls into this category.Quote from: chippieboy on September 17, 2025, 03:25:01 amThat this filter can only attenuate is clear. My warning is about making it attenuate too much. We may put arbitrary values in our mathematical model and draw pretty charts, but then comes the reality. A circuit will be of no use, if its impedance is so high the filter reacts to you changing position in a chair or it sinks signal below noise of your amplifier. Even if the theoretical charts are beutiful. Right?Quote from: chippieboy on September 17, 2025, 03:25:01 amTest jwet’s and my ideas first. See if either fits you.If not, you may amend them with one more change. Reducing R1 will let more power on the high end to pass. The outcome will not follow the equal loudness curve! But the part of hearing suffering most with age is high tones. Maybe boosting them a bit is a good idea. Something from 5.6 kΩ to 2.7 kΩ? I wouldn’t go lower than that. While in a theoretical model tremble never goes above bass, in your actual, physical system the lower end is already attenuated a bit.

author: EEVblog
author_rank: Unknown
timestamp: September 17, 2025, 08:26:10 am
content: Quote from: Analog Kid on September 16, 2025, 11:45:56 pmThis is not appropriate, especially in the beginners section. Please remove yourself from this thread.

author: EEVblog
author_rank: Unknown
timestamp: September 17, 2025, 08:28:24 am
content: Quote from: golden_labels on September 17, 2025, 05:32:31 amYes, this rule is basically for those who just want people to do their homework question for them.The OP's post definitely isn't even close to this, welcome

author: jwet
author_rank: Frequent Contributor
timestamp: September 17, 2025, 02:36:06 pm
content: This has been entertaining if not enlightening. My goals on this board are simply to help where I can. I enjoy electronics problems.Part of my duties in my first job was in sustaining engineering and manufacturing support at an industrial instrument company. My boss, a consummate mentor, thought this experience should be part of any journeyman engineer's toolbox. One of the common memes that developed was upon going out to the line to investigate a problem was something like "we're having a problem with this run". We'd like you to fix it but you "can't change anything". This became sort of a joke. I came to understand the perverse nature of the world this way.Here are a couple of ideas that I would try most likely. I think with golden_label's analysis, you have a near closed form solution to the operation.1. I understand your reluctance to use EQ and active filters. Consider using one only as a test bed, not to stay in the circuit but to help determine what sounds good to your subjective hearing. Once you determine what is missing, perhaps you can return to your circuit and golden_labels analysis and realize the filter. You could do something similar with some RC sub boxes and some experimentation if this appeal more.2. One of the difficulties with passive filters is that without amplifiers, you don't have reverse isolation and everything talks to everything else. You end up with all the dipoles coupled and having interactions, etc that can be seen in golden_label's analysis. We haven't even talked about twisting up phase and those interactions. With modern simulation tools and careful design you can de-sense for this to some extent. I think if you do step one, you can arrive at a "tweaking" plan to get to what you'd like with something a lot like your original design.My initial statement to remember that this is a passive filter and can't boost may seem trite. I was an Analog FAE and strategic apps director at Maxim for many years. I try not to assume any basic knowledge and like to put some of these basics on the table early to establish some common ground.Good luck with your circuit. I'm approaching 70 myself, my hearing has deteriorated badly- a lot of dubious decisions made in my youth probably. We baby boomers have to stick together- take care and good luck.

author: golden_labels
author_rank: Super Contributor
timestamp: October 05, 2025, 10:17:28 pm
content: I know it’s 3 weeks later, but in my condition things go on their own, slow pace. I hope it’s still relevant.I tested that with an actual circuit. I don’t have equipment to make a professional test, so this is basically my built-in sound card and some Python scripts.With 56 kΩ, 100 kΩ, and 470 kΩ for R4U (and therefore 0 Ω for R4B), here are the results. As predicted, in a physical, soldered circuit driving a finite impedance (the line-in input) we lose a lot on the low end.In the other attachment the same charts, but with a high-pass and low-pass parts separated.Ignore the 10 Hz part and falloffs on the sides. These are artifacts from how I was doing alignment and calibration.

author: chippieboy
author_rank: Newbie
timestamp: October 12, 2025, 12:13:49 am
content: Actually it is still very relevant!I have been hoping and waiting for more responses. I am very impressed and grateful in your test and response. I can actually see the difference the Pots make. After reading your findings I am a lot more motivated in this project. Thanks golden_labels

author: golden_labels
author_rank: Super Contributor
timestamp: October 12, 2025, 08:30:16 am
content: Note I did not use potentiometers. These are fixed resistors replacing a potentiometer at its most extreme setting. The “maximum” effect one may get. As opposed to just passing signal on the other end of the setting.Graphs are still only graphs. I offered them to not mislead readers with pure mathematical models. The ears will judge. These must be your ears, as it’s your taste.