I've built a repro of the H.B.E. Detox EQ and I like the way the tone stack sounds. But I'm confused about some bits at the end of the schematic.

Given this:



After the second opamp stage....

C10 seems like a bog-standard bypass cap, and R15 is to reduce the output before the pot.

But what is the point of R13, C9, and R14 and R15?

R13 is redundant..
(left there from another curcuit without other output series resistance..)

C9/R15 form a HPF..

C10/R14/VR4 form another HPF..

ευχαριστώ Antonis!   I suspected the 27R as an error, and thanks for the hints on the other two.

Those HPFs might seem complicated but if you ignore R13 (due to its extremely low value, compared with the other resistors), consider U1B output almost GND and C9>C10, then corner frequencies are 0.159/C9*R15 and 0.159/C10*(R14+VR4) respectively..

Given that the corner frequencies are so low (both <1Hz) that they don't actually have any effect on the sound, you don't need both of them. I'd get rid of R13, C9, and R15. The result will be sonically identical.

I wonder if there was some 'error in transcription' in the schema.  A HPF at the end, well that might be ok - but not at < 1 Hz! Wrong decade resistor?

 And then followed by the variable thing working with the level control...wazzat?  Odd.   

Quote from: PRR on June 03, 2024, 08:29:50 PM

Lol, nice.

Makes you wonder about the rest of the circuit, doesn't it? How many unnecessary parts are there in there?

Thanks gents. I can't speak to the original schematic's accuracy or lineage so these comments make sense.

I've seen and drawn enough schematics to smell that something is off, but not strong enough in circuit analysis to know why the smell is there

Quote from: PRR on June 03, 2024, 08:29:50 PM

Do those parts bring me joy? No. So I shall declutter.

You might find that neither R1 value brings you joy..
(try to make it bigger for more happiness..)

Also, move C1 across R3..
(more "quiet" input in the cost of VRef "polution"..)

Quote from: ElectricDruid on June 04, 2024, 06:49:37 AM

Quote from: PRR on June 03, 2024, 08:29:50 PM

Lol, nice.

Makes you wonder about the rest of the circuit, doesn't it? How many unnecessary parts are there in there?

It's definitely not a just this schematic thing either. Because I'm insistent on using a dumb layout for my builds, I'm always looking at what I can do to simplify/etc and make things fit nicer.

I think it's probably due to the fact a lot of things are borrowed from existing pedals with different stages, but just SO many schematics have absolutely useless parts - filters that have no benefit, doubled or even tripled coupling caps (that are so big they have no consequence on audio or noise), low-gain stages that are immediately followed by a voltage divider to cut out whatever was just boosted, etc.

It all makes sense when you look at what some of these snippets came from!

Having asked the (at the time, mostly rhetorical) question, I did actually have a proper look at the rest of it, and ironically, it's actually all pretty much spot on. About the only thing I could argue with is the R1 value that Antonis also noticed and mentioned already.

But I totally agree - it's definitely not just this schematic. You often see circuits where two or more snippets have been naively stuck together, with both an output coupling cap from one and an input coupling cap from the other. I call that style of thing a "frankencircuit". It might rise from the slab and live, but it sure ain't pretty! 

Quote from: ElectricDruid on June 04, 2024, 06:49:37 AMHow many unnecessary parts are there in there?

Why do you care?

If you build millions, the price per part approaches zero. SMD resistors especially.

If you build just one, the price per part is often much-much less than the value of the brain-pain to figure which parts are not needed.

Yes in DIY you do not charge yourself for thinking but it still has value. Hard to evaluate, but perhaps similar to the time we have spent discussing in this thread. We have *about* 1,000 words invested so far. I don't have a rate-chart for thinking+writing, but writing alone goes around 1,000 word per hour. Working alone is less time writing but more time thinking. So say we have 'thought' for about 1 hour. What are wages today? McD's offers $15/hr here for flipping. Targeting thinking should be worth more. Can you think-away $20 of parts in an hour? Not in a one-off DIY economy: the whole guts may not be $20. (Yes, the assembly of pointless parts is another work-cost and easier to balance against thinking-cost.)

Unnecessary parts reduction IS vital in medium production. Cars and planes, semi-fancy TV sets. These products can support hard-thinking designers but not radical production machinery.

(FWIW, my 2023 Toyota engine probably has more parts than my 1967 Mercury engine. 4-valve, variable cams, variable oil pump, two fuel injection systems.....)

(FWIW, Intel no longer counts how many transistors are in a CPU, so probably not how many are useless.)

You should keep either R13 or R14.  Something around 470 ohm (a precise *minimum* value depends on the the specific opamp).
It stops the opamp oscillating with capacitive loads ie. when driving cables.

https://ww1.microchip.com/downloads/en/appnotes/00884a.pdf

Quote from: PRR on June 05, 2024, 12:29:10 AMWhy do you care?

Because I want to create something beautiful! That phrase about "Perfection isn't when there's nothing left to add, but when there's nothing left to take away"? That's pretty much how I feel about it.
It doesn't have anything to do with the cost of components or the time spent thinking about how to get rid of them. It doesn't make "economic sense" (although economics is used to justify all sorts of nonsense). Luckily for me, I'm not in this for the money!

As far as caring, I suppose it depends on your perspective.

- Cost Conscious Capatilist? Be like Mike Matthews who used the cheapest components he could find in the bin. (Cf. the cap values use between BMP versions over the years) Don't include anything that is absolutely not necessary for the minimum viable product. Re-use the same building blocks over and over again because you already paid the design costs.

- Design Perfectionist? Agonize over the design details even though some of them may never affect the outcome. Choose parts on utility, performance and/or mojo for art's sake. Fret about the discourse on forums around your design and how it will be discussed.

- Wow-it-worked builder? Be amazed that your amateur design efforts yielded a device that worked within you rough design goals, thank the maker, and get a PCB made quick before it doesn't work anymore.

- Perenial learner and non-pro? Take all the feedback from threads such as this as valuable and become non-dumber with each thread.

I'm quite sure I've dabbled in each of these categories

In some cases, cost consciousness is not the main reason for eliminating "extra" components from cluttering up a design.  In some cases, the amount of space taken is important.  In some cases, board layout is simplified.  In some cases, being able to account for the effect of the components matters.  In some cases, reliability can be an issue that improves with fewer components.  I am not a believer in mojo as a design consideration.

There are a lot of considerations in a design and a lot of reasons for doing something different.  Otherwise, we wouldn't have the number of design topologies we often have.

Quote from: PRR on June 05, 2024, 12:29:10 AMWhy do you care?

isn't that what we do? if not us, then who?

Quote from: ElectricDruid on June 05, 2024, 06:04:10 AMLuckily for me, I'm not in this for the money!

Phew!  Thank goodness for that - I'm not alone!  If I were in this for the money, then building well over 100 pedals that I don't need and don't use would be really, really stupid...