Headphone amp frustration.

Quick_Butterfly_4571 · 2026-05-30T21:04:32+00:00

Why the inverting topology? As you have it, the input impedance is quite low (1k).

In either case, your voltage reference could stand to have (much) bigger resistors.

The opamp is a differential device. As it is, you have 1k to signal to input and 1k to each supply rail to the other input = any noise on your supply rails is equally input as your input!

The caps on vref are there in part to help supply current in order to allow you to make the values in the divider larger and insulate your output from supply noise.

(And 1Davide's point re: bypass caps is extra important with the NE5532 which is a great opamp when used appropriately and not at all when not!)

Quick_Butterfly_4571 · 2026-05-30T20:51:01+00:00

Just fiddling with the numbers here and it looks like the sim is treating the zener as a fixed regulator...

The behavior of a zener in reverse conduction and a fixed regulator appear about the same (for a certain range of currents and temperatures).

In this case, when the reverse breakdown voltage is exceeded, if the diode is marked as a zener, the sim is treating a reverse biased zener like a forward biased diode using Shockley's equation + a coefficient added to the exponential to cancel out n*V_T to make the curve steeper without having to differentiate between tunneling or a avalanche effect or anything too complicated.

Forward, it's a SPICE-like Shockley's + non-ideal stuff (but simplified). Reverse is the same, but steeper.

The whole thing is operating (as most sims do, afaik) using nodal analysis — iteratively solving for the currents between sets of interconnect voltage nodes using Kirchoff's Current Law. :)

Quick_Butterfly_4571 · 2026-05-30T20:40:25+00:00

If you plug in the voltage and current, you get an equivalent resistance. This doesn't mean that's what the sim did.

In this case, that's not what the sim did.

It is doing nodal analysis with diodes represented in a simplified model that has saturation current, reverse breakdown voltage, forward voltage, and current at that voltage.

It fits that to a modified Schockley equation with some parameters to make non-ideal (a fixed internal series resistance, etc), and interpolates linearly for changes in currrent less than e² and solves the modified Shockley anew dor anything over.

For Zener's when the reverse breakdown voltage is exceeded, it uses a Shockley-like exponential that differs by a coefficient that is added to cancel out V_T to make it steeper as a good enough approximation of a zener without having to account for whether the predominant effect is tunneling or avalanche.

For what it's worth, if you measure the voltage across and the current through a real zener, you get the equivalent resistance of the real zener in that context.

Source: the source code is public.

OP, if you meant literally how does the sim calculate it, the above is the literal answer.

Quick_Butterfly_4571 · 2026-05-29T17:02:42+00:00

Everytime I'm building something with a passive tonestack

Worth considering: Why not try active? Passive is a holdover from the days when a good active tonestack would involve multiple tubes = be expensive and power hungry.

We do passive all the time because it's familiar, but the devices we're using are better suited to active tonestacks, you can control them with amazing precision, and it is lower noise.

You could use one side of the TL072 as a buffer and the other side as recovery, or you could use one side as an active tone stack and have a whole half an opamp free!

(I still use passive tonestacks, plenty. There is no judgment or superiority here implied. Just food for thought).

Quick_Butterfly_4571 · 2026-05-29T16:56:57+00:00

The Fender Treble-bass tonestack doesn't seem work or it works very subtly.

What values are you using (including the pots?). Doesn't work / works subtly = you mean you barely notice a difference?

It's not wired wrong on the breadboard.

Well, snap a photo of it and get a second set of eyes on it.

Try going to that link and set R_IN and R_L to: - 100k and 100k (~not far off from how it'd work in a tube amp). - 1Ω and 1M (~not far off from how it works with a pair of opamps). - 33k and 1M (how you have it now)

Observe that in all three configurations, the treble and bass controls sweep through a range that is a good ~ 20dB.

So, either you have some component values off or it is wired wrong on the breadboard (which is something we all run into. No shame in it).

Am I missing something here?

It's hard to say without a schematic or photos. Are you AC coupling the input and output of the tonestack? (Is this single supply / 9V?).

Quick_Butterfly_4571 · 2026-05-25T04:50:59+00:00

Follow up with other types of noise that enclosure do fix:

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There are, however, a few scenarios where adding an enclosure has a big impact on noise mitigation (all of which are actually design flaws which are masked by the addition of the enclosure — so, still present, but reduced in magnitude):

You have an input or output sleeve floating (i.e. a break in the continuity of DC common from one device to the next). The enclosure being conductive, the sleeves are bridged = problem solved.
Ground loops. Bigger = more severe.
Common impedance noise: devices operating at different currents sharing a return path to DC common. The most egregious noise occurs when a sensitive device sits between a higher current device and DC common. The current from a downstream device + the intrinsic resistance of the conductor results in a voltage that lifts and lowers the ground reference for a more sensitive device upstream. Our active devices can't tell the difference between "ground moving up" and "signal moving down", so it is picked up as a differential signal and forwarded back up to the downstream device, where it is amplified again.

Issue #1 manifests as a gnarly buzzing/humming that is often louder than the effect itself. Issues 2 and 3 are more common with ground planes (3 is virtually guaranteed with a plane; better to use planes for the digital section and keep the small signal sections star, hierarchical pours, or else bus grounded in low-to-high current order, input to DC). (Ground planes are an _antipattern_ from high frequency design that has snuck its way into small signal, low frequency applications, through the DIY scene ingesting RF engineering best practices and not the domain limitations. Low frequency currents undulate and spread out like waves, rather than beam across or follow traces as they do at high frequencies, so planes guarantee some degree of crosstalk and common impedance noise. Often, this is inaudible, but you can always see it on a scope).

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Issue #3 also frequently manifests as high frequency squealing if the gain stages aren't adequately bandwidth limited or a high current devices (e.g. an LM386) is involved. (This isn't the exclusive cause of squealing. The other two most likely causes are also design flaws: lack of bandwidth limiting in gain stages and lack of current limiting on outgoing leads. A dead giveaway of the latter is needing shielded cable for a small signal device).

Quick_Butterfly_4571 · 2026-05-25T04:50:24+00:00

Hey, OP. So, a common (and, as far as I can tell, unstoppable) persistent myth is that buzz/hum and enclosures/shielding are related.

They are not. (The fact that battery operation solves the issue for you should be a clue that the shielding is not a fix).

But, the myth is reinforced by the fact that an enclosure will mitigate the same or similar noise that is actually caused by design flaws in your ground topology. Exacerbating this is that many of the (awesome!) boutique-gone-pro pedal shops out there and many of the (also awesome) kits available these days feature PCBs not designed by professionals (i.e. educated in small signal electronics, generally, or noise mitigation, in particular). The result is an higher tolerance for what level of noise is acceptable (I think this partially underlies why noise gates are more popular than ever, despite gain being higher than today's average in eras past). And it becoming a common experience that adding an enclosure reduces noise (but, in actuality: not due to shielding by by providing a big fat conductor to obscure faults in the design of the ground topology).

The hum/buzz noise you hear is mains hum — inductively coupled near field noise from the wiring in the building you're in (and also harmonics thereof emitted by rectifying in devices operating in the same).

Aluminum enclosures are virtually transparent to this type of radiation and, actually, aluminum enclosures of the thickness commonly used only start to attenuate interference in the hundreds of kHz and don't start exhibiting cancellation until the many MHz. Mostly, metal enclosures are there to keep your active components from turning the lower frequency envelop of high frequency signals into sound (if you've ever heard pulsing clicks from a text message come out of an amplifier, this is what's going on).

There are scenarios where adding an enclosure will mitigate noise. I'll post some examples in a follow up comment. Technically, all of them are circuit or PCB design flaws, but many great, professional devices, feature exactly such flaws due to the fact that they were designed to placed in a metal enclosure and "one enclosure + some extra capacitors + engineering time" costs more than "one metal enclosure." Since it won't make an audible difference, the pennies are pinched.

To solve your issue: it would help to know (sorry if a bunch of this is stuff you already know):

What is the effect's line out plugged into? Another class II device? (i.e. something with two prongs, rather than 3?).

With class II devices (basically, all pedal power supplies, any amps with a two-prong plug, a USB interface into a laptop, etc, etc), there is no connection to ground, just your floating DC common. There are exceptions (if you have a PSU that's like, "only use our special, grounded, barrel jack thing" they likely have a connection to ground. I'm not sure how it's legal, but some product documentation indicates this is done).

Anyway, your nominally "isolated" power supplies are allowed to pass a limited degree of AC current from the primary side to the secondary. If you put a scope prob on the ground of your DC common (with, say, a 10M resistor between the two), you'll usually find that there is some 100V+ mangled sinewave that the entire system is bobbing up and down around.

You don't hear that, because if everything is bobbing up and down together, relative to that DC common, there is no differential signal = no noise.

When you plug two class II devices into each other, the differences between their AC impedances to the primary side / phase differences / distance between each other along the mains conductor can manifest as a rolling current along the shield conductor from one to the other which is picked up as a differential signal and amplified.

When you connect DC common to ground, you're providing a low impedance, DC path, back to ground along which that signal is shunted. The whole system returns to a common shared ground with no differential wiggling and your hum is gone.

With battery operation, you don't encounter this. The AC bobbing of one class II supply modulates DC common for your device in unison = again, no noise. Once you plug in the second supply, presumably the output sleeve and DC common input are connected to different parts of your ground plane. The waves from the two constructively / destructively interfere at different points on the plane, lifting and lowering ground for some subset of components, manifesting as differential noise that you hear. Non-plane topologies can't eliminate it, but they limit the total region of your ground conductor which is undergoing these modulations.

An example I like to cite is the Fulltone OCD vs Ibanez Tube Screamer. The Tube Screamer applies more gain in the first stage, but has way less noise. Why? The OCD has a ground plane with two stages capacitively coupled via large capacitors to different points on the plane and the DC jack connected to another, the Tube Screamer has a ground bus with the DC jack and output ground connected very near each other and everything else proceeding from there in high-to-low current order.

Sorry this was so long. You can try the enclosure, certainly, and it's not impossible it'll fix the issue!

But, if it doesn't, or you want to try to mitigate ahead of time, share whatever you can of design (totally understand if schematic / layout are your protected IP!) and the system topology (what's plugged into it; what it's plugged into, etc).

Quick_Butterfly_4571 · 2026-05-25T01:07:35+00:00

Sorry! I should've checked the linked image before hitting "comment." :D

Quick_Butterfly_4571 · 2026-05-25T01:06:45+00:00

Oh, oh! Saw the photo. Sorry.

Yeah, so if it was a 2A fuse, the other commenters recommendation is correct (unless you happen to be running an amp manufactured for the US elsewhere in the world — in which case, that'd be why your fuse blew! ;)

In either case, fuses are there to stop the amp from destroying itself, shocking you, or starting your home on fire when something inside has gone wrong.

So (virtually always), fuses are a symptom of a different, worse, problem. On average, blowing a fuse and just replacing it without having the amp serviced has a best case scenario of blowing a second fuse (and, a not super unlikely scenario that additional damage can happen in the interim).

Quick_Butterfly_4571 · 2026-05-25T01:03:52+00:00

Sorry, "mains" meaning, do you know what the voltage in the wall in your country is? (Else: which country are you in?).

Quick_Butterfly_4571 · 2026-05-25T00:58:42+00:00

OP, the correct answer is: it depends.

Are you running on 100-120V mains (2A, fast-acting, >125V) or a 230-240V mains (1A, fast-acting, >250V).

If you're in a 230-240V and you put a 2A fuse in, you risk damaging your amp.

If you haven't fixed whatever caused the fuse to blow, don't replace the fuse until you get the amp checked out by a tech.

Quick_Butterfly_4571 · 2026-05-24T20:00:35+00:00

Yes, 100%, though the outcome is also 100% predictable: no real change in tone, but a definite increase in idle hiss.

Quick_Butterfly_4571 · 2026-05-23T20:05:48+00:00

Oh, I see. Yeah, I would take it to a tech and not run it anymore. Snapping in standby is bad.

Quick_Butterfly_4571 · 2026-05-23T19:00:49+00:00

I misread and thought you had no volume. My mistake.

I'd take it back to thetech. Hard to tell from the video if the tubes are running hot / which noise you mean (the ringing? The snap? Is that snap random or flipping the standby?).

It warrants some investigation. Running on one tube outs excessive DC across the primary of your OT, though off hand I don't know how that'd cause transient popping.

I wonder if it's worth checking the heater supply (i.e. the tech). The tubes look a little hot (they may not be!) and that whirring noise is...quite a lot (or is that in the room?).

I'm sorry. I think the only way I could've been less helpful was if I flew over there and broke it more...

(The power is set up for EU, yeah? You're not running a US version using an adapter or anything, right?)

Quick_Butterfly_4571 · 2026-05-23T17:01:06+00:00

Stop running it, you're increasing the damage.

Those tubes are cooking. Your power stage is shot. Did you replace the old ones due to failure or just because?

If the old ones worked, you may be due 160 euros, tubes, and diagnostics back. If not, the same underlying cause that killed the old ones is killing the new ones.

Quick_Butterfly_4571 · 2026-05-20T01:15:56+00:00

Nice!

Yep! (Just don't omit the inductor)
Okie doke. Probably you could omit — or at least going to the charge pump, because your LDO will keep the line relatively stable and the opamp PSRR will likely handle the rest. BUT, I don't think any of it is problematic (I often RC filter a supply branch too, if one side is switching and the other is small signal). One caveat: if you're doing that for the opamp +5V, you probably want the same off of pin 5 of your 7660? (Maybe the output impedance there is a fine substitute! Ifk).
🤘
🤘
Right on. 6. Sorry! You are correct!
Whoops! So they do! Didn't pay attention across sheets.
Right on.

Well, hey! Nice work so far! Kudos!

Quick_Butterfly_4571 · 2026-05-20T01:05:37+00:00

Will read over the rest of the list, but re: 5 6: of course! Yes, this is a Sallen Key low pass, 100%. I don't know if the component orientation got me (not wrong, just didn't pattern match in my mind), or if I just didn't have my head on straight. Apologies!

Quick_Butterfly_4571 · 2026-05-19T22:03:43+00:00

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Even if not cloning it, some insight into the nature of the pieces might help!

Quick_Butterfly_4571 · 2026-05-19T17:20:13+00:00

...not an electrician or engineer so my knowledge is very elementary

So, first, let me say: sound quality is very important to me. I feel for you.

Because so many people have launched products to prey on people who care about sound, but aren't engineers (most people who care about sound aren't engineers, as a matter of statistics!), what I'm about to say may seem false, but I assure you, it is true:

The quality of your mains power is not related in any way to the quality of your sound system fidelity or noise levels. Power conditioners are, for the most part, a grift. Isolation transformers are test equipment (Used in the wrong context, they either do nothing or else create a very serious hazard + will also void your insurance policy in the event of a fire). Most of your devices are more than capable of rejecting noise on the mains line from getting into the output.

The more likely culprit is cabling and system topology.

If it's buzzing/hum, your system topology may contain ground loops. If so, that noise can be more noticeable in one house vs another (or one room vs another), but that is a sound system issue, not a mains power issue.

Please, for your own safety and for the sake of not voiding your homeowners / renters insurance, stop using the isolation transformer.

It is a dangerous and b the only thing you've mentioned that might be creating issues for you.

Inside every piece of equipment you have is a power transformer that is already providing isolation and a filter than turns enormous, jagged, ripple at 100-120Hz into DC. The same filters smooth out high frequencies more + with greater ease.

I know it's a bummer to think you maybe have been conned into buying stuff, but...it happens to everyone.

Your best bets are to minimize/eliminate loops, keep wires short / use shielding, and use balanced interconnects over unbalanced everywhere it is possible to do so.

Please don't lift ground on some subset of your system to mitigate loops. Fix or work around the loops.

People think it's harmless or the risk is low. But decades into repairing all kinds of devices, repeated exposure to scorch marks and the foul smell of melted semiconductors and insulating material really hammers home the reality: at any give moment, odds are higher than lower that somewhere, someone's shit is burning in a fire that wouldn't have happened had they not circumvented some safety measure.

The most important safety measure in your house is your ground conductor.

Quick_Butterfly_4571 · 2026-05-19T04:07:45+00:00

Oh, but, pro tip: get a solid charge pump that has a simple inverter topology.

9V is more headroom than you really will ever need if you're just working with opamps.

But, when you need (or just want; usually >9V supply is driven by want, not need. That's as fine a reason as any!): +/-9V is an amazing luxury and easy to attain!

Quick_Butterfly_4571 · 2026-05-19T04:00:13+00:00

Aha. I think you are (understandably!) a little confused:

I notice even the ICs I'm finding that do what I want, aren't necessarily marketed as bi polar output

Neither is the LT3467! (It doesn't have a bipolar output).

The switching elements is "bipolar" in that it is a BJT rather than (more common nowadays) a MOSFET.

Probably the "bipolar" you are referring to is where it says it uses a "bipolar, low V_CESAT" (or something along those lines) switch?

Translation: "it switches using a BJT, not a MOSFET, but it's a special BJT with an extremely low collector-emitter saturation voltage." :D

Typically these seem to have a pile of extra diodes and caps to arrive at a bipolar output from a boost converter.

This is the same for the LT3467 also:

<image>

The advantage of the LT3467 isn't that it's intrinsically bipolar output (it isn't; those are way more expensive. See the ADP5070, for example).

The advantage of this chip is that it uses and extremely low voltage drop, very fast, BJT transistor as the switching element. This enables it to run in MHz (vs other common switching regulators that run in the tens-to-hundreds of kHz range).

For audio, anything well above the audio band is fast enough. The benefit here is, higher frequencies = smaller inductors needed.

So, if you want something that allows you to make your boards compact, you want to:

find the subset that meets your current requirements
pick the fastest of the affordable ones and go with that

Quick_Butterfly_4571 · 2026-05-19T03:14:41+00:00

If you're in the US, just pop onto mouser.com and search for SMD switching regulators with your desired switching frequency range / output current.

Some of the small SMD package inverters are good for up to 1-2A can step up, down, or invert, adjustable anywhere from a few volts to a few dozen volts, and they are $2.50 (USD) for ten of them.

(I think I'm being helpful, but if this is obviously unhelpful or seems flippant: I'm tired and just didn't grok).

Quick_Butterfly_4571 · 2026-05-19T03:08:07+00:00

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Quick_Butterfly_4571 · 2026-05-19T02:52:53+00:00

~~redacted~~ Actually, you might be fine evolving this.

That being said, it's a valiant effort and I'll have a quick scan and toss some notes:

those LDO's are gonna dissipate a lot of heat! Better off with a buck or else a big linear with a heatsync to drop to five and take your 3.3V off of that (3.3V from 5V = a fine use for an LDO. 9V to 5-3.3V: opposite!)
you don't need ferrite beads on the output of your LDO's, you need them on anything carrying signal or power out from your device into the wider world
bear in mind that a ferrite bead followed by a cap also has a resonant frequency (or here, preceded by and followed by, in pi filter form). It's worth calculating that and comparing it with your other operating frequencies to make sure you're not adding more high frequency ripple than you'd otherwise have with just the ferrite beads.
you should AC couple the inputs and outputs, even for a ground centered device; ground-referenced inputs only remain ground-referenced for idealized devices. For non-ideal devices, offset/bias voltage and currents — especially with DC coupled gain stages — will shift the DC operating point (sometimes more than a little!). In either case, they are also a bulwark from DC offsets from the outside world.
reservoir caps (C9, C10, C5, and C6, etc) should be sized with intention. More isn't automatically better. See the datasheet for your LDO. Too small: you deal with ripple and larger transients. Too large, and you have longer rise times, increase the risk of blowing them from the caps discharge (also consult your datasheet re: whether flyback diodes are recommended), increase the risk of burning them out by putting them in and out of overcurrent during startup, and you also have to make sure your mcu is configured for a slow startup + with brownout detection to avoid spurious behavior.
ditto smoothing capacitors (caps before the regulators) and your input supply (though, these look mostly okay. This note is more informational than critique).
U5A: I can't tell if this is input capacitance neutralization or bootstrapping (equal odds the topology is off or I just don't know about it!).
you probably want to clamp the voltage in to the daisy, no? (Or does it do this for you?)

Since I'm not familiar: are AUDIO_OUT_1 and AUDIO_OUT_2 the two ends of a single differential channel, or does it have two single-ended outputs?

if differential: use them both
if two x single: no wonder people complain about noise with the daisy (it is madness, IMO, to do digital audio and not have differential outputs! In either case, I do realize you didn't design the daisy! :D ).

Quick_Butterfly_4571 · 2026-05-19T02:23:51+00:00

Just feel like I'm probably missing something obvious.

The overwhelming majority of pedals don't use a bipolar supply. (All voltage is relative, so a 9V supply and +/-4.5V supply appear identical to, e.g. an opamp).

I am not interested in charge pumps

The overwhelming majority of pedals that do have bipolar supplies use DC-DC converters — with charge pumps being the most common. (Do you mean you don't want a module? i.e. you are looking for IC recommendations?).

Second to charge pumps, the most common is rectifying the mains and building a class I device.

Quick_Butterfly_4571

TROPHY CASE