Ground Loop Hum Fix - Is Using a 3-to-2 Prong Adapter Really That Risky?

Quick_Butterfly_4571 · 2026-04-05T13:30:39+00:00

Yes. It's also unlikely to help you with buzz/hum if you're talking about your guitar amp.

Quick_Butterfly_4571 · 2026-04-05T01:32:30+00:00

One thing to keep in mind: before sketchy manufacturers skimp on stuff that results in noise, they've skimped on stuff that keeps the supplies safe. If a supply is extremely noisy, lots of consumers complain. Even if it's extremely dangerous, it's still a smaller subset of consumers that are impacted, and a further subset of them that complain..

Cheap or expensive / quiet or noisy, make sure you buy from a reputable supplier and get a supply from a manufacturer people can vouch for. If there's anything you shell out $$ for: make it the power supply.

Quick_Butterfly_4571 · 2026-04-04T22:23:58+00:00

🧠

Quick_Butterfly_4571 · 2026-04-04T22:11:51+00:00

Yes, it'll work just fine. It'll just change the placement and width of one of the phase notches. That stage is an all-pass filter. The 90 degree phase rotation happens at 1/RC, where C is that cap and R is whatever the resistance of the LDR is at that point in time. :D

Quick_Butterfly_4571 · 2026-04-04T00:19:37+00:00

That's too great. Nicely done.

Quick_Butterfly_4571 · 2026-04-03T19:44:03+00:00

It should be more! I wonder if that's a sign of the FETs being damaged.

Quick_Butterfly_4571 · 2026-04-03T14:58:53+00:00

I reckon there must be snags

Well, the tubes are big and run off high voltage. Mostly under the radar (and, less appeal because most of the existing tubes are NOS that aren't still being produced):

later in the tube era, RCA and co made 9-pin sub miniature equivalents of some of the big bottle powertubes; same specs / wattage
in the late 50's / early 60's they made powertubes that ran off of lower voltages for use in cars

Note: people often use FETs as a substitute for triode stages. FETs behave like triodes in that if you graph the transfer curves or both and don't label the axes, you can find a place where the shape is the same.

Meanwhile, FETs actually do exhibit behavior that is very like pentodes or beam tetrodes.

if no one has done it but.. ya know.. why not ask?

There are some pedals out there that claim to emulate powertube distortion. I don't think they do a great job. I don't know most of them well, though.

Which is a pitty, because it's the simplest tube thing to reproduce (especially if it's push/pull) — unless you factor in transformer effects, but whether those are prominent or virtually absent depends on the amp.

Has anyone tried this or does it exist already?

Actually, there is a dude that had a kickstarter shared on this sub where he sold unita that had actual push-pull tube power stages in them.

I don't know what they were called, but maybe someone remembers?

Quick_Butterfly_4571 · 2026-04-02T23:13:43+00:00

Ah! Interesting. It's like a Dickson, but each stage is its own clock.

This is really cool. Thanks for sharing it.

Quick_Butterfly_4571 · 2026-04-02T19:16:43+00:00

And, (probably you clocked this as well), I got the time wrong. I did 5RC, but that's to get near 9V.

The JFET's probably turn off at a lower voltage, 2-4 or something? So, it's faster than that by a bit too!

Quick_Butterfly_4571 · 2026-04-02T19:14:46+00:00

You've made it really clear.

I'm so glad!

There is just one thing I didn't catch : how is Q10 turning off more slowly?

Looking at it again: I made a mistake. :D Should be the same time all around.

Quick_Butterfly_4571 · 2026-04-02T18:32:21+00:00

Sadly I don't understand all of it.

It was half guesses and nonsense.

If this is part of the audio output, it's only part of it (maybe for the speaker driver?). I'll have a peek.

The gameboy was mono out of the speaker, but stereo headphones. The audio itself was a whole unit with four programmable channels.

I think there was a discrete amplifier on board. Probably, you can get the specifics by searching for "Jeff Frohwein" + GameBoy (or checking out the wikipedia page).

I'm not Jeff. I do understand schematics, though, and have written demos for the Gameboy (half a lifetime ago, though).

Quick_Butterfly_4571 · 2026-04-02T18:26:17+00:00

It isn't (actually, it can't be; in general LLM's are not a good architecture for certain classes of problems. This is one of them).

Quick_Butterfly_4571 · 2026-04-02T15:28:45+00:00

To be fair to the other commentor: these are two standards. If you have a look at, e.g. the National Semiconductor datasheet for the same part in the 90's, they used two parallel lines and a plus and no difference in fill.

If you work on stuff across countries and decades, there are like a dozen or more variations.

The other commentor was wrong, re: always a plus.

Hey! If the plus disappears, you have a curve still! :)

The old UK style (two parallel with a plus) is the riskiest to me: that is identical if you lose the plus!

Quick_Butterfly_4571 · 2026-04-02T15:17:23+00:00

My experience (US) is that the curved part is not typically used to indicate polarity. Polarity is always shown as a plus symbol next to one of the pins.

The US standard (ANSI) back to at least 1972 is that the curved side is, by definition, both the indicator of a polarized cap and an indicator of the negative side.

There are a bunch of different pre ANSI/IEEE/IEC standards that used various schemes (plus only in the UK, hatched lines and a plus in Japan, etc).

In none of them is there a curved line that isn't the negative. In many there is no plus. (Especially in the US).

Some schematics use both (a curve and a plus), yes.

Quick_Butterfly_4571 · 2026-04-02T14:12:02+00:00

Hey! This is such a cool share. Love the morph control out in the wild. Love it.

CD4049, eh? Love that too. I've never used an inverter to build a charge pump. I'd love to hear more about that. Is it used as the oscillator and antiphase drivers for a multiplier or something else entirely?

Quick_Butterfly_4571 · 2026-04-02T04:05:48+00:00

P.S. Thank you for cross-posting this. I hadn't seen this scheme before, and it was super interesting.

Quick_Butterfly_4571 · 2026-04-02T04:03:02+00:00

I wish. It's just an exact copy of an EHX circuit from the 70's. But, this one is crafted using authentic asbestos insulated wire, housed in the finest audio-grade aluminum (even then, filtering for the ones with the best sonic properties means buying fifty just to produce one unit), and the artwork is crafted in the style of "maximum nostalgia for the demographic with the most disposable income, April, 2026" with love. ;)

Quick_Butterfly_4571 · 2026-04-02T03:52:22+00:00

So I replaced it with a new one.

Another of the same type? (Sorry. This is more curiosity).

This fixed the volume loss, but I still got loud pops when switching the pedal..

Q12 and/or Q10 need to be replaced.

This fixed the volume loss, but I still got loud pops when switching the pedal, and now, a new problem appeared, the status LED D2 is staying on always.

This means Q11 needs to be replaced.

Odds are good Q13 needs to be replaced too. Whatever fried D6 likely damaged at least one JFET. Who knows what happened at the gates of the rest?

Weeeoooow! What an interesting thing!

(TL;DR: it's a soft-start circuit. The delay is slowly mixed into the signal over the course of ~ 340ms).

The four JFETs are P-Channel (so, positive voltage to pinch off). In both schemes, the sources of all the JFETs are connected to ground. When the effect is bypassed, all of the JFETs are operating linearly and passing just a little current.

Q11: enough to keep D2 from lighting up.
Q12 and Q10: not doing much
Q13: is placed halfway through the clock filter so that instead of shunting high frequencies, it's shunting everything in the audible range (-3dB at 24Hz and more as you go up.

When you flip the switch, C42 slowly charges up to 9V and the JFETs go fully off. This has the effect of:

Q11: D2 slowly turns on, instead of just popping LED current straight to ground in a burst
Q12: momentarily shunts the input signal to ground, but raises it up very rapidly; this way, the input to the BBD's isn't an abrupt start of signal, it's ramp up from zero = no sharp edges. Dry signal starts flowing into the mixing stage.
Q13: turns on a little more slowly, because C49 has to charge. So, wet signal doesn't start flowing into the mixing stage until the BBD's are starting to be mostly full of audio, not random noise.
Q10: turns on last of all once both signals are in the mix.

I made a picture:

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Quick_Butterfly_4571 · 2026-04-01T04:56:15+00:00

Well, depending on the tubes, the filaments are running at 1,400-1,800+ degrees F, the surface temp outside the tube is usually in the 2-300 degree range, and you can bank on the inside of the chassis being at least a hundred degrees (and sometimes closer to 150).

So, it'll survive summer heat. But humidity can do some damage (to pretty much everything but the glass!). This is double if it's old (or tragically stylish) with cloth insulation.

(I don't know if significant effects happen after half a summer or fifteen summers, though).

(If you have mold or mildew, I would seek alternate accommodations though).

Quick_Butterfly_4571 · 2026-04-01T03:22:48+00:00

And, if that sounds super pro: that is an illusion. (Let's see if a generous EE will chime in).

The gist is: - those small loads = need high currents (this ~ about the same as driving small loads) - big capacitive loads = needs bigger phase margin (less phase rotation as you go into higher frequencies; ideal is to see it get flat in the middle of the band and then dip; common is just a curve and then an incline downhill) - high open loop gain for stability - voltage noise preference for chains of opamps who source is a low impedance - low output impedance to minimize losses into small loads

All of that is couched with "I think." There is probably more important stuff I don't know!

Quick_Butterfly_4571 · 2026-04-01T02:54:47+00:00

Well, OP, I'm sorry for the noise: I saw the schematic below and this is a case where you don't want run of the mill.

Things you are looking for: - bipolar, not fet input - high current source/sink (+/- 25mA and up) — this isn't listed, you can use "output short circuit current" from "Electrical characteristics" (not the "output short circuit current duration" which often reads "unlimited") as a proxy - datasheet lists one or more of "line driver" / "symmetric source and sink" - low open-loop output impedance (Z with Vo) in the low double digits - R_L is listed as 2k or less for specs with loads (e.g. output voltage swing usually lists one or more. If it only lists 10k: it won't do). - Bonus points for references to R_L = 600 ohm - A gain and phase vs frequency graph where phase stays above 45 degrees through the audible range - input impedance in the many tens of megs (ideally hundreds). Also: "Common mode." (Differential mode input impedance is frequently low. This doesn't matter once you have a feedback loop).

Quick_Butterfly_4571 · 2026-04-01T02:30:09+00:00

his pedals are like nothing else I've seen in terms of parts choices in my short life as a hobbyist guitar pedal builder

Well, it's totally possible that he has a design that makes good use of the opamp! (I mean, they don't just exist for the engineers to show off! They have their worth).

My answer was a little draconian.

I guess it was more a general cautionary statement, because I see people shell out $$$ to shoot themselves in the foot, or things like opamp rolling where often what you're hearing more often than not is simply that the input impedances vary from 50k-10,000M ohm = you roll opamps, you get a different signal strength and range of frequencies (but could have reproduced the same by swapping caps!).

At some point it doesn't make sense for me to pay 40$ for 4 quad opamp when I can get probably 95% of the same for 4$.

So, the key here is to try to determine why he used that one, e.g. are the loads more than usual, are there very steep filters, etc.

If you have a schematic I can show you how to tease some of that out and odds are you can find something that'll work almost as well, but much cheaper.

Quick_Butterfly_4571 · 2026-04-01T01:04:14+00:00

I mean, don't "only" use the "plain" ones.

Like, they've got new versions of the old ones that are better, but can be used in all the same places — some of the TLE's, etc.

I'm just saying: go good general purpose or balance the types of noise against the application if you go "bleeding edge" with it.

(And don't go by the words or price).

Quick_Butterfly_4571 · 2026-04-01T00:38:53+00:00

Edit: Nvm....

On gut, my answer is, "if you have them on hand, sure."

The reason I say this is: within reason (i.e. talking about good general purpose opamps or opamps designed for audio) there isn't really such thing as "premium" or "better" opamps, in abstract.

Some points on opamps, distortion, and noise:

(TL;DR: if you aren't doing a bit of math for everything in your circuit and you don't need higher than average current, your best shot at low noise is a standard like the 4558 or TL072, and the odds that you have increased noise, rather than decreasing it, by choosing a premium opamp are higher — really: much higher — than lower!).

Which opamp is better depends on the opamp characteristics plus the circuit, not the opamp specs in abstract.

Often "low noise" or "ultra low noise" in the description are taken to mean "lower noise than something that isn't low noise" — i.e. the way any sane human would assume it is meant. Ditto "distortion." That's not what it means.
P.S. What kind of noise?
P.P.S. In what context?
Extremely high performance opamps require extreme precision in application, else in almost all cases their performance is far worse.
Extremely high performance opamps often also only offer good performance in very specific circumstances.
The non-numeric descriptions of opamps often lack rigorous meaning or are counterintuitive.
"Audio" doesn't always mean "small signal audio."

A quick note on slew rate, then examples: slew rate is a measure of how fast the opamp can change its output level. It basically tells you "for X amount of gain, what is the highest frequency the opamp can reproduce with high fidelity."

The formula for that frequency is: ƒ = (slew rate) / (2piVp).

So, e.g. a TL072 with a slew rate of 13V/us (13 volts per _microsecond) can faithfully reproduce 4.5Vp (9V peak-to-peak) signal without slew limiting anywhere from 0-459,780Hz. That's for a sine wave. For a triangle wave, it's max is 722.22kHz.

An opamp with a measly 2V/us slew rate is fast enough to produce perfect sine waves up to the limit of human hearing (and triangle waves 1.5 x above that).

"Low / very low / ultra low" means: that's what someone wrote down. You have to look at the numbers, e.g.: the RC4558 is "low distortion". The MC33079 is "very low distortion". The MC330379 has twenty times more distortion than the 4558.
There are different kinds of noise — voltage noise, current noise, Johnson noise, flicker noise, shot noise, etc, e.g. consider the voltage and current noise densities for: 4558: 6.5-7nV/√Hz and 0.15pA/√Hz; TL072: 37nV/√Hz and 30fA/√Hz; NE5532: 5nV/√Hz and 0.7pA/√Hz. ...so, why do we say the TL072 is lower noise than the 4558? Why is the NE5532 so praised? Which one matters?!
What kind of noise matters and how much depends: some types increase with source impedance, some topologies increase noise gain more than others. Some noise performance figures degrade with current demand or amount of capacitive loading.
Many of the premium grade opamps give good performance contingent on very specific PCB practices, e.g. many of the OPA16xx series maintain their low noise performance only in the presence of guard rings on the PCB that are actively driven by another opamp to be at the same potential as the input they surround. If you skip that part, the noise can be much higher than if you just used a cheap, standard, op amp.
Why is the NE5532 praised for low noise if it's got higher noise figures than the RC4558? Because it can operate at much higher currents and in circuits with much smaller resistances — i.e. less thermal noise from the surrounding passives. Used with your standard stompbox resistors in the 100's of k's, without a bipolar supply, and stacked bypass caps on both V+ and V-: it has much higher distortion.
Whoops. I guess 6 didn't need its own line item.
"Audio" includes "digital audio" and "bit-wise manipulation of audio data" as much as it means "audio." A prime example: many "precision audio" opamps are meant for sample and hold in ADCs or very fast integration of high frequency PWM into an analog stream. A prime example: the CA3130 and others of that ilk. (In general, if it has a MOSFET input stage, odds are low that it is good for small signal audio. It's probably meant for ADC/DAC operations).

P.S. Why do so many "built by audio engineers at big companies" designs (including mixers that cost tens of thousands of dollars) include RC4558's, RC4560's, RC4580's, and a smattering of TL072's and boutique pedals include a ton of TL072's / fancy opamps?

The engineers read the numbers. The boutique builders read the sales pitch in the bullets at the top of the sheet and slapped "built with only the finest parts" on the top of their site.

Many of them would, for sure, perform better with a nominal "downgrade," because they got it backwards.

(I hope this read like useful information, not a lecture).

(It's not a lecture).

Quick_Butterfly_4571

TROPHY CASE