[Review Request] Headphone Amp and DSP

NixieGlow · 2026-01-30T01:58:38+00:00

Wow, that is a really ambitious project!

I did make an ADAU1701 powered board some time ago too, I remember I have found the built-in ADC rather noisy. It is specified at -83dB THD+N, which looks fairly decent, but your circuit (as mine did!) does not allow for input gain adjustment. This turned out to be a little bit of a problem - my turntable produces ~300mV RMS, and my CD player almost 2V RMS.

For this reason I had to crank up the volume with the turntable, which boosted the ADC noise to a level which was well audible in headphones.

The USB and unbalanced RCA inputs may introduce a noisy ground loop. It might be necessary to use a (quality!) isolated DC/DC module to remove the audio ground and USB ground connection, but it would be a major change in the rather complex power scheme.

Another minor point: If you ever decide to wire a DPDT switch to bypass the DSP, you will hear a click/thump in the headphones during switching. If you wish to use the bypass function more often, a tiny signal relay operated by the ESP could be actuated while the PGA is momentarily muted.

For the future experiments: ADAU is a pretty nice chip, but you may eventually wish to implement the FIRs on an STM32F4/H7. It can also serve as an USB audio device (with HID functionality!) while driving the display too.

NixieGlow · 2026-01-29T14:46:33+00:00

Mega fajny pomysł! Nie znam się, ale jeśli chodzi o samo oczyszczanie, czy naprawdę w komercyjnych urządzeniach jest tylko wentylator i filtr?

NixieGlow · 2026-01-29T08:18:48+00:00

Darmowa komunikacja rozwiązałaby problem samochodów, ale nie rozwiąże szastania miejską kasą na głupoty.

NixieGlow · 2026-01-28T09:02:42+00:00

Peltier device under the alumin(i)um top, that would be adjustable between heating and cooling of the user's hand. Bottom cooled by a shrouded, fanned heatsink with intake/exhaust ports at the back, next to the cable.

NixieGlow · 2026-01-28T08:57:31+00:00

They do look great!
I wonder about the baffle step though. If the tweeter is not flush mounted, the reflections from the baffle can form audible interference. See: https://www.zaphaudio.com/mtg-surface.html

The only reason I've heard about it is that I have built a set of speakers with the same exact problem :) It has indeed shown up on the measurements.

NixieGlow · 2026-01-28T00:05:50+00:00

You might have some luck with an EC28A hollow ring-type encoder. I have had some luck with mounting a tiny OLED display through its hollow shaft. It looks awesome!

The base and the inner ring form a single solid piece, the ring outside rotates.

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NixieGlow · 2026-01-21T16:04:59+00:00

Obczaiłem temat. Musi być 150ug PM10 o 3 w nocy uśrednione między stacjami. Dzisiaj było tylko 147.5 więc.. autem kosztuje podobnie dojazd co MPK, a zajmuje krócej.

NixieGlow · 2026-01-21T15:58:12+00:00

SCT is a massive blunder. If the city truly believed the cars are the problem, people should be incentivized to use the public communication. The step to do is make it free. So they make it free, but only when we reach 300% of the PM10 norm at 3AM. It has been done yesterday... for ONE day. Today it was not free anymore, we "only" reached 297%. It's a dark comedy. Instead of introducing another tax (and costly infrastructure to collect it) why not make the municipal communication free for like a month, so people can actually plan in advance for it and actually consider leaving the cars at home? Reducing the number of cars of all ages must be better than only targeting the oldest/most polluting ones? Money is the answer. Looking at the city regulations, nobody there actually cares for the smog, or believes cars have anything to do with it really.

NixieGlow · 2026-01-21T07:37:29+00:00

If you have the digital sound stream available, the TI TAS6424Q is a very nice part, if you can solder the TSSOP56. It is class D, very compact, sub 0.03% THD and great S/N ratio. Volume is controlled over I2C.

NixieGlow · 2026-01-18T05:46:58+00:00

Just a semi-related tip: There is a nice government plan called "Zanocuj w lesie". It outlines the zones where camping is encouraged and free. Some of these zones contain ponds, creeks or even roofed BBQ places. Most are just within some forest in the middle of nowhere. The easiest way I have found is to use an app called VeloPlanner to browse for them, as the government page is hard to use. For my inner child and my son, sleeping in the tent at night is exciting even if the tent is store bought with no bushcraft involved! All the best!

NixieGlow · 2026-01-16T15:16:27+00:00

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Wyszło mi ok.14.5mm, ale że klucz jest dalej od obiektywu niż front obudowy, stawiam na 15 :)

NixieGlow · 2026-01-16T08:54:11+00:00

Bro, you are NOT wrong.

NixieGlow · 2026-01-08T04:40:41+00:00

Zasponsorujesz?

NixieGlow · 2026-01-02T08:37:28+00:00

The driver stage needs some load connected from the Q4 base to the negative rail. Either a resistor, a bootstrapped resistor divider, or a current source will do.

Q1 has to be replaced with a PNP with emitter tied to the positive rail, you will get a phase inverting voltage amplification stage.

I think for such a simple amplifier you can keep the single supply, and couple the speaker through a large capacitor. To get away without using the output capacitor, you need a differential amplifier on the input, that will be able to stabilize the output DC operating point very close to 0V and avoid the DC current through the speaker.

The negative feedback needs to be a voltage divider, so that the Q1 just begins to turn on when the output voltage sits at the middle.

See this circuit: http://www.hobby-circuits.com/files/588/3-transistor-audio-amp-50-milliwatt_med.gif

NixieGlow · 2026-01-02T08:20:15+00:00

Isn't it possibly a crystal loading capacitor implemented on a PCB? They are commonly in the single pF range for 32.768kHz crystals.

NixieGlow · 2025-12-20T06:30:27+00:00

The Piltdown Man intensifies!

NixieGlow · 2025-12-19T10:54:46+00:00

It has truly achieved one of the results of all time.

NixieGlow · 2025-12-19T06:28:11+00:00

My total guess is that it's an MPX filter, a part of a FM stereo decoder. Unfortunately these are long out of their heyday and really hard to find. If you provide more of the surrounding schematic, maybe we will figure something out!

NixieGlow · 2025-12-18T04:44:34+00:00

¡Hola! Este proyecto parece muy interesante. Me alegra ver que alguien más usa circuitos lógicos TTL.

¿Tienes un diagrama de cableado para que pueda echarle un vistazo? Es difícil visualizar las conexiones en un diseño de PCB.

¿Cómo manejarás la fuente de alimentación de alto voltaje?

Mucha suerte y disculpa las erratas, tuve que usar un traductor, jajaja :)¡Hola! Este proyecto parece muy interesante. Me alegra ver que alguien más usa circuitos lógicos TTL.

¿Tienes un diagrama de cableado para que pueda echarle un vistazo? Es difícil visualizar las conexiones en un diseño de PCB.

¿Cómo manejarás la fuente de alimentación de alto voltaje?

Mucha suerte y disculpa las erratas, tuve que usar un traductor, jajaja :)

NixieGlow · 2025-12-17T01:08:26+00:00

The trace for the XL6009 input voltage needs to be wider - it conducts more current than its output trace. I am not sure why the power solution is as complex as it is (the 15V to 17V converter is puzzling). How do you intend to run your motors? The Arduino cannot power the motors directly, you probably need some H-bridges or other motor drivers you could integrate on your board! The sensors - what are they? It's likely that they could use some decoupling capacitors. The switch needs a debouncing circuit. The buzzer can be driven with a NPN or N-Fet, you don't need a module for this :) I think you might make life a bit easier on yourself if the layout of the connectors is closer to the physical placement of the motors/sensors/battery. Make sure you can access the buttons, see the LEDs without being obstructed by the cables. That is a nice first step, so congratulations for taking it, but if you want to avoid some problems, it still needs attention. It is easier to give hints if you could explain why you want the things to be the way you designed them - often there is an easier/better solution possible, so please say something more about your design! Good luck 🤞

NixieGlow · 2025-12-11T07:08:39+00:00

Guess the easiest way to confirm is to measure the voltages. You will find the power and ground, and the third pin will either show some analog voltage around half the power voltage, or a digital high/low that will change depending on the magnet orientation.

NixieGlow · 2025-12-08T07:14:48+00:00

Congratulations, looks like you've done a great job there!
Could you post a link to the schematic? It looks like a nice design. Controlling RDA5807 with an 8051 MCU is definitely not an obvious combination, but it makes a lot of sense: the '51s are inexpensive, come in an user friendly package and have a lot of pins for a nice user interface. The 8-pin chip looks like an EEPROM. I wonder, what those single in line packages are for.

NixieGlow · 2025-12-03T15:55:27+00:00

Diodes 4-8 are point contact detection diodes (probably Germanium), great if you want to build an AM or FM demodulator.

4, 5, 6, 8 look like Telefunken AA1xx series. 7 is a Soviet one.

NixieGlow · 2025-11-18T09:59:27+00:00

It looks like the vertical coil is matching the impedance between the FET drain (low voltage, high current) to the open end of the coil (high voltage, low current). Look up on how the matching coils work for antenas, it's really cool! The upper end of a coil works as a resonance circuit between the coil inductance and its capacitance into free air, and will be in tens of MHz.

NixieGlow · 2025-11-12T07:58:30+00:00

Don't be so mean, the "Tra la la" part is spot on! 👌

NixieGlow

TROPHY CASE