Once again - Small headphone amplifier - ty.ytka

tytka 10 Nov 2024 21:11 16 4128 Cool? (+40)

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TL;DR

A small headphone amplifier was updated from an earlier Douglas Self–based parallel op-amp design.
The amplifier section was simplified to two stages: a parallel voltage amplifier with 2 op-amps and a parallel current amplifier with 4 op-amps.
The new enclosure has internal dimensions of 105 x 160 x 35mm, and the amplifier board measures 97 x 159.5mm.
The rest of the circuitry—power supply, protection circuit, and DAC—stayed unchanged, and amplifier measurements were still to be published later.

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The design of this headphone amplifier, is actually an update of my earlier design presented here ( Link ).
Compared to the original version, a few things have been changed. The one that is immediately apparent is the casing.
The new housing is slightly larger. Its internal dimensions are: 105 x 160 x 35mm (with external: 115 x 168 x 40mm). The dimensions of the amplifier board are: 97 x 159.5mm. So, there is now slightly more space. The board itself is now not pushed into the profile, but fixed to the underside of the case, via spacers 5mm high. From the side edge of the plate to the side wall, there is 4mm of space on each side. The area of the housing that is adjacent to the mains voltage area of the PCB has been shielded with 0.5mm thick pre-shrink. The mains cable exit from the enclosure, is sheathed in a dedicated rubber bend.

All the work involved in machining the raw case to adapt it for this project was done by myself.
The lettering was engraved, using a small, simple CNC machine.
Just to pre-empt any questions, I would like to inform you that this time the engraving was not filled with paint on purpose. The idea was for the description to be there, but relatively subtle.

Now perhaps a little about the electronics.
For those who haven't seen the previous version, let me remind you that this is a design based on Douglas Self's project on achieving higher power when connecting operational amplifiers in parallel.
Compared to the previous version, the circuit has been slightly simplified. The amplifier section has been shortened to two stages. Why? The previous version was actually a fitting for a power amplifier realised on operational amplifiers, which I was working on at the time ( Link ). It was known that the circuit there (power amplifiers) would need more gain. However, in a headphone amplifier, such an elaborate circuit is not needed. So I decided to simplify it, resulting in only two stages remaining. The first is a parallel voltage amplifier (2 operational amplifiers) and the second is also parallel, but this time a current amplifier (4 operational amplifiers).
The rest of the circuitry, i.e. the power supply, the protection circuit, and the DAC, remained as before.

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Below are a few more photos showing the end result.

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However, I will take the liberty of posting the results of the amplifier measurements, as these will also be there, at a later date.

And one additional piece of information.
The previous enclosures have not gone to waste. They were used for other devices, which quite possibly will be presented here..

About Author

tytka wrote 737 posts with rating 1828 , helped 8 times. Live in city Pabianice. Been with us since 2006 year.

Comments

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Piottr242 11 Nov 2024 00:30

Each time, I am impressed by the aesthetics and refinement of the design. What is a U3 circuit in an oblong housing? The output protection? And a second question - do you mill the front panel in-house... [Read more]

Anonymous 11 Nov 2024 07:46

Workmanship as usual super. Those extensions for the switch and potentiometer are reminiscent of the construction methods once used for oscilloscopes. The schematic? It's gone from very contrived to... [Read more]

wiorekw 11 Nov 2024 10:06

Another very nice design, I'm very curious to see the measurements and how they compare to the previous, heavily developed version, how much the performance has deteriorated (or improved?). [Read more]

_Avalanche_ 11 Nov 2024 11:21

You, when you throw something in, it's always a masterstroke, your designs are top notch :) [Read more]

pikarel 11 Nov 2024 14:17

It is a pleasure to look at each of your projects, the attention to detail of workmanship is exemplary in DiY. You have improved the design by what you deemed worthy of correction. Obviously a big plus... [Read more]

Bucefal76 11 Nov 2024 18:22

You are already approaching the level at which I would spend the money to buy. A little more refinement to the lettering on the front panel and we have a commercial level of workmanship. [Read more]

katakrowa 11 Nov 2024 19:23

I see the paper under 230V is :-) . [Read more]

saly 12 Nov 2024 08:45

I'm impressed with the design and aesthetics. It looks great and probably works just as well. I noticed a little zonk in the schematic but that's probably a Czech error. https://obrazki.elektroda.pl/5456268400_1731396564_thumb.jpg... [Read more]

gulson 12 Nov 2024 09:39

Something wonderful. There is no denying that some users are waiting for the ty.tyk presentations with baked goods! I guess you haven't received the goodie kits from Elektroda yet, send a Parcel Post,... [Read more]

Anonymous 12 Nov 2024 10:14

If I were ever tempted to build an amplifier according to this topology, I would simplify it to: https://obrazki.elektroda.pl/1127212500_1731402858_thumb.jpg . - the first stage amplifier I would... [Read more]

tytka 12 Nov 2024 19:14

Thank you very much for your comments. Of course, I'm aware that the level of my projects is still not up to par with the commercial ones, but for the time being, that's all I can afford. And also... [Read more]

wiorekw 13 Nov 2024 15:03

If I'm not mistaken, this simplified version has slightly better performance than the previous, more elaborate version, especially the THD, which is almost halved. I'm already overlooking the fact that... [Read more]

katakrowa 13 Nov 2024 15:36

. Interesting... You must have something wrong. I would expect a lot more from analogue too. Can you give measurements of the card/interface itself? [Read more]

tytka 13 Nov 2024 22:44

In fact, this version came out slightly better in the tests carried out. But this may be partly due to the slightly changed measuring conditions. It doesn't look like anything is wrong. It's just that... [Read more]

siewcu 15 Nov 2024 14:05

Where did you get this enclosure from? I ordered some from the net, it was supposed to have these dimensions, it arrives and here's the trick, the dimensions are given for the outside and it's not possible... [Read more]

tytka 16 Nov 2024 15:04

. Today I had a moment, so I tested the original/more elaborate version (as it sits in a drawer one without a case) under the same conditions as now this revised, or simplified, one. For the line input: ... [Read more]

FAQ

TL;DR: For DIY headphone-amp builders, this update uses a 105 × 160 × 35 mm internal enclosure and a simplified two-stage audio path; the key measurement lesson is that "the PCM2705 is the bottleneck" on USB input, not the analog amp alone. It solves two common problems at once: cleaner packaging and a simpler Self-inspired topology without losing performance. [#21300397]

Why it matters: This thread shows how enclosure mechanics, protection, and source quality can matter as much as the amplifier schematic in a compact DIY headphone amplifier.

Feature	Updated version	Earlier version
Audio stages	2	More elaborate than 2
First stage	2 parallel op-amps	More complex arrangement
Second stage	4 parallel op-amps	Parallel op-amp output stage
PCB mounting	Fixed to case bottom on 5 mm spacers	Board pushed into profile
Measured result summary	Slightly better in retest	Slightly worse in same-condition retest

Key insight: Simplifying the amplifier did not hurt results in the author’s retest. The USB DAC section, especially the PCM2705 path, limited measured performance more than the revised analog amplifier itself. [#21305458]

Quick Facts

The new enclosure has 105 × 160 × 35 mm internal dimensions, 115 × 168 × 40 mm external dimensions, and the PCB measures 97 × 159.5 mm. [#21297550]
The PCB now mounts to the enclosure base on 5 mm spacers, with 4 mm clearance to each side wall, which improves fit and separation from the case. [#21297550]
The amplifier uses 2 parallel op-amps in the voltage stage and 4 parallel op-amps in the current stage; the supply, protection, and DAC sections stayed from the earlier design. [#21297550]
The output includes a 6.8 Ω series resistor, and the author says his lowest-impedance headphones are 44 Ω, not 16–32 Ω models. [#21300397]
The mains-side enclosure area next to the PCB is insulated with 0.5 mm pre-shrink, and the mains cable exits through a dedicated rubber bend. [#21297550]

How is this updated headphone amplifier different from the author's earlier Douglas Self-based version?

It keeps the same Self-inspired parallel-op-amp idea, but simplifies the audio path and changes the mechanics. The new version uses two stages instead of the earlier, more elaborate arrangement: a first parallel voltage stage with 2 op-amps and a second parallel current stage with 4 op-amps. It also moves the PCB from a slide-in profile arrangement to bottom mounting on 5 mm spacers inside a larger 105 × 160 × 35 mm internal enclosure. The power supply, protection circuit, and DAC remained essentially unchanged. [#21297550]

What is the U3 circuit in this headphone amplifier, and how does its output protection work?

U3 is the output protection circuit. It delays headphone connection at power-up and automatically disconnects the headphones if DC appears at the output. That protects drivers from turn-on transients and from fault conditions that could place direct current on the jack. In a compact mains-powered design, that function matters because a fault at the amplifier output can reach the headphones immediately unless a relay or similar protection stage interrupts the path. [#21300397]

How do you machine and engrave an aluminum front panel yourself using a table drill, mini grinder, and small CNC machine?

The author does it in-house with simple workshop tools. 1. Drill the front-panel holes with a table drill. 2. Refine and fit the openings with a mini grinder. 3. Engrave only the lettering with a small CNC machine. He also states that the engraving was intentionally left unpainted for a subtler look. This method produced the front panel on the shown amplifier without outsourcing the machining. [#21300397]

Why were two parallel operational amplifiers still used in the first voltage-gain stage of this simplified design?

They were kept mainly because the PCB was reused from the earlier version. The author says two parallel amplifiers in the first stage were “apparently not necessary in this case,” but the board layout gave him no practical alternative. He also notes that parallel devices still offer some advantages, even if this headphone amplifier did not strictly require them. So the choice came from board continuity first, not from a claim that the simplified circuit needed extra first-stage current. [#21300397]

What is a DC-servo circuit in an audio amplifier, and why do some builders support it while others avoid it?

A DC-servo circuit is a control stage that corrects DC offset at an amplifier output, usually with very low-frequency feedback, while leaving the audio band largely to the main signal path. In this thread, one commenter sees the servo loop as unnecessary complexity, while the author says DC-servo circuits have both supporters and opponents and places himself “in the middle.” He still chose to use it in this amplifier, so the debate here is about added complexity versus DC control, not about whether the circuit exists. [#21300397]

Why is there a 6.8 ohm output resistor in a headphone amplifier, and how does it affect 16-32 ohm headphones versus 44 ohm and higher models?

The 6.8 Ω resistor is there mainly to protect the op-amp outputs if the amplifier output is shorted. The trade-off is obvious: with 16–32 Ω headphones, a 6.8 Ω series resistor is large enough to change damping and interact with headphone impedance versus frequency. The author accepts that compromise because he does not use low-impedance models; his lowest pair is 44 Ω. That makes the resistor less disruptive in his actual use case than it would be with typical 16–32 Ω portable headphones. [#21300397]

What do the 1 ohm resistors at the outputs of parallel op-amp stages do, and why might removing them be risky?

The 1 Ω resistors isolate the paralleled op-amp outputs from each other. That helps share current and reduces the chance that one amplifier will fight another due to small offset or gain differences. A commenter wanted them removed or absorbed differently into feedback, but the author says he “would not dare” omit those 1 Ω parts. In a parallel stage, removing them can create instability or current hogging, especially when several output amplifiers work into the same load node. [#21300397]

How did the simplified two-stage headphone amplifier measure compared with the author's earlier, more elaborate version?

It measured slightly better when the author retested both versions under the same conditions. Another participant already noticed that THD looked almost halved in the simplified version, and the author later confirmed that this revised build came out “slightly better in the tests carried out.” He also warned that earlier comparisons may have been affected by changed measurement conditions, which is why the same-condition retest matters more than the first impression from separate reports. [#21305458]

Why does the PCM2705 appear to be the bottleneck in the USB input measurements of this amplifier?

The USB-input results stayed worse than the line-input results, and the author saw the same pattern on another device using the same PCM2705. He explicitly says the PCM2705 “turns out to be quite a bottleneck” in the measurements and later adds that he cannot achieve better results with this PCM even after comparing across projects. That points to the DAC path as the limiting block during USB tests, rather than the revised analog amplifier alone. [#21305458]

How can you check whether poor audio measurements come from the PCM2705 DAC, the amplifier, or the measurement card/interface itself?

You isolate each block and compare results. 1. Measure the amplifier from the line input. 2. Measure it again from the USB input using the PCM2705. 3. Measure the sound card or interface itself. The thread follows exactly that logic: the author posted a line-input report, a digital-input report, and then a separate measurement of the card/interface on request. That sequence helps show whether the limit comes from the analog amplifier, the PCM2705 USB DAC path, or the measurement chain. [#21302333]

What is RIAA equalization, and how would adding an RIAA stage change a headphone amplifier intended for vinyl playback?

RIAA equalization is a phono preamplifier stage that applies the inverse of the standardized vinyl recording curve and raises a cartridge’s low-level signal to a usable line level. In the thread, one commenter says the amplifier would feel complete to him if it included an RIAA stage, because that would let him listen directly from vinyl without adding more external boxes. The author did not plan to retrofit this amplifier, but said the idea could influence a future tube OTL upgrade. [#21300397]

NE5532 versus TPA61xx: which approach makes more sense for a small DIY headphone amplifier?

In this thread, NE5532-based parallel stages make sense if you want to pursue the author’s discrete DIY topology, while TPA61xx makes sense if you want a simpler small-form build. One commenter says that, if he built this topology, he would simplify the first stage from a dual NE5532 arrangement and would personally rather use “something like the TPA61xx.” That is not a measured shootout here. It is a design-choice contrast between a more elaborate DIY op-amp approach and a more integrated headphone-driver approach. [#21299568]

What safety measures should be used around the 230 V mains area inside a compact headphone amplifier enclosure?

You should physically insulate the mains area and protect cable exits from abrasion. The author shielded the enclosure section next to the mains-voltage PCB area with 0.5 mm pre-shrink material and routed the mains cable through a dedicated rubber bend. A commenter also noticed the insulating sheet under the 230 V area, and the author explained that the earlier version used foil instead. In a tight metal enclosure, spacing, insulation, and strain relief are the key safety measures shown here. [#21297550]

How do you choose an enclosure so the quoted dimensions match the usable internal space for the PCB?

Choose by verified internal dimensions, not by the seller’s outside measurements. This project fits because the usable inside is 105 × 160 × 35 mm, while the PCB itself is 97 × 159.5 mm, leaving only 4 mm to each side wall. One later commenter described the common failure case: ordering a case advertised by dimensions that turn out to be external only, so the board will not fit inside. In practice, compare PCB width, PCB length, wall thickness, and mounting method before ordering. [#21304213]

Where can you source the aluminum enclosure used for this project, and what dimensions should you verify before ordering?

The author sourced it from AliExpress. Before ordering, verify both the external size, 115 × 168 × 40 mm, and the internal usable space, 105 × 160 × 35 mm. Also confirm that your PCB dimensions leave room for side clearances, spacers, cable bends, and mains insulation. In this build, the PCB is 97 × 159.5 mm, so the fit is already tight in one axis. That is why internal dimensions matter more than listing titles or photos. [#21305458]

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