A custom full Dual Mono Class A integrated amplifier combines a phono preamp, Bluetooth, DAC, control logic, and speaker protection in a Modushop 1NPD02300B enclosure.
The phono stage uses a FET-input circuit inspired by Pioneer and Denon designs, and the power amplifiers are redesigned from the Pioneer A-09 with current, inexpensive transistors.
The build took seven months, and the DAC uses Cirrus Logic CS4334 converters with DIR9001 SPDIF decoders and a 4052 source selector.
Startup was mostly straightforward: the analog section needed only offset and bias adjustment, and the finished amplifier sounded clearly better than the earlier GainClone and Vincent.
The 30 cm-deep case left very little space, the heatsinks run around 75 degrees, the subtitles are too dim, and the DIR9001 needed delayed Reset timing to start correctly.
Generated by the language model.
Hello, By way of introduction: The amplifier is visually very similar to the Gamut design. It's a coincidence. Wanting to build an amplifier, I was looking for a damaged enclosure donor. The housing of the Koda AV-600X receiver caught my eye, but the knob was asymmetrically placed. I didn't find anything interesting, I started to draw something similar to this pattern myself, but with a symmetrically placed knob. There were many iterations until an almost final form emerged, very similar to the present one. Then I accidentally came across the Gamut DI 150 amplifier and noticed that it looks almost identical to the amplifier in my concept. I began to hesitate whether to change my idea to something else, because it is someone's technical idea, someone's industrial design. I started to study the regulations in this direction and it turned out that the law allows the use of an industrial design for personal use, so a stone from my heart and I started to act.
The inspiration for the new amplifier was the occasional purchase of the Vincent SV-121 amplifier and comparing its sound with my existing amplifier, described in this topic:
It turned out that acoustically Vincent is several classes above. Unfortunately, Vincent does not have a signal output, BT and, most importantly, a phono preamplifier.
This is where the search for an interesting scheme, design and construction began. The amplifier was built for seven months.
Its design consists of separate plates of individual modules, so that you can easily upgrade, upgrade, etc. Everything was packed into the 1NPD02300B housing purchased from modushop.
The input selector board is integrated with the phono preamplifier. In the phono preamplifier I used a circuit known from more expensive Pioneer and Denon designs with FET transistors at the input. The sound from this circuit is clearly better than the circuits I've made before (including the Emerald circuit, praised on the web). The board also has a signal output buffer for driving the previously presented Equalizer on IN-9 tubes. Scheme and selector board (for one channel) below:
The power amplifiers are based on the Pioneer A-09 scheme, but they have been redesigned to use currently available and cheap transistors. The board has been designed to be screwed together with the transistors on the heat sink. Below is the schematic and PCB for one channel.
The amplifier additionally has a Bluetooth receiver and a DAC also working in Dual Mono. The DAC board has been designed in such a way that it can be screwed onto the PA214 Bluetooth receiver (bought on a well-known Asian website). The signal in the SPDIF format is fed from Bluetooth to the DAC board. There, through the 4052 multiplexer, one of the three SPDIF (Coaxial / Optical / Bt) sources is selected, whichthen, through optocouplers, it is fed to SPDIF decoders independent for each channel, and from there to converters. The "primary" side of the DAC (Bluetooth receiver, selector, Toslink receiver) is powered from the power supply of the digital part, the "secondary" side (SPDIF decoders, converters) - from the output of the audio power supply switched on with a relay. The DAC is based on the Cirrus Logic CS4334 converters, because they are one of the few that can still be purchased on the market, and they are also easy to use. Below is the schematic and PCB
The amplifier's control section is not too complicated and was designed parallel to the front panel to handle the backlight, buttons, have a hole for the potentiometer axis, etc. It is based on the Atmega16 microcontroller.
A 10x5mm model bearing with a flange was fitted into the hole for the potentiometer axis, through which the potentiometer axis with a diameter of 5 mm passes. The bearing is the fulcrum of the axle.
Effects below. Control board:
Aluminum front panel of the case:
Panel with illuminated inscriptions under the glass (with a cutout for the IR receiver and the axis:
The control section is powered from a stand-by power supply connected to the power transformer switching system and soft-start. It is presented below:
The audio part is powered by the power supply system with independent circuits of power transistors, power stage voltage amplifier and preamplifier. In addition, through the relay, the DAC and BT are powered. The relay turns on only when selecting digital sources, so when playing audio from other sources on the DAC and BT, no voltage is applied. The audio power supply is shown below and is one of the two channels. The tile of the second channel is a symmetrical reflection.
And finally, the speaker protection board. This protection is supervised by the microcontroller that controls the amplifier and is screwed directly to the loudspeaker terminals.
In general, the launch did not cause any major problems. The analog part started right away. only adjustment of the offset, transistor currents was required and it was possible to twist. The mechanical part turned out to be a big problem due to the use of a slightly too small housing. On paper, everything was nice to fold, but in reality it was not so colorful. The housing also required a few modifications, especially the way the radiators were screwed to the frame.
In terms of electronics, I had the biggest difficulties with starting the DAC. The reason was my negligence, because I downplayed the requirement to delay the Reset pin relative to the power supply in the DIR9001 chip. It took me several hours to figure out what the source of the problem is. Using an inertial system on the Reset pin solved the problem.
Below are some photos of the construction.
I also add a video showing the operation of the amplifier icon illumination:
The amplifier, as in class A, heats up mercilessly:
problems - low brightness of the subtitles backlight - they were etched on the PCB and covered with a black soldermask. Unfortunately, this is not the best solution and I am in the process of looking for something else. If you have any ideas - please give me a hint - very little space inside - I chose a casing with a depth of 30 cm. On paper everything fit, in reality it would be much better to use a larger housing, - too small radiators - the amplifier heats up to about 75 degrees. I wish I had taken a 3U case - drifting with the temperature offset - the offset must be set on a well-heated amplifier.
Sound - no comparison to anything I've had before Previous GainClone or Vincent are far behind.
Regards!
About Author
Maciek_C wrote 249 posts with
rating 254 , helped 14 times.
Live in city Elbląg.
Been with us since 2016 year.
Beautiful construction.
As for subtitles, there are two methods:
1. Taking on photographic film. There are digital platesetters for film. Maybe someone who prepares stencils for offset printing using... [Read more]
katakrowa
20 Jul 2023 17:59
Very nice! I admire that you wanted to make "mirror" tiles.
You can easily sell for PLN 14,999 :-) [Read more]
CMS
20 Jul 2023 18:01
The construction of beauty. However, I don't like one thing, actually two, because these are two amplifiers in one housing. It's about the way of mounting plates with power amplifiers. While it... [Read more]
Maciek_C
20 Jul 2023 18:27
Thank you friends for the kind words :)
You do not need to unscrew - removing the transistors is not a problem, and to insert a new one, just slightly hook the ends of the legs. Checked ;)
The only... [Read more]
CMS
20 Jul 2023 19:38
Maybe white paper with the same negative print between the foil and the glass?
And possibly delicate white LED backlight during the day. [Read more]
bodzioluban
21 Jul 2023 07:35
The construction is ok, but aren't they sometimes ready-made modules or for self-assembly? But it looks nice. [Read more]
Maciek_C
21 Jul 2023 09:22
Thanks, I'm looking into it :)
Could you expand on the idea? I can't visualize it in my head.
Are you asking if I didn't use ready-made layouts or couldn't I use ready-made ones... [Read more]
LemuRR 11
21 Jul 2023 11:29
Prepare a 3D model of this glass so that the inscriptions are protruding (approx. 1 mm). Then find someone with a resin printer who will print it for you in transparent resin. The next step will be to... [Read more]
kris8888
21 Jul 2023 12:12
Great and thoughtful design, I'm impressed. Almost like it came from the factory. The only thing that puzzles me is that, due to the fact that the whole thing gets quite hot, is it right that the transformers... [Read more]
efi222
21 Jul 2023 15:16
Beautiful design. I always admire my own audio equipment.
When it comes to pictograms, I have been etching them on laminate for years. It's just that the laminate should be thin. Within 0.4, 0.6mm.... [Read more]
CMS
21 Jul 2023 17:38
I got the idea that you want the subtitles to be visible when they are not highlighted. :) And you want the opposite. My mistake. [Read more]
Maciek_C
21 Jul 2023 18:55
The dimensions of this tile are 74x154. As for 3D, I only have AutoCad and 3D modeling is a pain in the ass, but I'll try to prepare something tonight or tomorrow, thank you :)
That is why the... [Read more]
E8600
21 Jul 2023 22:21
It's been a long time since there was such a refined structure on the forum. I wonder how long it took from the idea to the implementation of such a miracle? [Read more]
LemuRR 11
22 Jul 2023 00:09
Insert a drawing in SVG format, I'll convert it to a 3D model in a few moments. I will also try to print, but I do not promise success, because the working field of the printer is "on contact".
As... [Read more]
mmaker
22 Jul 2023 00:12
I used to etch such inscriptions on a 0.3 mm laminate and put 3 mm smoked plexiglass on the front. It worked. If it's supposed to be glass, aim for tinted car window film. You probably have every 5... [Read more]
nyquist
22 Jul 2023 09:55
A fine piece of work! I admire and envy ;)
Out of curiosity, a question - what did you shield the power transformers with? Is it permalloy or plain copper? [Read more]
Maciek_C
22 Jul 2023 10:33
Thank you guys for your kind words and good advice :)
Buddy LemuRR 11, I am very grateful for your willingness to help, but I don't want to bother you for now - I will try what you suggested first,... [Read more]
efi222
22 Jul 2023 11:22
Let me have a "three cents" on tinted glass.
Just like my colleagues, I covered plexiglass with foils, but something was always wrong. And this is a bubble, and this is some kind of scratch on the plexiglass... [Read more]
gdkj
23 Jul 2023 18:54
Out of curiosity, C102 = 100µF/25V. Why so much value? [Read more]
FAQ
TL;DR: With 2x28V DC rails and a builder-verified "about 32W at 8Ω" in pure Class A, this FAQ helps DIY audio builders understand a full Dual Mono amp with modular boards, relay-switched digital power, and a DAC reset bug that blocked startup until Reset was delayed. [#20673453]
Why it matters: This build shows how to combine Class A sound, upgradeable module design, and practical troubleshooting in one custom integrated amplifier.
Alternative
What it achieved
Main drawback reported
PCB with black soldermask under glass
Labels stayed hidden when off
Passed too little light
Etched thin laminate + smoked plexiglass
Proven workable by multiple users
Needs darker cover material
Tinted glass
Most "pro" finish and cleaner surface
Can reduce IR range
Laser-printed film on foil
Simple and cheap
Print could show through
Key insight: The strongest lesson is thermal and mechanical planning. The electronics started easily, but the 30 cm chassis and undersized heatsinks pushed temperatures to about 75°C and made the build much harder than expected.
Quick Facts
Power rails were 2x28V DC, with 375 mA per branch and 1.5 A total quiescent current per channel. The builder calculated 2x35 W into 8Ω and 2x70 W into 4Ω, then checked that practice gave a few watts more. [#20662842]
Pure Class A operation was not full-scale. The builder later estimated about 32 W into 8Ω and 18 W into 4Ω before the stage moved into AB operation. [#20673453]
Thermal behavior was the main hardware limit. The chassis reached about 75°C after 1 hour of moderate playback into 6Ω speakers in a 23°C room. [#20687891]
The front-panel insert measured 74 x 154 mm. Suggested label solutions included etched 0.3-0.6 mm laminate, smoked plexiglass, tinted glass, and window-tint film. [#20663210]
The control section used an Atmega16, and the volume control was an Alps RK271 2x50k potentiometer. Spare JLCPCB boards, Gerbers, Eagle files, and MCU code were offered privately. [#20661024]
What does a full Dual Mono amplifier design mean, and what advantages does it bring in a Class A build?
A full Dual Mono design means the left and right channels are built as largely separate signal and power paths. In this build, each channel has its own mirrored audio supply board, and even the DAC works in Dual Mono. That separation reduces shared supply interaction, simplifies channel-by-channel servicing, and fits the builder’s modular upgrade goal in a hot Class A amplifier. [#20661024]
How was this Class A amplifier divided into separate modules, and why does that make future upgrades easier?
It was split into separate boards for input selector and phono stage, preamp, power amp, DAC, control, standby and soft-start, audio power supply, and speaker protection. The builder explicitly said the design uses separate module plates so parts can be upgraded or replaced easily. That modular layout also let him mirror one channel and keep digital, control, and audio sections physically distinct. [#20661024]
Why did the DIR9001 DAC chip fail to start correctly, and how do you delay the Reset pin relative to the power supply?
The DIR9001 failed because Reset came too early relative to the supply voltage. The builder said he underestimated the need to delay the chip’s Reset pin, which cost him several hours of debugging. He fixed it by adding an inertial delay circuit on Reset, so power stabilized first and the SPDIF receiver then started correctly. [#20661024]
How do you determine when a Class A amplifier based on the Pioneer A-09 topology leaves pure Class A and starts working in AB?
You determine it by watching output-stage current behavior as drive rises. 1. Load the amp with 4Ω or 8Ω resistors and feed a sine wave. 2. Measure voltage across the emitter resistors while increasing output. 3. Mark the point where emitter current at the sine peak no longer stays clear of zero and begins to deform. The builder used that method and estimated the transition near 32 W at 8Ω and 18 W at 4Ω. [#20687839]
What output power can this 2x28V DC amplifier deliver into 8Ω and 4Ω, and how were those values calculated and checked?
It can deliver about 2x35 W into 8Ω and 2x70 W into 4Ω. The builder said those values were first calculated with margin for bridge drops and ripple, then verified practically and came out a few watts higher. He also stated the amplifier runs from 2x28V DC rails with 375 mA quiescent current per branch. [#20687839]
How do you adjust DC offset in this power amplifier, and why does the offset drift with temperature until the amp is fully warmed up?
You adjust DC offset with potentiometer R9 on the power-amplifier board. The builder set the amplifier to about -100 mV when cold, which moved to about +20 mV after warm-up. The drift matters because this Class A design runs very hot, so transistor operating points shift with temperature until the chassis and output stage stabilize. [#20687839]
What is DC offset in a power amplifier, and why is offset drift a concern in a hot-running Class A design?
"DC offset" is a power-amplifier output condition that leaves a steady DC voltage at the speaker terminals, instead of a zero-centered audio-only signal, and its key characteristic is that heat can shift this voltage as transistor conditions change. In this build, offset moved from about -100 mV cold to +20 mV hot, so the builder had to set it on a fully warmed amplifier. [#20687839]
Which transistors were used instead of the original Pioneer A-09 parts, and how well do TIP3055 and TIP2955 fit this redesign?
The builder confirmed he used the transistors shown in his redesign, including TIP3055 and TIP2955 as the output devices. He also stated the Pioneer A-09-based stage was redrawn around cheap, currently available parts. In practice, the analog section started immediately and only needed current and offset adjustment, which suggests the TIP3055/TIP2955 pair fit the redesign functionally. [#20687891]
What is SPDIF, and how was it routed here between the Bluetooth module, 4052 multiplexer, DIR9001 receivers, and CS4334 DACs?
"SPDIF" is a digital audio interface that carries stereo PCM data as a serial bitstream, and its key characteristic is that one source can be selected, decoded, and then converted to analog by a separate DAC stage. Here, SPDIF from Bluetooth, coaxial, or optical first goes into a 4052 multiplexer, then through optocouplers to separate DIR9001 decoders for each channel, and finally into CS4334 converters. [#20661024]
Why was the DAC and Bluetooth supply switched by relay only for digital inputs, and what noise or power benefits does that give?
The DAC and Bluetooth supply was relay-switched so those circuits only receive power when a digital input is selected. The builder said the digital section’s secondary side is powered from the audio supply through a relay, and when analog sources play, no voltage is applied to DAC or Bluetooth. That reduces unnecessary powered digital activity inside the chassis and avoids keeping extra sections energized all the time. [#20661024]
What are the best ways to make illuminated front-panel inscriptions visible only when lit, using PCB soldermask, etched laminate, smoked plexiglass, tinted glass, or laser-printed film?
The best thread-backed options were etched thin laminate behind dark cover material or darker tinted glass. The builder’s PCB with black soldermask hid labels well when off, but blocked too much light. Other users recommended 0.3-0.6 mm etched laminate with smoked plexiglass, laser-negative print on film, or darker glass such as Antisol, because those methods preserve the hidden-until-lit effect better than weak tint. [#20663469]
Tinted glass vs smoked plexiglass vs car-window film: which works best for hiding IR receiver cutouts and front-panel labels in an amplifier?
Tinted glass gave the most professional finish in this discussion. One experienced user said foil on plexiglass kept producing bubbles, scratches, and artifacts, then switched to cut and polished tinted glass with better visual results. The main limit was IR: with 6 mm black Antisol glass, the remote range dropped to about 3 m, so darker hiding can cost remote performance. [#20663469]
How hot is too hot for a Class A amplifier heatsink, and what case size or radiator changes would help if the chassis reaches about 75°C?
About 75°C was high enough here that the builder regretted the chassis choice and wanted larger heatsinks. He used a 30 cm deep case and later said he wished he had chosen a 3U enclosure with bigger radiators. In practice, that means the thermal design was workable but undersized, especially for a 1.5 A per-channel quiescent current Class A build. [#20661024]
How do soft-start and speaker protection work in an Atmega16-controlled amplifier with standby power and relay-switched transformers?
The Atmega16 control board runs from a standby supply and manages transformer switching, soft-start, and speaker protection relays. The builder said the control section handles backlight, buttons, and power switching, while the speaker-protection board is supervised by the microcontroller and mounts directly at the speaker terminals. That setup lets the amp power up in a controlled sequence and disconnect speakers under fault conditions. [#20661024]
What model was the volume potentiometer in this build, and where can I get the preamplifier PCB files, Gerbers, or spare JLCPCB boards?
The volume potentiometer was an Alps RK271 2x50k. The builder said he still had extra JLCPCB boards left, including 3 power-amplifier and selector boards and 4 of the other boards, because the minimum order was 5 pieces. He also offered to share Gerbers, Eagle files, and microcontroller code, asking interested people to contact him by private message. [#20672089]
Comments
Beautiful construction. As for subtitles, there are two methods: 1. Taking on photographic film. There are digital platesetters for film. Maybe someone who prepares stencils for offset printing using... [Read more]
Very nice! I admire that you wanted to make "mirror" tiles. You can easily sell for PLN 14,999 :-) [Read more]
The construction of beauty. However, I don't like one thing, actually two, because these are two amplifiers in one housing. It's about the way of mounting plates with power amplifiers. While it... [Read more]
Thank you friends for the kind words :) You do not need to unscrew - removing the transistors is not a problem, and to insert a new one, just slightly hook the ends of the legs. Checked ;) The only... [Read more]
Maybe white paper with the same negative print between the foil and the glass? And possibly delicate white LED backlight during the day. [Read more]
The construction is ok, but aren't they sometimes ready-made modules or for self-assembly? But it looks nice. [Read more]
Thanks, I'm looking into it :) Could you expand on the idea? I can't visualize it in my head. Are you asking if I didn't use ready-made layouts or couldn't I use ready-made ones... [Read more]
Prepare a 3D model of this glass so that the inscriptions are protruding (approx. 1 mm). Then find someone with a resin printer who will print it for you in transparent resin. The next step will be to... [Read more]
Great and thoughtful design, I'm impressed. Almost like it came from the factory. The only thing that puzzles me is that, due to the fact that the whole thing gets quite hot, is it right that the transformers... [Read more]
Beautiful design. I always admire my own audio equipment. When it comes to pictograms, I have been etching them on laminate for years. It's just that the laminate should be thin. Within 0.4, 0.6mm.... [Read more]
I got the idea that you want the subtitles to be visible when they are not highlighted. :) And you want the opposite. My mistake. [Read more]
The dimensions of this tile are 74x154. As for 3D, I only have AutoCad and 3D modeling is a pain in the ass, but I'll try to prepare something tonight or tomorrow, thank you :) That is why the... [Read more]
It's been a long time since there was such a refined structure on the forum. I wonder how long it took from the idea to the implementation of such a miracle? [Read more]
Insert a drawing in SVG format, I'll convert it to a 3D model in a few moments. I will also try to print, but I do not promise success, because the working field of the printer is "on contact". As... [Read more]
I used to etch such inscriptions on a 0.3 mm laminate and put 3 mm smoked plexiglass on the front. It worked. If it's supposed to be glass, aim for tinted car window film. You probably have every 5... [Read more]
A fine piece of work! I admire and envy ;) Out of curiosity, a question - what did you shield the power transformers with? Is it permalloy or plain copper? [Read more]
Thank you guys for your kind words and good advice :) Buddy LemuRR 11, I am very grateful for your willingness to help, but I don't want to bother you for now - I will try what you suggested first,... [Read more]
Let me have a "three cents" on tinted glass. Just like my colleagues, I covered plexiglass with foils, but something was always wrong. And this is a bubble, and this is some kind of scratch on the plexiglass... [Read more]
Out of curiosity, C102 = 100µF/25V. Why so much value? [Read more]