DIY Sound Adventure: Amp Camp Amp (ACA) Project – The perfect entry-level amplifier for horns

Nelson Pass and the Amp Camp Amp (ACA) – A beginner DIY project for sound enthusiasts

Nelson Pass is an icon in the field of high-end audio amplifiers. The American engineer behind renowned companies such as Pass Labs and First Watt has been known since the 1970s for designing amplifiers with minimalist circuits that deliver outstanding sound quality despite their simplicity. His Class A amplifiers are particularly well-known, setting the standard for natural and detailed music reproduction. For DIY audio enthusiasts, Pass has launched the "First Watt" project, which focuses on low-wattage amplifiers with superior sound fidelity. The Amp Camp Amp (ACA) is one of the most accessible projects of this approach and a fantastic way to delve into the world of DIY audio building.

Source: https://www.firstwatt.com/

The Amp Camp Amp: An introduction to the Class A amplifier world

A few years ago I started looking for a suitable amplifier for my active system, which is equipped with BMS 4592 coaxial drivers and JBL 2360 horns . The BMS 4592 driver is an exceptional coaxial driver that has both a midrange and a tweeter membrane to enable precise reproduction over a wide frequency spectrum. In conjunction with the JBL horn, this allows you to cover a frequency range of 450Hz-20kHz. Due to the coaxial design, I therefore need two power amplifiers per speaker, and with stereo, I need four. That's where the Stereo Amp Camp Amp (ACA) from Nelson Pass came into play, or rather two in my case - a small but extremely interesting Class A amplifier that is ideal for my setup.

Technical data of the Amp Camp Amp:

• Power: 5 watts at 4 and 8 ohms
• Distortions:
• At 1 watt at 8 ohms: K2 at -50dB
• At 2 watts at 8 ohms: K2 at -45dB
• At 5 watts into 8 ohms: K2 at -40dB, 1% distortion
• Signal to noise ratio: 110dB
• Distortion at 4 Ohm: Higher than at 8 Ohm

Source: https://www.firstwatt.com/

For a detailed technical analysis of the ACA, I recommend the review on AudioScienceReview :
Audioscience Review ACA

What does this mean in practice?

• The Amp Camp Amp offers little power, but this is ideal for horns like the JBL 2360, as they are very sensitive and require only low wattages.
• The distortion increases noticeably at 5 watts at 8 ohms, but this is rarely or never relevant for high-efficiency loudspeakers in normal operation.
• The signal-to-noise ratio is excellent, which ensures clear and interference-free playback. There is also no hum.

Special features of the ACA in my version:

• 60,000 µF capacity per channel
• FET input and MOSFET output in typical pass design
• Two IRFP240 MOSFETs
• Short-circuit proof outputs

Materials and components

There are several ways to build the ACA. You can either order the complete kit or just purchase the circuit boards and select the components from various retailers at your own discretion.

I used the following materials for my construction:

1. ACA boards:
   Link: ACA boards with components

It is advantageous to purchase a circuit board that already has rectification and voltage stabilization (electrolytic capacitors) on board. Circuit boards can usually be found on eBay.

1. Modushop housing:
   Modushop Mini Dissipante Case: Link to product
2. Rear wall milling drawings:
   DXF drawing back: Link to file
3. Connections:
• IEC socket
• RCA jack
• Speaker terminals
4. Reichelt shopping cart:
• Thermal paste, metal oxide powder, 5ml, 1x
• Spacer sleeves, metal, hexagonal, M3, 5mm, 8x
• Toroidal transformer, 120 VA, 2x 15V, 2x 4.0A, 1x

In addition, M3 x 5mm and x8mm screws are required to attach the circuit boards as well as the transistors and the bridge rectifier.

Why a 15V toroidal transformer and not 18V?

A 15V toroidal transformer delivers around 21.21V (15V * 1.414) after rectification, while an 18V transformer delivers 25.45V. Since the electrolytic capacitors used are designed for a voltage of 25V, an 18V transformer could affect the service life of the components in the long term, especially in the event of voltage fluctuations. The 15V transformer therefore offers more safety and durability for the project.

Construction process

After some preparation time and waiting for the ordered parts, I was able to start building:

1. Soldering circuit board parts:
   I assembled and soldered the boards myself, which allowed me to keep complete control over the construction.
2. Drilling holes:
   In my father's workshop I drilled the holes for the spacers, the circuit boards, transistors and the rectifier.
3. Placement of the boards:
   At home I cut the threads and mounted the circuit boards.
4. Wiring:
   I then wired the amplifier and connected the back to all the connections. The wiring/connections are self-explanatory for experienced DIYers.

Conclusion: Sound quality of the Amp Camp Amp

The sound of the Amp Camp Amp surprised me in a positive way. It is very homogeneous , controlled , but not too tight. The stage image is excellent and impresses with depth and precision. However, it must be noted that the amplifier can only reach its full potential with speakers with high sensitivity. I also tested the ACA with speakers that have an efficiency of 90 dB , but that was not enough. It should be at least 95 dB efficiency so that the amplifier can achieve the necessary dynamics and volume.

If you are interested in an exciting DIY project and want to experience the characteristic sound of a Class A amplifier, I can only recommend building the Amp Camp Amp. It is a worthwhile project with excellent sound and plenty of scope for your own adjustments.

Greetings
Florian