Pass Labs ACA Power Amplifier

Introduction

The Amp Camp Amplifier ("ACA") is a simple amplifier that was designed to be built in a single afternoon. It was created by Nelson Pass for an event called "Amp Camp" in which a small group of people turn up and by the end of the day, they would have built a small amplifier.

This means that the build has to be straightforward, even for the first-timer. Indeed, the build is quite easy, and the very detailed build guides on the internet will certainly ensure that you get this up and running in no time at all.

A complete kit with everything you need is available at the Diyaudio Store - just be quick, because they sell out really fast !

Description

The ACA is a low power Class A power amplifier which is able to deliver 8W per channel into an 8 ohm load (at 3 % distortion), or 15W if operated as a monoblock. It is built in a very compact chassis, with a separate power brick supplying 24V DC to the ACA. This is a great idea for a first-time project as you don't have to mess around with potentially lethal voltages.  

The current version of the kit at the time of this post is V 1.6, which was the version I built. Other important specifications :-

Input impedance - 10 Kohm
Gain                    - 14 db
Damping factor  - 10
Output noise       - 100 uV

For a detailed description of the circuit, you can read what Papa Pass (as he is affectionately called in the DIY audio scene) has to say here.

Build Experience

I highly recommend this for the beginner. Like any kit, it would be helpful if you practice your soldering on a practice or waste board first. Everything you need is in the kit, except for soldering iron, solder and a multi-meter. I assume of course that you have basic tools around the house, like screwdriver set, pliers and a cutters to snip of the component leads.

This is a PCB kit, and the very low parts count means that you really can finish this in half a day if you are minded to do so. 

The kit includes both blue and red LEDs - choose your favourite colour. I friction fit those LEDs for fun, and settled for red LEDs.

Like any Class A circuit, this runs quite hot. It settles down at about 18-20 degrees C above ambient temperature.

Like anything in life, things can go wrong if you do not take due care. This kit is straightforward enough that your possibility of messing things up are quite low. The most difficult part of assembly is likely to be affixing the transistors to the heatsinks (installing the insulation pads wrongly could either short out your transistors, or cause them to overheat. The second most difficult part (for a beginner) is biasing the transistors. Well, for me the most difficult part was aligning up the chassis as one of the rails arrived bent out of shape.

In operation, the ACA runs hot - it needs every square inch of those heatsinks ! After about two hours of operation, the faceplate is quite hot to the touch too. There is no anti-thump circuit, and the ACA will make rude sounds when powering off, and a loud thump when switched off. If you are using sensitive speakers, this is loud enough to scare both pets and probably their owner too.   

Sonic Quality

The limited power requires careful partnering. If you have typical speakers (e.g. 86-89 db sensitivity), you certainly can’t go very loud with the ACA. However, the few watts on tap will suffice for background music, and even reasonable listening levels if you are sit close to your speakers and listen to less complex music. To extract the most out of this amplifier, high-efficiency speakers are highly recommended.

The ACA has a very pleasant smooth and warm quality that makes listening very pleasurable. I spent more than a week during this lock-down, paired with my Vivid Giya G4 speakers (hardly the easiest load). Used within its power limitations, the ACA actually sounds very good for the money invested (Let's face it - pickings are slim for Class A amplifiers below USD 500).

Soundstaging is reasonably good, with good dimensionality. The ACA has a very nice grain-free and refined sound that eludes most solid state designs. The only obvious weakness was a lack of bass control with a flabby and loose low-end. Given the low damping factor of the ACA and the unsympathetic load, this was hardly surprising. Time to try out the ACA with more suitable speakers !

A pair of Tannoy Kensington GR speakers proved to be a better match although it was evident that more power was needed. Although they are rated at 93 db sensitivity, the Tannoys are not known to be an easy load either. They sound quite good at background music levels, with a usable limit at a volume in between my usual listening levels and that. Pushed harder, and the amp begins to harden in sound, as well as lose its limited grip on the woofers. Perhaps my very easy to drive Zu Dirty Weekend II speakers would be a perfect match ? I'll update this post once I've tried that.

Conclusion

The ACA is a pleasant amp that is easy to build and an excellent introduction to the wonderful world of DIY. If you have reasonable expectations and a suitable partnering pair of speakers, you could end up with sonic bliss at a very modest budget. If you are expecting something really special with a sound that rivals First Watt or Pass Lab's offerings, you are going to walk away disappointed. 

Pass Labs ACA Power Amplifier
Available direct from diyaudiostore.com
USD 327