In this blog post I test the ACA Mini Amplifier by Nelson Pass. This is a kit based amplifier that retails for $110 USD from https://diyaudiostore.com/collections/aca-mini.
For an additional $25.00 the ACA Mini "Essentials" kit includes:
- 1 x ACA Mini PCB
- 2 x LSK170
- 2 x LSJ74
- 2 x Harris IRF9520
- 2 x Harris IRF520
This brings the total to $135 USD.
Nelson's philosophy is to sell the board and active devices separately to allow diyers to purchase their own parts and customized as they see fit. The completion kit is intended for beginners who want to get it up and running as quickly as possible without having to spend time sourcing parts on a bill of materials.
For the technical guide click here.
There's also a youtube video I found which reviews the amp.
Specifications
- 5w output power
- 2-Channel Stereo
- Power switch
- Includes 24V DC outboard power supply
- RCA input with set screw speaker output terminals
- Class A two-stage complementary push-pull amplifier
- Jfet input transistors
- Mosfet output transistors
- Current feedback loop (CFA).
The ACA Mini Amplifier does not include a volume control.
Before testing I confirmed that the amplifier was operating at the specified voltages by adjusting two trim pots according to the provided instructions. This was relatively straightforward.
Measurements
I tested the amplifier at the following output voltages.
- 0.06V
- 0.25V
- 1.00V
- 4.00V
I measured the following types of distortion.
- Harmonic
- Intermodulation (IMD)
- Gedlee Metic (Gm)
Harmonic Distortion
We see that at the 0.06V output H3 is at 0.19% for the 10kHz region.
The 0.25V output shows H3 at 0.016% for the 10kHz region.
The 1.0V output shows H3 at 0.016% for the 10kHz region.
For the 4.00v output we H3 at 0.035% for the 10kHz region.
Intermodulation Distortion
The 0.06v output shows IMD at -64dB.
The 0.25v output shows IMD at -75dB.
The 1.00v output shows IMD at -77dB for the 10kHz region improving to -81dB for the 20kHz region.
I attempted to test the amplifier at the 4.00v output however the amplifier did not have the output capability even with a larger power supply. So I had to set the amplifier close to it's maximum output voltage of 2.3v. We see IMD is limited to -58dB.
I then decided to power the ACA mini with my Chord Mojo which has much more output voltage than the Topping D10S used earlier.
The 4.00v output from the ACA Mini shows IMD at -38dB.
The amplifiers circuit board includes a removable jumper which defeats the negative feedback. Below is a comparison with the jumper removed using the 1.00v output.
With Jumper
Jumper Removed
We can see that distortion increases +10dB when the jumper is removed.
Gedlee Metric (Gm)
We see Gm is very low at only 0.0002 for the 0.25v output. This is comparable to my KT90 tube amplifier tested here.
Conclusions
We can see that the ACA Mini provides peak performance in the 0.25V to 1.00V output range which equates to about 1/16 watt. In this region we achieve around -75dB IMD distortion performance. This places the ACA mini in the ultra-low power category making the ACA Mini suitable for powering high sensitivity compression drivers. To highlight this I decided to connect the ACA mini to the Ciare CT440 Bullet Tweeter which has a of 110dB@1w. At 0.25V amplifier output the tweeter provided 91dB output at 1m.
In this configuration the ACA Mini provides slightly less distortion than the tweeter itself. To demonstrate this I measured the Gm distortion of both the amplifier and tweeter output as an overlay using 0.25V output from the amplifier. We see the ACA becoming a bottleneck for distortion at the 5kHz region where the two overlays converge, but otherwise the ACA distortion is lower.
I detected no hiss or buzz from the tweeter. This is rare since nearly all amplifiers I test require a fixed resistor L-pad to bring the amplifier out of the noise floor. This is not the case with the ACA Mini.
Subjective Listening
I tried the ACA Mini on some medium sensitivity speakers (89dB@1w) and found the amplifier lacked the clarity I've come to expect from other amplifiers such as Hypex. So I decided to see how it would fare connected to a high sensitivity tweeter. Listening the the Ciare CT440 and Fostex T96A super tweeters, the ACA Mini provided excellent overall sound quality for this specific use case.
Considering the cost of only $135 USD for the kit, I would recommend the ACA Mini if you are looking for an amplifier solution to power very high sensitivity drivers in a bi-amp situation. It provides a smooth sound character and very low noise floor when operating in the sub-one-watt range. Although I did not try this, the ACA Mini may work well with high sensitivity full range drivers, particularly with 8" full range drivers which can have sensitivities exceeding 100dB in the treble region.