diyAudio ACA MinMax Amplifier Instruction Manual

: June 8, 2024
: diyAudio

Table of Contents

diyAudio ACA MinMax Amplifier
Specifications
Product Usage Instructions
FAQ
Overview
Options
ACA MinMax Wiring
ACA MinMax Rear Panel Mods
References
Read User Manual Online (PDF format)
Download This Manual (PDF format)

diyAudio ACA MinMax Amplifier

Specifications

PCB holes and transistor locations match the Store chassis
35mm capacitor footprints for increased voltage rating (50V)
Compatible with TO-220 or TO-247 output devices
Requires an SPDT front switch to enable/disable bias
XLR input for balanced mono (balanced source required)
Recommended bias: 0.8 amp using a 36VDC power supply (max 48V)
Estimated power per channel: 10 watts @ 8 ohms (double for monoblock operation)
Dimensions of each PCB: 60mm X 127mm

Product Usage Instructions

Ensure all parts are included as per the bill of materials
Match PCB holes and transistor locations with the Store chassis
Install output transistors (IRFP140 and IRFP9140) or other compatible MOSFETs
Optional: Install gate stopper resistors R12, and R13 for better performance

Bias Adjustment

Use the SPDT front switch to enable or disable bias in the circuit. The recommended bias is 0.8 amp using a 36VDC power supply.
Adjust bias as needed within the specified range.

2nd Harmonic Distortion Adjustment

For adjusting the 2nd harmonic distortion profile, connect or disconnect jumper JP1 according to your preference for sound quality (warmer vs cleaner sound).

Power Supply

Ensure the power supply voltage does not exceed the capacitors’ voltage rating (50V caps in the kit).
Use the provided external 36V, 5A switching power supply or a compatible linear power supply.

Additional Filtering

If using a switching power supply, consider incorporating a SMPS DC filter like the P089ZB filter.
Alternatively, you can use the AmyAlice filter with your PCB fabrication from Gerber files.

FAQ

Q: Can I use different output transistors with the ACA MinMax?
A: Yes, as long as they are compatible with the chassis and have good thermal contact with heatsinks.
Q: How do I adjust the sound quality of the amplifier?
A: Use the optional RC network and jumper to adjust the 2nd harmonic distortion profile for a warmer or cleaner sound.
Q: What power supply should I use with the ACA MinMax?
A: It is recommended to use a 36V DC power supply, ensuring it does not exceed the voltage rating of the capacitors in the kit.

Overview

In 2021, Nelson Pass created the ACA Mini as a follow-up to the original Amp Camp Amp. The ACA Mini is a Class A complementary, push-pull amplifier running on a single-ended (not bipolar) 24V DC power supply, rated for about 5 watts of power per channel. The output transistors use small “board level” heat sinks on the PCB to dissipate heat, and the complete amp was intended to be run “naked”, without any additional chassis. The stated goal was to “make a nice little class A amplifier that brings some interesting circuit design elements at a much lower cost, while still appealing to more sophisticated audiophiles.”
At Burning Amp 2023, @PSz. asked Nelson Brock to adapt the ACA Mini into the diyAudio Store’s ACA chassis, to allow for higher power supply voltage and bias current. The ACA MinMax is the result of that collaboration.

Key Features and Specifications
The ACA MinMax uses two “mirrored” PCBs, with each board providing a single channel of amplification. They are designed to mount in the ACA chassis available from the diyAudio Store (Store) or ModuShop. Because of the increased heatsink capability, the MinMax uses a 36V DC external power supply. The output power with 36V DC and the recommended 0.8A bias is estimated (not measured) to be around 10W/channel into 8 ohms. If the amplifier is configured as a parallel or balanced monoblock, then output power will roughly double, depending on speaker impedance. Additional features of the MinMax are:

PCB holes and transistor locations match the Store chassis
Right and left PCB layouts to keep wiring clean
35mm capacitor footprints for increased voltage rating (50V)
Compatible with TO-220 or TO-247 output devices
Optional gate resistors (R12, R13)
Requires an SPDT front switch, (the original ACA uses SPST) which is used to enable or disable bias in the circuit
XLR input for balanced mono (balanced source required)
Rear switch can be used for parallel mono
Markings on the ACA chassis’ back panel can be modified for the MinMax

Specifications

Recommended 0.8 amp bias using a 36VDC power supply, other supply and bias options can be used (max 48V)
Estimated power per channel: 10 watts @ 8 ohms. Double for monoblock operation
Dimensions of each PCB: 60mm X 127mm

Schematic

One channel is shown
There are a few minor changes compared to the ACA Mini:

Change output MOSFETs, Q3 and Q4, to IRFP9140 and IRFP140, respectively
Reduce value of degeneration resistors, R8 and R9, from 0.75 Ω to 0.47 Ω
Add optional gate resistors, R12 and R13
Increase voltage rating of C2, C3, C4, C5 from 25V to 50V
Increase resistance of LED resistor, R0
Each channel has its LED

Here is the recommended list of parts (also called bill of materials or BOM)

Options

Output Transistors
The original ACA Mini specified TO-220 size output transistors. The MinMax kits come with IRFP140 and IRFP9140 transistors, which are in a larger TO-247 package. The primary benefit is lower junction temperature at higher bias due to better thermal transfer to the heatsinks. A wide variety of MOSFETs, including the original ACA Mini’s IRF520 + IRF5920, will also work without issue as long as they make good thermal contact and are isolated from the heatsinks.
Gate stopper resistors R12, R13
These resistors are optional and are only needed when the MOSFETs have high gate capacitance, which in theory could stress the JFETs if you run square waves through the amp, or in extreme cases cause ringing/oscillation. They are not necessary for any typical build of this design, but there is no real penalty for using them either. If you are of the “less is more” persuasion, leave them out. If you like the idea of a little extra JFET insurance, add them in. If you do not use these resistors, be sure to jump the pads for each resistor with solder or a small length of wire.
2nd Harmonic Distortion Profile Adjustment
Like the ACA Mini, the MinMax includes an optional RC network (R11, C6) and a jumper (JP1) which, when connected, reduces the 2nd harmonic distortion at the output (see discussion in the original ACA Mini article). In short, the jumper disconnected will have a “warmer” sound and the jumper connected will be a “cleaner” sound. If you do not wish to have this adjustment available, then the resistor, capacitor and jumper can all be omitted.
Power Supply and Additional Filtering
The MinMax was designed with 36V DC operation in mind, and the kit in the Store will come with an external 36V, 5A switching power supply.
The MinMax can be powered at the original 24V, or as high as 48V if that suits your goals for the build. In any case, the bias should be adjusted to keep the heatsinks below 60°C. The dissipated power is roughly equal to the PSU voltage times bias current, so more voltage will mean a proportionally lower bias for a given heatsink.
When using a linear power supply, you must check to ensure that its unloaded (open circuit) voltage will never exceed the voltage rating of the capacitors (50V caps in the kit). The on/off switch of this amplifier reduces the bias to zero but does not disconnect the supply from the filter capacitors.
If you are using a switching power supply, then you may consider using an SMPS DC filter in your MinMax like the P089ZB filter sold in the Store. Another alternative is the AmyAlice filter, which uses surface mount devices (SMD) and is not sold in the Store (you need to have your own PCBs fabricated from Gerber files available on the AmyAlice thread. Both the P089ZB and AmyAlice filters were designed by Mark Johnson. You will have to devise your mounting solution within the ACA chassis.

Construction notes
A step-by-step build guide is in the works and will be linked here in a future revision. The usual advice of verifying that you have all the parts needed before starting, stuffing the boards with components from smallest to largest, and checking the value of each part with a multimeter as you go, all apply here. The chassis wiring is expected to be the most difficult part of the build. The kit provides TO-247 output transistors with alumina ceramic thermal pads and thermal grease. The grease packet provided, spread equally between both sides of all four thermal pads is the correct quantity to use. It might seem like too little, but the purpose of the compound is to fill in the very tiny irregularities between the heat sink, pad, and transistor case. A thick coating reduces thermal transfer. High-quality silicone pads, for example, Keratherm Red, match the performance of the alumina + grease and are a convenient no-grease option, though they are relatively expensive. Lower- quality silicone pads are not recommended for this power level. If you are using output transistors with TO-220 cases, keep in mind that the screw needs to be isolated from the tab, so use a stepped isolation washer or a nylon screw.
Adjustment
Set-up and adjustment should follow the procedure in the original ACA Mini article or build guide, but with the following changes using a 36VDC power supply: Before first power up, be sure to turn trimmer potentiometers P1 and P2 so that they measure zero ohms between all three pads. This should mean fully counterclockwise, but there is a large variety in trim pot design, so the best way to avoid errors is by measuring and adjusting them just after soldering.
Output voltage balance: the voltage between Test Point 1 (labelled “Vo” on the PCB) and ground TP3 (labelled “GND”) should be between 17.2 and 17.8 volts.
Bias: For the recommended 0.8 amp bias, the voltage between TP2 (labelled “Vb”) and ground should be 0.38 volts if you are using the 0.47 ohm degeneration resistors (R8, R9). For those who wish to experiment with other biases or resistors, Ohm’s law is used to calculate the bias, current equals voltage divided by resistance.

Ultimately it is up to each builder to select an appropriate bias current for their build that takes into consideration the local climate, airflow around the heatsinks, and personal tolerance for hot metal objects in the home.
Best practice is to not exceed 60°C (140°F) external temperature. You should be able to place a hand on the heatsinks for several seconds without pain. The MinMax does not have temperature compensation and bias can increase as temperature increases. It is a good idea to periodically check the bias current and readjust as needed, especially when the weather is warm.

Rear Panel Mods
The ModuShop / diyAudio ACA chassis allows us to easily configure this amplifier as balanced or parallel mono. The ACA has a third option, bridged mono, that the MinMax does not support. The ACA 1.8 is an inverting amplifier that allows for a cool hack that connects the inverted output of one channel back to the input of the other channel. The MinMax (and Mini) are noninverting, and cannot do the same. We are reusing the ACA chassis, so the markings on the rear panel need to be modified to show the appropriate connections and configurations for this build. A graphic showing how to modify the rear panel is in the appendices of this document.
Stereo Operation
Wiring the MinMax for stereo operation is straightforward. Put the rear switch in the up position and make connections as you would any other amplifier.
Monoblock Operation
The MinMax can be configured for either balanced, or parallel monoblock operation. This combines the two amplifier channels into a single, more powerful channel. Balanced operation requires a balanced signal into the XLR connector and the rear switch in the up position (same as stereo). The speaker positive is connected to the output A positive (red) terminal and the speaker negative is connected to the output B positive (red) terminal. The parallel operation takes a single-ended signal into either RCA input with the rear switch in the down position, which connects the two inputs. The A and B outputs need to be connected in parallel by adding a wire externally between both positive (red) terminals. The negative (black) terminals are already internally connected. The speaker now gets connected to either set of terminals as you would expect, red to red, black to black.

Links

Nelson Pass’ original ACA Mini Article (PDF)
Recommended Parts List (BOM)
ACA MinMax Build Guide
ACA MinMax thread on diyAudio.com
ACA Mini thread on diyAudio.com
ACA MinMax kit at diyAudio Store

Appendices

Chassis Wiring Diagram
Rear Panel Modifications

ACA MinMax Wiring

For DIY Audio Store ACA chassis

ACA MinMax Rear Panel Mods

For DIY Audio Store ACA chassis

Use a permanent marker to black out the bridged mono markings and use stickers to change output connection polarity as shown below
Mark voltage as appropriate for your build