DIY Housing for 100W Audio Amplifier Module

This was a fun build around the Sure 2×100 Watt Amplifier Module. I spent most of the time on this project designing the chassis in SolidWorks and chasing around the cause of a fairly high noise floor. The final chassis which was very beautifully built by my friend Rodney.


Amplifier Specifications


The full specs for this amplifier are available on the seller page but the general specs are:

  • Power output (w/ 36 VDC power supply): 100W x 2 (6 ohms, THD 10%), 99W x 2 (6 ohms, THD 1%)
  • Frequency response: 20 to 20,000 Hz
  • Minimum impedance: 6 ohms
  • Fixed gain settings: 24 dB, 25 dB, 26 dB

Chassis Design


I designed the chassis as a typical rectangular design using SolidWorks. The main design feature is that I added some slots for ventilation on two sides. Since this amp was purposed for desktop use, I opted to remove the fan and use passive cooling. Rodney Bryant did an awesome job of bringing my design to life! Here are a few pictures he took while building the chassis:

Electrical Design


The wiring for this build was fairly straightforward, mostly switches and such. First, I opted to use a single 3.5mm input instead of an RCA inputs since. I used an external power supply and placed a resettable fuse on the positive power rail. I then wired three switches to the amp: a main power switch in the rear, a standby switch, and a mute switch. Since I opted for passive cooling, I removed the fan and wired an LED to the fan connector.

Noise Floor

The main issue I encountered was the high noise floor (loud humming when nothing is connected and even when something is connected and is at low volume). I lost the shielding benefits presented by a metal chassis since this chassis was made out of wood, .

Because I was using a 3.5mm input, I had to solder some connectors onto the board instead of using the already present RCA connectors. The noise floor was drastically reduced when I connected an input to the RCA jacks as opposed to my 3.5mm input. This at least narrowed down the issue to my wiring. I tried the usual recommendations (shorter wire runs, shielded cables, twisted pairs) but nothing seemed to work.

Eventually I realized that in all my testing I had the ground from the 3.5mm input connected to the volume potentiometer and then from there to the amplifier. Instead, I needed to connect the ground directly from the input to the amplifier, not the potentiometer. This design is closer to the “star ground” technique where all the grounds are tied to a central point. Making this simple wiring change immediately eliminated the high noise floor and made the amplifier usable.

Volume Control

The volume control knob used in the initial design was a dual 50K “audio taper” potentiometer. Volume control potentiometers must have a logarithmic response because the human ear doesn’t perceive changes in loudness in a linear fashion. Here is a great article about the nonlinearity of human hearing.

I discovered that all the potentiometers I ordered (first from Amazon then from Mouser) were very flimsy and also introduced some scratching noises (they were all graphite pots) sometimes when switching. This was the opposite of the smooth and heavy feel I wanted and why I bought a metal knob.

A stereo stepped attenuator is typically the “professional” solution to my problem but I was already over budget for the build and I didn’t have room to fit a large component. On a hunch I went to Goodwill and happened to find an old record player with a volume potentiometer having the exact characteristics I needed. I switched it out and it turned out to be a perfect fit — I didn’t even need to drill out the hole. Unfortunately I didn’t take any pictures when making the change.

Notes

Overall this was a fairly straightforward and fun build. My search for the perfect volume control potentiometer and my improperly grounded input caused the biggest delays in the build. Otherwise, this amp is a perfect addition to my setup!

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