- Build Rating
I've recently finished this power amp build for use with my bass rig. I'd built a fEARful 15/6 cabinet last year, and wanted something light and powerful enough to get the most out of the cab. I'd previously been using the power section of a Hartke LH500 (100w into 8ohms) with a C2CE nobelium (and a sansamp q\strip prior to that), and ultimately liked the idea of DIY'ing my whole rig.
It's based around the ICEPower 250ASX2 board in BTL mode, which should provide 500W into an 8ohm load. This is an OEM board that crops up in a number of commercial Class D bass amplifiers and combines the amplifier and power supply, providing a +/-24V unregulated power output to power any other circuitry.
The main part of this build were designing the input circuitry, most of it are the reference designs for BTL mode that are described in the 50ASX2 designers manual (there are 2 versions of this document kicking about, this one labelled 1-1b is more complete and has suggested components). The VU driver circuit came from fig.7 from Michael Fidler's page on VU meters, and I splashed out on Douglas Self's Small Signal Audio Design, which provided the design for the balanced/unbalanced input circuitry. I've learned a huge amount from this book (and this project in general), and it's left me more confident in my understanding of what's going on in audio circuits. I ended up implementing it all on various bits of protoboard, which is fine for this as a one-off, but I may return to this in the future. There's a power board that converts the +/-24V down to a regulated +/-12V, this is also the reference design from the datasheet.
I had a load of rotary switches kicking about, and wanted some kind of volume control on the box, so this ended up as a attenuation switch with a few fixed levels of attenuation. I'd originally intended this to be the main way (along with volume on preamps) of ensuring that the ICEPower's max input of +/-3.3V wasn't being exceeded. After the first test, and hitting some Class D distortion, I ended up adding a soft clipping circuit to the back of the attenuator switch. This can handle up to the +/-12V I was seeing coming out of my Nobelium, but is less than ideal for what should be a clean power amp. I'd like to experiment with a limiter, but this got this usable at higher volumes for now.
The fan might be overkill, as the enclosure is fairly big and leaves the datasheet-defined distance (14mm) around the ICEPower board, but I wanted some additional cooling for it and the linear regulators. It's just attached to the +12V rail, so just runs continuosly. The enclosure is a Hammond 1456NEK4WHBU.
I'm really pleased with how it's turned out - sounds fantastic, and I'm getting excellent thump out of the 15/6, and it's got more than enough power to reproduce the low resonant sweeps of the filters I use.
Bonus photo of it at the end with the rest of my DIY rig - a jazz bass build I've been using for years, a Schalltechnik Pumpernickel, Aion Spectron, C2CE Nobelium, the power amp and the fEARful 15/6. (TC electronics polytune, pedalboard and CIOKS SOL not DIY, but don't care to make any of those things).
It's based around the ICEPower 250ASX2 board in BTL mode, which should provide 500W into an 8ohm load. This is an OEM board that crops up in a number of commercial Class D bass amplifiers and combines the amplifier and power supply, providing a +/-24V unregulated power output to power any other circuitry.
The main part of this build were designing the input circuitry, most of it are the reference designs for BTL mode that are described in the 50ASX2 designers manual (there are 2 versions of this document kicking about, this one labelled 1-1b is more complete and has suggested components). The VU driver circuit came from fig.7 from Michael Fidler's page on VU meters, and I splashed out on Douglas Self's Small Signal Audio Design, which provided the design for the balanced/unbalanced input circuitry. I've learned a huge amount from this book (and this project in general), and it's left me more confident in my understanding of what's going on in audio circuits. I ended up implementing it all on various bits of protoboard, which is fine for this as a one-off, but I may return to this in the future. There's a power board that converts the +/-24V down to a regulated +/-12V, this is also the reference design from the datasheet.
I had a load of rotary switches kicking about, and wanted some kind of volume control on the box, so this ended up as a attenuation switch with a few fixed levels of attenuation. I'd originally intended this to be the main way (along with volume on preamps) of ensuring that the ICEPower's max input of +/-3.3V wasn't being exceeded. After the first test, and hitting some Class D distortion, I ended up adding a soft clipping circuit to the back of the attenuator switch. This can handle up to the +/-12V I was seeing coming out of my Nobelium, but is less than ideal for what should be a clean power amp. I'd like to experiment with a limiter, but this got this usable at higher volumes for now.
The fan might be overkill, as the enclosure is fairly big and leaves the datasheet-defined distance (14mm) around the ICEPower board, but I wanted some additional cooling for it and the linear regulators. It's just attached to the +12V rail, so just runs continuosly. The enclosure is a Hammond 1456NEK4WHBU.
I'm really pleased with how it's turned out - sounds fantastic, and I'm getting excellent thump out of the 15/6, and it's got more than enough power to reproduce the low resonant sweeps of the filters I use.
Bonus photo of it at the end with the rest of my DIY rig - a jazz bass build I've been using for years, a Schalltechnik Pumpernickel, Aion Spectron, C2CE Nobelium, the power amp and the fEARful 15/6. (TC electronics polytune, pedalboard and CIOKS SOL not DIY, but don't care to make any of those things).
