500W Class D Power Amp (ICEPower 250ASX2 based)

[fmcc]

Build Rating

5.00 star(s)

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).

 

[MattG]

That’s 100% badass! Great job, thanks for sharing!

 

[iamjackslackof]

At first I thought you repurposed an old enclosure of some kind. Then I realized you started with a blank one...

This is seriously one of the coolest DIY audio projects I've seen!

 

[Passinwind]

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).

Lookin' good. I was deeply involved with the early fEARful crossover design work and still talk to Dave Green fairly regularly.

FWIW, I've built DIY amps with both the 700ASC (now 700AS1) and 500ASP modules, The 700AS1 already has a decent regulated +/-15V aux supply, which really simplified things. One was a standalone 700 watt power amp and the others were all integrated bass amps. I still have the 500ASP build and use it more for acoustic guitar than bass these days, but it's an excellent bass amp too. I managed to use convection cooling on all my builds, as I despise fan noise, but a fan is certainly easier and lighter!

 

[Alan W]

Wonderful! Any build report that includes VU meters and mentions Douglas Self gets automatic love from me.

 

[Alan W]

Lookin' good. I was deeply involved with the early fEARful crossover design work and still talk to Dave Green fairly regularly.

FWIW, I've built DIY amps with both the 700ASC (now 700AS1) and 500ASP modules, The 700AS1 already has a decent regulated +/-15V aux supply, which really simplified things. One was a standalone 700 watt power amp and the others were all integrated bass amps. I still have the 500ASP build and use it more for acoustic guitar than bass these days, but it's an excellent bass amp too. I managed to use convection cooling on all my builds, as I despise fan noise, but a fan is certainly easier and lighter!

I hate fan noise too, but there are some really quiet (in the 25 db range) fans available. In projects I built where noise was an issue (power supplies for photographers was one niche) combining multiple small fans and really thinking about air flow, I was able to get some packages that you'd need to be in a really quiet room to hear from more than a few feet away. This also allowed me to do stepped cooling systems (where some fans stayed off until a specific temp. threshold was passed) without needing to resort to chip controlled systems. On the other hand, there's silence, and there's quiet, and I do love looking at boxes with big heatsinks. No fans in my stereo, and the supplies tend to be shunt regulated, which means you set them to deliver the maximum power you need, and what isn't needed warms the room. (Not on my power amps though—there is a limit.)

 

[fmcc]

Lookin' good. I was deeply involved with the early fEARful crossover design work and still talk to Dave Green fairly regularly.

FWIW, I've built DIY amps with both the 700ASC (now 700AS1) and 500ASP modules, The 700AS1 already has a decent regulated +/-15V aux supply, which really simplified things. One was a standalone 700 watt power amp and the others were all integrated bass amps. I still have the 500ASP build and use it more for acoustic guitar than bass these days, but it's an excellent bass amp too. I managed to use convection cooling on all my builds, as I despise fan noise, but a fan is certainly easier and lighter!

Yes! I referred to your builds again and again as I went through the process of making it. I'm pretty confident that it was them (along with any other ICEPower based builds on the talkbass forums) that originally inspired me to try this out. Thank you!

 

[fmcc]

I hate fan noise too, but there are some really quiet (in the 25 db range) fans available. In projects I built where noise was an issue (power supplies for photographers was one niche) combining multiple small fans and really thinking about air flow, I was able to get some packages that you'd need to be in a really quiet room to hear from more than a few feet away. This also allowed me to do stepped cooling systems (where some fans stayed off until a specific temp. threshold was passed) without needing to resort to chip controlled systems. On the other hand, there's silence, and there's quiet, and I do love looking at boxes with big heatsinks. No fans in my stereo, and the supplies tend to be shunt regulated, which means you set them to deliver the maximum power you need, and what isn't needed warms the room. (Not on my power amps though—there is a limit.)

The fan spec puts it at 24 dBA - I can barely hear it over any room noise when I'm not playing. I had been worried about it introducing electrical noise, and I did test it with the input circuitry before settling on how it's wired in.

 

[Passinwind]

Yes! I referred to your builds again and again as I went through the process of making it. I'm pretty confident that it was them (along with any other ICEPower based builds on the talkbass forums) that originally inspired me to try this out. Thank you!

Cool, and you're welcome. I had a bit of help from some great people along the way, including the US ICEpower rep at one point. I'm planning on doing a powered cab for acoustic guitar next, which should be a lot simpler and cheaper than the high powered bass stuff.

The fan spec puts it at 24 dBA - I can barely hear it over any room noise when I'm not playing. I had been worried about it introducing electrical noise, and I did test it with the input circuitry before settling on how it's wired in.

I'm not always too fussed about a little background noise in many/most playing situations, but I do play as the only amplified instrument fairly often, where being as silent as possible in between tunes is definitely a bonus. Plus, it's a real challenge and a way of differentiating my builds.

 

[Passinwind]

@fmcc : you might take a look at the online Traynor service manuals for the SB series amps, which use ASX2 power modules and a pretty simple optical limiting format. I've been using an SB200 head for my friend's weekly bluegrass/Americana jam for the last couple of years, it's actually the only commercial amp I own at this point.

 

[fmcc]

@fmcc : you might take a look at the online Traynor service manuals for the SB series amps, which use ASX2 power modules and a pretty simple optical limiting format. I've been using an SB200 head for my friend's weekly bluegrass/Americana jam for the last couple of years, it's actually the only commercial amp I own at this point.

Thanks, I've found the service manual and that's really useful to see - would you rate this kind of limiter design that uses the output from the power amp over something that strictly limits to +/-3.3V beforehand. It's not something I've particularly thought about before this project.

I've had a trawl through various threads on which amps use what icepower boards, but I think I'd only concretely known that the fender rumble amps are using *ASX2 boards, because I'd consulted the schematics for those when putting things together. Do you know any others that are using this board?

I've got schematics mostly drawn up for my input stage, power etc., and I'd like to finish those up. It is mostly just the reference designs, but I'd love to get any feedback on it, since I'm sure there are some common mistakes I've made.

 

[Passinwind]

Thanks, I've found the service manual and that's really useful to see - would you rate this kind of limiter design that uses the output from the power amp over something that strictly limits to +/-3.3V beforehand. It's not something I've particularly thought about before this project.

I've had a trawl through various threads on which amps use what icepower boards, but I think I'd only concretely known that the fender rumble amps are using *ASX2 boards, because I'd consulted the schematics for those when putting things together. Do you know any others that are using this board?

I've got schematics mostly drawn up for my input stage, power etc., and I'd like to finish those up. It is mostly just the reference designs, but I'd love to get any feedback on it, since I'm sure there are some common mistakes I've made.

I really like the results of the dual limiter approach in the SB amps, FWIW. You might also take a look at Andy Field's patents on the Genz-Benz and Mesa limiter schemes, which strive for something like tube emulation a bit. And then there's a very useful old designer's guide for the old ICEpower ASP series floating around online, which was a huge help when I built my first 500 watter.

Some other well known ASX2 module users: Genz-Benz Shuttle and Streamliner, G-K MB and MB Fusion series, Aguilar TH, and as you say some Fender Rumbles.