A DIY 500 Watt Bass Head

[Passinwind]

The parts stash in my hobby shop finally hit critical mass this winter and I decided to build myself an amp and try out a few circuit designs I've had kicking around in software models for a while now. I'm still pretty much of a rookie at printed circuit board design, so this was a good excuse to work on that skill set a little too. I want to say right up front: this particular project has great potential to fail spectacularly and I highly recommend not trying this at home!

I worked backwards from the extra rack case I had kicking around, which is two rack units high. It came from Modu in Italy, and since shipping two cases cost the same as one, I just bought two while I was at it. The other one got used in this project. I had an extra preamp board from this one as well. That board has an EFX loop and an aux input, and has proven itself on lots of gigs by this point. I am just not wired to ever build the same thing twice though, so I decided to add a single band fully parametric EQ and a variable high pass filter, which I designed as a daughter card that can mount on the same standoffs as the preamp board. In the future I may do some other daughter cards for more EQ options, compression, a crossover, etc. The prototype daughter card came out like this:

Here are the modeled responses for the high pass filter section, then the parametric EQ when set at ~480Hz:

The nominal PEQ range sweep is 150-1500Hz for now, but I may change that or make it switchable as things evolve.

I found a lightly used ICEpower 500ASP module, which appealed to me given my long time use of a plate amp that uses the 1000ASP. Between the two, I should be good to go for either stereo or biamped rigs, both of which I like a lot for certain things. So a couple of months ago I installed the module in the case, and tested it out with quite a few different preamps. I found that the addition of a Jensen input transformer as had been recommended to me by the original module owner works pretty well, so I've been rocking things that way for the last month or so. Here's the power amp section, and how much room I have left to work with for the preamp power supply and preamp boards:

Next, I sent the front panel off to Front Panel Express for machining, engraving, graphics infilling, and standoff installation for the two preamp boards. A week later I had the finished panel back in hand, which looks like this with all the knobs, jacks, and switches installed:

Just have to wire up the preamp section and I should be making some noise in the next few days. Pics to follow as things progress...

 

[TonyP-]

Nice

 

[kleinenenten]

Waaaaay above my head, but awesome! Congrats and sub'd!

 

[swamp_bass]

That's awesome! Love the look too. Blue on black is ice cold.

 

[drunkenDAYlewis]

Cool

 

[JGR]

Great work Charlie, looking forward to watching it come together. I've been thinking the same thing regarding using the Jensen in front of the power module, very interested to see how it turns out. Did you get cables/connectors with the module?

 

[gjbassist]

Waaaaay above my head, but awesome! Congrats and sub'd!

That's what I was thinking too. I didn't understand much of what was said, especially the graphs. Very cool though that you have the ability to build your own amp OP!
I'm also amazed that there is a place called Front Panel Express!

 

[Passinwind]

Great work Charlie, looking forward to watching it come together. I've been thinking the same thing regarding using the Jensen in front of the power module, very interested to see how it turns out. Did you get cables/connectors with the module?

The guy who sold me the amp module had the right crimp tools and connector sets, and was nice enough to break out all the connections I specified to pigtails. The Jensen in this one is the JT-11P-1, which I have been wanting to try for quite a while. I am still undecided about adding a DI and/or transformer-balanced line out for the preamp section.

 

[InsanityAmps]

Looking good Charlie! I too am looking forward to the finished amp. Also curious about how those ice modules perform, been thinking about trying one out myself...

 

[beans-on-toast]

Very nice Charlie.

Do you have any concerns about noise from the ICE module. Will you need a divider shield between it and the other sub-assemblies? I'm wondering if the Jensen transformer in close proximity will be an issue.

 

[Passinwind]

That's what I was thinking too. I didn't understand much of what was said, especially the graphs. Very cool though that you have the ability to build your own amp OP!
I'm also amazed that there is a place called Front Panel Express!

If you click on the graphs they get bigger and might make a little more sense. The first one represents starting with the knob full-down (green trace) and progressing to full-up (red one). Each step down hacks away a little more sub-bass, giving the option of boosting the regular bass control with getting too woofy and losing control of the cab. So instead of bass boost peaking at, say 40Hz with a standard bass control, the peak can be anywhere from 30-300 or so Hz -- wherever you find "punch" to live in your playing situation. At least that's the theory...

The second graph shows the range of boosts and cuts that the parametric EQ can effect at one given frequency, in this case right in the middle of the midrange zone. The parametric can interact with each, all, or none of the bass/middle/treble controls; again, in theory. Until a few years ago I would have had to build multiple iterations of these circuits in order to suss how they'd interact, but these days you can just throw most of it in free software and get both graphs and audio samples. You still need to understand the graphs to really get anywhere though, of course.

 

[Passinwind]

Very nice Charlie.

Do you have any concerns about noise from the ICE module. Will you need a divider shield between it and the other sub-assemblies? I'm wondering if the Jensen transformer in close proximity will be an issue.

I did have some concerns about that David, and very well may still make a shield/cage. The Jensen is double shielded and so far I haven't heard anything untoward though. I looked at several layouts using this module/transformer combination in supposedly high end home audio commercial builds, as well as the ICE designer's manual for the module. I have added ferrite suppressors on the mains power and speaker leads, yet another lengthy detour as so often seen in DIY builds. The preamp will have a completely separate power supply, FWIW.

 

[OldogNewTrick]

Charlie, tres cool !!!
Great feature set (variable HPF + FPEQ), not available in a commercial product, and at pro level execution.

True DIY, Passinwind-style.

Congrats.

 

[cableguy]

Charlie,

I can't wait to hear it.........Totally kick-ass. I really dig the integrated HPF.

 

[JimmyM]

It's a gift.

 

[astack]

You da man, Charlie.

There's been far too few DIY / design threads lately.

 

[bluesdogblues]

very interesting. good luck with your work

 

[1n3]

Impressive, as usual. The parametric and variable HPF seem like useful additions. Curious about one thing - what is the reason for transformer-coupling the pre and power amps?

 

[BbbyBld]

I did have some concerns about that David, and very well may still make a shield/cage. The Jensen is double shielded and so far I haven't heard anything untoward though. I looked at several layouts using this module/transformer combination in supposedly high end home audio commercial builds, as well as the ICE designer's manual for the module. I have added ferrite suppressors on the mains power and speaker leads, yet another lengthy detour as so often seen in DIY builds. The preamp will have a completely separate power supply, FWIW.

I recommend adding a shield to your preamp even if you don't hear noise. Also, it's a good idea to add an active low-pass filter between pre amp and power amp. Noise will excite resonances seemingly out of the blue and blow a class D amp. They sometimes don't warn about that because the majority of class D applications are captive systems.

As for your ferrite cores, that's a good idea for your speaker leads, but they usually work better if you wind them common mode style with more than one turn. How did you select the core?

I suggest using a fan, even if you don't think you need it, to keep THD and your ambient temp down.

 

[Passinwind]

As for your ferrite cores, that's a good idea for your speaker leads, but they usually work better if you wind them common mode style with more than one turn. How did you select the core?

Bobby,

Thanks for popping in and providing plenty to ponder. Heat and noise, not gonna do it...

The mains ferrite is very close to what the ICE manual specs. That snap-on on the speaker output is a temp kludge. I do have something very close to the spec'ed one for that as well, which is around 1.5" OD and will accommodate 3-4 turns.