Building a DIY poweramp - Build complete - w/ live recording

  • (Live recording link at end of this post if you just want skip ahead to check out volume etc of these little amps)


    .Apologies if this isnt suitable for the "other gear" section.. Wasnt really sure where to post it?


    I've kind of regretted not buying the KPA Poweramp from the get go and the release of the 3.0 FW finally tipped me over the edge. After filp floping on buying the Ambrosi, amp I finally decided to take the hard road and attempt build my own aftermarket poweramp 8|
    This wont be interesting to everyone but I thought I'd post my build as it progresses.


    Let the tale begin:


    I've built a few tube amps, a bunch of pedals and various other electrical things, and for me, sourcing the parts for any project has always been the most tedious part of the process. This one however wasn't bad at all. Everything was easy to find except for the transformer, and that an ongoing story (more on that later).


    I chose to base the amp around the well proven and acclaimed "gainclone" chip amp. I've heard these in person as home hi-fi systems and they have always blown me away. Very linear, great SPL's, consistent results and a simple build. I also chose this amp design as I suspect it is similar to what the Ambrosi amps are built around. I'm hoping to get close to the sound of the Ambrosi as everyone on the forum seems to be very impressed with them. The Ambrosi is very reasonably priced too.. . (I'm starting to wonder why I'm building my own as I type this! ?( ).. Anyway..


    I bought a well known "gainclone" kit online for the amplifier and power supply sections. Took about an hour to solder them all together. Piece of cake. :thumbup:


    [Blocked Image: http://thefreemusiclabel.com/s…225_183955%20(Medium).jpg]



    I found a 1/4" piece of aluminium for the back plate/heatsink at a local metal fabricator that sells offcuts. Cost me USD $12. Was a bit wide so I cut it down to fit in the kemper back and drilled holes to mount it to KPA frame. Its covers the output writing a little but I'm going to live with it as I'm not sure I can operate a grinder accurately enough to keep the obvious edge straight. :whistling:



    [Blocked Image: http://thefreemusiclabel.com/s…_185813%20(Medium).jpg%20]



    Next up was figuring out the best way to lay out the components. The big circle is the physical size of the largest transformer I'm considering. I'm yet to decide on the two layouts below:


    This layout moves the audio section away from the transformer (less induced noise) but puts the transformer far to one side (making the Kemper head very side heavy to carry).. It does allow the "monitor in" plug to sit directly above the KPA monitor out plug though which would make for a very short and tidy jumper cable.


    [Blocked Image: http://thefreemusiclabel.com/s…225_183631%20(Medium).jpg]


    This one has the weight nicely in the middle, but less distance between the audio section and the transformer.


    [Blocked Image: http://thefreemusiclabel.com/s…3809%20(Medium).jpg%20%20]



    Both would work. No sure which one I'll go with.



    Finally, I wont need a separate power plug as I've already put an internal outlet in the cavity to power my delay and midiraider. I might hard wire the amp once its all together, but the plug will stay while I test everything.


    [Blocked Image: http://thefreemusiclabel.com/s…225_185046%20(Medium).jpg]



    The final parts are due in a few days. Will update the build progress when they are thrown in.


    :)


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    EDIT 1-Mar: Put the second half of the build into the same 1st post. See below.


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    OK, so I finished the build yesterday. The second half of the story (it's a long post - sorry) :)


    The back plate looked pretty average so I decided to clean it up. Here it is beforehand - not very impressive looking.


    [Blocked Image: http://thefreemusiclabel.com/s…0AMP%202/back%20dirty.jpg]


    I cleaned the print/scribble of it with isopropol alcohol (comes right of in a few wipes, and gave the surface a good rubdown in one direction with some fine steel wool. 10 minutes later it was good to go.


    [Blocked Image: http://thefreemusiclabel.com/s…0AMP%202/back%20clean.jpg]


    Ok, now the important decisions. :huh: I had to commit to a layout. I asked over at the Ambrosi thread if the unbalanced weight of the KPA witht he amp transformer was an issue/hinderance in any way.. The answers were all a resounding "No". Because ofthat I decided to go with the layout that will give best amp performance (lowest possible hum or noise at no signal). I went for this layout.


    [Blocked Image: http://thefreemusiclabel.com/s…AMP%202/test%20layout.jpg]


    Oh yeah, I decided to go with the torroid transformer in the end. Even though mounting it was looking like it was going to be a bit of an adventure, it's electrical specs were exactly what I wanted. I decided that was the most important thing.


    So with that all decided, I marked up the backplate, double checked everything, and grabbed the drill. No turning back now.


    [Blocked Image: http://thefreemusiclabel.com/s…P%202/marking%20plate.jpg]


    Mounted the parts to test fit - and looks pretty snappy imo :thumbup:


    I originally bought a recangle power switch but went out and replaced it for a round power switch with an inbuilt LED. Cutting a clean rectangle out of 1/4" plate would not have been fun. The drill was much easier and made mounting the switch a piece of cake (and it looks very clean and tidy)


    [Blocked Image: http://thefreemusiclabel.com/s…202/back%20parts%20in.jpg]


    Wired everything up and did my best to keep the AC side far away from the DC/signal side. These little amps are really very quick to wire together. Even so, I took my time as I didnt want it to blow up


    [Blocked Image: http://thefreemusiclabel.com/s…ack%20components%20in.jpg]


    Decided to hardwire my previously added mains power cable to the amp to keep everything simple. One power plug to do it all.


    [Blocked Image: http://thefreemusiclabel.com/s…AMP%202/power%20cable.jpg]


    This is where I tested the unit for the first time. I used my standard first powerup routine - give everything the once over, install the fuse, plug it in, close eyes, turn head (silent prayer), cross fingers, and switch on. :/


    No puff of smoke...
    No fireworks...
    No zapping when I touch the backplate..
    No blown fuse... good, good, good.


    :thumbup:


    Tested the DC offset at amp output. I measure 1.6mV. A very good low reading! (kit says anything below 100mV is typical so I was pretty happy with 1.6)


    Next up was fitting it in the beast.


    [Blocked Image: http://thefreemusiclabel.com/s…KPA%20AMP%202/fitting.jpg]


    The damn transformer was a bit of a nightmare to fit. I knew this was going to be fiddly but it turned into a bit of a saga until I figured out a way that would work. The problem is that toroids have a nut on the inside to secure the transformer to the base, which when installing in the kemper is totally unaccessable. :cursing:

    Fiddled with it for about an hour and a half before I remembered there were holes on the kemper edges if I took the green hood off. Once I did that, fitting the transformer took less than 5 minutes. Lesson learned.


    [Blocked Image: http://thefreemusiclabel.com/s…sformer%20fiddling.jpg%20]


    Everything fits! Put the hood back on and I believe I'm done. :thumbup:


    [Blocked Image: http://thefreemusiclabel.com/s…0plate%20fitted.jpg%20%20]


    Testing a clean profile with a 1x12 guitar cab and it's very loud. The poweramp does not distort and I dont notice any coloration from it. Into my 2x12 it sounds even better (beefier) and the volume increases again. I can not imaging a scenario where using a 2x12 this would not have the volume needed - and still have more on tap. Insane.


    The amp itself is dead quiet which is very good news.


    With that I declared the build a raging success and proceed to play the Top Gun outro at wall shaking volume in celebration. :thumbup:


    [Blocked Image: http://thefreemusiclabel.com/s…20AMP%202/testing%201.jpg]


    [Blocked Image: http://thefreemusiclabel.com/s…MP%202/finally%20done.jpg]


    Ok. I typed way too much but I hope this is usefull to anyone else who decides to DIY their own poweramp.


    All in all total cost for this was ony $138usd. Took quite a few hours (damn that transformer mounting!) but it was a pretty fun project. Sourcing parts still sucks though. Speaking of - one last thing to do - need to go on ebay and find some symbol stickers for the "input" and "speaker out" jacks.


    Maybe I could DIY some of them?


    Thanks for reading!


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    (EDIT2 9-Mar: Added audio link below)


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    Live jam room audio sample through a 2x12 cab. Recorded on a stereo field recorder in middle of room:


    Live Jam with poweramp and 2x12 cab


    Thanks.

  • Very interesting :) I built a little DIY power amp for the KPA too - but I went with an external housing (that fits in the KPAs back though) and an external power supply :)

    MJT Strats / PRS Guitars / Many DIY Guitars -- Kemper Profiler Rack / Kemper Remote / InEar

  • I thought of doing just that (either with the LM or one of the TDA chips) but in the end decided it wasn't worth the hassle (as I'm very short on time these days) and just ordered the atm-70 Ambrosi.


    I mainly didn't want to deal with procuring the aluminum plate and didn't want a separate unit. Other than that, it's a rather straightforward project.


    What kind of a transformer are you looking at?

    I'm just trying to be as truthful to my experience and personal opinion that I'm clearly presenting only as a personal opinion no more no less in an honest and truthful discussion about equipment.

  • Very interesting :) I built a little DIY power amp for the KPA too - but I went with an external housing (that fits in the KPAs back though) and an external power supply :)


    Cool. Got any photos mDan? It's always good to see how others have put things together. Did you run into any problems that i should keep an eye out for?

  • I thought of doing just that (either with the LM or one of the TDA chips) but in the end decided it wasn't worth the hassle (as I'm very short on time these days) and just ordered the atm-70 Ambrosi.I mainly didn't want to deal with procuring the aluminum plate and didn't want a separate unit. Other than that, it's a rather straightforward project.What kind of a transformer are you looking at?


    I was expecting the aluminium plate to be a PITA too.. but got lucky and found a local store that had almost exactly what I wanted for a bargain price. :thumbup:


    I'm on the fence between a torroid transformer (22-0-22v 160va) or a EI traditional transformer.
    Finding an EI transformer that has enough power to run the circuit where it like to be and will fit in the KPA has been really tough. I finally found a used 27-0-27v unit (27v is nudging the upper limit for my chips tolerance), and it will fit in the KPA, but it's slightly undersized in VA department. It will either work and make the amp louder, or fail and exit this world in a puff of smoke 8o . I bought it just in case ($15). No guts no glory right?


    The torroid donut is perfect spec wise but will require some creative mounting to mount in the KPA (as you can see in the above pictures it's a touch too big to slide right in to the Kemper). I bought this one online and am waiting for it to arrive (expected tomorrow). I'll probably go with the torroid in the end but not quite sure yet.


    Safe to say I will test all this with a cheap speaker and use a phone for the initial audio source. The Kemper will only get hooked up once everything has been put through its paces.

  • Looks great. I'm building the same amp from chipamp.


    I wired up the transformer today and the blue LED came on. I measured 17.5V on the + and -. Don't know if thats enough voltage or not. I would like about 25V.
    The tranny came from a old Peavey near field monitor that was supposed to be 50W. I still need a back plate and some 1/4" jacks. This is fun. My day job is an Electronic Tech so I do have the tools
    but sometimes not the drive to repair stuff after work but this is fun.
    I'll post pics when done.

  • Hey Ibanez270. 17v dc after rectification is very low. I'm guessing your ac transformer is a 12v output which isnt really enough voltage for the LM3886 chip. It may work, but be very low output. You should go ahead and use if for now as it's not going to hurt anything.


    Something like this would be much more suitable: 22v-0-22v link

    If you have the room for a slightly bigger transformer something like this would also work and give more volume: 25-0-25v link (this one is slightly too big to fit in the KPA back slot though)


    I wouldnt go any higher than dual 25vac transformer for your build (would make dual 35vdc after it's rectified). Any more volts than this will start working the chip too hard.


    Got any photos of the amp?

  • I finished my build tonight. I understand why everyone keeps saying the Ambrosi is has plenty of volume. Holy crap it is loud through a 2x12 cab! Mine sounds great and I'm very happy. Will post photos etc of the last half of the build shortly.

  • great job, congrats. are you gonna make any sound clips or vids of the amp in action ?


    Thanks. Yeah, I was going to record something but realized there wasn't a whole lot of point. It just sounds like a really good, transparent poweramp!


    If I remember, next time I'm jamming with the drummer and bass player I'll get some audio/video to try and demonstrate it's substantial available volume in a band situation.

  • Looks great man. I keep blowing fuses :)
    Did you connect only one side of the power supply? I only have 3 taps coming off my tranny center is ground. I can't figure out the power supply AC1 and AC2 ?