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Mrboo's Diy "gainclone" Amplifier


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#1 markiemrboo

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Posted 02 December 2006 - 08:18 AM

Well, I've not made an appearance here for quite some time now, and here's why!

I have been building a mystery box. I used schematics from someone called 'carlosfm', and a soft-start (to try and prevent the toroid inrush current from blowing fuses / dimming lights) from Rod Elliot, and drew up some home made PCB's.

It's a two channel amplifier based on LM3886 chips, ~38WPC in to 8ohms and ~80WPC in to 4 ohms (I think, I forget... watts are over rated). Each channel has it's own regulated power supply, sitting on a 500VA toroidal transformer. Speakon sockets for the speaker outputs, RCA for input. Considering switching to XLR for input, not sure yet :)

2U rackmount case from Maplin, they sent me a used one at first and I had to wait a whole week for replacement! Urgh!

I measured the speakon's as 25mm, so got a 25mm holesaw... and the hole is too big, you can see the hole edge on one side :( I am going to try and fix this with some rubber edging if possible though. Also couldn't be bothered drilling a hole for the power LED :lol:

My camera seems b0rked sadly, so no pictures of inside yet. I might not want to show you all how bodged up it is anyway :D To insulate the bottom of the PCB's from the case they're currently bolted to badly cut and drilled wood, which I might well replace with some sort of plastic or perspex / plexi. I think this would look alot nicer. The screw holes on the PCB's are actually all over the place too, but it all works... and I have learnt from all these mistakes of course (considering redoing the actual amplifier PCB's with slightly updated PCB's, but this would mean redundant screw holes in the bottom of the case. Whoops.) and it sounds just as loud as my current commercial 60WPC amp (which I am also using as a pre-amp for this amp now aswell). No thump on turn off or on, no audible humming, I am not really an so called "audiophile", so I won't even try and go in to all that audiophile talk, but it sounds just as good as the commercial amp (which is nothing particularly special mind) I can say that much :P Can't hear any distortion even at full volume, which is nice.

The finished(ish) product
Picture_14.jpg

Picture_15.jpg

Early version of an amp PCB
early_pcb.jpg

Soft-start
softstart.jpg

A freshly etched and drilled PCB (i've since discovered Eagle can fill in blank unused space automatically to reduce etching time quite and allow more use out of the etchant!)
etched_and_drilled_pcb.jpg

Early single channel running on my bed (probably quite dangerously) to test it out
running_on_bed.jpg

The best part is that now I've finished it I don't know what to do with the dang thing! Do I trust my bodge work enough to use it? :)

Cost far too much to build :) probably about £130, but a fair amount of that was "one time" sort of stuff, like etching stuff and things. I think it should "only" cost me about £70 if I were to build another now. More money than sense perhaps. It has been quite a learning experience though I suppose :rolleyes:

EDIT: dunno what's going on with pictures. Seems you have to click them to see them now?

Edited by markiemrboo, 02 December 2006 - 08:20 AM.

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#2 hardnrg

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Posted 02 December 2006 - 11:19 AM

2U rackmount case from Maplin

thought that looked familiar hehe

Cost far too much to build :)

eh, but custom gives you that warm fuzzy feeling! :D

like the fat toroidal donut :)

i was gonna ask if you had an etching tank already, but you said you had to buy the etchin stuff, so did you get a "beginner's kit" tray thing or go for the full-on etching tank... did you use photosensitive etching or something else? printed circuit board, or drawn?

the first circuit picture looks like the components are ginormous! :lol:

as for the XLR or phono dilemma... there's not really much point imo to switch to XLR unless you have a balanced XLR output from a mixer/desk and the amp itself supports balanced inputs... otherwise it's really just a difference of connector... i prefer phonos, but if XLR makes it easier for whatever you're connecting then i guess that'd be cool...

oh yeah, the bodged speakon holes... you could always make a rounded-corner rectangular cutout around the speakon holes and bolt on a suitable small metal plate... would look like you planned it too if you did the cutout nice n neat :)

i wanna see the photos of the final internal assembly, messy or not!

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#3 markiemrboo

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Posted 02 December 2006 - 11:37 AM

thought that looked familiar hehe


Heh! It's not a bad looking case, the only thing I don't like about it is that the front plate doesn't support the front-middle of the top and bottom plates, so you can push down there and it actually bends slightly! Luckily with the PCB mounted on wood, the heatsink is a perfect fit... so it's acting as the support for the front too!

eh, but custom gives you that warm fuzzy feeling! :D


This is true!

like the fat toroidal donut :)


Strangely enough the 500VA was cheaper than the originally planned 300VA (from Farnell), so obviously I got that. It makes no "mechanical" noise either, which I am quite pleased about :) It is sort of heavy though at 4.6kg...(i know this probably isn't alot to most people but 1) i'm a god dang wimp and 2) i still think it's insanely heavy for what it is!) :)

i was gonna ask if you had an etching tank already, but you said you had to buy the etchin stuff, so did you get a "beginner's kit" tray thing or go for the full-on etching tank... did you use photosensitive etching or something else? printed circuit board, or drawn?


Coh, can't afford an etching tank! I just bought a polishing block and ferric chloride from Maplin, and used a cheapo cat litter tray to etch it in :D The PCB boards I got cheap off ebay!

EDIT: Oh, and the process I used. Basically, I drew the PCB's in Eagle myself, printed them off on my inkjet, took the prints to Staples and asked if they could photocopy for me and then used the toner transfer method. I dont know what paper they used (I just asked for glossy paper), but it worked flawlessly!

the first circuit picture looks like the components are ginormous! :lol:


Ha! I could try and compare it to a CD or something! Not as tiny as some of the Class T/D stuff, but still quite small really. All the important amplification stuff is done in the LM3886 (basically just a giant opamp), which makes it kind of easy for idiots like me to start, which is nice :) It's also got all sorts of protection built in, shorted output, thermal, over current etc.. even more suitable for starting out.

as for the XLR or phono dilemma... there's not really much point imo to switch to XLR unless you have a balanced XLR output from a mixer/desk and the amp itself supports balanced inputs... otherwise it's really just a difference of connector... i prefer phonos, but if XLR makes it easier for whatever you're connecting then i guess that'd be cool...


I only don't like RCA at the minute because with the RCA sockets I bought, it's near impossible to isolate the signal ground (outer casing on the barrel) from the case. I don't really want those directly connected to the safety earth. I also just like the plugs on XLR connectors! Much like I like speakon rather than a basic binding post :)

It also sort of makes it easier should I want to make things balanced in the future I suppose!

oh yeah, the bodged speakon holes... you could always make a rounded-corner rectangular cutout around the speakon holes and bolt on a suitable small metal plate... would look like you planned it too if you did the cutout nice n neat :)


I'll probably try this is the rubber trim doesn't turn out any good! Cutting metal is so horrible though!

i wanna see the photos of the final internal assembly, messy or not!


All I can get you webcam pics of that for the moment... or phone pics..... ha, no... go for the webcam option please :rolleyes:

Gotta nip out and be a taxi for the parents so I will do that when I get back. Watch this space..?!?!

Edited by markiemrboo, 02 December 2006 - 11:39 AM.

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#4 jammin

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Posted 02 December 2006 - 11:50 AM

I'm not going to pretend to know about all the electronics and stuff (there is not enough time in the day for learning!), but it certainly looks interesting.

Those toroidal transformers are certainly heavy though.

When I get back home for christmas I'll crack the casing off my Arcam Alpha 10P and take some pictures of the insides of that if you want?
(Just to see how the professionals do it :lol: )

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#5 markiemrboo

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Posted 02 December 2006 - 01:04 PM

I'm not going to pretend to know about all the electronics and stuff (there is not enough time in the day for learning!), but it certainly looks interesting.


Not to worry! I don't know much about all the electronics either haha :D

Those toroidal transformers are certainly heavy though.


They are indeed!

When I get back home for christmas I'll crack the casing off my Arcam Alpha 10P and take some pictures of the insides of that if you want?
(Just to see how the professionals do it :lol: )


I've already had my Cambridge Audio open ;) But post away anyway if you have the effort! :D It will surely make my DIY look even more bodgearific haha

Speaking of bodgearificness, here's a couple of shady webcam pics for you :)

Attempt at an overview
Picture_16.jpg

On the left is the softstart, the transformer is fairly obvious :) and just to the right and below the transformer is a little thing called a "disconnecting network". My understanding of this is that it sort of separates the earth of the electronics from the safety earth, but still provides a path to safety earth for the electronics in case of a fault (could be wrong here).
Picture_17.jpg


One of the amp boards here. I've used Cat5 cabling for the very short run between the PSU and amp and also for the input signal. The input signal could do with some shielded cable really, but it seems to work fine!
Picture_18.jpg

Regulated PSU's, with a cut up old very large P2 heatsink of some sort for heatsinking the regulators! You might be able to see that one of them is mounted not on wood, but a cut up plastic 5 1/4" face plate for a computer case. I ran out of wood :) It stands out so much!
Picture_19.jpg

Apologies for the terrible quality of the pictures!

Edited by markiemrboo, 02 December 2006 - 01:06 PM.

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#6 hardnrg

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Posted 02 December 2006 - 01:20 PM

:lol: terminal block action! hehe...

wood doesn't look as bad as i'd imagined... couldn't you use nylon spacers instead though? or even stacks of nylon washers?

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#7 markiemrboo

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Posted 02 December 2006 - 01:24 PM

:lol: terminal block action! hehe...

wood doesn't look as bad as i'd imagined... couldn't you use nylon spacers instead though? or even stacks of nylon washers?



It's not too bad. Better than the plastic front plate effort that's for sure! I could have used something like that yeah, I just didn't have any at the time (and still don't). I imagine that would probably be even better though :)

EDIT: Terminal blocks are a brilliant invention :D

Edited by markiemrboo, 02 December 2006 - 01:26 PM.

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#8 Sagittaria

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Posted 02 December 2006 - 05:54 PM

I'm not going to pretend to know about all the electronics and stuff (there is not enough time in the day for learning!), but it certainly looks interesting.



:withstupid: I dunno... but that certainly looks VERY VERY clean being built from scratch :o I bow down to your uber skillz :P But seriously, it looks like it came right off the factory belt :blink:

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#9 jammin

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Posted 02 December 2006 - 05:58 PM

:withstupid: I dunno... but that certainly looks VERY VERY clean being built from scratch :o I bow down to your uber skillz :P But seriously, it looks like it came right off the factory belt :blink:


It may look clean compared to some computers, but for an amp I'm not so sure :lol:

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#10 markiemrboo

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Posted 02 December 2006 - 06:13 PM

:withstupid: I dunno... but that certainly looks VERY VERY clean being built from scratch :o I bow down to your uber skillz :P But seriously, it looks like it came right off the factory belt :blink:

BTW: We missed ya! Your in the OCC Awards, so your going to keep your current couple of awards :P


lol thanks. I think the webcam pics might be a bit deceiving as to the cleanness of things, but it's probably not quite as bad as I make it out to be either really!

I'll head on over to vote for myself in the OCC awards then :D


It may look clean compared to some computers, but for an amp I'm not so sure :lol:


Mr Unforgivin might have just been talking about the outside and not the inside possibly, I think :D The mess wires are the main problem really I expect? I used sort of... scraps... anything I could find, so I couldn't really do nice lengths to be routed easily unfortunately. It is, believe it or not, FAR tidier than an old, broken PA amp I have sitting on the floor!

The 'star grounding' (virtually guarantee'd way of getting rid of any hum without knowing all about excellent PCB layout) doesn't help the mess of wires much either!

I should probably mention diyaudio for being so helpful with my idiotic questions too.

I'd love to describe the sound, but it would be a bit unfair at the moment. It's hooked up to my older speakers (for testing!) so it wouldn't really be an apples for apples (or oranges for oranges ;)) comparison really!

Maybe I could have my current speakers, one on the left channel of new amp, other on the right channel of old amp... then find a mono track and listen and use the balance control to switch between amps, or something....hmm!

Edited by markiemrboo, 02 December 2006 - 06:14 PM.

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#11 jammin

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Posted 02 December 2006 - 06:17 PM

Listening tests are pretty subjective.
It's a lot easier if you have several pieces of equipment to compare, then you notice all the differences more.

When we were getting rid of some CD players (we had too many :lol: ) from the house a while back, I plugged them all into my system to make sure they still worked and it certainly opened my eyes to how different things can sound.

Ended up keeping the old NAD player for my room because it sounded so good.

Edited by jammin, 02 December 2006 - 06:17 PM.

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#12 markiemrboo

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Posted 02 December 2006 - 06:38 PM

Listening tests are pretty subjective.


Very much so :)

It's a lot easier if you have several pieces of equipment to compare, then you notice all the differences more.

When we were getting rid of some CD players (we had too many :lol: ) from the house a while back, I plugged them all into my system to make sure they still worked and it certainly opened my eyes to how different things can sound.

Ended up keeping the old NAD player for my room because it sounded so good.


:D nice one! I don't really have that much different stuff to test at all, but it would be interesting to try all sorts of different things to see if / how different they sound.
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