Class A kit build

[MurphyUHF10]

Hi there,

Just thought I'd share my current project. 20w Class A mono-block amps..

The story began when I was so impressed with how good my Ultra-LD amp was, especially once I had converted it to a single input “power amp only” amp that I needed something as good, if not better, for High Frequency Tweeter duties for an active crossover setup..
So I decided on the Silicon Chip Class A design,, with a few refinements. I realised that to maximise the 20 Watts, improve channel separation, and hence improve overall performance, I needed to build them as ‘mono blocks’, with their own power supplies and enclosures..

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My first step was to source 2 locally custom built toroidal transformers (I wanted to stay away from Chinese Altronics/Jaycar transformers for this project)..
Next was to find 2 cases.. I looked around for pre-made cases, but most were from overseas or the wrong size for what I had in mind.. So I decided to build my own cases from sheet aluminium.. I found a local supplier who would cut my sheets to the required dimensions

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This was much harder than I thought,, and much more time consuming than I would have liked!
I'm really happy with the build quality and case layout. All spade terminals have been crimped AND soldered, and on some I have actually soldered them together once mated to each other! And of course, where possible I have not used them at all and soldered the wire directly to the board.. I could have technically 'hard wired' everything together, but serviceability and testing becomes a pain in the butt!

Another requirement I had was to get the speaker cable and input signal cable out of the case as soon as possible, at right angles to the power supply and amp board,, I was able to acheive this.. I've also got some 'foil tape' which I will use to tape the input cable to the base for extra shielding..

Setting the Quiescent current was a bit trickier than with class AB amps.. But I think I found a good point at which I left it alone (Any tips on this will be appreciated)..

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I'm playing an old Neil young HDCD, albeit in mono, which has a noticeable high-freq noise/distortion that rides on his voice during some passages when he 'yells', and I can notice that doesn't seem to be as prominent.. It seems a bit 'smoother' than usual..

Of course, these observations are just the beginning,, but so far so good :-)
And yes, the heatsink does get hot!

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The specs say 625mV RMS for full power into 8 ohms, so need to somehow workout a mental calibration on my DCX that way I know how hard the amp is being driven.. (Any ideas about this welcome!)
I'm about half way through building the second case :-)
I'll report back once I'm running them in in my main setup,,, shouldn't be too long!

Cheers,
CM

[JoMo]

Hi CM,
They look great I reckon!
I have built a pair of the earlier Silicon Chip 15W Class A amplifiers and I really like the sound of them.
I used a similar approach for the cases as you, very fiddly and time consuming. Currently doing the same thing again for a highish power Class AB amp.
Like you say things do get very hot! I am thinking I will have to build a set of low power Class AB amplifiers before next summer.
Cheers,
Joe

[MurphyUHF10]

Thanks Joe,

Yeah it is fiddly,, and I don't think I will attempt to build my own cases again :-)

Had another listen tonight (in mono) and I think it sounds great! Can't wait to finish the other amp and install it in my main system!! Eventually they will be used for an active crossover setup, but initally I'll use them in full bandwidth..

Did you find 15 watts was enough?? How do you monitor how hard the amps are being driven?

CM

[Boofers]

Nice work!

To see how hard an amp is being driven I use a function generator and oscilloscope.
If you don't have these tools, I guess you could just use your ears by turning it up until you
hear distortion. If at max volume you don't hear distortion you may still have some room left on
your amp, and a pre-amp would probably be the way to get the absolute max output from your
power amp. Or you could increase the gain of your amp.

Your case looks good man. Yeah it can be a lot more work to make your own case. Personally I
enjoy machining, and I like making something that looks different than anything else out there.
My latest project. Looks a bit like wood in this pic, but it is all aluminum.

[MurphyUHF10]

Nice one Boofers... That looks pro!!

You obviously have a good 'machine shop'.. Did you do your own folding?? How did you avoid scratches on the aluminum?? I used fine sand paper and steel wool, but I could not get all the scratches out :-(

Mine look a bit "DIY-esque",, but the build quality is good,, I'm happy.. The only tools I have for case work are a hack-saw, hand drill, set of files, and screw driver..

No I don't have an oscilliscope unfortunately,, i suppose I could use the level meters on my pre-amp?? (which is a modified DCX)

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Cheers..

[Boofers]

The DCX you have, is it the Behringer Ultradrive DCX2496? I found the datasheet for it, wow that is an awesome unit. It says it has a max gain of 15dB which works out to 5.6 times. So if your CD player has a 300mVrms output, your output at max gain would be 300mV(5.6) = 1.68Vrms. I’m not sure how that correlates to your level meter. How is it modified?

Hey thanks. The folded aluminum piece is something I salvaged out of some old industrial equipment. Yes I am a bit of a junk collector ha ha. It was already folded like that. I have a friend that works in a machine shop; he cut it up for me. All the other “machining” like making smaller brackets, I did like you; with a ban saw, files, rotary tool and dull hacksaw. I have a drill press and I did all the drilling and tapping myself. It actually has scratches all over it, but the grainy finish of the aluminum kind of hides them (and my camera is crappy, better pics are coming.) I guess my pieces are a “brushed aluminum” as opposed to a smooth finish.

I have been known to make projects in cookie tins as well

[MurphyUHF10]

Thanks Boofers,,


Yeah, It's an Ultra drive pro DCX-2496.. It has a 'pre-amp style' input/output board as part of the mods..


So I imagine when both the input and output is set to zero dB on the modded 'pre-amp' board, the signal level will be that displayed by the DCX meters,, which in turn is relative the signal coming from the CD player?? If thats the case, I need to find the output level of my Oppo :-)


Thanks..
CM

[JoMo]

Hi CM,
I have definitely found 15W enough so far...but I am listening in a tiny back room with the speakers (little full range ones) right next to me! I have turned it up too on the weekends and it seems to go pretty loud.
I wanted to fit a clipping indicator but completely ran out of room inside my cases. I was going to use one found at Marshall Leach's Leach amplifier site.
It has been a learning experience for me how little power you actually need in some situations.
Also I have heard it said that class a amps are subjectively louder than you would imagine going off the power rating, don't know if that is really true or not though.
Our shed is getting converted to a really nice room right now which will become kind of my area (pretty lucky I reckon!).
Like you I'm will be bi-amping eventually so I think a lot of the load should get removed from the Class A amps and I think it should be fine.
The good think with the class a amps which I have noticed is that they run cooler (the transistors/heatsinks) the harder you run them.
Still have to worry about clipping though.
I am definitely worried about heat once we get back to our Australian summer though.

Cheers,
Joe

[MurphyUHF10]

Originally Posted by JoMo

Hi CM,

I wanted to fit a clipping indicator but completely ran out of room inside my cases. I was going to use one found at Marshall Leach's Leach amplifier site.

Thanks Jo, I'll definately have a look at what I find!
I should hopefully finish my other amp over the weekend and then I'll be able to properly setup my mono-blocks and have a proper listen :-)

Cheers,
CM

[MurphyUHF10]

Ok..

I finally finished my Class A mono blocks project! It's been a long journey, slowly chipping away at it, a couple of hours here and there..

Overall I'm very satisfied with end result

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I am amazed at how good they sound.. Compared to my 2 modified A500's (permanently wired in bridged mono) they sound much better!! The treble, like the cymbals, high-hats, some piano and vocals are much clearer and smooth,, nowhere near as 'harsh' as my A500's..

Stereo Imaging is improved, and the vocals are pin pointed right in the middle and out in front,, I've never really been able to achieve this in my current system,, I'm wrapped!! I was sitting there wondering what had happened ,, I've only really noticed this at gtg's on other people's system..
Also I can notice more "silence" in between the sounds and instruments,, it's very quiet! I believe these amps have been helped by using a good locally custom made shielded transformer!


I still have a technical concern,, Setting the quiescent current was tricky, and I never really got a consistent result. How critical is it to have this value exactly as the specs suggest?? I'm about +/- 30mA

Cheers,
CM