'AK Mods' SC Class A 15W Amplifier

[Will]

A quick question to Alan, Shaun and Alex,

What output devices are you guys currently using?

[Deleted]

May 6, 2012 7:09:53 GMT Will said:

A quick question to Alan, Shaun and Alex,

What output devices are you guys currently using?

Hi Will
I would have to open mine to be sure, as it wasn't a new build when converted to fully DC coupled with the Speaker protection PCB added. The main thing here is that they are 30mHz devices.
I suggested the NJW devices as in the schematic, as they have higher minimum HFE than the older types, and you have a better chance of getting pairs with less HFE difference between types.
Regards
Alex

[Deleted]

May 6, 2012 7:09:53 GMT Will said:

A quick question to Alan, Shaun and Alex,

What output devices are you guys currently using?

Hi Will

I'm using 2SA1302/2SC3281 at the moment but I've had a bit of a measure round with some others listed and the NJW0281G/NJW0302G look best as they are closer matches HFE wise than what i have in at the moment.

the NJW have the same pin spacing but are a little smaller in the body compared to the others.

my vote would be on the NJW that Alex mentioned.

take care

[Deleted]

Hi Will,
I've been away for the past weekend which is why I'm a little slow at getting back to you............. albeit I'm slow anyway.
Shaun kindly gifted me a pair of 2SC3281/2SA1302 which I duly installed with a comedy of errors across the whole build, by the time Shaun had helped this old brain iron out all his monumental boobs I'd obtained a set of 2SA1943/2SC5200 which is what I'm running with now. Cricklewood have them at £1.70 each plus vat.
Must say I'm more than happy with my set up, SQ is exceptional so much so that I'm presently building up a complete Pre and Power for my elder son who was more than impressed when he heard it perform.
Still not got it cased up and even with all the flying leads I've used it's brilliant.
Alan

[Deleted]

I'd obtained a set of 2SA1943/2SC5200 which is what I'm running with now. Cricklewood have them at £1.70 each plus vat.

Hi Alan
Good work.

A note of caution about those transistors though. Make sure that you get them from a reputable source as Alan has done, as there are lots of inferior counterfeits flying around.
Kind Regards
Alex

[Deleted]

@will,

Sorry matey, I missed your posts. Fab work going in there and I'm looking forward to the bulid. Seeing the board art makes the whole thing seem more real.
Pricing is incredible value.
Very well done

[Deleted]

Hi all,
I've finally had to buy my aluminium for the 15W build as the scrap I'd hoped to use ain't forthcoming well perhaps my patience wore a little thin, here's hoping it duly arrives in time for 15 Watt mark 11, and my elder son will benefit.

The cost of materials for this bespoke case came in at about £60
not sure how that measures up to a store bought one.

Dimensions are 340mm X 340mm X 125mm and the thickness for all sides is 2mm, angle used is 15mm X 15mm X 2mm, U section for the heat sinking on the sides is 20mm X 20mm X 2mm and the fins
are 98mm. All permanent joints are fixed with 3mm pop rivets with self tapping screws for the case top.
Alex has a saying when he thinks you might be stretching yourself
with a build, "not for the feint hearted", and I reckon that about sums up going down this route.
So here are a few pics to show where I'm at right now.
Alan

[Deleted]

[Deleted]

Note that I've still a few holes to drill for vents in top and bottom and the modules are only sitting inside to give an idea as to how they fit.
All in I'm very satisfied with the way this case has turned out,
it could be used as is but I'll think a bit about how I'm going to finalise it, probably a nice wood front.
Alan

[Deleted]

Hi Alan

That's REAL DIY !!!

Kind Regards
Alex

[Deleted]

Hi Alan

great work and a lovely ''case''of how it should be done

good job

take care

[jonclancy]

Blimey!!!

[Will]

Errmm, hello again

A further delay, due to the PC that had eagle on falling over big time. Pretty sure it's the motherboard and as it's a socket939, not so easy to replace!

Anyhow, new laptop on the way, along with a desktop sata dock to get the info from the HDD's in the bost PC.

The design is pretty much there (as can be seen from the previous posts) so I reckon an afternoon's work should see it in a state where the files can be sent off to Shaun to ordered up!

[Deleted]

Finally got the beast caged ;D
and here it is.








Sounds better in the box than out of it, runs fairly cool so (Frans) I'll still need to have that domestic winter heating running.

Just the front panel indicator led and face plate to decide on.
Who's a happy bunny then?

Thanks Alex and everyone here at RG for your valued input my dear old Quad 405 is now pensioned off, Alex your modded SC 15 Watt Amp will keep me happy from here to eternity many thanks,
Alan

[Deleted]

Finally got the beast caged and here it is.

WOW!!!

P.S.
Do you have any heavier gauge leads/wire that you could use for the speakers ?

[Will]

Superb build Alan!

[Deleted]

True battleship design Everything a "power amp" conjures up in your mind.


Did you ever put time in the Clyde dockyards? Superb.

[Deleted]

Did you ever put time in the Clyde dockyards? Superb.

Has to be an aircraft factory Chris, all that beautiful aluminium!

Brilliant, Alan, top of the pile

Syd

[Deleted]

Hi All,
well it's been running almost night and day now since cased, one hiccup when I had a go at getting the front end balance right and setting the bias, I lost the left hand channel which was down to a bad joint on the output stakes.
Now Alex what effect does setting the bias at say 100mv rather than 250mv have? other than greatly reducing the heat output.
I set the bias at 240mv as per Shaun,s post and had in effect Frans' classic room heater.
I'm running at 100mv now, sounds to me no different than when it was at 250mv but the internal case temperature is a nice steady 40deg C whilst the surface of the LT Regulators is running at a about 10 deg above that, with the OPs a little less at 47 deg.
I feel that it could do with a better air flow through the case so I'll strip all the modules out and pepper the base and top with as many 6mm holes as will fit in easily.
Alex don't be misled by the speaker out cables as they are temporary extensions to allow me to work on the unit easily.
I will be re-cabling when the completed 15w unit goes into it's allotted niche above the AK/HA/PRE.

All in this is a cracking addition to my sound system and I'm well pleased with it.

[Deleted]

Hi Alan.
You will have greatly reduced power in true Class A, and it won't sound as good.The original design dissipates around 100W of heat in total, so that wouldn't even warm a Water Closet too well !
Provided the heatsinking is adequate, then the heatsinks will not be too hot provided there is adequate ventilation.The only time that I felt mine was running a little too warm, but still without problems, was when I originally had the Class A preamp sitting directly on top of it in a cabinet, with the perspex doors closed in summer.
Without the Class A preamp sitting on top this summer, I only opened the perspex doors a little on the hottest days, even though there wasn't a problem.My case doesn't have a ventilated top, so I just use M3 washers between the lid and the case. Efficient heatsinking will keep internal temps within reasonable limits.
Check that you have efficient thermal transfer between the output devices and the heatsink with your thermometer.
Regards
Alex

[Deleted]

Hi Alex,
thermal transfer is good, external face of sink at point of contact with device is 43 deg.
With probe on the device 48 deg. Of course this is running at 100mv not 240mv.
I'll improve the airflow through the case first and see how much that lets me lift the bias.
I'm feeling though that I'll require to beef up the depth of the sinks fins to run at 240mv.

I have to say though that at 100mv the SQ to my ears is exceptional and I would'nt dare lift the volume control beyond half way.
Regards,
Alan

[Deleted]

Hi Alan
Do you also have big rubber feet under it to lift it off the deck for improved air flow ? You could even fit rubber door stoppers ?
Kind Regards
Alex

[Deleted]

Hi Alex,
I've stripped it down to bare case, not so easy when you've hard wired the lot but its done.
I reckon the heat sink fins are not adequate given that the PSU regs are on the same sink.
I'm looking now at how to extend the fins from 20mm to 60mm.
It's all a learning process.
Big feet are in place.
Regards,
Alan

[Deleted]

Hi Alan
In the meantime, why don't you try increasing the bias to say. 150mA, and see if that gives a more acceptable temperature with the heatsinks ?
Regards
Alex

[Deleted]

Hi Alex,

In the meantime, why don't you try increasing the bias to say. 150mA,

Not easy to make those adjustments with case, sinks, guts and umbilical chords all carved up on the bench.
However I'm hopeful of getting it up to toaster strength in the next few days. I've cut the sink fin extensions from some ally' angle and if I'm allowed a little time to myself today after the gardening duties then I've a whole lot of drilling and popping to do before bedtime. So I'd better get some shut-eye pronto.
G'nite
Alan