'AK Mods' SC Class A 15W Amplifier

[Deleted]

Hi Alan,

Glad it works.
Cap C1 can only blow by 3 possibilities:
1: +/- wrong (unlikely) as it had a few volts on it.
2: over voltage (judging from measurements NO as it had a few volts on it.)
3: An AC voltage on it. (highly likely)
Blowing caps cannot be caused by the reg or anything behind it.

[Deleted]

Hi Frans,
many thanks, each time we err a little we learn a little I just wish I'd spent more time in class
and paid more attention it's good to have you all willing to help.
Regards,
Alan

[Will]

Glad to see that you got it sorted Alan.

An update of the amp pcb.

Jon sent me a .lib file for the round inductor earlier this week, so once the bonding on the kitchen wall has dried and the tiles are up, I can crack on.

I'd say mid Feb for the final layout, hopefully earlier.

[jonclancy]

I have ordered some thinner enamelled copper wire to rewind the inductors and test the layour on the .lib. Great thing is that it will be easy to adjust the pads as required.

[Deleted]

Hi All
Well January is a pretty busy time for me one way or another.

BUT

Bitten by the etching bug I thought that it might be fun to etch some boards for the Alex’s Class A power amp.

So I decided to build a Class A light with some but not all of Alex’s mods.

I left out the LS devices for obvious reasons (don’t have any) and used 560c closely matched and MPSA18 again tight match needed. I have some 2N3811A on order so that should be interesting.

I also used IN5819 in the LTP balance even though I have 2 SF12 left I want to save some surprises for the build with Will’s boards.
OP devices are 2SA1302/2SC3281 so no change there.

also a pair of Greg's double JLH where fitted along with the SC speaker protection unit.

i also etched Will's DC offset corrector (thanks Will) and have DC offset @ 26mV and 27mV so good going on that front.

anyone thinking of not using the speaker protection should be aware that it also eliminates switch on and off POPS and THUMPS so it's worth fitting for that alone.

Soooo how does it sound.

First of I had a listen with the DCB1 which sounded pretty good but when I changed to Alex’s class A PRE/HA woooooow just sounds superb.

Really wide deep sound stage and has that knack of letting the music through in the same way as the HA.

So anyone out there wondering how good Alex’s modded Class A is?

Well I’ve heard lots of amps and always preferred the sound of a good Class A but Alex's Class A combo IMHO to my ears in my system is the best I’ve ever heard bar non and a real bargain price wise.

The build can be a little challenging at times but there are plenty of people here to help so any one wondering

jump in the waters Chrystal clear and fantastically musical.

I'd like to thank Alex for sharing all of his hard work on this and other projects.

I'd also like to thank Alan for the kick up the BH to try making my own boards.

so I'm ready for the real deal with Will's boards which I'm sure will improve the sound still further.

Take care

[Deleted]

Bitten by the etching bug I thought that it might be fun to etch some boards for the Alex’s Class A power amp.

ETCHER OF THE YEAR
Well you certainly don't sit on your Laurels "Young Shaun"
skipping along the etching trail like a two year old.
Some pics would be nice, you'll be requiring more F4 board soon.
I'm afraid I've slowed down a bit here but have my traffos cased up and looking good, first thoughts were to have them all in the main amp case but reason prevailed and I've opted to use two redundant PC SMPS boxes. I think they like their new homes, gives them a room each, of course nil outlay on that one.
I've also finalised my PSU layout and cut two boards with a little help from our friends here at RG.
I'll stuff the PSU Reg. sections and fire them up first before adding the JLH components to the board then it's on to the main boards but that'll be another story.
Well done Shaun
Alan

[Deleted]

Jan 19, 2012 16:21:03 GMT drymdrum said:

Bitten by the etching bug I thought that it might be fun to etch some boards for the Alex’s Class A power amp.

ETCHER OF THE YEAR
Well you certainly don't sit on your Laurels "Young Shaun"
skipping along the etching trail like a two year old.
Some pics would be nice, you'll be requiring more F4 board soon.
I'm afraid I've slowed down a bit here but have my traffos cased up and looking good, first thoughts were to have them all in the main amp case but reason prevailed and I've opted to use two redundant PC SMPS boxes. I think they like their new homes, gives them a room each, of course nil outlay on that one.
I've also finalised my PSU layout and cut two boards with a little help from our friends here at RG.
I'll stuff the PSU Reg. sections and fire them up first before adding the JLH components to the board then it's on to the main boards but that'll be another story.
Well done Shaun
Alan

Hi Alan

just a bit of a quiet Xmas so i had time to play.

I'll post some pictures once I've located the camera so no worries on that front.

good plan on the separate PSU case front.

you could also build the psu and fire them up one at a time to make sure all is well. i built up the power amp boards that way and apart from a small transistor in the wrong place drama it made life sooo much easier to build that way.

Alan i kid you not you're in for a real treat with this amp and i say again with the PRE/HA it's the best I've heard.

it's going to leave your lovely Quad (and most other amps)gasping for air.

oh i forgot to mention I'm using my Sonus Faber Concertino Home 88dB 6ohm speakers with this amp.

anyone thinking that 15W is not enough well it drives my speakers very loud effortlessly.

just lovely and right sounding.

i really did want to thank you for re kindling my interest in board making

I'd not have bothered without your encouragement and support.

take care HIFI superman.

[Deleted]

Jan 8, 2012 11:11:25 GMT @shaun said:

Hi All

just to keep this available.

for the LS device GB so far i have

Will 2x LS352 and 2X LS313
Phil 2x LS352 and 2X LS313 (provisional)
Me 2x LS352 and 2X LS313 + 2 X LS313
Syd 2x LS352 and 2X LS313
Alan 2X LS352 and 2X LS313
Alex 2X LS352 and 2X LS313
Mark 2X LS352 and 2X LS313 (cyteen)
Jeff 2X LS352 and 2X LS313
Jon C 2X LS352 and 2X LS313

Kits GB

Chris
Syd
Phil
Will
mark (cyteen)
Jon C
me


Boards only GB

Alex
Alan
Jeff


anyone else?

take care

Hi I've just had another look at this and to be clear the PCB only list is for those just wanting the PCB.

The Kit includes PCB.

take care

[Deleted]

Hi Shaun
You HAVE been a busy boy ! Well done.
How did you find the front end balancing adjustment ?
Kind Regards
Alex

[Deleted]

I have some 2N3811A on order so that should be interesting.

Shaun
IF you bought them on ebay, are you able to cancel your order ?
A well known otherwise O.K. Asian nsupplier was supplying what appeared to be genuine Motorola 2N3811A where the HFE was abysmal, and FAR below published specifications.My friend david2vk got caught !
Regards
Alex

[Deleted]

Jan 19, 2012 20:09:44 GMT @sandyk said:

Hi Shaun
You HAVE been a busy boy ! Well done.
How did you find the front end balancing adjustment ?
Kind Regards
Alex

Hi Alex

thanks for the well done but it's really you that deserve all of the credit for a great amp. i just followed the instructions.

OK

the front end balancing with the IN5819 was pretty much like it was for the HA/PRE.

IE a little flake and took ages to settle down enough to balance it out but a little patience helped on that front.

the OP devices seemed to take around 1/2 an hour to stabilize and get just a little warm on the double sized U4 heat sinks.(my wife calls it the Belgrano).

I'm pretty sure that the SF12 will work better in that spot and do remember it being quite a bit more stable and sounding a little better also in the PRE/HA.

but as i say I'd like to compare with Will's superb Boards when I've built them.

also i tried to build from what i had to hand so the only order was some wire for the inductors,MPSA18 and 2N3811A.

oh the inductors

20 1/2 turns of 1mm which from Johns account is not enough.

i used the formers from the SC-HA HA which for some strange reason I'd saved. they fitted the board nicely.

25mm heat shrink was used to hold the wire nice and tight as per the instructions in the 20W build. i also used the jig suggested in that article.

well i have no way of measuring inductances that low so i went with the calculation.

scientific method employed

i felt the force Obi wan and it felt right so that's what i have in place.

OK

thanks for the heads up.

good point on the 2n3811A front but i did take the precaution of ordering just one first which i have in front of me.

HFE measures 395 which is on the low side of the data sheet 300-900 but higher than the low point on the LS352 -200 so not tooo bad. i think i may order another set of LS352 as these boards are going to my brother when i have Will's installed (birthday and Xmas pressie)

even with the 560C it just sounds amazingly good and has the same rightness of sound.

it's not a wham bam thank you mam thing but a slow proccess of hearing all of the hidden detail masked by previous amps and set ups.

it would have been nice to do a long review but it just does what the music tells it to without getting in the way.

it does seem to avoid adding the sugar sweetness that most of the Class A stuff i've heard seem to. it also has a tight top to bottom (oh the bass is just superb with non of the overhang that seems to be the norm with this type of amp) just a lovely sound and one which to buy comercially would cost a fortune.

the soundstage (when the recording allows) is just real and natural but focused.

i was thinking on finding a new set of speakers but it really sounds like i have.

a Rolls Royce amongst Holden Commodores (with open window air con ;D).

superb Alex and a testament to your skill and know how.

take care

[Deleted]

Hi Shaun
The SF12 will give you far better tracking here to. I altered a couple of resistor values in the final circuit to allow it's use.(200 +200 in the Q101 and Q102 emitters)
That 2N3811A sounds O.K. Most of mine from the 20 I got from a Motorola distributor many years ago were mainly between 300 and 400 HFE.(>$20 EACH back then around 1990)
The LS313 and LS352 just seem to give another small lift.
Regards
Alex

[jonclancy]

Jan 14, 2012 15:37:06 GMT jonclancy said:

I have ordered some thinner enamelled copper wire to rewind the inductors and test the layour on the .lib. Great thing is that it will be easy to adjust the pads as required.

You've been busy, Shaun!

I got home today to find the postie had dropped off my new reel of enamelled wire. An eBay bargain which cost more to post than the item itself!!

22 SWG = 0.711mm

I'll try a rewind tomorrow and see what result I get.

Cheers

Jon

[Deleted]

Jan 20, 2012 0:57:52 GMT @sandyk said:

Hi Shaun
The SF12 will give you far better tracking here to. I altered a couple of resistor values in the final circuit to allow it's use.(200 +200 in the Q101 and Q102 emitters)
That 2N3811A sounds O.K. Most of mine from the 20 I got from a Motorola distributor many years ago were mainly between 300 and 400 HFE.(>$20 EACH back then around 1990)
The LS313 and LS352 just seem to give another small lift.
Regards
Alex

Hi Alex

yes you are dead right on the SF12 but I'm saving that lift in performance for the main build.

yup 2N3811A are still expensive but around US$10 from tubie. it will be interesting to see how they stack up against the LS in the later build.

i do like the idea of going vintage on that set of tranies.

Hi John

You've been busy, Shaun!

the devil and idle hands over Xmas stuff.

i used 1mm 19awg for mine BUT i have no idea of how close they are to spec measurement wise.

what are you using to measure uH with?

i must sort something out soooooon.

good work on the inductor front.

oh one general question.

the 0.1uf in the zobel

is inductance in that cap important? should we be avoiding a wound cap there?

hmmm unsure

take care

[Deleted]

and before i forget

thanks John for the superb guide to metalworking.

i have my regs and output devices drilled, tapped a snug as a bug thanks to the info you posted.

great tips on the centering drill, tap lube, and various types of tap made things much more straight forward.

I'd have been lost without that info

thanks

take care

[Deleted]

the 0.1uf in the zobel is inductance in that cap important? should we be avoiding a wound cap there?

Any 100nF poly type cap with a rating of 100V or higher will be fine.
Alex

[Deleted]

20 1/2 turns of 1mm which from Johns account is not enough.

Hi Shaun
Considering that the original article used a 6.8uH choke with 24.5 turns of .8mm wire, I would expect 20.5T of 1mm enamelled wire to be more than adequate. I wanted a more extended frequency response than the original which I felt was rolled off too soon.
The SC editor's response to me about that rather (in my opinion) savage low input filtering was " belt and braces". It was aimed more about reducing possible RFI, which I have never found to be a problem.
As many people play high resolution files and SACD etc. these days, I feel it should have a wider frequency response than the original rather conservative design, and that applies to the Zobel network too. A later SC design had the input filter rolling off at 1.9MHz I am also using much faster 30MHz output devices instead of the original 4MHz devices.
Yes, I can hear the resulting improvement due to the wider bandwidth.
Regards
Alex
P.S.
Ideally, the Zobel network should take into account the impedance of the loudspeakers too. When SC designed the original amplifier, the majority of loudspeakers were likely to be 8 ohms nominal.
These days, 4 ohms is much more common due to the speaker designers preferring that, rather than 8 ohms which normally results in lower distortion in the output stage due to "Beta droop" with the output devices at 4 ohms and lower.

[Deleted]

Blimey guys!

I've barely even given this any thought. I'm hanging on for all good stuff from da boys wot know wot they're doin'.

[jonclancy]

Hi Shaun,

I'm using my Peak electronics LCR meter to measure the inductors. It's a neat bit of kit and you may recall I invested in a boxed set of DCA55 Semiconductor Analyser and LCR in a box set. Pricey, but I use both testers quite a lot and they are easy to use.

I'm glad you found something of use in y rambling metalworking post!! My Q.Max 7\16th punch arrived yesterday as well as the enamelled wire, so I'll be drilling and punching out the AK Class A chassis today. I'll post a pic of the box with boards fitted (but some PSU/JLH boards unpopulated) with hook up to be completed, so you can see how I'm approaching the build.

Cheers

Jon

[Deleted]

Ideally, the Zobel network should take into account the impedance of the loudspeakers too. When SC designed the original amplifier, the majority of loudspeakers were likely to be 8 ohms nominal.These days, 4 ohms is much more common due to the speaker designers preferring that, rather than 8 ohms which normally results in lower distortion in the output stage due to "Beta droop" with the output devices at 4 ohms and lower.

Hi Alex

yes i agree .

but for me the big problem is that Loudspeaker impedance tends to be a stated nominal value.

in reality loudspeaker impedance tends to rise and fall with frequency which makes the zobel tough to get right in all cases with all speakers.

i sometimes wonder how close manufacturers are to the reality with their published specs. I've used my SF 6ohm 88 dB speakers with 1.5Watt 45SE on 8ohm OP taps with no problems at all. 15W is a real power house compared to that and should be more than enough for most people.

makes me laugh a little when people use the number of Watts as a measure of goodness.

WE300b where designed to drive cinema speakers yup 8W to fill a room that size. it's more about having the right speakers (horns in the cinema case) than just plain power.

maybe it's more to do with how much power is actually used or needed to drive speakers.(IE not as much as people may think)

I'd say that anyone who is using speakers of average sensitivity and impedance should be more than fine with this amp.

are you still using the DCM's or is it Maggies now?

I'd be interested in hearing from people what speakers they intend to use. just interested

Just a word on heat

I'm using a U4 case which i was going to use for an F4 build and they really don't get very hot at all. overkill for this amp but i had it to hand so used it.

warm yes hot no.

when i remember the MF A1 i had (and loved @15 Tim De P W) well you could fry an egg on those heat sinks. IMHO U4 is a little over kill but the U3 disipante that Will mentioned looks spot on and will keep those OP devices nice and cooool.

i used a U2 for the PSU regs and that runs very cool but is again a little overkill but i wanted matching cases.

so no asbestos gloves needed with this baby.

i have read the original article (er several times) and what stands out for me is the amount of work that you've put in to getting the best out of this amp.

great work.

hi John

I'm using the Peak transistor tester and to me it was money well spent. easy to use and takes the stress out of wondering if I've made so error doing things the old fashion way. it's earned it's keep over and over again.

the LCR meter looks equally good but it's expensive for one off measurements.

on the heatsink drilling front

what ever way people decide to fix there regs and OP devices (tap or bolt through) my advice would be.

get the centering drill

it gave me consistently accurate holes every time and for the few quid it cost from ebay was an essential item.

also take a look at Johns post as there is some great pointers for newbies like me who did cookery instead of metal work at school.

yup my mates had a good laugh on that until they realized i was the only boy in the class. ;D ;D

mmmmmmmmmmm

take care

[Deleted]

Hi Chris,

I'm hanging on for all good stuff from da boys wot know wot they're doin'.

you and me together, I just love the way "The Coachman" is driving this project along and showing what can be achieved if you just put your mind to it.
Alan

[Deleted]

are you still using the DCM's or is it Maggies now?

Shaun
I am still using the DCMs because of their superb imaging when well driven.They are only 87dB /W @ 1 metre
Alex

[jonclancy]

Well, 40 holes drilled and punched. Chassis is configured for, well, every configuration of boards. Two more holes to drill in the front plate to mount the jack and we're done.

I'm out of stand offs and some shorter fasteners. eBay will provide. I will pick up a centreing drill bit while I'm at it.

Yes, wrong thead...

[Deleted]

Jan 20, 2012 11:45:45 GMT @sandyk said:

are you still using the DCM's or is it Maggies now?

Shaun
I am still using the DCMs because of their superb imaging when well driven.They are only 87dB /W @ 1 metre
Alex

Hi Alex

I've had a listen to the DCM time windows and as you say they are superb especially in the imaging department. a friend of mine still has a pair and swears by them.

Hi John

looking forward (er sort of) to your build completion.

I'm sure it's going to make mine look like a collision between a rats nest and a shopping trolley (Morgan Jones)

will it be configured for pre duties?

i used a 2.5mm centering drill and regular tungsten drill bit of the same size for 3mm taps.

but that is where the professional finish ends for me.

take care

[Deleted]

I've had a listen to the DCM time windows and as you say they are superb especially in the imaging department. a friend of mine still has a pair and swears by them.

Shaun
I think that Miguel may still be using the bigger brothers to mine.
Regards
Alex