'AK Mods' SC Class A 15W Amplifier

[Deleted]

Dec 29, 2012 1:52:14 GMT @sandyk said:

Thank you Alexander for your kind observations on my youthful inadequacies

Hi Alan
You are welcome. ;D
Actually, I bet that many others would have done just what you did . Your mishap may save others from making the same mistake.
I wish that I had followed my own advice on many previous occasions.
Kind Regards
Alex

[Deleted]

Would this stuff be ok to use for windings on the inductors?

[Deleted]

Dec 29, 2012 15:09:56 GMT @cjarchez said:

Would this stuff be ok to use for windings on the inductors?

Hi Chris
As it's enamelled it should be O.K. The seller doesn't give the length though.
You need a 2M length.
Regards
Alex

[Deleted]

Cheers Alex,

I was going by weight. I can't find the very large quantity I was originally avoiding to buy BUT by dividing the weight of the ebay bobbin into that circum-planetary roll I came out with an approxiamate length of 6.5 to 7 metres for said bobbins.

Since placing my Farnell order, typical, I've found a 70 metre roll @ 500g, here, which agrees with the above calculation too.

As long as it's electrically sound worth a go

Ta

[Deleted]

Chris
I just checked Jaycar for the roll I have, and it seems that a 50G roll will be around the length you mentioned.
Regards
Alex

1.0mm Enamel Copper Wire Spool

1.0mm
18 B&S
100g Weight
19 SWG
14 Approx L(m)

[Deleted]

A few pictures to speak a thousand music files.

[Deleted]

Fully hard-wired and operational on both channels.
It's been powered up and playing music now for about five hours solid.
Just to set the bias and ballance the front end but not yet as
I'm off down to partake of a little "Julia" and watch my favourite programme.
I will return later.
Regards,
Alan

[Deleted]

I will return later.Regards,Alan

Please do.
I am curious as to what you found wrong, and how you identified the cause.
Kind Regards
Alex

[Deleted]

I am curious as to what you found wrong, and how you identified the cause.

Ah! Alexander, my dear friend the year draws to a close and we look forward, hoping for better things ahead.
The truth is Alan's ever the optimist, when things are black they are never "Stygian Black" there's always a chink of light saying here I am.

So you advised checking rail voltages and as per my original post.......

I replaced the ground lift applied power to left side in forlorn hope that all would be well...........no joy.
Checked power supply first and noted 20.3v on positive side which lasted a few seconds before dying away to 1.6v.

I'd done just that.

Herein lay the problem............I replaced the LT1085 rechecked supply and all was good with the PSU.

I then thought about your next advise and was about to pull the board but baulked at the idea being not but a little averse at the thought of the work involved. Being ever the optimist as indicated already I decided that the fault could have been all down to the bad LT1085 and hesitantly powered up both channels, after all nothing ventured nothing gained.
Yes it could have all ended in tears, just glad it didn't.

I intend to run this amp alongside my AK SCMODS H/A PRE whilst building up the #2 AK SCMODS H/A PRE which will be it's mate.
So it'll get a good burn in before it goes to my son.
That could take a while as I've got to build the case first,
fortunately both PSU boards and amp boards are built with just your balanced phono stage to make.....perhaps by Easter.
Regards,
Alan

[Deleted]

Hi Alan

great work on the fault finding and the usual super quality build.

well done sir

take care

[Deleted]

Hi Shaun,
did we have to dispense with the sf12 ON OUR FIRST BUILD I was just about to back track and look see as I cant get the balance to zero?

[Deleted]

Dec 31, 2012 13:18:18 GMT drymdrum said:

Hi Shaun,
did we have to dispense with the sf12 ON OUR FIRST BUILD I was just about to back track and look see as I cant get the balance to zero?

Alan
Have you tried as per the note on the circuit? See if the balance gets closer with the top covered with either a top cover, or even a newspaper across the top with the test leads attached.
Post your voltage readings as shown at the bottom of the schematic if you can't get within a few mV of Zero .If you are VERY close to getting the adjustment, replacing the 120 ohms emitter rresistor with 130 ohms MF should also give you a little more adjustment.
Regards
Alex

[pagan]

Just so some might get ideas on easy chassis builds

this is my class A 20+ watt amp


Uploaded with ImageShack.us

Heatsinks with 4 aluminum angle pieces holding it all together..


Allan

PS other Alan (one L)
Nice job on the build

[Deleted]

Hi Alex,
ignored the Chrome warnings.
I've replaced R3 200R with 220R on both boards, that just dropped the mv value by 10 closer to zero.
So I'll have to pull the boards and replace the SF12s which is what I had to do with build #1.
Regards'
Alan
PS by the way the SQ is superb on this build also. Noise floor is again that black hole thanngy
Shaun and I have both found. I wont go through the catalogue of senior bits that led up to that conclusion..........black hole...........just say that at one point I exclaimed to myself.................."Bugger the ef'in thing is working, it's so quiet" then set the music to play, no sound if you don't put some music in it.
Regards,
Alan

[Deleted]

Hi Alan
At a later time, if you obtain some 130 ohm 1% MF resistors with a future order, you should be able to replace the VAS stage 120 ohm emitter resistor and re-fit the SF12.
The SF12 will enable better tracking at all temperatures, and makes for a much easier adjustment.
A typical silicon diode (not Schottky) has a temperature coefficient of −2 mV/¢ªC , so you can see how your much lower ambient temperatures would affect this adjustment until the amplifier warms up.Your SF12s will have a higher forward voltage drop than in more temperate climates.
A Schottky diode has a lower temperature coefficient, so it doesn't fully track the VBE of the VAS transistor with temperature, although it certainly helps.
Kind Regards
Alex

[Deleted]

Hi Alex,
that's an easier option than pulling the boards to replace the SF12s then a redo to put them back.
I've 150R at the moment but no 130R can I try the 150R? and this is the emitter resistor on the BC549.
Alan

[Deleted]

Dec 31, 2012 22:27:40 GMT drymdrum said:

Hi Alex,
that's an easier option than pulling the boards to replace the SF12s then a redo to put them back.
I've 150R at the moment but no 130R can I try the 150R? and this is the emitter resistor on the BC549.
Alan

Hi Alan
Try the 150, and if you have a couple of 1K, or 820 ohms try tacking that across it underneath.That should give you close to 130 ohms.Yes, that is the emitter resistor for the BC549.
I haven't had that coffee yet, but it should give you around an extra 50mV to play with.
Regards
Alex

[Deleted]

Dec 31, 2012 13:18:18 GMT drymdrum said:

Hi Shaun,
did we have to dispense with the sf12 ON OUR FIRST BUILD I was just about to back track and look see as I cant get the balance to zero?

Hi Alan

yes i had the same balancing issues and went back to the previous diode used in that spot.

but

I'd just come of the back of some intensive fault finding with Alex at the helm and decided to settle for what i had which sounds great.

however the SF12 sounded better in Alex's HA/PRE so I'm going to be fitting that for the new build. Alex's suggestion of using changing the 120r to 130r looks like it should work so I'll be doing that also.

I'm guessing that our cooler climate gives lower ambient running temperatures and may be the reason that 120 does not get us a balance.

have you set the bias yet?

great work

take care

[Deleted]

Hi Shaun
The attachment should help explain. It is at an ambient temperature of 25C. Say for example that your ambient temperature was 20C lower than here, then the forward voltage drop of your SF12 would be >40mV higher than mine.
If Alan verifies that changing the BC549C emitter resistor to 130 ohms corrects the problem, then perhaps all U,K. constructors should use 220 ohms for the emitter resistor of Q103 as suggested in the recently posted schematic, and 130 ohms for the emitter resistor of the BC549C ?
Kind Regards
Alex

[Deleted]

Jan 1, 2013 12:18:16 GMT @sandyk said:

Hi Shaun
The attachment should help explain. It is at an ambient temperature of 25C. Say for example that your ambient temperature was 20C lower than here, then the forward voltage drop of your SF12 would be >40mV higher than mine.
If Alan verifies that changing the BC549C emitter resistor to 130 ohms corrects the problem, then perhaps all U,K. constructors should use 220 ohms for the emitter resistor of Q103 as suggested in the recently posted schematic, and 130 ohms for the emitter resistor of the BC549C ?
Kind Regards
Alex

Hi Alex

yes i remember you mentioning ambient temp differences (rubbing it in for us chilly UK dwellers ;D) and i think that you are right on that front.

we had identical circuits when i had the SF12 in and i just could not get a balance. Alan also so it must be down to temperature.

I'll make the changes with my up and coming build.

take care

[Deleted]

Hi Alex,
we have balance up here in Alan's audio haven.............music is good...............all's right with the world.
I've not yet hooked it up to the LTs but it's so beautiful in it's presentation of a certain parties astoundingly good files even with these small speakers..
Changed to 132R on resistor #18 formerly 120R and was then able to get a semblance of balance...........will have to jig back and forth as she heats up to attain the perfect.
So Alex that looks like your recc for colder climes is a working mod.
Many thanks again
PS yes changed to 220R on Q103. also
Going below now for my N'erdy traditional steak pie tatties an' champit neeps and a glass or two.

[Deleted]

Hi Shaun,
just noticed you on line, yes set bias not sure if I've got it right though as barely warm on these big heatsinks. I think having the PSU Regs on their own sinks makes a big diff.
Happy New Year to you and yours.
Alan

[Deleted]

Hi Alan
I am glad to see that you can now balance the front end, as it will now perform pretty much the same (hopefully) on both chilly days and warmer days.I wouldn't have minded a little of your U.K. colder air yesterday, as I feel like crap when the mercury reaches 36C like yesterday, or higher.
If anyone is using the full on +5V JLH PSU with a toroidal transformer for ripping to, and playing files from a USB memory stick, or even powering an external DVD writer, I would recommend increasing the input filter caps from 4,700uF to 2 x 4,700uF in parallel, or perhaps a single 10,000uF 16V capacitor with 3A bridge rectifier diodes such as the Schottky 1N5822.
Don't tell Greg ;D , but rips then sound even better at the low end and slightly better all round,(verified by Chris in Spain) as does the playback of .wav files stored on the USB memory. Rips already saved on the same Corsair, don't sound quite as good as fresh rips of the same material after the increase in PSU filter cap value.
I had previously seen the improvement on my CRO after doing this with a big +12V 2A and +5V 2A PSU.
Regards
Alex

[Deleted]

as I feel like crap when the mercury reaches 36C

Ah! the joys of the warmer climes. ;D
What no air conditioning?
Cant say I've ever seen that sort of temp here Alex, we're happy to get into the low twenties and days of upper teens are bliss.

When I was a lad.........remember the forties Alex..........we ran around in our bare feet all day the sun was hot enough to melt the pavement tar and our mothers used margarine to get it off our feet when we went home. Never seen tar melt like that since those magic days.

Only fly in our weather ointment is of course the interminable rain
which has been a real pig this/last year.

Well I've been setting the front end balance and it's good on the left as I sit here, changed over to have a stab at the right now, got it sitting at 1mv and frightened to touch it as it's so finicky but I'm sure I can get to zero stable if I am ever so careful.
Thanks again Alex, just love this amps clean sound.
Moving on now to the HA/Pre, with the power amp I built the case before proving the built up modules, I can't see me going into the freezing cold workshop to build the case for the HA/Pre first so I'll stay up here in the cosy warmth and get the uncased HA/Pre working first, should get a bit of sun in March so Easter might see the whole project finalised.
Regards,
Alan

[Deleted]

A couple of Will's PSU PCBs. Tested and ready to fit on a much cooler day.
I am using MUR820 diodes, LT1085CT and LM333T 3A voltage regulators.(LM317T and LM337T would have also done the job O.K. , but would be working closer to their maximum ratings.)
The filter capacitors are 6,800uF 50V.
I didn't feel the need to use higher value capacitors, as the original S.C. design only used a single 5,600uF on each supply rail to supply both channels.
Alex