'AK Mods' SC Class A 15W Amplifier

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Hi Alan
I have revised the drawing to incorporate recent changes.
Will has already made provision for the larger 7815 and 7915 on this PCB.
Kind Regards
Alex

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Hi Syd,when I looked at the board to see where the 1M was placed then turned it over it was easier to take the trace from pins two and six back to the 1M to be sure I was at the right spot, since the 1M is actually across these pins. If you look closely at your board you can see the copper strips run to each end of the 1M. Here is Alex's latest offset circuit.Alan

Ah, all is now clear, on this bit anyway

Thanks Alan

Syd

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I have revised the drawing to incorporate recent changes.

That's really clear even for me Alex, great.

Syd

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Nice one Alex,
have to save your updated one to my schematics pile.
Alan

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Well having been a little bit under par recently my second build had stalled for a week or so however yesterday I installed the combo PSU/JLH modules, on test one PSU worked the other was OK on the Positive side but wouldn't rise above 16v on the negative side.
Resolved that tonight having found the output leg of the LT Reg was fractured, replaced LT1033 and both PSU modules are working fine.
Next up is the SPKR PRO unit but I don't expect to have any problems there.......time will tell.
Syd when you build up the SPKR PRO board make sure that you are working with the revised model. On my first build I was using the original circuit and Alex advised me of the mods for better operation.
Regards,
Alan

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Alex, I think you said at one point that the inductors that weren't used for the AK SCHA were OK for this build. Am I right? I've just found them, already wound in a bits box, also two OPA 134s whatever I had those for.

Thanks

Syd

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I was getting ready to order the traffos and checked the size to see if my case would do, didn't realise the 160 is so big, over 200mm!

I looked at the 120, its 85mm, which would fit the case, would it do the job?

Thanks

Syd

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Dec 23, 2012 10:06:57 GMT @juke said:

I was getting ready to order the traffos and checked the size to see if my case would do, didn't realise the 160 is so big, over 200mm!

I looked at the 120, its 85mm, which would fit the case, would it do the job?

Thanks

Syd

Actually, 2 x 80VA will do the job. 2 x 120VA would be good though.
What you have posted here doesn't sound right.
18-0-18 160VA from Jaycar and Altronics are either 104 or 110mm OD. An 18-0-18 80VA from Altronics is 90mm OD.
Perhaps Shaun can advise you about a suitable U.K. transformer.

Regards
Alex

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Dec 23, 2012 10:00:15 GMT @juke said:

Alex, I think you said at one point that the inductors that weren't used for the AK SCHA were OK for this build. Am I right? I've just found them, already wound in a bits box, also two OPA 134s whatever I had those for.

Thanks

Syd

Hi Syd
Those would work, but I think the wire is less than the 1mm thick specified for the Class A 15W.The S.C. HA specified .62mm wire with 21 turns.
I would wind on 20 1/2 turns of 1mm wire.
I recovered 2 x OPA134 with the SC DAC by using 2 x LME49710HA
at the output.
Regards
Alex

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Hi Syd
Those would work, but I think the wire is less than the 1mm thick specified for the Class A 15W.The S.C. HA specified .62mm wire with 21 turns. I would wind on 20 1/2 turns of 1mm wire.

Thanks Alex, I'll do that.

I recovered 2 x OPA134 with the SC DAC by using 2 x LME49710HA at the output. Regards Alex

That'll be where I got them too, mystery solved.

Syd

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Dec 23, 2012 10:19:01 GMT @sandyk said:

Dec 23, 2012 10:06:57 GMT @juke said:

I was getting ready to order the traffos and checked the size to see if my case would do, didn't realise the 160 is so big, over 200mm!

I looked at the 120, its 85mm, which would fit the case, would it do the job?

Thanks

Syd

Actually, 2 x 80VA will do the job. 2 x 120VA would be good though.
What you have posted here doesn't sound right.
18-0-18 160VA from Jaycar and Altronics are either 104 or 110mm OD. An 18-0-18 80VA from Altronics is 90mm OD.
Perhaps Shaun can advise you about a suitable U.K. transformer.

Regards
Alex

Hi Syd

yes those 160VA transformers are big but not 20cm from memory. where did you get that info from?

the only place that i see 200mm mentioned is in the specified length of the primary and secondary wires.

the size if the mounting hardware is specified and if you compare that to the photo of my PSU you will see that the transformer is only a little wider than the mounting hardware.

TBH i was lucky enough not to have a case for the PSU so it was only a question of buying one big enough to fit the transformers and PSU into.

from my way of thinking buying the transformers to fit a case is a little bit like putting the horse before the cart.

as Alex has said

anything from 80VA upwards will be fine.

i chose 160VA because the cost difference was not so big and i like to have over spec'ed where possible.

sure it seems expensive but once built it seems very inexpensive SQ wise.

take care

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Syd,

looks like you're leaving me behind! I've yet to order the bulk of the parts (pretty even RS/Farnell split), waiting for the xmas post to be over.

I'll probably order my transformers from the link Will gave in Poland, ( www.toroidy.pl/ ) Airlink's carraige out to me is a definite deal breaker

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Hi Syd and Shaun,
I've still to case one Airlink 160va for my build and it measures
no better still here's a couple of pics.................every picture tells a story.

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Syd,
120mm X 50mm is ample space you might need a little more hight for the bolt and base plate though
mine are in a duff PSU box just 150mm X 145mm
Alan

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What you have posted here doesn't sound right.

It wasn’t Alex! Shaun has cracked it, though I was sure I saw 205mm od somewhere on that site for a 160 traffo. Saturday night was a ‘nice glass of wine’ night though

from my way of thinking buying the transformers to fit a case is a little bit like putting the horse before the cart.

I agree Shaun, that’s why I was checking I do want to use the last of my Chinese HA cases, though it wouldn’t have made me use a wrong traffo.

Thanks again guys, until the next blip, Syd

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120mm X 50mm is ample space you might need a little more hight for the bolt and base plate though
mine are in a duff PSU box just 150mm X 145mm
Alan

Thanks Alan, I reckon they should just sqeeze in height wise, no problems footprint wise.

Syd

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looks like you're leaving me behind! I've yet to order the bulk of the parts (pretty even RS/Farnell split),

Hi Chris
I got a sudden burst of energy, won't last long
I have a list ready for Farnell, I was amazed, it's coming out over £60, and I thought I had most of the bits already! I might get stung by the traffo people, often do to NI if its other than Royal Mail. RS offer free post over £20, but they were looking £4.95 delivery for a couple of small bits that Farnell didn't have. Since located thebits on Ebay and RS no longer an option for me. The cost isn't an issue as regards the build we end up with, as Shaun says, its just how the small bits add up.

Syd

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Okay with the SPKRPRO board it works a treat after I found the fault. No ground on one side.
Had a break in the ground on one side.

Alex, the A/C sense on the spkr board, with my first build I dispensed with that function as did Shaun. To test board you connect a wire from positive DC in to one of the A/C sense diodes,
then after test time you connect pos. and neg. from one secondary winding to the A/C sense
points as permanent.
With my first build I have no shut-off bumps so I left the DC connection as at test.
However this build I decided to follow orders and tested on the DC then wired up from the 20v A/C.
I was suitably impressed at the speed of shut-off A/C over DC that I intend to alter my first build to conform to orders. With A/C shut-off the reaction time is milliseconds and that means when my finger hits the off switch the relay is open before my finger bottoms out.
With DC sense there is almost 1 second, very noticeable.
Shaun's point is that he don't like A/C in the amp box.
What are your thoughts on this and (are you A/C or D/C)?
Regards
Alan

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What are your thoughts on this and (are you A/C or D/C)?

Hi Alan
I use AC in the little delay PCB for the preamp, but only DC in the 15W itself. I am not too fussed about the slight shut down delay in the 15W. I just want it to protect my speakers quickly in the event of a fault because of being DC coupled. The turn on delay of the LS Protect board is a bonus.
With the preamp it's a different matter, as it has a quick shut down.
It's not that important to me though, because I switch both the preamp and 15W from the same PSU case.
Kind Regards
Alex

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Thanks Alex,
I think I'll probably revert this one back to DC since the A/C line travels the full length of the case and I've no problems with DC on my first build.
Regards,
Alan

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If anybody needs a copy of the original article, complete with the later simple modification, email or PM me.
Alex

P.S.
See reply 68

rockgrotto.proboards.com/index.cgi?action=display&board=m&thread=7461&page=1

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I have a list ready for Farnell, I was amazed, it's coming out over £60, and I thought I had most of the bits already!

Crikey that's good! I'm out of nearly everthing, my first Farnell order arrived today @ €160 (I am buliding two though), another on the way, a little one and the 'sinks....

Hopefully all parts and spare time will be with me soon

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Hi Alex,
enjoying TFTVWs at this moment, many thanks, helps to banish the blues.
Question.................no first
I powered #2 up on left hand channel and had nice music for about
four or five seconds then the SPKRPRO dropped out. Tried it a couple more times and had sound again each time (music was good no probs with the audio) then SPKRPRO dropped out. DC offset had been good less than a mv. but I'd made no other measurements.
Our past days have been a little fraught and I was called away.
On returning to the amp yesterday I had one of those moments which make you wonder if you should really be allowed to fiddle with stuff you know little about.
I thought pop a flying lead LS + output to left out + on SPKRPRO board and bypass the Relay.......I really liked that music but it kept on going away...............
Switched on, thought there's a resistor smoking.....don't it know smoking's bad for the health............and instantly switched off muttering bad things about myself.
So Alex, why did the ground lift resistor smoke?

Today I installed the right hand channel all nicely wired in and applied power............glory be we have music and it played away happily for around thirty mins till I pulled the plug. DC offset is below a mv on this side also. So looks good for the right hand side.
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.
Negative supply is good no probs with it.
Your thoughts would be appreciated.
Regards,
Alan

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Hi Alan
To state the obvious, you shouldn't have tried to bypass the speaker protect without first checking the supply rail and output voltages on that channel.
It seems likely that you may now have caused further damage, although the output devices MAY be still O.K. due to their heatsinking and ability to handle much higher currents .
I would first disconnect both the JLH and the incoming power from this channel and check that their voltages are correct again.
Then using your DMM, check for a very low resistance between the output terminal of this PCB and earth, as well as both supply rails to earth.
If the rail voltages are then O.K. and you can't see anything obvious, try lifting one end of each 100 ohm resistor to the BC327 and BC337 with a link temporarily fitted to the speaker socket, and a 1K resistor fitted to the Input socket.
Re-apply power from the JLH ONLY and check all the front end voltages.
Post what you find so that we can enlist the added intelligence of Will and others if need be.
Kind Regards
Alex

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To state the obvious

Thank you Alexander for your kind observations on my youthful inadequacies
I'ts always a when you jump over the "Pale" and fall into the shit.
I will follow your good advice and return soon, many thanks.
Should I blow myself up in the meantime,
I wish you all "A Guid New Year in 2013" ;D
Regards,
Alan