'AK Mods' SC Class A 15W Amplifier

[Will]

Some of us have built the 'AK Class A' headphone amplifier (here) which is based upon a 15W Class A amplifier, described in Silicon Chip magazine.

What made the HA considerably better than stock was the modifications that Alex made to it, including the use of the JLH and the LTP thermal balancing mod.

If you wanted a speaker amp of the same calibre as the HA, you'd be a bit stuck. The kits that Altronics and Jaycar used to sell are no longer available, although RCS is Australia do sell the pcb. However, the pcbs (in both the kits and from RCS) have to be modified to enable the LTP mods, which can be off-putting to some.

There has been enough interest in a speaker amp version of the AK Class A pcb's that I did to have a go at doing a power amp version. Having learnt a few things from that build (not least that the new pcb will be normal thickness!) I'm now not far from finishing a pcb for the 'AK mods' SC Class A 15W Amplifier.

A quick over view of the pcb:

• Slightly larger PCB than the HA (due to having to incorporate a zobel network on the output of the pcb).
  
• LTP modifications incorporated on the PCB, using LS devices.
  
• DC offset corrector incorporated onto the PCB, allowing a fully DC couple signal path*
  
• 1.6mm thick pcb, with 2oz tracks
  

*depends on your other components.

The inductor in the zobel gave me a problem, as I wanted an off-the-shelf inductor that would be easy to use by everyone building it. With help from Frans and Alex (thanks guys!) pointing me in the right direction, learning enough to understand what I wanted, I ended up at Jaycar, and now have the parts needed on the way. It'll mean winding your own, but very easy to do, especially if you built the Jaycar SCHA, and fit the standard inductors.

Jon Clancy has agreed to help do the inductor file needed to allow me to finish the pcb layout, so thanks to him as well. Soon as the bits arrive, I'll be posting them down to him. Thanks, Jon!

On top of the amplifier pcb, I intend to do a PSU board for the amp. The original amp had a lm317/337 based unit which used a transistor to handle the higher current required. I'd like to simplify this by using a psu per amp channel. This would allow the use of just a lm317/337 for the supplies (or the lt1084/33 if you wanted some headroom on the current capability).

I've posted these details just so everyone knows what's coming, and once I'm in the position to order in pcb's I'll let everyone know. As always, the boards will be supplied at cost, keeping this a proper hobby-diy effort. I hope to be ready to go in the new year.

I'll post more details as they are available, and welcome ideas, thoughts and suggestions, although I'm not going to move away from what is proven to be a good design.

Finally, thanks to Alex for all the info you've sent and the questions answered! Hopefully, we'll soon all be enjoying our own AK room heaters

[Deleted]

Hi Will

oooooohhhhhh yes I'm in.

the idea of having a speaker amp of the same outstanding quality as the HA/PRE is just ttttoooo tempting and a dream come true.

i think Air link may have an off the shelf transformer that may be good for this project.

I'll have a look see.

if so a GB would possibly save some cash.


any help needed logistics wise let me know.

take care

[Deleted]

Hi Will
I’ve just dipped over to Airlink and found that they have a 230V 160VA @ 20V and 22V which is not quite the 21V specified in the original article but not far off.

how close does that 21V need to be?

how about 2x

www.airlinktransformers.com/chassis_mounting_toroidal_transformers_standard_range/3-CM0160220.html

after a quick think I've amended the above to something more suitable

Any use?

Or is custom winding required?

The lucky Australians can order from the usual place but I think for us Europeans well were going to get killed on the shipping costs.

I’ll keep having a look round.

Take care

[Deleted]

Hi Will,

oooooohhhhhh yes I'm in. the idea of having a speaker amp of the same outstanding quality as the HA/PRE is just ttttoooo tempting and a dream come true.

didn't think you'd be posting this one before Xmas......................................................................... is you'r middle name Santa..................?
I'd better start saving the "bawbees" I've been waiting for this one so count me in.
Alan
Gee I just hate those sheep but you can't help but love that Shaun

[Deleted]

I'm sure the guys already know but yes, me too!

For anyone who may be thinking that 15w/c, even in class A, may be a little low for them;

I've been talking behind the scenes and apparently more power is possible with extra/different mods. So don't be shy!

[Deleted]

Guys
With a suitable power supply the Class A is able to deliver 25-30W into 4 ohms.( See attached .jpeg.)
My speakers are only 87dB/1W sensitivity at 1metre,but I have never been able to use full power at any of my previous addresses.
For best results I would recommend using either 2 of those 20 or 22V
toroidals referenced to, but my Harbuch transformer (PTT4639)actually has 2 x 21-0-21 secondary windings, so it is very close to using 2 separate transformers. You may be able to order something similar by paying a bit more.I would go for 22-0-22 if wanting to push it slightly harder, and you would then also be able to increase the output voltage another volt (or 2) if desired.
Most would be more than happy with 15W/Ch. into 8 ohms in Class A though. It will of course work off a single transformer, but it will sound better again with dual AC supplies.
Alex

Attachments:

[Will]

I think that going dual supplies will make for an easier build, long term, as you avoid having to build the circuit with the current handling transistor.
If the dual supplies are built using the LT 1083/33 regs, then twice the current than the LM317/337 will be available (helpful for those wanting higher power), so that is worth considering.

On the traffo front, I ordered some 18V, 120VA traffo's recently with the spec -

audio grade, noiseless transformer price You'll get transformer wound on high inductive, selected and measured core. Core and all the windings will be impregnated. Transformer will also has electric and electromagnetic shields, epoxy filled interior and mounting pads in price.

Two of these delivered is £1 more than the standard airlink ones, with goss and rfi bands added as an option. I would suspect that they would be a similar price for what we are after.

I reckon 22-0-22, 120VA per channel will comfortably see most requirements for power, channel separation etc met quite easily.

[Deleted]

Nov 23, 2011 19:07:31 GMT drymdrum said:

Hi Will,

oooooohhhhhh yes I'm in. the idea of having a speaker amp of the same outstanding quality as the HA/PRE is just ttttoooo tempting and a dream come true.

didn't think you'd be posting this one before Xmas......................................................................... is you'r middle name Santa..................?
I'd better start saving the "bawbees" I've been waiting for this one so count me in.
Alan
Gee I just hate those sheep but you can't help but love that Shaun

Hi Alan

I'm so glad to See you and Chris here and when i read Will's first post i though ''i knew a man who would really appreciate such an amp''.

but for me it's like saying ''how do you fancy building something you're unlikely to ever want to change''? anyone who has spent some time with the awesome ClassA HA/PRE knows exactly what I'm talking about.

well as luck would have it it's my birthday soon and my good lady wife has agreed to fund the transformers.

then Xmas

so the double whammy

you lot may know that i was planing an F5 but hit indecisiveness about the build. good thing as i have a 4U 300 disipante case un drilled and ready for action. for the PSU i thought on 2U of the same type (disipante)for the TX's and regs. also maybe squeeze in the pre transformers would be great.

Hi Alex

15W is going to be more than plenty for me '(don't forget I've got by for years on 6W) with my 6ohm 88dB speakers.

from what I've heard with commercial stuff the higher power class A amps can sound a little less good. the lower power sugdens for example IMHO sound much sweeter than their bigger stuff. same to with the first Musical fidelity A1 which i owned for a time 15W and great. the next variant pushed the same design to 20W and i felt that it had lost some magic along the way.

15W is going to be just fine and dandy.

Hi Will

are those transformers from Canterbury? they seem like a good price for what you get. I'd rather go with those specs (or my wife would) so some order details would be useful.

i could get some sort of idea of cost and post the results.

time to have a dig through the parts box but anyone who has built the Class A HA/PRE is going to be half way there component wise as the original circuit has more than a passing family resemblance to the HA/PRE.

i have some re reading to do

great news

take care

[Spirit]

Finally
This is what I've been waiting for! Glad to hear you're almost there
Good work.

If any group buys go ahead before early February, could you shoot me an email directly Will?
I'm buggering off to India for 9 weeks on Saturday
I should be able to check email at least once a week, and can organise paypal or whatever payment from there (will have a netbook with me).

[Deleted]

Hi Shaun,
not given you the low down on what's happening here for sometime so I'll PM you soon. I'm just about to drill seven hundred holes in FR4 board before teatime so I'll catch you again.
Regards,
Alan

[Deleted]

Hi All

is it a good time to as about the JLH config?

CL or no CL that is the question.

any answers gracefully received ;D

take care

[Will]

Hi Shaun,

On the JLH, it's only used on the front end, the power stage is powered from an untreated supply (shocking I know!)

So, looking at Alex's post here it looks like the CL is used.

Before anyone asks, here's a link to the loudspeaker protection board rockgrotto.proboards.com/index.cgi?action=display&board=m&thread=3977&page=63#92837

On the toroid quote, it was from Toroidy in Poland, as part of a rebuild of my B32s rebuild. Alex's suggestion of a traffo with twin secondaries is quite a good idea. Both sets of secondaries will be independent and isolated from each other, can help reduce space needed and may even be cheaper? Worth an email, but remember to ask for audio construction.

I think that Canterbury Windings will be more expensive, but you are buying craftsmanship, and made in the UK. Worth it if you are after a full, top-of-the-range build.

[Will]

Nov 24, 2011 3:46:15 GMT Spirit said:

Finally
This is what I've been waiting for! Glad to hear you're almost there
Good work.

If any group buys go ahead before early February, could you shoot me an email directly Will?
I'm buggering off to India for 9 weeks on Saturday
I should be able to check email at least once a week, and can organise paypal or whatever payment from there (will have a netbook with me).

Hi Phil, thought you might be pleased!

I'll keep you in the loop on any GB's for parts. I've found a good cheap supplier for the output devices, although you can buy them over the counter at Altronics. LS devices are easy to get now, as Mintech sell them online, so all good their as well.

You have a great time in India, sounds fab!

[Deleted]

Nov 24, 2011 18:08:31 GMT Will said:

Hi Shaun,

On the JLH, it's only used on the front end, the power stage is powered from an untreated supply (shocking I know!)

So, looking at Alex's post here it looks like the CL is used.

Before anyone asks, here's a link to the loudspeaker protection board rockgrotto.proboards.com/index.cgi?action=display&board=m&thread=3977&page=63#92837

On the toroid quote, it was from Toroidy in Poland, as part of a rebuild of my B32s rebuild. Alex's suggestion of a traffo with twin secondaries is quite a good idea. Both sets of secondaries will be independent and isolated from each other, can help reduce space needed and may even be cheaper? Worth an email, but remember to ask for audio construction.

I think that Canterbury Windings will be more expensive, but you are buying craftsmanship, and made in the UK. Worth it if you are after a full, top-of-the-range build.

Hi Will
thanks for the info (hey how did i miss that?).

I've heard good things about Teroidy in relation to the Audiosector chip amp build (many nice comments on price and quality). for my part I'm still up for the two transformer approach for no particular reason other than it feels good.

i think your suggestion of 21-0 -21 120VA with audio construction from the big T in Poland looks good .

take care

[Deleted]

Hi Shaun,
traffos for Xmas sounds like a plan..............................Airlink look good have you a price idea somewhere close in the ball park would let me drop hints to the boys.

[Deleted]

Just looked at that link 2x delivered £88
160VA 230v to 2x20v
1xES01 Electrostatiic screen
1xNM01 EMI/RFI Shield
That would lift a bit of aggro if Santa was willing. SWMBO aint as I've overspent my PM for quite a while.
New PC or refurbished PC has drained all my "Brownie Points"
That's what I get for listening to you Shaun.................pull the fuse indeed!

[Deleted]

On the JLH, it's only used on the front end, the power stage is powered from an untreated supply (shocking I know!)

Hi Will
We aren't talking about raw sewerage here, it has been partially treated ! ;D
I have used the CL set for approx.500 mA, which is way above what is required here. Doing that ensures a fairly fast JLH turn on.
For most people, the amplifier will stay in Class A at all times, This means that the current drawn from each rail will be a fairly constant value of a little over 1A per supply rail, and the PSU ripple will be reasonably low.Without the CLs in the JLHs for the front end power supplies, the LM317/LM337s etc. would also be subjected to a very high inrush current from the JLH on switch on.
There would appear to be an advantage to using the Greg Erskine designed dual JLH PCBs here, or a fairly similar design, although you could use side by side pairs of Jon and Will's designed PCBs. Also small heatsinks should be fitted to the current limiter transistors.
Alex

[Will]

We aren't talking about raw sewerage here, it has been partially treated !

I know, but it just seems strange after all that's been done and said when it comes to PSU quality and audio!

I pick up my inductor formers, wire and LS protection pcb tomorrow, so not long until we can get going.

[Deleted]

Quote: I know, but it just seems strange after all that's been done and said when it comes to PSU quality and audio!


Chris
We need to remember here that most modern SS amplifiers also have a very high PSRR, (Power Supply Rejection Ratio) and the front end and VAS stage (Voltage Amplification Stage) are the most susceptible areas.We have covered those areas.
People are of course free to experiment in that area, and try non CL JLHs feeding the complete amplifier modules.
However, I wouldn't do that unless using those beefier LT devices for the voltage regulators.
Alex

P.S.
I would recommend using 2 x 2,200uF 10V low ESR in parallel , not 2 x 4,700uF, in the CL version of the JLH.

Attachments:

[Deleted]

Nov 24, 2011 19:40:32 GMT drymdrum said:

Just looked at that link 2x delivered £88
160VA 230v to 2x20v
1xES01 Electrostatiic screen
1xNM01 EMI/RFI Shield
That would lift a bit of aggro if Santa was willing. SWMBO aint as I've overspent my PM for quite a while.
New PC or refurbished PC has drained all my "Brownie Points"
That's what I get for listening to you Shaun.................pull the fuse indeed!

Hi Alan

fuse what fuse ;D i never use em

I've had some transformers from Airlink in the past and they where delivered within two days. i have two 22V monsters in the Audiosector chip amp and the one in the DCB1 is also Airlink.

we could do a GB but i think that the follow on mail would kill off any potential savings.

nice quality transformers though.

Santa's going to be puffing and wheezing getting those donuts down the chimney.

my wife is used to me now and just asks ''what audio related stuff do you want for Xmas this year''?

and a happy new year.

take care

[Deleted]

fuse what fuse i never use em

Several years ago, workmen building a new electricity substation about a couple of hundred metres up the road , hit live wires and we were hit by a massive overvoltage for a short period.
It caused a flashover in the primary winding of the big Harbuch Transformer in the Class A , as well as the 15-0-15 toroidal in the same case. Luckily, the power supply regulators survived.
However the damage may have been much worse if the primary winding didn't have a fuse.In hindsight, the fuse value may have been a little on the high side.
Alex

[Deleted]

Nov 24, 2011 22:47:06 GMT @sandyk said:

fuse what fuse i never use em

Several years ago, workmen building a new electricity substation about a couple of hundred metres up the road , hit live wires and we were hit by a massive overvoltage for a short period.
It caused a flashover in the primary winding of the big Harbuch Transformer in the Class A , as well as the 15-0-15 toroidal in the same case. Luckily, the power supply regulators survived.
However the damage may have been much worse if the primary winding didn't have a fuse.In hindsight, the fuse value may have been a little on the high side.
Alex

Hi Alex

don't worry i was joking (in poor taste) and of course all equipment must be correctly fused.

safety must always come first,second and third or we may not get to hear the last thing we build.

so point taken.

take care

[Deleted]

Shaun
Harbuch Transformers have changed owners since I bought my transformer, and I am not sure they still make them.
I got lucky this time, because I had bought a spare !
I didn't have my surge suppressor in line at the time, as I had only recently moved in at the time .
Still, it's almost enough to make you cry when such a beautiful and relatively expensive transformer gets badly damaged.
Regards
Alex

[Deleted]

don't worry i was joking (in poor taste) and of course all equipment must be correctly fused.

X2 ;D

[jonclancy]

This'll be fun!

What sort of sinkage do you need?