'AK Mods' SC Class A 15W Amplifier

[Deleted]

Feb 27, 2013 11:32:32 GMT jeffc said:

Hi Shaun,

Those Tellurium Q speaker cables look terrific, might be a bit tricky though, as by the pictures they look to be quite stiff, to use them for internal wiring.... But if you have space... and the patience.... why not. They look to use evenly spaced wires which reminds me of these musings into DIY interconnect cables, which the designer liked best when spaced evenly about 19 mm apart.

forum.audiogon.com/cgi-bin/frr.pl?rcabl&1150905409

Interestingly the Eichmann cable pods I decided to use are low mass and made of tellurium copper. When I replaced the binding posts on my Wharfdale Diamond 8.4s with some years ago I remember there being a quite significant change in presentation for the better, amazing that changing something as basic as a binding post can have such an effect.... anyway, had some so used them..

I also have some skinned Eichmann Express 6 speaker cables (freebees) that I might try with this amp. I haven't done anything in the way of testing different cables with my DML panels yet, so might be time to investigate.

Problem with this wide-band Class A amp compared to my Class D Audio amp with steep HF attenuation via its output filtering will be the naked piezo tweeter cones I have in parallel with exciters on my DMP panels..... I'm expecting a bit of fun with this.... and have a feeling the Class A might not be too enthusiastic about driving them... but there are easy fixes should this prove to be the case.

cheers.. Jeff

Hi Jeff

I guess that what I was trying to say was that IMHO cabling especially is a personal thing.

Some say they hear no difference and some say they do.

In my experience they do so I tend to spend a little time trying them out for myself in my set up.

I still love the Eichmann bullets and was shocked at the difference that they made compared to my stock plugs. The same sort of cool, calm and collected sound that I had from the Tellurium Q’s but less so.

If you ever feel the need to try some new speaker cables then the Blue is a good place to start. It’s a little rolled of top and bottom compared to the black but in some cases that’s helpful.

The blue is modestly priced also

A few thoughts on the Tellurium cables

The IC are a little expensive but I managed to pick my two pairs up second hand (Xmas and Birthday Presies) which softened the blow a little.

Well there’s not too much info on the construction method or technical measurements.

And that’s fair play to them as ears do that so much better.

But being nosy I did had a quick measure of the IC’s capacitance wise and these do seem to be on the high side. But for my Buffalo/Cap less Legato with Alex’s superb Pre Power that’s no problem.

What I did find though was that the cable between my DAC and Alex’s pre made much more difference to the overall sound than the one between Pre and power.

So be prepared to experiment a little once you have things set up especially with those Piezo’s as your system my need a little nudge one way or the other.

What I will say is that combined with my PMC FB1+ the whole thing makes a very musically convincing and cohesive sound.

great work Alex

Ooooo isn’t it nice to get to the chewing over cables part Jeff?

Take care

[jeffc]

Ooooo isn’t it nice to get to the chewing over cables part Jeff?

Hi Shaun,

Sure is.........and stress levels are diminishing....
Alex has been helping me check voltages etc in the background to try and get the front end balanced... both of which are a bit off ... and still to figure what's with voltages, but with R18 increased from 120R to 130R (balance down from 65mV to about 16mV when warm) and then R3 increased from 200R to 220R (balance further drown to -7mV cold, and warm in the +1/+2mV range) we've decided close enough for the time being, and time to get it set up to enjoy some listening.
Just had to gnd connect heat sinks, lids etc during the week, now done (so L/R amp PCB gnds are now connected via the heat sinks and Al lid), and screw lids on to stop me from fiddling anymore.
With lids screwed on and amp running for a few hours, being 3mm thick Aluminium. the lids help spread heat from the heat sinks and with bias set a 240mV across the 1R resistors, no concerns about temperatures, so all good there. Wedding to attend today but with some luck I'll get to finally listen to it tomorrow. Really looking forward to that. And of course I can't thank Alex, Will and you guys enough for the privilege of being able to join the select few with this AK-mods 15W Class A amp.

cheers..jeffc

[Spirit]

Roughly what time tomorrow are you hopefully going to be listening to said amp Jeff?

[Deleted]

Mar 2, 2013 6:39:21 GMT Spirit said:

Roughly what time tomorrow are you hopefully going to be listening to said amp Jeff?

Phil
Do you want to be present when the Pyrotechnics start ?
Alex

[Deleted]

Great Stuff Jeff, 
I'll be all ears up here in the Northern Climes it'll be interesting to hear your verdict on your build and Phil's also if he gets to join you.
Regards,
Alan

[Deleted]

''Sure is.........and stress levels are diminishing....''

Hi Jeff

Yes it's surprising how challenging building Alex’s class A is considering its pretty much the same circuit as the HA/PRE but with higher currents involved. My build stretched me to the limit and I hope that yours did too because that’s how I learn. Alex got me through my build and had the patience of a saint which really helps when you feel the pressure. He reminded me of the Tellurium’s in that regard ‘’cool, calm and collected’’ when I was anything but.

The stress soon goes but the beauty of the sound is yours for as long as you want it.

‘’Alex has been helping me check voltages etc in the background to try and get the front end balanced... both of which are a bit off ... and still to figure what's with voltages, but with R18 increased from 120R to 130R (balance down from 65mV to about 16mV when warm) and then R3 increased from 200R to 220R (balance further drown to -7mV cold, and warm in the +1/+2mV range) we've decided close enough for the time being, and time to get it set up to enjoy some listening. ‘’

Yup agreed its close enough for listening tests and all work with no play makes Jack a dull boy.

‘’With lids screwed on and amp running for a few hours, being 3mm thick Aluminium. the lids help spread heat from the heat sinks and with bias set a 240mV across the 1R resistors, no concerns about temperatures, so all good there.’’

Looking good

‘’Wedding to attend today’’

Have a good day

‘’I can't thank Alex, Will and you guys enough for the privilege of being able to join the select few with this AK-mods 15W Class A amp.’’

From switch on my Class A build takes around 20mins to settle so remember to give her some time to warm up. It also improved once the components had settled down.

I still feel lucky every time I listen to Alex’s Class A HA/PRE/PW combination.

All good

So on to the listening Jeff.

Take care

[sean]

Hi all, Sean here (Dave K's amp builder). I've had a read through the thread and it looks like a great design, just a few queries...

- I do plan on fitting speaker protection of course, there's mention of a specific one on this thread. Can anybody point me towards where I can get a pair of the SPKRPRO boards? (dual mono build, so need 2 PCBs). I've seen a schematic for it on this thread already, any other info I need would be gratefully received. Or a recommendation for an alternative but equally good speaker protection module would be fine.

- For the JLH boards, should I use the current limiter feature? And if so, what current should limiting be set to?

- I can't seem to view any attachments, it says 'forum limit exceeded' or suchlike. Is there something i'm doing wrong or is the forum broken? If someone has the useful files (schematics etc) then i'd be very grateful if they could be emailed to me.

- in what I assume is an early version of the amp BOM, R23-30 were given as 10R, going by the schematic on p42 of this thread I assume that they are meant to be 1R?

- I can see that the LTP is trimmed with R10, so that you get a 0mV offset across LS313/352, any tips on trimming R17 to set the bias current? Should I be measuring bias current directly, or should I measure across the paralleled 1R resistors? (I work a lot with Naim power amps, setting bias on these when I service them means measuring voltage across the emitter resistors, I had assumed this amp is the same but best check first). And should R17 be adjusted down to 0R or up to 500R before powering up for the first time?

- from what i've read, for UK use R3 should be 220R and R14,18 should be 130R, is this right? Might they need to be swapped back to the original values following initial testing, or can these resistors be placed for good?

- Inductor-wise, it's 18.5 turns of 1.0mm enamelled copper wire for a value of 4.7uH, is there an exact former diameter that I should use? I've got an inductance meter so I can add/remove a turn to adjust it if I need to. What's an acceptable tolerance on the inductor?

- the GB parts that Dave sent me included 8 of the output transistors with HFEs marked on (4 of NPN and PNP). Is it best to use the higher gain ones or does it not matter? What is an ideal value to use? The '281s are 91 and 92 (2 of each) and the '302s are 94 and 110 (2 of each). Maybe the 2x92 and the 2x94? Do the colours of the stickers mean anything significant?

Already seen the mention of using an OPA134 and a 2n2 cap, will do that mod thanks.

If there's some sort of build manual or other thread that has all this info that i've missed, then apologies in advance!

Dave has chosen a matching pair of the Modushop 3U cases, and i'll be building the amps as monoblocks. I'll post some pics up in due course, when i've got something worth showing off

Cheers all,
-S

[Deleted]

Hi Sean

I’ll have a stab at some of the questions

The speaker protection board that I’ve used and I think most will be using are from Jaycar here

www.jaycar.co.uk/productView.asp?ID=KC5450&keywords=speaker+protection&form=KEYWORD

Or altronics here

www.altronics.com.au/index.asp?area=item&id=K5124

Yes I have the same forum limit message?

R23-30 are 1R 1W 10R was used for the HA/PRE.

In balancing the LTP Will’s super post for the HA/PRE tells you all you need to know about the front end balance. You should find two test points to measure from which makes things nice and easy.
I usually set my pots half way @250r then go from there.

rockgrotto.proboards.com/thread/3977?page=40#

it also gives the method for setting the OP bias but obviously the power amp has much more current on the op. 250MV is about right but you may want to try 200 initially just to be on the safe side. Then turn her up one you’ve checked that all is well.

Great post Will

On the inductor winding front Alan’s superb show and tell posts are a great reference so maybe have a look see at that.

rockgrotto.proboards.com/thread/7789/winding-small-air-cored-inductor

Great work Alan

Hope that helps

Take care

[Deleted]

Thanks Sean. - much appreciated.
If Alex wants to add anything that also would be greatly appreciated
TIA,
Dave.

[Deleted]

Mar 4, 2013 19:25:34 GMT @davek said:

Thanks Sean. - much appreciated.
If Alex wants to add anything that also would be greatly appreciated
TIA,
Dave.

Hi Dave and Sean
The answers from Shaun appear to be pretty comprehensive. In the original S.C. design the 1 ohm resistors were specified as 1 ohm .5W. I am using .6W MF types there , but I doubt that there will be any audible difference with carbon film if used.I don't see any real advantage in using 2 Loudspeaker Correctors, as channel separation is already way above that of most designs.
The output devices aren't that critical, and mine aren't matched nearly as well as those from the GB. Neither are the BC327 and BC337, although they should be reasonably close in HFE.
The "magic" comes from the very closely matched metal can duals in the front end, ( LS352 mainly) and the front end balancing.
In the JLHs, I am using 1 ohm emitter resistors and 1.8K base resistors for the CL device. The CL device should have a small heatsink fitted.
I am just out of bed ATM, and no wake up coffee yet.
Dave feel free to give Sean my email address and I will email him any additional schematics etc. that he requires .
Regards
Alex

[sean]

That's brilliant, thanks Shaun and Alex. And thanks to Dave for the inductor winding tutorial as well.

One last question (for now at least), is there an optimum value for the 2 large caps on the JLH boards? I was going to use 2x4700 but I swear that i've read somewhere on this thread that 2x2200 is a better option, can anyone confirm this or did I imagine it?

[Deleted]

Mar 4, 2013 23:16:01 GMT sean said:

That's brilliant, thanks Shaun and Alex. And thanks to Dave for the inductor winding tutorial as well.

One last question (for now at least), is there an optimum value for the 2 large caps on the JLH boards? I was going to use 2x4700 but I swear that i've read somewhere on this thread that 2x2200 is a better option, can anyone confirm this or did I imagine it?

Hi Sean
You should use 2 x 2,200uF when the Current Limiter section is used . 2 x 4,700uF sounds a little better where the current limiter isn't needed, such as PSUs up to +12V 2A where the current limiting is done by the preceding voltage regulator.In the JLH I would recommend a 2,200uF 10V low ESR in parallel with a 2,200uF 16V for both sides.If your source is already highly detailed,i.e. a little on the bright side, then 2 x 2,200uF 16V may be a little better. The original JLH CL section is a little lacking, which is possibly why JLH only gave values for up tp 100mA in the original article.
Regards
Alex

[Deleted]

UPDATED SCHEMATIC.



Uploaded with ImageShack.us[/b]

[Deleted]

The company I was waiting for 1mm enameled copper wire from has now said they are unable to supply this gauge for the foreseeable future but are able to supply either 0.8mm or 1.2mm.

Which would be more suitable and how many turns would it require to make the correct uH rating?

TIA

[gommer]

es.farnell.com/pro-power/ecw1-0/cable-cobre-esmaltado-1-mm/dp/1230985

[Deleted]

Mar 10, 2013 16:18:22 GMT @cjarchez said:

The company I was waiting for 1mm enameled copper wire from has now said they are unable to supply this gauge for the foreseeable future but are able to supply either 0.8mm or 1.2mm.

Which would be more suitable and how many turns would it require to make the correct uH rating?

TIA

Hi Chris
1.2mm wire will be hard to use on the former specified, so I would use .8mm. As your speakers are 4 ohms (?) I would use 18 1/2 turns.
I am now actually using closer to 3.3uH, not the 6.8uH of the original article which used .8mm 24 1/2 turns.
Regards
Alex

[Deleted]

Hi Alex,

Thanks for that, the spec. of my 'speakers is;

Sensitivity 88dB/1W/1m
Impedance 6 ohm (min)
Power handling (music programme) 75W
Frequency response 30Hz - 20Hz plus or minus 3dB (in room)

in case that changes anything.

Cheers,

Chris

[Deleted]

Hi Chris

i have some 1mm wire if you'd like some sent out to you.

take care

[Deleted]

Hi Shaun,

Thanks for the kind offer.
I have a paid back order so all I need to do is tell the supplier to send 0.8mm instead of the 1mm ordered.

Cheers,

Chris

[Deleted]

Mar 10, 2013 21:58:45 GMT @cjarchez said:

Hi Alex,

Thanks for that, the spec. of my 'speakers is;

Sensitivity 88dB/1W/1m
Impedance 6 ohm (min)
Power handling (music programme) 75W
Frequency response 30Hz - 20Hz plus or minus 3dB (in room)

in case that changes anything.

Cheers,

Chris

Hi Chris
I would still go with 18 1/2 turns of .8mm.
Regards
Alex

[Deleted]

Excellent.

Cheers Alex.

[jeffc]

Hi guys,

Nice to see Dave/Sean and Chris getting down and dirty amongst solder fumes...... building DIY cases was what made my life extra interesting and casing everything up where woodwork meets fibreglass meets metalwork meets wiring and fine tuning is what really stretches patience to the limit.

Anyways, amongst weddings, parties and work I did get to clean up downstairs on Saturday and set up the Class A amp, and get a have a decent listen on Sunday afternoon and Monday night. Not going to say too much yet sound wise...... mainly because I've been using a 10K stepped passive attenuator built into my Class D Audio amp and thus I had to be swap-out my all battery-powered WPK Sabre DAC for my Audio GD NFB12 Wolfson DAC to make life easy for volume control....

But....... with that limitation.... there's good signs....
- amp works OK with my DIY DML panels which is a nice surprise.... whether it has the grunt to rock the house without distorting I still have to test, but OK so far at normal to loud-ish levels
- amp is deathly silent at any normal to loud volume setting, absolutely nothing heard with my ear right on the panel directly in front of an exciter
- amp bass articulation is as good if not better than the Class D Audio amp

Until I get the Sabre DAC back in the chain its not fair to comment more as I'll be describing differences more likely to be DAC-related rather than amp-related....

Only minor niggle is the PSU transformers that were dead quite on my work bench power point are how humming.... levels seem to vary and not noticed at my listening seat but something I'll try and look into as it appears to be circuit related..... I think the house fuse box has 3 power circuits..

Roughly what time tomorrow are you hopefully going to be listening to said amp Jeff?

Hi Phil, as soon as I get the set-up better optimized I'll get you over for a listen...

cheers.. jeffc

[Deleted]

Mar 12, 2013 21:12:54 GMT jeffc said:

Hi guys,

Nice to see Dave/Sean and Chris getting down and dirty amongst solder fumes...... building DIY cases was what made my life extra interesting and casing everything up where woodwork meets fibreglass meets metalwork meets wiring and fine tuning is what really stretches patience to the limit.

Anyways, amongst weddings, parties and work I did get to clean up downstairs on Saturday and set up the Class A amp, and get a have a decent listen on Sunday afternoon and Monday night. Not going to say too much yet sound wise...... mainly because I've been using a 10K stepped passive attenuator built into my Class D Audio amp and thus I had to be swap-out my all battery-powered WPK Sabre DAC for my Audio GD NFB12 Wolfson DAC to make life easy for volume control....

But....... with that limitation.... there's good signs....
- amp works OK with my DIY DML panels which is a nice surprise.... whether it has the grunt to rock the house without distorting I still have to test, but OK so far at normal to loud-ish levels
- amp is deathly silent at any normal to loud volume setting, absolutely nothing heard with my ear right on the panel directly in front of an exciter
- amp bass articulation is as good if not better than the Class D Audio amp

Until I get the Sabre DAC back in the chain its not fair to comment more as I'll be describing differences more likely to be DAC-related rather than amp-related....

Only minor niggle is the PSU transformers that were dead quite on my work bench power point are how humming.... levels seem to vary and not noticed at my listening seat but something I'll try and look into as it appears to be circuit related..... I think the house fuse box has 3 power circuits..

Roughly what time tomorrow are you hopefully going to be listening to said amp Jeff?

Hi Phil, as soon as I get the set-up better optimized I'll get you over for a listen...

cheers.. jeffc

Hi Jeff

Great to hear that you have things up and running (thought that you may have gone tropo for a momemt )

But now that you have it’s a nice time to talk a little more about my experiences with Alex’s class A PA.

What have you in mind for the pre or volco side of things?

I’ve tried everything I have pre wise

Glass house AVC standard and C core

DCB1

6n6p 6n1p Aikido

10K switch pot in a box

And to be honest none of them could match Alex’s PRE/HA which topped the poll (by some distance) for Soundstage height width and depth. And also had rightness (natural sound) about the sound that my other pre’s just could not touch.

I’ve found that the passive approach sounded a little flat, dull and lacking in drive compared to my active pre’s. Not bad just not as good if you get my drift.

But that’s just what works for me with what I have and you may find otherwise.

IMHO the PA does need a little driving to get the best from it so maybe line up what you have pre wise and take a taste test before setting anything in stone.

As I mentioned previously I do think that the last little push cable wise is a good bet so a line up on that would be fun. Some don’t think cables make a difference but I do and I think that you may also.

It’s worth that little bit of effort to find the right sound balance for you, in your set up and to your ears.

Wishing I lived a little closed for a listen to those interesting panels.

I’m thrilled to know that your build turned out well.

Great work Jeff

Take care

[Deleted]

Hi Jeff
It's a shame we live in different states, as I would love to hear the Class A into those panels.
Several years ago with an earlier version of the Class A I had a quick listen through similar panels, that were using large pieces of cardboard.
It seemed worth going further, but unfortunately never did.
Regards
Alex

[Deleted]

Hi Guys,
I know you're all waiting with bated breath for a progress report on my amp so I just thought I'd mention that Sean expects to complete one amp any day now and it is on target for mid April completion. I'm looking forward to it .