'AK Mods' SC Class A 15W Amplifier

[Deleted]

soft start (i know no real need for one but i quite like the idea of having one anyway

Hi Shaun
A soft start circuit would be a waste of space,time and money here.
Soft Start circuits are mainly used for high powered amplifiers with huge banks of parallel capacitors of typically 80V to 100V rating that are capable of momentarily dimming the lights on the same AC feed.
david2vk's Class AB 100W, for example uses 30,000uF per supply rail. That is the kind of situation where a soft start circuit would be an advantage.
Regards
Alex

[Will]

on rectifiersI must say that my gut instinct goes with Alex re the Rectifiers. butin the interest of discovery maybe some of those nice Silicon Carbide ones would be good to try. it would be a shame for the sake of an extra few quid to not try them. let's not spoil the ship for a h'pence of tar.or if you make space on the board for TO220 rectifiers then people could choose there own poison. if we reach a consensus then a GB looks like a good idea.

Hi Shaun,

That's pretty much what I meant to say, really I'll for to220 diodes on the pcb, then everyone can choose their own poison, as it were. The mur860 are a big favourite over at diyA, and we also have the choice of hexFreds, stealths and SiC types to play with. Remembering the improvement that the UF4001 made over the 1n4001 in the jaycar PSU, then it's worthwhile experimenting.

To get the board up and running though, I'd agree with what you say, and go with what Alex is suggesting, and will be a good spec from the get go.

Also, as a clarification for everyone, the psu pcb will be essentially the original PSU that the SC class A used, but without the power transistor (so just a plain 317/337 reg with good heatsinking). This will mean you have to use two psu pcbs. You'll be able to use the higher current lt devices if you wish to have a larger margin of capacity, and also if you wish to run the amp with a higher bias than normally set. I'm not deviating from the winning recipe too much, just trying to make it more a straightforward build for anyone who wishes to build it.

[Deleted]

Hi Alex,
thanks for the hint to the more suitable speccy
diode at RS..............20 on order pending.......more bits to add of course.



IXYS
DHG5I600PA
Fast Recovery Rectifier 5A 600V TO-220AC 2.98V

[Deleted]

Hi Will,

If you are making an order with Jaycar, I might ask a favour of you.

Sorry Will I've been pondering my cash flow and I'd be better to concentrate on the bits at the input rather than output for now, so Ive shelved that idea..........with lots of stuff to buy if I dribble it all in it won't be so obvious in the accounts.
Besides I've a hankering to cut and build the Spkr protection board on a nice bit of FR4.
Regards,
Alan

[Will]

Nov 30, 2011 0:14:53 GMT drymdrum said:

Hi Will,

If you are making an order with Jaycar, I might ask a favour of you.

Sorry Will I've been pondering my cash flow and I'd be better to concentrate on the bits at the input rather than output for now, so Ive shelved that idea..........with lots of stuff to buy if I dribble it all in it won't be so obvious in the accounts.
Besides I've a hankering to cut and build the Spkr protection board on a nice bit of FR4.
Regards,
Alan

Hi Alan,

No problem, I'm sure someone here will be talking to Jaycar soon enough.
If you would like a scan of the loudspeaker protection pcb....

[Deleted]

Hi Will,

If you would like a scan of the loudspeaker protection pcb....

that would be very helpful..............schematic also
Regards,
Alan

[Deleted]

Nov 29, 2011 23:36:18 GMT Will said:

on rectifiersI must say that my gut instinct goes with Alex re the Rectifiers. butin the interest of discovery maybe some of those nice Silicon Carbide ones would be good to try. it would be a shame for the sake of an extra few quid to not try them. let's not spoil the ship for a h'pence of tar.or if you make space on the board for TO220 rectifiers then people could choose there own poison. if we reach a consensus then a GB looks like a good idea.

Hi Shaun,

That's pretty much what I meant to say, really I'll for to220 diodes on the pcb, then everyone can choose their own poison, as it were. The mur860 are a big favourite over at diyA, and we also have the choice of hexFreds, stealths and SiC types to play with. Remembering the improvement that the UF4001 made over the 1n4001 in the jaycar PSU, then it's worthwhile experimenting.

To get the board up and running though, I'd agree with what you say, and go with what Alex is suggesting, and will be a good spec from the get go.

Also, as a clarification for everyone, the psu pcb will be essentially the original PSU that the SC class A used, but without the power transistor (so just a plain 317/337 reg with good heatsinking). This will mean you have to use two psu pcbs. You'll be able to use the higher current lt devices if you wish to have a larger margin of capacity, and also if you wish to run the amp with a higher bias than normally set. I'm not deviating from the winning recipe too much, just trying to make it more a straightforward build for anyone who wishes to build it.

Hi Will

sounds like a plan to me and pretty much what i was thinking.

the original 317/337 PSU should be pretty good and a big step up from the unregulated supplies with big caps that Alex mentioned which nearly always IMHO sound a little er sloooow compared to a good regulated design.

i just don't see the point in changing something that should work pretty well and has stood the test of time.

from my experience you can't beat a good reg and the real bonus is that it's all regulated not just the front end. Add the JLH and LTP mods and it sounds pretty tasty to me.

i have the MUR860 in my Audiosector (large transformer small tank cap )and that sounds pretty good but I've not tried anything else so

fully regulated PSU that's just plain being spoiled and quite rare even amongst commercial designs.


take care

[jeffc]

Hi Will,

On PCBs, I'll certainly be needing some for PSUs as well as the amp, Phil too no doubt, and will be happy to go with the flow on design.

As I'll probably run out of PCB stand-offs, I'll need another shopping trip to Futurlec, and when I do I might get 8 of the Hyperfast, Soft recovery variants of the MUR860 to try (page bottom, and note they're CHEAP )

www.futurlec.com.au/test13.jsp?category=DIODESPOWER&category_title=Power Diodes&main_menu=DIODES&sub_menu=DIODESPOWER

cheers.. jeffc

[Deleted]

Nov 29, 2011 21:22:52 GMT @sandyk said:

Nov 29, 2011 17:11:21 GMT drymdrum said:

Hi Alex,
by slow recovery I take it you are referring to something like this.......................
Fast recovery/ultra soft diode 10A 1200V IXYS DHG10I1200PM

RS have them

Qty Unit Price
25 £1.24 each incl. vat
100 £1.11 each incl. vat
Alan

Hi Alan
I did mean soft recovery.There should be plenty of similar types available with lower ratings, and also a little less expensive.
Alex

e.g.

Diode,rectifier,fast recovery,10A,600V,TO220AC,DHG10I600PA

australia.rs-online.com/web/p/rectifier/0462221/

Alan
I have closely checked their specifications, and it appears that the forward voltage drop may be a little too high when using 20-0-20 toroidals.
Jeff and Will
I am not so sure that the MUR820/MUR860 are a good choice either, as they are not soft recovery types.
Alex

P.S.
What's the feeling about this one ?
australia.rs-online.com/web/p/rectifier/6393308/

[jeffc]

They look OK to me Alex,

You'll remember me discussing the 2A versions of these when constructing my computer linear supply, partly because specs looked really good, and they were only 32c ea for a lot or 25. Gotta stop looking for bargains, OCD.

cheers.. jeffc

[Deleted]

Hi Jeff
Let's see what Will and the others have to say about different diodes
before we jump in here ?
Alex

[Will]

My personal thoughts on the rectifier diode, at this stage, is to use something that can handle the voltage and current requirements, and prove the amp works. Cheaper the better, as tweaking can come after.

My mention of SiC diodes was mostly to make people aware of them, rather than recommend them, as I've not used them.

Alex, I think that the diode that Barrows mentioned (that thread with Erin in?) was the MSRF860, plastic case version of MSR860. The SiC diodes are mentioned at the end of the same thread.

Jeff, Everyone likes a bargain, and wants good vfm, so not OCD, just normal in my book!

I like finding the OEM of 'posh or expensive stuff. An example is F&T capacitors, If they looked prettier and had the words Mlytic on them, then they would cost more (allegedly )

[Deleted]

My personal thoughts on the rectifier diode, at this stage, is to use something that can handle the voltage and current requirements, and prove the amp works. Cheaper the better, as tweaking can come after.

Hi Will

that hits the nail firmly on the head and totally the way that i would do things.

I've always worked that way in the past as it saves the despair of watching those nice expensive components puffing nice blue smoke.

a simple bridge package is cheap and easy to use and one less thing to worry about should things go wrong.

these things can be changed latter pretty easily.

my thinking is pretty much in line with Alex's on the rectifiers.

from experience the quality of the reg used has FAR more influence on the sound than the rectifiers.

for unregulated stuff yes they do have a large impact on SQ

from playing with regs for a few years I'm note sure loads of money in that spot is going to be well spent. surely part of the reason for a regulated supply is to get round the need for expensive rectifiers and tank caps. thats one of the big plus points for using regulators IMHO.

let's not forget that we are talking load and LINE regulation here so before and after the reg are influenced.

my experience is mostly with tube based rectifiers and regs like the 6080. when i built the legacy 300B with a fully regulated supply and SS rectifier diodes i tried out quite a few options diode wise.

no real difference SQ wise IMHO but others may have a different take on things.

also it's part of the fun of a good build thread to have people report on changes they make and any effects on SQ they bring.

so keeping it sensible in the up and running stage gets my vote.

I'm happier spending cash where it may do more good.

take care

[Deleted]

Hi Alex,
Diode,rectifier,Fast Recovery,Ultra Soft,SBYV28-150
price is good........RS UK £0.23 each per100.
buying 25 saves £10 on the IXYS..................
that's a no.....brainer
Alan

[jeffc]

Guys,

Some interesting reading here, at least I found it interesting. Particularly on the SS Amp (Class AB in this case) Power Supply considerations including capacitor and diode types, plus if you look elsewhere you'll find stuff like interesting insights into how to make good and CHEAP alternatives to stepped attenuators for vol control.

www.aksaonline.com/faqs/faqs.html

cheers.. jeffc

PS. Alex, do you know what the amp input impedance is?

[Deleted]

do you know what the amp input impedance is?

Jeff
It would be a little less than 15K with mine, and less than 18K
with the original.Don't forget that most sources these days have an output impedance of way under 200 ohms, so the input resistor value could be even lower if you wanted a further small improvement in S/N, without loading the source.
Alex

P.S.
Hugh Dean loves to have his SS amps sound a bit like valve amps, so often tailors the harmonic distortion structure to favour even order harmonics for added warmth.Hugh also likes tube preamps and vinyl. Greg Erskine should be able to confirm this, as he knows Hugh personally, and is conversant with his designs which usually include a circuit technique called bootstrapping. ( see the D.Self book)

[jeffc]

The joys of going cheap.

The talema/Nuvotem 150VA 2x115V 2x20V transformers arrived today and look OK but were minus any mounting kits (which were specified they’d have) and any documentation. Their centre holes are potted and I have bolts to suit if trimmed in length, BUT on principal I’m going to hassle TorTech to send me 4 sets. No documentation would not normally be too much of a problem EXCEPT 2 have 99 production dates and 2 have 94 dates, and the wire colours are ordered differently.

On all 4

Primary: Black-White and Red-Green have continuity
Secondary: Brown-Orange and Yellow-Blue have continuity

Wire order on the 99 dated pair (from left to right as looking at them)

Primary: B-G-W-R Secondary: BR-Y-BL-O

Wire order on one 94 dated transformer

Primary: B-G-R-W Secondary: BL-O-Y-BR

Wire order on the other 94 dated transformer

Primary: G-B-W-R Secondary: O-BL-BR-Y (so end pairs reversed)

I’ll ensure Phil gets the 99 dated pair as these at least are identical, but I’m going to need help on how to wire the 94 dated pair, OR send this information to TorTech and ask them for clarification OR request that they send me ones with identical wire orders. I’ve checked the talema web site and downloaded their traffo catalogue, which has general info and specifications, but wiring colours of toroidal transformers described in this are different again.

Any advice welcomed.

cheers.. jeffc

[Deleted]

The joys of going cheap.

Hi Jeff

never mind that

i had a quick peek over on Talema's web site and as you say info is pretty sparse.

what i have noticed recently is that transformer primary and secondary windings seem to have different colour coding according to what market they are aimed at. US transformers seem to have a similar (but not identical) coding system to the ones you have. here in the UK we seem to have a pretty standard colour coding system.

any of you international members recognize these transformers?

sorry not very helpful but good work on getting them at such a good price.

take care

[Deleted]

Hi All

is it about time to start thinking on a LS device group buy?

if so I'd be happy to organize that if it helps.

take care

[Deleted]

Hi Shaun,
could you split your PSU positive and negative sections onto two boards that would make it easier to heatsink them inline.
Alan

[Deleted]

Hi Alan

yes i was just thinking that as i was scratching my head and wondering.

i was thinking that we could do two boards as a mirror with the 0V running down the center of the two boards.

I'll post you a quick sketch of what i was thinking.

take care

[jeffc]

Here's the transformers


2 @ the top - dated 99
2 @ the bottom - dated 94

- hopefully the primary/secondary wire colour sequences are clear
- the primary wires are solid so I guess simply extensions of the windings
- secondary wires seem very thin.....

Do they look right for the job?

Shaun, great if you want to buy transistors in bulk, top man.
Alex, did you fit JLHs in the amp or PSU case? Just thinking about PSU case size.

cheers.. jeffc

[Deleted]

Hi Jeff
Most toroidal transformers have information stuck on the side indicating the voltages,VA rating etc. Do these have printed info there ? The secondary windings are of course those with heavier gauge wire because of the much higher current (and lower voltage)in the secondary .
The JLHs for the Class A amp should be installed close to the amplifier PCBs
Alex

[Deleted]

is it about time to start thinking on a LS device group buy?

What's been keeping you?

Count me in Shaun, great you're doing this, again for us all

Syd

[Deleted]

Dec 2, 2011 18:38:32 GMT @shaun said:

Hi Alan

yes i was just thinking that as i was scratching my head and wondering.

i was thinking that we could do two boards as a mirror with the 0V running down the center of the two boards.

I'll post you a quick sketch of what i was thinking.

take care

How big do these heatsinks need to be?

Is it possible to have the regulators backing onto the same heatsink (with insulators), of the form we used in the A class hpAmp? With the rest of the single or dual board then worked around that.