'AK Mods' SC Class A 15W Amplifier

[Deleted]

Hi Alex

nice work

those boards look great

take care

[jeffc]

Hi Alan,

Great stuff mate... I've been chipp'n away at PCB stuffing and case building and am getting close to being able to start wiring all the pieces together. With 32C-36C heat waves for weeks and visiting folks over Christmas, it hasn't been particularly conducive to doing anything too challenging, more or less this amp. Deciding to try and make my own cases hasn't helped progress either.... but oh well, if I can drill and tap a few holes on the weekend they'll be done.

So..... time for a couple of questions for guess who.

Alex,

With Will's PSU boards, I've fitted 120R in R1/R4 and 2K trimmers in R2/R3 as calculated 1K8 would be about right for 20V, R5/R6 omitted.... this OK?
For C3/C8 I've fitted 10uf but have some 22uf if these might be better for the LT1085/LT1033's I have yet to fit.
With the 150-160VA transformers I have, I plan to fit 4n7 MKPs I have at C1, this value about right?

And I decided to fit SBYV28-150 diodes rather than the MUR1520's I had, bit tricky as their leads were 1.3mm dia and thus too wide to fit through the holes but ground leads to points to fit and soldered topside, which seemed to work OK. Being a safety nut, also added heat shrink to the exposed lead just in case I get in frenzy at any point and forget they're live. Assuming these diodes will be OK too.

I'll post some progress pics when I get a sec.

cheers.. jeffc

[Deleted]

Alex,With Will's PSU boards, I've fitted 120R in R1/R4 and 2K trimmers in R2/R3 as calculated 1K8 would be about right for 20V, R5/R6 omitted.... this OK?

Sounds O.K.


For C3/C8 I've fitted 10uf but have some 22uf if these might be better for the LT1085/LT1033's I have yet to fit.

I don't see any added advantage, but if they are normal electros it won't hurt unless you go higher in value.


With the 150-160VA transformers I have, I plan to fit 4n7 MKPs I have at C1, this value about right?

TBH, I doubt that those caps will make any difference with a power amp, as distinct from a preamp.


And I decided to fit SBYV28-150 diodes rather than the MUR1520's I had, bit tricky as their leads were 1.3mm dia and thus too wide to fit through the holes but ground leads to points to fit and soldered topside, which seemed to work OK. Being a safety nut, also added heat shrink to the exposed lead just in case I get in frenzy at any point and forget they're live. Assuming these diodes will be OK too.

I intended fitting those too, but baulked at the thought of trying to enlarge the holes, due to the small copper lands.
Regardds
Alex

[jeffc]

Ta Alex,

I intended fitting those too, but baulked at the thought of trying to enlarge the holes, due to the small copper lands.

Unfortunately I didn't check the datasheet for lead dia and when they arrived thought WTF do I do. Like you didn't want to mess with the PCB lands so only solution I could think of was to take to the leads with my grinder. Interesting I found that the SBYVs use copper leads

cheers.. jeffc

[Deleted]

Jan 15, 2013 21:17:25 GMT jeffc said:

Ta Alex,

I intended fitting those too, but baulked at the thought of trying to enlarge the holes, due to the small copper lands.

Unfortunately I didn't check the datasheet for lead dia and when they arrived thought WTF do I do. Like you didn't want to mess with the PCB lands so only solution I could think of was to take to the leads with my grinder. Interesting I found that the SBYVs use copper leads

cheers.. jeffc

Hi Jeff
BTW, I found the need for a 4 wire external PSU cable , due to the terminal blocks having + and 0V , and - and 0 Volts , a bit of a nuisance, so I removed the central PCB pin , enlarged the hole and fitted a 3mm screw and nut to it. I then soldered the underneath nut to the copper after scraping around the hole, and fitted a couple of solder lugs for using my existing +,- and 0Volts cable, as well as permitting earthing to chassis of the "earth" between the big electros with both PSU PCBs.
Regards
Alex.

[Deleted]

Re added solder lugs for 0 volts/earth.

Regards
Alex

[jeffc]

Hi Alex,

No doubt it's in here "somewhere", but can you please enlighten me on Cfb and what to do with it... Ta....

cheers.. jeffc

[Deleted]

Jan 21, 2013 21:12:27 GMT jeffc said:

Hi Alex,

No doubt it's in here "somewhere", but can you please enlighten me on Cfb and what to do with it... Ta....

cheers.. jeffc

Hi Jeff
Possibly because I haven't had a wake up coffee yet, but you better point me towards which section you are talking about.
Kind Regards
Alex

P.S.
Time permitting, Jon will get a chance late Thursday afternoon to quickly hear the Class A without that framing output socket in the DAC, and the front balance fully working.Last time I had just changed the diode to a SF12,when I got a letter from the Estate agent about a property inspection, and forgot about redoing the balance.

[jeffc]

Hi Alex,

I don't have Will's PCB artwork in front of me, but from memory there's 2 substantial topside lands/pads labeled Cfb that connect a transistor/resistor to GND positioned between the LS metal can transistors and the GND terminal block. What goes there, nothing/link/?

Also for the time being I have the nearby GND-LIFT fitted with a link.

I'm sure you'll enjoy a good chat with Jon over a nice cup of English Breakfast tea as well as some darn-fine audio

DIY cases holding me up still but almost there to start wiring everything up.

cheers.. jeffc

[Deleted]

Hi Jerff
O.K. You mean the Feedback capacitor that is installed initially for test purposes before we go fully DC coupled. It can be from 100uF to 470uF. The -VE side of this electro goes to earth/0 volts.
After it all checks out, it is replaced by a link.
Kind Regards
Alex

[jeffc]

OK Alex, thanks, that'll be it, shall do

cheers.. jeffc

[Deleted]

Jan 22, 2013 4:37:22 GMT jeffc said:

OK Alex, thanks, that'll be it, shall do

cheers.. jeffc

Hi All

My Alex Class A blew the candles out for it's first birthday party.

1 year of total sonic bliss.

thanks Alex it's still going strong and sounding superb.

I'll be starting work on Wills PCB boards shortly.

take care

[Deleted]

Hi Shaun
I am pleased to hear that it hasn't taken up smoking as it get's older.
I have made a start on Will's PCBs, but as it means that my own will not be working while I update it,so I keep putting it off.
I use mine for TV as well.
Kind Regards
Alex

[pagan]

Jan 29, 2013 2:06:34 GMT @sandyk said:

Hi Shaun
I am pleased to hear that it hasn't taken up smoking as it get's older.
I have made a start on Will's PCBs, but as it means that my own will not be working while I update it,so I keep putting it off.
I use mine for TV as well.
Kind Regards
Alex

Alex
just make another one and run them bridged mono....

or use the old one for test,,,, ie turn up the bias........

moooore class A

Allan

[Deleted]

but as it means that my own will not be working while I update it,so I keep putting it off.

Hi Alex

yes that's how I'm feeling right now.

take care

[jeffc]

Shaun,

With a little more persistence, your amp will hopefully have another cousin downunder in the not too distant future.

Alex, a question on the LT voltage regulators, which appear to have their tabs connected to either Vin or Vout, I'm guessing I'll need to insulate these if both are attached to the same heat sink

cheers.. jeffc

[Deleted]

Feb 1, 2013 7:20:36 GMT jeffc said:

Shaun,

With a little more persistence, your amp will hopefully have another cousin downunder in the not too distant future.

Alex, a question on the LT voltage regulators, which appear tp have their tabs connected to either Vin or Vout, I'm guessing I'll need to insulate these if both are attached to the same heat sink

cheers.. jeffc

Hi Jeff
Yes,you will need to use insulation pads such as silicon rubber ones stocked by Jaycar as kits.
Regards
Alex

[Deleted]

Shaun,

With a little more persistence, your amp will hopefully have another cousin downunder in the not too distant future.

Hi Jeff

Can't wait

good luck

take care

[jeffc]

good luck

Thanks Shaun, gonna need it

However...........

Matching PSU and AMP cases now built phewwww was that stress city I'm never attempting that again as much as Alan has provided inspiration with his amazing DIY efforts

Transformers and all completed PCBs now mounted in the cases, with PSU-PCB V regs and AMP-PCB power transistors screwed into heat sinks OK , tapping 36 holes into 4 heat sinks without a drill press and actually getting most to line up OK....... phewwww . Only broke off 1 drill bit that I couldn't remove, which required a heat sink to be flipped over, so I actually tapped a few more than 36 holes

20-0-20 transformer secondaries reading at about 22.5V AC according to my DMM
PSUs both working and set to +/- 20.2V DC and stable
Single +/-V JLHs also seem to be working OK with stable voltages

So........ now on to more wiring and more questions and more finger crossing.....

Alex, a couple of questions

Is the 100uf electro fitted to Cfb really needed for caution at power up or is it safe enough to go straight to a wire link?

Link currently fitted to 10R gnd lift position, leave it like this for testing or go straight to 10R? I'm assuming wiring layout will be sub-optimal wherever I do and I'll get hum.

SC Speaker Protection PCB, I have the 22K to GND closest to the PCB side replaced by a wire link and the 22K next to this replaced by a 47K (as I remember you suggesting) as well as the 47uf NP electro cap closest to these resistors omitted. This as it should be?

No doubt I'll do stuff that's probably not ideal but will go with my gut instincts at let you sort out the mess later

Taking a few pics along the way that I'll post once I have wiring under control.

Well.... just so you know I'm not fibbing.... cases with their lids off prior to fitting everything....

PSU case - front view

AMP case - rear view with RCA sockets and speaker binding posts + Rocky doing what he usually does - sleep


cheers.. jeffc

[Deleted]

Hey Jeff

that's great work and with the PSU up and running you are half way there.

i must say that I'm most impressed with your DIY case which looks most professional.

OK

''Is the 100uf electro fitted to Cfb really needed for caution at power up or is it safe enough to go straight to a wire link?''

my take is to go for the wire link in this position as the cap was only intended as a fall back position should any thing go wrong with the offset corrector. if you have high DC offset at switch on then you can check the rest of the circuit with the FB cap in place which i found helped Alex isolate the mistake that i made with the IP transistors (round the wrong way).

the amp also sounds way better with the offset corrector and no cap.

''Link currently fitted to 10R gnd lift position, leave it like this for testing or go straight to 10R? I'm assuming wiring layout will be sub-optimal wherever I do and I'll get hum.''

I'd put the 10R in as the ground lift is useful especially if you are driving the PA with Alex's superb PRE/HA. (i hope that you will be as it's waaaaay better than anything else I've tried to drive the PA with). the ground lift is useful with the PRE/HA and may stop you chasing hum that is caused by not having the resistor in place. My PRE/HA PA combo is soo quiet even with my 92db speakers it's hard to tell if it's switched on even with ear hole to the drive unit but you may get a little noise without the PRE connected when you test with the stunt speakers hooked up.

OK on the wiring

i noticed that Alex used screened IC wire to connect the long power runs from the JLH to the front end of the board with the screen connected to ground. i did my build that way and even though i did not try without i felt that if it was good enough for Alex then hell it was going to be good for me also. for the rest I've used nice THICK wire which made sense and works well.

if you can power one side of the board at a time and check that all seems well and then do the same with the other. no puffs of smoke of fire balls then power both and go to the front end balance and bias setting.

just the same method as with Alex's PRE/HA but with higher current on the OP transistors.

stunt speakers are great at this point

i used crap midi system speakers bought for peanuts at a local second hand store. no tears that way.

I'm hoping that the above makes sense and that all go's well with the cousin down under.

looking good Jeff

take care

[Deleted]

Hi Jeff
Nice work with the cases !
I would fit the 10 ohm earth link resistors and the 100uF FB cap.
Initial testing would be with a resistor close to 1K across the input terminals, and the offset corrector I.C. not plugged in.
If the DC out is only a few mV then you can replace 100uF with a link and plug in the I.C. The FB cap reduces the gain at DC to 1 x.
The output would be left O/C at initial testing to reduce the risk of damage if there is a fault.After setting up tests O.K. make sure that the Speaker Protector is wired in before connecting speakers or an 8.2 ohm 5W resistor as a dummy load.
With the original SC PCB , the mounting holes for the output devices were spaced so that you could drill holes right through the heatsink and use long screws. I also broke a tap when enlarging holes to 4mm for the top cover after couple of the originals became stripped due to years of use.
I cheated with the Speaker Protector. I just removed a 47uF and left the 2 x 22K in series. Don't forget to mod the Speaker Protector as described elsewhere to provide a front panel green LED.
With a fully DC coupled amplifier, it's reassuring to switch on and then have the green LED come on several seconds latger. It tells you that the Speaker Corrector is giving you the delay it's meant to at switch on.Of course, if there is some kind of DC problem, the
green O.K. LED won't come on, or it will go out.That tells you where to start looking if smoke doesn't ! ;D
It also gives you the chance to switch off much quicker if there is a problem, and reduce possible damage.
Kind Regards
Alex
P.S.
If the wiring from the JLHs to the PCBs is fairly short, don't bother with screened wiring. You could also use twisted wire for that and connect the earth one only at the JLH end.

P.P.S.
Did you get the all clear ?

[jeffc]

Hi Shaun/Alex,

All clear, thanks very much guys, and thanks for the encouragement, I'll proceed as suggested.

Alex, just one problem with the DC offset corrector, OPA134s are soldered in, I omitted using DIP8 sockets. So.. need to keep fingers crossed here. Luckily they're cheap enough from RS should any let out smoke.

I'll power up one amp PCB at a time as suggested by Shaun to half any damage due to any serious stuff-ups.

JLH wiring will be reasonable short as I have a single RG-PCB placed either side of the AMP PCB. Was planning on using a single GND wire from the output end facing inwards from the heat sinks as this will allow a direct and relatively short route to the GND terminal block on the AMP PCB. This will also have the incoming GND wire fitted from the PSU, so essentially a star point.

I'll have to track back for the green LED fault-tracker mod for the Speaker DC Protector.

OK...... onward with wiring............

Histology on the biopsies probably with the specialist today and booked in to see him Thu to discuss what was found . Hopefully nothing.... touch wood.

cheers.. jeffc

[Deleted]

just one problem with the DC offset corrector, OPA134s are soldered in,

Hi Jeff
I forgot about that.
So you may as well put a link in place of the 100uF cap and hope that previous lessons learned mean there will be no problems.
Kind Regards
Alex

P.S.
The mod involves cutting the track to CON3 right near it's LHS terminal ,and connecting the LEDs dropping resistor under the PCB from CON3 to the relay coil.CON3 is then used for + and - to the front panel LED.Once you look under the PCB you will see how easy it is to do.

[Deleted]

Feb 3, 2013 20:52:42 GMT jeffc said:

Hi Shaun/Alex,

All clear, thanks very much guys, and thanks for the encouragement, I'll proceed as suggested.

Alex, just one problem with the DC offset corrector, OPA134s are soldered in, I omitted using DIP8 sockets. So.. need to keep fingers crossed here. Luckily they're cheap enough from RS should any let out smoke.

I'll power up one amp PCB at a time as suggested by Shaun to half any damage due to any serious stuff-ups.

JLH wiring will be reasonable short as I have a single RG-PCB placed either side of the AMP PCB. Was planning on using a single GND wire from the output end facing inwards from the heat sinks as this will allow a direct and relatively short route to the GND terminal block on the AMP PCB. This will also have the incoming GND wire fitted from the PSU, so essentially a star point.

I'll have to track back for the green LED fault-tracker mod for the Speaker DC Protector.

OK...... onward with wiring............

Histology on the biopsies probably with the specialist today and booked in to see him Thu to discuss what was found . Hopefully nothing.... touch wood.

cheers.. jeffc

Hi Jeff

i hope that things go well on all fronts

take care

[Deleted]

 Hi Jeff i hope that things go well on all fronts take care

X2,
regards,
Alan