'AK Mods' SC Class A 15W Amplifier

[sean]

Hi Alex, yes they're definitely OPA134. With no speakers connected, preamp connected (Naim NAC82), op-amp removed, CFB left as a wire link, the offset on the speaker output is 770mV.

edit - just measured offset on the preamp and it's 1.5mV. OK, but not ideal.

With a 1k resistor on the input, offset is 79mV.

[Deleted]

Apr 23, 2013 11:37:48 GMT sean said:

Hi Alex, yes they're definitely OPA134. With no speakers connected, preamp connected (Naim NAC82), op-amp removed, CFB left as a wire link, the offset on the speaker output is 770mV.

Sean
How about a quick measurement into a 4.7 ohm load ? This isn't making much sense . I wonder if the Naim is AC coupled at the output ?
The offset correctors do seem to be working if the offset with a load is much lower than without it. With no FB capacitor the offset is being multiplied by the gain of the amplifier.


Regards
Alex

[sean]

No op-amp, 1k input resistor, 4R7 dummy load on output - 80mV. With the Naim preamp, it's back to 770mV or thereabouts again. I've never had any problems with the Naim preamp into any other power amps I have (various Naim amps, gainclone, NCC200s) so don't think that is a problem.

personal MP3 player brings the offset down to 33mV though.

How come there isn't an input capacitor on the class A amp, by the way?

[Deleted]

Apr 23, 2013 11:37:48 GMT sean said:

Hi Alex, yes they're definitely OPA134. With no speakers connected, preamp connected (Naim NAC82), op-amp removed, CFB left as a wire link, the offset on the speaker output is 770mV.

edit - just measured offset on the preamp and it's 1.5mV. OK, but not ideal.

With a 1k resistor on the input, offset is 79mV.

Is the DC out of the I.C. still -13.9V ?
IIRC, originally you had about double what I had, but it was only about 300mV ?.
Your front end readings seem pretty close, so I wonder if we need measurements further on.
Are the voltages at the 100 ohm base resistors of the 327 and 337 within a volt or thereabouts of earth ?

[Deleted]

Apr 23, 2013 12:00:47 GMT sean said:

No op-amp, 1k input resistor, 4R7 dummy load on output - 80mV. With the Naim preamp, it's back to 770mV or thereabouts again. I've never had any problems with the Naim preamp into any other power amps I have (various Naim amps, gainclone, NCC200s) so don't think that is a problem.

personal MP3 player brings the offset down to 33mV though.

How come there isn't an input capacitor on the class A amp, by the way?

The input capacitor is optional, as most source stuff is DC coupled these days and nobody had problems with DC coupling in the preamp using the offset corrector.
It was reported to sound better with no capacitor, and a fully functional offset corrector nulled the offset anyway within a few mV.
This amp is normally used in conjunction with the Class A preamp by most members. It is also DC coupled with an offset corrector of it's own, and the result is normally within a couple of mV at the speakers. Even with nothing plugged in, the DC out of mine is still not a worry, and the speaker protector is there too, in any case.
Didn't you previously have around 10mV offset with the Naim plugged in (earlier post) ? If an A.C. coupled preamp is used the input resistor value can be scaled down.
Try a 4.7K across the input terminal block in parallel with the input with the Naim connected.

[sean]

The op-amp isn't fitted - the voltage at pin 6 of the IC socket is about 10% less than the output DC offset value for each of the 3 inputs listed above. With the op amp back in place, it's back to giving 13.9V on pin 6 with a 1k input again.

Voltages at either side of the 100R resistors of the 327 and 337 are 0.68V/0.75V and 0.81V/0.90V w.r.t. ground, so they sound about right.

[Deleted]

Apr 23, 2013 12:20:13 GMT sean said:

The op-amp isn't fitted - the voltage at pin 6 of the IC socket is about 10% less than the output DC offset value for each of the 3 inputs listed above. With the op amp back in place, it's back to giving 13.9V on pin 6 with a 1k input again.

Voltages at either side of the 100R resistors of the 327 and 337 are 0.68V/0.75V and 0.81V/0.90V w.r.t. ground, so they sound about right.

Sean
Something is definitely weird here, as originally you had about 380mV at pin 6 of the opamp with a 1K input resistor . IIRC. it was about double what I measured with mine.
I am fresh out of ideas at the moment, so perhaps one of the others can chime in, as it's approaching my bed time.
Regards
Alex

Apr 18, 2013 at 10:47pm

Post by sean on Apr 18, 2013 at 10:47pm

With FB cap bypassed and no speakers, and 1k on input, DC output at speaker terminals is 2.7mV. Pin 6 at is at 376mV DC.

As above but with 8 ohm test speaker connected, DC output on speaker is 1.0mV. Pin 6 is now 396mV.

With a 4.7 Ohm 50W resistor connected, DC output on resistor is 0.9mV, Pin 6 is 394mV.

Looks like there's too much gain - the 4K7 and 1M resistors are definitely right though. I swapped op-amps for the other OPA134 and got the same results.


Read more: rockgrotto.proboards.com/thread/7461/ak-mods-class-15w-amplifier?page=48#ixzz2RHzykMTP

[sean]

I did have about 30mV of offset with the Naim hooked up previously, before I shorted out GNDLIFT and took the corrector feed from the other side of the Zobel. I know the Naim works fine, and it has an output offset voltage of 0.1mV now (it might have been on mute when I checked it before, oops).

tried a 4k7 in parallel with the input with the Naim connected, DC offset is now 214mV. Pin 6 still at 13.9V

[sean]

That's right Alex, that was before I bodged it up with wire links and suchlike though. Maybe I should try some new OPA134s?

[Deleted]

Sean
Something seems to have happened since those original readings I just quoted. The front end doesn't look too bad though.
Fart sack time !
Regards
Alex

P.S.
With the olffset corrector fitted, and a FB cap fitted, the DC out should be reduced by around 16 times ?
Good Night!

[Deleted]

‘’No op-amp, 1k input resistor, 4R7 dummy load on output ‘’

Hi Sean

Is that with the FB cap inserted? If it is then it seems a little on the high side to me. If not then try a 100uf ish cap in the FB loop to see if that changes anything.

How come there isn't an input capacitor on the class A amp, by the way?


Because we have the DC offset corrector.

Is the DC out of the I.C. still -13.9V ?


Are the measurements the same for both amps?

Sean I have a strong feeling that when we fine the problem that it’s going to be something small and we’ll go Ah that’s what it was.

As Alex has mentioned a full measure round may be useful at this time.

I would definitely check the 327/337 pairing as one or both of those could be the culprit as it was on one of my boards. Yup been there and had that fault so it does sound a little familiar.

My boards where self-etched BTW.

Nearly there main man

Take care

[Deleted]

Hi Sean

Just one last question.

Are your 327/337 heat sinked?

I’ve had a quick look at your build photo and it's a little hard to see.

Just to remind others

Those two devices do need to have heat sinks.

Take care

[sean]

Hi Shaun, no that is with a wire link in CFB and no op amp - i'll try out a 100uF cap in there with no op amp and report back. I've double-checked all the transistors against their pinouts from ther datasheets and they all look fine, is that what you meant by checking them?

I've measured the front end already, which seemed fine to me and Alex agreed. Is there anything you can recommend for useful measurements from the output end?

I'm only working on one amp at the minute - the fault was identical on both amps anyway, but now that i've found a fix for the oscillations I will get the other one fixed and then I can use that to test too.

edit - and yes, i've got some gold clip-on heatsinks fitted to the 327/337.

[Deleted]

I've double-checked all the transistors against their pinouts from ther datasheets and they all look fine, is that what you meant by checking them?


Hi Sean

Good start but no I mean measure the CBE voltages as I strongly suspect that one or both of those devices may have gone bye bye.i had the same sort of strange measurements when my 327/337 combo went pop.

I’d check the other amp to see if you get the same readings if net then it’s going to be much easier to find the fault.

But myself from a little experience I’d have just a little money on the 327/337 from what you have described.

Another tell tail and good for a quick and dirty is if only one of the OP devices is heating up. Those devices are robust and unlikely to fail so if that is the case then it will most likely be the corresponding 327 or 337 gone to heaven.

''and yes, i've got some gold clip-on heatsinks fitted to the 327/337''

thought i noticed some bling

Keep us posted

Take care

[sean]

I'll check them later then, thanks. Should I test with 1k input resistor, 4R7 dummy load, op-amp fitted and wire link in CFB? Or something else?

[Deleted]

Apr 23, 2013 14:25:19 GMT sean said:

I'll check them later then, thanks. Should I test with 1k input resistor, 4R7 dummy load, op-amp fitted and wire link in CFB? Or something else?

Hi Sean

Yes that’s how I would do it but maybe to check that both OP transistors are heating up first without the Dummy load.

Good work Sean

Nearly there

I can feel it in my bones

Take care

[Deleted]

Hi Alex/Shaun/et al,
did the mod on Will's boards, replaced the singed 10Rs and on fire up
had the pleasant surprise of music ....no good music coming out of the Wharfedale 6 ohms.
So that's a result, thanks Alex, not a difficult work around to do, I didn't lift the output block as I couldn't see
the point in making the job difficult.
Any point along the offending trace would be good to make the break which suited me, I suppose if you
had aspirations to submit your work for a gold star you might try to hide it under the block.........................
it's the music that counts for me.
Thanks again to all concerned in this project the music is top notch.
Regards,
Alan 
 

[Will]

Sean and Alan, really glad to hear that the 4-6 ohm speaker oscillations are now cured, and we know what the cause is.

Alan, is there any chance you could measure the dc on the output, see if it helps narrow down the potential problems that Sean is having? I must admit that I think he has a fried op amp if he has nearly rail voltage at the output. Had very similar fault once on a mini3 amp. Might be worth swapping it out for another single opamp.

[Deleted]

Hi Will,
yes I've looked at the DC on the output 
right channel has .8mv
left channel has .6mv
I think it was lower in January  when I first tested it, I might be thinking of the HA/Pre though.
Regards,
Alan

[sean]

Hi Will, yes i've got some new opamps on the way, should have them tomorrow - in the meantime I need to test the C/B/E voltages on the 327 and 337, and see how they are. Does anyone have measurements from their working amps that I can compare my readings to? Cheers for all the ongoing help!

edit - the op-amps arriving tomorrow are TL071, are there any mods to the correcor that I need to make to use them or are they just drop-in replacements for the OPA134? They don't sound as good from what you guys say, but they're just cheap bits to get the thing working properly.

[Will]

Apr 23, 2013 18:58:00 GMT drymdrum said:

Hi Will,
yes I've looked at the DC on the output 
right channel has .8mv
left channel has .6mv
I think it was lower in January  when I first tested it, I might be thinking of the HA/Pre though.
Regards,
Alan

Hi Alan,

Thanks for checking, and good to know that the 'Will PCB' has a working offset corrector. This means Sean has a fixable problem, and isn't fighting against a fault on the PCB. Good stuff!

[Will]

Apr 23, 2013 19:03:07 GMT sean said:

Hi Will, yes i've got some new opamps on the way, should have them tomorrow - in the meantime I need to test the C/B/E voltages on the 327 and 337, and see how they are. Does anyone have measurements from their working amps that I can compare my readings to? Cheers for all the ongoing help!

edit - the op-amps arriving tomorrow are TL071, are there any mods to the correcor that I need to make to use them or are they just drop-in replacements for the OPA134? They don't sound as good from what you guys say, but they're just cheap bits to get the thing working properly.

The TL071 will do fine for testing, or for normal use, without any change to the circuit, so just drop them in.
I'm afraid I've yet to build mine up (or the fully do the previous HA )so the guys here are better placed to help you.

I know that you are building this for DaveK, and he'll see you good, but with all this extra hassle, I reckon you should peruse the 'Beer Appreciation' thread and see if Dave can get you something nice from there! (I can recommend Paulaner by the way!)

[Deleted]

Great news that Alan's amp is now driving 6ohm 'speakers well with an easy fix.

Could someone explain the "Cfb" to me. Why do you link it out or conversely pop in a 100uf cap?

Cheers

[Will]

Apr 23, 2013 19:36:27 GMT @cjarchez said:

Great news that Alan's amp is now driving 6ohm 'speakers well with an easy fix.

Could someone explain the "Cfb" to me. Why do you link it out or conversely pop in a 100uf cap?

Cheers

The Cfb was a cap in the feedback in the original design, to reduce dc at the output (I think). It's here only to help if you have problems. When first firing up, I might be a good idea to have a 47-100uf 'any old type' capacitor in place. If you have no appreciable dc at the output of the pcb, remove the cap and link across using your finest cryo-treated platinum coated 9n copper. Or a capacitor leg.

[Deleted]

Thanks Will. I'll do exactly that.