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Avondale NCC300 amp project ..

I think he meant to type that he has -50mV offset - like the meter readings.
Must admit I'm surprised by this as I always match the hfes closely and have never had any issue with output offsets in similar topologies (to what I assume the NCC300 is).

I did always wonder why the input cap and feedback cap are shown the wrong way round on some NCC200 schematics:
https://obrazki.elektroda.pl/2640663400_1332688131.png

This is the correct way:
http://www.next.gr/uploads/47/power-amplifier-circuit-ncc200-Z-a.jpg
 
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S-Man, Alan's left DVM is monitoring the FE rail voltage of 50.0V.

FWIW, the input cap is sized for a film, so there's no polarity issue there. Alan's got it wire linked, so there' really no polarity issue. ;) Interestingly, Les has the FB cap for the NCC300 with the cap's ground end connected to the input ground.
 
flashgo said:
S-Man, Alan's left DVM is monitoring the FE rail voltage of 50.0V.

FWIW, the input cap is sized for a film, so there's no polarity issue there. Alan's got it wire linked, so there' really no polarity issue. ;) Interestingly, Les has the FB cap for the NCC300 with the cap's ground end connected to the input ground.
Click to expand...

Ah yes. I should have zoomed in. Thanks.
-5mV is absolutely fine. There is no neccessity to have a +ive offset.
 
S-Man said:
I think he meant to type that he has -50mV offset - like the meter readings.
Must admit I'm surprised by this as I always match the hfes closely and have never had any issue with output offsets in similar topologies (to what I assume the NCC300 is).

I did always wonder why the input cap and feedback cap are shown the wrong way round on some NCC200 schematics:
https://obrazki.elektroda.pl/2640663400_1332688131.png

This is the correct way:
http://www.next.gr/uploads/47/power-amplifier-circuit-ncc200-Z-a.jpg
Click to expand...
David
The DC offset was only 3-5mv but negative.
The negative -50V was the -ve rail voltage to the front end
BTW I cant find anything wrong with the regs form a 53v supply I can have it at 51.5 or wind it down to 40V and adding a load does not cause a drop out its rock solid

Funny you should say that about the FB and Input cap I noticed that about 6 months ago that the early diag shows it the way you say is correct and thats the way I had mine in all my amps ?, I looked at the Doug self Audio Amplifier design book and he has it the other way round same as the later Diags and the new NCC300 so I turned mine round but cant say that I noticed any difference ?, when you think about it its just DC blocking caps in the signal path so cant see that it would be that critical, but that said I would place it the way you stated for the input cap if it was polarised.

Alan
 
Marra
Yesterday I used TR1 HFE-292 / TR2 HFE-262

Input Cap
I have a wire link in the input cap position at the moment but I will fit 2 pins in there to experiment with different caps
In my previous tests I have always preferred the 10uf Mil spec Wet Tant but I have some 10uf films that I would also like to try, as below pic
Note that no cap is best if you are confident of your preamp output.
I'm not going to mess with the feed back cap its mil spec wet tantalum every time for me

20180406_090921 by Alan Towell, on Flickr

Alan
 
337alant said:
David
The DC offset was only 3-5mv but negative.
The negative -50V was the -ve rail voltage to the front end
BTW I cant find anything wrong with the regs form a 53v supply I can have it at 51.5 or wind it down to 40V and adding a load does not cause a drop out its rock solid
Alan
Click to expand...

That's what I would expect.
Seems like Graham's regs are not regulating (assuming I understood his description)

337alant said:
Funny you should say that about the FB and Input cap I noticed that about 6 months ago that the early diag shows it the way you say is correct and thats the way I had mine in all my amps ?, I looked at the Doug self Audio Amplifier design book and he has it the other way round same as the later Diags and the new NCC300 so I turned mine round but cant say that I noticed any difference ?, when you think about it its just DC blocking caps in the signal path so cant see that it would be that critical, but that said I would place it the way you stated for the input cap if it was polarised.

Alan
Click to expand...

Turning them round will not sound any different, but technically it's more correct.

If you think about Tr1 (NCC200 schematic)... it will have a base current of ~ 0.5mA/300 = 1.67uA. This has to flow from ground, through the 24K and 2K7 which results in TR4 base being at -44.5mV and the 24K/2K7 node being at -40mV.
The feedback cap voltage depends on how the voltages sum around the LTP, but if you assume perfect matching of the Vbes and the emitter resistors and they are running at the same currents then TR2 base = TR1 base voltage. TR2 base current is supplied from the output via the 27K feedback resistor.

In reality 40mV of reverse plolarisation is not going to bother any electrolytic.

Is Selfie's LTP made of PNPs? Then the caps should be the other way round.
 
Front end PSUs built and ready for installation ....

One issue is the spec on transformers for the FE (mentioned a few times across the forums) is 230v primaries and in this case 42-0-42 secondaries but mains, as with most in the UK hits 245v so the resultant secondaries are at 48.5 not 42. And after rectification you wend up with = - 66vdc. I didn't want to use 63v caps, not one to push my luck Ive installed 80v caps in the FE PSU to be on the safe side. Im sure these 63v caps would be OK in the short term but,,,,,I have seen litic caps go pop and what a mess...

27401687818_bcc78be189_h_d.jpg


All the components in the 300 FE PSU are min of 100v so all should be well.
 
S-Man said:
Turning them round will not sound any different, but technically it's more correct.
.
Click to expand...

Quite, the watchword here is leakage and if the cap doesn't leak in either polarity,
it does its job.

I took a hammering on another forum from a well known designer of amps on this
very subject but thinking about the old motto:

"If it's stupid and it works - it's not stupid"

Could we move on now please....
 
LesW said:
Could we move on now please....
Click to expand...

I was just trying to dispel the myth that the offset needs to be +ive. I also thought it might help others to have an explanation of why the offset occurs and how to calculate it.
If this is not appreciated then I can save my time and effort.
 
S-Man said:
I was just trying to dispel the myth that the offset needs to be +ive. I also thought it might help others to have an explanation of why the offset occurs and how to calculate it.
If this is not appreciated then I can save my time and effort.
Click to expand...
David, Marra , Les, Flash sorry for any Confusion caused you are correct and I over reacted to the negative offset, but IMO its all good discussion in the final development of a potentially superb amplifier.
I'll wire an amplifier up over the week end
20180406_153859 by Alan Towell, on Flickr
Alan
 
No problem Alan. The build looks up to your usual high standard. I look forward to your thoughts on the sound.
 
Looking sweet, Alan!
A couple component questions:
  1. Where do you get those little seats for the transistors? They look like a good idea at least for tweaking the offset with the BC546s.
  2. What's the power switch? It looks like it's elegant from the front but can carry full current on the back.
Thanks
 
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