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QUDOS - the brilliant new amplifier boards from Avondale

James Evans said:
Ah, I was still going for the 10% difference. Should they now be matched?
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James I haven't seen the Circuit diag for the NCC220 but it the LTP has a CCS Current mirror like the NCC300 then IMO no they no longer need an offset

Alan
 
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You should still be aiming for a 10% difference which I think is what Alan meant when he said "match".

I wouldn't be overly worried by a d.c. offset of 65mV, let's see what you get with the second board.
 
Thanks, I figured I can live with 65mV, second board testing to commence later. I did however notice the latest schematic has no mention of the 10% on it, whereas all previous versions have?
 
Chops54 said:
James, I had one board at 44mV and the other at 48mV. I’m happy at that and they sound fine.
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Thanks Si. I've added the vbe front end into the mix now and that's bumped the offset up a bit more. Now at 65 and 75. Still ok I think, if a bit high.
 
James Evans said:
Thanks, I figured I can live with 65mV, second board testing to commence later. I did however notice the latest schematic has no mention of the 10% on it, whereas all previous versions have?
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that's at 100mA if i'm understanding correctly? mine are still at 100mA but i am concerned the DC offset will go way up when i 'crank' the current up a notch or two to a target 120+ mA
 
Jonathan said:
that's at 100mA if i'm understanding correctly? mine are still at 100mA but i am concerned the DC offset will go way up when i 'crank' the current up a notch or two to a target 120+ mA
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I'm at about 130mA I think now. 116 + 13 on the front end.

Sacrificial speakers connected and been playing for a few hours. Sounds promising. Heatsinks warm. Soon be time to hook up the main speakers.
 
mine's at 100mA and sounding pretty fantastic i have to say - i can't imagine it getting much better ... or maybe it's the wet tants working their wet tant magic ...
 
I'm running the output stage of my 220s at 110mA so somewhat similar to yours James and I'm using VBEs for the front end though I've never measured their current draw. I have no idea what causes some builds to have a higher DC offset than other builds. Maybe it's just down to component choice?
 
I recently measured the current draw of the 220 front end (measured between the HCR200 output and the front end input); it was 12mv on the positive line.
 
TR1 should be 10-15% higher than TR2 as I recall to keep the offset low. You should be able to get it around 10mv or less. But anything around 50mv is good to go..I’ve never built an amp with higher offset.
 
DC offset mainly comes from the effect of the base currents in the LTP. Since both LTP transistors "see" the same impedance at DC (27K & 24K + 2K7), for lowest offset voltage you should match the input pairs for hfe.
The polarity of the input and feedback caps can be worked out from the direction of base current flow. For an NPN LTP they should be fitted with the negative terminal toward the bases.

Maybe Les has some special reason for doing something else, but the above is the correct engineering method.
 
I pulled some pins out of a 8 pin opamp socket and used them on the second transistor position,tried different transistors till the offset was under 10mV, just for fun.
 
On the subject of BC546, what is the difference between:

https://uk.farnell.com/on-semicondu...or-bipol-npn-65v-to-92/dp/2453320?st=BC546BTA

and

https://uk.farnell.com/on-semiconductor/bc546bta/trans-npn-65v-0-1a-0-5w-to-92/dp/2985341

They have the same datasheet as far as I can tell, but the "new" second one of those is 1/3 the price, so you can get 100 for £3.40 + vat.

I know the first ones states 110 HFE and the 2nd 200, but as they are both BC546B I'm assuming they are both in the 200-450 range in reality?
 
The expensive ones are cut-tape (and have formed legs); the cheaper ones are loose (and have straight legs).

They have pictures of both in the datasheet....
 
Thanks Jeff, I was going by the images on the farnell listings, obviously incorrect. The only hint on the two pages appears to be the "supplied on cut tape", next to the price. They are both shown as BTA, but ABU is the bulk version with straight legs according to the datasheet. All very confusing...
 
Sloppy descriptions by Farnell. Happens a lot, but then they are managing 100s of thousands of SKUs.

Or maybe just sloppy pricing. It could be they are both BTA, but one of them has ABU pricing....
 
ABU would fit with the 110HFE, so the more expensive would appear to be that one, but then that one is described as being on cut tape....
 
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