Well, I am very much relieved - I learnt a very interesting thing today! So I am posting it, in case it is helpful to others. 
Since changing over the Source resistors (R141 & 142 on the 8R 'as built' schematic) from the specified-for-the-4R-version of 0.12R to 0.15R (which reduced the bias current from 3a to 2.5a) - which was a relatively easy job - I have been unable to have a good listen to the amps (to try and judge whether their sound had been impacted by the reduction in bias). The reason ... a nasty buzz through the spkrs - which wasn't there when I was listening to the amps last winter (ie. before I did the res change).
Anyway, after a couple of weeks of looking at the open-topped amps and wondering how I could possibly get rid of the buzz ... I finally remembered the wiring layout that @Hugh used to specify in his early kit amps (which had their PSs separate to the amp PCB).
This was: spkr return ... goes to the PS 'star earth'! Whereas, on the AN PCB there are spades for the spkr signal & return - so that is how I've had these wires connected.
So I decided to disconnect the spkr return terminals on one amp, from the ground wires going to the AN PCBs ... and instead, clipped jumper wires from the spkr return terminals to a spare earth spade on each SLB PCB.
Voila - dead silence! Yay!
So now I will make up 4x spkr return wires to replace the jumpers ... and I can then sit down - at last!! - and listen to my AN4Rs again.
So, I would recommend others try this "Hugh trick" ... to see whether it lowers the sound floor in their build.
Since changing over the Source resistors (R141 & 142 on the 8R 'as built' schematic) from the specified-for-the-4R-version of 0.12R to 0.15R (which reduced the bias current from 3a to 2.5a) - which was a relatively easy job - I have been unable to have a good listen to the amps (to try and judge whether their sound had been impacted by the reduction in bias). The reason ... a nasty buzz through the spkrs - which wasn't there when I was listening to the amps last winter (ie. before I did the res change).
Anyway, after a couple of weeks of looking at the open-topped amps and wondering how I could possibly get rid of the buzz ... I finally remembered the wiring layout that @Hugh used to specify in his early kit amps (which had their PSs separate to the amp PCB).
This was: spkr return ... goes to the PS 'star earth'!
Whereas, on the AN PCB there are spades for the spkr signal & return - so that is how I've had these wires connected.So I decided to disconnect the spkr return terminals on one amp, from the ground wires going to the AN PCBs ... and instead, clipped jumper wires from the spkr return terminals to a spare earth spade on each SLB PCB.
Voila - dead silence! Yay!
So now I will make up 4x spkr return wires to replace the jumpers ... and I can then sit down - at last!! - and listen to my AN4Rs again.
So, I would recommend others try this "Hugh trick" ... to see whether it lowers the sound floor in their build.
Efkaristo, Nikos.
In terms of the new spkr return connection - it is simply that the wire from the spkr return binding post will now go to the SLB ground (not the AN4R PCB spkr ground).
Eυχαριστω και εγω Andy.
it seems there are two difference connections
Edit Nov 29, 2019 - Grounding scheme schematic (post 279)
and
Edit Apr 24, 2020 - Dual Monobloc Connection Diagram is using SLB and SFP boards:
it seems there are two difference connections
Edit Nov 29, 2019 - Grounding scheme schematic (post 279)
and
Edit Apr 24, 2020 - Dual Monobloc Connection Diagram is using SLB and SFP boards:
Attachments
Well found, Nikos!
Your 2nd diagram (the earlier one) is what I saw on the thread ... and that's the way I first connected up my system.
But I now have it connected, basically, like your 1st diagram ... although I'm not using an SFP board. (I use a thyristor, instead.)
In other words, the spkr -ve now goes back to the SLB ground. However, I don't - unlike that 1st diagram - have the AN4R PCB connected to mains earth (via the 'safety earth bolt on the case).
Hey guys
I would highly suggest wiring speaker
- return onto your power supply ground and not at the amp board for some reason when attached on amp board it introduces
Noise and almost overdriven distortion in my opinion weird but true!!
I was almost ready to give up on this amp until I figured this out.
Scott
I would highly suggest wiring speaker
- return onto your power supply ground and not at the amp board for some reason when attached on amp board it introduces
Noise and almost overdriven distortion in my opinion weird but true!!
I was almost ready to give up on this amp until I figured this out.
Scott
Last edited:
True! But as I noted in my post ... I never had the buzz in my initial build - so the "design" is basically fine.
It was only when I changed R141 & 142 on the 8R 'as built' schematic that I had a buzz problem. (Why this should've happened is beyond me!
)So maybe it's a problem which is specific to a particular build?
All I am suggesting is ... if you hear a buzz problem when you used the original spkr return connection to AN PCB - try connecting it to the SLB ground, before you give up on the amp.
Adason is correct; C124 is the feedback shunt cap and to protect it if the output swings permanently to a rail it should be a bipolar.
You can use a polarised cap of course as long as you ensure negative side is connected to ground.
Choice of brand here is entirely your choice; over the years the 'best' swaps around. I have no problem with the Panasonic, very good caps.
Andy,
Thank you; good catch on the earth loop!
Hugh
You can use a polarised cap of course as long as you ensure negative side is connected to ground.
Choice of brand here is entirely your choice; over the years the 'best' swaps around. I have no problem with the Panasonic, very good caps.
Andy,
Thank you; good catch on the earth loop!
Hugh