Hi Salas,
I've sorted out the gnd loop that goes via the volume and switch pcb by breaking the track on the sub-pcb in a couple of places. What hum is left is still there with that disconnected (and the inputs suitably loaded).
I will proceed with my track-breaking and get back to you...
Dan
I've sorted out the gnd loop that goes via the volume and switch pcb by breaking the track on the sub-pcb in a couple of places. What hum is left is still there with that disconnected (and the inputs suitably loaded).
I will proceed with my track-breaking and get back to you...
Dan
One other thing you might consider since this thing has a 220V power transformer is to use a small external transformer to buck the mains voltage. You have a 50% chance of phasing it correctly to buck the mains voltage the first time around, if it is boosted just reverse either (not both) the primary or secondary - whichever is easiest.
Mount the transformer and mains socket in a small metal box.
Ground the box through the mains cordset, and also connect mains socket ground connection to safety ground on your cordset. Install a 3A (or whatever the transformer is rated for) fuse in the fuse holder in the mains plug.
This or a variac will greatly reduce the losses in the amplifier's power transformer core which at ~10% high line probably have the transformer approaching saturation. Should you be in an area of the British Isles where mains voltages approach 250V this could be a real problem. (Harmonization indicates 230V, but I have heard that it can still be much higher in parts of the UK.)
Note that you can also use a transformer with taps and switch between them to get a closer match. (24VCT gives you both 12V and 24V bucking.)
If you choose a mechanically quiet transformer for the "buck box" you won't even know it is there, far quieter in a lot of instances than the typical variac, and if acquired ex stock, far cheaper too.
One other thought if the power transformer in this amp is approaching core saturation due to load and a high mains voltage it may be spewing harmonics in its magnetic field all over the amplifier. It may quiet down significantly..
See the schematic below:
Mount the transformer and mains socket in a small metal box.
Ground the box through the mains cordset, and also connect mains socket ground connection to safety ground on your cordset. Install a 3A (or whatever the transformer is rated for) fuse in the fuse holder in the mains plug.
This or a variac will greatly reduce the losses in the amplifier's power transformer core which at ~10% high line probably have the transformer approaching saturation. Should you be in an area of the British Isles where mains voltages approach 250V this could be a real problem. (Harmonization indicates 230V, but I have heard that it can still be much higher in parts of the UK.)
Note that you can also use a transformer with taps and switch between them to get a closer match. (24VCT gives you both 12V and 24V bucking.)
If you choose a mechanically quiet transformer for the "buck box" you won't even know it is there, far quieter in a lot of instances than the typical variac, and if acquired ex stock, far cheaper too.
One other thought if the power transformer in this amp is approaching core saturation due to load and a high mains voltage it may be spewing harmonics in its magnetic field all over the amplifier. It may quiet down significantly..
See the schematic below:
Attachments
Thanks for the suggestion Kevin. I have just picked up a variac in need of some repair and once done I can test on a lower voltage.
I've sorted the grounding as best I can now, and have certainly calmed it down a lot!
The attached pictures show the channels at startup (quite a loud hum) and then after the valves are operational. The relatively smooth wave you see is at idle is 50Hz.
Startup (noisy):
Warmed and at idle (still some audible hum):
I've also messed about with CRC and CLC filtering, but the choke I have is not up to the job full time (current handling too low) and is already too big to fit in the chassis!
Thanks,
Dan
I've sorted the grounding as best I can now, and have certainly calmed it down a lot!
The attached pictures show the channels at startup (quite a loud hum) and then after the valves are operational. The relatively smooth wave you see is at idle is 50Hz.
Startup (noisy):
An externally hosted image should be here but it was not working when we last tested it.
Warmed and at idle (still some audible hum):
An externally hosted image should be here but it was not working when we last tested it.
I've also messed about with CRC and CLC filtering, but the choke I have is not up to the job full time (current handling too low) and is already too big to fit in the chassis!
Thanks,
Dan
Very well done, ehmm Dan.🙂
What exactly did you do for the GND? How does the sorted out amp sound?
What exactly did you do for the GND? How does the sorted out amp sound?
All I really did was ground all the shields and the pcb in the same place. The main problem was the input selector and volume earthing. Once the chassis of those switches was disconnected and grounded separately from the input itself, things got a lot better.
There is still some noise, but I'm running out of patience (and ideas) with the thing to do much more.
As for sound, my garage is not quiet enough for a good test (freezer running in there etc.) but I will feedback when I have put it all back together and had a chance to plug it in somewhere quiet.
Thanks,
Dan
There is still some noise, but I'm running out of patience (and ideas) with the thing to do much more.
As for sound, my garage is not quiet enough for a good test (freezer running in there etc.) but I will feedback when I have put it all back together and had a chance to plug it in somewhere quiet.
Thanks,
Dan
A successful thread.
I think that you have done much, and this thread is a good guide for anyone with this amp. You sorted the heater voltage, the output stage bias, the bulk of the ground noise, and you upgraded the supply. Plus quality upgrade hints were given, and the circuit got known. With your soon to come performance description, I think that the original questions of the thread starter are covered, and more. A short, to the point, hands on thread. Very nice.
I think that you have done much, and this thread is a good guide for anyone with this amp. You sorted the heater voltage, the output stage bias, the bulk of the ground noise, and you upgraded the supply. Plus quality upgrade hints were given, and the circuit got known. With your soon to come performance description, I think that the original questions of the thread starter are covered, and more. A short, to the point, hands on thread. Very nice.
Check out this guy selling these in the UK! He is claiming the the MC-5881A is point to point wired. I've emailed him asking how he is dealing with the various safety issues they have on 240v.
Should be interesting:
http://yaqin.co.uk/mc5881a.htm
Dan
Should be interesting:
http://yaqin.co.uk/mc5881a.htm
Dan
Ha - ha! Right! Point to point, a super regulated PSU, and a hefty mark up to match! Ok Dan, you kick him first for lying through his teeth to the UK audio public and me next for they use Greek alphabet for their logo!😀
Sounds like a plan!
One other thing a friend of mine mentioned is that the transformers in this amp are all core-aligned. This could be the source of the 50Hz ripple on the output?
See their EL84 effort which has the mains tranny rotated:
Dan
One other thing a friend of mine mentioned is that the transformers in this amp are all core-aligned. This could be the source of the 50Hz ripple on the output?
See their EL84 effort which has the mains tranny rotated:
Dan
Yes it can give some hum when not in right angles. Do you still have audible hum with your speakers at listening position? Did you manage to listen critically at all?
Yes, it is still audible, more so on the more sensitive speakers I have in the living room (90db as opposed to 88). It's something like 30mv peak to peak. It would be a major cosmetic mod to rotate the mains transformer as it is not square and would not fit within the cover anymore. It must be induced mains as the basic sine is at 50Hz and supply ripple/ac heater noise or similar would be at 100Hz, no?
Dan
Dan
Supply ripple sure is 100Hz, but as I recall your heaters are AC fed, so still 50Hz for them. If we assume that your GND loops are all gone by now, it can be an electromagnetically induced residual that you pick up, or at least the best part of it.
I set this up last night in my daughter's room. It's quiet in there (when she's not around). I was surprised that actually the noise was mostly inaudible. I need to try it with other speakers, but overall the sound was enjoyable. It's actually got quite an involving feel and Eva Cassidy came over well at first listen. I'm not expert at describing sound quality, but I will arrange a listen with a friend of mine who is, so expect more to come.
If you replace those gray coupling caps with something better you will have a lot more to listen to.
One thing I didn't consider in lifting the ground between mains earth and the input is that other devices which are earthed reintroduce the loop. My original testing was with a battery powered cd player - no earth. It also works fine with my marantz cd player - again no earth as it is 2 pin only to mains.
How best do you think I can earth the input so that connecting other devices with three pin plugs don't cause a massive hum?
Thanks,
Dan
How best do you think I can earth the input so that connecting other devices with three pin plugs don't cause a massive hum?
Thanks,
Dan
Just got this amp from ebay. It runs hot like crazy - any chance of how to do the resistor thingy for someone who has ZERO electrical/electronics experience...?
I would advise you to print out page 1 of this thread with my instructions for lessening the heater voltage and dropping the bias, and off you go to your nearest TV repair guy or some other technically adept fellow. Its very easy for someone who has put a couple of circuits together before. You STAY away! Lethal voltages inside, even when not plugged to the mains.