Hi Terry
I'm still following this thread and happy to answer any questions. My MkIII is currently dismantled because I'm using the PSU components for another project I'm working on - so can't really show you a photo unfortunately.
Power supply is very basic consisting of a single transformer, two bridge rectifiers (one per channels) and two pairs of filter banks, again one per channel. I don't use any snubbers, bypass caps, bleed resistors, or anything of the sort. Total off-board capacitance = 16 x 4,700uF. 1000uF caps are installed onboard.
Inputs are wired directly to the board using microphone cable. The shield is affixed to the chassis near the RCA jacks, the other end is floating. I don't use any of the floating earth mods described in this thread.
I have two pairs of star grounds (one set per channel) formed using the copper bus bars of the main filter caps. The E1 grounds are tied together using heavy copper wire, which has the effect of connect both cap bank grounds where dirty currents circulate. The "centre tap" is taken at the centre of this copper wire. An earth ground wire is taken from this same point and connected through an earth loop breaker, and then attached to the chassis, at the same same point where the safety earth lug is also attached.
All wiring pairs are tightly twisted. Cable lengths are minimised, to the extent possible, while still maintaining good physical separation between low level signal, high level signal and high current power lines.
I can hear absolutely no 100hz hum. There is a faint hissing sound audible when the inputs are connected and no music playing. It is not the source because there is no hiss when using the same source with my P3A. I re-arranged the wiring to eliminate the hiss but was unable to hear any improvement. It is unobtrusive and doesn't detract from musical performance.
Its a wonderful amp and I hope you can sort out the last niggling issues.
I'm still following this thread and happy to answer any questions. My MkIII is currently dismantled because I'm using the PSU components for another project I'm working on - so can't really show you a photo unfortunately.
Power supply is very basic consisting of a single transformer, two bridge rectifiers (one per channels) and two pairs of filter banks, again one per channel. I don't use any snubbers, bypass caps, bleed resistors, or anything of the sort. Total off-board capacitance = 16 x 4,700uF. 1000uF caps are installed onboard.
Inputs are wired directly to the board using microphone cable. The shield is affixed to the chassis near the RCA jacks, the other end is floating. I don't use any of the floating earth mods described in this thread.
I have two pairs of star grounds (one set per channel) formed using the copper bus bars of the main filter caps. The E1 grounds are tied together using heavy copper wire, which has the effect of connect both cap bank grounds where dirty currents circulate. The "centre tap" is taken at the centre of this copper wire. An earth ground wire is taken from this same point and connected through an earth loop breaker, and then attached to the chassis, at the same same point where the safety earth lug is also attached.
All wiring pairs are tightly twisted. Cable lengths are minimised, to the extent possible, while still maintaining good physical separation between low level signal, high level signal and high current power lines.
I can hear absolutely no 100hz hum. There is a faint hissing sound audible when the inputs are connected and no music playing. It is not the source because there is no hiss when using the same source with my P3A. I re-arranged the wiring to eliminate the hiss but was unable to hear any improvement. It is unobtrusive and doesn't detract from musical performance.
Its a wonderful amp and I hope you can sort out the last niggling issues.
Last edited:
Yes I did. I also tried taking a test wire, first with a 10R resistor attached to one end and went across all ground points I could reach and when that didn't work I tried it without the resistor and had the same results. The hum only happens when something is plugged into the input.
Last edited:
Terry, this is another way to break a loop. Simply don't create one in the first place.
I really don't think it is a loop. The hum is there if using only one channel. It is like the amp is super sensitive to the input source. The main difference I notice about Mine and Ranchu's amps is he uses two bridges and I use one. I can try adding a bridge and splitting up the caps but I have used one bridge on almost every amp I've built and this is the first time I have had this issue.
Hi Terry-
Actually the MK III is "super sensitive." The amps gain is significantly higher than many power amps. It was designed to be driven to full output by an mp3 player. I reduced the gain of mine since I drive the amp with a preamp. That could account for the hum you hear if there is some hum coming from your source and the amps you are comparing it to have less gain.
Also I don't think you can rule out a ground loop just because you hear hum with only one channel connected. It's possible to have an input ground loop in a single channel amp.
Terry i would lower the gain , Imo this is a must, were you able to chk your supply noise , after lowering the gain if the hum is still there , take your wire and touching the amp chassis, touch the gnd on the source chassis ....
If you still have .....
Hi Guys,
I probably didn't say that right. When I say I don't think it is a loop, I meant one caused by connecting both sides of the source. Obviously I don't completely understand ground loops or we probably wouldn't be having this conversation. I am not presently driving the amp with a preamp. I am using a CD player directly into a 50K pot and into the amp. None of the other amps I have are displaying this hum so I believe the source is not the problem. As for gain, I believe the DX Super A is higher gain as it is louder that the MKIII with the same input signal and the MKIII is +-80V rails and the Super A is +-37V. The Super A is dead quiet.
How do I lower the gain?
Thanks, Terry
I probably didn't say that right. When I say I don't think it is a loop, I meant one caused by connecting both sides of the source. Obviously I don't completely understand ground loops or we probably wouldn't be having this conversation. I am not presently driving the amp with a preamp. I am using a CD player directly into a 50K pot and into the amp. None of the other amps I have are displaying this hum so I believe the source is not the problem. As for gain, I believe the DX Super A is higher gain as it is louder that the MKIII with the same input signal and the MKIII is +-80V rails and the Super A is +-37V. The Super A is dead quiet.
How do I lower the gain?
Thanks, Terry
The change Rudy shows reduces voltage gain from 120 to 21. Gains of 20-30 are typical for most power amps.
I also use a 1.5K for gain control. It gives a 30dB gain. The initial gain of 121x with the 390R (42dB) is OK to avoid the need of a preamp with low-powered sources (MP3 players). It has the drawback to amplify every little stray interference... big time.
Hi Terry,
Yes I hear a slight hum with my ear close to the speakers because there is no closet around it, and my audio entrance is near the transformer.
Look here, have many photos posted: http://www.htforum.nl/yabbse/index.php?topic=120317.0
Regards,
Rudy
Last edited:
Terry, did you try modifying the lifted ground traces? Btw, Make sure you use the correct PCB plan (i've joined the correct one)
Question to all: what are the components that should be part of the lifted ground? I've joined a picture that shows what actually uses the lifted ground. It's... it seems wrong to me.
Regards,
Martin.
Attachments
R17 should not be part of the lifted ground circuit but should go directly to the star ground. The same is true for R8b and R15 which are currently part of the lifted ground.
Steve
Hi Martin,
Yes, actually I just did it yesterday. I made two trace cuts and added two jumpers. I didn't include R17 in the lifted ground. I can include that if necessary. I also added a ground loop lifter at the earth ground. It is better but there is still a slight hum when something is plugged into to input. The left channel is worse than the right. I am attaching a pic of where I made the cuts and added the jumper. I installed the 1w/10R resistor at R30. I left the resistors that I have at the RCAs. I may try jumping those to see how it affects the hum.
Blessings, Terry
Yes, actually I just did it yesterday. I made two trace cuts and added two jumpers. I didn't include R17 in the lifted ground. I can include that if necessary. I also added a ground loop lifter at the earth ground. It is better but there is still a slight hum when something is plugged into to input. The left channel is worse than the right. I am attaching a pic of where I made the cuts and added the jumper. I installed the 1w/10R resistor at R30. I left the resistors that I have at the RCAs. I may try jumping those to see how it affects the hum.
Blessings, Terry
Attachments
Hi Terry-
You're right about R15 as it appears on the schematic. I was looking at the board layout as posted by Byron and there seems to be a connection from the left side of R15 to a trace going left across the board to the lifted ground. I haven't confirmed this on the actual board so maybe the posted image is wrong or I'm not seeing it correctly. Since you have your amp apart perhaps you can confirm that R15 is properly connected to the negative rail and Q7E.
Steve
- Status
- Not open for further replies.