Dx Blame MKIII-Hx - Builder's thread

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OK, I found one other thing. With everything connected the amp is dead quiet, but with nothing connected to the input I had a very bad hum. I found by grounding the case of the either one of the RCA's the hum would disappear. So I'm not sure it I handled it in the best manner but what I did was solder another 10R 1W resistor between one of the RCA grounds and the shield which is attached to the case. Hum is completely gone now and I can't hear any problem in the sound. It may be that something would work better than a resistor for that job and I am open to suggestions.

Thanks, Terry
 
Hi Guys,
As for the shared lifted ground, my thinking was to make the "lifting" point at the RCA jacks by inserting the 10R 1W resistors there and just installed a jumper at R30(31). That way C11 & 15 are no longer lifted. They just go directly to ground via the jumper. Maybe that is wrong thinking but it worked.

Hi Terry-
Gotcha on the use of the jumper and moving the lifting point. That's a nice simple solution. I'm a little confused about the other mod you described. Are you saying that the star point was connected directly to the case and that you insulated it to eliminate the hum? Did you also change it's position relative to the filter caps?
 
Hi Bonifis,

If you look at the pic below you will see a bolt in the center of the ground plate. That bolt extends to the bottom of the case and served as a hold-down bolt for the filter caps as well as my star ground. What I did to isolate the star ground was to drill out the hole in the plate and insulate it so it no longer made contact with the bolt. Then I moved the attachment of the center tap and star to either ends of the ground plate. I also added 0R15 5W resistors between the filter caps on each rail to change it to CRC. That lowered the ripple to 24mv. I'm glad I built this amp. It is a slightly more dynamic sounding than the Honey Badger. Very detailed and powerful. Nice amp.

Blessings, Terry
 

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Terry, that is a very clever idea: using a single bolt to form the star ground and secure the cap bank to the chassis. As I mentioned before, I prefer to form two ground points on the ground bus, and earth the ground bus to the chassis/mains earth via a loop breaker. Nonetheless I can appreciate the simplicity of what you've done.

For my next build I will tie down the caps using a nylon bolt with two small bolts at each end of the bus for the actual star grounds. I generally earth the loop breaker to the chassis, near the mains inlet, using the same bolt that attaches the mains earth lug.
 
On my side, i always had a 120HZ buzz that I've never tackled. Now that the cold season is back, I may try to figure it out.

This buzz is present with nothing connected to the amp and the input shorted. So it points to internal stray magnetic field. I know that my PSU wiring is far from perfect...
 
Hi LG,

I like it a lot. It has very good clarity and that kind of athourity that is tough to get without high power. I want to put in a speaker protection circuit mainly because it has a nasty groan after shutoff as to voltage winds down. I will probably put in a soft start too although I haven't needed it so far.
 
I think soft starts are a good idea for any torrid transformer over 300VA,
When working on my last amp with a 45ac-0-45ac 500VA transformer with 30000uF per rail, I could actually see the wire element in the supplies glass fuses buckle at switch on, this can't be good for there life span,
Regards
 
Attached is relative "blameless" grounding concept for unbalanced stereo amp sharing one common PSU for both channels. R1, R4 should be the same value, R2, R3 too (CMMR..). Other channel identically.
 

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Hi LG,

I like it a lot. It has very good clarity and that kind of authority that is tough to get without high power. I want to put in a speaker protection circuit mainly because it has a nasty groan after shutoff as to voltage winds down. I will probably put in a soft start too although I haven't needed it so far.

Thanks for the response. My DX Corp Amp (DX HR II) is fantastic, no problems, ever. Looks good and sounds great but as you say sounds a lot louder with more authority than it should. I like to encourage people to try Carlos' designs but the support and documentation is not the greatest (depending on the project). Anyway, congrats!
 
I wonder how many of you used speaker protection circuit for that project?!?

I did, but it creates a problem to my... let me explain.

I have an odd 120HZ buzzing. I just found that I have a ground loop that circles around my rear power transfo.

My amp is built as a dual mono, meaning everything is doubled and completely isolated... except for the star ground. and that'S the gotcha. My line inputs goes one to the far left and the other to the far right. I have a grounding post that is shared for both channels. It's located between the two power transfos. See on the picture that nicely done ground loop.

Of course I need to break open that loop. The thing is I cannot totally separate the two grounds since I have a shared speaker protection circuit that needs to be grounded to the channel it protects, ie both...

So I was wondering if I would prefer to have two speaker protection circuits, or simply get rid of the one that's there...

Here is a pic of the inside of the amp... of course feel free to suggest wiring improvements. I don't plan to redo the whole interior nor to replace the chassis but I obviously lack comme knowledge to properly route the wires.

Regards,
Martin.
 

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I wonder how many of you used speaker protection circuit for that project?!?

I did, but it creates a problem to my... let me explain.

I have an odd 120HZ buzzing. I just found that I have a ground loop that circles around my rear power transfo.

My amp is built as a dual mono, meaning everything is doubled and completely isolated... except for the star ground. and that'S the gotcha. My line inputs goes one to the far left and the other to the far right. I have a grounding post that is shared for both channels. It's located between the two power transfos. See on the picture that nicely done ground loop.

Of course I need to break open that loop. The thing is I cannot totally separate the two grounds since I have a shared speaker protection circuit that needs to be grounded to the channel it protects, ie both...

So I was wondering if I would prefer to have two speaker protection circuits, or simply get rid of the one that's there...

Here is a pic of the inside of the amp... of course feel free to suggest wiring improvements. I don't plan to redo the whole interior nor to replace the chassis but I obviously lack comme knowledge to properly route the wires.

Regards,
Martin.


Supply wires are not twisted, the loop is between it. Try to turn phase on one transformer also, maybe buzzing decrease.
 
I wonder how many of you used speaker protection circuit for that project?!?

I did, but it creates a problem to my... let me explain.

I have an odd 120HZ buzzing. I just found that I have a ground loop that circles around my rear power transfo.

My amp is built as a dual mono, meaning everything is doubled and completely isolated... except for the star ground. and that'S the gotcha. My line inputs goes one to the far left and the other to the far right. I have a grounding post that is shared for both channels. It's located between the two power transfos. See on the picture that nicely done ground loop.

Of course I need to break open that loop. The thing is I cannot totally separate the two grounds since I have a shared speaker protection circuit that needs to be grounded to the channel it protects, ie both...

So I was wondering if I would prefer to have two speaker protection circuits, or simply get rid of the one that's there...

Here is a pic of the inside of the amp... of course feel free to suggest wiring improvements. I don't plan to redo the whole interior nor to replace the chassis but I obviously lack comme knowledge to properly route the wires.

Regards,
Martin.
That's the problem with every dual or multi-channel power amplifier that is connected to a source with a common signal ground/return.
It has nothing to do with the speaker protection relays or relay driver circuit.

You have to reduce the currents in the power amplifier loops to such a level that the interference voltage becomes low enough not to be a problem.
This reduction of loop currents, generally requires extra resistors in the loops.
Reducing the imposed voltages generally comes from using low resistance return cabling.


BTW buzzing instead of hum usually indicates an amplifier ground connected to the PSU zero volts. It is the high frequency of the spiky charging currents that are being imposed on the audio ground reference.
 
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" BTW buzzing instead of hum usually indicates an amplifier ground connected to the PSU zero volts. It is the high frequency of the spiky charging currents that are being imposed on the audio ground reference."

+1 on a previous amp of mine, I had zzz sound on my speaker, the problem for me was that I had my main smoothing caps on my amp pcb which connected straight to the star ground point on my pcb.

I took out the 6800uF caps and substituted 1000uF local on pcb, and moved the 6800uF caps off board.

Regards
 
The buzzing is definitely picked-up by the audio cable that's running from the RCA connectors to the board's input lugs.

With everything connected, if I short the input at the RCA connector's end, it buzzes.

If I disconect the board input cable and short the input at the board, it becomes dead silent.

I do use the lifted ground resistor on the board.

I will try to find a screened two cores wire for the input just like S4G did and ground the screen to the chassis at the RCA connector end. I will put a 10R resistor between the audio ground and the chassis ground (screen) at the RCA connector's end too.

The screened wire should help isolate from noise pickup, and the extra resistor at the RCA end should help send the picked-up noise back to the ground.

I will post results. :)

Martin.
 
Hi Martin,

Did you get a chance to address some of this? I still have a very slight hum. It is very low and not noticeable when listening to music but it is still bugging me that it is there at all. None of my other amps do this. I tried attaching a resistor from star to case and it makes the hum worse. I have been working on other amps but I want to get back to this soon. I like the sound of this amp a lot so I really want to get to the bottom of this problem.

Thanks, Terry
 
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