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Dx Blame MKIII-Hx  - Builder's thread
Dx Blame MKIII-Hx  - Builder's thread
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Old 7th October 2013, 02:10 PM   #1901
ostripper is offline ostripper  United States
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Default Nice amp - a few tips ...

I'm glad you gave credit to the great Dr. Self. His topology can create great sound.

I read the WHOLE thread...
I have recently run into 2 of your builder's problems , #1. sticking #2. oscillation
either after sticking or with very high gain devices.

#1 - you have solved that just a reverse diode across the miller (called a "baker clamp")
Choose your diode carefully , one with high reverse leakage can increase distortion considerably.

#2.- The dreaded oscillation - you chose wisely.. 22R for the MJE-xx and 2.2R for the
outputs are perfect for these devices.
Increasing the value of the stoppers is at best just a "band-aid" and won't solve
the problem.
You must learn the ways of "Gain Margin and Phase Margin - measures of stability"
With one builders (still4given) oscillation , I could easily recreate it after "sticking" ...
the simulator recreated his problem EXACTLY as his scope showed.
Another builder used ultra high gain input transistors and I also recreated an
oscillating amp using (models of) his transistors.

(Attachment 1) is a stable amp , even after sticking just a very short burst
as the VAS transister "recovers". Notice it has -90 degrees phase.

(attachment 2) is the amp that really did oscillate in the "REAL world" .

In the second attachment the phase is -127 degrees - bad... 180-127 = 53 degrees "phase margin" .
78 degrees or more is where an amp becomes "rock solid" stable.
I increased that margin by increasing the emitter resistors on the input pair.
You can also increase this margin with a higher value VAS emitter resister or
increasing the current mirror's emitter resistor values.

Have you run a loop gain analysis of this amp ??

PS - These are not criticisms of your amp's design , it's design is standard
"blameless" and can be optimized. I want it (your amp) and it builders to
be .

Attached Images
File Type: jpg stable.jpg (56.2 KB, 327 views)
File Type: jpg unstable.jpg (59.6 KB, 323 views)
Wanderer - my travels.... Facebook - davyjones lockre -

Last edited by ostripper; 7th October 2013 at 02:13 PM.
Old 7th October 2013, 03:52 PM   #1902
a.wayne is offline a.wayne  United States
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Thanks for the input OS , I'm sure the mk3, will benefit greatly from simulations and mods ...

Old 7th October 2013, 04:12 PM   #1903
smartx21 is offline smartx21  Brazil
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Send a message via MSN to smartx21 Dx Blame MKIII-Hx  - Builder's thread
TG this thread is running global!
Old 7th October 2013, 06:30 PM   #1904
destroyer X is offline destroyer X  Brazil
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Default Despite nice the intention to cooperate

I would like to sign to forum folks that Andrew T and also Ostripper are not Dx consultants, nor representatives, so, their opinions are their opinions.

This amplifier is an evolution of so many other models, not anymore a copy from Doctor Self Blameless.... was inspired into the Blameless, but you can see it has some differences.

Friends will call these differences as clever evolution.... less friends will call this flaws.

Making some examination from the past i would be afraid to say these gentlemen, studying historical moments of this forum, do not seem to be all that friendly.

I do not use to read them, as often i could find not nice gestures from them.... they are in the ignore list, they post and i do not read them.... well, some of them have their own amplifier to worry about... some are modified, other are genuine copies.

Several group buys already runned, several foruns offered as group buy wide world.. something alike 280 guys assembled using the pcboards we have offered, others built etching the unit by themselves.... 3 guys reported troubles when clipping ....you see, this is a very little ammount of troubles people faced if you calculate the percent.... and many times is due to their part selection or failures when building.

Mr Smartx21, a very competent builder, a high end audiophile from Brazil, is also not a close friend...we had our troubles in the past.... when have criticisms, they use to come in group to "help".... all ignored land in group to help

You all know this kind of helping hand when the someone report problems.

Our forum has more than 200 thousand members...i have some members in my ignore list..try to imagine why?... well...it is a long story.... all them skilled, competent and nice guys...but not my friends....this was proved many times before.


Attached Images
File Type: jpg Untitled.jpg (48.2 KB, 292 views)
Restriction removed ; ttps://www.youtube.com/watch?v=3ioyOjAzDt0; https://www.youtube.com/watch?v=uFcnddpe3MU

Last edited by destroyer X; 7th October 2013 at 06:50 PM.
Old 7th October 2013, 07:18 PM   #1905
a.wayne is offline a.wayne  United States
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opps .....
Old 7th October 2013, 07:44 PM   #1906
BMW850 is offline BMW850  Netherlands
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I find it weird that there every time on positive comments and modifications, such comment comes.
It's not what you know, but who you know "and where they are"

Itís not what you know, but who you know ďand where they areĒ
Old 7th October 2013, 08:32 PM   #1907
still4given is online now still4given  United States
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Dx Blame MKIII-Hx  - Builder's thread
I have big troubles now but will seek help off line. I will post back after I have it resolved.

Blessings, Terry
Old 7th October 2013, 09:24 PM   #1908
destroyer X is offline destroyer X  Brazil
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Default Thanks dear Terry, much appreciated your comprehension

You see what happens when someone report problems.

I will reach 10 long years in this forum...i know forum people very well..their behavior..what they want..what they need and so on.

Much appreciated Terry.


Restriction removed ; ttps://www.youtube.com/watch?v=3ioyOjAzDt0; https://www.youtube.com/watch?v=uFcnddpe3MU
Old 13th November 2013, 05:24 PM   #1909
still4given is online now still4given  United States
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Dx Blame MKIII-Hx  - Builder's thread
Hi Guys,

I have finished building the boards I received in the newest group buy. I have a hum problem that we have been trying to resolve in the newly created builders thread but that has brought up some questions and since some or you who have built these may be following this thread and not that one I would like to bring your attention to it so that maybe you could chime in. The issue is that I get a bad hum as soon as I connect both inputs to a source. I tried several different sources and all do the same thing. so then I tested it with shorted inputs and as soon as I short the grounds of the input together I get the hum. A fellow member here, while trying to help noticed this.
"If posted pictures (schematic and PCB) are valid, than here are following faults:
To input signal ground should be connected only R01, C04, C06, C07 and one end of R31, 10R. To this point also should be connected input RCA ground , but input conector must be isolated from chasis. And nothing more!
To input signal ground can not be connected C15a, C15b,C11b C11a , it is AC short circuit from suplly voltage on signal ground (AC short circuit for 10R), via PSU main capacitors it is AC shorted to power GND! Also reconnect C01, R17, R08b to power ground.
In current state R31, 10R, have no function.."
That in is reply to the schematic and boards shown in this post.

After looking over the schematic I see that on these boards the grounds for C15a,b and C11a,b are all tied to the same trace as the input ground. Could this be causing the ground loop? Have any of you experienced the same thing I am?

Thanks, Terry
Old 13th November 2013, 05:51 PM   #1910
AndrewT is offline AndrewT  Scotland
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The Signal Ground is on the other side of R31.
R31 reduces the current flowing around the loop.
regards Andrew T.

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