Dx Blame MKIII-Hx - Builder's thread

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space2000

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Joined 2008
Hi,

I am using veroBoard. I decided to add few more transistors at output to test. Now sounding much better within very low volume, I can hear carlos high freq sound.

I think all problems are related to bias setup and also Oscillations. Now i can't set the bias as like before at 1mv. I wanted to set bias but it only can start from 22mv across R39, I went to check how to set bias, carlos deleted the video.

Anyone pls let me know how to setup Bias properly. I forgotten many things in 3 years. I made my Dx hariri on 2008 also used same transformer to test. It was nice working amp and lost it when i shifted my house.

Here is the pic of my amp. you can see the voltage is showing (-) sign. The mulimeter was connected to output and GND, it should not show (-) volt.
amp.jpg


amp2.jpg


Regards
Michael
 
C

currentflow

Member
Joined 2008
space2000 said:
Hi,

I am using veroBoard. I decided to add few more transistors at output to test. Now sounding much better within very low volume, I can hear carlos high freq sound.

I think all problems are related to bias setup and also Oscillations. Now i can't set the bias as like before at 1mv. I wanted to set bias but it only can start from 22mv across R39, I went to check how to set bias, carlos deleted the video.

Anyone pls let me know how to setup Bias properly. I forgotten many things in 3 years. I made my Dx hariri on 2008 also used same transformer to test. It was nice working amp and lost it when i shifted my house.

Here is the pic of my amp. you can see the voltage is showing (-) sign. The mulimeter was connected to output and GND, it should not show (-) volt.

Regards
Michael
Click to expand...

Hi,

Thanks for the photos. The insulating washers under the transistors on the heatsink are not visible. How did you insulate these? Also try connecting the heatsink to the 0V point.

The output stage bias transistor, which has R25 connected to its base, has to be mounted on the same heatsink as the output transistors as it forms part of the thermal feedback loop.

The DC output offset you have measured may be positive or negative. Minus 29 mV is acceptable.

Post #200 in this thread shows the method of setting the bias current correctly.

Is the veroboard the strip type or plain type? If you've used the strip type, instability can be caused by capacitive / inductive coupling between adjacent strips.

An oscilloscope would be an advantage in locating the source of instability.

Regards,
currentflow
 
S

space2000

Member
Joined 2008
Thanks. I finally found the problem. it was output resister that i had wrong value 470 ohm. I checked the old forum list and found that it should be 0.47, I jsut changed to 0.15 because i don't have any 0.47 ohm. Now its working fine. Bias not yet set cause no proper value on resister.

I want to confirm it again, it should be 0.47 ohm right?

Thanks again for your help.

Regards
Michael
 
C

currentflow

Member
Joined 2008
space2000 said:
Thanks. I finally found the problem. it was output resister that i had wrong value 470 ohm. I checked the old forum list and found that it should be 0.47, I jsut changed to 0.15 because i don't have any 0.47 ohm. Now its working fine. Bias not yet set cause no proper value on resister.

I want to confirm it again, it should be 0.47 ohm right?

Thanks again for your help.

Regards
Michael
Click to expand...

Hi,

I'm pleased to hear you've found the problem. The output transistors' emitter resistors, should be 0R47 (= 0.47 ohm), this being the specified value.

Don't forget to move the bias transistor (C16 connects across it) from the Veroboard to the heatsink as it needs to monitor the temperature of the output transistors. If this is not done, there is a very real possibility that the output transistors can continue to overheat and eventually suffer damage. The purpose of the bias transistor is to reduce the bias current in the output transistors should they exhibit an increase in temperature. This forms a thermal feedback system that regulates the amplifier's quiescent current.

Regards,
currentflow
 
S

space2000

Member
Joined 2008
Last edited:
lanchile

lanchile

Member
Joined 2008
space2000 said:
I am confused about R22 check this Diagram (http://www.diyaudio.com/forums/atta...read-dx-blame-mhiii-hx_numbered-schematic.pdf)

is this 2.7oh or 4.7oh? DX list shown 2.7ohm.

I also have one problem. when i power on the amp and no input connection, the output make sound (toooooo) like this. when i connect with Audio input, there is no sound and its ok.
What could be this?

Regards
Michael
Click to expand...

Well, R22 shows 4.7ohms. and the (toooo) noise can be a "ground" problem.
 
C

canonnica

Member
Joined 2011
space2000 said:
I am confused about R22 check this Diagram (http://www.diyaudio.com/forums/atta...read-dx-blame-mhiii-hx_numbered-schematic.pdf)

is this 2.7oh or 4.7oh? DX list shown 2.7ohm.

I also have one problem. when i power on the amp and no input connection, the output make sound (toooooo) like this. when i connect with Audio input, there is no sound and its ok.
What could be this?

Regards
Michael
Click to expand...

Don't be fooled, no official list or "BOM" has been posted. Everyone builds its own from the schematics.
 
BMW850

BMW850

Member
Joined 2004
youtube.com/results?search_query=grevingonline
I've changed 390R resistance to a 2.2 K resistor.
Now regulates my Fettle Preamp better, it was 4 steps on listening quality with the 41-step series attenuator of Acoustic Dimension.
Now it take 10 steps, much nicer listening with the with the 41-step series attenuator.

Have now ordered 2 pieces Charcroft Z-Foil Resistors 2K2 0.4 W.
For replacement of the 2.2 K TAKMAN Audio Metal Film Resistors.
Wonder what the differences will be.

I have told you all before:
Also I have the bridged with a wire (Silver/Gold) the 1uF capacitor and this sounds much better, sounding instruments now much looser/opener.
I will replace these 1 uF (MKT1813 1, 0uF Roederstein 100V) one more time and replaced by a 1uF Mundorf silver/gold/oil I still have lying around here.

I hereby wish like some help and suggestions from fellow forum members what a good capacitor is for this place in the signal path.
The rule is NO capacitor "sounds better" than a capacitor.

I ask this because I want to listen one more time to the passive Inductive Volume Control of TRIBUTE Audio again with “the best cap” on this place.

NOTE:
The bridged wire (Silver/Gold) with the 1uF, can only if you used a preamplifier and no passive volume control.
Then you have the chance you'll hear steps of the passive volume control.
Then you hear a bit of DC voltage with your passive volume control.
Most Preamps (99%) have an output capacitor that the DC stop.

You will be thinking how these Dx Blame MKIII-Hx sounds?
I do first some modifications to this amplifier, changed with some salt and pepper but I can say this amplifier sound good ... ;)

Regards,
Rudy
 

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