Dx Blame MKIII-Hx - Builder's thread

[BMW850]

@ Junie,
This amplifier you have built is beautiful, congratulations.

@ harmonicconverger
Sorry I cannot help you, but I find this amplifier earned a decent power supply.
If I look at the photo of the transformer, this transformer has had his best years.

Regards,
Rudy

[a.wayne]

Neat work Junie ..... what size transformer (s) ...?

[space2000]

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

[currentflow]

Originally Posted by space2000

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

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

[space2000]

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

[Junie]

Originally Posted by space2000

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

Thanks again for your help.

Regards
Michael

Yes it's 0.47ohm

[currentflow]

Originally Posted by space2000

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

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

[space2000]

I am confused about R22 check this Diagram (http://www.diyaudio.com/forums/attac...-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

[lanchile]

Originally Posted by space2000

I am confused about R22 check this Diagram (http://www.diyaudio.com/forums/attac...-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

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

[canonnica]

Originally Posted by space2000

I am confused about R22 check this Diagram (http://www.diyaudio.com/forums/attac...-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

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