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Old 14th February 2012, 08:59 AM   #11
AndrewT is offline AndrewT  Scotland
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If you change the bias current, then the Vbe will also change.
regards Andrew T.
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Old 16th February 2012, 09:47 PM   #12
DIYGEEK is offline DIYGEEK  United States
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I understand that, what I don’t understand and can’t seem to find is the recommended bias setting voltage or current for this particular amplifier, checked the Blame site, Youtube and this forum. I found that the MKIII should have 1mv bias voltage but I’m wondering if somebody knows this Blame supercharged amp or not.

OR is their some rule of thumb on adjusting bias of an amp… JAYCEE said “You should see a little more than twice this being drawn from the supply when the amp has no input.”

OK so if bias is 30mA then draw should be 60-80 mA but what is the starting bias current of the gain? Adjusting gain will vary the supply current as it should.

So here is a funny story and maybe a lesson too, I needed thermal past for the transistors and ran to the local box electronics store… purchased thermo past designated for computers and lathered up all to the transistors and insulators then mounted them tight. Well for those who know the moral of this story
CPU past is made with silver and is HIGHLY conductive… luckily I checked resistance prior to applying power… I had to replace the insulators and clean each transistor with alcohol to remove that awful sticky stuff… What a mistake…

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Old 17th February 2012, 11:35 AM   #13
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Think of the bias current as the idle speed of an engine. The correct way to measure bias current is to measure the voltage drop over one of the output emitter resistors. This will show you the current flowing through that transistor by V=IR. As there are two pairs of output transistors, your total bias current will be twice that.

The adjustment and measurement should be made with no output load, and with the input connected to ground.

The point of bias current is to avoid crossover distortion - that is, the point around which one pair of transistors turn off, and the others on the opposite rail turn on. How much bias current to use is somewhat subjective. I personally use 30mA.

I suspect you still have oscillation issues. Debugging those requires an oscilloscope. A quick indicator of oscillation though is that the Zobel resistor, R28, will get hot.
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Old 17th February 2012, 11:10 PM   #14
DIYGEEK is offline DIYGEEK  United States
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Thanks Jaycee

I do have a scope, digital dual channel 100mhz... so I'll take a look and see if their is anything strange... Actually this amp is working pretty good right now, its not overheating after an hour of use but I wanted to be certain of the ACTUAL bias current setting (if their is an actual calculated setting according to the designer)... I saw quite a few people setting OTHER amps to 30mA (For now I'll take that as a rule of thumb should the designer or schematic not call it out)...

Your catch on the incorrect bias transistor has seemed to solve the overheating problem

I’ve been through electronics but this advice is great bringing me back into the game. I wish to build several amps but I need to know everything on how this blame circuit works first. I’m reading Self’s book (or least trying to)

You have taught me Bulb tester, gain current and voltage measurements, idle current, oscillations. Thanks, that’s a great start for me.

NEXT I need to measure offset voltage, understand WHERE to take this measurement, what I am looking for, and HOW to adjust if it’s incorrect.

If anyone cares to have dialog on this issue I’m all ears…

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Old 17th February 2012, 11:41 PM   #15
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Offset voltage is basically the voltage on the output, when the input is zero volts. You will have a little bit of this. In practice, as long as it's under 200mV it's nothing to worry about.

Numerous things influence the offset voltage. Commonly, it is caused by slight mismatches in the devices used in the input stage (Q1 and Q2 in the DX Blame). Some amps do contain adjustments to eliminate it - some use a servo stage with an opamp as an integrator to remove it. Other amps just assume the offset is negligible. On the DX Blame, there's no adjustment for it.

So if you want to measure it, short the input to ground, and use a multimeter on DC millivolts to measure the speaker output. As long as it's below 200mV it's ok. If it's over that, you may want to try matching the hFE of Q1 and Q2.
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Old 19th February 2012, 02:43 AM   #16
DIYGEEK is offline DIYGEEK  United States
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That sounds pretty easy. I thought offset was the difference between the starting points of the positive cycle compared to the negative. I thought you had to measure it in the small signal section then adjust one side of the NPN or PNP side of the transistors… Kind of hard to explain here without paper…

I sat down with the amp today and finally gave it a proper sound audio test… This thing works great… sounds wonderful and is producing very small amounts of heat; I ran it wide open for 5 hours and no problems. Other builders may be interested to know I am using MJE3055T & MJE2955T for Q9 and Q10 then a MJE340 for bias transistor Q8. And the final output transistors Q11-Q14 are 2sc5200 & 2sa1943… I blew out Q9 & Q10 with a slip of my meter probe and had to use what I have on hand… Still works great and sonically sounds pretty good.

I’m thinking after a bit more of reading I will build another one… This time I will use the laser toner transfer method for making the PC board instead of laying it out by hand and I will make two at a time, left and right. Before printing the boards I wish to Photoshop the design and include one more NPN and PNP output transistor on each side. I BELIEVE that I can do this without modifying any other value or part on the board. Just make the board a little wider, move the pos and neg traces out to the sides and insert another transistor and resistors R23 & R26…. I will use MJL4281A & MJL4302A and everything else like the original schematic. All amps I build need to be PA rated so for overkill I would like to include another set of transistors.

Also I couldn’t resist and opened up one of the transistors 2sc5200 to look at the die size… its pretty much junk 1.5mm x 1.5mm die… I knew these transistors were not Toshiba but I figured it was a third party COPY with similar or same specifications… sadly its not. Its working great right now but the next amp will have new transistors for sure.

Attached Images
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File Type: jpg IMG_0706t.jpg (595.4 KB, 190 views)
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Old 19th February 2012, 07:35 AM   #17
DIYGEEK is offline DIYGEEK  United States
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I was thinking of something like this… I would call it DX Blame MKII Super Duper Supercharged amp… 1 extra set of output transistors…. I CAN DO THIS RIGHT?

The final layout will come once I have all the parts on hand so I can tweak the layout should a capacitor or fuse socket need just a bit more space.

I would power it with 45 volts (61 after regulation) 300VA transformer per side and make all the capacitors rated at 100 volts (80 volts for the storage caps 60,000uF total each amp)

Design goal would be 150 Watts output at 8 ohms for use in PA application of top high and mid speakers.

What do you think?
Attached Images
File Type: jpg dx-blame-Super Duper Supercharged A.jpg (284.5 KB, 180 views)
File Type: jpg dx-blame-Super Duper Supercharged B.jpg (320.1 KB, 174 views)

Last edited by DIYGEEK; 19th February 2012 at 07:46 AM.
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Old 19th February 2012, 08:09 AM   #18
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Here is all you need: Amplificadores DX Blame - Carlos Mergulhăo
Click on DX Blame Supercharged link and there you go - Schematic , PCB normal and inverted for the toner transfer method and two pairs of outputs and of course size of PCB.

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Old 19th February 2012, 08:49 AM   #19
DIYGEEK is offline DIYGEEK  United States
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Yes but I desire THREE sets of output transistors. So I just changed up Carlos design a bit, anyone see problems with this?
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Old 19th February 2012, 01:30 PM   #20
Bigun is offline Bigun  Canada
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Should be no problem to change it up with an extra pair of outputs. The thing with multiple outputs is that for good current sharing between these devices you want to ensure the devices are 'matched' so that they all turn on together and flow similar current. This usually means buying extra and testing them - something I've never done because I'm too lazy. Although I've never tried it, I think multiple output devices from the same batch will be fairly well matched already and then just use largish emitter resistors, say 0R47 to force the current sharing a bit more.

Nice build by the way. It was about 3 years ago I started my first amplifier (called TGM) along very similar lines to you. There's always something to learn with these projects, whether it's choice of parts, optimizing the circuit with a 'scope, finding and fixing a problem, layout out pcb, designing a better power supply, etc. A very enjoyable experience for the most part. For me, the experience was incomplete without building a speaker to go with the amplifier, something relatively simple from the Full Range forum.

I hope you realize that this hobby is very addictive

p.s. I noticed in your photo that you have a simple carbon-track open potentiometer for bias adjustment. A lot of older amps used them. The trouble is they can degrade over time, depending on their quality, so I use only sealed potentiometers and I use mult-turn types to make adjustment easier.
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Last edited by Bigun; 19th February 2012 at 01:32 PM.
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