Dx Blame MKIII-Hx - Builder's thread

[destroyer X]

muchas gracias senhor Carlos is spanish... in portuguese is

- "muito obrigado senhor Carlos"

Meaning in english is:

- "thank you Mr. Carlos"

But we can understand.... latine languages are easy, Italian, French, Spanish and Portuguese have the same root.

A picture from other forum, another MKIII Hx playing loud and clear.

regards,

Carlos

[a.wayne]

Originally Posted by AndrewT

Do you want to hear about the correct way to adjust the bias of DX amplifiers?

I'm all ears ...........

[Boscoe]

2SC5200/2SA1302 is compatible with this amp right?

[ByronInLawrence]

That's a lot of instructions, but worth following!

I have one more question for Carlos, or anyone else who knows the answer. I'm using a 56.4V power supply. Should I set the voltage drop across the 100ohm resistor to -4.4V, as I would with a 64V supply? Or should I calculate

64V : -4.4V = 56.4V : -3.88V

and set the voltage drop across the setup resistor to -3.88V?

[Junie]

Originally Posted by Boscoe

2SC5200/2SA1302 is compatible with this amp right?

Hey Boscoe, I'm using 2SC5200/2SA1973 OP at the moment, so far so good. been running two hour and no heat build-up yet...or maybe not.

[meanman1964]

it is the 2SA1943

[destroyer X]

in order to measure a voltage drop of 1 milivolt or a little bit less into one of the power transistors .... others will be reasonable too.... try to measure in one of your transistors that are in the positive side, the NPN transistors, as they have lower gain and this way your PNP will be fine with a little bit higher current in stand by mode.

1 milivolt measured over (voltage drop in DC milivolts, 200 milivolts scale) a 0.47 ohms resistor represents a current of 2.1 miliamperes...and this multiplied by your voltage from colector to emitter says you will be dissipating 118 miliwatts in each one of them.... so... 10 transistors may dissipate 1 to 1.5 watts (because PNP will dissipate more, the gain is bigger, the current there will be bigger, so the dissipation).... this is 21 miliamps in the output transistors, you have CCS sucking current too, but you should not worry too much, because if you have low offset and bias control trimpot adjusting the current means your amplifier is fine....this way go monitoring your output current in one of your transistors and everything gonna be all right.

If you dislike the method, despite i am saying it is the best way to keep things cool, then try 10 volts over your 100 ohms protective resistor and let it be this way (less cool, more heat).

I am suggesting this way because i know that people will never use the heatsinks in the size i suggest, so, people may face problems of heat due to small heatsinks (more elegant, more pretty, smaller represents smaller price too... but they are less efective) and adjusting with the smallest possible current in the output, you will be rid of some heat, not only in stand by mode but also during dinamic operation as you will change your operational point.

My suggestion to heatsinks is an aluminium blade of 4 by 4 inches (square) to each 10 watts.... power will depend your distortion, supply current, supply voltage and speaker impedance... each one will have a different case..... so, if your will operate 100 watts each channel, then you will need the equivalent of 10 blades to each channel if external heatsinks..if internal you should increase 50 percent the size...if use a blower you can use half the heatsink size....if you use more than one blower and tunnel ventilation, then your efficiency will be multiplied by 4 (minimum)... and heatsink size will reduce in the factor of 4 too.... this is for heavy duty..if you will use music...or a non constant tones being amplified (unlike pipe organ) .... then you will have twice the efficency and 10 by 10 centimeters (4 by 4 inches) will be good to 20 watts...... so, two blades will be good for 40 watts... three blades for 60 watts RMS.... the equivalent you know.

I hope i was clear..if not, then ask again.

regards,

Carlos

[destroyer X]

MJ777 from another forum tried two different solutions, the first one was increasing the heatsink size that resulted fine, temperature dropped to 42 degrées celsius (human fever).

He tried a second solution installing these transistors below the pcboard, this way they were screwed into the main heatsink and temperature result fine too.

regards,

Carlos

[a.wayne]

What PSU voltage is he running the MK3 with Carlos ...?

[canonnica]

Originally Posted by a.wayne

What PSU voltage is he running the MK3 with Carlos ...?

Words from the initial poster on foreign forum:

I'm using a rail voltage of plus/minus 68 volts dc from a 50-0-50 volts ac transformer rated 10 amperes.