LJM MX50 kit amp

[ekremy18]

Another question: one of the amp boards is warmer than the other when the whole amp is at iddle, i think quescient current of the warmer one is higher.

How can i fix this problem, if it is so?

 

[ekremy18]

Another question: one of the amp boards is warmer than the other when the whole amp is at iddle, i think quescient current of the warmer one is higher.

How can i fix this problem, if it is so?

İ measured the cement resistor voltages and i read 20mA at one of the boards and 100mA at the other, that is why one of the boards gets warmer when it is at iddle.

These are identical board how can they differ and how can i fix this situation?

 

[ivanlukic]

Biasing circuit uses fixed resistors. With that kind of biasing circuit variation in the value of idle current is inevitable.

 

[ekremy18]

Finally got enough stuff together to start building.
I still need the output terminals, which still are in transit.

The amp will be set in a partial class A. Also i'll ad a small board to make the setting of the quiescent current easier. In the current configuration thst procedure is quite difficult.

sir can you inform me about your board makes possible to set the quescient current?

i need to set too... my values are 20mA and 100mA for cement resistors.

 

[ekremy18]

I recommend using MX50SE version. Need to pay attention to, don't replace all parts, transistor.
But does not include high power output tube B817 D1047, output power tube according to preferences of free choice,
For example, TTA1943 TTC5200, or SANKEN 2SA1216 2SC2922. A1186 C2837
Pay attention to any other parts, can not replace. They have been optimized measures in the design. DC + - 45V

can you inform me about setting the quescient current?

my values are 20mA and 100mA for cement resistors.

 

[ekremy18]

Biasing circuit uses fixed resistors. With that kind of biasing circuit variation in the value of idle current is inevitable.

i understand you but i wonder is there any way to set the quescient current since the amp is very good

 

[Noise]

i understand you but i wonder is there any way to set the quescient current since the amp is very good

Adjusting the bias on the MX50SE is fairly easy. (The MX50 is a different amp, so only on the MX50SE)

On my MX50SE I have simply removed the 1k bias resistor and replaced it with a 2k trimpot. If you bend the center pin on the trimpot out and connect (solder) it to the outer pin, and then adjust the the resistance between the two outer pins to 1k, before mounting it in the amp. Then it should be safe to power up the amp, and adjust the bias to what you want it to be. See the pictures below.

It works fine in my amp, mine came with the opposite problem that you have, mine had extremely low bias, I think it was around 2,5mA. It's been a while so I don't remember the exact numbers. But the trimpot fixed the problem.

 

[ekremy18]

Adjusting the bias on the MX50SE is fairly easy. (The MX50 is a different amp, so only on the MX50SE)

On my MX50SE I have simply removed the 1k bias resistor and replaced it with a 2k trimpot. If you bend the center pin on the trimpot out and connect (solder) it to the outer pin, and then adjust the the resistance between the two outer pins to 1k, before mounting it in the amp. Then it should be safe to power up the amp, and adjust the bias to what you want it to be. See the pictures below.

It works fine in my amp, mine came with the opposite problem that you have, mine had extremely low bias, I think it was around 2,5mA. It's been a while so I don't remember the exact numbers. But the trimpot fixed the problem.

Thanks bro you save my life i have just bought the 2k trimpots and going to home

 

[ekremy18]

An accident occurred and emitter and collector tips were shorted for a moment, now i read huge dc voltage at speaker output about 34V dc, what is the possible reason and solution? Thanks in advance.

 

[vgeorge]

Check the output transistors first and then check that the small ones work too.
You don' t have to desolder the small ones, just check that BE voltage is around 0.6V

 

[ivanlukic]

Output transistors are destroyed.

 

[ekremy18]

Output transistors are destroyed.

İ checked output transistors after desolder them with a multimeter at diode section and observed voltage drop at both B-E and B-C terminals about 0.58V so they seem normal

 

[t-minik]

Hello ivanlukic!
If you look better you will see that the schematic is the same as yours. I have made some corrections on the original. The only difference is the emitter resistors on my own is 0R22 and your own 0R1

Hi

I just received an MX50SE Kit with 0R15 5w cement resistors and sanken C3264 + C1295 power transistors.

as i read some recent kits came with 0R1 or olders with 0R22, do you think that could be an improvement to change them for 0R1 ?
I'll set the bias to 15/20ma to ensure not to much power to dissipate.
I guess lowering that resistor could improve damping factor but i don't know if it's a risky trick.

best regards

 

[ivanlukic]

The circuit is based on Doug Self's Load Invariant Amplifier and that amplifier is a variation on his Blameless Amplifier. Doug claims that the distortion is (very little) lower with 0R1 but the higher values are perfectly OK! There will be no improvement with 0R1, even 0R33 is OK.

 

[t-minik]

Thanks for your reply ivanlukic.

Do you know what is the recommended biasing value for Self's Load Invariant amp?
I can't find it anymore but I think I remember ljm said 20ma was a good value for mx50.

Best regards

Edit // I bought an anorher Mx50x2 kit for pleasure/compare.
My step father's amplifier, where I put an Mx50x2 pair is really good.

 

[ivanlukic]

For CFP amps like MX50SE recommended bias is 15mA. With 0R1 that's 3mV between output transistors collectors.

 

[t-minik]

OK thanks a lot for your help.

In my case for 15ma bias it's 4.5mv between collectors (through 2x0R15).

best regards.

 

[ekremy18]

hi, i use my mx 50se kit with the gain of 12kohm/220ohm=55x, i wonder, is there any disadvantage with this configuration? i have not observed any issue and "hiss" due to high gain yet.

 

[ivanlukic]

It's OK.

 

[t-minik]

Hello.

Just a last question before wiring power supply and 1st start, to ensure a proper biasing it must be set without any load connected.
Am I wrong?

Best regards