Problems with Crescendo ME - Need help!

[Eccu]

Sorry peranders, but could you clarify these texts below, because I don't understand what you mean with this:

You easily test the offsets. Short C2 the you will get the voltage offset. Short C3 (disconnect the DC-servo) then you can measure the voltage across R2-R3. This will give you the input bias current. This current creates the startup transient before the DC-servo has started to work. The only way to improve the startup is to increase the time constants of R19/C7 and R16/C6. The amp itself will start slower than the DC-servo. The other solution is to use more matched input transistors and they MUST be monolithic.

Diy you mean that I must short-ciruit (add some wires) over C2 and C3 and measure voltage over R2-R3 -> and it is input offset or what? And what I must to do with that knowledge? Could you explain precisely?


or this:

Q2 "C" has higher gain, both DC and AC but BC550C/BC560C is out of production but you can choose BCx46C, BCx47C, BCx56C, BCx57C but beware of different manufactures have different data. ON Semi seems to be best. Beware of the voltage across the transistors! T7, T9 has very little, any transistor will do but the other ones, T1-T4 has 40-47 V. T5, T6 15-20 V

or even this:

C type is easier to work with because the base current are smaller but B will also work but it's possible that you must "tweak" some passive components values.

Q3, Red colour has slightly lower forward voltage but if you choose green or yellow you could be forced to increase R19/R20 some but the current aren't criticical but they should be around the calculated value most so the voltages(power) across the transistors don't get too low och too high. The feedback will take care of small differencies.

What disadvantages increasing R19/R20 will cause? And did you mean that I must increase also R19 (R16)? And why???? I don't understand.

 

[peranders]

Diy you mean that I must short-ciruit (add some wires) over C2 and C3 and measure voltage over R2-R3 -> and it is input offset or what? And what I must to do with that knowledge? Could you explain precisely?

If you measure the voltage and then multiply it with the amp's gain, this is the startup transient. The voltage divided with the resistance is the base current (the sum of NPN/PNP). What would you do with the knowledge? Just for curiosity maybe..

The second Q: If you want to change transistors, you must know how much voltage you have across them.

Increasing R19/R20 will decrease ALL currents but since you have green LED's you will get slightly larger currents, 10-20%, I'll guess. Why? If you want the same currents as original but it's not very important if all transistors have working conditions below their limits.

If you want this amp run exactly as intended you must adjust R19/R20 for 3.7 mA

 

[joensd]

Hi Esa,
I get a voltage of 36,84V over R18 and 36,71V over R21
Other channel is 36,85V and 36,58V.
I sent you an email with the other testpoints.

And don´t forget your voltages will be higher anyway cause your transformer voltage is higher than the Elektor amplifier.

Regards
Jens

 

[Eccu]

Thanks Peranders and Joensd.... this helps me lot....


Higher bias = higher heating, i guess.
So I wan't that bias to same level as original design, because I have rather small heatsinks. Smaller than original design in Elektor.


Happy easter holidays for every one!

 

[peranders]

You will get higher speed (slew rate) also...

 

[Eccu]

Now I have changed those R17 and R20 from 270ohm to 460ohm. Now bias is set to right level!

But is there any disadvatages by this changing? Will this reduce sound quality?


I get voltage over R18 or R21 about 22V, but Joensd get about 36V!



Now I have chance to get those red leds, but I am so lazy to change those led's from green to red. But anyway... should I change those or can I just let alone that R17 and R20 modification?

 

[peranders]

But is there any disadvatages by this changing? Will this reduce sound quality?

The transistors should work within their limits, this is the most important thing. Each amplifiying stage should allways be optimized in order to get the most out of it so to answer your question: YES and NO! and my own opinoin, NO.

 

[Eccu]

Thanks Peranders again!

Now my soul got the peace.



Still my other channel do not work. I have done some fault tracing and now I know that malfunction is on protection circuit. I must do some measurements because relays do not act.

At least both amplifier boards works very well.