Crescendo Millenium Problem

[yankur]

I've built Crescendo ME and found same problems for both channel. The offset voltage on first startup is >10V and then down to around 5V, and voltage on bias point=20v. What could be the problem?
I have measured the bias on T5 and T6, at around 3.7mA and bias for T7 and T9 around 35mA, which should be correct. Voltage on R34 and R35 is 50mV.

I have some MAT02 and MAT03. I want to change the input pairs with those MATs, but the voltage is too high. I will insert 20v/1.3W Zener in series with those MATs, is it possible? How to calculate the value for new R8 R9 R13 R14 and R6 R7 R11 R12 ?


Thanks,

 

[meanman1964]

Try with changing C3 to 330n and R4 and R5 to 1m from the amplifier board

 

[AndrewT]

Hi,
do not fit the MATs to the Cresendo.

The modifications required will be extensive and the benefit will seem inconsequential when you eventually get the altered PCB to work with MATs in place.

 

[yankur]

Thank you for the responses.
I found the solution from a book I read last night. I changed R6 and R7 with a 200 ohm trimpot, short C3 (disable bias), measure voltage at tp3, turn the trimpot until the voltage at tp3 between -10v and +10v. Then enable bias, the integrator works well now.
Voltage decreasing to 0.03V after 20s. Then I changed the value of C5 on the protection circuit to 88uF (4x22uF ... the value I have) to make the power on delay time much longer, make room for the integrator to take down the offset voltage below 50mV.

Does changing R6 and R7 with a trimpot decrease the performance?

I'd love to use the MATs next time, since I already have some (samples from Analog Dev). I made my own PCB using silk screen printing so I can modify the PCB as I need. Separate the input stage supply from the VA and output stage would solve the problem? Then I should decrease the value of R6-R9 and R11-R14 ?

Now finishing my crescendo, will try some musics tomorrow.

Regards,

 

[Elbert]

Rather than starting a new thread, I'm trying to revive this one as I have similar problems with my Crescendo ME...

When starting up Voltage in the output (TP3) quickly rises to -7 to -8 V, then it settles down to arround -1,28V after about 5 seconds. Bias voltage (from the protection card) rises as high as 18V.

The loudspeaker protection relays does not close and those two 5W 1k resistors on the protection card get warm.

I have not dared to keep the power on for more than perhaps 10s. No obvious fires or burned smell observed.

Does anybody have any idea what might be wrong or where I should start checking?


Everything is buildt according to the Original article in Elektor 2001.

As I'm not an expert on amplifiers , or analogue electronics in general for that matter, I would be really happy if someone can point me in the right direction..