Identify problem with Aleph P

Something went wrong:bawling:

I’ve had my Aleph P up and running for a while. But today, when I tried to measure the current of the pre amp itself, I ran in to trouble. Either I quickly shorted the circuit after the power supply or I did something wrong when I tried to do the measure itself. When I tried to measure the current I heard two small clicks, probably when one or two transistors blew up:hot:

This is not as bad as one could think since it’s just a prototype board and I will soon order new PCB’s.

But never the less, I have no problem to copy a schematic and make my one PCB and get an amp like this up and running. My problem appears when it is not function as it should or when as in this case, one or more components ran in to failure due to my one mistake. I really have a hard time to locate the problem itself, and I actually do not know where to start. And maybe someone could be nice and give me some hints where to start.

I actually do not know how to control if a transistor (9610, 610 ZTX450, ZTX550) is working as it should when it’s still stuffed to the board, or if it’s even possible to do so.

What I have measured so far is that I have a drop over R73 (original schematic, the first 3 Ohm resistor the current flows through from the PS) of 56 V on one channel and 48 V on the other channel.

I would be extremely happy if someone could give me some tips where to start.

BR

/loovet
 
I'm not sure I understand how you are measuring the current flow. If you are measuring the voltage drop across a 3ohm resistor with 60ma running through it you should be reading around 0.18v. If it's much higher than that you have blown the 3 legged fuses in the circuit. The higher the current the higher the voltage drop.

Bill
 

culture

Member
2006-10-15 1:40 pm
loovet said:
What I have measured so far is that I have a drop over R73 (original schematic, the first 3 Ohm resistor the current flows through from the PS) of 56 V on one channel and 48 V on the other channel.

Check the fets in the psu circuit... don’t really remember the values i measured when i
connected my psu board the wrong way around but i belive it measured something similar

cheers,

c
 

culture

Member
2006-10-15 1:40 pm
Bill Fuss said:
Hi Culture, are you assuming he is measuring the voltage from the PS neg to the resistor? If he's reading 48v like that then he's dropping 12v across the resistor, which would be 4 amps.

Bill

Oops, you're right...after taking a look at the schematic (and reading the post more careful) i realized i made some assumptions a bit too fast. i was measuring 60v plus and measuring between ps neg an the resistor indeed.

c
 
It depends somewhat on the board layout and the attenuator board, if you're using one. If you are using the Dantimax 3 board you will notice that all signal grounds are on the same PCB trace so this must be used as the starting point of your star ground.
Getting back to basics, the DC neg from the PS board has to be directly connected to this point, and the chassis has to be isolated from this point. The thermistor everyone is referring to acts as a 10 ohm res to protect your system from serious damage and personal injury from a fault or short anywhere in your system. Theoretically, your signal ground should be totally separate from your chassis ground for the quietest operation, but that can become dangerous.
Please tell us what boards you are using.

Bill
 
I'm also using the KK boards and have a problem myself, but I dont think it's related to yours, unless your problem is only in one channel and only on balanced out as mine is.
Basically, on the kk boards the INs and OUTs are physically very close to one another so you shouldn't have a problem picking up your star point from there, as long as the channels are isolated from each other totally. The problem is if there is another common point anywhere else, such as the Dantimax relay boards, then you have a second path where current can flow, and you no longer have a star ground. If you are using the Relvol 3 board or similar arrangement you must use that as your star ground point and run all of your grounds to that point, not to the KK board inputs and outputs, and also run separate grounds to the KK board grounds from that point. Dont forget your PS board neg must go to that point also.
The thermistors can be lifted off of chassis ground for test purposes to eliminate possible problems but should be used in every day use. The old trick of a .1uf film cap from the input ground to chassis ground is sometimes very effective, but don't ask me why.

Bill
 
I've been chatting with someone off-line about hum problems with this same circuit. Everything appears to be done correctly, and the layout is good, yet there is still some problem we've failed to locate. Every known grounding scheme has been tried, and a few that aren't. Does anyone know how sensitive this circuit is to component substitutions, how good the power supply rejection is, or any other reason it might be more susceptible to hum?

Exposing my ignorance, I also have to ask, what signal source would need significant gain in a line stage? Also, in a single ended application, would it be typical to single end the output, or to "ground" the cable shield to the output of the other half of the circuit?

IMO, the thermister is a clever solution to the safety ground problem, but would it be permitted in a commercial product under CE regs? Would it pass a proper high current ground bond test?