So when/if the rail fuses blow the 100ohm resistors will keep the circuit "working" to prevent full rail voltage on the speaker output ?
It should allow some properly oriented supply voltage across input stage.
I am sure that Hugh who is familiar with this circuit will explain it precisely.
I am sure that Hugh who is familiar with this circuit will explain it precisely.
Gee, Pavel,
Is there nothing you don't know about these circuits?
The 100R resistors also serve another important purpose; they permit initial testing of the amp without fuses under low voltage conditions. If there is a problem in the circuit which might otherwise threaten the output devices, it will show up as a large voltage drop across these resistors. This alerts the constructor to a problem, visually and by smell, and prevents large current flows, and preserves his valuable output stage. In the factory, of course, one would use a Variac, but this luxury is not afforded the kit constructor.
Cheers,
Hugh
Is there nothing you don't know about these circuits?
The 100R resistors also serve another important purpose; they permit initial testing of the amp without fuses under low voltage conditions. If there is a problem in the circuit which might otherwise threaten the output devices, it will show up as a large voltage drop across these resistors. This alerts the constructor to a problem, visually and by smell, and prevents large current flows, and preserves his valuable output stage. In the factory, of course, one would use a Variac, but this luxury is not afforded the kit constructor.
Cheers,
Hugh
Hugh,
What happens when there is short on the output for example and one of the rail fuses blow. Wouldn't the 100ohm resistors also burn ?
Thanks!
What happens when there is short on the output for example and one of the rail fuses blow. Wouldn't the 100ohm resistors also burn ?
Thanks!
Yes, Jean,
Certainly! But they will fail very quickly, and are elevated on the board so they do no damage there.
Remember, a fuse blowing makes no smell. A 0.6W resistor makes quite a pong, and you notice it immediately and switch the amp off. It is a warning, and serves very well, much like a gauge AND a lamp in an auto.
The AKSA was deliberately designed without protection other than fuses and fuse resistors. This is intentional; it is a design decision which will prompt debate, certainly, but this is not the only amp where protection is eschewed. Some are very 'high end', and I make no excuses. There is completely transparent protection out there available, it is true, but it is rare and quite complex, often with DC sensing, separate comparators, etc, and often it fails to protect things anyway. I made the decision to forgo such a circuit because as a kit amplifier this had to be simple and easy to make.
As ever, the devil is in the details.........
Cheers,
Hugh
Certainly! But they will fail very quickly, and are elevated on the board so they do no damage there.
Remember, a fuse blowing makes no smell. A 0.6W resistor makes quite a pong, and you notice it immediately and switch the amp off. It is a warning, and serves very well, much like a gauge AND a lamp in an auto.
The AKSA was deliberately designed without protection other than fuses and fuse resistors. This is intentional; it is a design decision which will prompt debate, certainly, but this is not the only amp where protection is eschewed. Some are very 'high end', and I make no excuses. There is completely transparent protection out there available, it is true, but it is rare and quite complex, often with DC sensing, separate comparators, etc, and often it fails to protect things anyway. I made the decision to forgo such a circuit because as a kit amplifier this had to be simple and easy to make.
As ever, the devil is in the details.........
Cheers,
Hugh
After no -one took any notice of my earlier comments, the off topic stuff is now here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=35984
There are 2 kind of resistor that I dont understand.
1. Base stopper. 100ohm and 10ohm in the final transistor. What is the function of these?
2. 10R ground lifters. PMA said a little about its function, a possible danger and how to solve it (putting 2 diodes parrarel in different directions?). A40 also uses this, said to have better offset. What is really the purpose about this 10R? 1/4watt enough? What makes this R burn, which ground difference? (From input different from amp's ground?
3. In the positive rail there is IN4148+100R, but none in negative rail. Imbalance?
1. Base stopper. 100ohm and 10ohm in the final transistor. What is the function of these?
2. 10R ground lifters. PMA said a little about its function, a possible danger and how to solve it (putting 2 diodes parrarel in different directions?). A40 also uses this, said to have better offset. What is really the purpose about this 10R? 1/4watt enough? What makes this R burn, which ground difference? (From input different from amp's ground?
3. In the positive rail there is IN4148+100R, but none in negative rail. Imbalance?
Hi Thanh,
I think you have shown a bootstrapped input. If it is then it's purpose is to increase the dynamic input impedance.
The bootstrap is needed because very low input impedances reduce the noise at the amp output but load the preceeding stage excessively. Bootstrapping tries to achieve low noise and high impedance. It has a down side due the positive feedback, it needs care to avoid oscillation and/or instability.
I think you have shown a bootstrapped input. If it is then it's purpose is to increase the dynamic input impedance.
The bootstrap is needed because very low input impedances reduce the noise at the amp output but load the preceeding stage excessively. Bootstrapping tries to achieve low noise and high impedance. It has a down side due the positive feedback, it needs care to avoid oscillation and/or instability.
Hi Thanh, AndrewT,
The 10R resistor is an earth float. It isolates the signal/feedback earth currents from the amp power earth currents, preventing the voltage drops from heavy currents from finding their way into the input/feedback circuits. This cuts hum and simplifies earthing. Often the 10R 'float' resistor is bypassed with back to back power diodes to ensure that in traumatic failure the resistor does not fail through overcurrent.
Cheers,
Hugh
The 10R resistor is an earth float. It isolates the signal/feedback earth currents from the amp power earth currents, preventing the voltage drops from heavy currents from finding their way into the input/feedback circuits. This cuts hum and simplifies earthing. Often the 10R 'float' resistor is bypassed with back to back power diodes to ensure that in traumatic failure the resistor does not fail through overcurrent.
Cheers,
Hugh
Hi,
where does this high (dangerous?) current enter the amp?
Could it come in through the input screen from a faulty ancilliary?
The mains safety earth and PSU grounding should ensure that the amp cannot generate a large circulating current by itself.
So tell me what is the problem that needs this diode or high power resistor cure?
where does this high (dangerous?) current enter the amp?
Could it come in through the input screen from a faulty ancilliary?
The mains safety earth and PSU grounding should ensure that the amp cannot generate a large circulating current by itself.
So tell me what is the problem that needs this diode or high power resistor cure?
AndrewT,
If the source is unearthed, that is, double insulated, and there is a traumatic failure, it is possible, and has happened, that the chassis of source will become live. This signal earth could take on live potential with respect to power earth. This will burn out the 10R resistor if back to back diodes are not used.
Cheers,
Hugh
If the source is unearthed, that is, double insulated, and there is a traumatic failure, it is possible, and has happened, that the chassis of source will become live. This signal earth could take on live potential with respect to power earth. This will burn out the 10R resistor if back to back diodes are not used.
Cheers,
Hugh
Thank you Heinrich......
AndrewT, you betcha it does. It is not uncommon for double insulated, cheap sources to fail in this way at the end of their lives, particularly if an electro fails and spews caustic electrolyte all over the pcb and power supply.
Cheers,
Hugh
AndrewT, you betcha it does. It is not uncommon for double insulated, cheap sources to fail in this way at the end of their lives, particularly if an electro fails and spews caustic electrolyte all over the pcb and power supply.
Cheers,
Hugh
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