A suggestion for SolidSnake:
Build your self a test PCB for this concept. If you are careful, you can leave out short circuit protection, which has the potential for confusing the issue. Build the PCB with holes for all the various resistors suggested above. If you have other configuration options in mind, larout leads for all of them but include jumpers.
Using different arrangements of jumpers, open traces (no jumber , no resistor) and resistors you should be able systematically reconfigure the circuit to evaluate what is working and what isn't. Oh yes, add more jumpers at positions where you want to measure current. The PCB will have a fairly large foot print, but that's good as it leave you working room. Also be sure to lay it out (since you have space to spare) so that the small signal stages are kept away from the out put stage. This is always a problem, but especially in an experimental layout where you don't want to deal with induced parasitics on top of the rest.
I've done this and found it helpful. CF output stages are not impossible for the non-pro (I'm an accountant by education for heaven sake!) but you have to be willing to try, try again. SPICE is fine, but you've got to build something, check it out, curse and grind your teeth, toss it out and build another one. I have had more unsuccessful than successful attempts, but I DO have a couple of stable CF amps around that I play nearly every day. I wish I could say WHY some work or don't work with more certainty, but then I'm only a non-pro hobbiest. Once you get a circuit and layout that does what you want - treasure it.
Build your self a test PCB for this concept. If you are careful, you can leave out short circuit protection, which has the potential for confusing the issue. Build the PCB with holes for all the various resistors suggested above. If you have other configuration options in mind, larout leads for all of them but include jumpers.
Using different arrangements of jumpers, open traces (no jumber , no resistor) and resistors you should be able systematically reconfigure the circuit to evaluate what is working and what isn't. Oh yes, add more jumpers at positions where you want to measure current. The PCB will have a fairly large foot print, but that's good as it leave you working room. Also be sure to lay it out (since you have space to spare) so that the small signal stages are kept away from the out put stage. This is always a problem, but especially in an experimental layout where you don't want to deal with induced parasitics on top of the rest.
I've done this and found it helpful. CF output stages are not impossible for the non-pro (I'm an accountant by education for heaven sake!) but you have to be willing to try, try again. SPICE is fine, but you've got to build something, check it out, curse and grind your teeth, toss it out and build another one. I have had more unsuccessful than successful attempts, but I DO have a couple of stable CF amps around that I play nearly every day. I wish I could say WHY some work or don't work with more certainty, but then I'm only a non-pro hobbiest. Once you get a circuit and layout that does what you want - treasure it.
Wonder if someone could answer my query?
My DoZ amp with a supply of 60V biased at 3A with 8 output transistors MJ21196, would 0R1ohm 5watt wire wound resistors be suitable as output emitter resistors or would they cause oscillation due to the inductance induced by the wire wound resistors.
Or would 0R1ohm 1watt metal film be more suitable?
I did some calculations and it seems the metal film resistors would be able to dissipate the heat well. But when looking at Rod Elliot's 300watt subwoofer amplifier module, I noticed he used 0R1ohm 5watt resistors for emitter resistors.
Also, with the emitter resistors forcing current sharing, would transistor matching still be needed?
Can someone clarify for me please?
Thanks in advance
My DoZ amp with a supply of 60V biased at 3A with 8 output transistors MJ21196, would 0R1ohm 5watt wire wound resistors be suitable as output emitter resistors or would they cause oscillation due to the inductance induced by the wire wound resistors.
Or would 0R1ohm 1watt metal film be more suitable?
I did some calculations and it seems the metal film resistors would be able to dissipate the heat well. But when looking at Rod Elliot's 300watt subwoofer amplifier module, I noticed he used 0R1ohm 5watt resistors for emitter resistors.
Also, with the emitter resistors forcing current sharing, would transistor matching still be needed?
Can someone clarify for me please?
Thanks in advance
Hi rchua77,
I have no experience with the DoZ, so do not know whether some inductance of the output emitter resitors will seriously affect stability. But I guess not; it is a low feedback design.
I still think 1W resistors should be enough in your case. Rod's 300W subwoofer design has a maximum output current of 56V/4Ohm=14A, divided by 4 output transistors per half. That gives 3.5A per transistor and resistor. In 0.1 Ohm resistors a maximum of 1.22W will be dissipated, so 5W gives a nice margin and 1W is really too small.
For your DoZ with 0.75A bias current per transistor, you get 75mV across the emitter resistor. For current sharing with unselected transistors this is marginal. Looking at Vbe differences for unmatched transistors I would like to see a bit more. Above 150mV you need not worry about current sharing anymore. So, I think you better use 0.22 Ohm resistors. These can still be 1W.
Steven
I have no experience with the DoZ, so do not know whether some inductance of the output emitter resitors will seriously affect stability. But I guess not; it is a low feedback design.
I still think 1W resistors should be enough in your case. Rod's 300W subwoofer design has a maximum output current of 56V/4Ohm=14A, divided by 4 output transistors per half. That gives 3.5A per transistor and resistor. In 0.1 Ohm resistors a maximum of 1.22W will be dissipated, so 5W gives a nice margin and 1W is really too small.
For your DoZ with 0.75A bias current per transistor, you get 75mV across the emitter resistor. For current sharing with unselected transistors this is marginal. Looking at Vbe differences for unmatched transistors I would like to see a bit more. Above 150mV you need not worry about current sharing anymore. So, I think you better use 0.22 Ohm resistors. These can still be 1W.
Steven
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