Hello, a few months ago I hve acquired a pair of active vintage studio monitors that have bout a milion and more problems, so I decided to keep the box and speakers and for now the crossover too, and redesign its amplifier to hopefully remove the original flaws and make it "better" too.
The original amplifier had a catastrophic PCB design even for the 70s when this thing would be manufactured, this thing would break into oscillation like as if it was built for that purpose. And at no joke frequencies!
The original design used high speed switching transistors, and despite that having its own problems and quirks I am willing to take chances and try to work with the original output pair. This means the amplifier will be, or rather, should be VERY FAST. The whole thing is rather low power so I am not concernet with burning the switching transistors with too much linear region power dissapation. And since these are switching transistors I have to work with a quasi complementary circuit.
Now as I mentioned earlier this thing is low power. Like 50W peak power. Originally this has a 25V+/- unloaded supply voltage, which considerig the size of the transiformer will droop signifficantly. So I dont expect to exceed some extreme actuall dissapated power. Also the heatsink on this thing is MONSTROUS. The load is 4ohms. I dont think a multistage current gain is required for this speciffic purpose.
So now that you have a wider context as of what and why I am doing, heres the schematic
Its quite chaotic so if any questions arise just ask me, and please do question me and my decisions. Maybe Ill come up with a better solution.
(Q11 and Q13 are not what the real parts are, but they are the closest to compare, Q3 mounted on heatsink for temperature sensing)
In LTspice I tuned this thing with a safe ish margin before complete breakdown while. It behaves calmly in extreme conditions like output voltage limitations, doesnt saturate the transistors within (least I havent noticed any too negative effects). It behaves well in the safe zone in the simulator while being quite fast. As that was the goal here. Fast responding amplifier with medium to low ish power but a very good quality.
EDIT: Yes I am aware my diff emitter current will drift with varying supply voltage, think of R29 as a dynamic resistor pulling exactly the same current no matter the voltage. I plan to put a depletion mosfet current source in there. At the time of design all I really wanted is for both diff pairs to drift equally in current if they absolutely have to
The original amplifier had a catastrophic PCB design even for the 70s when this thing would be manufactured, this thing would break into oscillation like as if it was built for that purpose. And at no joke frequencies!
The original design used high speed switching transistors, and despite that having its own problems and quirks I am willing to take chances and try to work with the original output pair. This means the amplifier will be, or rather, should be VERY FAST. The whole thing is rather low power so I am not concernet with burning the switching transistors with too much linear region power dissapation. And since these are switching transistors I have to work with a quasi complementary circuit.
Now as I mentioned earlier this thing is low power. Like 50W peak power. Originally this has a 25V+/- unloaded supply voltage, which considerig the size of the transiformer will droop signifficantly. So I dont expect to exceed some extreme actuall dissapated power. Also the heatsink on this thing is MONSTROUS. The load is 4ohms. I dont think a multistage current gain is required for this speciffic purpose.
So now that you have a wider context as of what and why I am doing, heres the schematic
Its quite chaotic so if any questions arise just ask me, and please do question me and my decisions. Maybe Ill come up with a better solution.
(Q11 and Q13 are not what the real parts are, but they are the closest to compare, Q3 mounted on heatsink for temperature sensing)
In LTspice I tuned this thing with a safe ish margin before complete breakdown while. It behaves calmly in extreme conditions like output voltage limitations, doesnt saturate the transistors within (least I havent noticed any too negative effects). It behaves well in the safe zone in the simulator while being quite fast. As that was the goal here. Fast responding amplifier with medium to low ish power but a very good quality.
EDIT: Yes I am aware my diff emitter current will drift with varying supply voltage, think of R29 as a dynamic resistor pulling exactly the same current no matter the voltage. I plan to put a depletion mosfet current source in there. At the time of design all I really wanted is for both diff pairs to drift equally in current if they absolutely have to
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The circuit has little control over the idle currents in Q8 and Q10. A small mismatch in the differential pairs will lead to a large variation in currents.
The two halves of a fully complementary amplifier need to remain well-matched across component and temperature variations. Look at Leach's amplifier or ostripper's "Spooky" amplifier.
Ed
The two halves of a fully complementary amplifier need to remain well-matched across component and temperature variations. Look at Leach's amplifier or ostripper's "Spooky" amplifier.
Ed
Thanks, yeah that appears to be an issue that the simulator doesnt anticipate. I assume the whole thing will be picky about transistors thats thankfully not an issue since I have a batch of 500. But I will be implementing a "current limit" for this stage as I do fear that this may become a larger problem down the road if it turns put to be a problem.
The entire VAS is questionable though. I want something with high enough gain as to avoid another current gain stage before the drivers or finals, but this particular vas I have gone with feels wrong to say the least. Any suggestion for a better VAS?
The entire VAS is questionable though. I want something with high enough gain as to avoid another current gain stage before the drivers or finals, but this particular vas I have gone with feels wrong to say the least. Any suggestion for a better VAS?
"...but this particular vas I have gone with feels wrong to say the least."
Ok. In what way does it feel wrong to you?
Ok. In what way does it feel wrong to you?
Its got quite the high current gain and if anything goes, well, bad, or the feedback gets messed up a little it has a lot of potential to send excesive problems down the road. Q10 and Q8 in a misfortunate state could end up conducting current trough heach other and then its just a matter of severity how much current it will be and since there are no current limits anywhere if it decides to send full supply it will send a lot of current frying itself and causing huge problems. Adding current limits should help, but then the question is if it is going to limit the slew rate severy and how it is going to impact impulse response.
None of this should ever happen in normal conditions, I may just be a little paranoid.
None of this should ever happen in normal conditions, I may just be a little paranoid.