Hi,
probably because it does not work with the cheap device options.
May work marginally with the medium price options
Will work with the expensive device options.
The reason for all this conflict is attrocious device choice by the circuit designer. If he can get this easy bit so wrong, then what other problems has he stored up in the schematic.
Did he build it? Did he debug it? Does he offer any guarantees?
Do you trust him to have got all else perfect with just the one mistake?
probably because it does not work with the cheap device options.
May work marginally with the medium price options
Will work with the expensive device options.
The reason for all this conflict is attrocious device choice by the circuit designer. If he can get this easy bit so wrong, then what other problems has he stored up in the schematic.
Did he build it? Did he debug it? Does he offer any guarantees?
Do you trust him to have got all else perfect with just the one mistake?
and Tino96 is confusing this situation as well. It's obvious that he's uncertain about some basic things. Btw, Tino! what happend to ya mate? Say something. Hi,
look at the +-60 supply rails, if they hold up well on full power then expect your Vpk to be about 50Vpk (60-10) into 8r equating to about 150W.
a further drop to about 48Vpk (60-12) into 4r equating to 280W.
If the power supply sags badly the 4r power could drop to as low half this projected power. i.e 100W into 8r and 140W into 4r.
So power output depends on how you build the whole amp not just the PCB.
look at the +-60 supply rails, if they hold up well on full power then expect your Vpk to be about 50Vpk (60-10) into 8r equating to about 150W.
a further drop to about 48Vpk (60-12) into 4r equating to 280W.
If the power supply sags badly the 4r power could drop to as low half this projected power. i.e 100W into 8r and 140W into 4r.
So power output depends on how you build the whole amp not just the PCB.
Hi,
500VA can support upto 300W of total output power.
A lateral FET amp can expect Vpk to be about 6V to 10V below loaded rail voltage.
If the +-60Vdc drops to +-56Vdc and then Vpk is just 6V below that then the output voltage will be about 50Vpk.
This results in 158W into 8ohms and 312W into 4ohms.
To allow the predicted 312W to happen the smoothing capacitance should be about +-40mF.
If the lower voltages result from the build the following may be a good prediction.
+-60Vdc falls to +-52Vdc when fully loaded. Allowing 10V below rail voltage gives 42Vpk at the load.
The predicted power delivered is now about 110W into 8ohms and 220W into 4ohms. Again +-40mF should be allowed for the 4ohm loading.
I would expect the as built amplifier to produce somewhere between these predictions.
The 8ohm nearer the higher value and the 4ohm nearer the lower value.
I suggest one might get about 140W into 8ohms and 240W into 4ohms.
If you use a 40-0-40Vac transformer you will get a bit less than these predictions and using a 45-0-45Vac transformer should get a bit more. A 500VA 40Vac could just about suit a stereo amplifier, but I would go to 750VA 45Vac for the higher power stereo version. Build a separate PSU for each amplifier fed from a common transformer.
500VA can support upto 300W of total output power.
A lateral FET amp can expect Vpk to be about 6V to 10V below loaded rail voltage.
If the +-60Vdc drops to +-56Vdc and then Vpk is just 6V below that then the output voltage will be about 50Vpk.
This results in 158W into 8ohms and 312W into 4ohms.
To allow the predicted 312W to happen the smoothing capacitance should be about +-40mF.
If the lower voltages result from the build the following may be a good prediction.
+-60Vdc falls to +-52Vdc when fully loaded. Allowing 10V below rail voltage gives 42Vpk at the load.
The predicted power delivered is now about 110W into 8ohms and 220W into 4ohms. Again +-40mF should be allowed for the 4ohm loading.
I would expect the as built amplifier to produce somewhere between these predictions.
The 8ohm nearer the higher value and the 4ohm nearer the lower value.
I suggest one might get about 140W into 8ohms and 240W into 4ohms.
If you use a 40-0-40Vac transformer you will get a bit less than these predictions and using a 45-0-45Vac transformer should get a bit more. A 500VA 40Vac could just about suit a stereo amplifier, but I would go to 750VA 45Vac for the higher power stereo version. Build a separate PSU for each amplifier fed from a common transformer.
Sean or anyone,
Looking at the schematic, what is the purpose of the 22K resistors (R16 & R17) from the gates to GND? How is this value determined and what is the best value to use? Are they just for some GND reference or to help discharge the gates? I think I have a good grip on what everything else is doing and why the values so am curious what these resistors are for.
TIA,
-SL
Looking at the schematic, what is the purpose of the 22K resistors (R16 & R17) from the gates to GND? How is this value determined and what is the best value to use? Are they just for some GND reference or to help discharge the gates? I think I have a good grip on what everything else is doing and why the values so am curious what these resistors are for.
TIA,
-SL
Hi,
I have read this is bad design.
R16 & 17 have been added to reduce the open loop gain and increase the openloop bandwidth.
The gain reduction helps with stability.
The bandwidth increase is supposed to help achieve low feedback ratios. It produces two poor results.
Firstly it reduces low frequency feedback which increases low frequency distortion, secondly it artificially increases the load seen by the VAS and increases the VAS distortion.
It is far better to use conventional means to compensate the amplifier and forget about low feedback ratios and the supposed benefits that may accrue.
Anyone want to challenge that view?
Using VAS to drive three FETs (and a resistor) without an intervening buffer!
I have read this is bad design.
R16 & 17 have been added to reduce the open loop gain and increase the openloop bandwidth.
The gain reduction helps with stability.
The bandwidth increase is supposed to help achieve low feedback ratios. It produces two poor results.
Firstly it reduces low frequency feedback which increases low frequency distortion, secondly it artificially increases the load seen by the VAS and increases the VAS distortion.
It is far better to use conventional means to compensate the amplifier and forget about low feedback ratios and the supposed benefits that may accrue.
Anyone want to challenge that view?
Using VAS to drive three FETs (and a resistor) without an intervening buffer!
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