Hi !
You should add a driver stage between vas and mosfets, they
have very big inputcapacity, resulting in high load to vas for higher
freqs. I recommend a pair of mje340/350, connected their emitters
with ~500ohm. This gives ~14ma current, so they will need small
heatsink. (Dissipipation ~1watt)
And you could make your vas bootstrapped, means making 2x 1.5k
out of r9, connecting the middle via cap (100uf) to output.
C10 might be too small, increase resistors in feedback/input, or
increase c10.
Mike
You should add a driver stage between vas and mosfets, they
have very big inputcapacity, resulting in high load to vas for higher
freqs. I recommend a pair of mje340/350, connected their emitters
with ~500ohm. This gives ~14ma current, so they will need small
heatsink. (Dissipipation ~1watt)
And you could make your vas bootstrapped, means making 2x 1.5k
out of r9, connecting the middle via cap (100uf) to output.
C10 might be too small, increase resistors in feedback/input, or
increase c10.
Mike
The load for your VAS is just a resistor. You will achieve far better results by bootstrapping or using an active CCS.
Just wondering why you have chosen such low impedance for your feedback network. It will mean that you need >>0.25W resistors for these components.
Ditch R29 and work out R2 so that you balance the currents flowing in the LTP.
I have seen practical success with driving these MOSFETs directly from the VAS with only <10mA VAS standing current. However, It's not a good design. You should either buffer the VAS (making kind of darlingtons with the MOSFETs), or run the VAS at much higher current and use a MOSFET or buffer/darlington for the VAS.
Just wondering why you have chosen such low impedance for your feedback network. It will mean that you need >>0.25W resistors for these components.
Ditch R29 and work out R2 so that you balance the currents flowing in the LTP.
I have seen practical success with driving these MOSFETs directly from the VAS with only <10mA VAS standing current. However, It's not a good design. You should either buffer the VAS (making kind of darlingtons with the MOSFETs), or run the VAS at much higher current and use a MOSFET or buffer/darlington for the VAS.
Hey djdamix;
Is C13 10 micro-farad? If so then this is way to big. In all of the amps that have this capacitor across the voltage multiplier (Q20 in your cct) the value is 0.1 micro farad.
The 10uf as it stands would place a huge load on your driver stage and cause the distortion you described.
Why are the gate resistors (100 ohms) shorted out and going nowhere?
Cheers
Is C13 10 micro-farad? If so then this is way to big. In all of the amps that have this capacitor across the voltage multiplier (Q20 in your cct) the value is 0.1 micro farad.
The 10uf as it stands would place a huge load on your driver stage and cause the distortion you described.
Why are the gate resistors (100 ohms) shorted out and going nowhere?
Cheers
Hi, why will changing c13 change the VAS loading? and why will this 10uF cause distortion?
I think the VAS load is r9//gate capacitance of output FETs.
r33 feeding the LED seems a bit small, 69mA flowing gives 5Watts!!! try something =>4k7. or maybe a 33v zener & cap as intermediate volts dropper?
regards Andrew T.
I think the VAS load is r9//gate capacitance of output FETs.
r33 feeding the LED seems a bit small, 69mA flowing gives 5Watts!!! try something =>4k7. or maybe a 33v zener & cap as intermediate volts dropper?
regards Andrew T.
jaycee=> I use MJE340 because they are rated for high votlage in fact that's the only reason.
MikeB =>I've increased C10 from 10pF to 100pF but it slows the amp too much
quasi =>I've remove the 10u and put a 0.1uF however it is still hard to have 20Khz clean sine wave. The 2nd output pair is not connected on the sim but is connected in reality ...
richie00boy =>I'll put an extra stage : IRF610 & IRF9610 and a 330 Ohm as on www.aussieaudio.com (AV800), I'll try that and tell you the results
Thank you all
David
MikeB =>I've increased C10 from 10pF to 100pF but it slows the amp too much
quasi =>I've remove the 10u and put a 0.1uF however it is still hard to have 20Khz clean sine wave. The 2nd output pair is not connected on the sim but is connected in reality ...
richie00boy =>I'll put an extra stage : IRF610 & IRF9610 and a 330 Ohm as on www.aussieaudio.com (AV800), I'll try that and tell you the results
Thank you all
David
My friendly advice here is that you look at other amp designs. It seems that you have drawn your schematic straight from your head.djdamix said:
Some inspiration can be found here.
If you check my 2 x 300 W mosfet amp, the QRV-01 (link above) is more or less an exact copy of it. Only the transistors are changed to smaller types.
jaycee said:
Ok, I had a look to the datasheets but the are "only" rated for 150V, I'd like to use until +/- 100V so 200V are recquierd that's why I choosed MJE340 but they do seems to be slow ...
However with a current source in the VAS maybe they won't have more than 160V I'll check in simulation.
David
Perhaps if this is a learning experience and you are trying to understand excactly what each component does, a smaller "size" circuit would be easier to work and experiment with. You could use parts you already have, and there is less risk of injury( 200V DC has just a bit of a bite 
), and smaller current surges when you make a mistake. No-one is perfect in that respect. This means that if something does goes wrong, you don't fry ALL of the components you have at once.
Upgrades are inevitable in DIY!
), and smaller current surges when you make a mistake. No-one is perfect in that respect. This means that if something does goes wrong, you don't fry ALL of the components you have at once. Upgrades are inevitable in DIY!thanks for your advice, that's the reflexion I had before starting. To begin I start with only one output pair of MOS with small voltage power supply filtered by small capa. (to keep a few energy amount). Then I had other output pairs and rise up the voltage only then when everything seems OK, I add big caps and measure power output
David
David
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