IRF9530 as pre output buffer

[wleediy]

I have seen the IRF510 as head phone amp output.
Can I use IRF9530, which I have on hand, as output buffer in the similar configuration to Szekeres amp? Of course, I hope to use 30-100ma instead.
Also, the input of IRF 9530 at 800pf is high, anyway to reduce the effect?

tks and rgds

William

 

[juma]

If you use it in common drain mode (source follower) and keep the output impedance of the previous stage low, input capacitance of the MOSFET is not a problem.

 

[wleediy]

Tks, I think that is the only way. Will try it out and see how.
Also, I always wanted to try a simple amp. Nwo with the most recent op amp sounding very good, May be I can try to use Op amp driving a single IRF type as a simple power amp. If I use just IRF 9530, I will need an output cap, while if I use 9530/530, then I will not need an output cap. Not sure which way it will sound nicer. I just need a few solid watts.

William Lee

 

[juma]

In attachment is a schematic that I've successfully used to drive 50 Ohms headphones. If you have higher Z headphones (300 or 600 Ohms) you can raise the power supply voltage and lower the bias current (by upping the R4 value). That will save some heat dissipation.

 

[Gordy]

...as output buffer in the similar configuration to Szekeres amp?

Experience suggests that this type of simple buffer requires good voltage regulation for best performance.

 

[wleediy]

Tks for the diagram! I am yet to try the sound of regulator to replace 7815/7915. I suspect 317/337 type is better, but I wud like to try a discreate type with max 2 transistors...

btw I am tweaking my pre amp now and playing with some op amps, I find that THS 4012 sounded best, better then 4032 and OP637.

best rgds

William Lee

 

[wleediy]

hi Juma,

I wanted to try using op amp driving the IRF9530 MosFet, and run the output stage with +/- 15V.
To modify your circuit, Can I just have R2 100K that connect the R1 220 ohm input R to the ground, but eliminating the R3 22K. Also, direct couple the R1 to the output of the Op amp. Do you use the TL431 as Zener here as I am not familiar with this device, is this a Zener or triac? how about using an LED to replace?

Tks and best rgds

William

 

[juma]

hi Juma,
I wanted to try using op amp driving the IRF9530 MosFet, and run the output stage with +/- 15V.
To modify your circuit, Can I just have R2 100K that connect the R1 220 ohm input R to the ground, but eliminating the R3 22K. Also, direct couple the R1 to the output of the Op amp. Do you use the TL431 as Zener here as I am not familiar with this device, is this a Zener or triac? how about using an LED to replace?
Tks and best rgds
William

Hi William, in attachment you got schematic for dual PS (+/- 15V). It's even more simple, it allows you to drop input coupling cap and MOSFET's biasing voltage divider. Price, but not very high, is to be paid for higher supply voltage: increased thermal dissipation on MOSFET (cca. 5W) and BD140 (cca. 4W), but that's easily handled with medium sized heatsink.

TL431 is shunt voltage regulator, but I use it here as voltage reference (2.5 V) for constant current source (CCS). You can use LED, Zener or any other means to make your favorite CCS. This circuit tends to sound better with more current flowing through MOSFET, but it's up to you to find a balance that suits your headphones the best.

 

[wleediy]

Tks Juma, seem the current thru the CCS is about 13 ma, so beter use 2 LED in parallel I think or I iwll play wiht it. Even 2 battery may be good!

best rgds

William

 

[juma]

Tks Juma, seem the current thru the CCS is about 13 ma

No William, the current through BD140 is 1.8 V / 6.8 Ohms = 265 mA. That's what I found good for my headphones (Sennheiser HD555).

I don't know what's your load for this buffer amp, but even with the very light load you should bias MOSFET with at least 50 mA. Anyway, this is very simple circuit and you should experiment a little - that's how you'll find what suits your needs the best.

 

[wleediy]

hi Juma,


Oh yes, the bias for the IRF is much higher, being the voltage across the R4 divided by it, I was checking the LED or Ref Zener load, as I have never used LED or work on any CCS.
Will try to build it up and tweak around and report it back to you here!

Tks again and rgds

William Lee

 

[AndrewT]

Hi Wleediy,
the BD also sources some base current into the 1k0 connected to ground. If the FET bias is 265mA and the hFE of the BD is ~100 then the base current is ~2.7mA.
The current through the 1k0 is 12.5mA.
2.7 mA comes from the BD and the remaining 9.8mA is sourced from the TL.

If you substitute a LED for the TL and consider that the Vref current is too high then change the 1k0 resistor to best suit your choice of components.

Remember that a red LED drops just 1.5 to 1.7Vf. The 6r8 must be changed to suit the actual Vref you select.