[ECM]

i'm working on buiding the szekeres mosfet amp and i have a couple of questions.

1) i'll be using a dual 8 volt supply to power the circuit. to add gain, i can use an opamp based circuit before the buffer, correct?

2) i added trim pots to the mosfet buffer to get rid of the output cap since i'm using a dual power supply. to couple the opamp circuit to the mosfet, do i need a coupling cap or can i use the trim pots to negate the dc offset coming out of the opamp?

3) if i need to use a coupling cap, can i get rid of the input cap since any DC that is amplifier will be blocked before it reaches the mosfet?

 

[Budgie]

1) Yes, providing the opamp is rated to work at that voltage range.(8+/
2) The bias voltage on the gate of the mosfet will also be on the output of the opamp. The opamp will try to "sink" the voltage, since opamps will try to hold their output half-way between the rail voltages (which is ground with a dual supply). It may be wise to use a cap here to protect the opamp, but you could also use this to bias the opamps output into class A, if you insert a resistor to limit the current into the opamp.
3) Yes, as long as the opamp input doesn't see too much dc on it's input.

I would suggest using an input cap for the opamp and then using the mosfet gate bias voltage to pull the opamps output into class A with a resistor to limit the current into the opamp.
-and always check for dc on the output before plugging your phones in.

 

[ECM]

so i would add a current limiting resistor in series with the opamp output to the gate of the mosfet?

if i then use the trim pots to bias the opamp into class a, won't that introduce some offset at the ouput of the mosfet?

or does the gate require negative voltage for a 0V ouput?

 

[morsel]

ECM, have you seen the M³ Project Announcement? It is more complex than the Szekeres, but you might get some ideas.

 

[ECM]

i glanced over the M^3 project yesterday. it did seem a bit more complex, but i'll go over it once more. still another question. i have an 18volt center tapped tranny that i'm gonna use in the power supply. would it be a good idea to regulate the +/-9volts down to +/-8 volts? i figure the regulator will "chop off" all ripple.

 

[Budgie]

Looks like the problem can be solved by putting the output stage inside the feedback loop like the M3 amp does. I think this goes beyond my limited understanding of how this works. I tend to glue two exsisting designs together, but a design like the M3 is a much better thought out concept.

I would adjust the pots to get the output set correctly, then measure the voltage at the gate and calculate the current into the opamp to see what value the current limiting for the opamp needs to be.

The regulators require the supply voltage to be several volts higher then the regulated output or they "drop out " and quit regulating. I would stay away from regulators with the transformer you are using. If you get hum you can add more filtering. 25 volt caps are pretty cheap.

 

[ECM]

thanks for all the help. i think i'm going to stick to the szekeres buffer without the additional gain stage since i use grado headphones. i realized it was quickly becoming complex and since this will be my first mosfet amp, i figured i would keep it simple. i'm gonna take budgie's advice and also simplify the power supply. i'll post pics when i'm done....i have had the parts for about a year and i'm finally gonna build it.

 

[ppl]

"In God we trust, all others we monitor, jam, or deceive." Scary sig ECM

 

[ECM]

ppl, you SHOULD be scared....it's people like myself that are watching you every day. J/K

actually, ECM stands for Electronic CounterMeasures. We "monitor, jam, or deceive" the enemy's electromagnetic capabilities.

To protect our assets we "monitor" the EM spectrum of a particular enemy and record/analyze their activity.

We are then able to "jam" their RF or IR based threats by either saturating the environment with RF/IR noise or using expendables (chaff for RF and flares for IR).

If we do not want the enemy knowing our whereabouts, we will "deceive" their systems.

But we have trust in GOD...

Added an Avatar...it's the ECM Crow.