[rickcr42]

Quote:

rick42, does your amp use an output cap?

sure does.1,000 uf elna in parallel with two 10uf SCR.

This for me was a better option than the dual supply with DC balance pot which by the nature of an adjustment means it could drift AND the cost for me of a dual power supply considering how much experimentation was involved would have been prhibitive.

At present I have a pre-regulator running at +30 volts followed by CLC filtration and a final shunt regulation stage for each mosfet at +24 volts.My source load is not a resistor but an inductor which raises the efficiency a bit though i can't say it sounds better than the simple power resistor.

JFETS and MOSFETS seem to like power and for me +12 to +15 volts wasn't getting it done.
But for a first build i would go the simple route with a basic DC supply of 13.6 volts@3 amps (your basic car battery eliminator supply) then the LM317 regs at +12 volts and keep the drain resistor in the 250 to 300 ma range.More is not rewally needed and I found this through using a 40 ohm 5W rheostat so i could "dialk in" as much or as little current as i wanted (inline with 10 ohm 10 watt resistor just in case i got careless and hit "0" ohms )

This particular amp sounds pretty darn good built "honest",straight from the headwize project library, but being so simple leaves a LOT of room for experimentation later on without undue complexity.

Once you KNOW the amp amd how it sounds ,then it is time to fiddle with current sources,power supply upgrades and mosfet gate isolation

 

[tangent]

Quote:

Voltage drop * current draw = wasted heat (in celsius)

You're collapsing two steps there. The answer is in watts, which have nothing to do with any temperature scale. You get temperature from the thermal resistance numbers, which are typically given in degrees C per watt. So yes, if you're numbers are in C/W, then 1W = 1 degree C. But if your thermal resistance nubmers are in degrees Fahrenheit per watt, you will be confused if you expect volts times amps to equal degrees C.

 

[silvervarg]

Thanks for correcting me. Looking back at my post I wonder how I could make such a misstake.
I did get it right in the example though.

 

[ECM]

can you get the same results as the dual jfet configuration rick42 used, using an opamp to isolate the MOSFET?

i have grado headphones so i'm not in much need of gain, but i would like to have that option later on and using an opamp will allow me to dial in some gain if i have to.

 

[silvervarg]

Well, perhaps not exactly the same, but close.
If you chose an OP-amp with JFET input it could be very close.

Will it be something like the DC-coupled gain stage in the Addendum of the Szekeres amp? (See image of
this DC-coupled gain stage).
With the difference that you change the 100K feedback resistor to something a lot smaller (e.g. 10K).

I ponder if I should build something very similar to this, so I would be interested if you wrote your progress and experience here.

 

[rickcr42]

Quote:

i have grado headphones so i'm not in much need of gain, but i would like to have that option later on and using an opamp will allow me to dial in some gain if i have to.

you can also do this with a jfet or even better a jfet/transistor compound amp stage (see the link to the national semiconductor jfet hanbook or the marshall leach "wideband preamp" file-both available in the DIY links section above)

My problem with using an opamp is I can not mentally get around adding such a complex circuit with many stages to the most basic and in my mind "pure" Class-A single device single stage buffer amp.It is the very simplicity of the design that makes the sound good and all attempts to keep the entire signal path a full class A are more inportant to me personally than the universal "plug it in and it works" ease of use nature of an opamp.with discretes you can choose where each element operates (bias,voltage,DC/AC,etc) but with an opamp the choices are made for you and exept for some class a biasing of the output are not user selectable.
the only "opamp' i would personally even consider using would be either the Pass Discrete Op Amp (jfet/mosfet :this one is mostly theoey and you must do some work on your own to optimise) or the Forssell 990 all fet class-A discrete opamp (pretty much verbatim ready to build and can even be assembled on a board that can plug into an opamp socket as a discrete replacement)

just some more options for you but again,some unsolicited advice :

Build the Szekeres "honest".that is straight from the project pages and with no modifications first to get a handle on the amp and optimise that stage for best sound.One you accomplish that you can then fiddle and tweak to get what you want or need from the design.
to make it complex from the start can have you pulling out your hair tring to track down hiss or hum (very power supply and layout sensitive) but with a single element /single stage it is easy falling out of bed (ouchy

)

 

[rickcr42]

I might add : really blows my mind how the obvious is sometimes overlooked by so many but this amp is one that a lot of folks overlook a simple truth.

Mosfet class a output stages can be looked at just like a class A triode and the same rules apply.Think about driving a SET output stage and apply that to the szekeres That is all I will say,gotta keep some secrets back but that should get the creative juices going......

 

[ECM]

rick42, if i were to use the opamp, i would bias it into class a because i was thinking along the same line as you were in keeping the entire path class a. what i didn't contemplate was that an opamp is indeed made of many smaller circuits inside the chip and that is the "opposite" of simple.

i will take your advice and build it as simple as possible first. then if i like what i hear, i'll do the other mods and input stage.

i'm curious as to how you were able to use an inductor as your source resisitor. once the field is built up, doesn't the impedance go to zero? and how does that improve efficiency?

silvervarg, i'll keep updating as i go. right now i have the voltage regulator and the amplifier circuits built. i'm debating whether to mod the regulator for higher voltage or to aquire a smaller transformer. i'm leaning towards a smaller transformer because of space constraints. i'm in my last month of college and work (i'm getting out of the Air Force) so i have plenty homework and paperwork to do between now and december. i will not be able to work on it as much as i would like to.

 

[rickcr42]

some background :

http://www.tubecad.com/articles_2002...ers/page3.html