What about a rickster legacy HA (it is like he is kicking me in the ass again!)? Here is a suggestion that he made to me a couple of years ago. I built it (using the original Szekeres schematic - Rick never found his drawing) and think that it does what we want. I am sure the gurus here can take it to the next level:
Re: AKG701
« Message sent on Jan 21, 2008, 10:01pm »
Sorry for the extreme lateness of the reply but I have been pretty much out of the "virtual" world entirely due to the pressing need to focus on the real world.....in other words been working my ass off to keep my way of life from going over a cliff and have had no time/desire to pursue any leisure activities no matter how much i would like to.
Anhooo.....
Will try to dig up the actual schematic which is hand drawn and get a copy to you when i get a chance but in the meantime a couple of points to consider :
1-the Szekeres is a ZERO gain device being a high current voltage follower which in laymans terms means an impedance converter (high impedance input to low impedance output) with enough current to drive a very low impedance transducer,the actual headphones,without loss of information.This means if your headphones are not in the 25-100 ohm area the Szekeres is maybe not the best design it being purely a current gain device.If you are unsure if you need the extra OOOMPH or if in facxt it is voltage gain you need the test is -
do the cans play loud enough but the bottom end to low mids sound a bit "light" ? If yes then the Szekeres may indeed be what you are looking for.If it is a lack of overall volume levels that you need then look to another design,one with pure voltage gain which in an ideal world (for me0 means a single ended triode design with an output matching transformer suited to the impedance of the actual headphones the amp is to be used with.
2-The reason for the JFET on the input is to isolate the gate of the MOSFET which being of a relatively high capacitance may/will have a problem with treble bandwidth due to the interaction of the driving device output cable and the input level control which will in combination with the previously mentioned gate capacitance will form a "classic" single pole low pass filter creating a top end rolloff.
By using an input buffer the actual intput to the amp will be a high impedance creating an easy to drive amplifier while at the same time creating a situation where the MOSFET itself "sees" a very LOW driving impedance.While the R/C single pole low-pass filter is STILL there (as it would be/is in ANY audio device) the "pole" of that filter is shifter so high into the ultrasonic range there will be zero effect on any adible frequencies hence you can say the filter is non-existing
3-Even though the amp is about as simple as it gets it is VERY dependant on what you power it with (piss poor PSRR due to the single ended nature) and the passive parts used to build the amp with (again due to it being single ended there is NO cancellation of signal between the negative and the positive sides so anything that goes in comes out pretty much intact-including the sonic signature of the surrounding parts)
4-layout is critical for best results with point-to-point wiring being the best method sonically.i can help with this also
bottom line is IF you have the voltage gain already in your system to drive your headphones to normal listening levels without hearing any strain and IF your headphones can make use of the additional current this amp is about as good as it gets and is my reference driver for all Grado cans and my sony MDR-2506 studio monitos.
Delicate highs,a "hint' of warmth plus enough slam to handle power rock or orchestral climaxes without sounding strained
good luck
rickster
(and if i slack on this bounce me another PM 0
Re: AKG701
« Message sent on Feb 2, 2008, 5:58pm »
"the Szekeres virtual engine with much lower power supply demands. Is this in line with your experience? It looks easy enough and I have enough gain elsewhere"
Yes and no.Richard Murdey (RJM) sometimes "can't see the forest through the trees" and so comes at things either from the the wrong direction as with his first attempt at a Szekeres Amp
www.geocities.com/rjm003.geo/rjmaudio/diy_head.htmlwww.geocities.com/rjm003.geo/rjmaudio/diy_head.htmlwww.geocities.com/rjm003.geo/rjmaudio/images/head_ps.gifwhere he not only has ZERO regulation even though the entire circuit is run Single-Ended Class-A but has the front end all wrong so he does what ?
Not clean up the circuit for full fidelity mosfet gate drive or add a proper supply but scrap the whole idea of a simple SE amp with good sonics in favor of one with NO front end to properly load the mosfet then adds the "flavor of the month" with the amb/tangent/etc crew "Active ground channel" that to be honest I am no fan of since it brings on WAY more problems than it cures and is seemingly only used by that crowd so they can copy the designs of others,add the third active channel then call it a "new" design with their name on it.
attach that puppy to a switch with a common ground and it blows up.Momentarily short the output by plugging in/unplugging headphones while the circuit is on ? blows up again......
Nope.The answer is proper design technique not fancy "workarounds" and if the output cap is a bother,and beleive me it is no impediment to good sound ( even though it LOOKS WRONG according to modern high fidelity audio convention ) sometimes the cure is worse than the disease and in this case it is.
The basic szekeres really is a wonderful sounding amp if you follow some basic rules and even Greg (the designer) will tell you all the variations in the articles addendum were done because folks just wanted them added (were always asking him for the additions) NOT because it made for a better sounding amp and the genious of the original article is he gives hints on where to take the amp but does not go into detail leaving it up to the builder to decide what the amp will be.
A "Quick and Dirty" rat killer amp would be to lose the volume pot entirely if the amp follows a device already having one and that provides a low output impedance (CD player,preamp,etc.) and replace it with a 51K or 100K fixed resistor across the input jack (from hot to ground) to set the input impedance then powered by a "can be purchased anywhere" Car Battery Eliminator linear regulated (NOT switching and NOT unregulated) of at least 5 Amps or so that you can likely borrow from someone just to see if the amp is to your liking and if so can then set out to do a proper power supply which to be honest is where the cost comes in to this "simple" amp
rickster