Case prep, especially when doing faceplate work... that's more time intensive... I don't do it all at once unless it's a plastic pocket amp, I spread it out over a few days or weeks...

And its not that expensive... even with the fancy case and meters and EVERYTHING it's still under $200. Yea, I've got a couple grand tied up in this "hobby" and most of the time is spent doing math not soldering or drilling... but I'm having fun! :)

the schematic is pretty much verbatim to the DC coupled gain stage one on the sezkeres project page... I scaled the crap outta the caps as stated, but that's it. I left out the output cap drain resistor, it actually thumps LESS without it. (but I'd unplug the headphones before turning on/off, or use a switch like I did - just because)

Also I'm rethinking the sealed box... I need to emulate a hot summer day and measure the temp again... I'm gonna sit it next to the radiator for a bit, 6" away it's 105f. I think I may need to attach the heatsinks to the lid with countersunk panheads. If that's not enough I'll take them off the protoboard and mount them directly to the rear ala TimH's version.

The schematic for the meters... no need, it's easy. take a dual opamp, wire some diodes to the inputs so when you turn the main volume pot down it doesn't decrease the signal, then either mathmatically calculate the diode drop and the boost needed to make 1.228V = 0.775V to come up with your gain setting, or do like I did just wire it for Gx11 and use input pots... feed it a 0dB test tone off a test CD, measuring the line level... turn the trimpots until it's calibrated.. this way you don't need to rectify the signal either. It's all just for looks anyway, there's nothing going out from here. :)

Someone explain the bias thing to me, please. My skills are pretty much nerbie and I'm lost in how, without the resistor divider, to control the stupid thing.

The voltage at the non-inverting input is therefore = 15*(220)/(220+100) which is approximately 15* 2/3.

Since the 4.7 uF cap near the op-amp inverting input is disconnected at DC, the opamp effectively sees only a 100k resistor to the output and therefore acts as a voltage follower at DC. Hence the biasing voltage of the mosfet is controlled by the voltage divider sitting below the input.

Btw - did you mail order all the caps? The only local place for esoteric caps that I know of is Lee's - only WIMAs and Orange drops, small capacitances as well. Where do you find surplus power supplies?

[Edited by Joobu (02-03-2001 at 07:29 PM).]

So the biasing of the opamps controls the mosfets too? I think I get it... since C1 isn't there blocking the DC from the opamp output, the 2nd resistor divider is redundant?

OOOH... ok now I just need to find some musical opamps that will take more V supply then... I want a 10V version. :) I guess I could power and bias them separately anyway... was just lost on how the mosfets were biased in this layout. I suck at all this. Back to the books. Thanks. :)

Yea man I mailorder everything... you have to, I mean I live in Manhattan but RS is about my only refuge for parts.

I keep running lists for what I want from most distributors, once I cross that "need it now" point, I submit my order.. If I'm ordering from overseas I tend to buy much more than I need so I never have to do it again.

supplies, check alltronics (not allelectronics) mpja, bunches of others... just stuff in "surplus power supply" into a search engine. I think Thingy has a good list, he's currently shopping for this stuff... maybe he'll share.

Of course, you can just change the value of R4 to control the quiescent current as well as altering the op-amp bias.

I am somewhat puzzled why R1 needs to be in the DC-coupled version. I would think Rg is sufficient.

"ok now I just need to find some musical opamps that will take more V supply then... I want a 10V version."
- What do you mean? Most op-amps I've looked at (other than low-power op-amps) can take up to ±15V, which means they're suitable for a 0 to +30V supply.

[Edited by Joobu (02-03-2001 at 09:35 PM).]

I too wondered that about the R1/RG... since there's no other connections it's just series resistance, so why both? Also R1 is the same value as the one to ground, maybe this is by intent? I'll bypass R1 and just run it thru the 220 ohm gate resistor and report back. The case is still open while I re-think my thermal management options.

[Edited by cmoy (02-04-2001 at 01:55 AM).]

Yea a diode per channel, else the meter response adjusts with the volume control.. anyway a 1k for the -in resistor and a 20k trimpot with the wiper tied to the low end for the fb resistor (I think a 10k woulda been enough though)

The meters are Sifam AL20-SQ w/12V festoon lamps... pretty expensive, $49 ea... just one meter costs more than the entire amp. But, they're made ready-to-go, for a 600 ohm line you just run a series 3.6K to the input and it's cailbrated. You can download the PDF on em from Sifam's website, only think is you have to do this long involved registration to actually get the freakin files...

BTW, which model Sifam meter is that? Where did you get it?

[Edited by cmoy (02-04-2001 at 06:20 AM).]

Have you built one of Kevin Gilmore's new amps yet? Do you plan to? Would like to know what you think, oh Mad Amp Builder!

Apheared, you have done some awesome job. You even have meters. My Szkeres amp does not have meter or indication lamp. Mine does not even have volume control (barebone output stage). I check whether if my amp is on or off by touching heatsinks. ...

Tomo

P.S. Apheared needs Portable Mr. Fusion just for his ultimate class-A amp.

Joobu, Lee's has also carries the full line of Solens (even the huge 68uF ones), and don't forget WIMA Black Box for the ultimate price ;).

[Edited by aos (02-04-2001 at 09:56 PM).]

And hey, thanks again for ignoring my request about what gfx/cad prog you use to make those diagrams. :)

aos, hmm damnit.. yea, same here... some are from sonic/tpc, some are from percy. Yea he takes awhile, but you do eventually get em... and if I learned anything from my stupid potentiometer adventure back in the beginning - this is not a hobby for the impatient. :)

Ben, yes. no. maybe. I dunno. It's a normal enough ss design, and I can alter it to be battery powered and drop the voltage and the current some, but kevin picked these components and values for a reason and I don't know how morphing it into a portable will work. Either way it probably won't compare to the AC version. I plan on ordering the parts though.

BTW, what model Sifam meter did you use? Where did you get it?

If I add the opamp to my Szekeres is it ok to back off on the opamp gain? The Szekeres seems to need just a little gain to work well. I've use a preamp before it and boosted just a little and it sounded great. I was thinking of a gain of five. Then the Mosfets probably wouldn't get so hot too.
Dan

Gain---guess I may have been misleading in our correspondance on the gain thing,not by intent,I just don't have that problem even straight out of my tape out jack.All my "sources" have good voltage and drive (those that did not DO NOW!)--in fact they drive 20 ft of interconnect just to get to my Szekeres amp
Average output is around 3 volts per

As far as heat goes,I addressed this with a big ass heat sink bolted to the INSIDE with holes punched in a 4X5 grid over it-still gets warm .

Power supply-man have I changed this a kazillion times.I finally have it down to the before mentioned "ancient" radio shack unregulated car battery eliminator,tad under 14 volts at five amps--to an LC "pi" filter using HUGE caps (lower voltage version of the zen amp psu,same values) and then --and this is where it still gets weird--either a series pass discrete regulator or a shunt regulator (parallel)---

Then we get into the actual AMP BOX (yes,two box setup for varios reasons)-series inductor of 100uh at 3 amps to 10,000 uf/10 uf/0.1 uf,one for EACH mosfet

All else is as it was last summer,only foolin' with the psu lately

GAWD THAT SUCKER IS BIG !

Mine is puny in comparison (3X8X6 amp,3X5X5 PSU),in fact my ZEN amp only comes in at 12X8X2 (X7.2 with the cage),BOZ is only 12X8X2,DAC is in two 5X5X3 boxes,all puny compared to this beast

And the headphone "mute" switch is a must,mine went in on build V2

Funny thing-I had those real cool aluminum cased power resistors in mine but like the sound of the Allen Bradley carbon compositions I got in there now (hand matched of course),a little warmer but still clean.

Letsee,oh yeah,caps

Angela film and foil in,solen fastcap and orange drop bypassing each mosfet psu,solen across the output elna

and.....Radio Shack mosfets,jacks,switches,wires,knob,chassis........

But the PAINT IS RUSTOLEUM !

charcoal gray hammer finish

and not to forget,dual inputs in parallel so I can jack in from my listening position or "daisy chain" sh*t off it.I have a Record Out "kill switch" on my preamp so there are no interaction problems and the BIG RIG can be used independantly

More information than anyone really needed ?

I tend to do that,and since I am easing back in from a self imposed semi exile I did not want to dissapoint anyone .

Good job man,I like the look a lot and

THOSE METERS RULE !!! AS does your proliferation of amps,not to mention the sheer dedication you have shown in a VERY short time (check his profile guys,you'll sh*t)

All the best man

Rick