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

I am not really concerned with Mosfet heat because I have big, nice Aavid heatsinks, but it seems that the opamp doesn't really need to be at a gain of ten. It might sound even better if a lower gain makes the Mosfet do more work at lower volumes. My Cmoy works fine alone at a gain of 5.
Dan

You and tomo are my szekeres motivators, like jaz is for the pocket amp.. I know tomo and his "CUTE" remarks and the whole function over form, but it does look DAMN COOL don't it? And yea, those source resistors are probably the most important part... got a high noise floor "hiss" in your szekeres? Change to a low-noise high wattage source resistor, I think this is the most important hiss factor. (I'm using vishay heatsinks and/or ohmite to220 style ones - haven't decided which I prefer)

Case prep is so much more involved than the circuit!! Just the base plate - like 18 drill holes that have to be exact, more than a couple mm and you're off finding bigger screws & standoffs... weight balance, heat management, wire routing, it's all part of the "amp" that we don't discuss much... but I built the amp in a couple hours and the case took weeks. Then desoldering the power amp rig to convert it to a szekeres chassis, another couple days. :)

Guess you missed "The meters are Sifam AL20-SQ w/12V festoon lamps".... you can get them direct from UK, there's also one US distributor listed on the website, Selco in CA. The meters - it's alittle more involved in that chu, but close enough. I know alot about this stuff, and then again I don't know crap. The diode drop is too much for line level, so another diode like in a resistor divider to compensate.. or tracking down some near-zero drop ones... and of course it's powered from a single supply so it's biased just like the gainstage on the szekeres and capacitive coupled. I found out also that if I just run parallel wires from the input jacks (including the ground!) that it doesn't vary the incoming signal with the volume.. even though the electrical connection is the EXACT same if I parallel off the back of the pot where they come in. Go figure, I'm a moron but cause & effect experimenting... heh. I'll figure it out eventually.

aos yea man... what can ya do? We're still ok, those caps are everywhere, stocked... and as far as I know the only cerafines they don't make anymore are the dual section ones the tube guys love (LOAs?) The ROAs are still made, as well as the level above those, Silmics. Next time you buy, just buy $100 more than you need. Sit on them. Or lets go in together and buy a batch directly, I have a tax id #. :)

Dan I pretty sure the mosfets are gonna work the exact same regardless of the gain. You can lower the bias divider and the source resistor to calm down that heat, but this isn't push-pull.. it has to move more than you'll ever possibly need to work. For REAL fun, try swapping that source resistor with an 8 ohm. You like your eggs fried or scrambled? Would make a great coffee warmer too... :)

[Edited by Apheared (02-06-2001 at 03:30 AM).]

quote:

---

The meters - it's alittle more involved in that chu, but close enough. I know alot about this stuff, and then again I don't know crap. The diode drop is too much for line level, so another diode like in a resistor divider to compensate..

---

Do you mean like this:

But if you are worried about compensating for the diode drop, have you tried putting the diode after the opamp like this:

While I like the TO-220 style power resistors I no longer have room on the main heat sink tomount the suckers,the chassis bottom is out too.That is where my new power supply regulator will be going and "it aint small"

But I have thought about eliminating the hiss with a capacitor or RC filter across the power resistor,should work but have not worked out the specifics yet

I am about at the point where this sucker will be buttoned up forever and just enjoyed for what it is-a really great amp for Grados.

The final frontier for me is the power supply.I have tried many,too many in fact.But the psu is so important to what this amp sounds like thatI have played with this more than any other part of this design (second was the current pot,could not get it to track between channels and went out of adjustment a lot)

Have been doing a lot of studying and have decided that for this design I will go with a Texas Instruments shunt regulator (TL431) combined with either a power mosfet or power transistor,single 10,000uf and 10-15 uf final filter,then a series resistor and 0.1-0.47 bypass cap for each amp mosfet.

Texas instruments has a web page with software and javas for doing the shunt reg formulas,pretty cool.

It will mean another big ass heat sink but I think I can cram one in.Maybe do a custom job,not sure yet

Oh,may change the volume knob too (hehe,already changed the switches too many times to get "the look" I was after)

What is the topology of your supply ?

[Edited by rickcr42 (02-06-2001 at 09:31 PM).]

Comes in the form of a 10-15 year old radio shack car battery eliminator.I had a bitch of a time opening this baby up-RIVETS !

But when I did I immmediately saw the potential.Way overdesigned.

No fancy designer parts but I think I could toss this baby out the window doing 90 on the highway and it would survive in working order.

Huge power transformer,huge rectifiers with like 16 guage leads soldered directly to some kind of glass encased inrush current limiter (at least I think so,looks to be protection for the rectifiers, charge current draw at turn on can wipe rectifiers out) . Other parts are a 4700uf cap with a 50 ohm 5 watt bleeder resistor.All this is wired up with the parts being connected directly to the output connector.Very heavy guage enameled wire and a heavy steel cabinet that Weighs like three pounds or more !

The line cord is only a two wire cord so I don't have to worry about ground loops

Modifications-a second 4700uf cap plus a inductor of 1-2 mh to make a "Pi" filter

AC rated caps across the power switch and transformer primary

Caps across the transformer secondary and rectifiers

maybe a new line cord and switch,not too esily done-everything is heavy guage point to point and twisted together wiring.I will keep the neon light

Possibly a ground post for a true earth ground (14 guage romex wire spiraled around the line cord tothe AC wall outlet.Would look "killer" if I encased the cord itself in a cloth cover like the old radios,OD green or sh*t brown would be in character

possibly lose the output connector and hardwire in a 3 ft 16-14 guage power supply cord.The other end would have spade lugs to match the barrier strip power supply input connector of the amp.

New rubber feet (missing),big bast*rds

And a paint job,musta been on someones workbench in the garage,took a beatin'--------thinking about black or brown wrinkletex paint put on REAL heavy,would fit the vintage look of the amp,hammertone paint with bakelite knob and simple toggle switches for power and headphone mute.Kinda looks '30s or '40s I think (the gold jacks kinda blow the effect but I am thinking a jack cover that flips down for the fron,screwed in back)

But hell,I got it free and always wanted to use it for something.All the power supplies exept one that I have used with this amp started with this supply

Rick

***************************************************************

JEEZ !

I'm looking at the back and it says "Clifford Industries" on the back,not radio shack,ooops

[Edited by rickcr42 (02-06-2001 at 11:06 PM).]

I have a 12V, 30A LINEAR REGULATED supply sitting in my closet.

It's waiting for when I build my szekeres. :)
It is quite difficult for me to pick the damn thing up.

It has a 450VA Hammond power transformer, 4 HUGE DO-5 stud-mount rectifier diodes, all feeding a discrete regulator circuit with 8 TO-3 Power transistors on huge heatsinks.

It's a beast.

What's even cooler is that it's got a built-in UPS :)

It has a medium-sized 12V SLA cell inside as well.

The thing was used originally to power an alarm system (hence the UPS), and I got it for free awhile back from a friend and it's been sitting under my bed.

I've only ever turned it on once, and have never loaded it.

I wonder how hot it'll get with a 30A load? >:)

- Thingy

Rings like a lunatic. Ringing you can hear, now that's bad. Feeding 2k squarewave pulse tones to it, they go in as "BEEP BEEP BEEP" and they come out "BEEPees BEEPees BEEPees" Again, I don't understand it but I know it's not a Good Thing. Might just be my layout, might still ring even with the resistor there... but I can't hear it so....

Dan (chu) I re-did the meter board... not it's just a standard layout single-supply OPA design. I used Gx11 and moved the trimpots to the volume pot locations... and yea -- DUH -- the rectifing diode after the gain is alot easier to deal, huh? :)

Rick, dunno haven't bothered tracing it out.. the "big beercan" is a single 10KuF, a couple 220s and some small films, there's a TIPx1 on it, and the power monkey is a 2N3055... this gets barely warm with a 800mA load but it looks 'sinked enough for the 3A rating just fine...

Drop the silly meters and the schnazzy rack case and it's a $80 amp... (time effort and blood reduce too - especially blood)

Drop the esoteric stuff and it's a $30 amp, complete.

See what happens when you want a specific thing? All the way down to the knobs and rubber feet were spec'd before ordering anything... even to get real IRFs not generic ST clones from RS. (as if it matters! :)

I need to redo the main board, I have to scoot the output caps farther from the source resistors.. the cap cases stay around 60c and I don't like that considering 85c is max... and I'll probably add another opamp and diode to the meter board and go fully rectified. But again, it's just for looks, as long as the needles dance with the music it exudes coolness. :)

Well, eventually... that's the plan anyway... the case is a pain to open, the back panel has to come off to remove the lid so you need table space... and my bench is littered with BJTs working on the 200mA discrete portable Class A.

Apheared, in the meter drive circuit, since the opamp runs from a single supply, is there an output capacitor? What value?

[Edited by cmoy (02-26-2001 at 07:06 AM).]