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Amp #7 - For the Young Sophisticate

post #1 of 8
Thread Starter 
So, I got this *rilly* good pair of cans but they didn't seem to respond to my metal 47 amp as well as the SR-325's did. I decided to build something a little more sophisticated for them.



This is built from another Hammond aluminum box, this one is a 1590BB. We have a machine at work called, aptly enough, the Timesaver. It is a huge belt sander with a belt 10' long and 8" wide. I bolted the the box together and ran it through the belts and the line between the lid and box disappeared. A coat of clear laquer keeps it from oxidizing. I made the knob from 1" aluminum bar stock and the LED is a super-bright red with a clear lens.



Inside is a pair of single opamps with a BUF634 inside their feedback loop. A third BUF634 provides the virtual ground. There are no caps in the signal path. There are 1uF Wima bypass caps at the opamps +/- pins and two huge Panasonic caps for filtering/resevoir. These things are insane - 12,000uF/16V and are 1"dia X 1" high. The amp runs for a second after you power off and the LED stays on even longer before fading out.



Here is it's normal operating position. There is a Markertek cable (purple!) going around the back of the monitor to the line out of the D-25S. The headphone cable goes along the front so its out of my way and free of stress on the plug.

At first I was running the amp with OPA627 opamps but, with only a 12V wall wart supplying power, they didn't really come to life. I switched them out with Elantec EL2044's and WOOOOOOO HOOOOOOOO!!! This is the liveliest bass I've ever gotten from an amp. Everything sounds so alive with this thing! Either that or it's the cans....

I'll have a PDF of it Monday - my Adobe Distiller isn't.

OK!
erix
post #2 of 8
very nice looking amp!

the enclosure has got to be one of the more rugged that I have seen. You certainly won't have trouble finding the volume either, I would wager.

Very nice. I look forward to seeing the schematics!

Driftwood
post #3 of 8
Elantec EL2044? I haven't heard of this opamp being used before!
post #4 of 8
Thread Starter 

EL2044 = Oh YEah!

I'm likin' this...

It's a nice part.

75mA output.
Able to run off as low as +/-2V or single supply.
Unlimited ability to drive capacitive loads (so they say).
5.2mA at idle.
120MHz bandwidth
325 V/us slew rate

It comes in a dual too, I think its a EL2244

ok,
erix
post #5 of 8
Where do you get your Elantec parts?
post #6 of 8
Thread Starter 

Rudy!

But Gerber Electronics in MA is easy to deal with - $25 min order tho...

ok,
erix
post #7 of 8
One must use Opamps With High Input Bias currents with caution with High Impedances on there inputs or the DC OFFSET can change with the Posistion of the Volume Control. When I use sutch a device I Buffer the Noninverting Input with a Pair of JFET's Connected as Source Followers. This allows High value Pots and resistors to be used on the Inputs. Otherwise use low resistence Pots and low value resistors in the Feedback loop. Boy I like that case.
post #8 of 8
Thread Starter 

Hold The Phones!

Well, at least the EL2044's....

On ppl's suggestion (and instruction - Thanx ppl!) I measured the DC offset of the EL2044's and got a max of 60mV at half volume. YOIKS! Needless to say I don't have them in the amp anymore.

It's too bad - they sound really good. I'd like to try to make them work.

ppl said:

Quote:
When I use sutch a device I Buffer the Noninverting Input with a Pair of JFET's Connected as Source Followers. This allows High value Pots and resistors to be used on the Inputs.
So I've been searching on 'suorce followers' and coming up empty. Lot's of references as to why to use them - none on what exactly it is.

Can someone explain this to me in erix terms? eg:

"Take the source of one JFET and connect it to the thing on the other JFET and hook em both to the doo-dad between part A and Slot B..."

What does this do? I mean does it trick the input of the opamp into thinking it's seeing low impedence?

Thanx!

ok,
erix
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