[OogeleyBoogeley]

hey all,
i finished my meta42 last week and it sounds great!!! the only thing is that the volume output is very weak. when i use my slimx with it (vol 40) the loudest the amp will ever get is the output on the cdp. the way i found this is by plugging in the amp, then the cdp and comparing. i have the amp plugged into my cd deck, the volume is more than adequate. is it the slimx or a possible problem with the amp?
also- the blue LED im using is incredibly bright. its a 5A led, and since im using 2x 9v, so using ohms law i got a 3.2 ohm resistor. i did that, and its still incredibly bright. any ideas on how i can fix that?
thanks in advance,
dan

 

[Arzela]

If you use a higher value led resister,
the led will be dimmer.

Are you sure the led is 5 Amps?
This seems very high. Can 9V batteries output that current. ? Maybe this is forward
voltage?

BTW: here's a handy web page:

http://us.switchzone.com/lamps.asp

 

[was ist los?]

What is the gain on your meta? Try a 10k ohm resistor for the led.

 

[OogeleyBoogeley]

sorry bout that, the gain of my amp is set to 5.
as for the resistor, thats what i remember, its a radioshack led that i had on hand when i built this. hope that helps.
dan

 

[OogeleyBoogeley]

oops

the LED was rated at 5 volts
silly me, i guess i need to recalculate...

 

[Arzela]

I would start with 5k or so. Then go higher
if it's too bright.

 

[OogeleyBoogeley]

i stuck in a 10k resistor, and i glows very nicely. it no longer burns out my retinas when i turn out the light

anyone have any suggestions as to why the volume is so weak? btw, the parts im using are (if this helps)
-8620 opamp
-el2001 x1
-18v batteries
-cascode to class a
-gain set to 5
hope that helps
-dan

 

[OogeleyBoogeley]

GAH, when checking for voltages, i think i blew out my opamp chip

is there any way i can check to see if its good or not? i guess working on amps when im tired isnt a great idea.
-dan

 

[Arzela]

I blew out a chip once. It actually
started smoking....

Anyway, check out tangent's site
for an op amp tesing circuit.

http://tangentsoft.net/audio/

 

[KTpG]

I also blew out a chip, and nearly melted down a TLE2426 rail splitter-- hat thing burned my finger to touch it.

What headphones are you using? Gain of 5 is kind of small if using anything with a fairly high impedence.

You say with your CD deck, volume is fine? Is this form the cd-deck headphone jack output? in that case, this would probably mean the singal is already amplified once and then is being amplified again by your META. If using the slimX, are you using a line out or the heapdhone jack out? I am not familiar with this device. Generally, line outs are line level and have little to no amplification, so, if gain is so small, you may not be getting much sound because of that. If you plugged it to the already amplified output, you may get more sound. BTW- sorry to hear about the chip... opamps are particular about testing them. When I test for current draw, I get this odd distortion and hiss like you would not believe.

 

[tangent]

Quote:

its a 5A led, and since im using 2x 9v, so using ohms law i got a 3.2 ohm resistor.

If it was indeed 5A (it's probably 20 or 30mA in actuality), that only tells you what its current tolerance is, not how high you must run it. Just for future reference, you generally run all LEDs well below their maximum level. 1 to 2mA is generally sufficient for most purposes.

As for the volts rating, that just tells you the minimum voltage you must put across it to make it light up. This is only partially related to the issue of picking a proper limiting resistor. The best practice is to just try different values until you get a brightness level you like. Don't try to engineer the thing. Save that effort for more important things.

Now, back to your volume problem: when you had that tiny resistor in series with the LED, I'm not surprised the amp sounded bad. You were nearly shorting the power supply out, which meant it couldn't do much besides run that LED circuit. Now you've fixed that and the problem remains, so I wonder if you damaged something.

You could try my op-amp tester, but it's only suitable if you use a true dual power supply or the TLE2426. If you don't have either of these ready at hand, finding a cheap replacement op-amp locally may be a better idea. Since you live near a Radio Shack, I suggest that you go get a TL082. It won't run well on a single 9V battery, but at low volume levels, it should at least tell you whether your current op-amp is working properly or not.

EDIT: One other thing: you claim you got a 3.2 ohm resistor from Radio Shack. No way. <shakes head> Either you're remembering the value incorrectly, or you've shifted some decimal places. If you've treated all the other resistors as lackadaisically, it might be good if you a) told us what resistor values you put in the amp and b) test each one to find out if that's truly what value actually went into the amp.

 

[OogeleyBoogeley]

sorry, actually the low impediance resistor i got from mouser, it was a vishay/dale.
the headphones im using are a pair of grado sr-80s.
im going to test the opamp sometime soon. what exactly is happening is: when i turn it on, it sounds fine for a few secons, then shortly after turning it on, the sound starts to break up. whenever i touch my finger to the ground on the chip, the sound is fine again. my amateur opinion is a ground problem when i tested the voltage at one of the pins. when i did that, the amp was on, and the chip was in the socket (doh!)
meanwhile, ill look for a replacement chip that i can use as a temporary replacement. i will probably order another ad8620 from tangent rather soon

thanks for all the help,
dan

 

[KTpG]

Did you use R2? I failed to put it in, thinking it wouldn't harm anything, and it did. My sound would be fine for 1-2 sec. then would cut out, cut back in, etc. If you do not have it in there, stick a 100k or so in! Now! Like Tangent asked- what values did you use all around?

 

[OogeleyBoogeley]

r1- 1k ohm
r2- 47k ohm
r3, r5- 22.1k ohm
r4, r6- 16.9k ohm
right now im using crappy resistors in r1, r2. the vishays i ordered from mouser are on backorder, so i stuck in some crappy radioshack resistors in place. since i damaged the opamp before i fixed the led, im not sure if there is a sound quality change.
thanks,
dan

 

[tangent]

Quote:

whenever i touch my finger to the ground on the chip, the sound is fine again. my amateur opinion is a ground problem when i tested the voltage at one of the pins. when i did that, the amp was on, and the chip was in the socket (doh!)

You may have killed the chip, true, but in general chips can take a fair bit of abuse. This isn't me telling you to beat your chips up, but simply that chips will often survive the occasional stupid short connection. Try the TL082: it'll cost you $2 to try, instead of $20 to try another AD8620.

I'm more in favor of an intermittent ground fault. It can either be that a low-impedance path to ground is higher than it should be (this is KTpG's issue, fundamentally) or something that shouldn't be going to ground being instead a high-impedance path to ground, so that it's not a dead short, which is why the amp runs for a few seconds before dying.

Quote:

Did you use R2? I failed to put it in, thinking it wouldn't harm anything, and it did.

Op-amp inputs need some DC path to ground. You could instead have jumpered C1 and also fixed the problem, since that lets DC drain from the inputs back through the pot. This is what the "input offset voltage" spec on the datasheet refers to. But fundamentally, R2 should still be there, because for DIY audio purposes an excessively high input impedance risks noise coupling into the inputs. It's the same issue as why we don't like pots over 100K.