[jboehle]

It's unclear from the schematic. I am using 2 9v batteries and creating virtual ground from the midpoint between them.

Do I tie input ground, output ground, and virtual ground all together? That's what I've been doing, but I get hum in the headphones. DC offset isn't *too* high, 17mV/28mV.

Also, I am using the matching RS pot. From the back my current hookups look like this:

HTML Code:

[left]unused-> + +-+[color=#000080]<-gnd out->[/color]+ +[color=#000080]<-out in->[/color]+ +<-in[/left]

My batteries are both good. One has 9v the other 8.9v. Both alkaline.

I've tried building this on plugboard and on protoboard, both versions have the same hum. Obviously I'm not doing something right.

Any pointers would be appreciated. I can post pics if you think they'd be useful.

-Jason

 

[MisterX]

Yes, all of the grounds connect to the "virtual ground."
Remember ground is relative not absolute.

Does touching any of the grounds make the humming stop?

 

[jboehle]

No, the humming is constant no matter if I touch the volume knob or not. It does get quieter if I touch the volume knob. It also increases in volume as I turn the volume pot up.

Here is a pic of how I have it laid out. Don't hesitate to point out my mistakes!

-Jason

 

[jboehle]

Oops, I screwed up my pic. C1 is a .15uF tantalum, not 10uF tantalum.

-Jason

 

[jboehle]

Also, OUTPUT GROUND is connected to basically the same spot as INPUT GROUND is.

-Jason

 

[bpribadi]

What you need to do is to shorten the input wire, or the best solution is to use coaxial, shielded cable for the audio input. The hum comes from the input signal, then amplified by the amplifier.

Try to make the connection between the source and the amp input as close as possible, use shielded cable or coaxial, and for my case, I wrap the pot with allumunium foil (cover the terminals with paper or plastic tape), and ground the allumunium foil. Guarantee, no more hum!

 

[rayofsi]

Quote:

Originally Posted by bpribadi What you need to do is to shorten the input wire, or the best solution is to use coaxial, shielded cable for the audio input. The hum comes from the input signal, then amplified by the amplifier. Try to make the connection between the source and the amp input as close as possible, use shielded cable or coaxial, and for my case, I wrap the pot with allumunium foil (cover the terminals with paper or plastic tape), and ground the allumunium foil. Guarantee, no more hum!

I dont think cables are the problem in this case.
http://www.redcobracable.com

 

[jboehle]

The hum is there even if I have no input source hooked up to the amp...

-Jason

 

[Earwax]

Quote:

Originally Posted by jboehle The hum is there even if I have no input source hooked up to the amp... -Jason

Does it hum with a source connected and on pause? I think because there is no external ground/earth connection the hum on these amps is actually worse without a source connected.

Also grounding the body of the pot to virtual ground is a good thing, but I don't know that it's the cure for what you are hearing.

 

[einolu]

that seems like a nice working layout you got there, a little cmoyesque

but it should do the trick. tell me if it pops like crazy when you are finished. Mine does

. One think you might want to try is mounting the solans onto the board since. I had them loose in the beginning, but I realized that its a little fragile, and then I hooked them up to the board. I can take a picture of my amp if you want, since I havent cased it yet (need a drill NOW!)/

 

[jboehle]

I actually do have them mounted to the board, I was just trying to keep the pic simple...

So anyways....I was a little fed up with my uncased clone and the hum it was producing, so I looked at the 'grado portable amp' thread that was posted recently and decided to build myself an A47 (or is it CHA47 or some other variation?). I used the first layout posted in this thread: http://www4.head-fi.org/forums/showt...threadid=31941. I left out the input caps, though. Thing is, after I completed the build, it had the same damn hum that my clone did! ARRGGGGGGGGHHHHHHHHHH! So I figured **** it I'm going to case it up and see it if it still has the hum. Lo and behold after casing it up and shortening all the input/output/power wires to fit well inside the case, there's *NO* hum. I mean absolutely NONE....turn it up all the way and no hiss or anything. I'm using 2 OPA2134's in it at the moment, because it's what I had on hand. I'm going to try it with some OPA2227s or OPA2228s when I can get them. Also going to build Tangent's op-amp tester circuit to see which of the NJR4556ADs I burnt up trying to build the clone, and if I have 2 left over that work I will try them too.

Too early to give a review of the amp. I'm going to take it to work and compare it against my self-built PPA. I will post some pictures when I post the review.

Any other suggestions for op-amps to try in the A47 circuit?

Also I think my clone build might've been subject to whatever interference or mistake on my behalf that the uncased A47 was, so once I get some more jacks and a case I will case up my clone as well and see if it is as silent as the A47.

Oh, and BTW, I do get the big-ass pop on turn on with my clone - from everything I've read it's a side-effect of the design of that amp. IMO, even though others will disagree with me, I just don't think it's a problem. I just can't fathom a respectable company like Grado releasing a product that would physically harm it's own line of $600+ RS-1 (or even the more expensive PS-1) headphones. I guess I'll take my chances with my SR-225s when I get it all cased up and see what happens...

-Jason

 

[MisterX]

Yup, Grounding the case (and the pot via the case) solves all kinds of problems with humming.

Glad to see you solved the problem. Can't wait to read the impressions of your new amp.

As far as what opamps to try in a 47 amp..... everything.
I like the 627s myself but that kinda kills the budget on said amp. The OPA204's and OPA602s also sound very nice in that amp.
Much more so then the OPA2227 IMHO

 

[bpribadi]

Quote:

Originally Posted by ufokillerz I dont think cables are the problem in this case. http://www.redcobracable.com

Well... I had hum problem with my CMOy also, and cable was the solution. Beside that ground the metal casing will also help, as it will "shield" the whole circuit. So the point is, every audio signal is better to be shielded, the input signal is the most important.