[CaptBubba]

Yay! My first wall-powered amp that I don't hate hate with a passion.

Pictures here

Key:
6: quick overview, low quality so dial up users can get a feel.
7: side view, pretty pointless
8: higher quality front view
10: rear-side view
14: overhead view of circuitry
15: overhead view of power supply
16: ugg, the actual circuitry below the amp section.
17: Circuitry below PSU, sorry about the blur

Simple Apheared-47 amp with OPA 227s as the gain and OPA 134s as the current boosting opamps, with removable gain resistors. Not much fancy going on in the actual amplification section.

The fun stuff is happening before the amplifier, both to the signal coming in and to the power.

First: the PSU

This PS cost me a grand total of about $6 out of pocket. All the caps, chokes, and the diode bridge was scavenged from broken computers and printers. It outputs +/- 10V with no current limiting, but due to the size of the transformer really isn't good for much over 200mA continuous. Ripple is 24mV. 1.46 fuse on AC input protects aganst short-circuits.

Next: The volume control

Because I can't afford a nice Goldpoint or something along those lines, I decided to kinda make my own. 16 dip switches in two 8-switch blocks make up the control, each switch routes the signal through a certain resistance (either 100k, 200k, or 1Meg), the switches can be used in combination to give just about any volume level that I could ever want. Worlds better than the cheap RS pot that you can buy, even using carbon-film resistors. I have to give credit where it is due though, I got the idea after noticing the level switch on Chu Moy's pocket amp project, and even though I multipied it by 8, the idea is still basically the same.

Because I'm at home and my CD player is in my dorm room three hours from here I can't really comment about the sound except that I can't tell anything really wrong with it. Mp3s played off my laptop still sound like mp3s, so I suppose that is a good thing, they seem a little more detailed, but that could just be me. It is very quiet when no music is playing, which is a good thing.

Oh, and before you ask, there is no off switch. This thing probably draws under 2W, so I'm not worried much.

Total cost (out of pocket): ~$30

Side notes:

Blue LEDs rock, they just do.
Predrilling holes in small pieces of oak makes nailing so much easier.
Opamps work better with they are: a. plugged in, and b. have power.
When all else fails, make sure the volume is turned up.

 

[Budgie]

I like that dip switch stepped attenuator idea. Very cool. It looks like you had some fun putting it together, too.

 

[CaptBubba]

I suppose I should say how much it cost for the volume control. Just at $10, without tax. All the resistors were matched to the limit of my dmm, and it was very easy to solder. It offers 24 different settings. My main concern is durability, I don't know how long those dip switches will last, but at $1.99 each I can afford to replace them, and if they are very short lived I may socket them.

I was going to put this into an actual enclosure, but now I don't think I will. I think it looks good enough to sit on its own like this. To kinda show off the DIY look.

You'll notice the extra space off to the side, behind the jack. I left this free so that I could at some point turn this into a multiloop amp (which is the reason I socketed the resistors).

Does anybody have any suggestions about the opamps to use? I like the 227/134 combo, but because The chips are separate, I am free to play around a bit.

I think just about anyone could build this amp with a bit of patience.

 

[CaptBubba]

Got back and hooked it up to my ns500v.

 

[Budgie]

I like the sound of the 134. I recently tried the opa 627 and 637 as well as the opa228 and I think I may be one of the few people around here who doesn't think they are great. I was swapping them in and out of the output stage of my CD player, and I was unimpressed. Maybe they sound better in other kinds of circuits? (I can't imagine why they would, though) I would be interested in finding out your opinion if you try them.
A friend of mine likes the LT-1364 from Linear.

http://www.linear.com/prod/datasheet.html?datasheet=79

 

[puppyslugg]

Budgie:
Quote:

I recently tried the opa 627 and 637 as well as the opa228 and I think I may be one of the few people around here who doesn't think they are great.

Oh, oh...Incoming!

I preffered the opa2228 to the opa2134. Thought the opa2228 was better balanced then the opa2134. This was in a cha47.

I've tried the lt1365, replacing a tl074 in a x-over and liked the results. But then again, wouldn't anything sound better than a tlo7x? I heard the lt1362 was thought to be better, even with it's lower bw. I think that was the one some people were using in the ART/DIO.

 

[Budgie]

Gonna keep my head down, and cover my ass for a day or two. Hey, maybe nobody will read this thread!

 

[ck42]

Puppy.....was gonna say, all the 1362 preferences I've heard have come from the DI/O list members.

I had the 1365 ready to install but ended up going with Jensen tranny outputs....so no op-amps.

 

[blip]

I love the dip switch idea! I had been thinking about doing the same thing with a rotary switch... but your method gives a lot more percision.

 

[CaptBubba]

Quote:

Originally posted by blip I love the dip switch idea! I had been thinking about doing the same thing with a rotary switch... but your method gives a lot more percision.

Crap, I almost forgot. Thanks for the praise btw. Radio shack has a 2 pole rotary switch which looks like it could easily be converted to a make before break type switch. Simply pop off the bottom plate and remove a little plastic spacer piece and I think it would work. 6 position, but I don't use more than that anyway.

For anybody wanting to use the dip switch idea: my resistor values were too low. I only use the 1-meg switches, I'll have to lower my gain to copensate. Perhaps unity gain.... The only problem is I am in my dorm room and don't wanna buy a big selection of resistors just to get 4 of them.

Edit: nm, I just switched the resistors I have already, now I am at 1.47 gain and it is much better.