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Another newbie with lots of questions

post #1 of 10
Thread Starter 
Hello all,
I have been going through the posts here and headwize for some time to decide upon a DIY amp for my Grado SR325 and SR80 phones. Must say that there are some very knowlegeable ppl out here.
I have a few constraints for the amp i want to build
-have never built an amp before so I dont want to take up something very complex.
-My budget is around 120 USD for a battery driven amp as I cant spend more than this now.
-I'm not looking for portable amp and size is not a big concern.
-The amp should be able to run on 2 9 volt batteries.
It would be great if somebody could clarify my following doubts keeping the above constraints in mind :
1) Can I build a an amp within my budget which can match some good sounding commercial amps like Corda HA-1, Creek OBH11 SE and Grado RA-1?
2) Was thinking of building either CHA-47 or Corda amp with high quality parts. Which one should I go for? Though I have only Grados now later I might buy some high impedence cans also. So I want to amp to be versatile.
3) most common question. Which OpAmp ???. I know that opa2134 and buf64 (CHA47)combinition is recommended for newbies as these are easy to deal with. But I wanted to use higher quality parts since at least for now I want it to be my only amp (i hope i dont get sucked into this DIY stuff ).
The epamps I have read about here (but no idea about which to use) are opa2134, AD823, AD845, LM6171 (corda), Opa 627 (headroom). Which opamp should i choose?
5) How much do these Opamps cost?
6) My location makes it tough to get any of these opamps. Could not even find opa2134. Are there any sites which would ship these components internationally for a reasonable price?

thanx for you help
RawHit
post #2 of 10
hey rawhit, nice to have you here.

First, if you arn't worried about pertability, you may want to use AA batts instead of 9v ones, as they last longer and are a bit easier to find.

1. Yes. The RA-1 actualy uses a design similar to the popular CMoy amp. The two biggest issues that can keep a DIY amp from being all it can be IMO are layout and soldering. Expensive parts without good layout can sound awful, while cheaper parts and a good layout can sound great (the RA-1 is an example of this, as it's parts are not the best, but it has a well designed layout)

2. If this is your first amp it would actualy be my suggestion to buy regular parts, but double the amount you will need. This way you can build your first amp and get experience troubleshooting, soldering and laying out parts, and then you can built the second amp and put those skills to use. Both those amps are good amps.

3. I would suggest you build using sockets. That way you can start with the easy to use 134, and later change out opamps without having to take out the soldering iron.

4. The phsycic emenations are coming to me...you ment to ask, ment to ask....ahhhh, so close...(did you forget a number or were you just messing with our heads? )

5. It depends on the model. I got my opa134s from digikey for around $3 each. Some people around here talk about experimenting with opamps that run in excess of $15-$20 apiece.

6. Sorry, but i can't be of help with this question.

any more questions? Don't hesitate to ask.
post #3 of 10
Quote:
My budget is around 120 USD for a battery driven amp as I cant spend more than this now.
If that includes tools, you might not be able to make it unless you build the amp correctly your first time. If you already have the tools, $120 will go a long way if you stick to simple designs. The more complex designs can require $50-100 in parts, which means you don't have any room to fail.

And for your other questions:

1. I have an OBH-11 and a Corda HA-1, and I've built amps costing less than each that are arguably in the same league sound-wise. It's hard to say exactly -- it isn't that one is better than another, but that they are both good quality and they sound different so they're good for different things. I prefer listening to my own amps, though, simply because I have more emotional involvement with them. The commercial amps are merely equipment to me.

2. Both designs will work for all headphones. Neither is tied to any particular headphone design. Personally, I would say that you should buy enough parts for two CHA47s and if you manage to get one working right the first time, you have lots of spare parts for your next project. If not, you can always try again. The nice thing about the CHA47 is that you can build on PCB, which is easier than on protoboard. Also, the parts for the CHA47 are cheaper than for the HA-1. I doubt you could buy all the parts for $120, if you tried to match the quality of all the components Jan Meier sells in his kits.

3. The OPA132 is a high-quality opamp. Yes, there are better designs, but the 132 is a fine chip to start with. There is a lot of audio equipment being sold today with inferior chips to the 132, so you're already above average if you go with this chip. I recommend the OPA2132PA specifically -- it's a dual version so it's cheaper than getting two singles, it will sound good with lower voltage levels than the 2134PA so you'll quickly recoup the cost in battery savings, and the more expensive 2132P doesn't sound better to me. If you go with the CHA47, you'll definitely have to go with dual-channel chips, by the way,

5. Under US$5/ea For a CHA47 you'll need two. For a CMoy amp, you'll only need one.

6. Digikey, Farnell and RS Components all ship internationally. You might be able to find the OPA2604, OPA2227, or OPA2228, all of which sound similar to the 2132, but are all harder to use. Some people prefer the sound of the 2228 to the 2132. The 2227 is generally regarded as inferior or at best a close rival of the 2132. Some people think the 2604 sounds a bit better than the 2132, but I can't hear any difference and it's definitely a tough chip to use. Not the toughest I've ever used, but the sound change doesn't justify using it unless you have to, IMHO.
post #4 of 10
Thread Starter 
hi guys,
thanx for your input.
I've decided to go for the corda circuit.
It might not be the easiest because it uses high speed opamps but i will give it a try.

CaptBubba, is there any site/document which gives info about designing good layouts?

Tangent I browsed your DIY amp guide at www.tangentsoft.net and its awesome, very comprehensiive and great starting point for newbies like me.

By the way my USD 120 budget was for parts plus basid tools like doldering iron and wirestrippers so you can say that about 90USD for parts.

Just a few more questions.
1) Are there any differences in differences in LM6171 and LM6172?
Which one should i go for?
2) Is it a good idea add a potentiometer or 2 way switch in the voltage gain part? This way I would be able tweak the circuit gain easily without modifying the circuit.

thanx
Rawhit
post #5 of 10
The LM6171 contains a single op-amp in a package while the LM6172 contains two op-amps in a package. While some purists would recommend using two LM6171, a single LM6172 is cheaper and more compact.

Just take a look at the datasheets:

LM is www.national.com
OPA is www.ti.com
AD is www.analog.com
post #6 of 10
I have two rules when laying things out, first to keep the signal path as short as possible, which means keeping the circuit small.

Second, I keep everything symmetrical. My left/right channels look exactly the same.

$90 should get you the parts, but they may not be the high-quality ones that you want. I really do suggest that you start with something simple, you may think "oh, this looks easy enough" when you look at the schematic, but when you have that soldering iron in hand and are trying to figure out WTF the amp you just built doesn't work.....its a bit different.

Just as a note, I got to audition a RA-1 a few days ago and found it amazingly similar to my cmoy I built for $30, except for a little wider soundstage.
post #7 of 10
Thread Starter 
so I should be getting the LM6172 then...I mean the dual version cant be sonically much different (inferior) to the single one. I went to
LM6172 availability website and these are available in different pacakges. I guess LM6172IN (D) is what I need.
CaptBubba, I will keep in mind your layout design tips while building the amp.
I know that Corda is a bit ambitious for a first project, but I have built both analog and digital electronic circuits before (I have done my BS in electrical engineering) so im aware that trouble shooting the circuits (particularly the analog ones) can be tough.
Its just that I havent built an amp before.

With the excellent knowlege and experience that the guys here have I should be able to sail thru.
Now on to acquiring the parts (which for me might be tougher than building the amp).
Its really exciting stuff
post #8 of 10
Ok, you should be fine then. I got the impresion that because you didn't already have a soldering iron that you had never soldered or worked on your own stuff before.

Generaly the dual version could have a rather minor bit of cross talk, and should be just fine. Just make sure to get the DIP version (is that what that "D" stands for?).
post #9 of 10
Thread Starter 
LM6172IN is the DIP version.

I sent a mail to Jan and he also suggested that since LM6171 is not much more expensive it should be prefered over the 6172 as it avoid problems like thermic cross-talk and excessive heat dissipation.

Also since the output stages of the opamps are biased into class-A current drain is rather high and one set of batteries
might not last longer than 8 ~ 12 hours.

So I guess 9 AA batteries should be preferable in this case.
post #10 of 10
Crosstalk rejection in the 6172 is something like 90 dB if I recall correctly, so that's a bit of a straw man.

The real reason Jan prefers the 6171 is the heat dissipation issue. This matters for the Corda HA-1 design because he biases the chips into Class A operation, which makes them draw as much current as they can (100 mA?). If you use the single-channel chip, you can get away without using heatsinks. With the dual, heatsinks are at least a good idea, and possibly even a requirement to avoid heat damage to the chips.

I don't think batteries are a good idea for powering a Corda HA-1. If I'm right on the current draw issue, the whole amplifier will draw something like 220 mA, and that's if you don't implement the HA-1's power supply as Jan designed it. (The regulators will add even more to the current draw.) With 9 AA batteries, you would want to run them in series, so you'd only get about 2850 mAh out of the whole set, giving only about 13 hours of run time. Running this amp could get expensive. I guess you could run 10 batteries, in two series groups paralleled for 7.5V at 5700 mAh instead -- still pretty expensive IMHO. 10 AA batteries every 26 hours.
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