[tiagoncalves]

Hello everyone, my first post here!
A few months ago I turned my attention to electronics, and I start to create some circuits. Audio circuits made all the sense to me, so I dig into the subject, and I tried to learn how to make an headphones amp. I already made a Cmoy, but now I would like to make a Apheared 47, for a pair of Grado's SR80i that I bought some days ago. This one doesn't need to be portable, it's for home use.
I'll show you the schematics that I plan to use:
 
 
 
I was thinking of a 6 gain, so are the resistors all right? And what about the capacitors size? I bought some 0,47uF 63 v 1% polystyrene caps, hope they are a good component for this amp. Also, I'm not a bass addicted, so I just want to have the real sound, not bass boost's or similar.
If someone could give me some opinion about this project, I'll be very grateful. I'm planing to use some zener diodes to check the battery level, when using the 9 v cells, just don't know yet what voltage I would like to have till the led's turned off.
The case will be in wood, something like this:
 

 
Should I use some interior shielding, maybe with some kind of conductive paint?
On the back of the case there will be the RCA connectors for the input and a DC socket for the wall wart. I read that it's possible to use a cheap wall wart for this kind of amp. What's your opinion?
Thanks a lot in advance! Regards.
 
Tiago Goncalves

 

[alfa147x]

Quote:

Should I use some interior shielding, maybe with some kind of conductive paint?
 

 
What about that foil stuff that guitar builders use? Seems a lot easier than paint 

 

[AmarokCZ]

Instead of conductive paint try copper or aluminium foil.
Resistors for gain 6 seems to be OK, and C3 should be sufficiant (but if I were you I would drop C3).

 

[tiagoncalves]

The problem with the foil, is the inner compartment, it will be tricky, but i'll try. Does it make much difference the foil/paint shielding?
In the Cmoy I used a 11 gain but it was to much! Is the 6 alright, or do you recommend even less? the source will be a android phone, or a macbook pro, both using flac music files.
And about the C3 capacitor, is it possible to drop it? What is the function of the capacitor?
Bye, thanks.

 

[AmarokCZ]

Difference between foil and painting could be quite large. Few weeks ago I used aluminium foil to cover plastic box but that box was made from 2 parts. So I had to connect those two foils together, I tried mechanical contacts, but it was insufficient (high resistance between those two foils). In the end I had to solde a wire between these two pieces of foil.
So I guess that painting could have high resistance to ensure proper shield, but try and see if I am right.
 
Gain 6 is quite high. Desired gain depends on phones you want to use primarily.
 
C3 is protection against DC voltage at input (and therefore on output).

 

[tiagoncalves]

So, I'll try to simplify the inside of the cover, and try to use copper or aluminum foil.
I'll use the amp to drive a 32 ohm grado sr80i headphones. Should I reduce the gain to what value? And the relation between the R1 and R2 resistors, should I use the R1 with 1 kohm and change the R2 to obtain the desired gain?
 

 

[AmarokCZ]

SR80 are quite sensitive, so lower gain should be sufficient, but I don't have experiences with Grados, so try some values of R2 and select the best one for you!

 

[Avro_Arrow]

Putting the amp inside a shielded case is not needed.
You can increase the input cap to 1uF.
A gain of five would be good for your headphones.
If this is going to be an "at home" amp, why are you 
bothering with batteries?
If this is going to be an "at home" amp, why not build a proper
power supply and ditch the rail splitter?
You don't need to use 47 ohm resistors for load sharing.
They can be much smaller, even 1 ohm is fine.
 
Good luck with your amp!

 

[tiagoncalves]

The amp will be used at home, but I want to have some portability through the house, so I can use it all around without having to plug and unplug the power supply. Also, to use on the outside, in the garden, where I don't have electricity. So, I'll have the DC power supply in one place, but if it's needed to listen to music somewhere else, I just switch to the battery. Is it a good idea?
For now, if I'm using a cheap wall wart, is it fine? Maybe in a few months I can try to make a better power supply. Any ideas, or circuits that you find better to use with the rail splitter in this schematic?
The shielding is to protect the circuit to some noise? I can drop it with no inconveniences? It would be much simple to build the case...

 

[Avro_Arrow]

Using both batteries and AC together is fine. You can use rechargeable batteries and build in a trickle charger.
These circuits are fairly resistant to interference, so no special shielding is needed. You can use some if
it makes you feel better but I think you could leave it out. Some people use metal cases, and some use
plastic cases and I have not heard of very many situations where there was any interference.
 
There is nothing wrong with using a rail splitter, but there are other and better options.
You can use a split battery pack to form the positive, ground and negative rails for
portable use but the best is a proper dual rail power supply for use in an AC only situation.
 
For the rail splitter in your schematic, the addition of an op amp for control of the buffer
will be of a big help. The BUF634 on it's own has an output impedance of around 10 ohms,
but in a feedback loop it is almost 0 ohms.
 
I see what schematics I have for you that would be good for this project.
 

 

[Avro_Arrow]

Trickle charger:
 
http://www.head-fi.org/t/185377/trickle-chargers/45#post_7100643

 

[Avro_Arrow]

Battery voltage indicator:
 

 
I can't seem to find the original thread...

 

[tiagoncalves]

Thanks Avro, thanks a lot. For now, I'll assemble the amp with a simple schematic, but I'll leave space for a future improvement, and using a charger and a battery indicator like the schematics you send me, looks great. I just need to receive the RCA sockets to start to build the amp.
Some more questions... Does it make sense to use some of the spare 0,47 uF polystyrene caps for bypass caps in the power supply? I read something about this, but didn't understand quite sure the importance of them.
And I would like to understand the utility of the 47 ohm resistors R3 and R4.
Tomorrow I'll make the wood case, and then I'll post the results!

 

[tiagoncalves]

The more I think about it, the more questions I have... and I like to understand the things that I do. I already went to the local library to read books about it, but not easy to find decent literature... About the gain resistors, it's just needed to have satisfy the equation (1+ R2/R1)= GAIN? Or how someone determines the value of the resistors? For example, is it the same using 4,7/1,0 kohm or 470/100 kohm, or 4,7/1 ohm resistor? Maybe there's some optimum interval...

 

[Avro_Arrow]

Yes, you can use the 0u47 caps in the power supply.
Your gain calculation is correct.
 
About the feedback and gain resistors.
Larger values of resistor use less current but induce more noise.
Low values give less noise but use more current.
I like to try and keep the parallel value of the gain and feedback
resistors to a value of 300 to 600.
Example: A 750 ohm gain resistor and a 3k feedback resistor give
a parallel value of 600 and a gain of 5.
 
The output resistors have a dual purpose.
The first thing they do is raise the impedance seen by the op amp.
Most op amps are not that happy driving loads less than a few hundred
ohms.
The second purpose (and their main use in this schematic) is load sharing.
When paralleling the outputs of some devices, they may try and "fight"
to control the output. But using a resistance, it reduces their tendency
to fight.