Quote:
Originally Posted by
dobby156 
As I said in an earlier post I intend to make a desk amp, as an oppose to a pocket amp, and ideally this will be the last one I need to build for a while so I am going all out.
Which earlier post? Why not tag this onto that thread? It is better to bump an old thread that you have a specific question about than to start another. At the very least link to it.
As JCX pointed out in an earlier post, not all opamps deal well with large swings on the inputs particularly when we swing near the rails (see this thread HERE, post 27,) which may limit their use as buffers if you want the CHA-47 to swing a lot of voltage. You are also limited to op amps which are unity gain stable for the "buffer" position (or the whole thing depending how you build the beast) but thats not too hard to do.
Now that thats out there, the real question of suitability boils down to how many volts do you want the amp to swing? Most people take no effort to learn and know this answer beyond "heh, as many as possible" which is a fun although somewhat expensive and challenging thing to design for. In many ways its wasteful... Designing an amp to swing 15vrms when you will never even use 1 full vrms...
How many volts do YOU want the amp to swing? After you know this, you can knock that off the list of questions before you build something... "will it go loud enough" yes/no. Its simple!
To figure it out, there are a couple methods. The best one is to ignore the pencil pushers, build the amp, and listen. Does it go loud enough? Yes? congratulations. whoever told you it dosnt have enough gain must have been doing something very different from you. If it dosnt, put it on the FS boards. Theres a lack of people building anything you cant find on a PCB these days.. give it some time and you are almost certain to recoup your parts cost on a slightly unusual build.
A slightly less expensive method is to take the headphones you have now, and measure how much voltage swing it takes to drive them to your desired level. Whats that you say, your meter dosnt measure below 0.1v? Thats a bummer, but it should probably tell you something....
The second method is to look up the rated efficiency of your headphones, which is typically given in db/mw or db@1v(note! rms or p-p) bust out the calculator and work it backwards.
Add a few db for headroom, and if you change source or find a particularly quietly recorded bit of music. Realistically, 12db will probably cover most people here. Many people will STILL find themselves within what they can cover with gain of 3, and possibly less.
Once you know how much voltage swing you need you compare that to the voltage output of your source. For most people the voltage going into the headphones will be lower than what comes from the source. Its funny how much money we as a group spend on attenuators which we set to -50db so that we can have a +20db gain stage for a net output 30db lower than the input voltage.
A lot of people like to build with more gain than necessary because when you have gain you can keep the volume knob wayyyy down which gives a feeling of massive power from an amp. Its an illusion. There are a few very REAL benefits to building an amp with lower (but still adequate!) gain VS higher. Real benefits VS an illusion.
With lower gain you can spin the volume knob up higher. This keeps the volume control in a better matched area. Even expensive pots have channel matching issues from time to time at low volumes... everything (even cheap stuff) matches well at high volumes.
With lower gain you have less "gain related noise" and noise in general. Simple.
Although its a little theoretical, GBP (gain bandwidth product) means you will have more bandwidth with lower gain. Newer opamps have sooooooooo much bandwidth that even with gain of "holly $#@% thats a lot" ! you still have way enough bandwidth but it never hurts to mention.
Edited by nikongod - 7/3/10 at 11:43am
