Hello everyone,

This is my first post to your forum and I hope to meet you all!

I've recently breadboarded a cMoy amp pretty much exactly as it can be seen here. http://headwize.com/?page_id=707

I made a virtual ground circuit using a couple transistors to get a +- 4.5v power supply from a single 9V source and it stays pretty stable plus or minus 10mv under a low impedance load.  I'm building this for a friend who lives far and uses a pair of 300 ohm senheiser 650s so it's a bit like making a costum suit for a man who's measurements I can't take in real life. In the meantime I've been testing this with a pair of 35 ohm headphones and I have to say I'm pretty impressed.  The op amp I used is pretty run of the mill, its a JRC4558.  It's very hard for me to get a hold of much else locally.  With my headphones this thing sounds pretty great but I'm a bit concerned of how it may actually perform on the 300 ohm cans that my friend has. 

Since higher impedance headphones require a bit more voltage and a bit less current than lower impedance headphones I'm trying to make sure this thing puts out a decent amount of voltage.  However, I'm a bit unsure as to how I could measure the voltage output reliably.  I currently only have access to a multimeter for measurements as my oscilliscope is dead. With the multimeter measuring AC and with the 35 ohm load I get an output of about 1.2. VAC @ 1000hz  with the the amp right at the point where it starts clipping.  I get roughly the same measurement unloaded. 

I also built a class AB stage with a couple transistors at the output of the cMoy amp that looks pretty much just like this.
 
http://media.soundonsound.com/sos/jun06/images/liveclassd4_l.jpg

At the output of this extra stage under the same conditions I get about 1.1 or 1VAC which is actually a tad bit less than what comes out of the cMoy amp alone but my idea was that perhaps the output stage would help provide a bit more current when needed.  At the same time I've come to learn that the 4556 op amp, which if I'm not mistaken is just a single version of the 4558, can actually provide a fair amount of current on it's own.
 
So basically my questions and concerns that I need some advice with are.

1.  Why am I barely getting 1.2V out of the op amp while feeding it to the point of clipping and even when unloaded.  Does this sound normal? Would conditions change under a higher impedance load?  Is the way I measured this reliable or am I doing it all wrong?

2.  Does the extra output stage sound like a good idea?  Is it unecessary? My idea was to provide some possibly necessary current when driving lower impedance loads but I'm not sure if this may negatively affect higher impedance loads which is what the amp's primary purpose is.  How is this affecting output impedance?  Should I just leave it out?

I would appreciate your input regarding this.  Would suck to send my friend an amp that works great for me but not very well for him. I'll be awaiting your replies.

Much thanks!
-Mimu

 

Anyone?

Ok... where to start. 
 
-  if you don't have access to a scope, testing your amp with a 50hz input will give more reliable results. Your soundcard can provide that. It's very difficult to say what kind of precision your meter truly has at 1KHz (unless it's a good one and made for it).
- The jrc4558 is basically a dual µ741. It can only push 25ma at best. Into 35R, that's 875mV, peak (1,75Vp-p). 
- Then, it cannot swing very close to its rails. At its best, it will not go closer than 1.5V from each rail. But it could be a lot worse and still be in specs. 
- The njm4556 is a completely different opamp. A much better one, with a lot more current output but also better specs all around.