tmbg
Well-known member
Hi folks!
Here's my eventual goal:
My portable recording rig consists of a couple of B* ADA-8000 boxes. I use all them wonderful mic pres and line ins of course, but I'm only using one pair of the 16 line outs I have for my reference monitors.
What I'd like to do is build a couple of headphone amps that can take a balanced line from the ADA8K and drive a couple of pairs of headphones. I will probably make a mono version and a stereo version. I did some playing around last night, breadboarding a couple of designs that I googled up, they all work, but not how I'd expect. I'd feed it a sine wave, and the output would be nice and clean, but once I hooked headphones up to the output, it got really horribly distorted.
So, basically I just want a little bit of guidance to make sure I'm on the right track here... the line outs of the B* are gonna be +4dBu balanced, so I should be designing so that maximum input level without the output clipping is roundabout 1.23V, correct? What sort of output level should I be shooting for? +20dBu? +24dBu?
If I'm doing my calculations right, +20dBu is 7.75VRMS, which is about 22V peak to peak, which is a reasonable swing for a +/-15V supply. +24dBu is closer to 35Vpp, which I can't do on +/-15V.
next, I need to know about output impedance. Seems like most designs mention using a 50R resistor in series with the output, is the goal to match Zout to the Zload of the headphones? If I'm using 32R headphones, should I be shooting for a 32R Zout?
How does the load of headphones affect the output of a good amp? If the unloaded output is a nice clean pretty sine wave, should the loaded output be a nice clean pretty sine wave of half the amplitude? My tests on the breadboard last night showed nasty distorted destruction when loaded.
7.75V into 32 ohms is 250mW. I'm using OPA4228's, which have an output drive capacity of around 45mW. Perhaps that is what was causing the distortion? I may have to look into either some higher drive opamps or possibly just a simple emitter follower that can handle that sort of drive and swing.
So, my plan at this point I think will be one op amp for debalancing and buffering, one with 16dB gain, and a 2N4401 or TIP31 or something for output drive.
Now that I'm done rambling, anyone care to comment? (PRR this means you, you naughty headphone guru!)
Here's my eventual goal:
My portable recording rig consists of a couple of B* ADA-8000 boxes. I use all them wonderful mic pres and line ins of course, but I'm only using one pair of the 16 line outs I have for my reference monitors.
What I'd like to do is build a couple of headphone amps that can take a balanced line from the ADA8K and drive a couple of pairs of headphones. I will probably make a mono version and a stereo version. I did some playing around last night, breadboarding a couple of designs that I googled up, they all work, but not how I'd expect. I'd feed it a sine wave, and the output would be nice and clean, but once I hooked headphones up to the output, it got really horribly distorted.
So, basically I just want a little bit of guidance to make sure I'm on the right track here... the line outs of the B* are gonna be +4dBu balanced, so I should be designing so that maximum input level without the output clipping is roundabout 1.23V, correct? What sort of output level should I be shooting for? +20dBu? +24dBu?
If I'm doing my calculations right, +20dBu is 7.75VRMS, which is about 22V peak to peak, which is a reasonable swing for a +/-15V supply. +24dBu is closer to 35Vpp, which I can't do on +/-15V.
next, I need to know about output impedance. Seems like most designs mention using a 50R resistor in series with the output, is the goal to match Zout to the Zload of the headphones? If I'm using 32R headphones, should I be shooting for a 32R Zout?
How does the load of headphones affect the output of a good amp? If the unloaded output is a nice clean pretty sine wave, should the loaded output be a nice clean pretty sine wave of half the amplitude? My tests on the breadboard last night showed nasty distorted destruction when loaded.
7.75V into 32 ohms is 250mW. I'm using OPA4228's, which have an output drive capacity of around 45mW. Perhaps that is what was causing the distortion? I may have to look into either some higher drive opamps or possibly just a simple emitter follower that can handle that sort of drive and swing.
So, my plan at this point I think will be one op amp for debalancing and buffering, one with 16dB gain, and a 2N4401 or TIP31 or something for output drive.
Now that I'm done rambling, anyone care to comment? (PRR this means you, you naughty headphone guru!)