47 headphones amp output impedence

[Thalis]

You can safely increase the power supply voltage to 30V providing the caps are rated at least 25V (16V would be sketchy).
In that case (for 300 Ohm headphones only) you can use 470uF and 220uF. Just have a close look at the diameter. They may become higher than you may want.

Yes the caps i managed to source are 25V but I do not know what the film caps are rated at and I could find any with the same size. From the board you can see the diameter will be the same for 220 and 470uF.

 

[EdW]

Ok I guess i could stick in larger caps... but max i have are 470uF. The recommended voltage is 12V but max as stated is 18V. Maybe I can push it with a 19V laptop brick?

Silly question is this +12V and -12V? Or +6V and -6V? With +/-12V you’d get about 7V rms O/P which with the sennheiser 600 at 105dB/v sensitivity would get you to 122dB spl. Pretty loud.

 

[Thalis]

Silly question is this +12V and -12V? Or +6V and -6V? With +/-12V you’d get about 7V rms O/P which with the sennheiser 600 at 105dB/v sensitivity would get you to 122dB spl. Pretty loud.

Oh its going to DC. So I bought a USB to barrel connector cable with a 5V to 12V convertor built in. If thats not recommended I guess i can order a SMPS. i do have a 24V DC lying about since I have retired the tube hybrid amp.

 

[RayDunzl]

From this calculator it would seem the bass might be affected.

I'd guess around a 6dB upward slope from 20 to 20k.

A little EQ could patch it up.

 

[EdW]

Oh its going to DC. So I bought a USB to barrel connector cable with a 5V to 12V convertor built in. If thats not recommended I guess i can order a SMPS. i do have a 24V DC lying about since I have retired the tube hybrid amp.

My mistake. I hadn’t noticed the power connector jack on the left of the schematic which does rather suggest a single supply. I’m not a great fan of running opamps with a single power rail but with just a single supply of 12V you’ll get approx 8V peak to peak O/P or 2.8V rms. => 9dB more O/P than the nominal O/P of the headphones at 1V rms. So for the Senn 600 this 114dB spl

 

[Thalis]

My mistake. I hadn’t noticed the power connector jack on the left of the schematic which does rather suggest a single supply. I’m not a great fan of running opamps with a single power rail but with just a single supply of 12V you’ll get approx 8V peak to peak O/P or 2.8V rms. => 9dB more O/P than the nominal O/P of the headphones at 1V rms. So for the Senn 600 this 114dB spl

Thank you for taking your time with the calculations but i wonder how would this translate for the AKG240 Mk2 and Senn HD280 pro. The SPLs are 104dB/V and 117dB/V respectively.

 

[jasonhanjk]

Crosstalk will suffer. Oh well, eventually you will out grow it.
Welcome to the DIY club.

 

[Thalis]

Ok thanks guys for replying and helping me... so just to recap... I am just going to replace the 220uF with 470uF caps so I can use my K240 and HD280 and use a 24V 2A SMPS.

I hope the unknown film caps will be not be a huge problem.

 

[solderdude]

The filmcaps will be above 16V so there is no problem there.

At 24V DC (+/-12V internally):
at 55 Ohm = 230mW
at 70 Ohm = 290mW
at 300 Ohm = 180mW

Enough to drive all of them.

 

[Thalis]

The filmcaps will be above 16V so there is no problem there.

At 24V DC (+/-12V internally):
at 55 Ohm = 230mW
at 70 Ohm = 290mW
at 300 Ohm = 180mW

Enough to drive all of them.

By the way I think all the resistors in the kit are rated at less than 1W... I am not sure if they are 0.5 or less. I only have 100k at 1W which I already have.

Thanks again solderdude. The LED I asked about in another thread is for this project

 

[solderdude]

Resistor ratings are fine.
I assume they are around 0.5W
Highest dissipation will be the 47 Ohm and these will be dissipating less than 50mW.

The opamps will do the heaviest lifting and will be disspating about 0.5W max with 55 Ohm headphones (less with higher impedances)
Of course under normal music listening the dissipated power will much, much less.

Gain = 3x (+9dB) so when using a 2V DAC only 120mW can be reached in 300 Ohm.
R4 and R9 need to be 3.3k (4x gain = +12dB) to reach max. power in 300 Ohm.

For the 100k to reach 1W it would need 316V across it.

 

[Thalis]

Resistor ratings are fine.
I assume they are around 0.5W
Highest dissipation will be the 47 Ohm and these will be dissipating less than 50mW.

The opamps will do the heaviest lifting and will be disspating about 0.5W max with 55 Ohm headphones (less with higher impedances)
Of course under normal music listening the dissipated power will much, much less.

Gain = 3x (+9dB) so when using a 2V DAC only 120mW can be reached in 300 Ohm.
R4 and R9 need to be 3.3k (4x gain = +12dB) to reach max. power in 300 Ohm.

For the 100k to reach 1W it would need 316V across it.

Right... so when I get that high impedance Senns or whatever i will source for caps and swap. Thanks again

 

[solderdude]

For high impedance headphones you could use the original values.
For the low impedance headphones I recommend higher values.
470uF/25V for output C is absolute minimum.

The lower the headphone impedance the higher the output capacitance should be.

 

[Thalis]

For high impedance headphones you could use the original values.
For the low impedance headphones I recommend higher values.
470uF/25V for output C is absolute minimum.

The lower the headphone impedance the higher the output capacitance should be.

Yep... the lowest I have will be the K240 Mk2 at 55 ohms... good thing I ordered extra 470uF caps

 

[solderdude]

For the lower frequencies the impedance rises so there will be a bit less lows roll-off than expected based on resistance
470uF will just do for K240.
I would also use at least 470uF for the power supply. Power supply caps are less important in this amp because the headphone out is not connected to virtual ground but to - voltage.
Because of this design choice the power supply must have a low ripple voltage.

 

[Thalis]

For the lower frequencies the impedance rises so there will be a bit less lows roll-off than expected based on resistance
470uF will just do for K240.
I would also use at least 470uF for the power supply. Power supply caps are less important in this amp because the headphone out is not connected to virtual ground but to - voltage.
Because of this design choice the power supply must have a low ripple voltage.

The HD280 Pro is 64 ohms so it should benefit a bit more with 470uF caps? And yes the PS caps are indeed 470uF. I would go higher but the enclosure is just not tall enough to fit bigger caps

Edit... no I got it wrong... 64 ohms would not benefit more from a bigger cap