47 headphones amp output impedence

[Thalis]

Ok stupid question and I hope someone is familiar with this kit...........

As per the title... is this put-it-together-yourself amp's OI 47 ohms? Looking at the circuit design I see 47R and 220uF caps at the output stage.

I hope the pic of the schematics is clear enough

 

[EdW]

The schematic makes more sense with pin 5 being the non inverting I/P. The data sheet confirms this. Then the pairs of opamps have their nominally identical O/Ps wired in parallel with 47 Ohms being used to allow for minor mismatches between the amps. Overall feedback is after the 47 ohms so O/P Z is very low. Gain of (10+4.7)/4.7

 

[Thalis]

The schematic makes more sense with pin 5 being the non inverting I/P. The data sheet confirms this. Then the pairs of opamps have their nominally identical O/Ps wired in parallel with 47 Ohms being used to allow for minor mismatches between the amps. Overall feedback is after the 47 ohms so O/P Z is very low. Gain of (10+4.7)/4.7

I won't be able to do anything about Pin 5 for the opamps as this kit already has a board with the printed circuits ready for assembling. All i could do was source better quality components before going ahead.

The gain you are calculating is based on any voltage? And any way to tell how low the output impedance is as you said its really low?

 

[RayDunzl]

My less educated guess might consider the reactance of the output caps as the impedance...

0Hz - infinite
20kHz - very low

 

[EdW]

No there is a mistake on the labelling in the schematic U1B and U2B. Pin 5 is the non inverting I/P and pin 6 is the inverting I/P. Both opamps U1B and U2B are wired as unity gain buffer stages so as to bolster the O/Ps of U1A and U2A respectively - potentially doubling the available drive current. The O/P Z will be substantially under 1 ohm in series with the blocking capacitor of 220uF

 

[Thalis]

No there is a mistake on the labelling in the schematic U1B and U2B. Pin 5 is the non inverting I/P and pin 6 is the inverting I/P. Both opamps U1B and U2B are wired as unity gain buffer stages so as to bolster the O/Ps of U1A and U2A respectively - potentially doubling the available drive current. The O/P Z will be substantially under 1 ohm in series with the blocking capacitor of 220uF

Ok so its just a mistake in the schematics on paper and not on the board as I have seen many videos on the assembling and no mishaps. Nice to know the output impedance will be that good. Thanks!

 

[Thalis]

My less educated guess might consider the reactance of the output caps as the impedance...

0Hz - infinite
20kHz - very low

Are you quoting the possible FQ?

 

[RayDunzl]

Are you quoting the possible FQ?

I don't know,

Being less educated I don't know what FQ might be.

Let's see:

https://en.wikipedia.org/wiki/FQ

No help there.

 

[Thalis]

I don't know,

Being less educated I don't know what FQ might be.

Let's see:

https://en.wikipedia.org/wiki/FQ

No help there.

oops sorry... Frequency Response... meant to type FR but then i shouldn't use acronyms

 

[Thalis]

Forgot to add that i am going to give this thing 12V if that makes any difference.

 

[solderdude]

When driving lower imp headphones I recommend to increase the output caps to 1000uF and and the power supply caps to 2200uF.
The 5532 isn't really designed for low impedance loads though. Even the output current doubling trick doesn't help much.
Nice opamp for amplification, not very suited to deliver current.

On 12V it will output about 3.2V and 60mA max. Due to the 22 Ohm current limiter and opamps current limit the output voltage will drop on 32 Ohm headphones to about 1.5V or so

So 30mW in 300 Ohm
80mW in 120 Ohm
80mW in 32 Ohm

 

[RayDunzl]

oops sorry... Frequency Response...

Having a capacitor in-line with a a transducer will affect FR to some degree.

Simplistic calculation for 220uF:

https://www.allaboutcircuits.com/tools/capacitor-impedance-calculator/

20Hz = 36 ohm

200hz = 3.6 ohm

2000 Hz = .36 ohm

20000 Hz = .036 ohm

 

[Thalis]

When driving lower imp headphones I recommend to increase the output caps to 1000uF and and the power supply caps to 2200uF.
The 5532 isn't really designed for low impedance loads though. Even the output current doubling trick doesn't help much.
Nice opamp for amplification, not very suited to deliver current.

ok noted... but I am not going to use any IEMs with this. My headphones are 55 to 70ohms and I plan to get a Senn HD600 or 650 one fine sunny day. Another problem is I cannot fit bigger caps as the housing is not very tall and its sized to fit in those caps in the schematics.

 

[Thalis]

Having a capacitor in-line with a a transducer will affect FR to some degree.

Simplistic calculation for 220uF:

https://www.allaboutcircuits.com/tools/capacitor-impedance-calculator/

20Hz = 36 ohm

200hz = 3.6 ohm

2000 Hz = .36 ohm

20000 Hz = .036 ohm

Thank you very much for this....

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

 

[EdW]

When driving lower imp headphones I recommend to increase the output caps to 1000uF and and the power supply caps to 2200uF.
The 5532 isn't really designed for low impedance loads though. Even the output current doubling trick doesn't help much.
Nice opamp for amplification, not very suited to deliver current.

Same thought crossed my mind about drive capability. The nominal value of the short circuit current from each amplifier is 38mA but could, worst case, as low as 10mA. (Rather unlikely).
So best stick to Sennheiser or similar high impedance headphones!
I did look for better opamps with the same pin out but TI’s website is down at the moment

 

[solderdude]

For the 55 to 70 Ohms you need to increase output capacitance. For HD600/650 is will play decently loud.
Consider changing the PS voltage to 18V or 24V when driving high imp. headphones. In that case the current caps (provided they are at least 16V rated) will be good enough.

 

[Thalis]

Same thought crossed my mind about drive capability. The nominal value of the short circuit current from each amplifier is 38mA but could, worst case, as low as 10mA. (Rather unlikely).
So best stick to Sennheiser or similar high impedance headphones!
I did look for better opamps with the same pin out but TI’s website is down at the moment

I did order a pair of original TI NE5532P

 

[Thalis]

For the 55 t0 70 Ohms you need to increase output capacitance. For HD600/650 is will play decently loud.
Consider changing the PS voltage to 18V or 24V when driving high imp. headphones. In that case the current caps (provided they are at least 16V rated) will be good enough.

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?

 

[Thalis]

This is the size of the board...

 

[solderdude]

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.