Headphone amp

[Overclocked]

Currently In stock Ive only got, LM741, LM1458, and TL082. I would rather not buy anything else if I dont have to, because I really have no cash

But which of these would be good to amplify a small signal to drives headphones? (of course at the output there would be a transistor driving the headphones)

Or would I be better off desigining a headphone amp around transistors?

 

[akg]

i think 741 can drive high impedence headphone directly..
for low impedence type (low efficient) try a class A type

 

[audioguru]

If you use a single transistor to drive headphones then it will pull the diaphragm off-center due its DC current, possibly causing damage or distortion. Headphones should be driven with AC signals without any DC current.

The old LM741 and LM1458 are noisy, distorted and can't work properly above only 9kHz. The TL081 might be noisy rejects from making low noise TL071 opamps but is much better than those others.

Look in Google for the Headbanger projects. They use an LM386 1/2W power amp IC that performs very well when driving headphones with an impedance of 16 ohms or more at headphones levels. One project has bass boost.

 

[Overclocked]

I just found 2 LM386's in my stock =D

 

[audioguru]

If your headphones are 8 ohms, then add a series resistor to them to prevent blowing-up your ears.
Did you find the Headbanger projects? I helped fix one. :lol:

 

[Overclocked]

Yea I found it!

I really dont know what type of headphones the person will be using, since Im building it as a gift.. (its a secret santa type thing.)

 

[audioguru]

When Santa comes here, I won't tell him what you are building for him and I'll let you know what type of headphones he uses. I've never built anything for him. I just leave some cookies and milk for him. :lol:

The projects discuss and include a power-reducing resistor for each earphone. Good luck.

 

[Overclocked]

Is it ok to subsitute the 100k pot for a 50k pot? Thats all I have in stock right now.

 

[audioguru]

Is it ok to subsitute the 100k pot for a 50k pot? Thats all I have in stock right now.

Please post the schematic you are using to see if the value of its input cap is big enough to pass bass. Its value probably must be doubled.

The volume control pots should have an audio taper (logarithmic), not linear.

 

[Overclocked]

**broken link removed**

Its the Volume Control Pot. The pot Came from a pair of computer speakers, so its probably Audio Taper (the amp was in the speakers itself, you know the cheap kind you can get for like 8 bucks).

 

[audioguru]

Your 50k pots are fine for volume controls. The LM386 doesn't need input capacitors. This project doesn't have bass boost.

There are much newer Headbanger projects with bass boost on my broken old hard drive and on the 'web. A Japanese expert measured one and reported how its extra negative feedback caused overload and distortion in its input stage, and how to correct it. The distortion when corrected and driving common 32 ohm headphones was surprisingly extremely low.

 

[Overclocked]

hmm so should I use the one thats in the LM386 data sheet for bass boost?

 

[audioguru]

National's bass boost circuit is designed for tiny speakers. It has too much boost (its resistor value is too low) and at too high a frequency (its capacitor value is too low) to boost the bass on quality headphones. :lol:

 

[Overclocked]

Hmm I see that, it peaks off at a certain point rather than something like a low pass filter.

Hmm what if I were to use the 20X gain (27db), but use a LM1458 or eq (because it runs on a single supply) That has the low pass filter for the bass init, with a gain of Say 1?

That way, when the bass boost is on, it will be a low pass filter and roll off from that point. The reason for the gain of 1 is because we dont want to amplify it too much.

When you fixed the amp, was it loud? The gain was at 11 (21dB according to the headbanger) If it was loud at that point, I might reduce the gain (but the data sheet says you can only go down to 20..

I also noticed it says for a 6dB bass Gain to increase the 10k resistor to 15k

EDIT: I really cant get a 0.033uf Cap. I do have 0.022uf and 0.01uf how ever. I have .1 uF if thats big enough. how do I select the cap and resistor? Calculate X or select the freq I want it to amplify and calculate values from that ?

 

[Dr.EM]

Any headphone larger than those in ear things won't need a bass boost (and often the device that those type of earphones are connected have some built in).

 

[Overclocked]

Whats the difference between Dual Ganged and Dual Pot, Potentiometers?

Arent they both the same thing? Both Have seperate connections for each pot.

 

[audioguru]

I see that, it peaks off at a certain point rather than something like a low pass filter.

National's bass boost circuit uses a small output capacitor that causes the response to rolloff -3db at 80Hz with an 8 ohms load. The max boost will be a little more if its value was larger or the headphone's impedance was higher.

what if I were to use the 20X gain (27db), but use a LM1458 or eq (because it runs on a single supply) That has the low pass filter for the bass init, with a gain of Say 1?

I don't understand why you want to add an opamp.
The LM1458 is a dual lousy old 741 opamp that is noisy, distorted and runs into trouble above only 9kHz.
Any opamp can operate from a single supply if its reference voltage input is connected to a half-supply voltage divider made with 2 resistors and maybe a decoupling capacitor. The LM386 power amp already has the half-supply voltage divider built in, so its inputs can be grounded.

When you fixed the amp, was it loud?

It was very loud if the input signal was adjusted to a high level. The circuit's gain affects how much bass boost it will have. Its output level is adjusted by its input level.

I also noticed it says for a 6dB bass Gain to increase the 10k resistor to 15k

I recommend 15k.

EDIT: I really cant get a 0.033uf Cap. I do have 0.022uf and 0.01uf how ever. I have .1 uF if thats big enough. how do I select the cap and resistor? Calculate X or select the freq I want it to amplify and calculate values from that ?

If you want some bass boost, I recommend 15k and 100nF (0.1uF) like this:

 

[audioguru]

I have some Dual Ganged pots that use a single knob to adjust both together. Other Dual Pots use two concentric knobs to adjust each separately. Maybe your supplier means the same things. :lol:

 

[Overclocked]

Well Im ordering from mouser, https://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=229061.
Its one knob adjusting 2 pots.

Thanks for the info audioguru! How did you make that graph btw?

So if I increase the output cap to say 1000uf, I would get a better response?The headbanger has the output cap at 470uF.

Hmm so really, I could keep the headbanger design the same, and just modify the RC circuit on pin 1 to give a better bass response.

 

[audioguru]

Well Im ordering from mouser ...
Its one knob adjusting 2 pots.

That pot is fine.

Thanks for the info audioguru! How did you make that graph btw?

I copied National's then pasted it into Microsoft Paint and fiddled with it for a couple of minutes.

So if I increase the output cap to say 1000uf, I would get a better response?The headbanger has the output cap at 470uF.

1000uF causes a -3dB drop ito 8 ohms at 20Hz and will be fine. Some new 1000uF/16V caps are nice and small and I'm glad I found them.

so really, I could keep the headbanger design the same, and just modify the RC circuit on pin 1 to give a better bass response.

Yes. Add 100 ohm resistors in series with each output to protect your hearing. :lol: