De-Amplifying Audio Signal

[StudentSA]

Hi All,

I have a audio signal from an amplifier, that is meant to drive a power speaker. I now what to use this signal into a sound card.

Is there a simple solution?

Kind Regards,
StudentSA

 

[Nigel Goodwin]

Two resistors as an attenuator.

 

[MikeMl]

Elaborating a bit. I would use three resistors: one to act as dummy load for the amplifier, and then two to make the attenuator:

Word of caution. If the speaker is driven from a bridged amplifier, you cannot hook SpkrL to the grounded input to the sound card. Unfortunately, computer sound card line and mic inputs use the computer case as a common ground, which is connected to earth ground via the power line. I have had to use a transformer to provide isolation to solve this problem.

 

[Nigel Goodwin]

Elaborating a bit. I would use three resistors: one to act as dummy load for the amplifier, and then two to make the attenuator:

Word of caution. If the speaker is driven from a bridged amplifier, you cannot hook SpkrL to the grounded input to the sound card. Unfortunately, computer sound card line and mic inputs use the computer case as a common ground, which is connected to earth ground via the power line. I have had to use a transformer to provide isolation to solve this problem.

You're making life far more complicated for yourself

You don't need a dummy load, UNLESS it's a valve amp - and even then it's going to be very expensive and will make a good toaster as well (for any decent size amp).

A bridged amp is no problem, you simply take the out put from one of the speaker connections, and the other from chassis.

 

[StudentSA]

Thanks Nigel,

Two 1/4 watt resisters should do the trick like this?

(Audio+) ----- (10k Resistor) --X-- (1k Resistor) ----- (Audio-) X

I would then tap a points X to my sound card?

Kind Regards,
StudentSA

 

[MikeMl]

...A bridged amp is no problem, you simply take the out put from one of the speaker connections, and the other from chassis.

Except for the fact that the output of a bridged amp is biased at about 6Vdc (Half the supply voltage). You might need a DC-Blocking capacitor.

Also, I have tried running solid-state amps unloaded (or loaded only by ~5k) and the audio output was not well behaved. It had a lot of overshoot/ringing on it...

 

[KeepItSimpleStupid]

IMHO, the signal CAN be noisy too if a High Power amp is used.

 

[Ramussons]

Hi All,

I have a audio signal from an amplifier, that is meant to drive a power speaker. I now what to use this signal into a sound card.

Is there a simple solution?

Kind Regards,
StudentSA

Are you planning to copy Tapes or Vinyl Records?

Ramesh

 

[Nigel Goodwin]

Except for the fact that the output of a bridged amp is biased at about 6Vdc (Half the supply voltage). You might need a DC-Blocking capacitor.

It would be a reasonable idea, but by no means essential, as the input you're feeding will (or SHOULD!) have such a blocking capacitor for that very reason.

Also, I have tried running solid-state amps unloaded (or loaded only by ~5k) and the audio output was not well behaved. It had a lot of overshoot/ringing on it...

You need to use better amps

 

[Nigel Goodwin]

IMHO, the signal CAN be noisy too if a High Power amp is used.

Perhaps slightly noisier (depending on the quality of the amp), but bear in mind you're attenuating the noise along with the signal so it should be negligible.

 

[4pyros]

They do make level converter boxs just for this:
https://www.google.com/search?q=spe...22&espv=210&q=speaker+to+line+level+converter

 

[cowboybob]

This strikes me as easy and safe:

https://www.google.com/imgres?imgur...a=X&ei=8RafUtfyAYj12QXZ94G4Cg&ved=0CGkQ9QEwBg

(even if an awkward url )
No problems from potential ground loops, awkward biasing, etc.
And cheap to implement.

 

[Nigel Goodwin]

And cheap to implement.

?????

Four expensive wirewound resistors, and two expensive transformers.

Compared with four cheap small resistors.

'Cheap' is relative - and in relative terms your solution is VERY expensive

The transformers will also alter the frequency response somewhat, where as the resistors are perfectly 'flat'.

 

[Nigel Goodwin]

Thanks Nigel,

Two 1/4 watt resisters should do the trick like this?

(Audio+) ----- (10k Resistor) --X-- (1k Resistor) ----- (Audio-) X

I would then tap a points X to my sound card?

Sorry for the slow reply - but yes.

The right hand X is ground, and should connect to both computer and amp, the middle X is the audio feed to the sound card.

 

[dr pepper]

Nige's suggestions are of course spot on.

You could if you were picky use a matching transformer, car radio dealers usually have them, its just a pair of transformers to drop the voltage in a little tube with speaker wires one end and phono sockets the other, this sorts out the dc issue (built in caps), reduces noise (attenuates it) and isolates the 2 devices to avoid any earth loops.

**broken link removed**