Sound Cards & Amplifier Speaker-Out

[Ryan]

I was wondering if it might be safe to attempt to use a small amplifier's speaker-out connection with a sound card's line-in jack if the amplifier's volume is set low, to no higher than 1/4 of the maximum volume to prevent clipping.  Would this be safe for the sound card?  I'm using a Sound Blaster X-Fi, and I'm in the process of re-recording Karl Bohm's Beethoven symphony cycle from vinyl, and wanted to get better response in the lower frequencies, especially at the start of the 5th symphony's third movement.  Using the line-out didn't give me a good result with that part of the box set, and I've wanted to make that part of the music more audible without having to crank up the volume.

[Geek-9pm]

No, not recommended. If the level gets above a half volt the input circuitry can become damaged. Instead, use a "pad' made of resistors.
I use a tin box witrh potentiometers to keep the level in the millivolt range.
On just try two 1 Meg ohm resistors in the line in plug.
As for the loss of the low end, you may be using the wrong type of pre-amp for the pickup of the turntable. This is a common reason for low end loss.

[Ryan]

Due to your advice, I decided to put the connectors back on the tape-out jacks.  I will not attempt to connect the sound card to the speaker-out jacks again.  I guess I'll have to find a proper pre-amp instead then.  I definitely want to protect the sound card.

Edit:  I just checked the input after switching back to the tape-out, and I am getting great sound still.  Thankfully I stopped before too much damage may have occurred.  Would the input volume level have affected the number of volts?  The input volume was so low that none of the recorded peaks got anywhere near the 0db mark.

[Geek-9pm]

If the system was calibrated, 0 dbm means an defective voltage of 0.7 volts AC on a 600 ohm load. But I do not think it is calibrated.
Tape heads require a good preamp. They need bass boast. That is the way magnetic pickups work. Lower frequency, lower output. The standard settings for tape is NAB, but for a turntable it is RIAA.

http://www.electronics-lab.com/projects/audio/045/index.html

Do a Google on Tape Head Preamp.

[Ryan]

I have since changed my setup.  Instead of the small amplifier I had been using, I decided to replace it with an amplifier that I had stored in my bedroom closet for a few years.  However, this is a big amplifier that can be used with surround sound systems.  I'm using the monitor output, instead of the tape-out.  Is this output okay to use?  I have the amplifier hooked up to a graphic equalizer, and then from the equalizer to the sound card.  I'm not using the speaker outputs (I can't use them anyways as they're the wire input variety), and the equalizer cannot be used with the tape output jack.

The equalizer I have has seven frequency sliders for the left channel, and seven more sliders for the right channel.  For both channels they are 60hz, 150hz, 400hz, 1khz, 2.4khz, 6khz and 15khz.  I'm trying to increase the first three frequency settings and decrease the last three in an attempt to get the RIAA curve accurately.  Would you be able to help me with this, Geek-9pm? 

[Ryan]

I have since changed my setup.  Instead of the small amplifier I had been using, I decided to replace it with an amplifier that I had stored in my bedroom closet for a few years.  However, this is a big amplifier that can be used with surround sound systems.  I'm using the monitor output, instead of the tape-out.  Is this output okay to use?  I have the amplifier hooked up to a graphic equalizer, and then from the equalizer to the sound card.  I'm not using the speaker outputs (I can't use them anyways as they're the wire input variety), and the equalizer cannot be used with the tape output jack.

The equalizer I have has seven frequency sliders for the left channel, and seven more sliders for the right channel.  For both channels they are 60hz, 150hz, 400hz, 1khz, 2.4khz, 6khz and 15khz.  I'm trying to increase the first three frequency settings and decrease the last three in an attempt to get the RIAA curve accurately.  The graphic equalizer is a Kenwood KE-205.  Would you be able to help me with this, Geek-9pm? 

[Geek-9pm]

The RIAA equalization curve has operated as a de facto global industry standard for recording on disc.
http://en.wikipedia.org/wiki/RIAA_equalization
The center frequency is 1000 Hz.
You need about 20 db bass boast and about 20 db treble cut.
If you have a old amplifier that has tape head inputs, it already has the equalization built in. 
 

[Ryan]

At any rate, I have decided to boost the lower frequencies and leave the others at the middle position.  My sound card's bass response seems to be much lower than that of my stereo, affecting some of my classical recordings.  I think I might have some re-recording in the near future. 

With a flat response, I have a hard time hearing the bass decently at the start of the third movement of Beethoven's 5th symphony, on Karl Bohm's cycle with the Vienna Philharmonic.  This is the primary reason why I have raised the bass.  I can hear that bass fine on my stereo without making the volume loud, but not on the computer.