Clipped waveform below 0 dB

[J_man]

I've just started recording records with Vinyl Studio. With the recording level set at 90 I noticed that most files peak at -1.5 dB. Audacity confirms the peak level at -1.497 dB (suspicious that they are all exactly the same peak level). Zooming in on the waveform I can see some clipped peaks. Am I overloading the line in of my soundcard? Recording at higher and lower level (peaks still below 0 dB) also shows occasional clipping of the waveform, but at higher or lower levels.

Equipment: turntable -> phono preamp (Ifi Zen phono @ 60dB gain) -> Sony amplifier -> rec out -> Asus Xonar Essence STX II soundcard line in. Recording in 24 bit 96 kHz, wasapi mode.

The cartridge has 0.3 mV output @ 3.54 cm/sec, with 60dB gain that would be 300mV, and because the phono pre is connected in balanced mode this becomes 600 mV. This will probably be more on most records, but line level of cd players is about 2V, you would expect that the soundcard is able to handle this. And digital clipping should be at 0dB.

Has anyone ever encountered a problem like this? Specific to this soundcard?

[Paul Sanders (AlpineSoft)]

Hi,

Is that WASAPI exclusive or WASAPI shared please?

[J_man]

Wasapi exclusive (Windows: allow exclusive use, Vinyl studio: exclusive, polled)

[Paul Sanders (AlpineSoft)]

OK, thanks.  In that case, it's hard to think of a plausible cause.  It doesn't sound like you're overdriving either your phono preamp or your soundcard, but you could try reducing the phono preamp gain by one step to see what effect that has.  I suspect, though, that the recording level will then be a bit too low.

Also, does you PC have a built-in soundcard with a line-in socket?  If so, you could try that in an attempt to identify which component in the chain is responsible for the clipping.

[J_man]

Datasheet of the Cirrus Logic AD chip used in my soundcard (CS 5381) specifies around 5.65Vpp for full scale output. Absolute maximum rating 5.7V, I think this is Vp, so 10.4Vpp.

I did some testing with an old test record and 300 Hz signals at 45, 68 and 85 micron. According to my true RMS multimeter, this gave me a signal of 1.92, 2.92 and 3.55 V RMS. 1.92 VRMS = 1.92 * sqrt 2 * 2 = 5.43Vpp. This should be close to full scale, and was indeed recorded unclipped. The second one is 8.26Vpp, and this signal was clipped (at -1.5 dB with the recording level set to 90 and at -3.2dB when set to 80). Logically, the third signal (about 10Vpp) was even more clipped.

So, it looks like the clipping is due to overloading the input of the sound card. Lowering the recording level does not help. Easiest would be to go back to unbalanced connection for the phono pre with loud records. First thing I will try though is loading the cartridge a bit more from 250 Ohm to somewhere between 100 and 180 Ohm to see if this lowers the output of the cartridge just enough to avoid clipping on hot records.

[Paul Sanders (AlpineSoft)]

Good, thorough analysis there.  A pair of inline attenuators (-10dB or so, sounds like) between the phono preamp and your soundcard might also be a workable approach.