All decks with a calibration system look out for the flattest response, but that setting isn't the best one, meaning with best, the highest output and lowest distortion. But that's another thing for an in-depth article...

The graph shows that the optimal bias is -20, which is a somewhat weird thing for a "pro" tape, because the best tapes do usually need much more bias than that. But this tape seems to go backwards in this regard...

This suggests that unless you can adjust bias easily on your deck, you may find that this tape is significantly overbiased in your machine.

As the output in HF (High Frequency) is high, I’ve set the Rec EQ at Low to compensate that. Ok, let’s see the frequency response with these settings:

Ok, we see a clear HF rise which goes up to about 5dB (which is quite high!) at its maximum point. However, for us enthusiasts that are mostly 40 yr-old and above, we should not care about what happens above 15 kHz because we can hardly hear it... and in this regards the tape rises for about 4 dB (which is still high). To fix this, a good EQ would be a nice help, because you can apply a EQ curve while recording (not when playing) because this way you can make the recording flat... and thus be perfectly playable on any walkman or deck without having to mess with an EQ.

Can a flat response be achieved with the calibration controls? Well, yes. Just adding bias the HF starts to fall off. At bias 0 the response is somewhat flat, but with a rise in the HF with about 3 dB of rise between 2 and 10 kHz.

Now the big question: what’s better? To record at the optimal bias setting (to get the highest MOL and SOL) or to record ad bias 0? (to get the flattest response). The answer is in the THD. Let’s check it out:

As you can see, the THD (blue column) indicates higher values (worse) for default bias than for optimal bias. While at default bias the THD is 1,88%, at optimal bias it's 1,25%. Both values are high but it's clearly worse at default bias.

1,88% is still under the standard 3% allowed for acceptable results, but it's close to. That means that with many machines that have fixed bias (usually associated to the type I position) the distortion will be close to the limit, even recording without going further than the reference (which I consider the Dolby mark, not the 0 VU actually). So with this tape it's really necessary a good deck with 3-head and variable bias to extract the best of it.

Just as a reference, let’s see how a simple’89 TDK D performs in regards to the THD:

The THD for this TDK D is only 0.41%.
This is less if a quarter of the NAC’s THD, both at the same bias setting (neutral).

Surprised, huh? Yes, they have a very low THD even at bias 0 (at their optimal bias is even lower). You read it well: 0,41% in the same conditions (default bias). You guess it.

Noise

There’s no info about the background noise nor the dynamic range of this tape. I’ve tested it in both situations: playing the blank tape and recording it with no signal at all. for that I've used the same deck: my SONY TC-K909ES. The difference is the erasing head, which causes (adds) the bias noise. You may or may not figure it but the noise is lower when the tape is blank.

The noise is usually measured at 15kHz. If you want to know more about noise, please read my article about that. In this tape it's about -65 dB (below the reference, Nak scale) using the recording head and almost -68 dB by just playing a blank tape. These values are just in line with other similar tapes.

You can see that the blank tape is quieter than if you record (even with no signal at all) over it. That’s something I already noticed many years ago but I didn’t have any way to take profit of this. Now I have, and it’s called AIWA XK-S9000. With its BTOR (Blank Tape Optimized Recording) system you can record over blank tapes without using the erasing head, which is responsible for this difference.

A real test

To end this article, I’ve set the optimal bias, added an EQ curve and recorded a song that you can listen or download. To record it I’ve used this:

• Source: ARCAM irDAC. FLAC file (no high resolution).
• Deck: AIWA XK-S9000 restored and calibrated by Sergi, to use the B-Rec. SONY TC-K909ES serviced and upgraded by ANT Audio.
• Capture: Creative X-Fi Elite Pro at 24bit/96 kHz. Adobe Audition CC. Exported as MP3 VBR at about 192 kpbs average without common stereo, high quality encoding.

Both recordings were made so the levels went up to Dolby level in majority peaks, leaving only the highest peaks to occasionally go up to +2-3 dB over Dolby level.

As you can hear, there are clear distortions here and there, like at 1:45, 7:25. 8:00. 8:50, 9:00, 10:08 and many moments where dynamic compression (of the very song) is somewhat high and causes heavy and clearly audible distortion.

[UPDATE 2018/09/19] As user Kirunavaara says in Tapeheads.net, the design is almost an exact copy of an old Ampex, as seen in C-90.org:

Conclusion

After all these tests, I can conclude that the Audio Pro is a decent quality tape that might work well in a deck that allows the user to adjust recording parameters, but it probably won't work well in a deck with fixed bias and level settings.

The shell doesn’t look as a professional product and the performance doesn’t look as that either. A professional tape should have a professional shell (or at least an excellent one) and a high performance, high (or even neutral) bias with a very flat response... which is not the case for this Audio Pro.

To make this tape perform as a pro tape you have to make very strong adjustments that not many machines can do: very low bias, push the Rec Lavel Cal and use a quite steep EQ curve. Duplicating machines don’t have such kind of adjustments. With a neutral bias you can get a decent, somewhat flat response but the THD suffers a lot, getting into dangerous values, near 2% of THD. Comparatively, a simple ’89 TDK D outperforms the NAC Audio Pro by far.

So the Audio Pro is not really a bad tape. I'd say it just has a weird performance. But the rise of HF might be actually a good thing with many walkmans and other devices with deficient high-frequency response on playback (see Techmoan's review of the Toshiba Aurex Hi-Res deck, for example) as it will compensate that poor performance.

For cassette enthusiasts, however, I can’t give this tape a good score due to all these reasons. The best thing (for NAC) is that this tape is not made by them but by Saehan. I hope the brand new tape that NAC is developing and will be released late this year can be much, much better... (let’s hope that).

A big thanks to Terence (aka Wilhelm at Tapeheads forums) who has given me a lot of help with this article and, of course, to Vincenzo who is always helping me with technical and grammatical corrections. Also to Pacific Stereo for his corrections and suggestions.