Stephen H
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- Oct 23, 2022
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I recently had the idea to use my hi-spec ADC in combination with a USB sound card to have a go at creating my own measurements. This is the first time I've done this, so keep in mind there may be some mistakes etc. This is not my area of expertise.
Above is my beautiful Akai GX-620.
I replaced all the dodgy Akai transistors and swapped out the electrolytic capacitors whilst I was at it. I have always been quite surprised by how good this thing sounds when listening to the source. However, I do find there is a bit too much bass when recording to tape. This could be caused by many things. I don't imagine it is particularly well calibrated, and the tape heads have probably seen better days. However, I've wanted to have a go at testing and measuring something for a while now, and this felt like the perfect starting point.
Devices used for capture:
Documentation & Notes:
1Khz Tone:
I was very surprised to see how similar it is to Amir's review. It is more or less identical. So nothing much to see here!
Moving on to a 32-tone signal:
Seems to be behaving as I would expect. Although, one thing to keep in mind, the above is not offset to 0db.
Frequency Response:
Nothing exciting to see here. Good and flat as you would expect.
Moving on to something more exciting... the below measurements show the tape deck hooked up between the DAC and the ADC.
First I measured the pre-amp alone (not recording).
1Khz Tone:
The above result was better than I was expecting! I reduced the voltage down slightly to not overload the pre-amp board, hence why the fundamental is 10db down. Also bit of mains noise going on (50hz here in the UK) and some harmonic distortion.
32-tone signal:
Doesn't seem too bad, noise floor would be much higher (almost 30db) if you were to offset the results for calculating SNR etc. Looks like the same mains noise going on.
Frequency response:
Seems flat enough to me. Little roll-off either end, but nothing major wrong here.
Now, the really interesting part! Let's record these tones to tape and see what happens. I used SM911 tape from Thomann.de
1Khz Tone:
Harmonic spikes have increased in size. Noise floor has also raised due to tape hiss. Personally, I am still pleasantly surprised by this result. I was expecting to see absolute carnage!
32-tone signal:
I'm afraid you guys are more or less on your own with the above result here. There is seemingly lots of distortion and the noise floor is uneven around 50-100hz. Also, you can see the frequency response is looking a little sloped down from the bass side of things. More below.
Frequency response:
Yeah, this is pretty ugly compared to what we're used to seeing on this website. As I have always thought, there is elevation in low frequencies. There is also a bulge around 50hz which I'm guessing it due to mains in the UK. I wonder if this would show any improvement with re-capping the power supply board? I have only re-capped the pre-amp board so far.
All in all, I was surprised by the above. I was expecting it to be a total horror show. Apart from the uneven frequency response, it doesn't seem too bad. However, as I first mentioned, I am not an expert in this area by any stretch of the imagination. I am merely a hobbyist.
I hope anyone who took the time to read this post has managed to get something out of it. Unfortunately, I won't be the best at explaining what it all means. However, I will say that, I was always surprised by how good this deck sounds. Obviously it doesn't sound like a modern day DAC, but I would never expect it too. I always had noticed the recordings from the deck had more bass, and sounded a little muddy when compared to the source. It's nice to see the data back this up by showing the uneven frequency response of the recording when compared to the pre-amp only measurement. Unsurprisingly, apart from the SU-8 alone, none of the above measurements look like what we're used to seeing here.
As a small side note. I can highly recommend the AK5572 ADC. What a fantastic little board that is. Coupled with the SPDIF converter here, the result is a truly fantastic performing ADC once boxed and fed with a decent PSU. I cannot remember exactly what PSU I used now, however it was a small silent-switcher style design. Clearly the above demonstrates that all working too!
Above is my beautiful Akai GX-620.
I replaced all the dodgy Akai transistors and swapped out the electrolytic capacitors whilst I was at it. I have always been quite surprised by how good this thing sounds when listening to the source. However, I do find there is a bit too much bass when recording to tape. This could be caused by many things. I don't imagine it is particularly well calibrated, and the tape heads have probably seen better days. However, I've wanted to have a go at testing and measuring something for a while now, and this felt like the perfect starting point.
Devices used for capture:
- Desktop PC running Windows 11.
- SMSL SU-8 v2 DAC.
- Nihtila ADC AK5572
- SoundBlaster SB1240
- Multitone software.
Documentation & Notes:
- The SMSL DAC has already been reviewed and measured courtesy of our friend Amir: Click Here
- Audio Precision results for the ADC can be found on Tomi Nihitila's website: Click Here
- Toslink connects from the ADC to SoundBlaster 1240.
- SoundBlaster Connects to PC via USB.
- SoundBlaster responsible for Digital-to-Digital conversion only.
- Bitrates all set to match between DAC and ADC etc (96khz).
- Measurements have not been offset to 0db.
- ADC is being used single-ended and not balanced which loses 6db of dynamic range.
1Khz Tone:
I was very surprised to see how similar it is to Amir's review. It is more or less identical. So nothing much to see here!
Moving on to a 32-tone signal:
Seems to be behaving as I would expect. Although, one thing to keep in mind, the above is not offset to 0db.
Frequency Response:
Nothing exciting to see here. Good and flat as you would expect.
Moving on to something more exciting... the below measurements show the tape deck hooked up between the DAC and the ADC.
First I measured the pre-amp alone (not recording).
1Khz Tone:
The above result was better than I was expecting! I reduced the voltage down slightly to not overload the pre-amp board, hence why the fundamental is 10db down. Also bit of mains noise going on (50hz here in the UK) and some harmonic distortion.
32-tone signal:
Doesn't seem too bad, noise floor would be much higher (almost 30db) if you were to offset the results for calculating SNR etc. Looks like the same mains noise going on.
Frequency response:
Seems flat enough to me. Little roll-off either end, but nothing major wrong here.
Now, the really interesting part! Let's record these tones to tape and see what happens. I used SM911 tape from Thomann.de
1Khz Tone:
Harmonic spikes have increased in size. Noise floor has also raised due to tape hiss. Personally, I am still pleasantly surprised by this result. I was expecting to see absolute carnage!
32-tone signal:
I'm afraid you guys are more or less on your own with the above result here. There is seemingly lots of distortion and the noise floor is uneven around 50-100hz. Also, you can see the frequency response is looking a little sloped down from the bass side of things. More below.
Frequency response:
Yeah, this is pretty ugly compared to what we're used to seeing on this website. As I have always thought, there is elevation in low frequencies. There is also a bulge around 50hz which I'm guessing it due to mains in the UK. I wonder if this would show any improvement with re-capping the power supply board? I have only re-capped the pre-amp board so far.
All in all, I was surprised by the above. I was expecting it to be a total horror show. Apart from the uneven frequency response, it doesn't seem too bad. However, as I first mentioned, I am not an expert in this area by any stretch of the imagination. I am merely a hobbyist.
I hope anyone who took the time to read this post has managed to get something out of it. Unfortunately, I won't be the best at explaining what it all means. However, I will say that, I was always surprised by how good this deck sounds. Obviously it doesn't sound like a modern day DAC, but I would never expect it too. I always had noticed the recordings from the deck had more bass, and sounded a little muddy when compared to the source. It's nice to see the data back this up by showing the uneven frequency response of the recording when compared to the pre-amp only measurement. Unsurprisingly, apart from the SU-8 alone, none of the above measurements look like what we're used to seeing here.
As a small side note. I can highly recommend the AK5572 ADC. What a fantastic little board that is. Coupled with the SPDIF converter here, the result is a truly fantastic performing ADC once boxed and fed with a decent PSU. I cannot remember exactly what PSU I used now, however it was a small silent-switcher style design. Clearly the above demonstrates that all working too!
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