Akai GX-620 Reel to Reel Tape Deck Measurements

[pseudoid]

...I've always loved the light-up buttons on these decks. Not that I'm unhappy with my Akai GX-635D!

....

OMG! The dust-cover for the reels is was such a clever idea. Never had seen them before.

 

[JaccoW]

OMG! The dust-cover for the reels is was such a clever idea. Never had seen them before.

I can imagine a lot of them got damaged over the years or some people didn't like them all that much. I've come across some collectors who thought they looked ugly.
Personally, I love the way they look and it makes it a lot safer to keep the reels on over time and not have the tape covered in a layer of dust.

Though like many parts on these machines, original parts are expensive, with these dust covers going for $200-300.

 

[guenthi_r]

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.

View attachment 239012

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.

Chain of devices (in loop): Windows PC -> SMSL DAC -> Akai GX-620 -> Nihtila ADC -> SoundBlaster 1240 (via Toslink) -> Windows PC

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.

The below measurements shows the SMSL DAC plugged directly into ADC:

1Khz Tone:

View attachment 239013

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:

View attachment 239014

Seems to be behaving as I would expect. Although, one thing to keep in mind, the above is not offset to 0db.

Frequency Response:

View attachment 239015

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:

View attachment 239017

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:

View attachment 239018

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:

View attachment 239019

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:

View attachment 239020

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:

View attachment 239021

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:

View attachment 239022

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!

For comparison, measurements from a Compact Cassette Deck (not High-End):
Pioneer CT 737

 

[SmackDaddies]

the Akai has a very 'warm' bottom end for sure

"warm sound"? That is what my DAC is for.



My work here is done.

 

[Prince Valiant]

Thanks for the post. It's always nice to see something other than DACs.

Oh, sorry -- as Lt. Columbo* used to say (more or less): ahh... one more thing...

@Stephen H on the off-chance that you don't know about it, there is an incredible resource of technical information on pretty much all things radio/audio/tape in one spot at www.worldradiohistory.com

More to the point, besides the hifi rags of the golden age (High Fidelity, Stereophile, Stereo Review, and - one of the best of all, Audio), there are collections of magazines (and some books) on studio and consumer tape recordin to be found there, too. The scanned document databases have been OCRd and are even searchable Quite the treasure trove, and a great way to spend a rainy day. Hey! We're having a rainy day today...

Kind of them to do so, that's a lot of correction work.

 

[Jim Shaw]

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.

View attachment 239012

...

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.

I have a vintage Pioneer R2R deck here. It works just fine (arcane audio measurement term). It sits, ready to go, with a case of unused Ampex-branded tape nearby. It has cables for line in/out, dangling from it.

For the life of me, I cannot find any reasonable use for it.

Yeah, it's 'cool' (arcane audio term) to watch the reels go roundy-roundy while the meters go wiggy-waggy (arcane audio terms). It goes ka-thunk nicely when buttonpushins are gafingerpoked. The little lights all work. It doesn't spill tape and it rewinds like a damn typhoon. The audio? It sounds okay, but any untrained listener could hear slight differences* from a high res source. (*Different, not so much worse.) Noise level is, I suppose, reasonable for 1970 home gear.

For the life of me, I cannot find any reasonable use for it. Does anyone have a sensible idea for its beneficial use?
(I've had several suggestions that it could make a useful buoy anchor or a stage weight. It seems of more value than that.)

No, I won't gift it to you.

 

[pseudoid]

I have a vintage Pioneer R2R deck here. It works just fine...It sits, ready to go, with a case of unused Ampex-branded tape nearby. It has cables for line in/out, dangling from it.
For the life of me, I cannot find any reasonable use for it....... Does anyone have a sensible idea for its beneficial use?

• Load it up (RTFM first for a brush-up) with your fave music of choice
• Plug-in the outputs to pre/amp/speakers/headphones
• Invite/warn other occupants in the house
• Dim the lights
• Locate libations and/or other mood-heightening aides @ arms reach
• Sit down in your sweet-spot
• Adjust Volume (as needed) and press Play
• Enjoy the music
• Report back, when reel reaches end...

Or your money back!

 

[amirm]

No, I won't gift it to you.

I was getting my hopes high as I was reading your post and then landed into this brick wall!!!!

 

[iraweiss]

I have a vintage Pioneer R2R deck here. It works just fine (arcane audio measurement term). It sits, ready to go, with a case of unused Ampex-branded tape nearby. It has cables for line in/out, dangling from it.

For the life of me, I cannot find any reasonable use for it.

Yeah, it's 'cool' (arcane audio term) to watch the reels go roundy-roundy while the meters go wiggy-waggy (arcane audio terms). It goes ka-thunk nicely when buttonpushins are gafingerpoked. The little lights all work. It doesn't spill tape and it rewinds like a damn typhoon. The audio? It sounds okay, but any untrained listener could hear slight differences* from a high res source. (*Different, not so much worse.) Noise level is, I suppose, reasonable for 1970 home gear.

For the life of me, I cannot find any reasonable use for it. Does anyone have a sensible idea for its beneficial use?
(I've had several suggestions that it could make a useful buoy anchor or a stage weight. It seems of more value than that.)

No, I won't gift it to you.

Looks like my Pioneer RT-707. I still use it to play recordings made in the 1960s and 1970s.

 

[iraweiss]

• Load it up (RTFM first for a brush-up) with your fave music of choice
• Plug-in the outputs to pre/amp/speakers/headphones
• Invite/warn other occupants in the house
• Dim the lights
• Locate libations and/or other mood-heightening aides @ arms reach
• Sit down in your sweet-spot
• Adjust Volume (as needed) and press Play
• Enjoy the music
• Report back, when reel reaches end...

Or your money back!

Add saw blades to the reels and use the high torque motors to cut wood.

 

[fpitas]

Add saw blades to the reels and use the high torque motors to cut wood.

Hmmm. Horror movie material right there. Death Reel 2022!

 

[mhardy6647]

Thanks for the post. It's always nice to see something other than DACs.

Kind of them to do so, that's a lot of correction work.

I am still wondering who's footing the bill. It is an enormous repository of (from my perspective, at least) incredibly valuable information.

 

[JayGilb]

Hmmm. Horror movie material right there. Death Reel 2022!

A combination of Saw + Mission Impossible where the tape self destructs and then ejects a saw blade at high speed.

 

[Jim Shaw]

I was getting my hopes high as I was reading your post and then landed into this brick wall!!!!

Imagine the crating and freight bill to send the machine to Seattle! It'd be like mailing a pallet of used paving bricks. )

 

[pma]

Nice job, @Stephen H !
Just for fun, below is a measurement of $40, contemporary product, cassette "recorder" GPO 162B

Portable Cassette Recorders | Best Portable Cassette Player

Portable Cassette Recorders, Best Portable Cassette Player,portable recorders for car,record and listen back to your cassettes.

www.gporetro.com

"Measuring cassette" was prepared by using SONY CFD-V21L CD Radio Cassette Recorder. CD with measuring signals was copied to the SONY EF-90 CC cassette in the CFD-V21L.

Below is the result. The "measuring cassette" played in GPO 162B, phone output to my Audio Buffer https://pmacura.cz/buffer_en.html (high input impedance) and then to E1DA Cosmos ADC. Recorded in AA to make a wav file. Wav file analysed by REW (BTW REW is by far the best software for correct THD+N measurements).

BTW, distortion (at $40 cost) is not much higher than that of GX-620. But the GPO 162B has horrible frequency response, as it is probably intended as a speech recorder. However, if one makes an EQ filter in AA, it is usable. Please also note the shift in frequency from 1kHz to 1.04kHz. Speed difference, can again be corrected in AA. The more primitive is the component, the more qualified operator it needs!!

 

[pma]

the noise floor is uneven around 50-100hz.

As the tape is moving, there are slow changes that FFT reads as increased noise floor, due its averaging nature.

 

[pma]

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:

I am very sorry, but the distortion numbers are wrong. It seems you have used Distort or Deltawave software by @pkane , and it has a fatal error. The amplitude of harmonics for distortion calculation must be related to amplitude of fundamental frequency and not to 0dBFS. I have already seen it several times at ASR, as the software by Paul starts to be a standard here. For this reason, such elementary mistakes should be corrected in SW.
The THD cannot be better than -49dB, please see explanation below. On the other hand, THD+N or noise is also not calculated properly. For distortion analysis, it is much better to use REW, as it produces reliable results.

 

[restorer-john]

The THD cannot be better than -49dB, please see explanation below. On the other hand, THD+N or noise is also not calculated properly. For distortion analysis, it is much better to use REW, as it produces reliable results.

The same THD+N issues ocurred with VA. (visual analyzer)

 

[pkane]

I am very sorry, but the distortion numbers are wrong. It seems you have used Distort or Deltawave software by @pkane , and it has a fatal error. The amplitude of harmonics for distortion calculation must be related to amplitude of fundamental frequency and not to 0dBFS. I have already seen it several times at ASR, as the software by Paul starts to be a standard here. For this reason, such elementary mistakes should be corrected in SW.
The THD cannot be better than -49dB, please see explanation below. On the other hand, THD+N or noise is also not calculated properly. For distortion analysis, it is much better to use REW, as it produces reliable results.

View attachment 240056

Fixed by selecting the correct units (dBr). The software provides dBFS display as that’s needed by another feature.

 

[restorer-john]

There should never be a situation where THD+N is less than THD alone.