Akai GX-620 Reel to Reel Tape Deck Measurements

[solderdude]

I did try both tape types. Made more or less no difference to measurements!

Still it is worthwhile measuring the FR at -10dB instead of 0dB.

You can use the tape you currently have and adjust bias so that FR is flat when measured at -20dB or if it makes little difference at -10dB.
After adjusting bias to that tape simply continue using that type of tape.
FR at 0dB may not measure linear while it could be at -10dB. Given treble levels rarely make it above -10dB it should sound good while lower frequencies can still be 0dB or even over it.

 

[Stephen H]

Still it is worthwhile measuring the FR at -10dB instead of 0dB.

You can use the tape you currently have and adjust bias so that FR is flat when measured at -20dB or if it makes little difference at -10dB.
After adjusting bias to that tape simply continue using that type of tape.
FR at 0dB may not measure linear while it could be at -10dB. Given treble levels rarely make it above -10dB it should sound good while lower frequencies can still be 0dB or even over it.

Ah OK, thank you for the suggestion. I may try this some point soon and post an update!

 

[DSJR]

This is a very long time ago and memory is vague as R2R was a sideline for me. Isn't response done at -20dB? (can't remember the exact amount in nano-Webers?). back then, there were differences in boas requirements for different tape brands and I remember that despite the costs even then, we sold huge amopunts of Maxell UL(?) and TDK 'Audua' 10.5" reels.

I had a high speed 'IEC' two track Revox B77mk2 and it was a lovely thing to look at and watch working, but that was it really, as I never used mine for live recording as my pal did. I was cash strapped in the mid 90's and sold it along with my Nak CR7. The friend who bought them (along with other Nak machines I had over the years) passed away over two years back and his museum of mint condition audio gear was sold off and resold at no doubt huge profit to collectors I suspect. His wife was only too happy to get her house back...

 

[AndreaT]

Hi, great review! As I remember in my days of cassette tape recording, choosing a recording level for the line signal (from the preamp and phono input) was a difficult task. Too high a level would spoil the fr, too low would worsen the s/n ratio…I am glad it is now a digital Era.

 

[30 Ounce]

The low frequency bump is “head bump”. A common issue with tape recorders and something the pros had to deal with for years. http://www.endino.com/graphs/

 

[voodooless]

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!

Stop making people happy with a dead sparrow Looks like you can't buy these boards anymore. I was super excited to see a good ADC board, went on to look for it, to be massively disappointed by the lack of availability

But otherwise, nice writeup! Cool to see the old tech in action.

 

[Stephen H]

Stop making people happy with a dead sparrow Looks like you can't buy these boards anymore. I was super excited to see a good ADC board, went on to look for it, to be massively disappointed by the lack of availability

But otherwise, nice writeup! Cool to see the old tech in action.

Yeah it's a shame regarding availability. I would email the owner of the website (Tomi Nihtila) if you were interested in getting your hands on one. He is a really helpful guy and may be able to say when and if he'll be making more boards. I know he has struggled due to AKM availability.

Glad you liked the writeup

 

[Stephen H]

The low frequency bump is “head bump”. A common issue with tape recorders and something the pros had to deal with for years. http://www.endino.com/graphs/

Ahh fair enough, thanks for the knowledge tip!

 

[Shiva]

Nice review. I have the same DAC and its nice to see that it still measures well.

 

[mhardy6647]

No probs

Tape speed = 7.5ips.

Recording level set at 0db (deck runs at 250nWb/m reference fluxivity).

Indeed a lot of variables here! Far too many for me to investigate. The deck could certainly be tweaked and further calibrated to improve performance. However, it's good enough for me!

Although reel to reel is a priori more forgiving of high signal levels than cassette*, the frequency response should be broader (and possibly??? flatter) at, say, -10 dB than at zero (and, that, of course, assumes that the meters are accurate -- unless you calibrated the signal level at the record amps some other way?).
EDIT: Oooops, I guess the -20 dB thing was already amply pointed out by others -- sorry!

Bear in mind, e.g., that cassette frequency response specs, back in the golden age , were determined at a signal level of -20 dB.

________________
* The bandwidth, and particularly the HF headroom of cassettes, was incrementally improved over the years by a variety of (clever) tricks, Dolby HX Pro being perhaps the most effectual (and also, probably, the last gasp of the technology just before the digital asteroid hit).

Dolby C and HX Pro frequency response by Mark Hardy, on Flickr

I believe I lifted this from an old h/k cassette deck ad from the very early days of HX Pro. I, rather irresponsibly, didn't document the reference when I scanned that image some years back.

 

[Stephen H]

Although reel to reel is a priori more forgiving of high signal levels than cassette*, the frequency response should be broader (and possibly??? flatter) at, say, -10 dB than at zero (and, that, of course, assumes that the meters are accurate -- unless you calibrated the signal level at the record amps some other way?).
EDIT: Oooops, I guess the -20 dB thing was already amply pointed out by others -- sorry!

Bear in mind, e.g., that cassette frequency response specs, back in the golden age , were determined at a signal level of -20 dB.

________________
* The bandwidth, and particularly the HF headroom of cassettes, was incrementally improved over the years by a variety of (clever) tricks, Dolby HX Pro being perhaps the most effectual (and also, probably, the last gasp of the technology just before the digital asteroid hit).

Dolby C and HX Pro frequency response by Mark Hardy, on Flickr

I believe I lifted this from an old h/k cassette deck ad from the very early days of HX Pro. I, rather irresponsibly, didn't document the reference when I scanned that image some years back.

Thanks for the info!
I will try and get round to running the frequency response test at -20db to see how it performs. This has been suggested by a good few people now, so it's definitely something to explore.

If I do get round to it, I will be sure to update this post.

 

[mhardy6647]

I do feel compelled to point out that -20 dB wasn't (at least as I understand it!) chosen as a marketing ploy to make tape recorder performance look artificially better than it was. The notion (again, as I recall) is that -20 dB levels are a reasonable simulation of the average signal levels for properly recorded music. That said, I like to point out to my students that the average isn't always the best statistic to employ to evaluate a population of... many things. I like to show 'em a photo of our erstwhile (now, sadly, departed) kitty Zappa and point out that he was grey* on the average.

________________
* or gray, if one prefers.

 

[mhardy6647]

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...

_______________________
* Peter Falk as Lt. Columbo:

 

[Stephen H]

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...

View attachment 239153

_______________________
* Peter Falk as Lt. Columbo:

Awesome! Thanks for sharing

I will have a look through the website later

 

[AndreaT]

Yes…-20 dB on the average music signal was my VU meter(s) visual reference when making cassette tapes with TDK SA…some CD players from the 1990’s had a “peak search” option to show the peak level in the CD and allow a calibration of the cassette recorder. It did not always work. Finally, it seems to me that early CDs were mastered like…tape, with an average level way too low. But in the digital world it is the low level signal the more difficult to reconstruct for the DAC…

 

[tomchris]

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...

I am trying to locate a review index covering the japanese Stereo Sound magazine. Any ideas?

 

[Ralph_Cramden]

Yeah, but they sure do look great. I keep mine for that reason alone.

 

[Matias]

Wow that frequency response! It is an "always on" equalizer. It explains an experience I have years ago comparing a tape desk to a modern DAC. I wonder if other tape decks also have this response.

 

[30 Ounce]

Wow that frequency response! It is an "always on" equalizer. It explains an experience I have years ago comparing a tape desk to a modern DAC. I wonder if other tape decks also have this response.

That is probably a result of over bias. Proper bias for each type of tape was necessary for extended and proper high frequency response. I think if the bias was set properly this would have excellent frequency response well past 20khz.

 

[Matias]

That is probably a result of over bias. Proper bias for each type of tape was necessary for extended and proper high frequency response. I think if the bias was set properly this would have excellent frequency response well past 20khz.

Still, if a user can have a bad bias in his unit, other users might as well. And I wonder if other brands like Revox too.