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Thread: Revox A77 recording level question

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    Revox A77 recording level question

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    Does anyone know at what recording level the Revox A77 was set up for when the machine left the factory new.

    Was it 0 db using a 185 nWb/m test tape
    Was it 0 db using a 257 nWb/m test tape
    Or was it some other level. Or was the level changed during the production run as better tapes became available

    I have always believed it was set up for 0db = 185 nWb/m and also that any voltages mentioned in the manual were written for this level.
    But am I right?

    If anyone can shed any light on this I would be gratefull for the info.

    Thanks Demto

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    I've restored seven A77 machines, including new heads and used a TEAC YTT-1003 test tape which has a fluxivity of 185nW/m at 400hz. All but the last 2 machines set up as per the Revox manual. As for the last two machines, somewhere along the line, the test tape got damaged. These last two machines were set up with a MRL 21T204 test tape which is 250nW/m. This results in setting the VU meters at +3dB above Revox spec. The actual difference between 185nW and 250nW is 2.61dB but the 0.4dB difference is below the significant resolution of the A77 VU meters as well as being frequency dependent. The calibration frequency affects the fluxivity. Since I can't insert links, go to MRLTAPES website. The home page gives their address on top so you'll know you are at the correct website, It is 165 Wyandotte DR. San Jose, CA. Click on the link in the second paragraph of that page, "Choosing and Using
    MRL Calibration Tapes for Audio Tape Recorder Standardization". It will open a pdf file. Go to TABLE 4 on page 8 of that file, "Reference Fluxivity Conversions". Also note the reference to section 5.2.4.1 on page 11 which explains the "Effect of Reference Frequency"

    This website is extremely detailed but contains a plethora of information on magnetic tape. Ignoring the difference in fluxivity may result in having a hell of a time setting up the bias as the tape may saturate at high frequencies.

    Jay McKnight's papers are extremely informative.

    Hope the above is useful.
    Robert

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    hi Robert, thanks for replying
    I looked at the MRL site a couple of weeks ago and yes, it is very interesting

    Do you have any info about my question ?

    In the absence of anyone who has definate info to the contary I am going to assume that A77s left the factory calibrated at either 180 nWb/m @
    1000 hZ= 0dB or 185 nWb/m @ 700hZ= 0dB. As they both amount to the same thing it doesn't matter which they used.

    Have you ever tried setting the A77 to a higher reference level than +3dB.... Say +6dB for example?.... If so how did the A77 sound 0k or did it run into any headroom problems,bad distortion etc. I know some people have tried it and say it is 'ok' but no real info apart from 'ok' ?

    Mark
    Last edited by demto; 03-27-2019 at 08:07. Reason: spelling check

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    Your 185nWb/m assumption is in agreement with mine. I realize I didn't mention that directly although it was implied. I also agree with you that 180nWb/m at 1khz and 185nWb/m at 700hz is six of one, half a dozen of the other. As stated in my previous post, all but 2 of my machines were biased with a 185nW/m Teac test tape. However, in all modesty, I was unaware at that time about the different fluxivities. You're right also about the voltages in the manual. A few years ago, I became aware that Revox lowered the playback level to 1.55v but I made no changes to the previously set up machines based on the premise that if it works, don't fix it.

    I did, at one time, accidentally not correct for the +3dB difference between the 185nWb/m and the 250nWb/m fluxivities. The result was an inability to bias the machine to get a relatively flat response to 20khz; it fell like a rock somewhere above 13khz. Of course, the type of tape used can also affect that. Finding the cause was somewhat frustrating and when found, my first thought was "dummy". I never did intentionally experiment with over biasing. Ten years ago when I started messing with restoring A77 tape recorders, I knew little to nothing about their modi operandae. Even after a decade, I'm not about to challenge the expertise of people like Willi Studer and John McKnight.

    Personally, I don't see an advantage to over-biasing a tape recorder despite that it may work with one type of tape but may fail drastically with another. It's kind of similar to over clocking a computer microprocessor. Sure, it'll run faster but it's more prone to instability. Similarly, nitromethane added to the gas in a car will add power but at the cost of engine life.

    So, yes, the A77 will perform OK with higher bias but at the cost of some high frequency response. However, most will not notice that as there isn't much musical content above 15khz, not to mention that most people above 40 can't hear that high anyway and if they were exposed to very high SPLs, like rock concerts or industrial noise, many can't hear above 13khz by the time they get into their 30's. As for distortion, such a practice can drive that to several percent. Typically, the machines I've set up have less than 2% (spec is 2% at 7.5 ips) but that's measured at 500hz. At 10khz, the first harmonic is 20khz, at the threshold of human hearing. However, at say, 1khz, we can have 2khz, 3khz, 4khz, etc., the sum of which can audibly change the sound of the fundamental. However, this isn't as serious as it may appear as without knowing how the fundamental sounds (especially in music), no comparison can be determined, audibly. Loudspeakers are notorious for their harmonic distortions which is one reason why people prefer some speakers over others - subjectivity. That is one very good reason why all but the highest priced speakers will publish THD figures. By speakers, I refer to the speaker by itself, NOT a speaker system.

    I do have a Tektronix harmonic distortion analyzer (AA501) but measuring THD at high frequencies from a tape recorder is impractical since the tape noise will interfere. The analyzer measures THD+N, total harmonic distortion plus noise. In other words, anything but the fundamental is displayed and in all probability, the noise floor will exceed THD.

    The process used to set bias makes assumptions, most likely the tape used. (See Table 6.5-69 in the manual) As engineers say, when the goal can't be attained, check the assumptions. I use a sweep process, recording an appropriate sweep from 20khz to 20hz fed to the input and monitoring the output, with the recorder in NAB. The result of bias and EQ adjustments can be seen on a computer screen instantly. The hardware and software is LMS by Linear-X.

    One thing I did notice is that the high frequency response can change from the beginning to the middle of the tape. One major cause was found to be a badly worn pinch roll. I would suspect a new pinch roll could give the same error if the pressure between it and the capstan was not correct. An incorrectly set or worn pinch roll can even cause frequency response variations between two consecutive sweeps. Taking about a minute to run a sweep from 20khz to 20hz, at 7.5ips, that's 37 feet of tape. I've seen it happen with much shorter consecutive sweeps between 20khz and 10khz. Such an affected pinch roll and/or adjustment can wreak havoc on trying to set the azimuth of the heads but in many cases, despite that, the machine can sound OK. Keep in mind that none of these machines of mine, despite some having old pinch rolls, produce audible WOW, even on sustained violin passages. However, a 10 khz or higher signal seen on a scope will drift, wiggle and bounce. This is caused by variations in the intimate contact between the tape and the heads. Under ideal conditions, the scope trace should be as stable as if it were coming directly from the generator.

    In the heyday of tape recorders, reel to reel or cassette, many people preferred a certain brands of tape. Neglecting subjective preferences, the difference in sound from various tape brands was mainly the result of the relationship between the recorder and the tape. namely bias. Based on that, if one sticks to the same brand of tape, the machine can be set to spec or even overbiased if the tape can handle it and performance will be consistent.

    I'm 76 and can't hear above 12khz yet I've spent hours setting up these seven machines to perform their best; a couple actually exceed factory specs. A couple are biased for older tapes. Why? Nostalgia. When I record 50's and 60's rock n roll, those old tapes and their decorated boxes add a certain flavour to the music. Then there's the mesmerizing effect of those rotating reels. Like fancy wheels on a car which add little to its performance, they look cool.

    Not knowing what type of meters you're using to measure these high frequency voltages, be aware that some DMMs don't give accurate results above 3khz to 5khz. While some will, it's best to check them with a constant voltage output signal generator.

    I have a website dedicated mostly to loudspeakers but there are some pages on my experiences with the venerable A77. Unfortunately, I have yet to meet the required number of posts to allow adding hyperlinks. I could split this post into several and meet that requirement but that would be cheating and is against my principles.

    Good Grief, I'd best stop here. My apologies for the diatribe. In defense, consider MRL's website and all they're doing is explaining the magnetic recording process. Consider the loudspeaker. A visually very simple device. Yet, an engineer can come up with analyses and data that mean nothing to anyone but another engineer. A guitarist doesn't care about the physics of a vibrating string, only how it sounds. Which, in conclusion, brings me to my pet expression on audio, which is, "It's unfortunate that so many pay so much attention to their audio systems that they miss the music."

    Robert

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    Hi Robert

    No, it wasn't higher bias I was enquiring about, it was have you ever tried using the A77 at a higher reference level.

    For example....the ORIGINAL reference level was 185 nWb/m = 0dB
    250 nWb/m = 0dB equals +3dB above the original reference level. Most A77's are now set at this reference level as recording tapes have got better over the years and the A77 can take advantage of this. (more signal, less less tape hiss and noise etc)

    The question is can this be pushed further by using an even higher higher reference leval (eg 355 nWb/m= 0 dB equals +6 dB above the original reference level for an even better signal to noise ratio)

    Have you ever tried it on any of youe A77's ?? ..Some people say they have and say it works ok but don't give any more info. Do you have any info
    on this or have you never been tempted to try it ?

    Changing the subject, just coincidence but I am 77 years old and I do have an interest in loudspeakers. I built some about 20 years ago and have updated them since. They are 3 way with a ribbon tweeter, 4" midrange, and a 10" bass uni. The crossover is 24 dB octave electronic between bass and midrange and passive between midrange and tweeter.

    Mark
    Last edited by demto; 03-28-2019 at 08:48. Reason: spellcheck

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    Please accept my apology for using the incorrect term, over biasing. Referring to my previous post, paragraphs 1 and 2, I should have used a term such as "a higher reference level". I did mention the possible consequence of using a higher reference level, but then focused too much attention on one of the consequences, namely bias. The initial consequence of using a higher reference level is tape saturation. That may be why Revox lowered the playback reference level to 1.55v. Another possible reason may have been to compensate for the higher reference level tapes (250nW/m). Perhaps the use of 185nW/m was being discarded in favour of the 250nW/m reference.

    I hadn't become aware of Revox's lowering the initial level to 1.55v until long after setting up the last machine. That may have eliminated subsequent level adjustments problems in the setup process.

    In paragraph 2, I mentioned my not correcting for the +3dB difference between 185nW/m and 250nW/m. According to my service logs, that compensation was made later in the setup procedure. In section 6.3.3 (Playback level from Test tape), the output voltage was set to 2.9v (+3dB above 2v) and in section 6.5.4 (Record Level), the 200mV was set to 290mV. I keep service logs as my memory doesn't always serve me well.

    Is your asking about higher reference level purely academic or does it have an intended application? If academic, I can understand that but if it is to be applied to an A77, my first thought would be to avoid it as it has repercussions all through the setup process.

    From what I understand, a higher reference increases the signal to noise ratio. With new tapes, I've not noticed tape noise unless the tape is blank. I do have a working Advent Model 100 external Dolby processor that dates to the 70's but have not used it since then and only on my SONY TC630D, which I still have, working but in need of heads. I know, good luck. I have considered sending them to JRF Magnetics for re-lapping.

    Robert

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    Hi Robert
    I have my A77 in storage together with my test equipment as I am staying with my sister while the purchase of a new home for me and all my goods goes through. So I can do nothing right now
    When I am settled in at my new place I think I will completely calibrate the A77 to a +6 dB recording level which is +3 dB higher than it is at the moment.
    Then I can try it out thoroughly and see what I think......keep it or go back to +3 dB ? Who knows only time will tell
    Last edited by demto; 03-29-2019 at 13:56. Reason: spellcheck

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    If you use a higher reference level check that the electronics can handle the higher level peaks. I seem to remember that the A77 doesn't have as much headroom as the later B77.
    JRP Music - Audio Mastering and Restoration
    http://www.jrpmusic.co.uk

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    Quote Originally Posted by jamesperrett View Post
    If you use a higher reference level check that the electronics can handle the higher level peaks. I seem to remember that the A77 doesn't have as much headroom as the later B77.

    Thanks for your reply

    You are right about headroom. I dont know what the headroom is on a B77 but I did measure my A77 about 14 years ago and dont remember the exact figures but after I had already set the reference level to +3 dB I think that the recording headroom was about 8-9 dB and the replay headroom was about 5-6 dB. I can only try the +6 dB reference level and see how it sounds.

    The so called +3dB reference level level was set with a + 3dB calibration tape but due to Revox specifing the playback level at 1.55 V = 0dB for reference levelit is actually only +2.2dB.

    If +6dB has problems then I might try a genuine +3dB or +4dB. I can only try and see how it goes

    Mark

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    In my experience professional recordings have peaks 10-14dB above the reference level.
    JRP Music - Audio Mastering and Restoration
    http://www.jrpmusic.co.uk

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