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Thread: Tascam 388 Calibration and Bias

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    Tascam 388 Calibration and Bias

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    I have a Tascam 388 and have just recently bought an MRL 21T204 calibration tape. I' hopig to check the bias and set the operating level (both record and reproduce levels). I need help from some of you who've been though this before. It would be great if you could walk me through the process. I also have a Heathkit model IG-18 signal generator at my disposal. It will generate frequencies up to around 100,000Hz. I have a manual as well. I need help determining which inputs will allow me to calibrate which level, and what I will calibrate using the MRL tape (I'm assuming the playback level?). Thanks in advance for any info you can impart.

    Matt
    Last edited by migrant worker; 12-06-2009 at 11:44.

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    Matt, welcome!

    The MRL tape is used to calibrate the playback level and playback frequency response.

    You use your own fresh tape to calibrate the record level and record frequency response and to set the bias.

    The manual is really good on the step-by-step process (i.e. it is laid out in the steps as you should go through them).

    A suggestion: rather than try and lay out/recreate detailed instructions here, Teac already did it. Crack open that manual and put your specific questions in this thread if you hit stumbling blocks as you go through the steps.

    Also, look at the sticky thread on calibration at the top of the thread list.

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    Hey, thanks for the quick reply! Here's one other question. I'm planning to use Quantegy 457 (a +3 db OL tape as I understand it). Currently, all I have on hand is Quantegy 456. Would it be okay to go ahead and calibrate with 456 since it has the same fluxivity etc. or should I wait until I order the 457? Would there be any significant difference? Thanks for any of your opinions!

    Matt

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    My opinion, and hopefully others will chime in, but the only real difference between 456 and 457 is the thickness. 456 is a 1.5mil class tape and 457 is 1mil. Both are classed as a +6 tape. 406/407 are the +3 1.5mil/1mil equivalent tapes. You can certainly calibrate with the 456. Any differences in level calibration should be minimal. There may be very minor differences because of the thicker tape and it may effect the tape tension which can effect levels to and from the tape at the heads, but no harm setting it up with the 456 and getting a feel for the process. You'll at least get each track/channel consistent with each other which is one of the major goals of calibration. I do suggest you re-bias when you get the 457 (make sure it is Quantegy, not Ampex), or take a look at RMGI LPR35, a good quality 457 replacement. 456, 457, LPR35 are all considered "bias-compatible", but there will be minor differences for sure.

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    Cool! Thanks again...

    Thus far, my understanding has been that 456/457 are indeed +3 db standard operating level (SOL) tape, but have a maximum operating level (MOL) rating of +6 db.

    Comments made by Dave (A Reel Person) in the thread below from the Tascam forums led me to this conclusion...

    http://tascamforums.com/index.php?sh...aded&pid=62409


    I think he's right that there is substantial confusion over SOL and MOL. Personally, my impression so far is that SOL is, on average, where you want to keep your meters to prevent tape distortion (which, of course, is not always an effect you want to entirely eliminate, but I'll leave that concept for discussion in another thread). And the MOL value (in db) corresponds to the maximum fluxivity (in nWb/m) inherent to a given type of tape and is the point past which that tape distortion will begin to occur.

    Is this correct?

    If so, I think what I need to know is... which of these two measures of operating level is pertinent to the calibration procedure? Will I be using SOL or MOL to calibrate?

    One reason for my confusion is that at US Recording (where I bought my MRL tape) they refer to the MRL 21T204 tape as having a refrence fluxivity of 250 nWb/m, but they state that this corresponds to an "operating level" of +3 (without differentiating between SOL and MOL).

    Wait... I think I just found the answer (let me know if this is wrong)...

    In the 388 manual under the "RECORDER/REPRODUCER SECTION ADJUSTMENT" heading it says: "if you use 250 nWb/m tape, adjust the trim for -10db reading."

    On the spec sheet (below) for the MRL tape it states that the frequency response section was indeed recorded at -10 db

    http://home.comcast.net/~mrltapes/pub101.pdf

    So, it looks like they're just referring to them by reference fluxivity and not really using the OL (in decibels) at all. Is this correct?

    Finally, thanks again for all your help. I have downloaded TrueRTA for use in frequency analysis. Once I get this working, I hope to contribute to the forum by putting together a pretty comprehensive tutorial on calibrating the 388 using the MRL tape and this software (complete with pics, etc.).

    Sorry for such a rambling post, and...as always...thanks again!

    Matt

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    Ah.

    Okay.

    Yes. Before you just referred to "OL" and I assumed you were talking about the tape classes +3, +6, +9 etc.

    Yes indeed there is a difference between SOL and MOL.

    You can set a machine up for ANY SOL with any level of test tape...185nWb/m, 250, 355...if you play a 1kHz tone back on a 250 tape and set the repro level for "0" on the VU, and then put the 185 tape on and reproduce the same tone the meter will show about -3VU...put the 355 tape on and the meter will show about +3VU. So the idea is that you set the deck up so that the meters show "0" at an average signal level that takes advantage of your recording tape's headroom without pushing the tape into noticeable distortion (unless you are going for that). 250nWb/m is indeed +3, which is the SOL of the +6 classed 456 tape...+6 over the Ampex standard (which would be 0). So setting the repro level to 0VU at 1kHz using a 250nWb/m tape will set the deck up as you typically would for 456/SM911, etc.

    Switching gears for a sec...If you tend to like to run levels hot maybe you set the record levels so that the meters show -2VU when you are recording a -10dBv 1kHz tone. "-10" because the 388 mixer is a -10 mixer. That's 0.316VAC RMS. That way when you are setting your record levels when recording program material, your input trims will be turned up higher to get your average levels to 0VU. So you see, calbration accomplishes several things:

    1. Gets your meters calibrated to a known standard
    2. Gets your tape electronics consistent with each other...makes it so what you record on 1 track would sound the same in level and character (i.e. record and reproduce frequency response) if recorded on a different track
    3. Sets your meters to show 0VU for the average peaks in the program material you record for the sound you are wanting to get out of the tape you are using


    I hope that makes sense and I hope I'm accurate in what I'm saying. I'm looking to others to chime in and correct me or if I'm wrong lest I lead you astray.

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    Alright,
    Well... I've got the head aligned (used a software x-y scope for this) and I've set the record level of the meters with a test tone. Now I need some clarification about how to adjust the Reproduce Frequency Response. Dave mentions this in another pne of his posts I think:

    https://homerecording.com/bbs/showth...&highlight=388

    ...but is not really specific about how it's done. I have read section 1-5-2 in the manual and I'm not sure that I understand what to do or what the figure that is associated with it is trying to convey. If you've done this step, would you mind explaining what it is I'm trying to achieve and which meters I'll be looking at while performing the calibration, etc.

    Thanks!

    Matt


    PS: Read your post on restoring your 388 today... nice job (very thorough) ! WOuld love to see some pics of the finished product.

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    Arrow I'll cut to the chase:...

    I found I got the flattest Repro Freq Resp on the 388 by setting the proper level using the 12.5KHz test tone. The adjustment is limited to one pot, with which you must adjust to get the flattest average freq resp over the whole bandwidth. It's a balancing act. Try that and tell me if I'm wrong.
    Quote Originally Posted by Lt. Bob
    ... subtleties of sound make a difference to those who really listen.

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    Thanks Dave!

    I think I was able to get it done. If I did it right, it just involved adjusting all eight VU meters for each channel while reproducing the 12.5kHz tone using the appropriate trimpot (either 124 or 224) for the corresponding channel. All of them seem to read the same now, but as you alluded to in the other thread, there are a few points along the frequency curve at which they read inaccurately (i.e., the various frequency tones on the MRL tape are recorded at -10dB, but the machine reads less or more for a couple of the frequencies upon playback). If you think 12.5 works best to achieve the closest reading across the curve then I'll stick with that.

    As always... I'f I'm doin' wrong, please let me know!!

    Matt


    PS: If any of this post just seems like reiteration of what's already been said, it's just so that others can follow should they want to use it as a reference.

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    Now I've just got to wait a couple of days for some 457 tape to come in before I can set the record level. (I could use this 456, but I figure as long as I'm goin' to all this trouble I'll wait so as to get the right calibration with the exact tape). Although, as I've set the machine up (at +3 over 0), I really wish I could get my hands on some 407 (as I think it might give more of the results I'm looking for on drums and electric guitar).

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