LOW FREQUENCY COMPENSATION adjustments....?

[miroslav]

?LOW FREQUENCY COMPENSATION? adjustment??

OK?so I recalibrated my Otari BIII yesterday using a fresh new MRL tape and the Visual Analyzer software (http://www.sillanumsoft.org/) in place of a hardware scope (the VA software works great).

Now?everything went well, and I did a full alignment/calibration, covering everything accept the actual head height, zenith and head wrap. The deck is almost new, with literally no use, and based on the fact that all my other adjustments were relatively minute, I felt no need to mess with the physical positions of the heads at this time. I also didn?t touch the Erase head.
I?ll check and save those alignment procedures for the next time.

Now, the only one thing I wasn?t 100% clear about was the Low Frequency Compensation adjustment.
I think I know it?s basic purpose, but the full ?Alignment Procedures? manual I have talks about setting it for the ?flattest frequency response??though it requires the use of 20Hz to 200Hz tones from the oscillator?and there is no way to get all of those tones ?flat?.
Meanwhile, the ?Operations and Maintenance? for this deck says to run the oscillator at 0VU Source, and then adjust the LFC pot to match 0VU when at TAPE while running in Record mode. It never mentions which frequency(s) to use.

What I did was something a bit different.
I ran my Left channel in TAPE mode and my Right Channel in SOURCE mode while in Record.
Then, using the VA software, I was able to monitor the frequency response of both channels. That way, I adjusted the LFC so that my Left (TAPE) curve was matching my Right (Source) curve?however, there were level differences between 20Hz ? 40Hz ? 100Hz ? 200Hz.

IOW?if I match at 20Hz?then the TAPE curve was several dB higher at 40Hz...if I match at 40Hz?it was lower at 20Hz by same amount.
When I looked at 100Hz and up to 200Hz?the LFC adjustment had les effect, and the TAPE/SOURCE curves were matched throughout, so I concluded that the LFC was mainly for the sub-80Hz range.
So what I did was to find the mid-point between the lowest and highest points curve positions at the frequencies from 20Hz to 80Hz.
What I ended up with was -3dB at 20Hz as my lowest differential between TAPE and SOURCE curves?and +3db differential at 40Hz. The differentials at 30/50/60/80Hz were less than that.

To me?that was the only way to obtain the ?flattest frequency response? as mentioned in the manual.

Turning off the LFC?at 20Hz ? 40Hz there was greater +dB hump on the TAPE curve when comparing to the SOURCE curves?which means the deck was boosting those frequencies above their source levels without the LFC.

So?is that what the Low Frequency Compensation is meant for?to trim/flatten the low frequencies as close as possible to their source levels?
I?m assuming it isn?t meant to be used to lower them severely below their source levels????

Anyone have a better/clearer explanation of the LFC adjustment from what the manuals describes?

I just want to make sure I didn?t mess it up?.

[Tim]

 The peaks and valleys you are getting in the low end are called ?head-bumps?.  They are a function of the play head geometry.  Your low-end EQ control has no effect on them as you noted.  You were correct to set the low-end EQ so that the mid-points are flat relative to mid-band level (1 kHz).  Also the drop-off at 20 Hz is normal and a function of the 15 ips tape speed and also somewhat due to the play head design.  High-end heads, such as the Flux Magnetics heads, have almost no head bumps and also have a more extended low-end.

Be very careful when contemplating a head alignment. Not only is very difficult to do correctly, it can get you into trouble in a hurry:  As tapes run across heads, they rather quickly start creating a groove in the head. Once this ?channel? starts to form, one should never attempt to re-align the head.  The tape will jump in and out of the channel and also have ?spacing-loss? (tape not perfectly flush with the head surface).  Playing tapes in this re-aligned state will soon permanently ruin the head.   Bottom line is that heads installed at the factory are typically very well aligned and that alignment will never change or drift (unless you drop the deck down a long flight of stairs).  I have never observed a head-alignment screw turn all by itself, even after shipping.  If you put on a head azimuth test tape and observe a large error (on a deck untouched from the factory), then it?s the tape that is out of alignment and not the head.  Tape mis-alignment (called tape skew) can have many causes and needs to be repaired to remedy the alignment error. 

If you observe a small azimuth error when playing a test tape, just leave it alone (unless the head is new or near-new).  Azimuth tapes are not all identical and have a margin of error.  Same goes with head height, zenith and wrap.  If the head is factory original, just leave those adjustments alone.  If you ever hear a loss of high-frequencies (or any discrepancy in the audio signal) in a factory-original tape deck, 99.9% of the time it will due to something other than head alignment!   

One exception:  Professional heads (such as the Flux Magnetics) have ?relief-cuts? in the face of the head. This prevents a groove from forming in the head as it wears.  In this case, it?s possible to make small azimuth adjustments in a head that has many hours on it (without ruining the head).  However, like I mentioned before, if the head was correctly aligned when new, the alignment will not change and not be needed.

Tim

[miroslav]

Your low-end EQ control has no effect on them as you noted.  You were correct to set the low-end EQ so that the mid-points are flat relative to mid-band level (1 kHz).

Hi Tim

When the LFC circuit was turned on, it actually did allow me a good amount of correction of the low end with the trim pot. I could increase or decrease the lower frequency TAPE levels relative to the SOURCE levels. I just wasn't 100% sure where to set the trim pot following the manual?s described process.
What I did was to adjust the LFC trim so that the TAPE level was as close to the SOURCE level ?as possible? across the 20Hz to 200Hz range?with most of the differences being in the 20Hz-80Hz range?above that, the TAPE/SOURCE levels stayed matched regardless of the LFC trim.
Since I was using 0VU at +4dBm as my source signal level (SOURCE  & TAPE) for all the tones I was testing(20Hz-200Hz, 1kHz, 10kHz, etc)...my assumption was to then also set the low frequencies at that same level....which should result in a "flat" response across all?right?

I guess what I am asking is...should the LFC be adjusted so that the low end frequencies are maybe higher or lower than what I am seeing on the SOURCE?
The fact that they call it a LF "compensation"...seemed to imply that the LFC is there to add/increase the lows...but that's the part that I was not 100% clear on, so I just went with my own common sense, and did what I described above.


AFA the physical head alignment...I agree with you, and someone else once told me that too...that most likely, the heads are fine out of the factory, and mild bumps/movement shouldn't change their position.
What I did when checking the azimuth was to first mark the position if the alignment screws with a fine-point permanent marker...that way I would know where I started from.
After several passes, the Record and Playback heads ended up almost in the same spot.
There was a minute difference in screw head position that was less than 0.1mm...you could just b-a-r-l-e-y make out the position change when looking at the thin line I made down the side of the screw head and top of head block surface.
However, the additional 4-Track head was off just a pinch more...maybe 0.2mm.

These heads are brand new, so no worries about tape groove yet, and I was thinking of just checking out the rest of head position adjustments just to be sure.
While I agree with you that it?s not easy do the physical alignment correctly, and I'm sure they do a great setup at the factory, I also know that sometimes people make mistakes or rush things.
This deck had an unsoldered capacitor leg that was causing intermittent Record issues on the left channel...so I know stuff can slip past QC.
I figured that I should give it a onceover, just to check out the deck from A-Z, and just doo some of the tests they outline in the manual for head alignment just to see how they come out, though I wouldn?t mess with them unless I saw something really way off.
Ten I probably will not touch the physical head alignment for a long time...except the azimuth.
I?ve read and heard from some people that the azimuth can be off a bit with different tape brands/types...is that true? I mean, they seem to always suggest doing the azimuth as part of the daily/regular alignment per session.

Thanks
Miroslav

[Tim]

With your deck in record mode (tape monitor), adjust the LFC so that the "head bumps" (peaks and troughs) are symmetrical above and below your 1KHz reference level.  So that the bump is say +2 db and the trough is -2 db (rather than +3 db and -1db).  You can't do anything about the bumps, just adjust them up and down so that they average out to the same level as 1KHz. And ignore the 20 Hz level : it's normal to be rolled off. 


<<<<Then I probably will not touch the physical head alignment for a long time...except the azimuth>>>

Once your record or play heads start to wear (creating a groove) you can't do any further alignment. Even minor azimuth adjustments.  If you were to have have "relief-cut" heads installed in your deck, such as the Flux Magnetic, you in theory can do minor azimuth adjustments, but it's still not recommended because it will still create a new wear pattern which will compromise head-to-tape contact.

[miroslav]

...just adjust them up and down so that they average out to the same level as 1KHz. And ignore the 20 Hz level : it's normal to be rolled off.

If not the 20Hz...then which LF between 20Hz and 100Hz would you use as the "calibration frequency/center frequency"...
?or should it be an average of them all between 20Hz and 100Hz?


<<<<Then I probably will not touch the physical head alignment for a long time...except the azimuth>>>

Once your record or play heads start to wear (creating a groove) you can't do any further alignment. Even minor azimuth adjustments.  If you were to have have "relief-cut" heads installed in your deck, such as the Flux Magnetic, you in theory can do minor azimuth adjustments, but it's still not recommended because it will still create a new wear pattern which will compromise head-to-tape contact.

OK...I get what you are saying.
So then I would want to be 100% sure right now, while the heads are new, if all the head alignments are as good as they can be...and then after that, leave them alone for their lifetime.

It does say at the end of the "Alignment Procedure" manual to only perform the Height, Zenith and Wrap adjustments when heads are replaced...but they kinda stick the Azimuth alignment out in front of the initial electronic calibrations, right before the Reproduce electronics are adjusted...
so it wasn't very clear if it?s part of the ?daily? alignments.

While I've used R2Rs for many years?besides the basic "front-end" electronic adjustments, I always use to have the decks set up by a tech experienced with that stuff?it was just the easier/safer thing for me.
But since I do have a good electronics background and I'm comfortable with doing most things around my studio, I wanted to make the leap into full-tilt alignments/calibrations for a long time now...but I was just chicken!!! :-D
Whenever I considered the physical alignment...I was nervous for exactly the reason you mentioned.
I can see how it is VERY easy to totally mess it up, especially if you're not experienced at it, and you have to have a real good eye, steady hand and really know what to look for.
This Otari deck and the two manuals really made it a LOT easier for me to dive into the deep end of the pool.
The electronics open up for easy access, and if you follow the procedures with care and eyes wide open...it's not too complicated. I learned a LOT more about the whole process since I got this deck and manuals...and of course, thanks to the tips/suggestions from people like you!

Next I'm going to do a full setup on my 16-Track...and I'm glad you mentioned the concerns about head wear VS making physical alignment changes.
On my 16-Track...all the screws have paint on them, to prevent movement/adjustment without "breaking the seal"...so I will leave them be and just focus on the electronics. Luckily, even on my 16-Track, the heads are like-new...no visible head wear.

Thanks again!
Miroslav