Biasing for Studer A807 using RMGI 468 tape

[squasher]

I am a newbie to R2R and am about to make my first attempt at calibrating my recently purchased Studer A807 (Mark II).

I've purchased some of the Tape Project's RMGI 468 pancakes to play around with and have read the Studer A807 Service Manual.

The bias adjustment procedure is done by 1) applying a 10kHz input signal 2) increasing the bias until the maximum output voltage is reached and 3) continuing to increase the bias until the output voltage drops by an amount (Delta U) from Max.

Delta U is tape dependent and is specified for a variety of common tapes (of the day?) in a table near the end of the chapter.

Unfortunately, RMGI 468 is not listed in this table (see attached file). How do I determine what I should use for this tape type?


Thanks in advance.

[docb]

Same as BASF PEM 468.

[ironbut]

Here's a link to a link to the RMGI data sheets.
Since you're new to this stuff (and everybody is a one point or another), the page that's linked has a lot of good info regarding tapes and such.

http://usrecordingmedia-store.stores.yahoo.net/archives.html

[High and Outside]

There's one additional way to approach biasing your deck for this tape: Use 1KHz instead of 10KHz. As you described, reduce the bias until the output goes down; then increase bias until the output peaks; then continue increasing the bias until the output declines by a quarter dB (0.2 dB if you really want to split hairs.)

This method works for any tape, any machine, any speed, and will get you to the point of minimum modulation noise and minimum distortion.

[squasher]

Thanks for the additional tip, Paul. I remember reading that same tip elsewhere (though I can't remember where. an MRL paper?).

One thing that I'm puzzled by with regard to this tip, however:

Wouldn't I be better off biasing / calibrating by using the larger voltage / db swing? My intuition tells me calibrating to a 0.2 db difference would be more susceptible to noise margins than calibrating to a 3.5 db difference.


One other question regard to calibration: The calibration process starts (in many cases) by applying a 1kHz sine wave with reference voltage of 775 mV to the input (and measuring test points / adjusting trimming pots as appropriate). What's left unsaid is whether this reference voltage is 0.775 V RMS or peak to peak. I'm assuming it's an RMS measurement.

Thanks again,
Chris

[High and Outside]

Thanks for the additional tip, Paul. I remember reading that same tip elsewhere (though I can't remember where. an MRL paper?).

Yes, it is discussed in one of Jay McKnight's papers available on the MRL site.

One thing that I'm puzzled by with regard to this tip, however:

Wouldn't I be better off biasing / calibrating by using the larger voltage / db swing? My intuition tells me calibrating to a 0.2 db difference would be more susceptible to noise margins than calibrating to a 3.5 db difference.

You have correctly identified the one advantage of using 10KHz: the larger swing. I have never really heard or read any argument in favor of using 10K, but I assume that's the reason. It's theoretically possible to dial it in more precisely because of the wider swing.

I say "theoretically" for several reasons. If you are using a VU meter, it's probably more accurate within 1/2 dB of zero than it is at minus 3.5, and if you're using a meter like a Fluke with a good digital readout then you can get 0.2 without any difficulty. But still, let's say you have a way of metering 3.5 dB accurately. That 3.5 dB target may or may not be accurate. It is based on an average sample of the tape, and the optimum bias for a given piece of that formulation will vary a lot more at 10K than it does at 1K. More than that, the figure is based on the length of the record head gap. And not the physical length, but the effective length, which varies as the head wears. Now with your Studer, you can be pretty sure that the gap (when the head was new) is what they said it was...unless it has been replaced. Studer always made their own heads, and the tolerances were very tight. However some manufacturers didn't make their own heads and sourced them from a variety of OEM's. I know of some instances of MCI machines of the same model number with heads sourced from different OEM's, and with different head gaps.

Anyone who wants to use 10K and rely on the tables may do so. I won't argue. But I want to bias my machines critically, and I have chosen to do it at 1K. To condense a very long story to a few words, I will say that I arrived at this after having biased my machines by many methods, including by ear for a long time, and proving to myself that I could get it just as accurate by doing 1K a quarter dB over.

Remember what we're trying to accomplish here. We aren't optimising 1K. And we certainly aren't optimising 10K, or we wouldn't be shooting for a point 3.5 dB away from the peak. We are trying to get it into the notch that represents the minimum modulation noise and the minimum distortion. Any method that hits that minimum is a good method.

[Dave Cawley]

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From my dim and distant memory.  If you peak bias at 10Khz you will find you are under bias at 1Khz.   Under bias at 1Khz will give you distortion exactly where you don't want it.  Peak bias at 1Khz normally gives low distortion.

I would suggest peak bias (not over) at 1Khz and use the record EQ to get the 10Khz level correct.

Now this is based on my experience with tape more than a few years ago, and I'm up for anything more modern?

Regards

Dave

[High and Outside]

Well, we differ by two tenths of a dB.

[Dave Cawley]

Hi Paul

What I meant was not over bias, like several dB's.  Clearly 0.2 is good, and shows you are on the peak in the right direction.  I'm always amused with cassette recorders that use bias to change EQ.

Regards

Dave

[High and Outside]

 
I'm always amused with cassette recorders that use bias to change EQ.

Hi Dave,

Yes that was always amusing.

You obviously have a handle on how this is supposed to be done: adjust bias for minimum distortion, then adjust EQ for "flat response within the machine's specifications" (to quote the announcer on the STL test tapes, a voice I've heard about a million times.)

Of course changing bias changes the HF response too. It's just backwards to use it that way, because you what you wind up doing is setting your machine for poor distortion performance and worse modulation noise. But many low cost decks--cassette and reel to reel too--don't provide all the adjustments we like to have. Or they make them very hard to find. So people change bias because it's there, and it certainly affects the HF.

I'm spoiled working with decks that have all the adjustments readily available. And that came with service manuals that lay out the procedure. I like it that way.

[Dave Cawley]

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Hi Paul

In the late 60's I prepared tape heads for samples.  I would do bias runs measuring output level and distortion and then frequency response.  Hard polishing on an Arkansas stone would often "sharpen" up the gap.  The customers were all down market such as Grundig and Fidelity and they were more interest in frequency response and rarely mentioned distortion. In fact automatic distortion meters had not been invented and they were manual with deep notches and many twiddly knobs!   The temptation to quote a bias figure that was clearly a tad too low, was simply irresistible.

The company was Marriott Marnetics in Penryn Cornwall England, and "old man Marriott" built a shining new factory with a penthouse suite overlooking the river.  He sold to IBM and then I moved away.  But that was 40 years ago, the factory still exists but is a shadow of its former self.

Regards

Dave