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Old 15th Dec 2021, 3:34 pm   #1
ButchJames
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Default Revox A77 Over Bias Figures

Hello all, has anyone got a list of over bias figures in dB for the Revox A77? I have the usual figures from the official service manual, ie for AGFA PE 36, Scotch 201 etc. I was thinking about the figures for AMPEX, BASF, TDK, and Maxell. I have some data, but I'm hoping that someone out there has done all the collating of figures?

Cheers.
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Old 15th Dec 2021, 4:57 pm   #2
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Default Re: Revox A77 Over Bias Figures

There are figures in the later B77/PR99 etc manuals for some later tapes.
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Old 15th Dec 2021, 6:34 pm   #3
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Default Re: Revox A77 Over Bias Figures

We used to adjust the bias for 1db drop at 10 Kc/s.
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Old 15th Dec 2021, 10:42 pm   #4
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Default Re: Revox A77 Over Bias Figures

@Timetape, @lesmw0sec

Thanks guys for your input. Yes, I'll check out the later service manuals! Cheers.
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Old 16th Dec 2021, 7:50 pm   #5
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Default Re: Revox A77 Over Bias Figures

Some useful info can be found in the attachment.
Attached Files
File Type: pdf A77_bias_setup.pdf (404.7 KB, 100 views)
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Old 16th Dec 2021, 8:28 pm   #6
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Default Re: Revox A77 Over Bias Figures

Quote:
Originally Posted by mitajohn View Post
Some useful info can be found in the attachment.
Great stuff John, many thanks!
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Old 18th Dec 2021, 1:47 pm   #7
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Default Re: Revox A77 Over Bias Figures

Its the tape not the machine that dictates the overbais. Rough rule of thumb is 1.5dB at 30ips, 3dB and 15ips, 6dB at 7.5, 9 at 3.75 etc RTM say 4.5 at 15ips for SM468 tape. Go head if you can adjust to half a dB and more importantly can hear it!
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Old 18th Dec 2021, 6:16 pm   #8
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Default Re: Revox A77 Over Bias Figures

I'd go more for 4dB overdrop at 7.5 and below, otherwise the HF crushing is likely to be something chronic...
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Old 19th Dec 2021, 11:11 am   #9
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Default Re: Revox A77 Over Bias Figures

From memory, A77s have issues with the higher-bias and flux density stock such as Ampex.

It's worth checking. I have an A700, which struggled, but back in the day, Agfa PEM 368 and 468 was ubiquitous, so I only needed to replay Ampex (and then very occasionally) rather than record on it.
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Old 19th Dec 2021, 11:50 am   #10
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Default Re: Revox A77 Over Bias Figures

A bias table is not necessary.
How do I set the bias settings?
Uses a new and good tape.
First set the EQ to minimum. For an A77 this applies for fast speed CCW and for slow speed CW.
Then the input signal at 1000Hz at -20dB
Make a recording and adjust the recording volume with trimmer P503 to also get a -20 dB signal on the output.
Then set input 10,000 Hz to -20dB and turn the bias (P720 - 721 - 722 - 723) from CCW to CW so you get a strong signal, but go further, over the top, so you get -20 dB, output.
Check the 1000 Hz is it - 20 dB?
Make a correction.
Then check the 10,000 Hz and the signal up to 20,000 Hz
Finally make a small correction with the EQ.

If this is not possible, check the azimuth setting. Perhaps this setting is wrong or false.

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Old 19th Dec 2021, 12:31 pm   #11
Ted Kendall
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Default Re: Revox A77 Over Bias Figures

A77s could bias the new generation tapes all right, but erasure wasn't always complete - I found the head fitted to the A700 and B77 better in this respect.

The overdrop figure is a compromise in any case - too little and distortion and dropout cause trouble, too much and the top end disappears once you put any level on the tape, especially at low speeds.
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Old 19th Dec 2021, 12:39 pm   #12
Simondm
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Default Re: Revox A77 Over Bias Figures

Thanks - I sit corrected.

There are two versions of the A700's analog audio cards and control circuit, incidentally. I no longer have the tech ref manual, so I can't remember the full detials (but it is available on-line).

Sadly mine is the earlier version, and that definitely had issues, such as being able to put audio on the record heads when it shouldn't, and the trimmers falling apart mechanically. The later boards and logic control are much better.

S.

PS: Ted, you have a PM...
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Old 20th Dec 2021, 3:12 pm   #13
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Default Re: Revox A77 Over Bias Figures

Interesting contributions above, thank you all.

I was interested to see the kind of range of 'over bias' figures Revox quoted.

The apparent reason Revox adopted this method was to -

Make setting up comparatively easy, and to get the best compromise for S/N, and distortion. Otherwise, we'd have to catalogue a series of bias voltages (at the head), and the corresponding flat-response de-emphasis (PB EQ), then analyse the various S/N and distortion figures.

What average technician has the time and resources to perform this? Not many.

BTW, personally I'm not the kind of person to worry over +-1dB at 10Khz etc. I too doubt that would be heard, although I have seen others in other forums 'concerned' over a +0.5dB to 1dB midband rise!?
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Old 21st Dec 2021, 3:33 am   #14
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Default Re: Revox A77 Over Bias Figures

I think the older method was to bias at a mid frequency such as 1 kHz rather than a high one. Not different in principle. I understood the shift to 10 kHz was because the higher frequency is far more responsive to small bias changes so the result is a more accurate setting of bias.

Setting according to a bias voltage at the record head can only be very approximate because even new heads varied in inductance and their inductance changed with head wear, so a far better indicator was head bias current rather than bias voltage across the head. Then of course tapes vary in their bias requirements. In the end it's about the particular tape, and as Ted says, it's a tradeoff.

If we're recording and playing back a tape in Dolby C for instance, a 1 db round trip error at 10 kHz will be far more audible than 1 db error with no NR, for the NR exaggerates such errors, which was always a downside of such NR.

Another thing is that recorded high frequencies act as "bias", adding to the total bias signal. This is especially true with eg: slow speed recordings. Strong highs can overbias the tape and we can get an inversion effect. The more highs we add, the weaker are the highs on the tape. But the temptation in setting recording levels in a deck which we know doesnt have great response in the highs can be to record at a higher overall level. The result is increased distortion in the lows and mids, and even more crushing of the highs. The crushing of the highs increases perceived tape hiss, the very thing we were trying to alleviate. Dolby HX Pro was servo bias which sensed the extra "bias" which the recorded highs were contributing at any given moment, and reduced record bias to compensate. It came into its own with slow tape speed recordings.

Of course these days this is more historical as these days few actually record to magnetic tape either at home or professionally. Possibly the most common recording to magnetic tape these days is for digital data archiving. We might think our family photos and recordings are archived "in the cloud". They may be archived somewhere on a magnetic tape.

Last edited by TIMTAPE; 21st Dec 2021 at 3:45 am.
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Old 21st Dec 2021, 3:21 pm   #15
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Default Re: Revox A77 Over Bias Figures

Quote:
Originally Posted by TIMTAPE View Post
Another thing is that recorded high frequencies act as "bias", adding to the total bias signal.
Agree with the bulk of your feedback, thanks.

However, I'm not confident that any audible HF content 'adds' to the bias at all? At least not in the sense I am about to suggest ...

The bias 'carrier' is a high voltage, high frequency sinewave where its peak 'sits' somewhere (linear) mid point in the B vs I (non-linear) curve of the head/tape combination. {B=flux density, I=magnetising current} The audio signal being very small in comparison raises and lowers the bias 'carrier' amplitude much like amplitude modulation - although mathematically it is not the same.

Of course this ac bias technique is far more effective and efficient than any theoretical dc bias that could be employed. It's all been research long ago as you guys will know.

I suppose the over-bias technique could be applied to all machines, including cassette decks - it's an interesting idea. Revox certainly applied it to their B215 and A721 machines judging by the service manuals. Incidentally, I also have a B215.

Sincere thanks for your input.
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Old 21st Dec 2021, 6:23 pm   #16
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Default Re: Revox A77 Over Bias Figures

When I was first at Leevers Rich in 1969 we mostly used EMI 77 tape or near equivalent, and biased 2db over peak at 1 kHz. Later, in the early seventies, with the newer types of tape becoming available such as 3M 262 we biased at 10kHz, 4db over at 7.5 ips, 2db over at 15 ips.

What users did once they had the machines was of course entirely up to them, we were only concerned that the machine would meet spec with a range of adjustment in hand.

The bias meter read current so that a user could return to a known figure for a particular tape if they used more than one type, few did.
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Old 21st Dec 2021, 7:32 pm   #17
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Default Re: Revox A77 Over Bias Figures

Quote:
Originally Posted by ButchJames View Post
Quote:
Originally Posted by TIMTAPE View Post
Another thing is that recorded high frequencies act as "bias", adding to the total bias signal.
Agree with the bulk of your feedback, thanks.

However, I'm not confident that any audible HF content 'adds' to the bias at all? At least not in the sense I am about to suggest ...
Its not too hard to test it. Disable the bias signal. Record say a 400 Hz signal at normal level. Now mix in varying levels of white noise and listen to the effect on the playback of the 400 Hz tone. ... Dolby HX Pro worked because HF content could unhelpfully overbias cassette tapes. HX Pro helped to counter this by reducing record bias, in the presence of such heavy HF content.
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Old 21st Dec 2021, 9:02 pm   #18
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Default Re: Revox A77 Over Bias Figures

Thanks TimTape for the pointer, I have just downloaded a 1984 article on this. Will print it out and read it later at leisure - I like to study these things.

Cheers!
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Old 22nd Dec 2021, 4:54 pm   #19
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Default Re: Revox A77 Over Bias Figures

@TimeTape

Interesting stuff Tim. I've just read an article written by Jensen & Premanik.

Okay, so I understand much of the fundamentals, but didn't consider the HF content acting as an additional bias influence on low frequencies as the audio spectral components 'ride' on the bias signal.

To clarify ...

Basically the HF bias is large (~80Khz ... 120Khz, ~10v ... 100v) , and the classical model is that the audio current raises and lowers the HF bias B-field average which gives us the audio impressed on to tape. (Forget EQ for the sake of simplicity)

However, since audio also contains (lower) HF content, let's say 10Khz ... 20Khz, this too can influence the bias levels for lower frequencies also contained with the audio. And in turn a change MOL at various frequencies.

Interesting stuff indeed.

Having said that, generally audio spectra follows the 1/f power law {-20Log(f)}, so I guess the influence of audio HF content isn't as damaging as implied? Otherwise all machines would have to be designed for this?

Back in the 1980s, I was aware of HX Pro, and only briefly what it did.

Cheers.
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Old 22nd Dec 2021, 6:36 pm   #20
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Default Re: Revox A77 Over Bias Figures

Actually it may be 10Log(1/f) if it's power? Apologies.
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