Studer A807 calibration

[Linnovice]

Hi all, I have a Studer A807 machine that is being fed from my Tascam M50 mixing desk. One of the great conveniences of the 807 is the ability of calibrating for individual tapes direct from the counter keyboard. As I use a few different tapes (Quantegy, Maxell, 3M996, etc) I tend to calibrate as necessary when I change. This involves keeping a signal generator and millivolt meter to hand and swapping cables as required. My question is this: the M50 is fitted with onboard test tones of 40Hz, 1kHz and 10kHz. Can I use these to calibrate the 807? By patching across these signals it would save the hassle of swapping the cables around every time I need to recalibrate. That doesn’t sound much but in a small but packed listening room it takes some dexterity and flexibility. Not readily available at 69!
I appreciate that it’s not going to be as accurate as the generator/meter option but it’s only for my own use. Not commercial. Any thoughts? Mike

[TIMTAPE]

I'd see it more as a short cut way of setting up the machine as opposed to checking the complete response. For example we might align a machine for flatness at those spot frequencies but if there's a problem such as a dirty or badly worn head we may be able to get the response apparently right at 10kHz but in the process create a large peak in the response somewhere below 10kHz, and the 10kHz response will be poor at higher signal levels. We might also have compromised distortion levels in the lower frequencies. There's really no substitute for a full check of all parameters.

[Ted Kendall]

Back in the day we used to check the line-up of the A80s in the assenmbly channels using the desk oscilliator and the PPM. If we sent at PPM4 and it came back at PPM4 at 1,6,10kHz, we reckoned we were good to go. Mind you, there was proper channel maintenance every couple of weeks and we checked or re-set the bias every day, but that's more a question of method than kit. If you can check go and return levels at 1 and 10k, that should be fine for day-to-day checking. The 6k was an insurance against having selected the wrong curve on the tape machines - we worked to IEC internally, but made despatch copies to both NAB and IEC.

[Linnovice]

I take your point TIMTAPE but we are not talking about full calibration and alignment. They are set both physically (azimuth etc) and internally by trimmers on the incoming levels. These are obviously set against generator and millivolt meter. I check these every couple of months or so and the heads are kept scrupulously clean all the time. Remember this not a singular machine, it’s one in a collection so does not get heavy usage. I’m just looking for a quick and convenient method when I use a different tape formulation. Bias, playback and 10k eq. All just zeroed in from the keypad. I suppose the answer is to get off my butt and give it a try!
Watch this space . . .

[Linnovice]

Well, what do you know. It does pay to get off the butt sometimes. I’ve just tried it out and it works a treat! The M50 is a great little mixer. It has a fold back facility on some of the VU meters which allows me to monitor the settings. Great stuff.

[TIMTAPE]

Originally Posted by Linnovice

I take your point TIMTAPE but we are not talking about full calibration and alignment.

Sure. What I meant by "the complete response" was "at all frequencies" when aligning record for a different tape formulation. A good confirmation could be cranking up the gain, and therefore the noise, on one - or several - of your M50's mic preamps and listening to the source/tape result. I often did this as a quick check, even using a machine's own mic preamps when it had them. More revealing than just the 40/1K/10K test.

[Linnovice]

As I understand it, when changing tapes on the A807, first the tape bias is adjusted, then the record level is set to the system level (.775v at 0dB), then the treble (10kHz -20dB input adjust up/down to read output at 0dB) and finally check the bias. That’s what I’ve been doing and the results are very good (to my ears). The treble input frequency varies with tape speed, ie. 8k at 3.75, 10k at 7.5 and 12.5k at 15. As I mainly record at 7.5 the 10k output on the mixer seems to work fine. The manual doesn’t require a sweep to be made.

[TIMTAPE]

Many handy test procedures/techniques arent listed in official service manuals.
I just offered FWIW an extra, quick and easy listening confirmation method that I have found useful.

[mitajohn]

I remember a very practical way of calibrating a pro tape recorder, after the typical procedure, using a kind of pink noise signal. You record the noise and compare by ear the tonality between in/out switching. This procedure was implemented in the Dolby SR noise reduction system named as "Dolby Noise" auto compare feature. This provided a continuous A/B comparison between the reference pink noise generator signal of the system to that off tape.

[Linnovice]

Yes TIMTAPE, I do appreciate that. I wasn’t disputing what you were saying, rather confirming what I do is correct. So, with that in mind. When you say ‘crank up the gain’ do you mean so as to record a ‘white noise’ to compare when recording? I’m a bit confused (nothing unusual there). The basic instructions on the mixer are to calibrate each channel by setting the slate on the buss master sliders to 0dB on the VU meter by playing the 1k tone and adjusting accordingly. You record that signal for a couple of minutes and then play it back and adjust the gain on the channel so that the playback level is 0dB. What you out is what you get back. Hence my confusion. If I up the gain will it not throw all those settings out?
Excuse if it’s a dumb question but I’m on a learning curve here that’s lasted around four years so far and I’m still trying to catch up!

[TIMTAPE]

Yes as mitajohn says, it's a kind of pink or white noise signal from the mic preamp and it's purely a listening comparison between source and tape playback.

I'm not familiar with the mixer. The manual should tell you how to insert a tape machine into it but the simplest way is to listen to the tape machine's output from its headphone socket, switching between source and tape.

[Linnovice]

Right chaps, I think I have the gist of what you are saying. I’ll give it a try later and let you know the results.