Revox B77 recording calibration

QA40x
1

Hi everyone,

Just sharing an interesting experience.

While calibrating my Revox B77, I couldn’t find the usual bias peak, making the standard bias adjustment impossible.

Instead, I monitored the frequency response in real time with the QA403 and alternately adjusted the bias and EQ trimmers until I obtained the flattest response.

The result was excellent: by ear, I could no longer distinguish the tape from the source, which is more than good enough for my personal use.

One interesting point: during the measurements, the QA403 Exposhirp output was connected to the recorder input, and the playback head output was returned to the QA403. At 3¾ ips, the tape path introduces a delay of about 340 ms.

I initially thought this delay had to be entered into the QA403 software, but it only worked with the default settings. The QA403 appears to recognize its own test signal automatically, even after such a long delay.

Very impressive! :+1:

2 Likes
2

It’s R2R calibration that I bought the asylum for specifically :o) So did you just keep looping different frequencies watching the response graph or use something like white noise? Good to know there are still a few reel heads out there!

3

Me and probably many others would want to know the technical specifics of the calibration procedure. While you are working with a Revox, I am about to re-cal a TEAC X-1000R. Certainly there are some differences in setup, but many may be the same. – best regards!

4

The beauty of the QA’s FR function is the constant repeating and speed of processing. You can basically tweak pots in real time for bias, eq, level etc and see the results a second after.

Even azimuth is easy, as you’ve got two channels in essentially real-time.

It’s a fantastic unit/system for tape aficionados wanting to tweak their ancient decks for the best performance.

Just did a lovely Akai GX-625 and I have a 4 track Teac to do soon.

5

Hi,

There are many adjustments on an R2R recorder, but the most important are head azimuth, playback response, and record calibration.

For head azimuth, simply follow the service manual. Personally, I’ve never had to adjust it.

For playback equalization, you normally need a dedicated calibration tape. These tapes are expensive and should match both the recorder and the tape formulation (LPR35, LPR90, etc.). For personal use, this is often not worth the cost. I use two Revox B77s, and tapes recorded on one play identically on the other, so I don’t adjust playback EQ.

The greatest improvement usually comes from record calibration, which includes:

  • Bias adjustment

  • Record level

  • Record EQ

Ideally, you need only an audio generator and a true RMS voltmeter (reading in dBu is a plus).

The standard bias procedure is to record a 10 kHz, -20 dBu signal, monitor the playback output, find the maximum level, then continue increasing the bias until the level drops by about 5 dB (depending on tape type and speed).

Record level is adjusted with a 1 kHz, 0.775 V signal.

Finally, adjust the record EQ with a 12 kHz, -20 dBu signal. Since increasing the bias reduces the high-frequency response, the EQ trimmer compensates for this.

It may sound complicated, but the real objective is simply to obtain the flattest possible frequency response.

This is where the QA403 becomes extremely useful.

Run a Frequency Response measurement (20 Hz–20 kHz swept bursts) and watch both channels in real time while adjusting the bias and record EQ trimmers for the best response.

The service manual remains the reference for the adjustment sequence and target values, while the QA403 makes the optimization much faster and easier.

I honestly think the procedures described in the service manual were designed around the test equipment that was commonly available at the time: an audio generator, a true RMS voltmeter, and perhaps an oscilloscope. High-performance audio analyzers were extremely expensive and generally found only in professional laboratories.

With a modern analyzer such as the QA403, you can directly observe the recorder’s frequency response in real time and optimize the adjustments much more quickly. The service manual still provides the correct sequence and target values, but today’s measurement tools make the process far more straightforward.

:grinning_face:

3 Likes
6

Adjusting the bias level for the flattest record/playback frequency response (FR) does NOT guarantee the optimal bias setting for a tape deck.

Calibrating a tape deck is not rocket science, but you must follow the correct procedure sequence as explained in your deck’s service manual.

The first major step is the playback adjustment. This requires one or more full-track calibration tapes made by a reputable source on a studio deck. MRL is the industry authority, but there are reliable suppliers in Europe as well.

Yes, professional calibration tapes are quite expensive. However, if you can afford a QA403, open-reel tape decks, or 3-head cassette decks, this should not be a major obstacle.

You cannot skip playback calibration, as these settings serve as the reference for all subsequent adjustments (record head azimuth, bias level, record EQ, and record level). If your playback adjustments are off, your record adjustments will be too.

The QA403 is incredibly useful for record calibration and renders the traditional “over-bias” method obsolete.

When adjusting bias, your goal is a compromise between the lowest Total Harmonic Distortion (THD) and the lowest modulation noise. Therefore, you should start by measuring THD at 315 Hz (0 VU), which is the standard frequency used for tape deck THD testing with analog distortion meters.

Next, switch to FR at -10 dB and adjust the record EQ for the flattest record/playback frequency response.

Note: With older tape formulations, FR was adjusted at -20 dB due to high-frequency tape saturation. However, with modern formulations or new RTM/ATR tape, adjusting FR at -10 dB is perfectly fine.

If you cannot achieve a flat record/playback FR, you can start tweaking the bias level to flatten the response. If the FR rises a lot at high frequencies, you can increase the bias level. Afterward, however, you must check that THD levels have not significantly increased. If they have, it indicates that the record EQ circuit requires modification to suit modern tape formulations.

7

I fully agree with you.

Just to clarify, my goal with these two decks was not to restore them to their original factory specifications, but simply to achieve the best possible result for my own ears.

Once I reached the point where I could no longer distinguish the source from the playback when listening through a good pair of headphones, I considered the calibration good enough for my personal use.

Since I did not have a calibration tape, I skipped the playback adjustment. Even so, I obtained essentially the same good result on both decks, so I did not feel the need to pursue an improvement that I would probably not even be able to hear.

In any case, thank you very much for describing your bias adjustment method. It makes a lot of sense and seems much more practical than trying to find a bias peak that is sometimes impossible to find.

:+1: