[krayzie]

Okay I just bought a used Harmon Kardon CD291 deck that has a fine bias knob on it... what's the best way to adjust this? The user's manual mentions nothing about adjusting this other than the fact that going negative makes recordings duller and going positive makes recordings brighter...

I'm using Sony Super Metal Masters and TDK MA-XGs... just wondering what bias settings do you guys use? Thanks!

 

[lini]

Well, I don't know that particular HK tape deck, but in general it's fairly easy to adjust the bias on three head decks: Just record anything and use the tape monitor switch on the deck to instantly and directly compare the input to what's been recorded. Then you change the bias setting slightly and listen, whether it's closer to the original or not. Thus you can gradually approach the optimum setting for the tape. With two head decks it's much more inconvenient, as one doesn't have the live monitoring feature at hand. For this type of deck, I'd recommend to make a test recording with different bias settings in certain intervals (for example you could start with a low setting and turn it up a certain degree every 20 seconds or whatever - and it could be wise to note the according counter display in case it's not a real time counter...), which you can compare to the original in a separate playback cycle later on.

Checking the azimuth and demagnetizing the heads and capstan might also be a good idea on a used tape deck, btw.

Greetings from Munich!

Manfred / lini

P.S.: And be careful: The rubber roll that presses against the capstain shouldn't be treated with aggressive chemicals for cleaning purposes. So while isopropanol will work well for cleaning the heads and the capstan, it would dry out and harden the surface of the rubber roll, which is not good...

 

[krayzie]

Hey thanks for the tip!

It's a two head deck by the way so yeah it was not very convenient but nevertheless I think I got the bias adjusted right... wish it had that auto calibration feature...

 

[mkmelt]

To get the best performance the tape deck needs to be callibrated for a certain brand of tape. This includes both the recording bias and the Dolby noise reduction record/playback levels. The recording bias needs to be adjusted for flattest response for your particular choice of tape for each type. Since your deck has a bias fine tune control, you can perform some matching of your deck to a particular brand/type of tape as follows:

If you are unsure of what brand of tape the deck was originally set up to use, try different tapes until you find what works best.

To fine tune the bias, record some off-channel FM interstation noise, as this hissing sound approximates pink noise. Set the recording level at -20 db on the recording meters, and record a minute or so of noise on a fresh tape, The Dolby noise reduction should be left turned off. Then play back the tape while switching between the tape and the same FM source, try and match the levels if possible so you are hearing a difference in tonal balance and not just volume.

If your deck's bias is properly set for the tape, the recording will sound almost identical to the FM source. If it sounds too dull or too bright, your deck is not properly set for this brand of tape, then try adjusting the bias fine tune until you find a setting that gives you a recording that sounds as close as possible to the original.

If you want to have your deck fully recallibrated for a specific brand of tape, including Dolby calibration, a good repair shop can do this in under an hour. If your deck needs other work done to it such as having the belts replaced, it makes sense to have the deck callibrated for your favorite tape formulations at the same time.

You can do this with a two head deck, it justs takes longer to get the bias set properly than with a three head deck where you can monitor the recording as it is being made and hear the results of any bias adjustments in real time as you switch back and forth between the source and the tape being made.

 

[krayzie]

So is it a rule of thumb that a 3 head deck can outperform a 2 head one in terms of recording / playback quality?

 

[mkmelt]

A three head tape deck theoretically will have better performance, in terms of top end frequency response, than a two head tape deck because the head gap width requirements are not the same for recording and playback functions. A two-head deck has a combined record/play head and an erase head. A three-head deck has separate heads for recording and playback, plus the erase head. In a two-head deck, the gap in the head is by necessity a compromise between what would be the head best gap width for a recording head and what head gap width gives the best playback response. So this would lead to the conclusion that a three-head deck is always better, but this is not always true as we shall explore.

By comparison, if we look at a reel to reel tape deck, all of the tape transport parts and especially the tape path is much larger than for a cassette deck. This gives a big advantage when trying to construct a machine with separate heads for recording and playback. There is plenty of room to place a separate erase head, followed by the recording head, followed by the playback head in the tape path between the two capstan/pinch rollers that are used in this type of closed loop system. Here too tolerances have to be very good for proper alignment between these heads. On many reel to reel decks, each record and playback head has its own azimuth adjustment so that it can be finely aligned once installed in the tape deck.

In the case of a cassette machine, everything is much smaller and the design of the cassette itself is limiting in that the record/playback head must fit into a very small opening in the center of the cassette shell. The erase head fits into its own opening to one side of the record/playback head. There is not nearly enough room to fit the typical separate record and playback heads together inside of the cassette shell, so originally this limited cassette decks to just two heads.

Eventually the Japanese, I believe it was Nakamichi, engineered a true three head cassette deck by building a deck with miniature separate recording and playback heads. This design was very expensive to manufacture, and also created the problem of getting/keeping good alignment between the recording and playback heads.

Most of the three head decks that followed solved this by packaging the separate miniature record and playback heads into a single housing, with a fixed azimuth alignment between the record and playback heads. This was both good and bad. Good in that if the heads were aligned correctly during manufacturing, then they would always perform optimumly. Bad in that if the heads were manufactured even a tiny bit out of alignment (cassette deck head azimuth alignment tolerances are unbelievably tight for proper performance to be achieved), that record/playback performance would always be compromised, with no way to correct this misalignment.

A two-head deck does not have this alignment problem because the record head is also the playback head. As long as this one head is aligned with the tape, performance problems due to record/playback azimuth alignment errors are not a factor.

In the case of the cassette tape recording, the results you get are very much dependent on the quality of the cassette shell mechanism as well as the tape. Also, the quality of the parts used in the deck and the manufacturers quality control has more to do with the results than any one design approach. There were some very good two-head decks and some very bad three head decks. There were also some very good three head decks, however, these were typically much more expensive tape decks.

The original audio cassette was designed by Phillips for use as a portable dictation device, or for recording lectures. As soon as people bought the first Norelco cassette player/recorders they proceeded to make cassette recordings of their favorite music to take along on trips etc.

The trip from being a low-fi portable dictation medium to challenging the vinly LP for the consumer market lasted 10 or 12 years. Along the way one estimate stated that during this same period the Japanese tape and electronics industry spent the equivalent of what the U.S. spent on the Apollo program to make the audio cassette a true music medium.

The cassette never did overtake the LP, but it killed off the 8-track and also was the death of reel to reel tape for the consumer market. By the early 1970s, using type II tape and Dolby B noise reduction, the typical consumer deck could make a copy of an LP that to most people was identical to the record and was portable.