Advent 100A Dolby B noise reduction.

tapepath

Well-Known Member
Feb 20, 2014
31
14
213
#1
Anyone who has collected prerecorded tapes from the Barclay Crocker company has become aware of the limitations imposed by the necessary decoding of the Dolby B noise reduction used by that company. Over the years I've assembled quite a few Dolby B units from Teac, Sony, Advent and Integrex and felt that the Advent 100A and the Integrex were the most neutral sounding. I began a project to try and learn about how the Dolby designed circuit works and maybe find a way to improve the sound. I built a replacement regulated power supply for the 100A and replaced all of the components (with the exception of the transistors) in the two playback processor boards and the final output circuit boards. I also used Advent's service manual to carefully adjust the critical Dolby threshold levels and meter calibration adjustments. These changes produced a significant improvement in the transparency and freedom from grain in the way my tapes now sounded. The next step is to see if I can build a vacuum tube output stage to replace the one Advent used and see if the change is what I'm after. Here is a photo of the test setup, the Dolby unit is feeding a tube preamplifier I built for this experiment. DSC_0278.JPG
 
Likes: astrotoy
May 25, 2010
1,113
381
525
SF Bay Area
#2
Hope you succeed!

Larry
 

tapepath

Well-Known Member
Feb 20, 2014
31
14
213
#4
Thank you, Larry.
The goal is to decode old tapes, ones that I've collected over the years, especially the Barclay Crocker and the Philips recordings. After making some adjustments to the feedback resistor I was able to play some music. In this photo I'm using my Stellavox Sp7 to play a Barclay Crocker tape and listen to the playback on some old headphones and excluding some low level humming heard the result. I must say not bad at all for a first try.
DSC_0286.JPG
 

tapepath

Well-Known Member
Feb 20, 2014
31
14
213
#6
Perhaps a bit of information about the Barclay Crocker tapes would explain my attempts at getting the most sound quality from them. The company was in operation from 1977 until 1986 and was based in Poughkeepsie, New York. During that time they produced approximately 350 titles. Most of the selections were of classical music with a few jazz artists represented (Marian McPartland and Joe Venuti) and one or two selections from Joan Baez. They would obtain a license from a record company and receive a master tape which they would evaluate and if approved would make a 1" tape duplicate on an Ampex machine. This 1" tape would then feed a series of duplicating machines and operating at four times playing speed (industry standard was copying at sixteen times playing speed!) produce between 200 and 500 copies of each tape. The tapes were offered in simple but well made boxes with plenty of enclosed information about the performance and played at 7.5ips in the 4 track format. From testimonials by former employees every copy was carefully done and a high level of quality control was the standard. The majority of tapes used the Dolby B noise reduction system to reduce tape hiss by approximately 10dB by compressing frequencies starting at 5.0kHz during recording and subsequent expanding these same frequencies on playback. The later tapes were produced with the dbx noise reduction system. While dbx tapes could be played without a reciprocal playback decoding those recorded with Dolby should be played properly decoded.
Some of the record companies whose material they produced were Deutsches Grammophon, Halcyon Records, L'Oiseau Lyre, Mercury, Pro Arte and Vanguard and others. The tapes sold for around $8.00 to $11.00 and considering they were a third generation copy represented an excellent audiophile bargain. In many ways these are the progenitors of today's audiophile tape offerings from The Tape Project and Analogue Productions. While not everything is state of the art (some are jaw dropping exceptional) every one is much better than the original vinyl version. So a combination of the interesting program material (lots of film scores by Bernard Herrmann and Miklos Rozsa) and very high sonic quality make them appealing to me.
 

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