Advent 100A Dolby B noise reduction

[Deleted User]

During the early '70's audio enthusiasts who liked making tapes on their open reel machines would often add outboard Dolby B components to reduce the effects of tape hiss. In 1972 I bought a Teac AN180 unit that I still have and occasionally use. The majority of tapes that I recorded on my Sony TC850 (which I still have, as well) were recorded as 2 track 7.5ips and Dolby B encoded. Many companies offered them, such as: Sony, Nakamichi, Panasonic and even Concord. One unit that has retained a good reputation is made by the Advent company, first as the model 100 and later the 100A and the smaller 101. There was a kit Dolby B unit offered by the British enthusiast magazine, Wireless World. This later became known as the Integrex Dolby unit and sold through the Barclay Crocker prerecorded tape company.

[Deleted User]

I wanted to see if I could make some improvement in the way the 100A sounded by changing the 16 volt power supply and updating the signal path components. The first step was to remove the original power supply and relocate the AC fuse from inside the chassis to the back panel and gain some room. Then using Schottky fast recovery diodes, Elna Silmic II capacitors (bypassed with 0.12uF Panasonic film types) and a Rohm 16 volt linear regulator build a new power supply.

[Deleted User]

Next I removed all of the unneeded terminal brackets and rerouted wires needed for the pilot lamp and drilled some mounting holes installed the new supply.

[Deleted User]

After letting the unit stay power on for 24 hours I decided a listen was in order. I connected it to the Stellavox SP7 and began playing a Barclay-Crocker tape of Mahler's Symphony No. 1 recorded originally on the Unicorn label. I'm pretty familiar with the way this sounds and I must say it sounded quite good. The recording has tremendous dynamic range and power. Even with just a reworked power supply it sounded very good. Now on to removing some parts in the audio chain and see what changes that makes.

msg

Man, cool project Ken.
What kind of wire is that you used for the connections? Is that silver with custom insulation?

[Deleted User]

Thank you, there's a lot of good sound in these old tapes. It takes some effort, but it's fun.
The connecting wire is something I've used for many years. The wire itself is called Wonder Wire and is extremely hard to find. It's a silver plating over solid copper and I cover it with a very thinTeflon sleeve and use shrink tubing for the ends. I have always polished the wire before I use it, I use Flitz. Wonder Wire is the same wire that was used to make the leads on the original Wonder Caps. I've wired everything with it, my preamp and amps and like the results.

honestaquarian

I remember the Advent noise reduction unit was well regarded back in the day. The legacy of Henry Kloss lives on

tonyp063

KennethSwauger wrote: »

This folks, is how cable management is done.

Lovely stuff Ken.
I don't comment much, but I enjoy reading, and learning things that I've not known, because of your tape posts.
Thanks.

[Deleted User]

You're welcome, I know all this is way out of date, but it was very relevant in it's day. There were a few tape recorder companies that had built in Dolby B and others that had DBX noise reduction. Phillips offered their prerecorded tapes in Dolby B versions as well as London. Then the cassette market took over from the open reel format because it was more convenient to operate (fidelity was not as good, but it got close) and because of incorrect Dolby record and playback levels the reputation of what the system could do deteriorated. Many people switched the Dolby function off when they listened to an encoded cassette tape.

[Deleted User]

Here is one of the circuit boards that begins the Dolby decoding process. I've removed all of the original resistors and most of the capacitors. I left the original polystyrene small value capacitors. All of the original resistors have been replaced with Dale RN Series metal film versions, a few are "thin film" types. Also the two variable trim resistors have been replaced as well.

JayCee

Excellent work, as usual, Ken. Coincidentally, saw this on CL earlier today.


https://spokane.craigslist.org/ele/d/spokane-teac-dolby-noise-reduction/7101346326.html

Seller claims, "Works. I haven't used it since Peter Frampton had hair."

[Deleted User]

Hi JayCee,
Thanks for the comments, I try and take my time when I'm doing this in order to avoid making an error. I measure each resistor to make sure of the value, then clean the leads with Flitz, bend the leads and carefully solder the part in place with the resistance value up so it can be read later if needed. Then I check continuity with the extra leads to another part of the circuit to make sure it's connected. I've done this for so long I can tell by the "beep" the Fluke meter makes when the connection is good or not. Then trim off the excess leads and check that part off of my master list.
I'd venture the Teac Dolby units were the most popular, they looked great with the pale blue meters and there were plenty of the decks sold so people wanted the matching component. The AN300 was a great looking unit, with four meters.

mhardy6647

Neat-o.

JayCee

I had a friend in high school that was a straight A student in the early to mid 80's...National Honor Society, Dean's List, etc. Partied like hell...the only B he ever got was in PE. Only child and, as a reward for good grades, his dad would let him pick out stereo equipment each semester.

Gear consisted of SL-10 "Close and Play" linear tracking turntable, SL-P8 CD player, Nakamichi cassette deck 1 level below the Dragon, Pioneer integrated something or other w/cool blue LED meters and speakers a friend of his father's produced that were highly regarded in Germany. Great gear for the time and, especially, a high school kid. But, what sticks out was his SAE Inpulse Noise Reduction System (transient noise elimintor) and DBX Dynamic Range Expander.

Pretty sure this is the SAE:


and can't remember for sure but DBX was similar to this:


His music room was in the basement and he had concert posters all over the wall, lava lamp, 3 way "disco" stoplight (only thing "disco" allowed) that would react to the music. Add some good ole German bier, wine or......fill in the blank......and we were mesmerized. Oh, and the music was to die for! Rock and metal was standard fare.

Mike was the guy that introduced me to hi-fi. Friday at school, I'd give him a TDK SA, or SA-X when I had the cash, chromium dioxide (CrO2) cassette, my preferred tape, and he'd record whatever album I'd want. If I was lucky, he'd compile a mixed tape of his best songs off his VAST LP collection. Monday, the gift of music would return to me.



Forgive me for the slight derail and trip down memory lane but reading your posts really takes me to a different place in life. A budding photographer in my former days, I appreciate your compositions and eye with the pictures that superbly lend to telling each story you post.

[Deleted User]

Not at all, that's exactly why I began the thread, hoping to trigger some memory in another person. Please post away, it's great reading about other people's audio experience. Your description kind of reminds me of the way myself and my friend Lance would collect music. We'd each buy a few new records, listen to them for a while then swap them and tape record what the other person bought. The problem with me was that I used a 2 track recorder and recorded at 7.5ips so it sometimes cost more to make the tape than actually buying the record. Fortunately I worked at a stereo store and was able to buy tape at a reduced price.
I was so persnickety that I wouldn't write anything on the tape boxes, I would type a half sheet of paper with the album title and track selection and put it into each box. I just numbered each reel box with a label maker and then did a file card with the artist name and record title and put it into a file card box (which I've somehow lost). But, I still have the portable typewriter I used.

JayCee

Very cool. Did you work in Polk HQ and for many years? Also, apologize if I don't remember, did I meet you @ '12 Polkfest? If so, we may have a common acquaintance from my past.

mhardy6647

The reanimation of this topic finally led me to dig out the Advent 101 in the basement and snap a coupla piccies. I found this - not surprisingly - on the swap pile at the Harvard, MA town dump. I noted today that someone had apparently tried (and failed) to sell it for 35 smackers (presumably at a yard sale) -- or, perhaps, someone paid 35 smackers for it and it failed to meet their expectations!

At any rate, this is the first time, I think, that I've actually even pulled it out of the box.

DSC_8408 (2) by Mark Hardy, on Flickr

DSC_8410 (2) by Mark Hardy, on Flickr

JayCee

Wow, you and your dump finds. Even boxed for your return trip home. I used to live in Portsmouth, Dover and Gonic, NH. Never found anything...anywhere. Well, quite a few used lobster shells. Did, frequent trash heaps as a kid w/my dad. Too long ago to remember finding anything.

mhardy6647

Heck, I have even picked up a few nice things from the dumps up this-a-ways. Not in the same league as Boston's "MetroWest", although not bad.

... but I digress.

Viking64

mhardy6647 wrote: »

... but I digress.

You never.....!

[Deleted User]

I started working for Polk in '95 and worked there for 15 years. Actually I still work for them in a very minor capacity. I help with their Amazon customer questions and review responses.

JayCee

KennethSwauger wrote: »

I started working for Polk in '95 and worked there for 15 years.

Polkfest '12 I spoke w/Al Baron (very nice guy) during the BBQ at Stu's house and we started connecting the dots. I worked for Matsushita (Technics/Panasonic) and, on occasion, Nakamichi in the military audio club @ Mainz Kastel, Germany, approx 84-85. Room after room of every manufacturer you could possibly think of. It was a high school kids dream. I even hired Mike, guy I mentioned above, to work for me on busy weekends.

I mentioned to Al that I was there the day Mr. Polk and entourage came through as Polk speakers were introduced to the overseas mil market. Had no idea who "Matt" was or Polk to be honest. Turns out, Al said he was on that trip. Pretty wild to think we'd passed each other in the hall all those years ago. He also went on to say they later poached the JBL rep that worked a room or two from me. Apparently, the JBL guy went on to be an east coast regional manager (based in Virginia?) and retired from Polk, as I recall, around 2010? For the life of me I can't remember his name but a good guy.

Any of this ring a bell and do you know who I'm talking about? Made me think, what if I had taken a different route in life and ended up w/Polk.

Flashing back...I'll admit, on slow days, I'd abandon my post and head down to the Polk room and sit there amazed with the original SDA-SRS. Polk was teamed up and marketed w/NAD, my first exposure to both brands. I'd bounce between the Infinity room and Polk listening to the best of each offering. It's the reason, twenty something years later, I located a pair of 1.2tl's (among many other SDA's and products) and am sitting here typing as I listen to them. Fully upgraded and better than the day they were produced.

[Deleted User]

Yea, Al Baron is a really good guy. He was one of the first people I met when I started working for Polk. At the time his desk was directly behind mine and I noticed a Spike Jones record leaning against his desk lamp. I thought to myself, "Now that's an interesting record!" As I got to know him better I found out he once worked in an audio shop on a caribbean island (I can't remember which one) but that explained all the bright floral print shirts he always wore.

JayCee

Any idea who the JBL guy I worked with is?

[Deleted User]

Sorry, don't know who that would have been.

[Deleted User]

Some necessary capacitors arrived from Digikey so I could continue replacing parts on the Dolby processing circuit board. The goal is to replace all of the electrolytic capacitors in the signal path with Panasonic film types. This is not easy since the film capacitors are considerable larger than the original. This resulted in some fairly creative placement using both sides of the circuit board. I also wanted to bypass each of the larger value capacitors with 0.12uF for improved high frequency capability. The large grey capacitor on the right hand side is a 45uF unit, the smallest I could source.

[Deleted User]

Now to put things back together and see if I can calibrate the two adjustment potentiometers and check the distortion.

[Deleted User]

The modified board checked out fine once I learned what to do. The procedure involves sending a 7.6mV 5.0kHz signal into the circuit with the noise reduction turned off. The FET is shorted to ground and a voltage measurement is made at another part of the circuit. Then the noise reduction is turned on and one of the adjustment potentiometers is adjusted until the output drops by -10dB. Then a second adjustment is done with the FET in and out of the circuit which should result in a 2dB change in the setting. Overall unity gain is checked out as well as THD.
This photo shows the parts removed from the second Dolby process board.

[Deleted User]

I've finished the second Dolby process board and after repositioning a 10uF capacitor in both boards was able to fit them both back into the supporting brackets. After some fine tuning of the calibration potentiometers everything seems to be working correctly. I'll let everything settle in for a day or two and re-listen to the Mahler tape and see what sounds different. Then progress on to the output amplifier circuit board for parts replacement. I've been thinking about trying to adapt a vacuum tube based output circuit from a Knight preamp kit I've got.
Here's a photo of the two circuit boards in place in the chassis.

pitdogg2

Impressive stuff Ken

[Deleted User]

Thank you, I appreciate your comments. I've just got done doing an afternoon's listening session to the 100A and am very pleased. The two Dolby process boards are done and calibrated and I decided the best way to evaluate the results would be to do a quick capacitor change on the output board. So, I took out the 1.0, 10.0 and 47uF caps and put in place a combination of Elna Silmic II and some classic Black Gate 47uF replacements. The Black Gate electrolytic capacitors were discontinued a while back and were excellent sounding. I still have a few remaining from older projects.
This unit has a unique way of doing quite comprehensive A-B listening comparisons. I have a second, unchanged, 100A that I can swap out the various circuit boards. And since each process board is individually calibrated and are easily taken out and replaced it's handy for back and forth listening. My trusty set of ears belong to my wife, she's a skilled listener and isn't overly burdened with what specific gear is being played. Just how it sounds or doesn't sound. I played a Barclay Crocker tape that has a guitar trio. I first played a selection with original process and output boards without telling her anything about what was specifically playing. Then I swapped the boards and replayed the same selection. Without a second's hesitation she said, "Sounds warmer with nicer string tone, and you can hear more of the reflections of the room. Plus the balance between the three guitars is more believable. And there's a reduction in brightness or glassiness (my word)."
That pretty much summed up what I was hearing, also. I went on to play a Mary and Jimmy McPartland B-C tape and just forgot about being an audiophile and just started listening and enjoying what was being played. The realness of each instrument was clearly heard and just sounded great.
I'll now see about doing more with the output circuit board, which is really a nice clean circuit design. I think it's a differential input design for the first stage and a couple of transistors after that. No EQ or filtering that I can see, it uses TIS 97 transistors which were the high gain low noise choice for the early '70's. The Moog synthesizer used them evidently and Advent has carefully matched the ones used here. A pleasure to work on; I'm only gilding Henry Kloss's well made lily.