Advent 100A Dolby B noise reduction

[Deleted User]

The next step in this project to build a better sounding Dolby B is to see if I can figure out how to take the signal from the Dolby processor before it's amplified by the original output stage and instead amplify it with a tube stage. But before I could do that I needed some kind of experimental platform for testing. I had available an older Knight-Kit of a tape deck record and playback set of electronics I could configure as a playback only circuit. I stripped everything off of the chassis and painted the frame and AC transformer's laminations and bell housings. Then I rebuilt the power supply and added some capacitance to the high voltage B+. Then I replaced all of the resistors and capacitors in the circuit board and installed some new RCA sockets. Here is a photo of the test setup, with the Advent Dolby unit on the left and the Knight on the right with test leads going from the Advent to the inputs on the Knight.

[Deleted User]

This is a photo of the Knight by itself, I've replaced the AC power cord, the fuse holder and the two volume potentiometers. I had to figure out how to mount four additional filter capacitors, so I used a piece of Masonite and cut out openings for the capacitors and volume controls. To add a bit of color I painted it with Modelmasters model car paint. I wanted to learn about how negative feedback worked in changing the frequency response, distortion and gain. The original design was for playing back NAB response curves for the particular playback head Knight used in their companion tape transport. I experimented with chasing different values of resistors and capacitors until I was able to duplicate the classic Ampex playback curve.
But for this project I needed a flat response, no EQ needed. I began trying different values of feedback resistors and measured the output, the distortion and how it effected frequency response. I've settled on using 1.0mOhm in the feedback loop.

[Deleted User]

I was able to listen to how things sound this morning and excluding some low level humming (not surprising) things sounded quite good.
The signal left my tube preamp and went to a venerable NAD 3020 integrated amp (soon to be replaced with the less cumbersome headphone amp) and played back through my dad's old Audio Technica headphones (fitting).

[Deleted User]

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.

[Deleted User]

Now trying to spruce up the Advent a little bit. I have a second unit that has a nicer faceplate and knobs, so I removed it and polished up the faceplate, knobs, decorative rings on the knobs and the locking nuts.

[Deleted User]

I've calibrated the Dolby level meters and checked all the other parameters. I've cleaned up the real wood cabinet and put everything back together. So, this one's finished.

pitdogg2

One of these days Ken when you're more dedicated to audio you might get good at this stuff.
(Ducks from the flying trash can)
🤣😆🤣

honestaquarian

pitdogg2 wrote: »

One of these days Ken when you're more dedicated to audio you might get good at this stuff.
(Ducks from the flying trash can)
🤣😆🤣

I think he paid somebody to do all of this

Hey watch the shoe!!!
(dang he has a good aim! )

[Deleted User]

The second half of this project is nearing completion, the tube based preamp that takes the signal from the Dolby processor and provides sufficient gain to be connected to my system. The idea was to not use the transistor based amplifier found in the Advent and have some fun and learning experience at the same time. Since the original Knight tape preamp used several tubes in the recording and bias sections which I didn't need there were some adjustments needed for the high voltage B+ and the heater supply. They were too high and the dropping resistors had to be changed to bring everything within the original design. After doing this I added the tube shield over the first stage tubes and painted the bottom plate and added feet and sound damping material to the underside of the plate.
I've done some initial testing for gain and distortion and found that the overall gain was 37dB and the lowest distortion was 0.56% for the left channel and 0.66% for the right channel using, curiously enough the Shuguang and JJ 12AX7 and 12AT7 which were quite a bit lower in THD than the Electro Harmonix. I've ordered some very low noise tubes for further testing. I wanted to to keep the negative feedback as low as possible and balance frequency response and distortion against raising the feedback. Initially when I began measuring the frequency response there was a significant roll off around 75Hz and very flat at the upper range. I did some testing of increasing the value of the final interstage coupling capacitors and found that going from 0.15uF to 2.0uF made a dramatic improvement. In fact the frequency response is within 2dB from 16Hz to 28kHz and for most of the audible range the amount is less than 0.1dB. I know full well that sound quality is what counts, but measurements are important also.

[Deleted User]

JayCee

Awesome work, Ken.

[Deleted User]

Thank you, I learned a lot doing this one.

[Deleted User]

I thought I would see if I could make some measurements of this preamp using different tubes. Keep in mind this is a linear amplifier there is no equalizing needed, what goes in gets increased by 37dB and appears in the output. I tried three different new production tube sets (a single 12AX7 followed by a single 12AT7) one set a combination of JJ Electronics (12AT7) and Shuguang Electronics (12AX7). A second set with both tubes made by Electro-Harmonix made in Russia. The third set I chose were from RAM Labs, a company founded by Roger A. Modjeski and, as far as I know, the most thoroughly tested vacuum tubes available. I chose the super low noise versions of both tubes.


This is a frequency response graph I plotted, with the JJ/Shuguang on the top, the EH in the middle and the RAM tubes on the bottom. For the test frequencies I selected 440Hz (middle A on a piano) and the octave frequencies above and below. These test frequencies ended up being 13.75Hz, 27.5Hz, 55Hz, 110Hz, 220Hz, 440Hz, 880Hz, 1760Hz, 3520Hz, 7040Hz, 14,080Hz and 28,160Hz. These test frequencies represent the even harmonics of a tone centered around middle A when played by a piano. The left side is divided into 1.0dB increments.
Mr. Modjeski maintained that the tubes themselves didn't have a way of changing the sound. What does change is how each circuit performs with different tubes. As you see by the lines the top two allow the circuit to begin a slight emphasis of upper frequencies. The lowest line has a gentle roll off with the difference between the two extremes adds up to 4.2dB at 28,160Hz. In my opinion clearly audible after careful listening. I hope this is interesting.
As an aside, the preamp with RAM Labs tubes has -3dB limits of 4Hz and 50,00Hz, not bad for an old technology.

JayCee

I did a double take when I realized the plots were by hand. Haven't "graphed" anything by hand in probably 30 years! You must've dug into your "if there's ever a pandemic, semi-logarithmic hard copy stash"?

This is interesting...based on Mr. Modjeski's insight and confirmation you hear the respective tube's affect on the circuit.

Thanks for the after midnight read.

KennethSwauger wrote: »

As an aside, the preamp with RAM Labs tubes has -3dB limits of 4Hz and 50,00Hz, not bad for an old technology.

Very cool!

[Deleted User]

Yes, I found a box of graph paper from Keuffel and Esser (K-E) on eBay a few years ago and borrowed a set of fine point markers from my wife.
Curiously in testing for THD (total harmonic distortion) at 500Hz the JJ/Shuguang was the lowest with 0.6%, the EH next at 0.7% and the RAM at 0.8%. Probably all within the same statistic grouping of essentially the same reading. I'm going to see if the THD changes after the tubes have played for a while.

Spencem

Just spent the last 30 minutes admiring your work, Ken. Outstanding! I just unearthed the 100A I used to use with my TEAC A-6010 that I just got running again: needed left take-up reel motor. The 100A has one missing knob, a slightly bent face panel, and one missing channel. Would appreciate advice on where to start trouble shooting or recommendation of service tech in northern California. Thanks very much.

[Deleted User]

Hello,
Welcome to Club Polk! Thanks for the comments. I would remove the top cover and try removing and reseating the circuit boards, it's easy the boards just pull up. Advent used a conductive grease on the pins which becomes less conductive over the years. Hopefully this would solve the problem.

Spencem

Ken, YOU DA BOMB!!!
Removed the top as you suggested and found the PLAY "A" card loose. I removed all 6 cards, tightened the spacing on the connectors on the motherboard and replaced the cards. It now works perfectly! Thank you!!
I did find that one of the round white plastic spacers that aligns the insertion of the RECORD "B" card is missing. Do you know a source or substitution for that? I also discovered that my unit is a model 100, NOT a 100A.
Now for some fun. Thanks again.
Michael

[Deleted User]

That's great, Michael, I'm glad you were able to sort that out successfully. The 100 is very similar to the 100A, I have both. There are a few differences, the 100 uses a different model transistor throughout the record and playback circuit from the 100A. In fact the transistor used in the 100 is the 2N3391 the same one used in the Tapesonic deck I have and the Moog synthesizer. This might be the reason some people have said the 100 sounds slightly better than the 100A.
I'm afraid I don't know of a source for the missing PCB guide, but there are usually a few of them for sale on eBay for fairly reasonable prices. Nice way to get parts.

Spencem

Thanks for your comments Ken. Good news! When I removed the top cover to reseat the cards, I had not removed the bottom. When I did that today (carefully removing the pilot light before tipping the front), I found the missing parts jammed into the wires behind the front panel. The unit is noisy and there is distortion, so it probably is time for come capacitor renewal. Best.

Spencem

By the way, your mention of the Moog brought back memories of the Different Fur Trading Company's mix-down/concert at the SF Morrison Planetarium entitled "Black Holes and White Dwarfs", which attracted the most unusual and diverse audience I've ever seen. Little did many of them know it was actually about astronomy!

[Deleted User]

I'm glad you found the missing part. Have you set the playback calibration levels in the unit? Does your Dolby encoded tape have the reference tone? When you say distortion and noise do you mean on playback or is it there when you try and record?
I'm not familiar with Different Fur Trading Company, were they a San Francisco band? I see there's a recording studio with the name Different Fur.

Spencem

Yes, I have calibrated using the tape and find that the VU meter levels on the deck are the same as I marked on the A-6010 years ago. I must have stored the tape well! It was when I began the record calibration process that I heard the noise and distortion. I tried playback of some tapes I recorded with the unit years ago using Dolby switched in and out; noise present both ways. Physically removing the unit from between preamp and deck eliminated it. I did this with the top still off and wiggling the input board exacerbated the noise. I did not wiggle any of the other boards. The volume levels of that tape were lower when thru the unit compared to without (physically removed, not switched out). Is this unit like the Dynaco PAT-5: noisy with the top off? I'll try it again today with the top back on to see if that helps.
Yes, Different Fur was a recording studio. They brought their fur-adorned mixing board and a Moog to the planetarium and 2 guys (probably Gleeson and Vieira) did live mix-down while the planetarium projected a light show. My guess is that was some time around the very late 60s during the Haight-Ashbury scene. I was in school at UCSF at the time and lived half a block from Golden Gate Park.

[Deleted User]

The Advent uses a steel enclosure, I had to turn off my bench lights when making distortion and noise measurements. It should be quieter with the top and bottom covers on. But, there could be a problem with the power supply's filter capacitor.
I found a photo of Patrick Gleeson and Herbie Hancock working on a synthesizer at Different Fur studio. The studio is still in business, boy if those walls could talk!

SIHAB

Awesome thread!!

Spencem

I remember him as having longer hair.
Been busy with other stuff: it rained in California...nobody knew what to do...
Maybe I will have time later today to check out voltages, etc.
Back later.
M

Spencem

Hey Ken,
More good news! I sprayed all of the contacts with DeOxit and then sprayed with contact cleaner/lube: all the noise is gone. I wiggled the boards and there was no more noise or distortion. I even swapped out the burned out lamps for some LEDs I had laying around. The lights are now green rather than amber, but the unit looks and sounds great. I'm in the middle of installing rain gutter leaf guards, so won't have time until probably Tuesday to do the record calibration. I am really enjoying listening to some of the old tapes I made. Earlier tonight I found the one I made of the last night at the Fillmore West with Tower of Power and CCR that I taped off KTIM July 4th, '71. Santana also performed but copyright law prevented broadcast; I later got it on their third album. Now if I can just find that tape of my wife's grandfather singing greek songs.
Best wishes and thanks again for your help. Stay healthy.

[Deleted User]

That is good news! I know what you mean by listening to old tapes. My buddy and I would exchange records and tape them, we had similar enough taste in music that we could swap albums. So, now I can listen to what the turntable, tone arm and cartridges that I was using then sounded like. I wish I had jotted down the specific combinations on the playlist I put in each tape box.
Take care!