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Anybody still use DBX-3BX units?
Anybody still use DBX-3BX units?
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Old 9th October 2017, 10:04 PM   #1
Chetopia is offline Chetopia  United States
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Join Date: Sep 2014
Location: Gainesville
Default Anybody still use DBX-3BX units?

Recently I purchased a DBX_3BX Dynamic Range Expander on EBAY. It came with the service manual and after a couple of weeks I finally figured out the schematic and the adjustments in the service manual. First I noticed the schematic and the manual didn't jive as far as the chips called out to check test points. It was something to do with my units serial numbers being higher than 4000. The IC chip numbers were not correct for the measurements that were to be made in the manual.
I recapped my DBX-3BX then re-calibrated it and now it sounds great!
Couldn't live without it!


Here is my calibration scheme! I hope it helps others too.

There is a little more than I posted here and if your interested just email me and I'll send the rest. The unit is so well built that I just ran through the rest of the tests without having to do any more adjustments.



RMS chip adjustments. “Using an Oscilloscope”
Low frequency.
100HZ input and -15dBV (0.178 VAC RMS)
Check the voltage at C10 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R195 for a balance +/- in the signal.
Check the voltage at C20 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R243 for a balance +/- in the signal.
Mid frequency.
1KHZ input and -10dBV (0.316 VAC)
Check the voltage at C10 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R230 for a balance +/- in the signal.
Check the voltage at C20 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R163 for a balance +/- in the signal.
High frequency.
1KHZ input and -10dBV (0.316 VAC)
Check the voltage at C10 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R214 for a balance +/- in the signal.
Check the voltage at C20 (4.7uF NP cap)(right side) capacitor input for symmetry and adjust R179for a balance +/- in the signal.

Then adjust the transition level for each frequency. (Low,Mid,and Hi frequencies). I used a multi-meter here.

R58 is the low freq. level at 100HZ input and -15dBV (0.178 VAC RMS) input this should be adjusted for 0 volts. 0A15 pin 1.
R76 is the mid freq. level at 1KHZ input and -10dBV (0.316 VAC) input this should be adjusted for 0 volts. 0A16 pin 1.
R98 is the high freq. level at 10KHZ input and -27dBV (0.045 VAC) input this should be adjusted for 0 volts. 0A17 pin 1.
R98 is the high frequency transition level adjustment and this is where I disagree and think the highs should be dialed down a bit. Instead of zero volts I have adjusted mine slightly above this by turning the pot CCW while listening to an album and reducing the enhancement. Set the Expansion control to 1.5 and adjust to your liking.

Voltage Controlled Amplifier Level Adjustments. Oscilloscope works best here.

Set the input to 1 VAC RMS for the left channel.
Low freq(100hz) adjust R69 for 1 VAC output
Mid freq(1000hz) adjust R90 for 1 VAC output
High freq(10000hz) adjust R109for 1 VAC output
Then the right channel.
Low freq(100hz) adjust R70 for 1 VAC output
Mid freq(1000hz) adjust R91 for 1 VAC output
High freq(10000hz) adjust R110for 1 VAC output
Distortion is the next test and usually is fine after doing these adjustments. Published is 0.03%.
I hope this helps bring some of those older units back to life!
Analog will never die! Just gets renewed!

Last edited by Chetopia; 9th October 2017 at 10:18 PM.
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Old 10th October 2017, 08:38 AM   #2
dotneck335 is offline dotneck335  United States
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Join Date: Dec 2012
I am wondering how you use this unit---is it in-line on your system always? Or just select recordings?
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Old 10th October 2017, 12:24 PM   #3
Chetopia is offline Chetopia  United States
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Join Date: Sep 2014
Location: Gainesville
Default Long story shortened...

Since the adjustments I made it really does sound great between my ADC Sound Shaper-3 equalizer and Class-A amplifier. It really does expand the range of the sound. Unless you get one of those albums that was recorded too loud!
I normally set it between 1.3-1.4 and the transition is set to give just one or two LED's in the red during loud passages.
I normally listen to albums on the line of Bob Seger, Dire Straits, Eric Clapton, showing my age here. I am really old school!
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Old 10th October 2017, 04:32 PM   #4
RJM1 is offline RJM1  United States
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Location: Titusville, Fl.
I still have a dbx117 that I bought brand new in the 70s. I replaced the 741 op-amps with LME49710's (a Jim Williams mod), re-caped and modified the power supplies a couple of years ago. It works much better now. I worked for dbx for 6 months while they were making the 3bx. Showing my age.
You might want to replace the 4558 op amps in your 3bx with something better as op-amps have been greatly improved the the last 40 years.

Last edited by RJM1; 10th October 2017 at 04:51 PM.
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Old 10th October 2017, 07:50 PM   #5
Chetopia is offline Chetopia  United States
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Join Date: Sep 2014
Location: Gainesville
Default I agree!

I was always taught that the noise starts with the first circuit and becomes less of a problem as things move along.
I don't agree totally, remove as much as can be possible along the way was my theory.
I love my system and it is 50% home made!
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Old 10th October 2017, 11:06 PM   #6
Pano is offline Pano  United States
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Location: SW Florida
Anybody still use DBX-3BX units?
Moved to Analog Line Level
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Old 11th October 2017, 03:51 AM   #7
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by RJM1 View Post
You might want to replace the 4558 op amps in your 3bx with something better as op-amps have been greatly improved the the last 40 years.
YES....please.....4558s are among the worst opamps available these days.....
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Old 11th October 2017, 12:57 PM   #8
Chetopia is offline Chetopia  United States
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I must have one of the later models (S/N-20,000+). My schematic and the service manual doesn’t line up as to what chip number it calls out for the signal it wants to test. When I looked over the schematic again I could not find a RC4558 in the signal chain.
The input buffers and frequency splitters (Low,Mid,and Hi) are LF351 and LF353 chips. The DBX-RMS chip samples the signal after the frequency select filter then gives it a full wave rectification with peak detection (RC4558's), then sends this to the transition and expansion level controls (more RC4558’s). Once these varying DC signals have been compared to the human adjustment the varying DC voltage is applied to the VCA to control the amount of amplification that is produced at any given time.
I was really impressed with the method DBX used for the VCA amplifiers temp stabilization. They had two matched transistors wrapped in a foil and glued to the other two wrapped in a foil and all four of them stuck together for one VCA. From there the the VCA amplified signal is fed to two FET drivers at another LF353 for the output of the DBX through a 4.7uf NP cap to the RCA connector on the back.
I mentioned that last capacitor because when I recapped it I wanted to stick with film caps and didn’t like the idea of putting electrolytics back in the signal path. I found a Wima cap in stock at Mouser for $3.10, (MKP4-4.7/250/5p27) this seemed reasonable at the time. Then the mailman dropped them at my door. Each one of them was about the size of a cigarette pack! They sure looked smaller in the Mouser pictures!
I bet it was fun to work at DBX? I always wanted to work in the high end audio business but got sidetracked into research and have been there ever since. As you can tell I still have the itch!!!
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Old 15th October 2017, 12:35 AM   #9
tom2016 is offline tom2016
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Back in the day when I was single I built several 200 watt per Channel amplifiers which were connected to active Crossovers and their own speaker drivers that would simply shake the walls of my house. When I played movies like The Terminator 2 movie I would switch this unit in and it would simply blow anyone away who came over to listen to it. Explosions would ruffle your pants. I still have that unit somewhere, maybe it's time to resurrect it. Thanks for the post, it brings back a lot of fun memories!
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Old 18th October 2017, 10:31 AM   #10
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by Chetopia View Post
.........the output of the DBX through a 4.7uf NP cap to the RCA connector on the back. I mentioned that last capacitor because when I recapped it I wanted to stick with film caps and didn’t like the idea of putting electrolytics back in the signal path. I found a Wima cap in stock at Mouser for $3.10, (MKP4-4.7/250/5p27) this seemed reasonable at the time. Then the mailman dropped them at my door. Each one of them was about the size of a cigarette pack! They sure looked smaller in the Mouser pictures!
The BEST 4.7 uF cap I have found is one from Mouser: Panasonic ECW-FD2W475J. It's a polypropylene, and large, but not TOO big---25.3 x 11.7 x 16.4mm; 15mm LS---it will fit on most boards, and I think it is far superior to any electrolytic or polyester.
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