DBX 120

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>>>I love all these one sentence explanations for how products work...

Sorry JR. It has been a few years since I did it, but I danced thru one of the dbx boxes with an O-scope and the explanation I gave is what I saw.

Look at the waveform chart for patent #4700390, that's pretty much what I saw...

Here is one of the four PV patents on this:
http://www.pat2pdf.org/patents/pat7136493.pdf


>>>They all work basically the same I think.

That's what I get for thinking... :oops:
 
[quote author="SSLtech"]they just use a zero-crossing detector and mute every alternate half-cycle. (which mis-tracks occasionally and randomly flips the polarity[/quote]

FWIW/BTW, had a closer look at the 120XP-schematic and saw they just do it just as the BOSS FX: they don't mute, but flip polarity.
See the arrangement around the TL072's on page 2: with the BJT conducting the gain of the section is -1, and opening the BJT brings the +2 gain into play, resulting in +1.

The OC-2 happens to do -0.5 & +0.5, but the principle is the same ( http://www.schematicheaven.com/effects/boss_oc2_octave.pdf).

Bye,

Peter
 
[quote author="StephenGiles"]Have you seen this?

http://www.rolls.com/data/rp221man.pdf[/quote]
Hey nice, yet another one doing it like that.
Looks like it makes little sense do DIY here with so many incarnations around.

http://filters.muziq.be/type/octaver/1down/2down/pedal

Stating the obvious, likely they don't all sound the same though, there'll be details (filtering and other stuff) that makes one box nice and the other one less usable.
 
[quote author="SonsOfThunder"]>>>I love all these one sentence explanations for how products work...

Sorry JR. It has been a few years since I did it, but I danced thru one of the dbx boxes with an O-scope and the explanation I gave is what I saw.

Look at the waveform chart for patent #4700390, that's pretty much what I saw...

Here is one of the four PV patents on this:
http://www.pat2pdf.org/patents/pat7136493.pdf


>>>They all work basically the same I think.

That's what I get for thinking... :oops:[/quote]

No worries.. If that's the same patent I'm thinking of, it's not really a bass enhancement box per se, and despite my personal dislike they sold a lot of units, pretty much to do just that.

For a little, "it's a small world", I interviewed and hired Elon there. He was an engineer reporting to me when I managed the mixer engineering group.

JR
 
There's also this which was purpose built for guitar
http://files.muziq.be/schematics/eh_dloctave.jpg

A bloody awful scan, but what you have there is a fundamental extractor where the output is always a sinewave, whatever is fed to the input. The ICs top right are CA3094!
 
there are like a thousand comp articles at AES, everybody wants to build a different compressor for some reason.
still searching dbx, but meanwhile, check this beam comp out from valve research, Austrailllllllllia, Graeme Cohen
Who said, "Do we really need another compressor?"
They were right.





beam_comp.jpg
 
[quote author="JohnRoberts"]I love all these one sentence explanations for how products work.. :roll: [/quote]
in conjuction with the schematic (linked in the first post) I didn't really think that I was being too vague... -I was trying to post what I read the circuit as doing.

On sheet 2, IC 13 is a pair of flip-flops (IC13a and IC13b) the outputs of which switch a couple of 2N3904's. -Wait. maybe that's not actually a mute...

It's been a while since I looked at this circuit, but we own several 120s, and I can testify that the LF output drifts in and out of polarity depending on the input level or the settings... so a polarity-opposed pair will drift through cancellation, reinfircement, cancellation, reinforcement... etc.

Keith
 
I hadn't looked at the schematic until now, but was just puzzled as to how one could get subharmonics out of muting alternate half cycles.

Zero-crossing detectors can come in various forms. Usually I think of them as generating unidirectional pulses in the vicinity of the zero-crossing. Here, it's just a high gain amp, and it clocks the toggling FF which is only triggering on negative-to-positive transitions---thus producing an f/2 square wave. That drives a reverse-connected Q as a shunt switch, which flips the polarity of the amp and hence the signal.

So every other full cycle of the input signal is polarity-flipped, hence producing a lot of energy at f/2, along with some cuspy high frequency stuff as well.
 
[quote author="SSLtech"]On sheet 2, IC 13 is a pair of flip-flops (IC13a and IC13b) the outputs of which switch a couple of 2N3904's. -Wait. maybe that's not actually a mute...[/quote]
Hi Keith,
Right, the 2N3904's are switching the non-inverting opamp inputs to ground or not (so 'muting' the 'direct' non-inverting path) resulting in polarity-swaps or not for the complete section of opamp+BJT, this because of the inverting input is also looking at the same signal (by means of a few resistors)as the direct non-inv. path. So the BJT is actually 'manipulating' a diff-amp; a diff-amp that has both its inputs tied to the same signal.
It's been a while since I looked at this circuit, but we own several 120s, and I can testify that the LF output drifts in and out of polarity depending on the input level or the settings... so a polarity-opposed pair will drift through cancellation, reinfircement, cancellation, reinforcement... etc.
Click to expand...
My polarity-words in previous mails were just about that local circuit-snippet mentioned above, but I can very well imagine that the whole process of dividing & swapping & filtering gives rise to a lot of non-aligned 'dry' (f) & 'wet' (f/2) signals :wink: :oops:
[quote author="bcarso"]So every other full cycle of the input signal is polarity-flipped, hence producing a lot of energy at f/2, along with some cuspy high frequency stuff as well.[/quote]
Yep, that's what must be happening based on looking at the circuit. The subsequent filtering gets rid of the high-frequency stuff, but I guess the generated octave down will quite be lagging the original. ... and so can give all kind of enforcements & cancellations, as indicated by Keith.


And about why/where/if the 120-ish circuits differ from the 'usual' octave-down-FX-boxes:
The 120XP seems to differ from the FX-boxes in their use of the dual-FF 4013 in that it has two 'catching areas' (frequency-wise), each with their own flip-flop & polarity-swap section ('near' 60 & 90 Hz, generating 'near' 30 & 45 Hz),
while the FX-boxes do one (& two) octave(s) down.
Maybe that's one of the key ingredients to the supposedly different results ?

Regards,

Peter
 
[quote author="bcarso"]I hadn't looked at the schematic until now, but was just puzzled as to how one could get subharmonics out of muting alternate half cycles.

[/quote]

While I'm not commenting on any single box, and need to apologize to Keith for offering my own one sentence descriptions, lots of these boxes make people think their cheesy systems are going lower because they hype low freq distortion actually in the system's passband. Many loudspeakers generate gobs of distortion at LF so listeners often mistake the distortion for the presence of lower frequency components.

JR
 
Ah yes, I see it now. I looked briefly at it some time ago and read it as a mute but I see it as a 'flipper' now.

Yes there's some higher-frequency 'fizz' in there where the flips happen, but that gets filtered out fairly heavily, just downstream. The signal is amplitude-related to the fundamental just above it, and -since it is filtered to a narrow 'target' band before flipping, it can be filtered to a similar 'target' band one octave lower fairly easily and reliably.

These things certainly do "the bizzniss" though. There's not a single major motion picture which doesn't enhance its BOOM in the LFE channel by the judicious application of these boxes.

We have THREE of them on our dub stage, and I'd LOVE more!

Many of the 'musical instrument' boxes use a PLL arrangement to generate a single-tone signal, then use an envelope follower to track the amplitude, and often some required filtering. A completely different approach.

The Rolls box is a mimic of the dbx approach, IIRC.

Keith
 
A friend of mine's wife came into the bedroom one evening and asked if he thought she should get breast augmentation surgery. -He asked how much it cost and almost choked when she told him.

-So he asked if she'd tried the "tissue paper trick". -She said:

"Tssue paper trick... -What's that?"

"Well" -he explained- "You take a piece of tissue paper and you rub it in the gap in-between, once or sometimes twice a day, over maybe a year or two."

"Does it actually work?" she asked...

"Well, it worked for your butt, didn't it?" He replied.

That was about 2 years ago, and he finally made it out of the ICU last week...

Keef
 
[quote author="SSLtech"]We have THREE of them on our dub stage, and I'd LOVE more![/quote]
Keith, just curious, that are all '120'-types ? Must admit I have lost overview a bit of the several models out there...
120, 120A, 120X, 120XP, 120X-DS
(if I understood it correctly that last one is digital and a consumer-oriented box with rca's)

Many of the 'musical instrument' boxes use a PLL arrangement to generate a single-tone signal, then use an envelope follower to track the amplitude, and often some required filtering. A completely different approach.
Click to expand...
As it happens I ran across the schematic of the BOSS DF-2 a while ago, sounds like an example of what you wrote.

Wayne, please tell us that pic was photoshopped.... :cool:

Regards,

Peter

[edit: added type-number]
 
Do we have a schematic of how Aphex did their "Big Bottom"?
Click to expand...

Yep, jokes aside, does anyone have a schematic for this ?
I believe it's a compressor for bass frequencies that is added back to the main signal but I'd really like to see a circuit.
 

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