dbx 166xl schemo...

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Kingston

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jef111 said:
Yes, I know, but paralelling of VCA's is for another PCB it would be crazy here...

Add an 8-pin socket and make an adapter. I did this with GSSL too. Plenty of room in the PCB in this area for it.
 

jef111

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I continued to play with noise suppression and found that the main source of noise was the VCA control circuit...
VCA is relatively sensitive to noise (or any other modulation) on EC inputs and TL074 is a silly noise generator here. OPA4134 is much better performer at this point of view and after replacement of U6 the noise dropped clearly to half or maybe less (by eye on scope). Also HF stability is better - there is no need to add a 1ohm resistor anymore - and THD+N also dropped slightly. Some another filtering caps can be added to control circuit to help reject noises from pots. I'll post some details later.
 

Kingston

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A definite upgrade to this type of design is modern PSU regulation. Replace those lowest of low end things with LT3081ET and LT3091ET. They have a large amount of pins compared to the usual pairing of LM7815/LM7915 but don't need significantly more parts. Not a drop in replacement but close.

They have the performance of jung regulators aka "super regulators". These would have very likely solved the noise you were seeing and definitely just about any spurious oscillations you might have encountered.

I'd start with that before any opamp swapping.
 

jef111

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Fine, I'll try those LT's, thank you for suggestion. I also have some LT1086, it's older generation but still better than 78xx and direct replacement. But don't have a negative complement :-( If I would upgrade an PSU, I would also add some RC snubbers in parallel to diodes and to main filter cap.

But noise what I'm seeing comes really from control circuit (yes, It could come from bad PSR of opamps, I know, but...) If I short EC pin of VCA to ground, then noise drops. Also after replacement of U6 the noise significantly dropped.

I finally received scheme of XL version from Harman. And maybe I found source of strange HF noise. C29 is shorting an output of LM339 at HF and when inverting input is connected to this point it could cause some unstability. It means that 1R resistor in series with C29 should stay. I also added some blocking caps to outputs of pots GAIN and TRESHOLD to reject it's noise when turning and 1n cap in parallel to R77 for better stability of U6-A. Let's look at attached scheme.
 

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Kingston

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I just came up a with a really good additional feature for this compressor. Fast and easy to implement. I did this for DBX 166XL but the same can be done for all the variants.

Feedback vs. feedforward switch for the sidechain with single pole on/on (SPDT) switch per channel. There's just enough room on the front panel when using miniature switches.

The idea is simple. Sidechain signal comes either directly from the input (stock setting) or from post-VCA opamp.

feed-forward wiring:
INT_SC1 -> SC1_SOURCE (internal sidechain into the C21 0.33uF, or SC enable switch)
feedback wiring:
CLAMP1M -> SC1_SOURCE (post-VCA opamp out - also known as CLAMP in the schematic into the C21 0.33uF, or SC enable switch)

Then do the same for channel 2. Note that this modification involves a single trace cut for each channel.

It's RMS based feedback so the effect can be subtle at times but in my opinion it's better than the original. It's a self-balancer for the RMS circuit basically. Kind of like RMS regulator. Attack shape sounds way better.
 

jef111

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Yes, very interesting thing. It changes a knee shape and compression ratio anyway.
 

sr1200

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I know this is an ancient thread by now, but is there a "final" version of the schem with all the mod additions/replacements. Also, with the SMD components on this thing, how easy is it to add components to the board (and if you have any pics of the mods as far as how you went about implementing them on the board) that would be awesome. I just picked one of these up for $20 (was pretty beat up) got it back working stock (with the exception that the second channel output is very low). Was going to just replace the stock amp for the output and hope for the best, but if i can make this thing more usable, id love to try it out! Thanks.
 

Script

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The limiter's changed behaviour is the single most useful and immediately noticeable modification.

When driving hard and cranking up the output, the limiter stays acceptable with bass frequencies still sounding 'defined', whereas before it went into useless sonic fart mode (--maybe that was intended as a feature [for hip-hop?] who knows--)
__
'Final versions' of schematics and modifications is all above. This is not a long thread. Would need to locate components and trace respective parts of boards thoroughly anyway (SMD 2mm) before changing anything.

Best to trace both channels at once (partially different layout). Component designators in (all but one) schematics above correspond to left channel.
 

Script

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Oh, I also changed C7 from 22uf to 100uf to lower the cut-off frequency there (0.36Hz to 0.08 Hz). Maybe marginal cos not many, if any other, elcos in audio path, but easy enough cos thru-hole.
 

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