Kingston
Well-known member
jef111 said:
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.
dbx 166xl schemo...
- Thread starter dementedchord
- Start date
Help Support GroupDIY Audio Forum:
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.
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.
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
Well-known member
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.
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.
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.
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.
Attachments
Kingston
Well-known member
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.
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.
sr1200
Well-known member
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.
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.
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.
Hey guys, here are some photos of my upgraded 166's. I think it's pretty visible which parts were soldered and added.
Enjoy it!
Enjoy it!
Attachments
uuwave
New member
Does anyone have a copy of the schematic for a dbx166xl? I checked the dbx download site
https://dbxpro.com/en/downloads/en/schematic
which has the 166a but not the 166xl. I did not see it in the forums here. Hopefully someone here has a copy. Thanks.
https://dbxpro.com/en/downloads/en/schematic
which has the 166a but not the 166xl. I did not see it in the forums here. Hopefully someone here has a copy. Thanks.
I have a issue with unstable output level on the right 2nd channel.
Level spikes!
Screenshot shows when i engage bypass - everything's fine.
No settings (knob tweaks) are affecting this problem.
I try both TRS and XLR cables.
Left channel is fine.
Please help to find problem.
Level spikes!
Screenshot shows when i engage bypass - everything's fine.
No settings (knob tweaks) are affecting this problem.
I try both TRS and XLR cables.
Left channel is fine.
Please help to find problem.
Attachments
Johnny1234
Well-known member
Only difference is the xl uses a differential op amp input and the plain 166 is an unbalanced opamp input. You should be able to trouble shoot the unit with 166 schematic.
99% of the time you will find dirty switches that cause problems such as yours.
Hello, try to look at IDC flat cable from rear to front board. Try twisting the cable slightly and continuously look at the output to see if there is any change. If not, you will need to pull out an oscilloscope or other signal analyzer, in an emergency, a computer sound card at a higher sampling frequency will also help. Look at CV (pin 2) of VCA, input (pin 1, before 20k resistor) and output (pin8) and you will see where the problem comes from.
Am electronic rookie. What switches you talking about? Bypass switches etc?
Can i clean them with something like Kontakt 60?
Thanks 4 reply.
Brian Roth
Well-known member
Spend several minutes pressing/twisting all switches for a minute or three to hopefully clear out any crud.
Bri
Bri
If I may recommend, do not pour anything in until you find out the exact cause of the problem. Only clean the switches (or pots) when you find a problem inside. Some kinds of cleaners can sometimes cause more harm than good.
As Brian said, repeated pressing/twisting all switches (and ribbon cable) should help to find out a source of problem, if it's electro-mechanical. If you find it to be only electrical, then look at pins of VCA by scope/soundcard. At input before resistor and output pins you should see a full amplitude of signal. On CV pin you should see only DC voltage (some millivolts), there souldn't be any ripple or peaks nor fast changes. These actions will tell us whether the problem is before the VCA, after the VCA or in the control circuit.
As Brian said, repeated pressing/twisting all switches (and ribbon cable) should help to find out a source of problem, if it's electro-mechanical. If you find it to be only electrical, then look at pins of VCA by scope/soundcard. At input before resistor and output pins you should see a full amplitude of signal. On CV pin you should see only DC voltage (some millivolts), there souldn't be any ripple or peaks nor fast changes. These actions will tell us whether the problem is before the VCA, after the VCA or in the control circuit.
Kontakt 60 is safe for circuitry, it's professional spray for pots cleaning.
Unfortunately, cleaning pots and switches doesn't help. Strange that i hear little crackles. Sound like bad potentiometer. But if spray doesn't fix it (like i did many times with many pots before on other my hardware devices) that only replacing may help, and i don't have spare parts here (Ukraine, shit happens)
Unfortunately, cleaning pots and switches doesn't help. Strange that i hear little crackles. Sound like bad potentiometer. But if spray doesn't fix it (like i did many times with many pots before on other my hardware devices) that only replacing may help, and i don't have spare parts here (Ukraine, shit happens)
