DBX 290 reverb processor noise

[tedsorvino]

Hi everyone.

I recently found an old digital reverb unit DBX 290.
It has this simple but hard to find 4 pin din AC power supply (attachment).
Simple 9 vAC 20VA (2.2A).

Since the original EU model is expensive and hard to find I made one out of 230v ac primary / 9v ac secondary 20 VA.
But when I used it there was an audible constant white noise blanket particularly audible on louder signals.
It's not acceptable so I guess this is not normal even if the unit is not a great one.

I replaced all power supply filter caps and the same noise is still there.
Needless to say that i used the same leads, same test procedure etc that i'm using with other units with great results.

My question is what could go wrong with a simple ac adaptor since there is no filtering on the external ps unit.
Is it a matter of lower quality ac supply?
I mean do you think that DBX use better quality stuff for its cheaper series?
Is it a matter of current rating since i'm using EU voltage?
Is it something else, somewhere else in the circuit?

If you have any ideas please advice

I attach the simple original supply diagram i followed

 

[NoisyIndividual]

An AC adaptor is unlikely to create "white noise".
Can you post a recording of this noise? It may well be digital distortion due to faulty RAM or DAC.

 

[tedsorvino]

An AC adaptor is unlikely to create "white noise".
Can you post a recording of this noise? It may well be digital distortion due to faulty RAM or DAC.

Thanks for the reply.
Even if at the moment i can’t send anything - i think i will do sometime tomorrow - but i guess what you said abiut bad DAC seems really probable. Ram would cause glitches or other artifacts and all sort of functioning issues. Nothing like tgat here.

This noise is like a layer of brown (or whatever type of) noise, under the clean and really good quality reverbed signal.
When the effect is taken back lower with the unit’s mix pot the noise is lower.
The same if i take back the output pot.
And when the effect mix is at its lowest the niise is not there.

It has to do with the digital reverb path.
Maybe something with the internal digital clock syncing?

The schematic is online if you want to further advice.

 

[NoisyIndividual]

Bad RAM most likely.

Edit: If the bad RAM only affects the lower bits, it will appear as a noise under the signal as you describe.
So every sample will have the fine details scrambled, while you can still hear clearly the rest.

 

[tedsorvino]

Bad RAM most likely.

Edit: If the bad RAM only affects the lower bits, it will appear as a noise under the signal as you describe.
So every sample will have the fine details scrambled, while you can still hear clearly the rest.

In case it is a bad part of the ram ( i wonder how this could happen without affecting other more common components) can i replace the chip without losing all the data it contains? I mean it’s impossible to find the info online.

 

[NoisyIndividual]

The RAM I'm talking about is just the audio RAM the unit uses to create the audio signal delays needed in reverb processing. It is not used elsewhere.

But this is so far pure speculation based on experience.
It may not be the RAM at all.

Which is why a recording might be useful.

A frying chip making noise, which it could be, sounds very different to faulty RAM. The ears can tell.

 

[tedsorvino]

That sounds promising. I attach the schematics in order to make things more specific. Hopefully i ll post a sample tomorrow

 

[RoadrunnerOZ]

There are three tantalum caps listed - C15, C17 and C52 - C15 and C52 are on the input to the 7805 regulator U7 and C17 is on the +5VA supply to U22 pin 7- the D/A converter CS4330. These (or one of these but I’d change them all) may have failed.

 

[tedsorvino]

There are three tantalum caps listed - C15, C17 and C52 - C15 and C52 are on the input to the 7805 regulator U7 and C17 is on the +5VA supply to U22 pin 7- the D/A converter CS4330. These (or one of these but I’d change them all) may have failed.s

Seems like a much easier fix. I will try it soon

 

[NoisyIndividual]

If you make a recording, maybe you could record a burst of audio with a long delay. Then it may be obvious if the same noise is repeated each delay, or the delay is noisy itself.
If you see what I mean.

 

[tedsorvino]

Here are my files.
One with just the continuous annoying noise floor set loud.
The other with a track played through the unit.
I experiment a bit with the decay, the in and the out level along with the reverb mix

 

[RoadrunnerOZ]

The attachments don’t seem to play

 

[tedsorvino]

Sorry about that. Please try these links

https://jumpshare.com/s/go2nEQArh4xBRhsDuOH1

https://jumpshare.com/s/c49suyz7AxtHYI2Dapqj

 

[NoisyIndividual]

That noise doesn't sound like bad caps or frying silicon.
More like something digital or SMPS, it has a 277Hz or thereabouts based tone in there as well as the white noise.
I don't recognise it.

 

[tedsorvino]

That noise doesn't sound like bad caps or frying silicon.
More like something digital or SMPS, it has a 277Hz or thereabouts based tone in there as well as the white noise.
I don't recognise it.

At least it’s obvious what i have to avoid. Even if i have seen cheaper pedals with this amount of noise floor, i think it’s improbable to sell a semi pro unit with this noise floor out of the factory.

 

[RoadrunnerOZ]

There seems to be some sort of feedback occurring at the end of the music track as well as the noise. The noise itself has a distinct lower audio overtone to the high frequency noise
I’d suggest tracing through with a scope - see if you can find the area of noise generation. Check the sends to the A/D U17 and if clean the returns from the D/A U22 first to see if it’s noise going into or coming out of the effects process zone.

 

[tedsorvino]

Thanks for the help and the observations.
I don’t think i will try to change such a huge ic for such a problem. I think for the time being i can survive with its flaw. A bit of more discreet effect in the mix, along with a bit of noise suppression, will do the trick

 

[NoisyIndividual]

There's a remote possibility the low audio tone (138Hz, not 277Hz I mentioned earlier) is coming into the device rather than being generated by it.
Running it standalone in a different location may eliminate this, but probably not, actually.

I also can't believe this was how it left the factory, unless there is now some kind of grounding issue, and the noise is generated within a earth loop or some such.
Or just that your gain structure is bad, resulting in a lot of noise overall, but that sounds unlikely too.

 

[RoadrunnerOZ]

There is an input noise gate which comes before the reverb (not the gate for gated reverb) check that this is on. The Small and Large buttons are pressed simultaneously until their LEDs light - then the first four room type buttons are used to select the gate threshold. If only room is lit, your gate is off - if all four are lit you are at the highest noise threshold. See if the noise you are hearing switches off when you have the highest threshold. If it does then the noise is pre reverb and is either coming in from external source or possibly is being generated by the opamps in the pre stages.

 

[RoadrunnerOZ]

With any reverb I never pass the full original audio through the unit as an insert but send to it from an Aux send bringing it back into the mix on its own channel(s), full wet, reverb sound only - thus eliminating any artefacts from the unit itself in the original signal.