Hardware compressor has a fluttering sound

[wearashirt]

I've been using a hardware compressor (DBX166XL, 2 channel) for my livestreams.

I recently noticed that channel 2 has a fluttering sound.

I emailed the dbx co. to ask for tips, as the product has been discontinued even for service. They kindly said that it may be the compressor potentiometers.

If anybody here has repaired this kind of equipment, do the issue and hypothesized repair sound sensible?

[Tim Gillett]

Potentiometers in older gear often cause problems as can switches. Just working them up and down or on and off for say 10 times can sometimes improve them enough to identify them as a cause. It may also be dying capacitors. Any chance of an audio sample? It can speed up diagnosis greatly.

[Martin Walker]

Hi wearashirt!

As Tim says, it's always difficult to diagnose audio problems based on written symptoms - can you record some audio demonstrating what this sounds like?

However, personally I find it hard to fathom that this fluttering sound could be caused by faulty pots.

Martin

[Hugh Robjohns]

Agree with the others here, text descriptions aren't a substitute for hearing the problems.

However, 'fluttering' suggests to me a rapid variation in volume, and that inherently points towards a problem wither with the VCA itself, or with its control signal.

The control signal is derived from the audio itself, of course. So the first practical test I would do is to play steady tone through the unit, adjust the threshold to achieve 4-8dB compression, and see if it still flutters. This effectively tests the VCA along with the ratio and threshold controls. If you're still getting fluttering then the problem lies in those areas.

If it sounds clean then that suggests a problem with the dynamic aspects of the side chain, the attack and, particularly, the release control time constants. An overly fast release time constant will easily cause a fluttering effect, for example.

Dirty or unstable pots could cause the side chain signal to vary randomly. But I would be more inclined to suspect a problem with dried out electrolytic smoothing capacitors either in the side chain circuitry or even in the power supply.

In any case, if you're sure the unit is set up correctly and yet performs differently to your other unit, it is going to need the services of a qualified technician with a service manual. Locating the faulty part(s) is one thing. Realigning the unit after repair is another!

[wearashirt]

Locating the faulty part(s) is one thing. Realigning the unit after repair is another!

Does realigning pertain to the unit's performance as a stereo processor?


Thanks everyone for the healthy responses. I will deliver that audio example in a few hours using acoustic and sine wave examples. I heard it first and most conspicuously with the bass guitar.

Maybe another adjective would be a "flapping" sound that adulterates the output sound. Perhaps imagine a torn speaker cone? Even if I raise the threshold to bypass compression, the flapping sound is there, mostly towards the decay.

My own senses tell me it has little to do with the compression action, but perhaps a busted circuit component. I was hoping there may be something in compression devices that bust out regularly.

When cheap compressors like these bust out - are they worth reparing in your opinion? Like, if the cost of repairing the channel is already 1/4 of the way to a new, used unit. (this is 2nd hand).

[Hugh Robjohns]

Does realigning pertain to the unit's performance as a stereo processor?

Yes, most definitely.

Thanks everyone for the healthy responses. I will deliver that audio example in a few hours using acoustic and sine wave examples. I heard it first and most conspicuously with the bass guitar.

Setting too fast a recovery time on a bass signal can often cause problems because the side chain tries to follow the amplitude of the fundamental waveform itself, rather than the overall longer-term level changes.

Applying a high-pass filter to the side-chain can alleviate this kind of problem if you need a fast recovery time for a musical effect.

When cheap compressors like these bust out - are they worth reparing in your opinion?

It depends on what is wrong, how much it will cost to repair (in components and time), how valuable the unit it to you and your workflows, and the cost and availability of a suitable alternative. Obviously, if the repair costs a substantial proportion of the unit it could be argued that it's not worth it.

[wearashirt]

Here are the audio samples.

https://www.dropbox.com/sh/r4hxolrp86e8 ... h97ma?dl=0

Once again, the problem is in Channel 2 of the compressor, and is noticably heard in bass sounds. If you have little time, pls go straight ahead to bass guitar, bass synth, and kick audio files.

Mid-high frequency examples don't inherit the audio defect (guitar and high synth examples).

Notably, If I enable the Stereo Couple button on the hardware device, channel 2 solely uses the circuit and front panel settings of channel 1, and the audio fluttering problem disappears. If the button is disabled and consequently Channel 2 runs as independent mono, that's when the problem comes in (i.e., as heard in the examples above).

[Eddy Deegan]

If I enable the Stereo Couple button on the hardware device, channel 2 solely uses the circuit and front panel settings of channel 1, and the audio fluttering problem disappears. If the button is disabled and consequently Channel 2 runs as independent mono, that's when the problem comes in.

Two compressors in mono mode, one on each channel, work independently; each monitoring the incoming signal level and applying compression based on their threshold/type/ratio etc. settings on that channel only. Each time compression is applied this will affect the relative amplitude of each channel to the other and depending on the threshold/ratio settings could be fairly radical and/or frequent.

Two compressors in a linked stereo configuration will synchronise their actions regardless of which channel causes that action. So if the bass level on channel 1 triggers the threshold then the resulting compression will be applied to both channels simultaneously, using the threshold/type/ratio etc. values of whichever compressor is selected as the master control.

You have two channels of bass - does this mean it was recorded in stereo? If not, and it's two parallel mono tracks then each compressor must be set to the exact same settings as the other in order to avoid variations in the sound of one channel causing issues when played against the other.

If your live-stream outbound audio is stereo and the compressor is on that buss then you should have the Stereo Couple button activated.

[wearashirt]

If I enable the Stereo Couple button on the hardware device, channel 2 solely uses the circuit and front panel settings of channel 1, and the audio fluttering problem disappears. If the button is disabled and consequently Channel 2 runs as independent mono, that's when the problem comes in.

Two compressors in a linked stereo configuration will synchronise their actions regardless of which channel causes that action. So if the bass level on channel 1 triggers the threshold then the resulting compression will be applied to both channels simultaneously, using the threshold/type/ratio etc. values of whichever compressor is selected as the master control.

The audio samples were recorded separately. I played a loop in my DAW, sending out audio to the compressor and back into Input 1 of my interface. Then, just simply swapping the jacks from one channel to the other, in order to control for factors.

This is the dbx 166XL. Is what you said a more accurate description of how it works? Maybe this lends a clue to the faulty piece or part? When I peek inside the circuit, it's like two conjoined twins, each channel having a mirror equivalent on the other side i.e. the other channel. (I looked as hard as I can trying to find something faulty, but the circuit and soldering all looked perfect. IC's were smaller than I'm used to seeing elsewhere though.)

[Drew Stephenson]

Notably, If I enable the Stereo Couple button on the hardware device, channel 2 solely uses the circuit and front panel settings of channel 1, and the audio fluttering problem disappears. If the button is disabled and consequently Channel 2 runs as independent mono, that's when the problem comes in (i.e., as heard in the examples above).

In my ignorance, this would lead me to look at the actual controls for channel 2. Have you given everything a thorough tweak, twirl, slide, switch etc.?