DBX Question

[deckard]

I have an old PORTA 05 4-track that I haven't used in years. I just broke it out the other day and have a question about the DBX circuit. Something seems a little odd to me, so if someone can tell me if this is correct or incorrect behavior, it would be most appreciated!!!

Tape playback is fine with DBX on (sounds like it should). However, with no tape playing, no sources connected and all faders (and effect returns) up, I get a TON of hiss/white noise when I switch DBX off. It doesn't matter what I have selected to monitor (CUE, EFFECT, REMIX, etc.). It’s as if I am playing back a tape that was DBX encoded with DBX off. After switching it back on things are quiet again. It's as if the DBX circuit is being applied to everything, not just tape playback.

Is DBX not purely a method of tape noise reduction? I know it works over the full frequency range and not just the highs like Dolby, but should I hearing any difference when not monitoring tape playback? I honestly don't remember if this is how it’s supposed to be or not.

If it is busted, would it be worth it to get it fixed or should I just buy a new one?

Please help! Thanks!

 

[dementedchord]

for gross simplification... DBX is a multiband compressor used for encoding and decoding tape... the filter exisits whether tape is playing or not.... all it's really telling you is there's a fair amount of noise being produced somewhere in front of that filter...

 

[deckard]

OK, thanks. That makes sense. I guess I just need to confirm whether or not the noise I hear when all levels are up and DBX is off is normal... Anyone else out there have a PORTA 05 or similar who could tell me if this is normal for this deck or if something is wrong?

 

[Tim Gillett]

you might get better answers in the analog only forum.

Does seem strange.

Normally I'd expect only to hear a reduction of tape preamp hiss plus tape hiss when a tape was inserted and play pressed. I tried a model 488, the only one I have with me at present and it did as I expected.

The dbx should only affect the record drive amp (compression for encoding) and the tape play back preamp ( expansion for decoding). Are you absolutely sure the effect happens with anything other than tape playback?

Tim

 

[deckard]

Here's exactly what I am doing:

No sources are plugged into any inputs.

All effect returns and input faders are all the way up.

Headphones plugged in and all the way up (or using line out to mixer/amp, doesn't matter).

No tape playing (doesn't matter where the monitor switch is anyway).

With DBX on, all is quiet.

With DBX off, I hear what almost sounds like pink/white noise.

Again, doesn't matter whether the monitor switch is on CUE/EFFECT/REMIX.

 

[dementedchord]

Normally I'd expect only to hear a reduction of tape preamp hiss plus tape hiss when a tape was inserted and play pressed. I tried a model 488, the only one I have with me at present and it did as I expected. Tim

that's true to the degree that the circuit is quiet to begin with.... if the noise is being generated between the repro head and the compander you would hear a difference....

 

[Tim Gillett]

that's true to the degree that the circuit is quiet to begin with.... if the noise is being generated between the repro head and the compander you would hear a difference....

Yes but it should only be heard in play mode. In all other modes the repro circuit should be automatically muted. Maybe the mute function is faulty (board fault).

Tim

 

[deckard]

See, that's how I thought it should work as well. For instance, on a normal 2-track stereo cassette deck, you don't hear a difference in the source input when you switch Dolby on & off. You only hear the difference during playback, or if it's a 3-head deck and your monitoring the playback head during recording.

Could it be that the noise is normal, but just exasperated by having everything all the way up? I am kicking myself because I don't remember if this is how it always worked, or maybe I just never turned DBX off before!!! lol

Anyway, I was just hoping someone had the same or another PORTA unit from that vintage and could verify this for me. Thanks for everyone's input! If no one is able to check this, do you think if I call TASCAM I could get someone on the phone who would know?

 

[The Ghost of FM]

do you think if I call TASCAM I could get someone on the phone who would know?

20 years ago, yes.

Today...I highly doubt it.

It sounds like a monitoring relay or transistor switching circuit inside your unit is stuck in a sync position.

That amount of hiss is perfectly normal with all the faders up as it's a Portastudio after all, not a Neve console!

To not hear that noise with no tape running and at full out levels is wrong.

The bigger question here is this;

Can you still use the unit to produce your music with the expected normal sound quality it produced in the past? If so, I wouldn't lose any sleep over this issue.

Cheers!