Low Hiss in Effects Sends/Returns on DP24SD

[Cosmic]

Hi All,

I am trying to figure out if I am over-reacting or what?

The 2 "effects outs" feed a stereo reverb unit (balanced +4 in/out) which comes back through channels 7/8 on the recorder. So far, so normal...

What is driving me a bit batty is the low but noticeable hiss in the effects chain when 7/8 are brought up to unity gain.

I unplug "effects-in" cables on channels 7/8, no noise on the channels, so the channels are clean.
I turn down output from reverb unit, no difference, still low hiss at unity.
I re-cable reverb directly into the mic chain (ext mic pre/comp/reverb) and the chain is quiet on that channel.
I set the effect 1/2 feeds and the masters on my desired channels first to 127 each, then to 100. Same thing.
I do a factory re-set on the DP24SD to see if I have forgotten any feeds in an open position that might contaminate the effects chain...no difference.

The one set of controls that kill the hiss is the "digital gain" controls on channels 7/8. When I dial them down, the hiss goes down, but now I lose the level as well.

So, gain staging in some form appears to be off.
I am usually very comfortable with pro vs. consumer gain levels and what will match and what won't, except here it's as if there is something terribly obvious I am overlooking.

The reverb box gets +4 to keep it happy. Since the effects-outs on the DP24SD are TS, consumer-level line, I use a Radial J+4 Line Booster to re-balance the signal to XlR +4 going to the reverb. (The Radial has been tested as well, in and out of the line, and it, too, is clean)
The reverb signal comes back to channels 7/8 via XLR-to-TRS +4 line in, with the internal pre-amps on the '24 turned down all the way.
All should be happy, right?
The reverb signal is fine otherwise. At unity gain it rolls in on the meters in the sweet spot, equal to the outbound level of the track that is being fed to the effect. And yet, there is still the low hiss.

Thanks for reading, any suggestions?

C.

 

[Phil Tipping]

The reverb box gets +4 to keep it happy

I know you said the booster sounds clean on its own, but if your reverb unit accepts consumer/unbalanced levels, you could try running the TS sends directly into the reverb and see what happens.
Update: Just a couple more points now I've woken up
The send controls are attenuators so these usually need turning up as high as you can. No point attenuating and then having to amplify later on; every stage of (analogue) amplification will contribute some hiss.
You could try eliminating all the external units by running the sends directly into the inputs of the DP. Obviously, watch out for feedback, e.g. use PRE sends and turn track faders down.
Finally, thank you for providing a really clear description of the problem, and what you've tried so far. Much easier than "xyz doesn't work"

 

[-mjk-]

Are you monitoring on headphones?

 

[Cosmic]

Thanks for the replies.

The reverb is a pro unit that only has XLR +4
Originally I did run TS-to-XLR cables from the 24 sends to the reverb, and of course this dropped the level going in to the reverb by the expected amount, when sending a -10 signal to a +4 input.

While I then had to turn up the reverb output, and add a bit more on the "incoming" 7/8 faders on the 24, it didn't alter the ratio of signal to noise (or hiss) by any audible amount.
It just gave me less maneouvering room by having less overall return volume to work with before 'pinning the pots', thus the Radial box to even things out for mixing and provide normal headroom.

The sends are back to 127 from now on, good to get confirmation on that.

Phil, I like the idea of running the sends straight into the return channels, to check the integrity of the feed. I will try this when I am back home again, being careful not to loop the signal.

For monitoring, I have examined this with both my AKG cans and my Adam monitor speakers, and they reveal about the same small amount of return hiss, relative to signal volume, listening distance and such.

What is maddening is that it is low enough that it falls exactly in that little grey area where you say: 'Who will hear it?" when you listen to a track or two, but then you start thinking of the cumulative contamination when adding more tracks...to me, it's the audio equivalent of noise in the black levels when you do digital photography: it's usually only when it is gone that you realize the effect it had to begin with.

I would hate to think the output stage of the reverb has tube- or output stage issues, since it is almost brand new, but you never know. If I do the loop with the send/return on the 24 and it is completely clean, there are few other things I can think of.

I hope my description will act as a bit of help for anyone who may just be getting into using the effects sends, particularly as it pertains to using +4 effects units.

Really appreciate the input and ideas so far.

C.

 

[Mark Richards]

If this helps any, about 5 years ago someone had complained about minor low level hiss on the headphone circuit. So I made some tests on my DP-24.

As I recall,

The result confirmed that on my unit all the analog line circuit throughputs performed to the S/N specifications stated in the OM: -90 dB Stereo Out, Sends 1 & 2, and Monitor out; but -82 dB on the Headphone circuit.

Test Gear was set to output 0 dB = .775v; Input Knobs full CCW (line). Each circuit measured first with Faders and Knobs full off: then with Faders and Knobs full on. Sine wave test tone was either 400Hz or 1kHz (don't recall now, but probably 1kHz).​

 

[Cosmic]

Thanks, Mark, yes the headphone 'out' on the 24 is not exactly world-class.

I have little faith in most headphone outputs on such gear, as they tend to be underpowered and noisy. I have a Samson multi-output headphone distro amp that I use, and even with that relatively humble gear, there is quite a difference.

C.

 

[Cosmic]

Haven't abandoned the quest, just haven't been back to the setup to try further tests yet.
Stay tuned...
C

 

[Cosmic]

OK, a resolution of sorts:

By using the Radial J+4 variable booster box from the Tascam's Sends into the reverb, I have hit a balance that works. It seems the reverb unit just wanted "more gas" on its input. What comes back through the Tascam return channels is now agreeably quiet unless I crank everything to silly levels.

Given the hybrid nature of the Tegeler Reverb (it's +4 in, transformer, then tube, digital reverb chip, then tube and trans out again at +4) it is bound to add a touch of 'something', but noisy it isn't.

What threw me on this is that I was never knowingly at the extremes of low/high on anything where the idea of "added noise" would come naturally, with obviously poor gain settings. The reverb just seemed to want a louder input to work with within the +4 range. The Radial did this very well.
(BTW I highly recommend that Radial J+4 box if you need an interface to bring consumer units up to +4 for any purpose. It has 2-channel TS-, RCA- and Mini-in, going out to (2) XLR +4, with a variable gain pot to boost things. Not bottom-end cheap, but well worth the money.)

That's the best explanation I have for my hiss issue.
Thanks for all the help and interest, hope to have added a little something here that someone might be able to reference.

C.