Hiss

[Biskit]

Slightly ambiguous title but here's the issue: With some mixer-PA combinations I find I'm getting an unacceptable level of 'hiss' noise at the speakers. The worst combination is a small Behringer mixer (apologies I don't have the model to hand but I suspect it's immaterial), and a RAMSA PA consisting of tops and subs, via the correct RAMSA SP2A sub processor, and a Thomann TSA 4-700 amp (two channels each for sub and tops). When the amp inputs are turned up to full, there is barely any hiss at all with the mixer master fader at the bottom, but as soon as this is pushed up to 0dB, the hiss becomes intolerable. The only solution which seems to work is to turn the amp inputs down to about half way, which of course means having to drive the mixer output harder to achieve the same audio level.

 

I don't believe it's down to any single component in the system: I've discounted the obvious such as cable noise or faults by testing with the mixer connected by just short link leads, and also tried every conceivable combination of ground lift at mixer end and/or processor, and between processor and amp. I've also tried unbalanced connection into the processor, but nothing makes any difference. Connecting the mixer direct to the amp channels driving the tops (ie. bypassing the processor), the hiss is the same, suggesting the mixer is at fault, however I have tested the mixer with another amp (an old Carlsbrough unit, with the same speakers) and it is quiet as a mouse. Signal straight into the amp from a line-level source (phone) is also hiss-free.

 

So - I'm thinking it may be some kind of compatibility issue between the mixer output stage, and the processor/amp input stage in this case. Mixer outputs and amp inputs both claim to be 'balanced'. I think this processor might pass the input signal straight to the top outputs without actually processing it (ie. only actually processing for the sub output, which is mono), which might be why I get the same result with or without it in the chain. The only thing I can think of is to try a line isolating transformer, but thought I'd gauge opinion before buying one to try. Might it help? Where should it be connected, and what grounding arrangement should be used? Any recommendations or ones to avoid, or build my own? Any other ideas/suggestions?

 

Many thanks!

[sandall]

Very strange. Hiss is usually what you get with the gain turned up on an unterminated input. It's hard to see how grounding problems would produce hiss rather than hum / buzz.

 

One possibility is that the Thomann amp has an unusually high input impedance. Have you tried running unbalanced in & out? If that makes no difference & you are handy with a soldering iron you could try a 10k resistor across an input lead. Failing that, if you have the adaptor leads to hand you could use 2 passive DI boxes back-to-back to make a line transformer (it would presumably work with active ones, but I've never tried).

 

I am assuming you have set up gains in the right order, so that 0dB on PFL corresponds to 0dB on mixer out & 0dB on amplifier inputs. I would be surprised if you could achieve this with both mixer output & amp/processor input set at full without destroying your speakers.

[Shez]

Hiss is often the result of a gain structure issue.

 

When the amp inputs are turned up to full, there is barely any hiss at all with the mixer master fader at the bottom, but as soon as this is pushed up to 0dB, the hiss becomes intolerable. The only solution which seems to work is to turn the amp inputs down to about half way, which of course means having to drive the mixer output harder to achieve the same audio level.

One important thing to note here (apologies if you already know) is that the controls on the inputs of the amp are attenuators and have no effect on the maximum volume that the amp can output - they're there simply to deal with different source levels. As a general rule of thumb, set your mixer up to get a good healthy output level and then turn up those controls on the amp to achieve the required volume in the room.

 

Some mixers have quite noisy mix buses; arbitrarily turning the amp controls all the way up will show that up very clearly. Run the mixer with the levels at which it's designed to work best and it'll sound much better. Winding the amp's controls down is not in any way a bad thing to do.

[Brian]

As sandall says, it sounds like your gain structure is wrong. You'd normally run a mixer so that, at 'normal' voulme, your faders all sit around the 0dB mark with any final level adjustment taking place on the amp attenuator.

[Biskit]

Very strange. Hiss is usually what you get with the gain turned up on an unterminated input. It's hard to see how grounding problems would produce hiss rather than hum / buzz.

 

One possibility is that the Thomann amp has an unusually high input impedance. Have you tried running unbalanced in & out? If that makes no difference & you are handy with a soldering iron you could try a 10k resistor across an input lead. Failing that, if you have the adaptor leads to hand you could use 2 passive DI boxes back-to-back to make a line transformer (it would presumably work with active ones, but I've never tried).

 

I am assuming you have set up gains in the right order, so that 0dB on PFL corresponds to 0dB on mixer out & 0dB on amplifier inputs. I would be surprised if you could achieve this with both mixer output & amp/processor input set at full without destroying your speakers.

What you suggests is consistent with my thoughts - I couldn't see how it could be a grounding issue as there is no hum/buzz, it's definitely 'hiss'. I realised after posting that the SP2A outputs are actually unbalanced (the inputs are definitely balanced though), so actually ordinarily the amp is getting an unbalanced input albeit with leads less than half a metre long from the SP2A. I did try balanced and unbalanced direct into the amp (bypassing the SP2A) and the hiss is there in both cases.

 

Gain-structure wise, the hiss level seems to depend only on the setting of the mixer master fader, it doesn't make any difference where individual channel faders or gains are set, or indeed whether or not there are even any inputs connected. I just want to be able to run, as normal, with mixer output fader at 0dB and amp inputs at anything up to full (0dB) in order to maximise the available audio level without having to overdrive the mixer outputs. There will be occasions where such a high output won't be required, and I happily turn the amp inputs down to allow for a sensible gain structure on the mixer on those occasions, but at present I'm having to do this as a matter of course just to keep the noise acceptable, which doesn't seem right. I can get the same level of amp gain without the noise using a different amp, which is why I wondered about compatibility or perhaps impedance issues.

 

I'll give the DI box option a try when I get chance later today.

 

E2A - I was aware of the amp inputs being only attenuators, and I do generally set up as you suggest using the amp input controls to set the final level when the mixer gain structure is optimised, but I just feel I'm getting more noise than I should in this scenario, for any given output level.

[MarkPAman]

I realised after posting that the SP2A outputs are actually unbalanced (the inputs are definitely balanced though), so actually ordinarily the amp is getting an unbalanced input albeit with leads less than half a metre long from the SP2A.

 

How is this lead wired? I'm guessing the SP2A is jack outputs?

 

You may get better results (if you're not already doing this) by using a cabe that is wired with tip -> XLR Pin 2 (or tip if TRS) & sleeve -> Pin 3 (or ring on TRS) with Pin 1 (screen on TRS) unconnected at the amp end. This way the amp "thinks" it has a balanced input connected.

[sandall]

Not wishing to appear Behriphobic, but have you tried a different brand mixer with this set-up? One possibility might be a cooked output IC, with the level of hiss related to the amount of audio current drawn. The little Behri mixers have totally inadequate heat dissipation for their regulator chips (I've had to replace a few), so that bit of the PCB can get quite warm.

[timsabre]

Not wishing to appear Behriphobic, but have you tried a different brand mixer with this set-up?

 

Not even a different brand mixer, just a different mixer would be worth trying. Behringer is often slated for having lots of hiss but it just seems to afflict certain units, so it would be definitely worth trying something else if you can beg or borrow for testing.

[Biskit]

Thanks for the suggestions, I will try again with a different mixer when I get chance.

 

The SP2A outputs are indeed 1/4 TS Jack, and the leads used are wired tip-pin2 and sleeve-pins1&3; I have also tried tip-pin2 and sleeve-pin3 with pin1 left unconnected but this seems to make no difference at all.

[sandall]

The SP2A outputs are indeed 1/4 TS Jack, and the leads used are wired tip-pin2 and sleeve-pins1&3; I have also tried tip-pin2 and sleeve-pin3 with pin1 left unconnected but this seems to make no difference at all.

In that case it's very unlikely to be a grounding problem, as pin 1 disconnected gives you a floating quasi-balanced output. Tim is right that not all Behringer mixers are hissy, but some certainly are.

[mackerr]

Hiss is often the result of a gain structure issue.

 

When the amp inputs are turned up to full, there is barely any hiss at all with the mixer master fader at the bottom, but as soon as this is pushed up to 0dB, the hiss becomes intolerable. The only solution which seems to work is to turn the amp inputs down to about half way, which of course means having to drive the mixer output harder to achieve the same audio level.

One important thing to note here (apologies if you already know) is that the controls on the inputs of the amp are attenuators and have no effect on the maximum volume that the amp can output - they're there simply to deal with different source levels. As a general rule of thumb, set your mixer up to get a good healthy output level and then turn up those controls on the amp to achieve the required volume in the room.

 

Some mixers have quite noisy mix buses; arbitrarily turning the amp controls all the way up will show that up very clearly. Run the mixer with the levels at which it's designed to work best and it'll sound much better. Winding the amp's controls down is not in any way a bad thing to do.

 

What Shez said. Your mixer is not the quietest thing going, and needs good gain structure through the system to keep the noise down

 

Mac

[richardash1981]

One possibility is that the Thomann amp has an unusually high input impedance. Have you tried running unbalanced in & out? If that makes no difference & you are handy with a soldering iron you could try a 10k resistor across an input lead.

Have you tried this suggestion? The noise level in electronics outputs depends on the load impedance they are driving, and in inputs on the source impedance they are fed from. So the difference could be that the Carslbro has lower input impedance (which is reducing the mixer output noise), or that the Thomann amp inputs need to be fed from a low impedance source to give low noise (and the mixer isn't giving that). I know that my little Behringer mixer has bad hiss on the mic inputs at max trim unless there is a low impedance microphone connected to the XLR input - which makes the jack line connectors rather useless at high gain!

 

Are we sure the problem is actually hiss (white noise) and not a high frequency (>10kHz) oscillation? They can be hard to tell apart by ear (especially once you can't hear the 2nd harmonic), so recording a bit of the noise (acoustically) may tell you more, even if it's quite crude. Oscillation in the power amp input stage would be a whole different bunch of possible causes!

[laolu]

Dear all:

 

I thought the same when I started reading this thread: something might be oscillating! Taking all the above expert opinions into account, especially about gain structure and impedance mismatch, there could be a third problem, namely that the combination mixer+processor is generating HF. Since the mixer is quiet (as a mouse ;)) in combination with another amp.

 

I had a similar "hiss" issue with a Beyerdynamic shotgun. Adding another 5meter coil of XLR cable solved the problem. Obviously enough extra capacitance and / or inductance to shut down "radio station".

 

Try it.

 

Norbert

 

 

[mackerr]

there could be a third problem, namely that the combination mixer+processor is generating HF. Since the mixer is quiet (as a mouse ;)) in combination with another amp.

 

What is the voltage gain of each of the amps you are comparing? If one of them is higher gain (not power) it will have more apparent hiss than an amp with equal power, but lower voltage gain.

 

Mac

[Biskit]

Thanks for all the replies guys. I had another play last night, including using another desk (a Soundcraft EFX12) which was noticeably quieter. The Carlsbro amp which I'd used as a comparison may well have a lower voltage gain, I don't have the literature to hand to check, but I take the point this is a possibility. The issue of noise produced being relative to the impedance being driven may well come into play, so I will definitely try the 10k resistor idea at some point just to see what happens. I did a quick phone recording and it is definitely 'white noise' rather than an HF oscillation.

 

In summary, I suspect I was reacting to an absolute 'worst case' scenario which can probably be worked round in practice by more diligent gain structuring and careful selection of equipment.

 

Thanks again for all the input (none of which was noisy ;) )