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bos3812 4th August 2012 03:02 PM

Hissing sound after switching off NAKSA70
Recently I bought a new minidsp 4 x 10 HD digital filter. Before that I had the 2 x 4 digital filter from Minidsp. I want to build a active 3-way speaker so I needed a new filter. After connecting everything it worked like it should be. But the strange thing came after switching off the Naksa's. I heard a loud hissing noise through my speakers. The noise fades away in 7 seconds. It seemes that there is a signal on the input of the Naksa's which fades away when the elco's discharge. While my 2 x 4 Minidsp had no problems before I wrote Minidsp a email about this issue. They said they have never heard of that with any costumer. They asked me if the NAKSA's has a AC coupling input capacitor. I am a newbee on electronics so I can't give Minidsp an answer on that. Normaly I would have asked Hugh but I know he is recovering from being in the hospital, so who can give a answer on my question and maybe someone has a suggestion on the issue.

PaulBysouth 5th August 2012 04:07 AM

The Naksa70 has an input capacitor (of 1.5uF). Try disconnecting the amplifier input, power on, wait 30 seconds, and then power off. Does it still make the hissing sound? This is to isolate the problem to either the amplifier or the input (ie the DSPs).

bos3812 6th August 2012 04:15 PM

Thank you Paulbysouth. I am sure it's not the amplifier because when I connect my old Minidsp 2x4 it's not hissing when switching off the amps. The problem is related to the Minidsp 4x10. Minidsp doesn't have a answer on that immediately. They asked what amps I use and if the amps has AC coupled input capacitors. Now I can give them a answer. I hope we can solve this issue.

sureshm 12th August 2012 09:23 AM

I have had the exact same problem for the last year or so. My source is a modified squeeze box v3. I could never get to the bottom of it. I even sent Hugh a recording of the hiss and he was surprised by it. He sent me some suggestions to identify the cause but by then I gave up on it assuming it is not damaging my speakers. My best guess now is it is to do with the timing of the discharging of the output cap on the source and input cap on the amp (hence will vary by source) but I am no engineer.

Ian Finch 22nd August 2012 09:04 AM

AKSA and NAKSA designs have no output relay, muting switch or other switch in the signal path to influence the signal or give cause for concern about sound degradation. However, virtually all modern commercial amplifiers will have some means of output disconnection at switch-off so you would not hear the sound which otherwise would likely be present there too, along with a loud thump and other noises that are commonly made as these amplifiers are de-powered and the DC supply voltages decay unevenly.

Logically, the Minidsp 4x10 lacks a muting feature or presents a very different impedance to the earlier model but when used with a typical commercial amplifier fitted with output relays, any difference would not be noticed. Though I don't have a complete NAKSA design to compare, most of my amps also lack loudspeaker relays for the same good reason. Some DACs and even preamps seem to have had a similar "dying breath" effect at switch-off with those too.

The solution unfortunately, is to either enable internal muting of the DSP by turning it off first, add a load sensing power board that permits switching both units off simultaneously or modify the DSP to mute its output with no signal present or other cue from the amplifier switch-off. There, the influence of switch distortion can be considerably less. The manufacturer, of course, will not like this approach or the suggestion there is any flaw in their DSP design. :2c:

rabbitz 23rd August 2012 05:43 AM

Isn't it a matter of turning off the power amp > pre amp > source in that order and opposite at start up. That usually solves any issues from the pre or source but of course does not solve the thump or fart in the power amp. All my AKSAs had a slight turn off thump but not a problem.

All my current commercial amps have relays on the speaker output so would never be able to hear any turn off issues.

Ian Finch 23rd August 2012 02:06 PM


Originally Posted by rabbitz (Post 3136654)
Isn't it a matter of turning off the power amp > pre amp > source in that order and opposite at start up....

Hi rabbitz, ordinarily, I'd go along with that sequence as each device usually isolates itself immediately at shutdown. However, when the amp shutdown takes a few seconds, it will remain active and often with sharply increasing gain as its power supply dwindles. Meantime, if power to the DSP, DAC etc. is cut and its output dies sooner, you may well hear some noise. If a standard preamp with isolating relays was in that chain, I think it should then become the first device in the chain to shut down, at least eliminating all the input sources.

I'm sure a lot of audiophiles also fit devices like very basic kit preamps, passive volume controls, DACs, CD players etc. directly to the amplifier without muting or isolation of any kind at power up/down so I guess that we have to be aware in pursuit of the cleanest audio chain, that there may be issues like this one.

It doesn't worry me what happens after I shut down either. NAKSAs are fine in that respect but virtually all solid state amps once made quite severe on/off noise and I can understand bos3182 being a bit concerned when his new Minidsp seems to have an issue that wasn't there before. Perhaps he'd like to comment on the ideas so far.

davewantsmoore 24th March 2013 12:30 AM

Hi. Hopefully OK to bump and old thread... I have encountered this same issue.

Minidsp 10x10
ME-550-II (Australian poweramp with no output relay/mute)

5 seconds or so of soft static when the amp is turned off.
Turning the DSP off first is not possible due to turn off thump from minidsp.

My question. Is this ok? I will not harm the amplifier? or speakers?

The amplifier has both AC and DC coupled inputs available. I am using the AC coupled input (I assume this is 'correct').

Thanks anyone still lurking around! :thumbsup:

PS - As noted... do not have this issue with MiniDSP 2x4

EDIT: Oh, just realised which forum this is in... hope Mr Dean doesn't mind.

AKSA 24th March 2013 04:38 AM

Hmm, it might damage the amp (and possibly the tweeter!) if the buzz has high frequency artefacts and IF the amp passes them through.

That said, most amps, including the NAKSAs, have a LP filter in the input stage which reduces such artefacts to very low levels. I would expect your ME would have such a feature. However, I would expect the best approach would be to turn off the amp, THEN turn off the source. That means the buzz near gets to the amp and the speaker.

Dave, time to move to a NAKSA!!



davewantsmoore 24th March 2013 05:21 AM

Ah, thanks for the reply. I will definitely try an amp of yours one day soon. I have plenty of need for more amps ('active' XOs)

Sorry, if I wasn't clear.... (re: your 'best approach') I am turning the amp off first... and when I turn the amp off, I get about 5 seconds of (very light) hiss through the speakers. It certainly isn't at an SPL that is worrying (barely audible from 5 metres away).

When I noticed this thread, I thought it was a good opportunity to check my assumption that it wasn't a problem.


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