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AV30R 700:7r - Hissing Surround Channels

Discussion in 'TAG McLaren Audio Owners' Forum' started by Mike-D, Mar 20, 2005.

  1. Mike-D

    Mike-D
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    For some time now I've been using an AV30R very successfully as a stereo preamp. The on board DA conversion improved my CD player and the 5.1 output from DVD invariably sounded much better than the stereo even though it was mixed back down to two channels.

    The recent clearout via HiFiBitz seemed too good an opportunity to miss and I acquired a 700:7r with a view to finally moving to surround sound. Impatient to get going, but reluctant to lash out lots of cash on surround speakers I've set up four very cheap rears and temporary cabling to allow me to experiment wth positioning and the relative merits of 7 and 5 channels.

    Finally I get to my problem/question. The surround channels have a constant hiss whenever they are active. This is independent of the volume but is loud enough to ruin the experience with anything except high volume movies. I would like to think that this could be due to the rubbish unscreened cables I'm using but I'm not convinced. I don't want to start shelling out lots of cash if it's an irresolvable problem. Based on what I have heard so far, music listening could continue in stereo only and I might be better of bridging or bi amping my front channels and adding just two rears for movies only.

    I know that the SN ratio of the surround channels on the AV30R is compromised compared with it's bigger brothers but was expecting more than this. Can anyone confirm whether this should be expected with this set-up, or perhaps suggest what the problem might be?
     
  2. Kenny Glasgow

    Kenny Glasgow
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    Mike

    If the front L and R channels are hiss free then it would suggest that it is a cable issue.

    Have you tried swapping the surround inerconnects and speaker cables for the L and R cables? That will help you to decide if it is the cables and not the speakers or the amp.
     
  3. Polecatpete

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    Hi Mike, I'm sorry to be the bearer of bad news but the problem will not go away. The hiss on the surround and center is due I believe (correct me if I'm wrong!) to the dac's in the AV30 controlling these channels being of lesser quality than the ones controlling the left and right. You could possibly reduce the noise by adding less sensitive speakers but you will not eliminate it unless you switch to an AV32 or above.
     
  4. Mike-D

    Mike-D
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    Well guys I done some more experimenting. I substituted my trusty Quad 405 and original cabling, for the surround channels. The hiss is still there. I could be missing something but I think this proves the AV30R is to blame.

    I would welcome any second opinions and theories but it looks to me as despite my recent investment, I still have a stereo system.

    I know that nobody here has had a good word to say for the AV30 but it still amazes me that TAG could have produced something this flawed. It was the budget model (in TAG terms) but if I can't put up with the noise, I am surprised that anyone else could either.
     
  5. Kenny Glasgow

    Kenny Glasgow
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    Mike

    Must say that I'm surprised too :eek: If you have not been put off by TAG then you could probably get £400-£500 for your AV30 and buy a new AV32R from HiFi Bitz while they still have them.

    You will NOT be disappointed by the AV32's stereo and surround performance :smashin:
     
  6. roversd1

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    TheAV30's stereo DAC is exactly the same as the original 96 kHz AV32R.


    The AV30 DACs that control the surround modes are different and have a lower (or is it higher?) signal to noise ratio, hence some hiss was noticed.

    There used to be a resistor mod offered by TMA but I dont know if IAG can offer this anymore.

    The 7x700 has a very low (or high) signal to noise which makes for a quiet amp.
     
  7. Stevesky

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    There is a cheeky way to resolve this. Buy a set of inline attenuators for the hissing channels and plug them into the input side of your amp. You can get a set from here:
    http://www.rothwellaudioproducts.co.uk/html/attenuators.html

    Next recalibrate your surround channels - they will be about 10dB too quiet but just push the levels up in the setup screen.

    You now get a reduction of hiss of about 10dB, this should be enough to make it inaudible. Only downside is that your AV30R will run out of headroom on the surround channels at volume levels around 0dB. This means that the fronts will get louder but the surround won't when it runs out of headroom.

    Do I get a medal? :D
     
  8. Polecatpete

    Polecatpete
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    You certainly do Steve! If only I had spoken to you when I first bought the AV30! Still, at least I have a DP now... Hopefully should be getting a replacement that works soon too! :D
     
  9. GrahamMG

    GrahamMG
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    A working DP is a whole heap better than an AV30.....IMHO

    Steve, John and I discussed headroom over a Chinese the other night and I hadn't realised until then that you could run out of headroom on a TAG processor before banging the end stops of the volume knob (figuratively speaking). I wonder why it just didn't have software limited to +5dB? (on the 32/192R)??

    Having "played" over the weekend I am rethinking my theory that the TAG amp clips before the Bryston, on the 192 it does but it appears the other way round on the DP...... Can the processors be subtly different at max volume?

    To summarise, the 250x3R or AV clips at +5dB and the 6BSST or AV clips at +7dB using the AV32RDP but with the AV192R the figures are the other way round..... I haven't checked if anything else might explain this but they are calibrated accurately and TMREQ was off....... Funny thing is with TMREQ set up and on I get +10dB out of the Bryston but only +5dB out of the 250x3R on either 192 or 32RDP....... I'm going to lie down now, its been a headache of a day........

    Come on Steve explain that one to me........
     
  10. Stevesky

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    A 250x3R has a gain of 28.8dB, so 1.64volts RMS in will give 253watts into 8 ohms out. An AV32R/192R (with all speaker calibration trims etc. set to 0dB) gives out 1.5 volts RMS at volume 0dB, so the volume control needs to be set to +1dB to start overloading the amplifier if a 0dBFS (digital full scale) signal is fed through the AV.

    The AV will start clipping internally at around volume +7.5dB (3.5volts RMS) which is more than enough to drive most amps into overload. If an amp had a gain of 28.8dB then you would be delivering 1155 watts into an 8 ohm load by then... bit more than a Bryston or TAG can deliver into 8 ohms!

    I suspect when you did your tests the speaker calibration trims were not the same on each unit. When ever doing any form of tests like this EEPROM reset the units to ensure everything is at unity gain. Use a digital source like a test CD or the output from an audio analyser for your reference signals, to avoid any analog domain inconsistencies.

    Why does the AV volume go up so much? Some recordings are quiet and require more gain to get the desired listening level. As an example lets pretend that the loudest passage on a quietly recorded CD is -6dB. The AV will only start to clip internally if the volume is set to +13.5dB.

    Enough of fun with decibels for one day! :)
     
  11. Mike-D

    Mike-D
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    Well if it works Steve you certainly deserve one.

    It's a fairly small room so I shouldn't think running out of steam at max volume will be a problem. I'll send off for some of the little gizmos right away and will report back on how I get on.

    It does make me wonder though if its that simple, why TAG didn't build in attenuation in the first place. It seems that it would be a better compromise for most users. If they used to offer a modification it must have been a common complaint.

    Thanks to all for the help.
     
  12. tedy

    tedy
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    Since you brought this up , is it possible to know ( or calculate) how many volts RMS the AV32R DP outputs at different levels? Lets say at -10db or at +3db.
     
  13. Stevesky

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    In a very simplified (and mildly inaccurate) form use the following:

    voltsRMSOut = (100^(vol setting/40))*1.5

    ^ = raise to the power by.

    So for +3dB vol setting would be (100 ^ (3/40)) * 1.5 = 2.11 volts RMS
     
  14. GrahamMG

    GrahamMG
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    Hi Steve...
    I think I'm learning something here, I know the gain of the Bryston is different to the TAG (I thought it less but.....) but into a 4Ohm set of speakers the Bryston goes louder as SPL's are higher (same test track, both processors calibrated to 75dBC). Thing is that TMREQ seems to have an effect on max volume with the TAG running out of puff earlier with it on and the Bryston not.... Is the protection circuit on the TAG set to ensure no damage and therefore ultimately limited internally (for very good reasons no doubt) and the Bryston just a little less conservative. All this bearing in mind that all this is very very loud........!!!

    The DP and the 192 are definitely different though.....Maybe just the VFD calibration?
     
  15. Stevesky

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    The output of the AV processor is not affected by the load attached to the amplifier. How "loud" something will go before it goes into overload is down to the amplifier module. It also depends what you consider as overload, if it's when the amplifier shuts down then in the case of the TAG it's distortion would of been blipping into about 10% for a while... not too good for the speakers. I suspect Bryston have decided to allow the amp to distort until it just goes out of the safe working area of the output drivers. Pro speakers are typically more hardy than hifi ones so what's a bit of distortion... expecially at radio 1! :D At very high SPL's it can be hard to hear a few percents worth of distortion as typically the speaker itself can be introducing that on lower frequencies.

    On a 4 ohm load it will make the 250x3R trip out earlier as in reallity it will be delivering (approx) twice the power. If it's a reactive speaker load it's quite likely it dips well below 4 ohms so the actual power the amp is delivering maybe quite alot...

    The 32DP and 192R code wise and hardware are identical, give or take the odd extra capacitor into the PSU - there has to be some form of calibration difference that is causing your variations. However if doing such a test I would always eeprom reset the AV so everything is back to 0. Use a resistive dummy load on the amplifiers and an audio analyser to do a 1KHz sine wave amplitude sweep for the input. Get the analyser to plot distortion vs power for the amplifier output, first one to reach 1% THD is a big girly! :D
     
  16. GrahamMG

    GrahamMG
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    Hi Steve.
    Yep understand all that but of course no-one listens to a sine wave for musical entertainment now do we....... In real world I'm just trying to get my head around why thye are sooooo different. having had a cup of tea, I wonder what the power rails on the 250x3R can actually deliver as I was also thinking about the OB1's and the lower bass distortion thing we were discussing, having out loud I think very low impedence loads might be involved here where the Bryston behaves better?
     
  17. Stevesky

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    You were driving both amps into distortion, even if it's only for short period times or results in compression on bass notes. All it proves is that the Bryston is willing to churn out distortion for longer!

    The key thing on an amplifier is keeping the output drivers in the various safe working zones and making sure the thing doesn't get too hot. On the TAG we monitor overload, DC offset and temperature and if any of them hit the various thresholds we pull the power on the module. The overload is the trickiest situation to monitor as a few short bursts of overload can be ignored, but repeated heavy overload is bad news for output drivers and speakers. There was an engineering change that made the 250w module drive into 4 ohms and below better as the hardware layer fault protection was a bit trigger happy on early modules (in the 250 there is software protection and a 'all hell is breaking loose' hardware protection).

    Really it comes down to can an amplifier deliver its quoted power *undistorted* into the load. Once you're into the blurry area of when an amp is being pushed into the overload area then it's up to the manufacturers strategy of when they pull the plug! To measure an amps true capabilities use an audio analyser on a resistive load, not a set of speakers, your ears and the volume knob... there are too many factors in the equation to come to any valid conclusions. If you want to be extra paranoid regarding distortion vs power measurements use a multi-tone (AP system 2) or a Belcher distortion test, it all comes down to various sine waves going through the amp anyway... just like music!
     
  18. GrahamMG

    GrahamMG
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    yep.....understand all that as well, music still is far more varied than a test sine wave...... :D

    So what can the 250x3R rails deliver into vanishingly small impedences such as an ATL speaker at low frequences? I wonder if you actually got to zero and the amp said "what the :confused: was that" and a big beefy Bryston said "bring it on" etc.???
    Of course those pug ugly things you have got now won't suffer the same fate but I am curious to try and understand why......
     
  19. Stevesky

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    Apart from testing a wider spectrum at once in a more psuedo random way, there is no difference compared to discrete sine waves. After all music is just made out of (lots of) sine waves. The Belcher test was based on this theory.

    The power that the PSU can deliver in a 250x3R will not colapse under a tricky load, assuming that load isn't a screw driver across the output terminals! A 250x3R can deliver a short burst of just over 1kW into a 2 ohm load, but as impedence drops it does become trickier for the amp to drive. That's where some of these monster amps with huge arrays of output drivers and big mains transformer can deliver the goods better into these wacky loads.

    As I said under normal working operations (low distortion, loads that are not silly impedence etc.) both amps should be pretty close. Once you slowly tread into the dark world of overload then all bets are off as it will depend on the manufacturers protection scheme. If you need tons of power then buy a bigger amp!
     
  20. Bolle

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    I have no idea what you guys are discussing but it is fascinating to follow!
     
  21. GrahamMG

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    Hi Steve.
    Yep that still makes sense to me! :thumbsup:
    Having brushed up on ATL theory without recourse to the holy bible yet (Pete), I can see where the mechanical impedence might get to 0 without too much difficulty, of course your never get that with a SUB/normal full range speaker..... I suppose that is why Bryston amps are built like they are........ We know the amps are pretty close after that round of testing but of course we never tested them with ATL......
     
  22. kraal

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    Hi Mike,
    Any thing to say about your tests? Because I have the same problem here.
     
  23. Mike-D

    Mike-D
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    Nothing to report yet. I'm still waiting for the attenuators to arrive.

    Mike
     
  24. Mike-D

    Mike-D
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    I'm pleased to report that adding Rothwell attenuators between processor and power amp has had the desired effect.

    The hiss from the surround channels is now so low that you can't hear it unless you put your ear right against the speaker. The volume is still as loud as I need and as far as I can tell there is no reduction in sound quality.

    Note for Krall, At the moment I still have some very cheap speakers for the surrounds and no centre. I can't therefore give a definitive opinion that there is no impact on sound quality. All I can say is that I'm convinced enough to go out an buy some proper speakers. Even with the current setup the sound is now quite impressive. If you have the same problem, I would not hesitate in giving the Rothwells a go.
     
  25. Kenny Glasgow

    Kenny Glasgow
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    Mike

    Great result :thumbsup:
     
  26. kraal

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    Good.
    I will try to find those tweeks here or something "equal".
     

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