Can I hook up 2 dacs into 1 stereo amp?

[allsorts]

Got a new 24"TV recently but sound out is not so good. I want to run my pvr (optical )and dvd (coax ) into my 09Zero dac and leave the macmini running music into my Caiman dac. I won't be running the 2 dacs at the same time into the Bantam Gold. Not wanting to change the analogue cables each time i change source, (too much twisting and pulling on the connectors can't be good) I thought that using some RCA double adaptors might do the job? Secondly, Will I lose any SQ using them? Or is there some black box that would do the job better? Cheers and love this forum for all the wise advice so generously proffered.

[Reid Malenfant]

Hi Greg, S/PDIF relies on the line impedance being 75 ohm, any connection that effectively parallels that will to put it bluntly f*ck the impedance up, even though the phono plugs should be pariahs :doh:

You need to work something else out fella. Mismatched impedances at these frequences result in reflections (we are talking RF due to the rising & falling edges of the digital waveform) in cables & a big difference in sound quality.

Don't do it...

[allsorts]

Thanks Mark. I think i should have made myself clearer. I have pvr/dvd attached to one dac and mac/cd attached to another. Each has a set of rca cables out to the amp. Most of the time i have the mac to caiman to amp setup. When playing pvr or dvd i use the Zero dac. I disconnect the caiman rca cables at the line in of the amp and connect the rca cables from the Zero. Reverse the process to go back to music. Only one dac is ever turned on at a time as they have to share the same power point. I thought that the double adaptors would reduce chance of damage from pulling the tight connections out regularly. Greg

[alfie2902]

What about knocking up a 2 input 1 output switch box that will allow you to switch between both Dacs? They can be bought for a few £ but not sure of the quality of cheap ones.

Or buy a preamp perhaps something like this http://www.ebay.co.uk/itm/Passive-preamplifier-ALPS-Potentiometers-passive-preamp-/190584080598?pt=LH_DefaultDomain_0&hash=item2c5fb248d6 might do the job.

[isj1202]

Would it be possible to plug them all into the Caiman and use the switches to choose source.

An optical to coax convertor (http://www.ebay.co.uk/itm/Optical-TOS-Digital-Audio-Coax-SPDIF-Phono-Converter-/200617526172?pt=UK_Computing_CablesConnectors_RL&hash=item2eb5bc839c) would change one of the optical outputs to coax

[allsorts]

Ta for that info. I think i have located the problem. It seems to be the Bantam Gold. When I turn it on with the speaker cables attached, the channel strength according to the PVR which is connected to the aerial drops by 20-30 % and i lose the digital signal to half of the available channels. SD and HD channels are both affected. Nothing is affected by the dac or the bantam gold if i turn them on without the speakers attached. I initally thought it was the Caiman not playing nice with the pvr and attached the Zero Dac which seemed to work a little better. When i attached my Topping T90 amp I managed to pick up one group of channels more than with the Bantam, but still lost about 30% of the available channels. Is it because of the small size of the amps or is something else happening here? :scratch:

[dave2010]

Re the input cable issue, I was going to suggest a cheap audio switch from Maplin - but I can't find the one I've got in their list at present.

Something like this might do - http://www.maplin.co.uk/four-way-stereo-audio-switch-box-9328 or this http://www.cables4all.co.uk/4-port-rca-audio-switch-box---manual-switching-898-p.asp

Mine was I guess intended for linking PCs and games consoles into audio systems - but I hide it under the TV. Quality is adequate for purpose.

If you're desperate for high quality units there are some specialist suppliers, but my guess is that in many cases you'll just pay a lot more, and maybe get a more solid unit, but not much audible difference.

Re running your Bantam Gold - how have you run the power cable? I suspect you have run your signal cable close to some other cables, so that when you turn your amp on it interferes somehow with the feed into the TV equipment. Try using better screened cable for the TV and PVR feeds. This site is quite helpful re cable - http://www.satcure.com/tech/cable.htm

I'd say that if you've got moderately decent cable, then it's maybe not worth changing, but if you've got some real rubbish stuff, then upgrading the cable could make a big difference. I'm assuming you have a decent outside aerial and it's the downfeed that's the problem. If you haven't then you need to check out the TV aerial, and get it up to scratch. Is it for terrestrial or satellite use?

[allsorts]

I think my cables etc are ok. Over here we have about 5 TV stations each of which send out about 2 or 3 SD and 1 HD digital channels, Different programming for each channel by and large. We have 3 of the stations coming from one transmitter and 2 stations transmitting from a slightly location, So aerial direction is a bit of a compromise. My signal strength is showing high 80s % but drops to mid 50s when the dac /amp/ speakers are running.The quality bar doesnt change at all. I have had various integrated SS amps (Lux, Maranantz, Rotel)before that worked ok in the same situation which is why i think it might be something to do the mini size of the bantam gold.

[dave2010]

Seems very odd to me. Didn't spot you are in Australia. The behaviour of the amp shouln't really have an effect further back down the chain, unless it's faulty, or generates lots of interference,

Do you get the same effect if the Bantam is on, but not connected to the video sources? Could it be a radiation effect due to the switching in the amp?

[Ali Tait]

My thought also.

[allsorts]

Haven't noticed anything abnormal when running the amp. I have the tv connected to the macmini (vga)with the dac and amp running and no issues. Its only when i try to use the tv coax out or the pvr optical out. Playback via pvr of recorded shows is alright but i lose anything i might be recording.
The antenna wire is r6 quad, most of my connections are Belden or decent quality shielded wires.

[dave2010]

Haven't noticed anything abnormal when running the amp. I have the tv connected to the macmini (vga)with the dac and amp running and no issues. Its only when i try to use the tv coax out or the pvr optical out. Playback via pvr of recorded shows is alright but i lose anything i might be recording.
The antenna wire is r6 quad, most of my connections are Belden or decent quality shielded wires.Suppose you don't connect the TV kit to the Bantam so there's no a backward path to the video feed, but you feed the Bantam from another independent source to drive it with something. Do you still get the problem? Is it program related in any way - i.e if you just have the amp on is the signal strength OK then?

It does sound more and more like a radiation thing, but one other possibility (unlikely though) is that it's a power supply issue. There are two possibilities here. One is that you've got the switching power supply for the Bantam (or other associated kit) close to your signal cable, and its the PSU which is causing the problem. The other is simply that when you have everything on the load on your power system drops the voltage, so that your TV kit doesn't work as well - but that's very, very unlikely except in areas where the power supplied from the generators is poorly regulated - unless perhaps you've got some dodgy internal mains wiring in which case that could happen.

To check if it's a radiation thing try siting the Bantam well away from your feeder cable to the TV and on a different power socket, if possible, and see if the problem persists.

[allsorts]

The signal from the pvr to the tv isn't affected until i attach the speakers to the speakers. If i turn on the dac and amp without the speakers attached everything is fine.The amp is on a rack 1metre above the floor where the aerial cable is. The psu for the amp is resting on the rack as well.

[dave2010]

The signal from the pvr to the tv isn't affected until i attach the speakers to the speakers. If i turn on the dac and amp without the speakers attached everything is fine.The amp is on a rack 1metre above the floor where the aerial cable is. The psu for the amp is resting on the rack as well.Doesn't that kind of amp have some power transistors at the back end which switch at high frequency, and then go through a simple (maybe) smoothing circuit? When you don't have the speakers connected there will be no current flow, so connecting them in will cause output current to flow, and also there will be a power drain on the PSU.

Why don't you contact the people at Temple Audio and ask them for advice? There does have to be a rational explanation of all this. Perhaps your unit is faulty?

[allsorts]

Can't see anything at back of amp. It does have a boost facility through on button to increase signal from ipod etc. This is off. Coax from cd player into dac works fine as well its just the pvr and tv that cause the problem. Will contact temple audio to see if its a known problem. Will hook up to a AV receiver over weekend if possible and see what happens.
Thanks for all your help Dave.

[Reid Malenfant]

Doesn't that kind of amp have some power transistors at the back end which switch at high frequency, and then go through a simple (maybe) smoothing circuit? When you don't have the speakers connected there will be no current flow, so connecting them in will cause output current to flow, and also there will be a power drain on the PSU.

Why don't you contact the people at Temple Audio and ask them for advice? There does have to be a rational explanation of all this. Perhaps your unit is faulty?
Yes, It's a Class T amp ;) It has a switching output stage followed by a 12Db/octave reconstruction filter that should prevent high frequency output from the amp. If all is well there will be current flow as you put it in the filter but not after the amplifier as there are no speakers connected.

It sounds to me as though when you connect the speakers that the leads to the speakers are acting as an aerial & radiating VHF frequencies. This shouldn't normally happen with the filtered output (but it's not perfect), there is a possibility that you might have an open circuit capacitor (part of that output filter) on one channel.

While the amp may be switching at up to & beyond 1MHz it's actually switching a square wave so there is the distinct possibility of harmonics being generated up to 50MHz or more. If there is any problem with the output filters then you effectively have a radio transmitter :eek:

[allsorts]

I think i understand that in part. I just turned on the gain option on the bantam and have now picked up one of the two remaining "NO Signal" station sets. When i turned it on the pvr signal quality dropped from usual 85% to just 65%. With the gain off, it dropped to 55% and i couldnt get the picture. Only leaves me with Channel 2/21/22/23 set ABC (similar to your BBC) without a signal. May have to invest in a better aerial (20 years old) as the last channel set i have picked up now is sent from same transmitter tower as ABC. Must be a stronger signal?

[Reid Malenfant]

Yes, get a better aerial & new coaxial cable :eyebrows: 20 years :lolsign:

Make sure you get a wideband aerial fitted so you can receive all Freeview channels. Don't listen to the idiots that will try to sell you a "digital" aerial, it's just a wideband one you need.

Just their excuse to milk you for more money :eyebrows:

[allsorts]

Will do. Ta

[dave2010]

Greg

Do you still get analogue TV over there?

If your aerial is 20 years old then the cable might also be of similar vintage.

Here in the UK you can sometimes get away with using an analogue aerial, providing its bandwidth is wide enough, or the stations you want are in the right range of your existing aerial. Rather than replacing our TV aerial, which was here before we came, we simply switched to digital, and that solved quite a few problems. However digital reception also depends on the connected devices, and I found for consistency and good picture quality across several PVRs and TVs that using an aerial amplifier improves the overall situation. I did this after noticing that we had a better picture with a feed via a PVR rather than straight into the TV. Our TV has a very good picture, but is weak in the RF input section, so i give it a boost, and that fixes the problems. I also discovered quite a while ago (and to my surprise) that some cable gives much better results. I'd bought a digital extension cable thinking it might come in useful, and I found that this gave much better results that the TV coax I was using before. Before that I thought that coax is coax is coax ... Not so! Some may give much better results, though it does depend on quite a few things. It's just possible that replacing the coax could make a significant difference - though putting really high grade cable will only give marginal improvement over good cable. If the cable was previously optimised for analogue, then it may not be great for digital. Apart from lower loss, a good cable should have better screening, which could reduce the air borne interference which you may be getting from your Bantam.

I suspect that there is something wrong with the output filtering on your Bantam, and that's affecting the tv feed.

[Reid Malenfant]

Oh crap, I didn't realise I was giving Freeview advice to an Australian :doh:

I could be a country mile out with my aerial suggestion ;) Sorry!

[allsorts]

We have Freeview here otherwise known as free to air digital terrestrial IIRC. I dont have satellite pay tv. Analogue is being phased out by 2013 or so. What we do have though with Freeview is industry supported nobbling of TiVo so that machines bought here have a lot of functions like ad jumping disabled. We also have the ABC HD station dedicated to 24/7 news instead of showing decent programs that would benefit from HD. The commercial channels use HD to run 40 year old programs and the one sports channel that was on HD is now SD. So much for the hype around HD free to air. We have more stations showing more rubbish most of the time. Rant over.

[Reid Malenfant]

Hi Greg, I'm afraid to say that I assumed you were in the UK when you were talking about Freeview ;) Disregard what I said & go with whatever an aerial rigger suggests where you live :)

In the UK they had the old analogue transmitters grouped into different bands (A - E) & when they started broadcasting Freeview they had to use different frequencies on the same transmitter. As they were out of band (could have been a higher & lower frequency) a wideband aerial was needed.

No idea what you need chap so apologies for the mis-information i gave previously!.

[allsorts]

I'll be getting some quotes next week for a good aerial and shielded coax (r6). They sell so called "digital" aerials over here too!

[Reid Malenfant]

I'm not sure what good coaxial cable would be called where you are, sorry once again ;) Over here though some of the best stuff is known as CT100...

What you should have installed is not you normal common or garden coax which has frankly bloody pityful screening, but stuff like CT100 which has an overall copper foil as well as the braid to do the job :)

CT100 is used in satelite installations, it's performance on lower frequency TV transmissions is superfluous!

[allsorts]

I was intending to get quotes on RG6/U quad shielded as per this advice from the Digital TV Govt site:
RG6/U Quad-shield is the minimum requirement under the latest Australian Standard for digital TV antenna cabling and for all TV antenna cabling for apartments/units (MATV). It is also used for the distribution of Cable TV (CATV) and Satellite TV (SATV) in residential or commercial premises. It features a copper-clad-steel inner conductor. Single-shield, dual-shield and tri-shield versions of RG6/U are available but do not provide adequate EMI shielding. This replaces older spec RG59 cabling
I haven't come any reference to better cables though.

[Reid Malenfant]

Greg, as long as whatever coaxial cable you fit is good enough for installing on a satelite dish it will be good enough for the best TV installations :)

Again as I know nothing of things antipodean you might like to make sure you need to install 75 ohm coaxial cable ;) Satelite stuff is indeed 75 ohm, in fact things may well be quite standard as the only 50 ohm installations I know of are for transmitters such as Amateur Radio & CB etc...

[allsorts]

I've never seen any coax sold here other than 75 ohm. Will be getting quotes from well established installers only. There are some here who have been around for decades. One even makes their own antennae here in Melbourne. And there's me, thinking we had no manufacturing capability left in Oz after outsourcing it all to China.

[dave2010]

Greg

I agree with Mark's earlier comments about rubbish 75 ohm cable which used to be used for analogue systems. The impedance rating does not tell the whole story. There are issues about attenuation and screening. The best cable has low attenuation and good rejection of interference.

I wouldn't completely agree with the statement about satellite cable being good enough, as there are differences between different satellite cables, but it is largely correct if you are going to use the cable for terrestrial TV use. There are some cheap satellite kits which use only moderate quality cable - though in reality it is probably good enough. I replaced some old cable by new stuff, and in terms of signal strength there was virtually no difference. However, it may be that interference rejection is better, and also long term reliability may be improved. Water/moisture can get into cables, and some degrade faster than others.

The site I mentioned earlier - http://www.satcure.co.uk/ - has good advice re cables, though don't go overboard on gold plated stuff etc. Some cables have aluminium screening - not so good, and some tend to degrade over time. You need to get a reasonable balance between cost, quality and your application area.

A good cable and aerial (that may need attention too) should give you a strong, relatively noise free signal, and may give some protection against any injected noise. There may be no perfect protection against high levels of RF noise.

If it is the case that your Bantam is radiating and that is getting into the TV circuits and causing a problem, then I'd say it probably has a fault. Mine is only a few feet from the TV, which is (mostly) fed by a satellite feed, though we also have Freeview fed from an old analogue aerial on the roof (it works!) and I've not noticed any serious problems when the Bantam is on.

Now that you've made me think about this though, I can test this out further, but I'm not expecting there to be any problems.

I do think you may need to take this up with Temple Audio if you can verify that the amp is radiating.

[allsorts]

Had the antenna technician in today. Tested the signal and noticed a drop off as soon as the Bantam Gold amp was turned on. The tv screen read out goes from 74% quality to zero. Signal strength remains unchanged. I have a quad shielded coax flylead from the wall outlet to the TV. He said no antenna is going to fix the problem and it was probably the amp's transformer. So upside is I don't spend 350 dollars on new aerial, wiring etc. but still stuck with one group of channels not working. I will contact Temple Audio UK and see if they have a solution.

[allsorts]

I never got around to mentioning that John from Temple Audio suggested new cabling and moving the bantam gold around to see if it made some difference. Unfortunately no real improvement. I was wondering and i have asked him "if adding some ferrites to the speaker cable would help"?

To refresh: When i turn on the BG amp there is no effect on the tv. Its only when i attach the 14awg speaker wires that i get a "No Signal" message on the tv. It doesn't happen when i use my Yamaha AS-300 amp.

[allsorts]

Well i went and bought this Lepei TA-2020 amp, just to see if it would alter my ongoing tv signal quality dropout problem: http://www.ebay.com.au/itm/280791825779?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649.

It came in 10 days, I plugged it in and NO drop in signal, all stations now play and the sound is plenty loud and clear for just powering a centre speaker.
I will retire the Bantam Gold for a future bedroom system.
Also the ac/dc supply unit that came with the BG died a couple of days ago, after 7months of use. BG has decent SQ but not entirely convinced of its other merits.
The Lepei is ugly as sin, has a blue light on volume knob that could be used as an emergency beacon, little finesse with volume adjustments, but does the job i need it to do. Time will tell if it goes the distance but for AU$24 delivered, if it lasts 12 months I will be happy.