joseph@topaz.ARPA (Joseph) (05/15/85)
I am trying to connect the earphone jack of my television set to an unused auxilliary connector on my NAD 7020 receiver. When I just build a cable with a mini phono plug on one end (for TV) and an RCA plug on the other and connect the units, I get a LOT of hum. I have been told by an electrical engineering friend that some kind of matching transformer is required because of the huge impedance difference between the TV output and the line level input of most receivers. Does this sound reasonable? Both the TV and the receiver are plugged into the same outlet to try to avoid ground loops. What other problems might be causing this (the TV and the Receiver are in perfect working order)? When looking for a transformer, what should I ask for? Do companies make products that do this? Can I for example buy a box to connect the ouput from my FM tuner walkman to an integrated amp? Thanks for your help Seymour
wmartin@brl-tgr.ARPA (Will Martin ) (05/17/85)
> I am trying to connect the earphone jack of my television set to an unused > auxilliary connector on my NAD 7020 receiver. When I just build a cable > with a mini phono plug on one end (for TV) and an RCA plug on the other and > connect the units, I get a LOT of hum. There are several problems with this set-up. I don't think that a "matching transformer" is the cure, though it might function to solve some of the problems. It would function more as an "ISOLATION transformer" here, to isolate the TV circuitry from the hi-fi audio stages. You are probably getting lots of power-supply leakage into that earphone jack from the TV circuits, which, given the poor bass response of most TV audio stages or speakers or small earphones, is never apparent until you feed it into a higher-quality audio system. You are probably also getting some DC offset and maybe 60 Hz AC from the TV into the heart of your audio system's preamp, which can wreak havoc with various elements of the circuitry. (I believe that it can damage volume and other pots, for example -- a friend with a Mac amp was told this at one of the old Mac free clinics when they replaced his volume pot; they said DC from a malfunctioning tape deck had caused the pot to become noisy. Since they were replacing the pot free, with no sales pitch or income involved, they had no reason to lie to him about that...) Also, you are amplifying the signal AFTER it has passed through all the (usually) poor-quality audio stages inside the TV. The signal you are then feeding to your audio system has already been distorted and band-limited, so you are not getting the best possible sound that your TV's detector/demodulator circuits can give you (often surprisingly good). > Both the TV and the receiver are plugged into the same outlet to try to avoid > ground loops. I fear you might be *causing* a ground loop, especially if both are equipped with three-prong grounding plugs. The shield of the cable would cause a ground loop. Maybe try a quick experiment, and disconnect and isolate the shield at one end (and then reconnect it and disconnect the other end). One or both of those options might reduce hum. > Do companies make products that do this? A company called "Rhodes" (or maybe "Rhoades"?) has been living off this need for decades -- they make a doohickey called a "Teledapter" to do just this. They have (at least had) several models, and also did market a TV-audio tuner for a while. Look in the little ads in the back of any of the slick hi-fi mags for their ad. The best thing is to grab the audio signal before it gets into the TV's own audio stages, and also strip off any garbage like DC offsets or stray AC power-line noise or leakage. This is usually done by getting the signal at the volume-control pot, and checking it with a DC-coupled 'scope. Some fancy TV's make it hard to do this -- I have a Zenith System III set (a real dog!) where the volume pot has only DC on it -- it just varies a control voltage and the real audio signal is buried down inside one of their overpriced modules! But if you can get clean line-level audio at your TV's volume pot, you can tap it off to a line-level-output jack which you can install anywhere convenient. (Suggestion: don't put it on the back panel were you'll have to disconnect it to take off the rear for servicing!) Will Martin USENET: seismo!brl-bmd!wmartin or ARPA/MILNET: wmartin@almsa-1.ARPA
mlf@panda.UUCP (Matt L. Fichtenbaum) (05/20/85)
>> I am trying to connect the earphone jack of my television set to an unused >> auxilliary connector on my NAD 7020 receiver. When I just build a cable >> with a mini phono plug on one end (for TV) and an RCA plug on the other and >> connect the units, I get a LOT of hum. >The best thing is to grab the audio signal before it gets into the TV's >own audio stages, and also strip off any garbage like DC offsets or stray >AC power-line noise or leakage. This is usually done by getting the signal >at the volume-control pot, and checking it with a DC-coupled 'scope. CAREFUL CAREFUL CAREFUL CAREFUL! KNOW WHAT YOU'RE DOING BEFORE YOU DO THIS! It is common nowadays for TV receivers, especially the $300 for 19" consumer kind, to derive their operating voltages from the AC line using a power supply without a transformer. In other words, "ground" within the TV is one side of the power line, or sometimes ~100V DC away from one side of the line. IF YOU PLACE YOURSELF BETWEEN TV GROUND AND A REAL GROUND IT CAN KILL YOU. If you connect TV ground to a grounded audio system it can blow a fuse or one or both pieces of the electronics. If you connect TV ground to an ungrounded audio system it can place lethal voltages on the audio equipment's front panels. Sets with earphone jacks isolate the earphone jack through a (cheap) audio transformer so as not to put dangerous voltages where people can touch them. (UL approval requires a certain amount of isolation, too.) Direct audio connections are kept within the cabinet. If you don't KNOW that your set has a power transformer, and is thus isolated from the line, don't attempt to find a direct audio output. Sorry to clutter up the whole net with this, but I think it's important. -- Matt Fichtenbaum "Our job is to rescue fires, not put out your cat."
brown@nicmad.UUCP (05/26/85)
> The best thing is to grab the audio signal before it gets into the TV's > own audio stages, and also strip off any garbage like DC offsets or stray > AC power-line noise or leakage. This is usually done by getting the signal > at the volume-control pot, and checking it with a DC-coupled 'scope. > > Some fancy TV's make it hard to do this -- I have a Zenith System III set > (a real dog!) where the volume pot has only DC on it -- it just varies > a control voltage and the real audio signal is buried down inside one > of their overpriced modules! But if you can get clean line-level audio > at your TV's volume pot, you can tap it off to a line-level-output jack > which you can install anywhere convenient. (Suggestion: don't put it on > the back panel were you'll have to disconnect it to take off the rear for > servicing!) WARNING: Doing the above may be hazardous to your health! Some TV sets don't use any kind of isolation from the AC line, ie, one side of the AC line gets connected to the TV chassis/ground circuit. The headphone output is done after a audio transformer. If the ground of a Hi-Fi system is attached to the chassis of the TV, it is hard to say which piece will 'blow-up', if not both. CAUTION IS ADVISED!!!!!!!! -- |------------| | |-------| o| JVC HRD725U Mr. Video | | | o| |--------------| | | | | | |----| o o o | | |-------| O| |--------------| |------------| VHS Hi-Fi (the only way to go) ({!seismo,!ihnp4}!uwvax!astroatc!nicmad!brown)
kann@savax.UUCP (kann) (05/28/85)
> Some fancy TV's make it hard to do this -- I have a Zenith System III set > (a real dog!) where the volume pot has only DC on it -- it just varies > a control voltage and the real audio signal is buried down inside one > of their overpriced modules! But if you can get clean line-level audio > at your TV's volume pot, you can tap it off to a line-level-output jack > which you can install anywhere convenient. (Suggestion: don't put it on > the back panel were you'll have to disconnect it to take off the rear for > servicing!) If you are hacking inside your TV set trying to get clean audio you should be acutely aware of the 120VAC danger. Most (almost all) TV sets today are "hot chassis" sets, where the chassis is directly connected to one side of the AC line. If the set has a non-polarized plug or the outlet is wired wrong, you could *DIE* connecting to the volume control or any other point inside the set. Even if you can work around this problem, you are still likely to get hum and buzz from the sweep circuits, anyway. You would do better buying a separate TV audio receiver or a TV monitor/receiver with the connections built in. Monitor/receivers, may still be hot chassis sets but they incorporate special 120V isolation circuitry for the direct audio/video connections. If you are hacking a Zenith set, you may be in luck. They were one of the last companies to go hot chassis, and they are willing to send a schematic *FREE* to anybody who calls with a chassis number. It should be a simple matter to tap the correct point on the volume control IC. But again, BEWARE, if your set is hot chassis or you cannot figure out whether it is or not, STAY OUT OF IT. USENET: decvax!savax!kann Erwin Kann Sanders Associates Nashua, New Hampshire (The opinions expressed herein are those of a person who thinks he knows what he is doing and still gets zapped once in a while :-O)