Problem with L1550 transmitter {support transcript}

I purchased an Amphony transmitter/receiver/amplifier from you on June 10. I put the unit onto my stereo system on June 25. Since then, the transmitter stays on continually and transmitts a signal to the receiver causing random crackling in my rear speaker. According to the manual, the transmitter is supposed to shut itself off if no signal is detected. Do I have a faulty unit, and if so how do I get it exchanged?

Unfortunately we have had a myriad of problems reported with this device .... What you are seeing has to do with random electronics noise being picked up by the transmitter. What most people do is turn off the power to the unit when not in use which we must admit is rather tedious. Do you have any 5.8GHz interferance such as 5.8G cordless phones? Unfortunately what we can do now is offer you a full refund of your purchase price if this problem is unbearable since most likely any other unit we send you will have the same problem. We apologize for the inconvenience.

Problem with L1520 transmitter {st}

Thank you for your offer. I considered returning the unit, but in fact, when in use it works very well.

The problem I am having is that there is a continuous signal from the transmitter; it does not turn itself off. But, I don't have any interference from any external devices.

I then removed the input leads to the transmitter, and found that to be the problem. Apparently my fancy 7.1 channel Yamaha receiver must be keeping a miniscule (probably less than a milliwatt) amount of power from the power supply on in order for the remote to operate. That is enough for the transmitter to think it has an input.

My current solution is unplugging the power from the transmitter; I just realized that I can use the switched outlet on my Yamaha to drive the power supply and accomplish the same thing.

It is interesting that you are having so many units with problems since there are many on-line places where the unit is being sold. In fact this new class-T amplifier concept is so effective that several new audio devices are incorporating them especilly in portable devices

It's interesting that you bring that up since this has in fact perplexed me as to why they chose Tripaths class-T architecture (well other than cost). True that these new class-T amplifiers do seem revolutionary in offering low THD, intermodulation distortion, SNR, dynamic range etc concurrent with high efficiencies. In actuality though just how significant is power efficiency for this application? It would seem that the L1520 audio quality far out weighs amplifier efficiency since these devices are not battery powered anyway. It would also seem that unless they were planning to slap an energy star label they might have just stuck with the highly linear, tried and tested class AB. In any case I don't believe the issue with these is with the audio amplifier or codec but rather in the RF radio architecture/design. When it works it works beautifully with good audio quality which is quite impressive for a wireless solution. Problem however is that there's no in-built adaptive co-channel or adjacent channel interference avoidance scheme. On of the earliest demos we set up was completely unworkable since we had a 5.8GHz cordless phone that was causing interference to the L1520 (interestingly the phone still worked fine since it had frequency hopped spread spectrum interference avoidance). Then there's the problem you identified which has to do with poor input squelch circuits (if it even exists) - the squelch circuit is supposed to disable the audio output if the input signal is below a pre-defined threshold (which should eliminate the hum and crackling due to noise at the input).

We are due to test some new improved models including an improved version of the L1550 (50W version) so hopefully these will be more robust.

I have some experience in this field, but hardly the level of expertise that you have. I got my knowledge from many years of trying different amps, pre-amps, etc. to get high quality sound, a little playing with modifying or building some equipment, but mostly by listening to people like you who do know the specifics of circuit design.

So far my 5.8GHz phone has not interfered with the L1520 (the phone is located only 5 feet from the receiver), and with about 15 feet between transmitter and receiver, unexpectedly good sound. In my initial test of the unit, I transmitted the main speaker signal to my main speakers (I still use some very "beefy" 30 year-old Altec Lansing speakers) the sound quality was really excellent. My rear speakers are also three-way Altec Lansing studio monitors, and those also performed very nicely. The sound is not what I would expect from a decent class AB amp, but at least as good as many medium quality commercial amps.

As I use the unit, I'll try to give you some additional feedback. At the same time, if you find the L1550 to be a better performiong device, I'd be interested in purchasing that unit.

BTW, I originally was looking for a receiver that would provide a low-level output (one shortcut in the 1520 that is unfortunate), with the intention of feeding the signal to a really low distortion amp. There are several available commercially, and I have some circuit designs that my son-in-law and I have experimented with; he has been going back to using tube designs while I have continued toward the digital media. In any case, an output directly from the receiver would be a great plus in my opinion.

I'm sure you are aware of some other companies that are also putting out similar transmitter/reciever/amp combinations (maybe they are all built by one company). Some also in the 5.8GHz range and a couple still down at 2.4, and one at 900MHz. I expect that during the next couple of years we will see some new designs that overcome some of the shortcomings that we are experiencing currently.