Folks,
There's been a lot of talk about issues with the new 536HP's external speaker output. There are now at least three separate threads concerning this and, although I did reply in one thread and pointed out most of what follows, I still see much confusion and mis-information concerning this issue. In essence, we're talking about issues occurring when that output is connected to something OTHER than a real passive external speaker, like an external amplifier or input to a PC soundcard, even an external powered amplified speaker. Problems such as low audio, distorted audio or no audio at all.
There is a reason for this. As UPMAN has said, the external speaker output in the 536HP is not like it was in the older models - it uses a BTL amplifier arrangement - BTL stands for "Bridge Tied Load". In essence, it means that what amounts to two mono amps are fed together with one having an inverted signal relative to the other. Think of the difference between -3 and +3 as being a total of 6 (the BTL; don't confuse this with "adding" the two "out-of-phase" signals and getting a cancellation, 0, as what we are doing is dealing with the "total difference" between the two signals and using that) versus the difference between 0 and +3 being just 3 (as in "normal" single ended grounded shield amps). That "difference" is what counts in the end at the speaker. So, essentially what you get is a very powerful audio signal for driving a passive speaker. The problem is, that is ALL it was designed to drive! It was NOT designed with any intention of being connected to a single ended device (with the shield grounded as with most consumer amplifiers, PC soundcard inputs, etc.).
So what happens when you connect a BTL amp to a single ended amp or other input device (one that has the shield grounded as in most consumer electronic gear)? Well, you end up grounding out or "shorting" one side of that BTL signal. Depending on the design of the BTL amp, this will cause, at minimum, low audio and/or distorted audio and at worst permanent damage to the amp.
So how do you connect a BTL amp output to a single ended input like a PC soundcard or external amplifier? WHATEVER YOU DO you MUST make sure that NEITHER the "tip" (center conductor of the speaker jack) or "ring" (outer ring of the speaker jack) are in any way directly connected to ground!
The BEST solution is to use an audio isolation transformer between the 536HP external speaker jack and the single ended device input. This would effectively isolate the 536 speaker amp output from the single end device from a DC perspective and still pass the audio. Each lead of the primary side of the transformer is connected to the speaker amp output (again, neither is grounded!) while the secondary side of the transformer has one lead grounded to the shield of the connecting cable between the device being fed and the other lead connected to the cable center conductor as you normally would expect in single ended connections. Such a transformer automatically isolates the 536 output from DC as well so would protect it from any DC voltage present from the device being fed (see the following "second best" solution for more explanation of why this may be needed).
The SECOND BEST solution, used especially when you do not have or cannot get an audio isolation transformer, is to use ONLY ONE LEAD from the speaker output jack of the 536 and leave the other lead floating - not connected to anything. So, what you would do, for example, is simply modify a standard mono audio cable such that the shield is "broken" at the end connected to the 536 so that there is no shield connection between the 536 speaker jack and the cable, and instead, connect the shield to some other point on the radio for ground (like a convenient chassis screw or whatever). The other end of the cable then would be connected to the input of the device being fed as normal. This would still yield plenty of audio for the external device to use and not short out anything at the 536 BTL output. Now, one other thing to consider here - that is, whether or not there is any DC voltage coming from the device being fed. If there is, as in some mic inputs present on PC soundcards (which is there in order to power some microphones that need external power, "phantom power") then you should also add some form of DC isolation to block the DC voltage from getting fed back to the BTL amp circuit. Again, depending on the design of the circuit DC voltage present on the line may or may not cause damage but it is not really good to have it there in any case. The easiest way to block DC and pass ac audio is by using a capacitor. A capacitor in series acts like a short at high ac frequencies but more like a resistor as the frequencies get lower until at DC it completely blocks the signal (like an "open circuit"). To pass audio frequencies well you should use something like a 10uF capacitor but you could probably get away with anything down to 1uF and do ok. Most caps in this range are "polarized" electrolytic caps and, therefore should be connected so as to "point" the positive lead to the most positive source of DC (like the device being fed such as the PC mic input) or you could use "non-polarized" electrolytic caps if you have access to those. So, simply solder the positive lead of a 10uF (or something more than 1uF, not critical) to the "tip" center conductor of a standard mono audio plug and the negative lead to a standard shielded audio cable center conductor. This plug would go to the device being fed (such as the PC mic input); the opposite end can just have a normal mono plug connection and would be connected to the 536 external speaker jack.
Why did Uniden choose this "new" means of external speaker amplification? Well, it's not really "new" - it has been used for awhile for professional land mobile radio (LMR) gear and is an efficient means to increase audio output power without needing more, or much more, supply voltage. That's the primary reason. As long as a passive unpowered speaker is the only thing that is connected to this output it will perform very well! The problem is that it was not designed to be connected to anything else; in professional gear, that is fine as such gear is usually only installed by special trained technicians and is not altered by the end users. Hobbyists, of course, are a different story. The other part of the problem is that the 536 was designed WITHOUT any form of analog audio line output as the older models up until now had been. Users became used to using that line output (aka "Record Out") for connection to their PC soundcards for streaming and recording. Now, without it, users naturally turned to using the external speaker output instead - hence the problem since that speaker output now uses the BTL design! It was a case of two design changes from older models which caused unintentional use issues among end users who were "trained" on the older models.
I think that Uniden expected that users would no longer need the "record out" jack as they now could record audio on SD cards. Also, I am not sure but I think that Uniden also expected to use the USB data port to pass digitized audio from the receiver for use if one wanted to pass that audio to the PC for streaming or remote operation, etc. I don't know this for certain and I haven't yet read of anyone using this method and I also do not own a 536 nor have I read the manual or specs. However, it seems likely, to me, based on the hardware and design approach; it may not yet be supported in the firmware and external software yet available but it may be supported in the hardware. Until and/or unless this approach is fully supported, users will have to rely on that external speaker connection and use the interfaces and precautions I have outlined above when connecting that output to anything other than a passive external speaker.
By the way - THIS HAS NOTHING WHATSOEVER TO DO WITH THE ISSUES CONCERNING THE HEADPHONE AUDIO OUTPUT PROBLEMS EXPERIENCED WITH SOME 536HP UNITS! The headphone audio issue has been addressed by UPMAN and Uniden in general and appears to be a manufacturing defect that affected some, but not all, units coming from the factory. The external speaker output and headphone outputs use COMPLETELY SEPARATE CIRCUITS and the problems with each are UNRELATED.
-Mike
There's been a lot of talk about issues with the new 536HP's external speaker output. There are now at least three separate threads concerning this and, although I did reply in one thread and pointed out most of what follows, I still see much confusion and mis-information concerning this issue. In essence, we're talking about issues occurring when that output is connected to something OTHER than a real passive external speaker, like an external amplifier or input to a PC soundcard, even an external powered amplified speaker. Problems such as low audio, distorted audio or no audio at all.
There is a reason for this. As UPMAN has said, the external speaker output in the 536HP is not like it was in the older models - it uses a BTL amplifier arrangement - BTL stands for "Bridge Tied Load". In essence, it means that what amounts to two mono amps are fed together with one having an inverted signal relative to the other. Think of the difference between -3 and +3 as being a total of 6 (the BTL; don't confuse this with "adding" the two "out-of-phase" signals and getting a cancellation, 0, as what we are doing is dealing with the "total difference" between the two signals and using that) versus the difference between 0 and +3 being just 3 (as in "normal" single ended grounded shield amps). That "difference" is what counts in the end at the speaker. So, essentially what you get is a very powerful audio signal for driving a passive speaker. The problem is, that is ALL it was designed to drive! It was NOT designed with any intention of being connected to a single ended device (with the shield grounded as with most consumer amplifiers, PC soundcard inputs, etc.).
So what happens when you connect a BTL amp to a single ended amp or other input device (one that has the shield grounded as in most consumer electronic gear)? Well, you end up grounding out or "shorting" one side of that BTL signal. Depending on the design of the BTL amp, this will cause, at minimum, low audio and/or distorted audio and at worst permanent damage to the amp.
So how do you connect a BTL amp output to a single ended input like a PC soundcard or external amplifier? WHATEVER YOU DO you MUST make sure that NEITHER the "tip" (center conductor of the speaker jack) or "ring" (outer ring of the speaker jack) are in any way directly connected to ground!
The BEST solution is to use an audio isolation transformer between the 536HP external speaker jack and the single ended device input. This would effectively isolate the 536 speaker amp output from the single end device from a DC perspective and still pass the audio. Each lead of the primary side of the transformer is connected to the speaker amp output (again, neither is grounded!) while the secondary side of the transformer has one lead grounded to the shield of the connecting cable between the device being fed and the other lead connected to the cable center conductor as you normally would expect in single ended connections. Such a transformer automatically isolates the 536 output from DC as well so would protect it from any DC voltage present from the device being fed (see the following "second best" solution for more explanation of why this may be needed).
The SECOND BEST solution, used especially when you do not have or cannot get an audio isolation transformer, is to use ONLY ONE LEAD from the speaker output jack of the 536 and leave the other lead floating - not connected to anything. So, what you would do, for example, is simply modify a standard mono audio cable such that the shield is "broken" at the end connected to the 536 so that there is no shield connection between the 536 speaker jack and the cable, and instead, connect the shield to some other point on the radio for ground (like a convenient chassis screw or whatever). The other end of the cable then would be connected to the input of the device being fed as normal. This would still yield plenty of audio for the external device to use and not short out anything at the 536 BTL output. Now, one other thing to consider here - that is, whether or not there is any DC voltage coming from the device being fed. If there is, as in some mic inputs present on PC soundcards (which is there in order to power some microphones that need external power, "phantom power") then you should also add some form of DC isolation to block the DC voltage from getting fed back to the BTL amp circuit. Again, depending on the design of the circuit DC voltage present on the line may or may not cause damage but it is not really good to have it there in any case. The easiest way to block DC and pass ac audio is by using a capacitor. A capacitor in series acts like a short at high ac frequencies but more like a resistor as the frequencies get lower until at DC it completely blocks the signal (like an "open circuit"). To pass audio frequencies well you should use something like a 10uF capacitor but you could probably get away with anything down to 1uF and do ok. Most caps in this range are "polarized" electrolytic caps and, therefore should be connected so as to "point" the positive lead to the most positive source of DC (like the device being fed such as the PC mic input) or you could use "non-polarized" electrolytic caps if you have access to those. So, simply solder the positive lead of a 10uF (or something more than 1uF, not critical) to the "tip" center conductor of a standard mono audio plug and the negative lead to a standard shielded audio cable center conductor. This plug would go to the device being fed (such as the PC mic input); the opposite end can just have a normal mono plug connection and would be connected to the 536 external speaker jack.
Why did Uniden choose this "new" means of external speaker amplification? Well, it's not really "new" - it has been used for awhile for professional land mobile radio (LMR) gear and is an efficient means to increase audio output power without needing more, or much more, supply voltage. That's the primary reason. As long as a passive unpowered speaker is the only thing that is connected to this output it will perform very well! The problem is that it was not designed to be connected to anything else; in professional gear, that is fine as such gear is usually only installed by special trained technicians and is not altered by the end users. Hobbyists, of course, are a different story. The other part of the problem is that the 536 was designed WITHOUT any form of analog audio line output as the older models up until now had been. Users became used to using that line output (aka "Record Out") for connection to their PC soundcards for streaming and recording. Now, without it, users naturally turned to using the external speaker output instead - hence the problem since that speaker output now uses the BTL design! It was a case of two design changes from older models which caused unintentional use issues among end users who were "trained" on the older models.
I think that Uniden expected that users would no longer need the "record out" jack as they now could record audio on SD cards. Also, I am not sure but I think that Uniden also expected to use the USB data port to pass digitized audio from the receiver for use if one wanted to pass that audio to the PC for streaming or remote operation, etc. I don't know this for certain and I haven't yet read of anyone using this method and I also do not own a 536 nor have I read the manual or specs. However, it seems likely, to me, based on the hardware and design approach; it may not yet be supported in the firmware and external software yet available but it may be supported in the hardware. Until and/or unless this approach is fully supported, users will have to rely on that external speaker connection and use the interfaces and precautions I have outlined above when connecting that output to anything other than a passive external speaker.
By the way - THIS HAS NOTHING WHATSOEVER TO DO WITH THE ISSUES CONCERNING THE HEADPHONE AUDIO OUTPUT PROBLEMS EXPERIENCED WITH SOME 536HP UNITS! The headphone audio issue has been addressed by UPMAN and Uniden in general and appears to be a manufacturing defect that affected some, but not all, units coming from the factory. The external speaker output and headphone outputs use COMPLETELY SEPARATE CIRCUITS and the problems with each are UNRELATED.
-Mike