Bcd536hp external speaker connection considerations and precautions

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Mike_G_D

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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
 

kellykeeton

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Excellent write up mike. This would be great material in the wiki. Also I am sure that some member with time.. Could doodle up some schematics to graphically illustrate the cable shorting and isolation tranformer. Which is easily picked up at radio shack or salvaging old Walkman parts,
 

an39511

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I am test feeding the BCD536HD using a modified patch cable. As Mike_G_D said, I only connected the center tap and the shield is floating free. To improve the sound level I placed a 3.3K resister inline on the center tap. This allows the volume on the scanner to be set to 13 without over driving the audio.

You can hear the feed at Cleveland Police Dispatch, Brook Park and SW Suburbs Police and Fire until 3PM PST (2/11/14).
After that time I will reinstall my regular equipment.
 

kellykeeton

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So, Uniden listened to Mable but not the Live Audio feed provider!

While that is worth a chuckle, I would disagree. The lack of analog audio and the use of industry standard practices being the higher contributing factor. At the same time, the product is early release, and there are "features" coming out in weeks ahead that might counteract the comment about live scanner feed users being forgotten. For example because audio is in digital format the ability to run it into the computer without a analog transmission wire aka the "lack of a record jack" that everyone is "so angry they will return the scanner about" is obviously something we will gain, per the functionality of the "app".

While it brings to question, the "beta testers" that were used and this not identified, it is odd that by teaming with RR and the large live feed users - more documentation about this wasn't provided day one... To calm down the lynch mob.
 

SCPD

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While that is worth a chuckle, I would disagree. The lack of analog audio and the use of industry standard practices being the higher contributing factor. At the same time, the product is early release, and there are "features" coming out in weeks ahead that might counteract the comment about live scanner feed users being forgotten. For example because audio is in digital format the ability to run it into the computer without a analog transmission wire aka the "lack of a record jack" that everyone is "so angry they will return the scanner about" is obviously something we will gain, per the functionality of the "app".

While it brings to question, the "beta testers" that were used and this not identified, it is odd that by teaming with RR and the large live feed users - more documentation about this wasn't provided day one... To calm down the lynch mob.
Well I'm not returning mine, will be watching for a solution. I'm sure as you stated, this will be fixed.
 

LIScanner101

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While I appreciate the OP's thorough and well-thought-out post, I find this truly baffling and puzzling. It basically comes down to Uniden literally ignoring the mainly hobbyist base these scanners appeal to. They KNEW that MANY people stream audio and to leave this feature out with no explanation is quite bluntly a boneheaded move.

I also find it very telling that UPMan didn't come out with this explanation and once again it had to come from a dedicated RR member.
 

WaveJam

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While I do appreciate this post. What lingers in my mind is. What affiliation is Mike_G_D with Uniden that he can announce "Best" and "Second Best" solutions? Technically he is correct, yes. But still these solutions (if implemented incorrectly) could leave you with a warranty voided/toasted scanner. His post sounds like recommended solutions that I do not think Uniden would agree with.

I personally would like an Uniden approved solution to help us feed streamers with this issue, without the possibility of hosing up my scanner. Let me be clear. I'm NOT saying that Uniden should "create" the solution. Just maybe "guide" us with an approved cable/transformer for us to use that will not damage the unit.
 

JamesO

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Somewhat a cross post of comments I made in a different thread.

Seems that the audio amp design and output configuration varies and is important on how external speakers, amplified speakers, amplifiers, sound cars, Bluetooth A2DP devices are connected.

Some of the issue is the stereo connection that some of the jacks have, some it has to do with the Amp design, some it has to do with ground loops.

Seems to me that Uniden needs to write some form of Technical Bulletins and/or Application Note for connecting external speakers, amplified speakers, line amplifiers, sound cards and USB audio from the newer model scanners. This documentation should be broken down by scanner model number and explain how the outputs are wired, mono or stereo, the Amp design and specs/impedance requirements.

^ UPMan/,Uniden hint, hint, hint.

Best if it comes directly from the source rather than having one of the forum members backwards engineer this. A set of Technical Bulletins/Application Note for products might even help out Customer Service and head off a lot of workload. Just come up with a standardized format and likely you can use this same template across the products lines. I have some ideas if anyone is interested.

I can also see the same Technical Bulletins and/or Application Notes cover things like the USB Audio connection, streaming, connecting and using the GPS unit, AGC configuration and how it should work and how to best set it up, updating the database, dealing with the wireless interface on the 536 and MANY other single task focused items.

This would save Uniden and the customers a lot of frustration in the long run. Invest in some time to set up, write, edit these Bulletins/App Notes and then have a library for them that can be accessed online. This way the users can help themselves easier, configuring their equipment wrong or possibly even damaging it.

I have also seen and set up similiar systems where there are both "public" and internal bulletins that could help Customer Service do a better and more consistent job.
 

markab

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While I do appreciate this post. What lingers in my mind is. What affiliation is Mike_G_D with Uniden that he can announce "Best" and "Second Best" solutions? Technically he is correct, yes. But still these solutions (if implemented incorrectly) could leave you with a warranty voided/toasted scanner. His post sounds like recommended solutions that I do not think Uniden would agree with.

I personally would like an Uniden approved solution to help us feed streamers with this issue, without the possibility of hosing up my scanner. Let me be clear. I'm NOT saying that Uniden should "create" the solution. Just maybe "guide" us with an approved cable/transformer for us to use that will not damage the unit.

I don't think it was Uniden's intention to have its 536 owners use the speaker out jack as a streaming source (it makes for a poor choice). If you use the headphone jack you avoid the BTA issue all together. Some don't want to do this because of the noise issue which is still being fixed by Uniden. Although I can't speak for Uniden I can't see them sanctioning a method of using the speaker out jack to stream audio. It would be opening a can of worms for them and I don't blame them.
I would suggest the following options A) stream with noisy headphone jack B) wait to get your headphone jack fixed and then stream with it or C) wait for the Siren app (it may offer other streaming options).

Mark
 

NYRHKY94

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Markab:

Agreed. I'll be waiting for option #2 after receiving my 536 replacement from Uniden. I would have preferred option #1 (Line Out/REC).....but ;-)

Mike
 

nr2d

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Someone posted a caution note they found I think in the BCD356HP user's manual. I found this reference:
http://forums.radioreference.com/uniden-scanners/278121-bcd536hp-discussion-thread-16.html#post2073456, post 314. I basically states don't connect anything other than a passive speaker to the external speaker jack.

I was also just thinking that maybe Uniden was asked not to provide a "Record Out" jack. In another forum here on RR an operator of a live stream was contacted and asked to take his live stream down until a special event was over. He did take it down.

I may be reaching but it wouldn't surprise me if this just might be the reason that there is no "Record Out" jack on the BCD536HP.

As for UPman not coming out with an explanation just remember he works for Uniden and is very well limited as to what he can say or divulge.
 

DPM

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Question. Is it ok to use an external speaker (Yaesu SP-55, 4 ohm impedance) just to listen to the audio. Are the above comments regarding if you are streaming audio?. If so, should I use the headphone jack?. Did not have this type of confusion with other scanners. Please advise, thanks
 

N0UDG

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While that is worth a chuckle, I would disagree. The lack of analog audio and the use of industry standard practices being the higher contributing factor. At the same time, the product is early release, and there are "features" coming out in weeks ahead that might counteract the comment about live scanner feed users being forgotten. For example because audio is in digital format the ability to run it into the computer without a analog transmission wire aka the "lack of a record jack" that everyone is "so angry they will return the scanner about" is obviously something we will gain, per the functionality of the "app".

While it brings to question, the "beta testers" that were used and this not identified, it is odd that by teaming with RR and the large live feed users - more documentation about this wasn't provided day one... To calm down the lynch mob.
I think UPMan mentioned in a previous post (about the headphone issue) that the radios used by the Beta Testers did not have this problem.

I guess it was a mistake made at the factory latter during the ramp-up to mass production.
 

Tryton

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I won't buy this scanner BECAUSE it does not have a line out.. As a feed provider, as well as having 2 XT's $100.00 cheaper that have it. Makes me wonder the thinking behind the exclusion.
 

MesquiteWx

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A even better solution is since this scanner has network capabilities it would be nice to have a built in source client or web server and completely bypass having to connect it to a sound card all together. It would be strictly an IP based wireless connection to whatever source you choose to serve.

I am still scratching my head why a record out jack was removed all together.
 

nr2d

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Question. Is it ok to use an external speaker (Yaesu SP-55, 4 ohm impedance) just to listen to the audio. Are the above comments regarding if you are streaming audio?. If so, should I use the headphone jack?. Did not have this type of confusion with other scanners. Please advise, thanks
I'm using an old Radio Shack speaker I used for many years. It works fine. I don't stream so I can't offer any suggestions.
 

pkneeyahx

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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.
Mike.. I've built a cable using your method, and another one using a 3.3k ohm resistor. Both are providing great audio from the rear jack. I am however getting a slight high pitch buzz.. similar tone to a ringing in the ear. any leads on how to eliminate this? Thanks

Adam
 

Skypilot007

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I applaud Uniden for using the BTL audio amplifier scheme. As mentioned by the OP it is very effective and has been in use for some time in the commercial LMR type of radio gear. This has been one of my main complaints about uniden scanners for a long time, the lack of audio output power and the tone quality. I can't wait to hear what the quality of the sound is like thru a commercial grade passive mobile communications speaker. I hope I'm not disappointed again. I am encouraged by what I'm reading.
 
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