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PostPosted: Fri Feb 01, 2008 3:43 pm 
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I have the DHS Mobile Theatre System (MTS) installed in my 2004 BFT. I am in the process of replacing the 13" 12VDC color TV (Model TVS-13DC1). Both TV and MTS were distributed by River Park, Inc., of Elkhart, IN, and were factory installed by Born Free when the coach was new.

The old TV had a mono audio output and was connected to the MTS amplifier mono input with a single cable with RCA-type plugs on each end. My new TV has stereo audio out. Is there some way I can hook up this new TV to the MTS amplifier and get a stereo signal to the speakers? Has anyone with the DHS MTS done this?

Thanks,
Bill

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PostPosted: Mon Feb 04, 2008 12:23 pm 
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Bill, I'm not sure I completely understand your setup and whether your MTS has a stereo input and whether you have a left and right speaker. So, please pardon my wordy response.

1.) If your MTS has a stereo input, then you can play both the right and left audio out channels from your new TV through the MTS. If you have a left and right speaker you should then hear stereo. You can probably connect the TV audio outputs into the MTS with either two RCA coax cables(red & white) or with a mini stereo cable, depending on which cables work with your system.

2.) If your MTS doesn't have a stereo input, you can still simultaneously play both the right and left audio outputs from your new TV through the mono input of the MTS. When you do this you'll get "complete" sound and won't be missing any sounds that were present in the right or left channels. Radio Shack and other sell a "Y" adapters that will allow you to merge the right and left channels out of the TV into one channel to play through the MTS. There are "Y" adapters available with RCA jacks, mini stereo jacks or even with both types of jacks.

Hope this helps.. Larry

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PostPosted: Mon Feb 04, 2008 5:50 pm 
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Larry, thanks for responding.

Here is my dilemma. The old TV had a single (mono) RCA audio out connection on the rear. A single RCA cable ran from this connection to a mono RCA input connection fo TV on my MTS amplifier. The new TV has no "audio out" but does have a 3.5mm stereo headphone jack. I want to hook up my new TV going from the headphone jack to the MTS amplifier.

I tried using a male stereo 3.5mm to RCA mono plug adapter (see image below) or this link to the Radio Shack part: http://tinyurl.com/2mmtwx

I got sound from the TV through the MTS to the speakers but it was a very low volume (not like the old TV) even with the volume adjusted to maximum on the new TV.

I'm wondering why I'm getting such low volume. Is the output power from the headphone jack less than what the old TV RCA audio out was? Or by combining the right and left signal from the headphone jack to a single channel RCA mono signal using the adapter, did I decrease the signal to the MTS amplifier such that I can't get adequate volume from my MTS speakers?

I'm hoping to get this mono audio connection working properly (just like the old TV did). Any and all suggestions appreciated.

Bill


Attachments:
File comment: Mono RCA Plug to 3.5mm Stereo Headphone Jack Adapter
3.5mm Stereo to Mono RCA.jpg
3.5mm Stereo to Mono RCA.jpg [ 5.47 KiB | Viewed 6587 times ]

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PostPosted: Mon Feb 04, 2008 10:30 pm 
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Bill.. I recently replaced my old CRT TV with an LCD. I am using the headphone output (mini stereo jack) to drive my sound system and it works fine. My sound system consists of an inexpensive computer speaker system with two small speakers and a small subwoofer. This sound system has a built in 30 watt amplifier that can be adjusted for volume.

That being said, I really don't know why you can't get enough volume. Joining the left/right channels with the mini stereo to RCA jack you have seems like it should work. Joining the two signals shouldn't diminish them. I've used the headphone output from TV's before for sound and it has always had enough power to play through an amplifier. You could test your headphone output with a pair of headphone to see if it is outputting enough power. If you can hear good sound in the headphones, then there should be enough power to drive an amplifier.

Some TV's have volume adjustments through the set up menu on the remote. It's a longshot but it's possible that you can control the volume setting from out the headphone out through the setup menu but it seems unlikely.

Does your MTS amplifier have an adjustable volume control? If so, is it turned up all the way? I'm pretty sure you already checked this but I'm just mentioning it in case it happens to be turned down. By the way what is an MTS amplifier?

Anyway, it just seems like what you are trying to do should work. You should be able to drive your speakers by sending the headphone out signal into an amplifier. So, I'm confused and probably not understanding your setup.. Larry

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Owners of a 2001, 22ft, Rear Door Born Free
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PostPosted: Mon Feb 04, 2008 11:11 pm 
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Larry, I agree that what I've tried should work, but...

Maybe the output from the headphone jack on my tv is just too little and I need some kind of line driver to bring it up to what will work with my theater system amp.

My MTS is distributed by River Park, Inc. and OEM installed by Born Free. When it is working, it is nice - integrates the tv, fm/am stereo w/cd/mp3 player, cd changer, and dvd player, all fed through six surround sound speakers.

The system looks like this (see attachements):


Attachments:
File comment: DHS Mobile Theater Feature Sheet
Mobile Theate Feature Sheet.pdf [85.72 KiB]
Downloaded 466 times
File comment: DHS Mobile Theater RV Troubleshooting Guide
mobile_theatre_rv_troubleshooting.pdf [44.17 KiB]
Downloaded 461 times

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2004 Born Free 22' Built for Two
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 Post subject: Impedance Mismatch
PostPosted: Tue Feb 05, 2008 1:05 am 
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Bill,

I have a solution to your weak audio problem but it will involve some work on your part. Are you an electronics experimenter and handy with a soldering iron?

The cause of you weak audio problem going into your MTS amplifier from the 1/8" stereo headphone output jack on your new TV is what they call an impedance mismatch. Stereo headphones are normally designed with a earphone impedance of 8 ohms so that it what the output audio output circuit of the TV is designed for. The stereo audio input to your MTS amplifier is most likely a high impedance input design of most likely 1000 ohms or more. This is a large mismatch and will cause you the weak audio problems you are experiencing.

The solution is an impedance matching network built using two 8-ohm to 1000-ohm impedance matching transformers. I had to design and build such a network to solve the same problem I had trying to amplify the stereo headphone audio output from my Windows XP laptop to drive the stereo audio amplifier in my Epson media projector.

Below is a photo of the network installed in a small 3"x2"x1" plastic case. Below that you can download a .pdf file copy of the schematic that shows all parts required that can be obtained from Radio Shack.

Qty 2 P/N 273-1380 Audio Matching Transformer: http://www.radioshack.com/product/index.jsp?productId=2103254&cp
Qty 2 P/N 274-248 1/8" Stereo Phone Jack: http://tinyurl.com/3d3ftc
P/N 270-1801 Plastic Enclosure: http://www.radioshack.com/product/index.jsp?productId=2062279&cp
P/N 42-2387 1/8" Stereo Phone Plug to 1/8" Stereo Phone Plug Cable (6' long): http://tinyurl.com/26c3lv
P/N 42-2550 1/8" Stereo Phone Plug to 2 RCA Phone Plugs Adapter Cable (3' long): http://tinyurl.com/33sfqx


Attachments:
File comment: Click on the blue Download button to the right to download a printable copy of a schematic for the Audio Impedance Matching Network
Audio Impedance Matching Network.pdf [4.21 KiB]
Downloaded 255 times
File comment: Photo of Audio Impedance Matching Network including both Interface Cables
Audio Impedance Matching Network.jpg
Audio Impedance Matching Network.jpg [ 129.04 KiB | Viewed 6524 times ]

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Bill Hemme - Spencer, Iowa
E-mail: whemme@earthlink.net
2002 Born Free (Ford E-450 V10) 26' RSB
2005 Chevrolet Malibu LS - toad


Last edited by whemme on Thu Feb 07, 2008 2:33 pm, edited 6 times in total.
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PostPosted: Tue Feb 05, 2008 1:10 pm 
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Bill, thanks for joining in and for the excellent description of my problem and recommended solution. The links and images are a big help, too. I appreciate all the effort that went into your post.

After some research of my own, I was thinking that what I needed was a line driver to increase the voltage between the source (headphone jack) and the load (system amplifier). But matching the impedance between the source and the load as you recommend may have the same effect by increasing the signal power (wattage) from the source.

Since your solution is a passive design (not requiring external power like a line driver does) , I think I'd like to try it first. However, I'm not very adept at soldering, etc., so if such a transformer is available commercially, I'm more inclined to purchase one than build it. I'll look around and see if I can find one.

Thanks again, Bill

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2004 Born Free 22' Built for Two
towing a 2008 Smart ForTwo

Escondido, CA


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PostPosted: Tue Feb 05, 2008 3:28 pm 
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Well, I just got back from some more testing. Funny thing is I can get great volume through the system from the headphone jacks of portable devices such as a transistor radio and an mp3 player, but not from the headphone jack of this LCD flat panel HDTV. Now I'm not so sure it is an impedance mismatch problem, but maybe more of an insufficient voltage problem. If that's the case, then perhaps a line driver that boosts the output voltage is the correct solution. Am I on the right track?

Bill

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2004 Born Free 22' Built for Two
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PostPosted: Tue Feb 05, 2008 4:49 pm 
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Bill.. it just might be there is a problem with the headphone output on the TV or that there is something wrong with the RCA cord/mini stereo adapter you are using.

You can check for signal strength at the headphone output on the TV by plugging in any headphone and seeing if you get sufficient audio volume to your ears.

If you get sufficient audio volume, then the problem may very well be in the RCA cord/mini stereo adapter. If you could find a way to again listen to the signal coming out of the RCA cord with the same headphone you could determine if this was the problem. For example, you could plug a male RCA to female mini stereo adapter into your current adapter. Then you could plug the headphone into this 2nd adapter and listen to the volume. Similarly you could test your current RCA cable/mini stereo adapter with a headphone. For that you'd need a female RCA to female mini stereo adapter. You could put this on the end of your RCA cable and then plug the headphone into that and listen.

I'd also like to mention that I've had some trouble with mini stereo jacks that didn't make a good connection into female mini stereo sockets like the one on your TV. The troubles I experienced were intermittent connections to never working at all. The cause was probably that the mechanical dimensions of the mini stereo jack and the mini stereo socket didn't match up properly.. likely due to a failure in manufacturing tolerances. So, it's also possible this is the problem.

Bill Hemme is correct that impedance matching will maximize power transfer. I'm an electrical engineer (34 years at Hewlett Packard) and have also designed such networks. However, in a lot of years of driving various speaker systems with mini stereo headphone outputs, I've always gotten sufficient signal strength. I've never had an experience like Bill where it wouldn't work without an impedance matching network. But it's certainly possible that Bill is right and you do have this kind of a problem. You might try to see if it's one of the problems I mentioned above before resorting to making/buying an impedance matching network.. Larry

PS.. Bill.. When you get this all solved let us all know and put us out of our misery! Good Luck!

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Owners of a 2001, 22ft, Rear Door Born Free
often seen towing a fishing boat.


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PostPosted: Tue Feb 05, 2008 6:22 pm 
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Larry, let me start by saying that I am a retired auditor/accountant (32 years with the Department of the Navy). As such, I don't know a whole lot about things electrical or electronic (although I worked alongside electronics engineers for a good part of my career :shock:).

rv4fun wrote:
Bill.. it just might be there is a problem with the headphone output on the TV or that there is something wrong with the RCA cord/mini stereo adapter you are using.


I think I can rule out the cord and adapter because with same I got adequate audio volume through my system speakers with both a transistor radio and an mp3 player hooked up via their headphone output jacks.

rv4fun wrote:
...You can check for signal strength at the headphone output on the TV by plugging in any headphone and seeing if you get sufficient audio volume to your ears.


I do get sufficient audio volume to my headphones when plugged into the flat panel HDTV stereo headphone jack.

rv4fun wrote:
Bill Hemme is correct that impedance matching will maximize power transfer. I'm an electrical engineer (34 years at Hewlett Packard) and have also designed such networks. However, in a lot of years of driving various speaker systems with mini stereo headphone outputs, I've always gotten sufficient signal strength. I've never had an experience like Bill where it wouldn't work without an impedance matching network. But it's certainly possible that Bill is right and you do have this kind of a problem. You might try to see if it's one of the problems I mentioned above before resorting to making/buying an impedance matching network..


This may be true, but everything I read says that maybe no so today with solid-state transistor amplifiers.

"The transistor utilizes maximum voltage transfer more efficiently than maximum power transfer. For maximum voltage transfer, the destination device (called the "load") should have an impedance of at least ten times that of the sending device (called the "source"). This is known as BRIDGING. Bridging is the most common circuit configuration when connecting audio devices. With modern audio circuits, matching impedance can actually degrade audio performance."

Which leads me to believe that the headphone jack source voltage from my flat panel HDTV is not sufficient to begin with for anything other than a set of headphones. That's why I think that a line driver that boosts voltage may be the solution. Make sense, or am I all wet?

Bill

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Barb & Bill
2004 Born Free 22' Built for Two
towing a 2008 Smart ForTwo

Escondido, CA


Last edited by bcope01 on Wed Feb 06, 2008 10:24 am, edited 1 time in total.

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 Post subject: Impedance Mismatch
PostPosted: Tue Feb 05, 2008 11:25 pm 
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Bill,

You are right in that you are not getting sufficient audio voltage levels into the two stereo channel inputs to your MTS amplifier from the 8 ohm headphone outputs from your TV. That transformer impedance matching network I posted earlier will result in a significant increase in the audio voltage levels into your MTS amplifier.

The voltage gain will be the square root of the ratio between the matching transformer's secondary impedance over the it's primary impedance.

That ratio is 1000 ohms/8 ohms = 125. The square root of 125 is 11.8. That means that the audio voltage at the output of each channel of this impedance matching network will be 11.8 times the input voltage. That is a increase of 42 db.

Probably just the increase in audio drive voltage to your MTS amplifier that is needed.

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E-mail: whemme@earthlink.net
2002 Born Free (Ford E-450 V10) 26' RSB
2005 Chevrolet Malibu LS - toad


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PostPosted: Wed Feb 06, 2008 10:13 am 
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OK Bill, I'm convinced. I need an audio impedance matching network. Are there also some diodes (?) that I can see in the image of your transformer impedance matching network project box? Also, did you mean Radioshack P/N 42-2387 (vice 42-2420) for the 1/8" Stereo Phone Plug to 1/8" Stereo Phone Plug Cable (6' long)? I think P/N 42-2420 is a mono cable.

Thanks,
Bill

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 Post subject: Posting Mistake
PostPosted: Wed Feb 06, 2008 6:45 pm 
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Bill,

You are sooo correct about me having the wrong P/N for the 6' long 1/8" Phone Plug to 1/8 Phone Plug stereo cable having the wrong Radio Shack P/N - the correct P/N is 42-2387 as you indicated.

Also those 'diodes' in the photo that you see are actually Qty 2 1000 ohm, 1/8 watt resistors that I used in my network. Each of the resistors was installed across the blu & grn 1000 ohm output leads of each of the two transformers. Their use is optional. If the input impedance of the stereo audio inputs to your MTS amplifer is real high such as 10,000 ohms or higher, then it may be a good idea to add those two resistors. However, if the input impedance is lower in the range of 1000 to 5000 ohms, then the resistors should not be necessary. Leaving the resistors out of the circuit will actually increase the voltage gain of the network somewhat.

I will correct my earlier post along with correcting the schematic shortly. Thanks for catching my error.

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Bill Hemme - Spencer, Iowa
E-mail: whemme@earthlink.net
2002 Born Free (Ford E-450 V10) 26' RSB
2005 Chevrolet Malibu LS - toad


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