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Headphone adapter for the MP790

Brain_Recall
Brain_Recall Page Icon Posted 2005-02-11 11:13 PM
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This is a quick explanation on how to build an adapter to use a standard set of headphones on the MP790.

The MP790 is unique in the fact of its headphone jack. The jack itself is a combo-jack that allowed the use of a propritery headset, both with speaker and microphone.
The jack requires a stereo sub-mini, 3/32", plug. See this page for an example: http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%...
Standard headphones fit into a stereo mini, 1/8", jack. See this page for an example: http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%...

The MP790's jack outputs in mono only. Each stereo plug is segmented into three sections. The largest section, is towards the cord. The smallest section is at the tip, and there is also the middle. Jacks are segmented interally the same way. Depending what kind of components you got, it may take a little prodding with a multi-meter to determine which wire connects to which. (Check the pictures in the above links to get an idea of what I am talking about.)

So, our connections are like this: connect the large-segment of the 1/8" jack to the large-segment of the 3/32" plug. Then, connect the middle AND the tip portions of the jack to ONLY the middle of the 3/32" plug (We do this so that both left and right channels of our headphones are driven from the one mono source).

I used a jack ripped from an old Gateway telephony modem and a plug from a failed TI Graph Link project (all TI calculators use the 3/32" plugs/jacks). My method for figuring the above out was trial and error. You can do this too, since you won't cause any damage if you miss-cross a wire. Here's a very, very bad scan of my adapter: http://mars.walagata.com/w/brainrecall/adapter.jpg I used hot-glue and heat shrink as a strain-relief.

Please ask if you can't understand any of this or if you have a question!
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Snappy! Page Icon Posted 2005-02-12 9:01 AM
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Gee ... this is nice. Hmmm ... I'm thinking maybe one of the prongs or something in the jack of my MP790 is shorted. My headphones does not have any sound and my recording has a buzzing sound in it.

I think I will go and check out the jack and see if something is shorted.

btw, do you know the size of the AC power adapter's jack? there is the internal and external diameter.
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Brain_Recall
Brain_Recall Page Icon Posted 2005-02-15 9:15 AM
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You, or more likely me, might have the wires crossed. Try switching them around and make sure not to have them touch. I haven't tried recording with the adapter in place, but I can understand why it's giving static. The tip is one part for the mic, the other part is a clip on the side of the socket. Kinda a wierd system, but it seemed to work.

As for the jack, this is about the closest I could find for it.



The supply is supposed to be 9v with a current rating of 1 amp.

I myself am using a modified supply from my 680. The cord on it must of been rolled over a few times by an office chair, and the connector was pretty much shot. I cut it off, and searched around the house for a plug that fit the 680's docking station and the 790. (The brick for the 680 is rated 12v at 2 amps. This isn't a concern, though, since the 790 has an internal voltage regulator that could take upwards of 24v without complaining.)
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Snappy! Page Icon Posted 2005-02-15 9:45 AM
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Gee ... this is good to know, that the MPs will take voltage above 9v up to 24v (??)

I got a DC plug around O.D. 5mm. I.D. 2.5mm ... that is after measuring rather inaccurately with a ruler the NEC DC adapter plug ...

I hope its correct, as I will be wiring it up with a 8XAAbattery holder and using that as a portable charger. The 8XAA will give 1.2*8 = 9.6v ... so I'm glad you mentioned about the internal voltage regulator!

9v@1A ... the 2400mAh batts should give me a charge of 2hrs ... almost enough to fully recharge the MP790.
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Brain_Recall
Brain_Recall Page Icon Posted 2005-02-15 10:36 PM
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Well, lets say it could take 24v, but I'm not willing to test it.

As for the batteries, make sure they can stay above 8v under load. That's the minimum the 790 needs to charge the battery, otherwise it simply won't. (charging Li-Ion is a little tricky, but the 790 will stop charging if it doesn't have the available power)
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stingraze Page Icon Posted 2005-02-16 2:15 AM
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I've built a 2.5mm to 3.5mm converter myself with some simple noise filter.



In case you don't know, the black part is the capacitor, and the tan one is another capacitor.



(DSC04822.JPG)



(Dsc04824.jpg)



(Dsc04825.jpg)



Attachments
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Attachments DSC04822.JPG (7KB - 59 downloads)
Attachments Dsc04824.jpg (18KB - 54 downloads)
Attachments Dsc04825.jpg (20KB - 56 downloads)
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Snappy! Page Icon Posted 2005-02-16 8:32 AM
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Brain_Recall - 2005-02-15 8:36 PM

Well, lets say it could take 24v, but I'm not willing to test it.

hehe ... it would be really toughie to try and explain that to NEC or any repair shop!!

Then again, I could always say "ok, I plug in a 24VDC@1A power source ... so eat me!" ... but I think that will so void any warranty I have left!!

Brain_Recall - 2005-02-15 8:36 PM

As for the batteries, make sure they can stay above 8v under load. That's the minimum the 790 needs to charge the battery, otherwise it simply won't. (charging Li-Ion is a little tricky, but the 790 will stop charging if it doesn't have the available power)


ic ... minVDC ~=8VDC. I hope the batts can take it. I have a bad feeling about those 2300mAh NiMH AAbatts. From my exp with solar cells and Marine Deep Cycle SLA (Sealed Lead Acid) batt, the Ah or mAh are usually pegged at a certain discharge rate. ie 20hr discharge or something. This translate to a certain amount of load current that produces that amount of available power. eg a 100Ah@20hr SLA should translate to 100/20=5A of load running for 20hr. This is the sweet spot that is rated ... ie if you run on a load less than 5A, you may get more than 100A of current from the batt or be able to run for 20 hours or more (usually lesser), but if you run at say 10A instead, you can be guarranteed not to get 10 hours of batt life. More likely you will get only 6~7hrs if at all.

In that sense, I saw this article where NiMH AABatts are tested and shown that it looses its charge fast than expected after sometime when running at 1A current draw. As a result, its actual power throughput is less than the expected amount given for 2300mAh batts. Its actual power turned out to be more equivalent to 1700mAh batts.

ok, some of you might be wondering how power can be equated to mAh, (P=IV??) hehe ... well, the way, it was calculated is to take integrate the (actual voltages of the cell terminals when under load * the current (which stands at around 1A)) to give the power given. The total sum of power provided is then used to calculate the current drawn I=P/V () ... then this current is divided by the duration of load test to find out the TRUE mAh rating.

Well ... after all these talk, it boils down to testing out the batt pack to see if it holds up! ... Will update you guys on it later ...
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Snappy! Page Icon Posted 2005-02-16 8:39 AM
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stingraze - 2005-02-16 12:15 AM

I've built a 2.5mm to 3.5mm converter myself with some simple noise filter.

In case you don't know, the black part is the capacitor, and the tan one is another capacitor.


Coolie!! ... hmm ... so is it a RC filter or something? I kinda skipped too much basic electronics class back in college to remember enough to make sense!!

Say, how about getting a heat shrink wrap thingie to cover up the parts? ... then you have a nice inline filter.

Suggestion, while you are at that, why not add a cheap rheostat and you have volume control too?

MP790-=====---------------------[RCFILTER+RHEOSTAT]---------------+----O
2.5->3.5 |
+-------------O
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stingraze Page Icon Posted 2005-02-27 6:13 PM
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Actually, it's more like a low pass filter
Like you said, I think I'll make a rheostat with a variable resistor that's lying around in my junk parts stock


Now, I must get on with my homeworks....

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abyssknight
abyssknight Page Icon Posted 2005-07-29 9:43 PM
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Could someone explain this to me?

I just bought a 790, but I don't know much about electrical stuff...

Anyone care to enlighten a newbie?

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abyssknight
abyssknight Page Icon Posted 2005-07-30 12:14 AM
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btrimmer Page Icon Posted 2005-07-30 2:57 AM
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abyssknight - 2005-07-30 12:14 AM

Would this do the trick?

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5...

Not quite that easy. The part you linked is a mono 3/32" to 1/8" adapter. You'll need a 3/32" (2.5mm) to 1/8" (3.5mm) STEREO (Ring-Tip-Sleeve, or RTS for short) adapter, plus some other things. Why? Because the headphone port on the MobilePros is actually a Headphone/Mic combination jack. Headphones take one channel, microphone takes the other.

[Actually, the MobilePro jack isn't even RTS; it's a 4-conductor RTSC plug. Sleeve is common ground, ring is audio out, tip is audio in, and the collar is record control. See that little brassy bit on the outside of the headphone connector on your MobilePro? It actually does something... If you short that brass connector to ground, apparently the recording software automatically opens on your MobilePro. I have not tried this myself.]

Basically, you have three choices for using headphones with your MP780/790:

1) Build and adapter, as people have described in this thread.

2) Buy a wired Cell phone headset. Most cell phones use a 3/32" RTS jack for wired headsets, which should be compatible with the MP780/790 headset jack. Just make sure the RING condictor is the headphone channel, not the TIP. Otherwise you're trying to play sound out the microphone and bad things happen to the mic when you do that.

3) Buy multiple off-the-shelf adapters and stack them up, in the following order:

(A) 3/32" to 1/8" Stereo adapter
(B) 1/8" Stereo to Left/Right 1/8" Mono adapter
(C) 1/8" Mono to 1/8" Stereo adapter

i.e (EDIT: ASCII grafiks r teh sux0rz)

(MobilePro) <--- (A) <--- (B) <--- (C) <--- (Headphone)
(Microphone)---------------^


(A) is necessary because I have yet to find a 3/32" Stereo to 1/8" Mono L/R adapter. (B) takes the two channels (normally left/right for a stereo signal, but for the MP780, it's audio out and audio in) and splits them in to two separate jacks. So now you have a separate (mono) headphone jack and (mono) microphone jack. (C) takes the mono signal and sends it to both channels of stereo headphones. This is technically optional, but without (C) you only get sound out of one ear.

I will warn you, though, that while this option works, it places a good deal of strain on the jack on your 780/790. You basically have 3" or so sticking out the front of your unit, pulling down on the jack. It might even be better to use a short 1/8" RTS extension cable, just to take some of the weight off. (I'll have to try that now that I think about it. )

If you want to buy the parts at Radio Shack,

Part (A) is Radio Shack #274-373:
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%...

Part (B) is Radio Shack #274-375:
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%...

Part (C) is Radio Shack #274-882:
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%...

Edited by btrimmer 2005-07-30 3:08 AM
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abyssknight
abyssknight Page Icon Posted 2005-07-30 8:43 PM
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So, if I understand this correctly... as long as I don't plug a mic into that plug for testing, I should be safe, because the mic is the only thing that could get fried?

I may end up learning some basic electronics, as stacking all of those adaptors might be a bit of a weight, as well as a pain.

How is the audio anyways?
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