solderdude
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Post by solderdude on Jan 9, 2015 12:27:09 GMT
As mentioned before 2W is already more than sufficient.
If you want to be anal about resistor quality (and the price difference isn't that much really) then you should go for the metal-oxide versions. Any cheap 2W resistor will already be good I suppose.
Perhaps when you order 2 different value resistors from the same seller you could save on shipping.
If you have enough room to house the 10W resistors and they are the cheapest ones than these will do as well. In any case the amplifier won't be delivering more than 1W per resistor even at full output power.
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sekar
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Post by sekar on Jan 10, 2015 23:08:43 GMT
I've finished assembling and soldering my adapter. What about very first impressions they are positive. Noises and a bit buzzing have gone or just at some practically inaudible level. I made two separate groups of inputs for balanced XLR output and for 1/4" socket to provide scheme as straightforward as possible without any internal commutation. K601 shines now with it's large soundstage. I may put pictures if anybody interested in that subject. Thanks to Frans for help.
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pete
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Post by pete on Mar 8, 2016 18:15:25 GMT
I came to the article via a Google search and then followed the link to this thread - hope it's still ok to use it to ask questions so much later.
I've just ordered a board that combines a signal generator with an amplifier - it's actually intended for alarm systems although I don't think that matters. The crucial part is that it has no volume control, and it's rated at 60W into 4 ohms.
I need to use it with a speaker that's already installed up a mast, rated at 30W, 4 ohms. When I ordered the board I thought this would be ok, that it would be like running something off a higher-current power supply where the device only pulls what it needs. But I've since realised that this is probably a mistaken understanding, and the extra power (remember, no volume control) would damage the speaker.
Can I solve this with a high-power resistor or two? Either in series with the speaker or set up as a voltage divider? If so, what resistance would you recommend? It would be difficult to replace either the board or the speaker, so I don't want to risk damaging either of them.
Thanks,
Pete
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solderdude
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Post by solderdude on Mar 8, 2016 21:12:21 GMT
Hi Pete,
Well the generator will supply 15.5V into a 4 Ohm load. To do so it will probably have 24V power supply rails (or +/-24V depending on the circuit)
What you could try is reduce the power supply voltage.
You will need to have around 11V (into a 4 Ohm load) For this a power supply voltage of 17V (or +/-17V depending on the circuit) would do.
However I have no idea how the tone generator will deal with this. That was option 1, lowering the power supply voltage.
Option 2 is to find where the tone generator output is and the input of the power amp circuit is and attenuate that signal so the amp doesn't reach 15V any more but reaches 11V instead. This takes some knowledge of the circuits but would be the safest way.
Then option 3 is to add a resistor in series with the speaker. you will have 15.5V and need 11V so you need to drop 4.5V over the resistor and then end up with 11V over the speaker. The speaker is known to be 4 Ohm but can be a little higher or even lower. Lets assume it is 4 Ohm. To drop that voltage the series resistance needs to be 0.41x that of the speaker. So 1.6 Ohm.
This isn't a standard value but 3.3 Ohm is and when you have 2 of those in parallel you end up with 1.65 Ohm. There will be 4.5V over the 1.65 Ohm = 2.7 W. As the load is spread over 2 resistors each one will dissipate 1.35W.
So you will be safe when using 2 resistors of 3.3 Ohm in parralel with a rating of 3W each (5W or 10W is also fine).
The amplifier will 'see' 5.6 Ohm load (better) the speaker will have max 30W on it and the resistors won't become that hot.
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pete
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Post by pete on Mar 8, 2016 22:45:38 GMT
Thanks for the reply! This is the board in question: www.ebay.co.uk/itm/OSD-23-Dual-Tone-Siren-Driver-Board-Bell-Steady-269-2152-/321952679933?hash=item4af5e007fd:g:rp0AAOSwCQNWdDAnIt takes a nominal 12v input; the fact that it actually states up to 15v is good for me as I know my system gets up to 14.8 when the battery bank is charging. The application is a maritime signalling horn, so it's unlikely to run for more than a few seconds at a time. But as loud as possible is good I don't think lowering the input voltage is a good approach, especially given the variation between charging and resting (14.8 and, say, 12.5). I'm quite happy to go the external resistor route. Or, given that the board looks pretty chunky and old-school, it should be quite easy to modify it (your option 2) - but I'm not sure which bit would be the tone input to the amplifier portion. Thanks, Pete
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solderdude
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Post by solderdude on Mar 9, 2016 12:48:05 GMT
It seems to be a bridged amplifier that's why it works at 12-15V and can still supply a high AC voltage
Adding the 1.6 Ohm resistor in series may be the easiest way.
instead of 2x 3.3 Ohm in parallel a 1.8 Ohm or 1.5 Ohm resistor may also be enough. a 5W rating should do.
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pete
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Post by pete on Mar 11, 2016 11:56:08 GMT
Thanks again - I've bought a pair of 5w 3.3ohm resistors which I'll connect in parallel, in series with the speaker. Just waiting for the board to arrive from the US now Pete
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Post by caracciolo1961 on May 9, 2016 13:40:08 GMT
Hello, I would like to connect the cash chopin sp-1680 3.5 W RMS www.cobraspa.eu/en/Loudspeakers/chopin.html to my desktop via the connectors line-in or mic-in to record the program. Can you tell me the resistance values R1 and R2 Thank you
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solderdude
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Post by solderdude on May 9, 2016 14:28:56 GMT
Speakers should be connected to the headphone out or line-out of the desktop amp.
Where do you need the line-in for and what do you want to record ?
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Post by caracciolo1961 on May 9, 2016 18:18:56 GMT
Chopin sp-1680 2x3.5 W RMS is a special stereo radio. On my desktop runs MIXXX program (it is a virtual dj). MIXXX must listen to all radio programs and then send them to the streaming server. To do this I need to connect my radio to the input sound card of my desktop with the blue stereo connector (Line-in) or in the worst cases with the pink mono connector (Mic). Sorry for my English
thanks
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solderdude
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Post by solderdude on May 9, 2016 18:47:52 GMT
So you have the line-out of a radio connected to the line-in of your desktop. Or do you have a radio with speaker-out and do you want this connected to the line-in ?
Is the line-in currently overloaded (clipping/distorting) when connected to the line in ?
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Post by caracciolo1961 on May 9, 2016 18:58:22 GMT
i have a radio with only speaker-out and i want this connected to the line-in
thanks
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solderdude
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Post by solderdude on May 9, 2016 20:42:28 GMT
Is it a portable radio with a headphone out ?
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Post by caracciolo1961 on May 9, 2016 20:44:06 GMT
no only speaker-out
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solderdude
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Post by solderdude on May 9, 2016 20:52:20 GMT
As you do not plan to drive a headphone but only want to attenuate the signal to an optimum level I suggest you use a 1k or 2k or even 5k potentiometer. With it you can adjust the radio to a decent level without distortion and then adjust the input level to the soundcard with that potmeter. The radio doesn't have to 'loaded' with a speaker alike resistance value.
A stereo potmeter or 2 single potmeters will do. Could be a log type or even a linear type. Isn't that important here.
You can also use resistors. As I have no idea of the output level of the radio itself and how well it sounds at any given volume choosing the resistors could be guess work.
The one to ground may have a value between 47 Ohm and 1k for instance with the resistor from the output of the radio to the input of the line-in of 1k Ohm.
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