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Old 5th November 2012, 08:11 PM   #1
RonM440 is offline RonM440  United States
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Default Speaker to Line Level Converter

Anyone have a schematic they could share for an easy to build speaker-out to line-out converter?

Something like this:
Jaycar Electronics
or this:
Amazon.com: XANTECH SLLC1 Speaker Level To Line Converter: Electronics
is what I want to build myself.

The unit will be hooked to a PC soundcard, so I don't really need 35w capacity (as for the commercial units). Seems like it would be an easy PCB project, even for Noob like me.

Thanks for any help.
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Old 5th November 2012, 10:15 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

It is very easy and you can build into the plugs that go into your soundcard.

All you need to do is wire a series resistor and a parallel resistor into the plug.
Typically say 2.7K series and 100R parallel to get a line level from high level.
However reduce the series to say 1K if your running at low volume.

rgds, sreten.
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Old 6th November 2012, 10:08 AM   #3
RonM440 is offline RonM440  United States
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Thanks Sreten, however I don't think that will do what I'm looking for. I need the line-out voltage to remain at a fixed level even if input (volume)is changed.
I'm using this for a home PC-based Jukebox. I want to be able to change the volume to a set of speakers at the jukebox using the jukebox software, however the line-out is to be connected to a FM transmitter, so it needs to remain at a fixed level even if changes are made to the speaker volume.
The FM transmitter just broadcasts what ever I'm playing on the Jukebox so I can also listen with other stereo equipment, such as my WP7 phone while I'm cutting the grass, etc.
I was hoping to find a PC sound-card with a simple analog stereo line-out, but most just have speaker outs now days. I don't need surround, just stereo speaker out, with a stereo line-out. The older cards may not have drivers for Win7 and even less likely for Win8, which is what I'm using for the jukebox PC.
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Old 6th November 2012, 10:39 AM   #4
phofman is offline phofman  Czech Republic
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Soundcards have line-outs, to be hooked to active speakers (with internal amplifier), or higer-impedance headphones.

IIUC what you want is two pair of stereo outputs - one with volume control and another at fixed volume. Any multichannel soundcard would do, with software routing to the second pair of outputs and independent volume control per channel. It would be a simple task in linux, I have no knowledge of windows. Perhaps others will help you with the routing.
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Old 6th November 2012, 11:09 AM   #5
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Seems to me, with the jukebox software as the playback device, and changing the jukebox volume, it will change the volume for everything downstream. Maybe an AGC automatic gain control circuit is what is needed for the FM.
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Old 6th November 2012, 12:57 PM   #6
RonM440 is offline RonM440  United States
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Hi Sofaspud, Yes that is the problem with most current sound cards. They are calling speaker-outs line-outs, when a true line-out is a fixed pre-amp like line voltage (maybe .2v or so) to be used with a down-stream power amp or active speakers. I power the jukebox speakers with an old Kenwood M1. Sounds pretty good going to a set of Optimus Mach 3s. I went with the Mach 3s becasue they have horn mids and tweets, and I covered the 15" woofs with grids. Makes a pretty durable set-up for portability as well, and a lot cheaper than Peaveys or such. Being a jukebox, I just designed it to thump moreso than for fine audio quality.

I think I may have found a VERY cheap soundcard that will work though. It has what they call a AUX Out, which I think may be a real line-out. Don't know how good the sound quality will be, but for $10 thought I'd give it a try. I'm only getting FM quality sound on this side anyway. It has Win7 drivers at least, and hoping it may work on Win8. I may just make this box dual-boot anyway. I like Win8 because I have a 21" Touchscreen monitor on it.

I guess as an alternative I could go with using S/PDIF out, and then a external DAC between it and the FM transmitter. I'm pretty sure that S/PDIF is steady signal being digital. This is a little more costly, but I did find a DAC at NewE for about $50.
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Old 6th November 2012, 01:07 PM   #7
RonM440 is offline RonM440  United States
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Great thinking Sofaspud! I'm sure I can find some schematics for a usable AGC! THANKS!
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Old 6th November 2012, 02:36 PM   #8
phofman is offline phofman  Czech Republic
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Quote:
Originally Posted by RonM440 View Post
cards. They are calling speaker-outs line-outs, when a true line-out is a fixed pre-amp like line voltage (maybe .2v or so) to be used with a down-stream power amp or active speakers.
Actually it is the other way round. They are calling line-outs speaker-outs. They have no internal amplifiers, just a simple buffer made of operational amplifier, often serving as the output LP filter at the same time.

The question of volume control is a different one. IMO it is easier to output full level signal to another pair of soundcard outputs than to play with some AGC down the stream. Or avoid the PC volume control alltogether and feed your active speakers via some 5k potentiometer or use active speakers with built-in volume control.
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Old 6th November 2012, 05:57 PM   #9
RonM440 is offline RonM440  United States
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Yes, I had it back a$$wards.
I think I've resolved my problem by using an old Chaintech AV-710 sound card I had stuffed away in a closet. I actually even got the drivers to work (with the control panel) in Win8. It has physical jumpers that you can set between speaker-out or line-out as well. Also has S/PDIF-out that I can use at the same time.
I think this will be a clean solution for what I need. Thanks all!
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