Audio Level Conversion and Down Mixing - Page 1
Forum section: electronics / projects

author: james2k2
author_rank: Contributor
timestamp: June 14, 2015, 08:52:11 pm
content: Hi All,I have a project idea in mind, and I'm certain it's been done before but can't find exactly what I'm after. Just looking for some direction really.I am looking for converting high level audio output to low level for an amplifier, but in doing so down mixing from stereo to mono without affecting the source.Please see attached a block diagram of what I mean. I'm not looking for a schematic right now (unless somebody has one already), just if what I want to do is possible. A few parameters:Output is 40W/channel, impedance is 4 Ohms on the two main speakers. Obviously my other choice will be to convert the high-level to low-level and then run two amps, but I'd rather not as this will introduce excessive costs for the benefit.Thanks in advance.

author: fcb
author_rank: Super Contributor
timestamp: June 14, 2015, 09:18:00 pm
content: One way of doing it is to use transformers, and then mix the output of the transformers (passively).Another way is to use a differential amplifier on each speaker, then mix the outputs. You'd need to pay attention to system grounds etc...

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 14, 2015, 11:29:19 pm
content: There are very simple and inexpensive solutions to this problem.But a major issue is the exact nature of the source.Specifically, are those speaker outputs GROUND REFERENCED? or are they BRIDGED?Now, you may not know the answer to that question, but if you can reveal the make/model of the amplifier, we can probably look it up.This sort of thing is done all the time in auto sound installations where they take the speaker-level signals and convert them to line-level.And combining L+R to create monaural is as trivial as a couple of 5-cent resistors.

author: moffy
author_rank: Super Contributor
timestamp: June 14, 2015, 11:48:02 pm
content: For a robust and easy solution use the transformers suggested by "fcb" but you can reduce the signal level into them with a couple of resistors, that way you can use standard signal level transformers.

author: Mike Warren
author_rank: Unknown
timestamp: June 15, 2015, 12:10:54 am
content: These are a very common and readily available item.Look up "speaker to line level converter".Make sure to get a transformer coupled version, then you can mix the 2 channels by shorting them, and if you want to keep the speakers connected I recommend you open it up and remove the load resistors.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 15, 2015, 12:16:03 am
content: Quote from: Mike Warren on June 15, 2015, 12:10:54 amYes. And while you have it open, you can add a couple of resistors (likely different value than what you are removing) to sum the L+R together to create monaural.The simple and inexpensive solution to summing L+R stereo into monaural depends on knowledge of the source and load impedances. Once you know those, the solution becomes very simple.

author: james2k2
author_rank: Contributor
timestamp: June 15, 2015, 05:10:05 am
content: Wow! Thanks all so far for the great response. As it happens, this is an auto (car) installation. Basically, my car actually has a mounting hole in the boot for a sub and amp as there is a Bose option, but unfortunately it's one of those that's factory only - that and I can't find second hard anywhere. I believe the Bose option uses a different head unit, but this isn't really an option either as it ties to the integrated sat nav that uses a separate display higher in the dash.Any way, back to topic. Looking at amps for the job, it's just occurred to me that they have a bridge option built in. So rather than having the high->low converter do this job, just literally build my own high->low with high impedance close to 10K? My estimations suggest this should be enough to leave the source signal mostly in tact with the system impedance being 3.99 Ohms. Is 10K a good amount to do the level conversion, or should it normally be higher/lower? Thanks again everybody!

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 15, 2015, 05:46:23 am
content: Quote from: james2k2 on June 15, 2015, 05:10:05 amNot sure what that means. Do you mean the the OUTPUT of the amp has a bridging option? If that is what you mean, it has nothing to do with your original question. Or does the amp have an option to take L and R stereo inputs and sum them internaly to monaural? Note that is a COMPLETELY DIFFERENT thing than the amp having a bridged output option.QuoteDo you mean "high-level to low-level"? Or do you mean "high impedance to low impedance". Please be more precise so we can all stay on the same page.QuoteNot clear why you are converting impedance at all? This is a signal voltage level problem that has almost nothing to do with impedances. If your source is a power amplifier, then it has a VERY VERY LOW output impedance (in order to drive speakers). And the input to your sub-woofer amplifier will have a rather high impedance input. An impedance of the kind most power amp inputs have will have virtually zero affect on the source (the very very low output impedance amplifier).QuoteWhat does "system impedance mean"?I would simply use a potentiometer (pot) to reduce (and simultaneously CONTROL) the attenuation. Trying to pick fixed values for the attenuator just seems like a lot of work for nothing, IME. What are you doing for a filter (to send ONLY bass frequencies to the sub-woofer)? That seems like a much more significant issue than impedance or even signal voltage levels.IMHO, you are "majoring in minors" and skipping right over the more significant design issues, namely: (1) whether the source is bridged/floating or ground-referenced, and (2) how you are "crossing over" the signal to send only bass frequencies to the sub-woofer amplifier.

author: james2k2
author_rank: Contributor
timestamp: June 15, 2015, 07:20:19 am
content: Thanks for your reply. I'll do my best to answer your questions.For info, the amp I'm looking at (for sub use) is an FLI FL450S.The amp has a stereo input, and then the output can be stereo or bridged. The amp also has a built in crossover with adjustable LFE. I cannot answer if the bridged output option will cause the amp to convert stereo to mono, or if it will simply take a differential, or even just say the Left channel.High-level I'm referring to is the output to a speaker, whereas low-level is equivalent to line-level. So my conversion is from (in my case) a 40W/ch amp to a level that is acceptable to use as a line-input.When I'm speaking of impedances, I'm referring back to parallel resistances. My head unit amp is designed for a 4 Ohm load per channel (this is what I mean by a system impedance), and if I'm using a resistor network or some form of other design to do the high-level to low-level conversions described above, I would have thought this will also have some form of impedance as well. All I was asking for here really is if the resistance of that converter is high enough to not change the 'system impedance' and therefore still able to drive the original rear speakers I'm planning on tapping from.On that note, I have just seen another photo of that particular amp and it has built-in high-level inputs, but again, can I tap from the speaker outputs and leave the original speakers in place, or would the the conversion affect the load too much and cause my head unit amp problems?Finally, the source I could not tell you how it's designed, sorry.

author: calexanian
author_rank: Super Contributor
timestamp: June 15, 2015, 02:38:20 pm
content: D.I. Boxes with speaker level Inputs are easy to find. Get two and then sum the outputs together. You can simply make up an adaptor cable with a couple 600 ohm resistors directly tying the two channels respective hot and cold conductors together Via the resistors and there you go. This is a common solution to get your signal to drive your remote line speaker system when the stage material is in stereo and all you have are remote stage speakers to pull a signal from.

author: james2k2
author_rank: Contributor
timestamp: June 15, 2015, 02:50:29 pm
content: Thanks for your informative reply calexanian!It will definitely come in use if my first idea fails. I've actually gone and bought a different amp to my original spec, as I'm simply not needing the level of power that other amp was offering. Just for info really, all I'm wanting is to add a bit of depth to my music, nothing over the top. I've bought a Sony Xplod XM-SD12X. And as it happens, the manual shows exactly the design I'm after, all built in!http://pdf.crse.com/manuals/2586733111.pdfDiagram C shows the set-up I'll be using. It has all the level conversion built in, with adjustment. It also supports stereo to mono conversion for sub use as pictured in Diagram 2. Bagged the unit for only £15 (~$25USD) delivered.What do you think?

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 15, 2015, 04:38:11 pm
content: Wow, you got it all. All the built-in functions you need at a very low price!

author: james2k2
author_rank: Contributor
timestamp: June 15, 2015, 07:21:23 pm
content: My thoughts exactly, providing I can now find a sub to fit the space I have available, I should be good.Annoyingly, the design only really allows for the Bose module, or a super slim one costing in excess of £100. As there's only around 600 of my car in the UK and even less of them have the Bose upgrade... finding the sub is proving challenging. :-(Thanks again everybody who has input. I'll no doubt be back if my plan fails! :-)