Need to connect high power stereo to device expecting headphone level. - Page 1
Forum section: electronics / projects

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: May 06, 2015, 09:47:16 pm
content: I have a bought a RF audio transmitter/receiver kit so I can listen to music on my deck from the stereo in the house. Could not run speaker wires.The receiver outside works perfectly running the speakers on the deck with a phone connected to transmitter via the supplied cord with 1/8" plugs on each end.But I wanna connect the input of the transmitter to my 100w stereo amp without blowing up it's input or shorting out the neg speaker terminals on the amp. Need to have speakers connected in the house going simultaneously and would prefer a use a passive circuit if possible. Looked around the net and nothing grabs me.Any ideas?

author: David_AVD
author_rank: Super Contributor
timestamp: May 06, 2015, 11:03:44 pm
content: Almost all stereo amplifiers (of the hi-fi variety) will have the speaker negative terminals are signal ground potential.A typical hi-fi amplifier uses a series resistor (often 330 Ohms, 2W) from the L & R speaker output +ve's to the headphone socket tip and ring. The headphone socket sleeve is signal earth.This forms a simple divider with the headphone's impedance (in the range of 8 - 32 Ohms usually).You can do something similar by using 10 Ohm resistors in place of the headphones and suitable "upper" resistors to give you the level you want.For example, 100W RMS into 4 Ohms is 20V RMS at the speaker terminals. ( sqrt of (100 * 4) )Now, (20V / (10R + 330R)) * 10R is about 0.6V RMS at the junction when the amp is running at full tilt. You can adjust the resistor values for other outcomes.Make sure you calculate the dissipation in the upper resistor as in my example it would be a bit over 1W.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 06, 2015, 11:35:02 pm
content: If you had revealed the identity of your 100W amplifier, we might have something to work with.Nevertheless, there are a couple of possibilities:1) Use a speaker to line-level converter gadget. They are trivial and easy to DIY, or you can buy them cheap.http://forum.blu-ray.com/showthread.php?t=1625732) Most traditional stereo receivers have a "Tape Out" or some other line-level output before the power amp section.

author: IanB
author_rank: Super Contributor
timestamp: May 07, 2015, 12:09:55 am
content: If the transmitter has a microphone input then I think it will be expecting a very low voltage input signal. Even a "line level" input is probably going to be too much for it.As mentioned above, giving make and model numbers of all items involved would enable more precise answers and avoid guesswork.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 07, 2015, 01:38:19 am
content: Many modern amplifiers are full bridge designs. Therefore, you'll have to use the chassis as ground.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: May 07, 2015, 02:00:18 am
content: Quote from: NiHaoMike on May 07, 2015, 01:38:19 amThis is the concern I have. It's a modern Denon brand amp. Three or four years old. Don't have the model number with me. Will google and see if I can find it. Only really wanted to know how to do this with -any- amp.Quote from: IanB on May 07, 2015, 12:09:55 amThe transmitter appears to be designed to accept the headphone signal from a mobile phone(cell phone or tablet) not microphone. Sorry for not making that clear.http://www.jaycar.com.au/Sight-%26-Sound---Home/AV-Transmitters/2-4GHz-AV-Senders/Stereo-2-4GHz-Digital-Wireless-15WRMS-Audio-Amplifier-System/p/AR1895Looks as though I'll need audio coupling transformers and some resistors to accomplish this. Thanks guys.

author: David_AVD
author_rank: Super Contributor
timestamp: May 07, 2015, 02:16:42 am
content: Quote from: NiHaoMike on May 07, 2015, 01:38:19 amIn general that's true, but not for the majority of (relatively) low power hi-fi amplifiers in my experience.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: May 07, 2015, 02:55:23 am
content: Quote from: David_AVD on May 07, 2015, 02:16:42 amGood to know.But we cannot really know from the model number. I'll have to check to see if the neg terms are ground on the amp with a MM next time I'm there.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 07, 2015, 05:25:42 am
content: If you can look through the top, see if you can count the number of output inductors. Full bridge will have at least two per channel while half bridge will probably only have one. Also beware that full bridge designs (and even some half bridge designs) generally have the outputs floating at about half the DC bus voltage above ground, which can be mitigated with series capacitors. (Note that you should check for bias voltage with something actually playing since some amplifiers power down the output stage with no input signal.)

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: May 07, 2015, 07:23:29 am
content: Quote from: NiHaoMike on May 07, 2015, 05:25:42 amThought about using series caps but figured, with the resistors, that I would have some nasty filter network in there.

author: JuKu
author_rank: Frequent Contributor
timestamp: May 07, 2015, 10:34:23 am
content: Quote from: David_AVD on May 06, 2015, 11:03:44 pmCaution! Very few modern amps have this. Some do, but verify before trusting neg. terminal = signal ground.

author: David_AVD
author_rank: Super Contributor
timestamp: May 07, 2015, 10:53:04 am
content: Quote from: JuKu on May 07, 2015, 10:34:23 amWe must work on different hi-fi amplifiers.

author: JuKu
author_rank: Frequent Contributor
timestamp: May 07, 2015, 11:04:45 am
content: Maybe. On digital amps, neg. terminal is seldom ground, and analog, it usually is. I see more and more digital amps nowadays, and I'm rather sure a newish Denon would be one.Another caution: Digital amps might expect regular loudspeaker load. For example, an amp I tried to measure a while ago went into protection when the negative terminals of L and R channels saw each other trough a 50k resistor (not sure about impedance or hf properties of said resistor, though).

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 07, 2015, 02:43:36 pm
content: Quote from: Ed.Kloonk on May 07, 2015, 02:00:18 amThere are many generic questions that do not have generic answers, at least not in the Real World.If you have a bridged (non-ground-referenced) output, you could use an isolation transformer to solve the problem.http://www.radioshack.com/ground-loop-isolator/2700054.html#q=isolation&suggest=1&start=7

author: paulie
author_rank: Frequent Contributor
timestamp: May 07, 2015, 05:04:01 pm
content: Bridged or single end, simple resistors will suffice with virtually any amp out there. Your transmitter has fixed input impedance so a resistance 5x or 10x higher would be a good starting point.

author: SeanB
author_rank: Super Contributor
timestamp: May 07, 2015, 05:48:27 pm
content: If it has a headphone socket then it will have the socket as the common, and most likely at ground. Otherwise you will find on the back panel a tape record or line output, which is basically the speaker audio before the amplifier, already at line level and perfect for the transmitter, all you need are 2 RCA connectors and the end for the transmitter. That way you can control the outdoor volume separately from the indoor, and even turn the indoor to zero and still have the outdoor working.