High and low level inputs

dayneger · « **on:** 29 Mar 2003, 04:57 pm »

I've been thinking about making a simple amp for a bedroom sub, probably based on one of the designs for National's LM3876 or something similar (suggestions welcome!).

What's the difference, electrically-speaking, between high and low level inputs? For this projekt I'd definitely need high level inputs, but have no idea what I'd need to add to the circuit to handle this (I've built an AKSA, but unfortunately have no EE skills of my own

).

Thanks! Dayne

mgalusha · « **Reply #1 on:** 29 Mar 2003, 08:21 pm »

Dayne,

It seems like you are talking speaker level (high level) vs line level (low level) inputs.

The big difference is that line level is usually about 2 volts (full volume), while speaker levels are much higher, 1 watt into an 8R speaker is 2.83 volts and of course goes up from there.

This page has a circuit that should meet your needs: http://www.hut.fi/Misc/Electronics/circuits/speaker_to_line.html

HTH, mike

dayneger · « **Reply #2 on:** 30 Mar 2003, 07:56 am »

Hi Mike,

Thanks, that helps a lot!

That should work great for the bedroom system, with a trimmer in the R2 position to adjust the level to the main speakers.

The site you referenced mentioned:

"Remember that this circuit is designed only to be used with normal amplifiers which have common ground for speaker signals. It is not suitable to be used with bridge type output car stereo amplifiers. "

I was planning on making one sub for the bedroom and one sub for the car. Do you know what the difference here is with the car amp

mgalusha · « **Reply #3 on:** 30 Mar 2003, 04:39 pm »

Most amplifiers have a common ground in that the both of the negative speaker terminals are connected to the amplifiers ground. Amplifiers designed for bridged operation have one negative connected to ground and one of the positive terminals connected to ground.

Normal amp: + --__gnd__-- +

Bridged amp: + --__gnd__+ --

This allows the channels to run in series, increasing the output power. What the article is warning about is that if you connect the speaker outputs to a premap input (or sub input), nearly all of them have a common ground. Obviously connecting the two negative posts on a bridged amp together will short out one of the channels.

mike