Adding microphone to line input

[4pyros]

You have a problem if you have a stereo amplifier but only one sub speaker. The sub needs to have its own mono amplifier because you cannot connect the left speaker and the right speaker outputs together to get mono for the sub.

You have another problem because you do not have a low level signal to feed the amplifier for the sub, then your stereo speakers will also play (and maybe get damaged) the very low and powerful sub frequencies. The stereo amplifier usually has highpass filters at its inputs.

I put together a crossover circuit that feeds the inputs of your three power amplifiers:

That seems to discount the passive subwoofer crossovers in the link I posted.

 

[audioguru]

That seems to discount the passive subwoofer crossovers in the link I posted.

The passive crossover circuit you posted uses an inductor with an iron core that has resistance, is not linear and saturates. Its low impedance HUGE electrolytic capacitor has poor tolerance.
Since it is second order then with matching second order highpass filters for the stereo speakers it results in a notch at the crossover frequency or a 3dB peak.

Each stereo speaker needs to have its own highpass filter as part of the crossover system. A mixer circuit is needed to feed the additional subwoofer amp and the active line level crossover circuit is the best way to do it.

 

[audioguru]

Double post deleted.
I pressed the "submit" button and it looked like nothing happened so I pressed it again.
Most other forums warn about that and show the reply that you just made.

 

[4pyros]

The passive crossover circuit you posted uses an inductor with an iron core that has resistance, is not linear and saturates. Its low impedance HUGE electrolytic capacitor has poor tolerance.
Since it is second order then with matching second order highpass filters for the stereo speakers it results in a notch at the crossover frequency or a 3dB peak.

Each stereo speaker needs to have its own highpass filter as part of the crossover system. A mixer circuit is needed to feed the additional subwoofer amp and the active line level crossover circuit is the best way to do it.

Ok so its not that good but it works.
I would agree that doing at the low level is better but I don't think he has that option right now.

 

[audioguru]

I guess he will overload one channel of his stereo by connecting the sub speaker plus the stereo channel speaker to it. Then if the amplifier is not destroyed, the deep bass sounds will come from both stereo channel speakers (which might destroy them) plus come from the sub speaker (but only on one channel).

 

[stuee]

thanks again guys for all your valuable advice,
I was thinking if i just ran the sub from 1 speaker output as the bass is mainly on both channels anyway and the amp will be easily powerful enough.
Im trying to prevent getting another amp just for the sub if possible.

 

[flat5]

AG, may I ask, what op amps would you use for your crossover circuit?

 

[audioguru]

AG, may I ask, what op amps would you use for your crossover circuit?

I have used TL07x audio opamps for most of my career. They have low noise and distortion and a full output bandwidth to 100kHz.
They are TL071 single, TL072 dual and TL074 quad opamps. The schematic does not show the plus 5V to 12V and minus 5V to 12V power supply.

 

[audioguru]

I was thinking if i just ran the sub from 1 speaker output as the bass is mainly on both channels anyway and the amp will be easily powerful enough.
Im trying to prevent getting another amp just for the sub if possible.

If the sub speaker is connected parallel to one channel's stereo speaker then the amplifier power and current are doubled for that channel and the very low and powerful bass frequencies will be played by the stereo speakers which might destroy the stereo amplifier and speakers. If you add simple passive LC highpass filters to the stereo speakers like the one posted for the sub speaker then the sound will have a notch or a peak at the crossover frequency that sounds bad.

The TV in my computer room has a sound system that is pair of stereo speakers and a subwoofer. It has 3 amplifiers and a crossover circuit similar to the one I posted. It sounds and feels great.

 

[4pyros]

I guess he will overload one channel of his stereo by connecting the sub speaker plus the stereo channel speaker to it. Then if the amplifier is not destroyed, the deep bass sounds will come from both stereo channel speakers (which might destroy them) plus come from the sub speaker (but only on one channel).

AG I most say I am surprised you have never seen these before.
They came out at the beginning of the TV surround sound craze as an easy way to upgrade.
They use both channels of an amp so as not to damage it.
But as I just remembered they use a dual voice coil speaker, witch the OP probably does not have.

 

[audioguru]

AG I most say I am surprised you have never seen these before.
They came out at the beginning of the TV surround sound craze as an easy way to upgrade.
They use both channels of an amp so as not to damage it.
But as I just remembered they use a dual voice coil speaker, which the OP probably does not have.

But he does not have an amplifier for the subwoofer speaker. He wants to connect it in parallel with one of the stereo speakers which will feed damaging powerful low frequencies to the stereo speakers (because they do not have highpass filters) and will be overloading that amplifier channel.

Many years ago I made a Motorola SQ surround sound project for my record player and bought some records recorded with SQ surround sound. I still have the SQ project, record player and records but I have not played them for about 30 years.

I have an 18" VERY heavy and powerful pro-sound subwoofer that was at a special clearance price. I still have it, I never made an enclosure for it and never played it.

 

[4pyros]

But he does not have an amplifier for the subwoofer speaker. He wants to connect it in parallel with one of the stereo speakers which will feed damaging powerful low frequencies to the stereo speakers (because they do not have highpass filters) and will be overloading that amplifier channel.

Did you look at my post#19?
The outputs of the amp go to the sub first, the lows get crossed out before it goes back out to the speakers.

 

[stuee]

So as the output from the amp is 1000w (so they say) it will be far too powerful for my need and i will be keeping the amp at around 50% all the time and using the mixer for the volume control.
Is it not possible to set a channel to say 60% and use that one to feed the sub and put a filter inline for the low freq to the sub and leave the others as is as i still want all the sound frequencies to the normal speakers

 

[audioguru]

Did you look at my post#19?
The outputs of the amp go to the sub first, the lows get crossed out before it goes back out to the speakers.

Your post #19 is a link to flea-bay and hundreds of items. Which one?
None of them have any details about what they do and none have any spec's.

1) How can you connect TWO amplifier outputs (STEREO) to only ONE sub-woofer speaker?

 

[audioguru]

So as the output from the amp is 1000w (so they say) it will be far too powerful for my need and i will be keeping the amp at around 50% all the time and using the mixer for the volume control.
Is it not possible to set a channel to say 60% and use that one to feed the sub and put a filter inline for the low freq to the sub and leave the others as is as i still want all the sound frequencies to the normal speakers

As I am typing, my unfinished reply gets posted.

You do not have a power meter so you do not know the output power of the amplifier. The volume control does not limit the output power and is not calibrated and the input level is also not calibrated. Since you will be overloading one channel of the amplifier then it might be destroyed.

The stereo speakers need highpass filters so that they play the frequencies above what is filtered for the sub-woofer. Without highpass filters then the response will have a peak at the crossover frequency that will sound "boomy". with proper lowpass and highpass filtering then the amplifier will not be overloaded and the speakers will safely play what they are designed to play.

You have no control about how much power will be fed to the stereo speakers (it depends a lot on the input signal) so if the stereo speakers play the low frequencies they might be destroyed.

 

[4pyros]

Your post #19 is a link to flea-bay and hundreds of items. Which one?
None of them have any details about what they do and none have any spec's.

1) How can you connect TWO amplifier outputs (STEREO) to only ONE sub-woofer speaker?

Like this;
**broken link removed**
or this;
**broken link removed**

 

[4pyros]

So as the output from the amp is 1000w (so they say) it will be far too powerful for my need and i will be keeping the amp at around 50% all the time and using the mixer for the volume control.
Is it not possible to set a channel to say 60% and use that one to feed the sub and put a filter inline for the low freq to the sub and leave the others as is as i still want all the sound frequencies to the normal speakers

Yes it is possible, but subs need a lot more power then the other speakers.
You may have to run one channel at 90 % and the other at 30%.
So it may or may not be acceptable for your needs.
The only way to make sure its going to be good is to go with a separate amp.

 

[audioguru]

Like this;
**broken link removed**

It is for TWO subwoofer speakers but he has only ONE.
It is 12dB so either there will be a notch in the sound or a peak of 3db at the crossover frequency. The active circuit I showed does not cancel nor peak frequencies.

or this;
**broken link removed**

It doesn't have enough inductors.
NO Spec's: Power rating? Impedance? Crossover frequency?

 

[stuee]

Would any of these amps be appropriate for my sub if i split the line out before it goes to the main amp?


https://www.ebay.com/bhp/subwoofer-amplifier-board

 

[audioguru]

The ebay Chinese ratings for the TDA2030A amplifier board lie.

12VAC minus a 1V rectifier diode drop makes +16VDC and the other 12VAC makes -16VDC.
The datasheet for the TDA2030A shows the power at only 12W per stereo channel into 8 ohm speakers. The subwoofer output power will also be only 12W into 8 ohms.