[SOLVED] help with PA amplifier connection

[zsolt1]

HI,
how to get signal from BTL amp , in order to feed with signal an other amp (sub woofer with tone correction preamp) ?
There is no other signal source from first device which is a multimedia player in a Dacia Duster . Has only 4 speaker outputs in BTL connection . (It has an SPDIF output on the back , but is to much effort to work around that...)

 

[BradtheRad]

To mix signals I have an assortment of Y-jumpers. They're made from shielded coax cable. They end with various types of plugs and jacks. Maybe two phono plugs and one phono jack. Or vice versa. I also have adapters for phono type, 1/8 inch phone, 1/4 inch phone, mono, stereo, etc.
Is that the sort of thing you need?

Or do you need an auxiliary switch box for your abundant sources? Plug your source cables in the back. The box might have room for 2 or 3 or 4 sources. Then one cable goes to your amplifier. Press a button to select a source. Stereo has twice as many cables and connectors, naturally.

 

[zsolt1]

neither ,the Duster has basic audio system . To ad an amp in the back ,I cant manage to get proper signal from the system
I tried to get signal from left speaker using 1 to 1 transformer, At >20% of volume the speaker behaves like short circuited. I added a series capacitor with the primary coil to decouple any dc.. it works but with this filter like configuration i don't get the proper signal to feed the amp , thous it has own equalizer

 

[FvM]

I tried to get signal from left speaker using 1 to 1 transformer, At >20% of volume the speaker behaves like short circuited. I added a series capacitor with the primary coil to decouple any dc.. it works but with this filter like configuration i don't get the proper signal to feed the amp

Sounds like using the wrong transformer type.

Ideally, a sub-woofer amp would be driven with sum L+R. I would prefer difference amplifiers over transformers.

 

[zsolt1]

ok , i have to outputs in BTL connection L+R : +L,-L,+R,-R . Should i add +L with +R , and feed the amp with that signal using GND reference?

 

[FvM]

Using the single ended speaker outputs is a possible option.

Question is about the topology of the bridge amplifier. It can be either single supply with respective DC bias at outputs or dual supply with about zero DC bias. I would use a series capacitor to remove DC bias in any case.

The other question is if the output bias voltage is stable and e.g. doesn't have superimposed 100 Hz hum.

Most likely it doesn't, but if there are any unwanted interferences, a true difference amplifier would be needed.

 

[zsolt1]

I belive that the audio system has only single supply 12 Vdc inside, and outputs are from BTL amps for shore.
would be this a solution :

 

[Audioguru]

Your circuit diagram is correct if the capacitor and resistor values are properly calculated.

 

[zsolt1]

calculate ? what about 1 K for resistors and 1 uF for caps ?

 

[Audioguru]

Two 1k resistors are fine and if the amplifier input impedance is 10k ohms then a single 1uF cap will pass frequencies down to about 16Hz. The amplifier might already have an input capacitor.

 

[zsolt1]

this would make the input part of the circuit look like this

(tranzistor set up to work on 12 v)

 

[Audioguru]

A subwoofer amplifier does not have a bass and treble tone controls circuit that is used with an ordinary wideband amplifier. A subwoofer amplifier is fed with an active lowpass filter that might have its cutoff frequency adjustable and it might have a phase switch.

 

[zsolt1]

phase switch not used, active filter (something with TL082 .. ) not used , replaced with the above circuit , all refurbished in a hurry .
it's ok for my needs ... just need to add a small tranzistor at input, something is wrong with the level ...thous sound seems to be good now

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what if i add something like this

 

[Audioguru]

If you have a subwoofer speaker and amplifier for it and a stereo amplifier and speakers for it then two tone controls circuits are used in the stereo amplifier with active highpass filters and the subwoofer amplifier has a matching active lowpass filter.

Why did you feed the low impedance emitter of the transistor instead of the high impedance base?
Why did your transistor have such low voltage gain?
Why was the transistor biased wrong so that it was almost cutoff?
I simulated it:

 

[zsolt1]

its a single amp driving single speaker .The speaker is not exactly for a sub , it's more for the mid - low range . Don't need that kind of bass sound , i find it uncomfortable , the amp is only about 18 W . Just a little extra for the existing audio . I'm not planning to change it because the device is bluetooth capable (beside the car is new )
Well you say that a low input impedance is needed in a previous post . Seems logic to me since i take the signal from 4 ohm speakers .. don't know how the fact that BTL amps drive those speakers affects that ..
Oh, i intended to bias the tranzistor with 6 V , 100k both r1-r2

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i simulated something myself ....

 

[Audioguru]

Your L and R signals are coming from the same amplifier then their DC voltages are the same and only one input capacitor is needed.
Your circuit is very distorted because you do not know how to bias the transistor correctly.
Here are simulations, one with a single 100k DC feedback resistor and another with the base biased at 6V:

 

[zsolt1]

Hi, i took R and L from front +R and from front +L, i 'simulated' that circuit with a couple of components i found in a junkbox... i will get a proper amp in the and ...

 

[zsolt1]

the amp is ok , works fine from phone, not at all when i connect it to the car

 

[Audioguru]

the amp is ok , works fine from phone, not at all when i connect it to the car

That is odd because the output level from the speaker outputs of the car amplifier is much higher than the output from a phone.

 

[zsolt1]

probably it's something i messed up when i took the signal ... i tested with a 2Kohm head phone , there is almost no signal . I have to an mount all an test connections right in site