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Old 2nd July 2005, 10:14 PM   #1
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Default btl to line level

Hello all,
I have one of those cheap Pyramid booster amps with eq in my pick-up and I want to use it to drive a car power amp. My problem is that I am not sure how to properly attenuate the speaker sigal from the booster amp without causing damage to the chip. It uses the toshiba TA7270P chip in BTL mode.
I know that I cant directly take the signal from a single output to ground because I will get approx, 6 vdc along with the signal.

How can I convert from BTL to line level?


Thanks
Lawrence
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Old 3rd July 2005, 09:40 AM   #2
gfiandy is offline gfiandy  United Kingdom
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The correct way to do it would be to use a balanced to single ended circuit using an opamp. However since you state this is a cheap component I am assuming you don't want to have to build an active circuit to convert to single ended.

If you have a ground for the piramid amp that you can get to - say the RCA input ground you could use a bit of a bodge that would probably work.

To remove the DC 6v use a cap to couple the signal a 100uF cap with a voltage rating that can cope with the ouput level (say 25V) should be fine. Make sure the +ve end goes towards the amp as it has the +6v on it.

from the other end of the cap connect a 20K reistor in series with a 1K resistor to the ground on the RCA. The resistance of 20K will ensure that not much current flows. The ouput voltage will be divided by 2 buy using only one of the ouputs and by a futher approx 20 (Slightly more in fact) by the 20K to 1K. Take the output for the next stage from the joint of the 20K and 1K and the ground from the RCA connector. (if you want more output you can reduce the 20K or increase the 1K, I wouldn't go below 10K as you will start to draw significant current)

A word of warning. Whilst this should work and only puts a very light load on the power amplifer, some BTL amps don't like asymetric loads and can go unstable. If this happens there is the possibility that the amp will be destroyed.

Clearly you try this at your own risk.

To reduce this risk you could replicate the circuit and put it on the other ouput as well. (Just leave the output unconnected) However there is still a risk of instability as you are driving a capacitive load even though it has been decoupled by the largish resistors. So this is definately a try at your own risk design.

If you do try it, put your hand on the heats sink for the amplifers when you turn it on, if it starts getting hot very quickly turn it off imediately as it has gone unstable. Don't put you finger on the amplifer chip itself as there is a risk of buring yourself and worst case it could even blow which would really hurt.

Regards,
Andrew
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Old 3rd July 2005, 10:17 AM   #3
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the circuit you describe will work OK. Its such a light load that I would not worry one bit about it only being on one "side" of the amplifier output.

Now, I don't see the point in putting this on the output of the amplifier? why not take the signal before it hits the Pyramid amplifier? If you are using some other function in that Pyramid unit, why not just find the datasheet for the TDA-whatever chip it has, open the unit up, and connect an RCA output in parallel with the input to the amplifier chip?
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