Help with line out tap from power amps ..like radial jdx

[dschwartz]

Hello all!
I would like to add a power amp in to my cabinet simulator..
I have been investigating, but there's not much info about this kind of stuff...
I don't want a load box or power soak, just a line level tap into the power amp output..(i.e. the speaker must be connected)
Most circuits that do this are a simple voltage divider that  takes  the amplifier high voltage output down to line level..it seems too easy, but if you use it with a bridged amp, it can destroy one of the power devices.
Checked the radial jdx, and it has a small audio transformer to isolate grounds, solving this issue, but there's no schematic available.. according to the manual, the speaker level signal goes straight into the primary.. i would assume there should be some kind of divider or current limiting because i dont think a small trans could handle 200W of power through it..
I wonder if a simple resistor before the transformer will take down the voltage to workable levels..

Has anyone knows hoy the radial jdx input section works?

Cheers!!

[wavley]

Hello all!
I would like to add a power amp in to my cabinet simulator..
I have been investigating, but there's not much info about this kind of stuff...
I don't want a load box or power soak, just a line level tap into the power amp output..(i.e. the speaker must be connected)
Most circuits that do this are a simple voltage divider that  takes  the amplifier high voltage output down to line level..it seems too easy, but if you use it with a bridged amp, it can destroy one of the power devices.
Checked the radial jdx, and it has a small audio transformer to isolate grounds, solving this issue, but there's no schematic available.. according to the manual, the speaker level signal goes straight into the primary.. i would assume there should be some kind of divider or current limiting because i dont think a small trans could handle 200W of power through it..
I wonder if a simple resistor before the transformer will take down the voltage to workable levels..

Has anyone knows hoy the radial jdx input section works?

Cheers!!

Check out the Ted Weber website, I just used a small DI transformer they sell to add a line out to a Bassman and it indeed had a voltage divider before it, the schem is somewhere on the site.

[dschwartz]

I couldn't find the schem, or the xformer..
I suppose i could use any 1:1 audio transformer with the desired frequency response...at least inside the radial, there's a cheap yellow small one..

[dschwartz]

OK
my goal is to:
- have a speaker to line level adapter that would take a wide range of power ratings and would accept single ended or bridged outputs without the need to switch something..
- reliable, the resistors should not burn or the transformer
- Decent frequency response..at least 60hz to 8kHz
after using intuition and some brain function i came up with this circuit:



i´m assuming the power amp output as a 1khz sinusoidal signal .. in2 can be 0 volts (single ended) or the inverted version of IN (bridged amp)
The signal goes to the speaker, as normal, and it is tapped by a 15k resistor. both sides are divided via 15k/(200+pot) and then limited by the zener diodes to protect the transformer..with a 10k pot the range is enough for amps from 1W to 320W (of course if the pot is high with 320W, the zeners will clip the excess..the attenuation range is from -10dB to -44dB

here´s the frequency response vs gain:




and here´s the input voltage vs the output voltage (10Vpp, 12W@8Ohms)green is input and blue is output at the secondary:



here is at 50Vpp (312W@8Ohms) :



the current through the resistors is very low, in the order of 1.5mA@50Vpp




but at 3Vpp(1W) the current is about 50uA...

now, my questions for the gurus!!!
- does this need extra protections?
- do i need to find a sweetspot to the resistors values so i keep the current high enough for the transformer to work?
- what cheap widely available transformer would you recommend? (i simulated a 1:1, 600 ohms, 1000 turns, 290mH per coil transformer, which i think it´s pretty standard like EI14 or EI 18 audio isolation trasnformers..
- i observed that if i left  In2 at 0 volts (single ended) voltages at the top resistor were just half of the input value (of course, it makes a voltage divider with the other 15k+pot), but the secondary voltage was just 6dB higher than the bridge config..which is fine to me..just to make sure i´m not going to burn the device..

..safety precautions?
i will add some kind of led light alert for "connect speaker!!!!"

[PRR]

You got it.

_I_ would omit the diodes. Overvoltage will do no harm with those large series resistors. And if the level is high, the guy at the sound board will complain.

> here´s the frequency response

With what transformer?? Real or simulated?

You don't need a "minimum current". You do need an impedance low compared to what the transformer was designed for. As you see the bass runs lower as the pot value goes down. I would guess this is some low-cost "10K:10K" part.

[slacker]

In the schematic isn't the ground wire between the primary and secondary sides of the transformer defeating the the point of using one? Or does R15 stop bad things happening if the negative of the the speaker isn't ground?

[dschwartz]

You got it.

_I_ would omit the diodes. Overvoltage will do no harm with those large series resistors. And if the level is high, the guy at the sound board will complain.

> here´s the frequency response

With what transformer?? Real or simulated?

You don't need a "minimum current". You do need an impedance low compared to what the transformer was designed for. As you see the bass runs lower as the pot value goes down. I would guess this is some low-cost "10K:10K" part.

About the diodes, i was just thinking on protecting the following active circuits..i dont want an input voltage higher than the power supply. Maybe just some 1n4148 to rails  protection on the secondary would be enough..

The transformer is simulated..the model includes type of iron, permeability, etc, number of turns, impedance, inductance. The iron specs i left as default, and i used a 1000 turns, 600 ohms, 290mH each coil..as in cheap isolation transformers..

[dschwartz]

In the schematic isn't the ground wire between the primary and secondary sides of the transformer defeating the the point of using one? Or does R15 stop rbad things happening if the negative of the the speaker isn't ground?

You're right. I forgot to cut that connection..its for "gnd lift"

[dschwartz]

You got me thinking..

10k:10k vs 600:600
Pros and cons?

[PRR]

> 10k:10k vs 600:600

Make your resistance network somewhat lower impedance than the transformer nominal impedance.