# Bi-amping with integrated and power amps

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#### Thomasjunta

Hi everyone,
I would like to passively bi-amp my speakers with integrated amplifier (mid & high frequencies) with 1.6V output, and power amp (woofers) with 1.0V input. Would you have any suggestions what to add between them to match the volume level? The woofers are significantly louder when driven by the power amp. I was thinking of attenuator (e.g. Rothwell) or passive pre-amp/volume control.
Cheers

#### rayma

Would you have any suggestions what to add between them to match the volume level?
The woofers are significantly louder when driven by the power amp.

Yes, add a 10k stereo attenuator before the woofer amplifier inputs.
An adjustable control would be best to set the level you want,
but you can replace the control with a pair of resistors in each channel
after you decide exactly what level you want.

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#### Thomasjunta

Thank you for reply. Is there a formula that I can use to see that 10k attenuation is sufficient?

#### DF96

10k is not the attenuation, but the total value of the attenuating potential divider.

#### Thomasjunta

you can replace the control with a pair of resistors in each channel
after you decide exactly what level you want.

Is it possible to define resistors value simply based on ohms law? Using Voltage Divider Calculator I worked out that I would need 10k ohm & 6.04k ohm resistors (in theory) to reduce the Voltage to 1.0. Thanks

#### rayma

Is it possible to define resistors value simply based on ohms law? I would need 10k ohm & 6.04k
ohm resistors (in theory) to reduce the Voltage to 1.0.

Sure, that's how you do it, this is a voltage divider. It's best to take into account the input impedance
of the amplifier also, which is in parallel with the resistor to ground, since that will affect the result.

A 10k in series is fine, then calculate what R to ground (in parallel with the input impedance)
gives you the loss that you want.

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#### Thomasjunta

A 10k in series is fine, then calculate what R to ground (in parallel with the input impedance)
gives you the loss that you want.

The integrated amp output is 1.6V/250ohm, while the power amp input is 1.0V/33kohm. Am I right with my calculations that resistors 4k7 and 10k would provide 1.0V output and approx 4.1dB damping? I used Voltage divider online calculator.

I will try to measure the sound level of the integrated and power amps and see if the difference is somewhere near 4dB.

#### rayma

The integrated amp output is 1.6V/250ohm, while the power amp input is 1.0V/33kohm.
Am I right with my calculations that resistors 4k7 and 10k would provide 1.0V output

I think that the 1.6V and 1.0V figures are the amplifiers' input sensitivities,
which is the input voltage needed for full output.

Even with 1V into both (different) amps, that does not assure equal output voltages.
Even with the same output voltages, that does not assure equal acoustic outputs
from the main speakers and the woofers.

However, you can still try that attenuator at the inputs of the woofer amp.
With the 4.7k shunting the 33k input impedance of the woofer amp, this would give a net 4.11k to ground.
Then the series 10k would give a loss factor of: 4.11 / (4.11 + 10 ) = 0.29 loss factor,
or a dB loss of -10.75 dB. That would make the woofer sound roughly about "half" as loud as before.

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#### AndrewT

For passive bi-amping you are better to start with 4 amplifiers that have the same gain.

If later you decide to experiment with different amplifiers for the upper frequencies from the lower frequencies, then you may have to look at equalising the gain of the different amplifiers.
Hopefully by the time you have tried 4 equal gain amplifiers and found how well they work (or not) you will have learned how to work out the gain of any of your amplifiers and how restricted your options are in altering the amplifier gain. Changing gain changes the stability margins !!!!!!!!!!!

Changing the sensitvity by adding on a fixed attenuataor at the input is an easy option.

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#### Thomasjunta

Thank you for help. I have measured the sound level difference with iMM microphone and the power amp is 12dB louder than the integrated.

#### rayma

Thank you for help. I have measured the sound level difference with iMM microphone
and the power amp is 12dB louder than the integrated.

Ok, then try the circuit in post #8. It should be close to what you need.

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