Connecting an Amplifier to a Receiver Without Pre-Outs

[Big Daddy]

CONNECTING AN AMPLIFIER TO A RECEIVER WITHOUT PRE-OUTS

If you want to use an external amplifier to power your speakers for better fidelity, you need a receiver with pre-outs for the different channels. However, if your receiver does not have any pre-outs, you may still be able to use an external amplifier by adding high-level speaker output to low-level RCA input adapters. Because speaker level outputs have higher voltages than line level signals, you cannot directly connect the speaker outputs of a receiver to the line level inputs of an amplifier. You will need a High-to-Low Adapter like the ones listed below:

Converter.jpg Metra.jpg Stinger.jpg

Speaker level signal voltages are usually a few volts up to 30-40 volts, depending on the receiver’s power output. The line level inputs on an amplifier normally require around 0.3 volts up to 2 volts. Therefore, signals from the speaker level outputs should be attenuated by 20 dB or more before they are fed to line level inputs of an amplifier. You can build a voltage divider circuit yourself by adding a couple of resistors in series with the signals as in the following diagram:

Image created by Big Daddy



CALCULATION OF VOLTAGE AND RESISTANCE

Let

Vin = Input Voltage in volts

Vout = Output Voltage

R1 = Resistance of the first resistor in Ohms

R2 = Resistance of the second resistor in Ohms

Then,

R2 = (Vout/Vin x R1) / ( 1 – Vout/Vin)

and

Vout = Vin x [R2 / (R1 + R2)]

If you want to calculate the level of attenuation in dB's, you can use the following equation:

dB = 20 x LOG (Vout/Vin)


Example 1:

Assume the speaker level output voltage of a receiver is 20 volts and the amplifier line level input voltage requirement is 1 volt. Furthermore, assume the input impedance of most amplifiers is around 10,000 Ohms (10k Ohms). Calculate the resistance of the two required resistors.

R1 is normally should be equal to the input impedance of the amplifier (consult the amplifier's manual).

Therefore,

R1 = 10,000 Ohms

and

R2 = (Vout/Vin x R1) / ( 1 – Vout/Vin) = (1/20 x 10,000) / (1 - 1/20) = 526 Ohms

Since such a resistor may not be available, you can use a resistor that has a resistance close to 526 Ohms. Any resistor with a resistance within 10%-20% of the required value should work.

Output Voltage:
Vout = Vin x [R2 / (R1 + R2)] = 20 x [526 / (10,000 + 526)] = 1 Volt

Attenuation in dB's:
dB = 20 x LOG (Vout/Vin) = 20 x LOG(1/20) = 26 dB's


Example 2:

In the previous example, assume the voltage coming out of the receiver is 30 Volts and the input voltage requirement of the amplifier is 2 Volts.

R1 = 10,000 Ohms
Vin = 30 Volts
Vout = 2 Volts

Substituting in the above equations will yield:

R2 = 714.3 Ohms
Vout = 2 Volts
dB = -24 dB's


Noise & Hum:

Unbalanced (RCA) inputs/outputs of audio equipments are very sensitive to noise and hum, especially when you connect something else to them. Usually at short distances there should be no problems in using the adapter.

The High-to-Low Adapter can be made cheaply. However, there are some inexpensive and some high-end options available that you can purchase from Amazon or Ebay. Make sure they have sufficient power tolerance. Some of them have additional noise blocking circuits. If you get any kind of noise problems, use an isolation transformer. You can find additional information about noise blocker and ground loop isolators in the following thread:

https://forum.blu-ray.com/home-theat...isolators.html.


Power (Watts) and Voltage (Volts), and Resistance (Ohms):

The formula that relates Power to Voltage and Resistance is given in the following. This relationship is one of the conclusions of Ohm's Law.

Power = Voltage^2 / Resistance = V^2 / R

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High-Low Adapters for Purchase:

http://shop.ebay.com/i.html?_nkw=speaker+to+line+level
http://www.hometech.com/hts/products...html#AT-SPKCNV
http://www.amazon.com/s?url=search-a...ters&x=18&y=22
http://www.parts-express.com/pe/show...&vReviewShow=1
http://www.nextag.com/speaker-line-l...rter/shop-html
http://www.audioc.com/library1/subaccess.htm
http://www.installer.com/hilo/


High End Hi-Lo Adapters:

http://ochomeaudio.com/p-136-adp-12-...l-adapter.aspx
http://www.soundscapeav.com/russound/adp-1.html
http://www.crutchfield.com/p_543ADP1...r.html?tp=9070

http://www.crutchfield.com/p_161LC2I...i.html?tp=2001


Audio Control 6-channel Adapter:

http://www.crutchfield.com/s_161LC6I...e.html?tp=6870


Resistors:

http://www.bcae1.com/resistrs.htm
http://www.alliedelec.com/search/sea...resistors&pn=1
http://www.parts-express.com/resistors-index.cfm
http://www.radioshack.com/search/ind...resistors&sr=1

REFERENCES & ADDITIONAL INFORMATION

http://www.tkk.fi/Misc/Electronics/c...ne_to_mic.html
http://sound.westhost.com/p-list.htm
http://www.avsforum.com/avs-vb/showthread.php?t=1079042
http://www.epanorama.net/circuits/speaker_to_line.html
http://www.termpro.com/articles/faders.html