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Go Back   Blu-ray Forum > Audio > Pre/Pro, Amplifiers and Separate Systems

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Old 01-11-2011, 12:08 PM   #1
Big Daddy Big Daddy is offline
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Default Connecting an Amplifier to a Receiver Without Pre-Outs

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:




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







*************************************************



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

Last edited by Big Daddy; 01-18-2011 at 02:09 AM.
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Old 01-11-2011, 03:14 PM   #3
rarredoa rarredoa is offline
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Where does the wattage of either the receiver or the outboard-amplifier come in to play in regards to the resistor values/wattage?

FYI: My R1 is 47 kohms and my R2 is 1 kohm, both quarter-watt resistors, with an assumption of 20 volts as the receiver's speaker output.

By my calcs, I get an attenuation of 33.6 db, which is equivalent to an input voltage to the outboard amp as 0.416 volts. Would this be decent small-signal going into the EPX3000?

Last edited by rarredoa; 01-11-2011 at 05:00 PM.
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Old 01-11-2011, 11:51 PM   #4
Big Daddy Big Daddy is offline
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Quote:
Originally Posted by rarredoa View Post
Where does the wattage of either the receiver or the outboard-amplifier come in to play in regards to the resistor values/wattage?

FYI: My R1 is 47 kohms and my R2 is 1 kohm, both quarter-watt resistors, with an assumption of 20 volts as the receiver's speaker output.

By my calcs, I get an attenuation of 33.6 db, which is equivalent to an input voltage to the outboard amp as 0.416 volts. Would this be decent small-signal going into the EPX3000?
I have no idea how they calculate the wattage ratings of the high-low adapters that are sold in the marketplace. However, I know that Power is related to Voltage according to the following formula:

Power = Voltage^2 / Resistance

In your case, the assumption of 20 Volts is equivalent to 50 Watts for an 8 Ohm speaker and 100 Watts for a 4 Ohm speaker.

For 30 Volts, you will get 112.5 Watts for an 8 Ohm speaker and 225 Watts for a 4 Ohm speaker.

As far as the resistances are concerned, we use the following formulas:

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

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

dB = 20 x LOG (Vout/Vin)

In your case,

R1 = 47,000 Ohms
Vin = 20 Volts
Vout = 1 Volts (we are assuming that the input voltage requirement of your amplifier is 1 volt)

The above formulas yield:

R2 = 2,474 Ohms or 2.5 kOhms
Vout = 1 Volts
dB = -26 dB's

If we assume the input voltage requirement of your amplifier is 2 volts, then the above equations will yied:

R2 = 5,222 Ohms = 5.2k Ohms
Vout = 2 Volts
dB = -20 dB's

Last edited by Big Daddy; 01-12-2011 at 07:10 AM.
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Old 01-12-2011, 12:00 AM   #5
crazyBLUE crazyBLUE is offline
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Evening Big Daddy

Interesting new thread / Sticky you have created & I would think would be popular with people that don't have pre-outs on there receivers.
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Old 01-12-2011, 12:05 AM   #6
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Quote:
Originally Posted by crazyBLUE View Post


Evening Big Daddy

Interesting new thread / Sticky you have created & I would think would be popular with people that don't have pre-outs on there receivers.
I will do anything to waste people's money.
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Old 01-12-2011, 12:12 AM   #7
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Quote:
Originally Posted by Big Daddy View Post
I will do anything to waste people's money.
Not So ~ You wasted a lot of my money & it was more then worth it ~ You can waste my money anytime
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Old 01-12-2011, 12:15 AM   #8
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Originally Posted by crazyBLUE View Post
Not So ~ You wasted a lot of my money & it was more then worth it ~ You can waste my money anytime
The diagram and the formulas in post #1 look too complicated. In reality, all you have to do is attach two speaker wires to the receiver and two RCA cables to the amplifier and solder two resistors between the speaker wires and the RCA cables. You can buy resistors real cheap from your local Radio Shack.
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Old 01-12-2011, 12:24 AM   #9
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Quote:
Originally Posted by Big Daddy View Post
The diagram and the formulas in post #1 look too complicated. In reality, all you have to do is attach two speaker wires to the receiver and two RCA cables to the amplifier and solder two resistors between the speaker wires and the RCA cables. You can buy resistors real cheap from your local Radio Shack.
That would be an easy thing to do I would think as long as you get the correct resisters. I guess as far as the soldering is concerned, If the person can't solder I am sure they could find someone to do it.

Am curious ~ By doing it this way, There would be no loss of quality on the sound ? Remember I am the newbie on this
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Old 01-12-2011, 12:34 AM   #10
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Quote:
Originally Posted by crazyBLUE View Post
That would be an easy thing to do I would think as long as you get the correct resisters. I guess as far as the soldering is concerned, If the person can't solder I am sure they could find someone to do it.

Am curious ~ By doing it this way, There would be no loss of quality on the sound ? Remember I am the newbie on this
Even a child can do soldering. You can buy a soldering gun from Wal-Mart for less than $10.

I have never used a High-Low adapter as my receivers/preamps have always had preouts. I have heard from others who have used them that the quality is ok. As long as the resistors are good quality and the wires are properly isolated, I don't believe you will hear any significant degradation.
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Old 01-12-2011, 12:43 AM   #11
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Quote:
Originally Posted by Big Daddy View Post
Even a child can do soldering. You can buy a soldering gun from Wal-Mart for less than $10.

I have never used a High-Low adapter as my receivers/preamps have always had preouts. I have heard from others who have used them that the quality is ok. As long as the resistors are good quality and the wires are properly isolated, I don't believe you will hear any significant degradation.
Then I am sure in the future this Thread can save people money by not having to upgrade there receiver to add an external amplifier to there system.
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Old 01-12-2011, 12:49 AM   #12
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Quote:
Originally Posted by Big Daddy View Post
The diagram and the formulas in post #1 look too complicated. In reality, all you have to do is attach two speaker wires to the receiver and two RCA cables to the amplifier and solder two resistors between the speaker wires and the RCA cables. You can buy resistors real cheap from your local Radio Shack.
Yea u're starting to waste my money, by giving me the green light to externally amplify my front-stage!
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Old 01-12-2011, 12:52 AM   #13
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Quote:
Originally Posted by rarredoa View Post
Yea u're starting to waste my money, by giving me the green light to externally amplify my front-stage!
I plead guilty as charged.
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Old 01-12-2011, 02:44 AM   #14
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Quote:
Originally Posted by Big Daddy View Post
Even a child can do soldering. You can buy a soldering gun from Wal-Mart for less than $10.

I have never used a High-Low adapter as my receivers/preamps have always had preouts. I have heard from others who have used them that the quality is ok. As long as the resistors are good quality and the wires are properly isolated, I don't believe you will hear any significant degradation.
I believe that, in theory, the more circuit elements an analog signal has to go through, the more resistive losses you have and the higher possibility for signal noise, particularly harmonic noise if you have switching elements. In this case you have quarter-watt resistors (heat losses),no switching of any kind and travel through relatively short wire-runs, so I believe that degradation of sound quality are negligible (I hope!).

We'll see.
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Old 01-12-2011, 03:22 AM   #15
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Quote:
Originally Posted by Big Daddy View Post
I have no idea how they calculate the wattage ratings of the high-low adapters that are sold in the marketplace. However, I know that Power is related to Voltage according to the following formula:

Power = Voltage^2 / Resistance

In your case, the assumption of 20 Volts is equivalent to 50 Watts for an 8 Ohm speaker and 100 Watts for a 4 Ohm speaker.

For 30 Volts, you will get 112.5 Watts for an 8 Ohm speaker and 225 Watts for a 4 Ohm speaker.

As far as the resistances are concerned, we use the following formulas:

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

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

dB = 20 x LOG (Vout/Vin)

In your case,

R1 = 47,000 Ohms
Vin = 20 Volts
Vout = 1 Volts (we are assuming that the input voltage requirement of your amplifier is 1 volt)

The above formulas yield:

R2 = 2,474 Ohms or 2.5 kOhms
Vout = 1 Volts
dB = -26 dB's
So when do the 90 watts * 7 channels at 8 ohms, 20 hz-20 khz actually come into play? When no other channels are playing? The power-supply for the receiver is something like 6.5 amps, 120 VAC, which equates to 780 watts. Are receiver amplifiers wasting away significantly more than 150 watts (780 w - 630 w) in the form of heat, signal processing, lights, etc?

I tried to measure the speaker-level voltage using my true-RMS voltmeter, but I am not getting reliable readings. I get around .6 to 0 volts AC. I was expecting around 20 volts. And according to W=(V^2)/R, to get 90 watts I need a speaker-level voltage of 26.8 V. Hard to tell what kind of power my receiver is putting out.

Last edited by rarredoa; 01-12-2011 at 03:34 AM.
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Old 01-12-2011, 05:34 AM   #16
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Originally Posted by rarredoa View Post
So when do the 90 watts * 7 channels at 8 ohms, 20 hz-20 khz actually come into play? When no other channels are playing? The power-supply for the receiver is something like 6.5 amps, 120 VAC, which equates to 780 watts. Are receiver amplifiers wasting away significantly more than 150 watts (780 w - 630 w) in the form of heat, signal processing, lights, etc?

I tried to measure the speaker-level voltage using my true-RMS voltmeter, but I am not getting reliable readings. I get around .6 to 0 volts AC. I was expecting around 20 volts. And according to W=(V^2)/R, to get 90 watts I need a speaker-level voltage of 26.8 V. Hard to tell what kind of power my receiver is putting out.
On the average, most receivers are only using a few watts during normal listening levels. If your receiver is listed for 90 watts, that is what it is capable of producing on a continous basis without significant distortion. You won't need 90 watts for normal listening levels.

Most receivers/amplifiers can produce significantly more power for a fraction of a second during peaks without clipping. If you turn the volume up so that your receiver or amplifier goes into clipping, you may damage the amp, the speakers, and most importantly, your hearing.
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Old 01-18-2011, 08:19 PM   #17
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Originally Posted by Big Daddy View Post
On the average, most receivers are only using a few watts during normal listening levels. If your receiver is listed for 90 watts, that is what it is capable of producing on a continous basis without significant distortion. You won't need 90 watts for normal listening levels.

Most receivers/amplifiers can produce significantly more power for a fraction of a second during peaks without clipping. If you turn the volume up so that your receiver or amplifier goes into clipping, you may damage the amp, the speakers, and most importantly, your hearing.
I completed my signal converter. L/R channels attenuate by 27 db and center channel attenuates by 26 db. Center input voltage is 1.03V and L/R input voltages are about 0.82 V (I designed different input voltages because I will be using different amplifiers, for L/R and Center.

Next question: I am pretty sure that whatever signal-noise coming out of the receiver's speaker outputs will also be amplified. Does anyone know how to design noise filters that might eliminate this noise?

I did some research and used input parameters of 24 kohms for input-impedance (the speaker-level signal to be converted to RCA) and a low pass cutoff frequency of 25 khz, so as to filter out any noise higher than my audible signal.

I think a first-order lowpass filter at 25 khz should be sufficient, but I could be wrong. Any suggestions?

Last edited by rarredoa; 01-18-2011 at 08:28 PM.
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Old 01-18-2011, 08:33 PM   #18
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This is the link to the site that calculated my values

http://www.the12volt.com/caraudio/crosscalc.asp

I used a low-pass cutoff frequency of 25 khz, to filter out any high frequency noise higher than the audible signals. The impedance value I used was the input resistance to my signal impedance, which is 24 kohms.
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Old 01-19-2011, 12:09 AM   #19
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Originally Posted by rarredoa View Post
This is the link to the site that calculated my values

http://www.the12volt.com/caraudio/crosscalc.asp

I used a low-pass cutoff frequency of 25 khz, to filter out any high frequency noise higher than the audible signals. The impedance value I used was the input resistance to my signal impedance, which is 24 kohms.
Why didn't you use the impedance of the speakers/subwoofer? After all, any low frequency or high frequency sound/noise has to come out of your speakers/subwoofer.
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Old 01-19-2011, 02:58 AM   #20
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Quote:
Originally Posted by Big Daddy View Post
Why didn't you use the impedance of the speakers/subwoofer? After all, any low frequency or high frequency sound/noise has to come out of your speakers/subwoofer.
I'm glad u mentioned that. Originally I thought that the HF noise from the receiver's outputs would be converted to low-level and then enter the external amp's input stage and get amplified to the speakers. But if I match the filters' impedance to that of the speakers all it does is put an 8 ohm load in series with 24 kohm, insignificant attenuation.

However, the HF distortion/noise issue is a thing of the past. I used my line-level converter and used the rca outs to plug in to an old sony receiver with multi-channel inputs. While I wasn't expecting extra headroom or fidelity, I was listening for any audible noise, even at loud volumes. To my surprise, no noise whatsoever. So if a low-end sony receiver acting as a power amp generated no audible noise, then I think I can get away using my line-level converter to feed quality power amplifiers!

My only installation concern is that these power amps have removable ends, so that the component width can be brought down to 17 inches from the specified 19.

Can the ends of the front-plate of your EPX3000 be removed? In other words the rack mountable holes be removed?

Last edited by rarredoa; 01-19-2011 at 03:00 AM.
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