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Old 20th December 2012, 06:45 PM   #1
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Default Hi Level Amp output connect to RCA input

Hi there,
I have an AV amp which has multi channel pre amp outputs. This allows me to run a Phono lead from the front left and right pre outs to the Aux input of my Hi Fi amp. This allows me to enjoy full surround sound and also the benefits of Hi Fi when listening to vinyl / CD.
The issue I am now faced with is I want to replace my AV amp with one that no longer has multi channel pre outs so will lose this facility to connect to my Hi Fi amp.

I know you can get high level to low level adapters for car audio but I am concerned the quality will not be there for my needs.

Has anyone any experience in this field to help me to either build a bespoke adapter or help me with the design / build.

Regards
Pete
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Old 20th December 2012, 08:14 PM   #2
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You'll typically want to pad the signal level down by about 15 to 20 dB, a factor of 7 to 10, for home audio. Each channel will have a series resistor followed by a shunt resistor, with the series resistor about 8 or 10 times larger than the shunt. For modern solid state electronics the series can 10K Ohms and the shunt 1K Ohms.

For a quick test, don't even bother putting it in a box. Just cut an old interconnect cable in half and solder the resistors in the line.

All good fortune,
Chris
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Old 20th December 2012, 08:37 PM   #3
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What is the model of the new amp? There is likely a way to modify it to bring pre-amp signals out.
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Old 21st December 2012, 07:53 AM   #4
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Hi Chris / Firechief, Thanks for your replies

In answer to your points, the amp I am looking to buy is the Sony STR DN1030. I have called Sony support who say the only way to output line level to my HiFi amp is via the stereo line outs but connecting this way will send all surround effects to these speakers as well which will dilute the surround effects.

As for building the attenuator, I have a few questions:

1.Do the resistors have to withstand a certain power. The amp can output 100 watts into 8 ohm
2.Do the resistor have to be made of a certain material for the use I have in mind
3.Will this method of attenuation affect the frequency spectrum in the signal

Thanks for your help with my query and I look forward to your replies.

Pete
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Old 21st December 2012, 08:37 PM   #5
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Quote:
Originally Posted by mixter_v View Post
1.Do the resistors have to withstand a certain power. The amp can output 100 watts into 8 ohm
2.Do the resistor have to be made of a certain material for the use I have in mind
3.Will this method of attenuation affect the frequency spectrum in the signal
Here's how to calculate resistor wattage: The amplifier's output voltage at full tilt boogie is E=sqrt(PR) or about 28.3 Volts. This voltage applied to 10,000 Ohms is a power dissipation of P=E^2 /R or about 800/10,000 or .08 Watts, reasonably small.

Maybe even too small - a little more current through the speaker relay might help keep it refreshed. Maybe scale the resistor values smaller by a factor of 10; you'll still be able to use ordinary 1/2 metal film resistors perfectly safely. And maybe reduce the shunts to 50 or 75 Ohms for a little more attenuation. You'll have tons of bandwidth anyway.

All good fortune,
Chris

Last edited by Chris Hornbeck; 21st December 2012 at 08:40 PM.
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