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Old 27th January 2010, 12:43 AM   #1
lgreen is offline lgreen  United States
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Default How to DIY Attenuated Interconnects?

My office system is way too loud. It is a cd player (no volume control) to BOSOZ preamp to amp to speakers. I can only get 1 click on the preamp volume controls and its still too loud. I need background music at low volumes and some vol control wiggle room.

What I'd like to do is install some attenuation into my CD->PRE or PRE->AMP interconnects without messing up the treble response. Seems a series resistor in the + lead and a resistor to ground should do it (talking about the RCA connection here). But what should the values be and where should I install this?
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Old 27th January 2010, 02:27 AM   #2
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You can calculate the values here: 50-Step Shunt Attenuator Resistor Calculator - Neville Roberts

Normally, you should only need appropriate value series resistor as the shunt resistor is usually already inside the equipment.

Here's the adapter I made for a customer (using Cardas parts) who needed additional attenuation with my amps: 22k series resistor (nude Vishay) will add 6dB of attenuation with 22k shunt that was in the amp.
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Old 27th January 2010, 03:44 PM   #3
AndrewT is online now AndrewT  Scotland
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if you want -20dB then the resistors are in a 9:1 ratio. -40dB requires a 99:1 ratio.

For this to hold true the input impedance (Rin) of the amp must be much greater than the grounding resistor (Rg).

Take a 9k and 1k pair. At first sight this gives the 9:1 ratio and should give -20dB.
attenuation = 20 * log [1 / (9+1)] gives -20dB.
Now take the input impedance of the amplifier into account.
Rin=100k.
attenuation = 20 * log[1//100 / (9+1//100)] gives -20.1dB. not too much of an error.
Rin=10k.
attenuation = 20 * log[1//10 / (9+1//10)] gives -20.7dB. the error is 7times bigger.
The closer Rin to Rg the worse the error gets.
The pre-amp/source sees the attenuator and the 22k amp as a 10k001 load.

Using 9k1 and 1k the attenuator has an output impedance of 901r and creates -20.35dB when Rin=22k
You can check whether that 901r source impedance causes an HF roll off problem.
The pre-amp sees the attenuator and 22k amp as a 10k001 resistive load in parallel with the the cable capacitance.
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Last edited by AndrewT; 27th January 2010 at 03:55 PM.
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