voltage drop in line-level signals

[Serow]

I was wondering how you calculate the voltage drop in line-level signals from a home CD player if they pass through a series resistor. If I had a cmoy, gain of ten, with a 470K resistor in series between the cd player and the opamp + input, what would the voltage drop, percent change or decibel change be? Also, how would this change depending on the line level voltage.

Thanks.

 

[kwkarth]

You would be forming a voltage divider with the input impedance of your amp. So you need to know what the impedance of that sink is before you can calculate to drop across the resistor. It would be directly ratiometric based upon the ratio of the 470k and the impedance of the amp. More or less, because the internal resistance (impedance) of your source is also a factor in the total circuit.

 

[Joobu]

If one uses the OPA134, a FET input op-amp, the input impedance of the op-amp alone is on the order of Gigaohms.

Assuming for the instance there is no other resistor connected to the +input of the op-amp, a 470k resistor in series would make no practical difference.

Now, if you had a resistor inline between the CD player and the amp (Rs) and a resistor from the input of the amp to ground(Rin), then the voltage ratio of the amp to the CD is calculated as:

Rin/(Rin+Rs)

 

[Serow]

Thanks guys, Joobu hit the nail on the head. I was looking at cmoy's pocket amp article, where he shows the Rs as 100K and the Rin as either 100K or 470K. He then says that the 110k Rin will drop the input level by 50% or 6 dB, and the 470K Rin will drop the level by 15dB. This page has a good explanation of the decibel and how to convert a voltage change into a decibel change as follows:

G=20*log[Rin/(Rin+Rs)], which gives a result of -15.12 dB for the 470K, just like cmoy says!

 

[danny]

There are other considerations than the voltage drop, that you need to be considering. Specifically, offset voltages (and drift),input noise ( both current and voltage), and parasitic input capacitances. As you go to the large resistors, the stray capacitances start forming low pass (generally) filters that will start rolling off your highs, and affecting the high frequency phase relationships. If your inputs are not matched (including symmetric layout), these parasitics will be different for each channel, further degrading your sound.

Also, the type of resistor you use can make a difference in the noise floor of the amplifier. Use good quality metal film on your input, I would not be surprised at all to be able to hear ( and measure) a noticeable difference in the amp, by changing the type of input resistor that you use, especially if you are using 470k.

 

[Serow]

Thanks for the reply Danny. Obviously I am not a professional electrical engineer-for me this is just for fun, and if the amps and stuff I build happen to work, I'm happy.

That said, I am using a 4.7K metal film series resistor with a 10K volume pot and 100K Rin in my portable amp, and it seems to work well. I originally built the amp with a gain of 3.3, but the volume range I use was at the start of the pot rotation. The series resistor gives me a drop in voltage of about 1/3. I can't imagine using the amps people build with gains of 10 with low impedence headphones!

 

[danny]

Hello Serow,

Maybe that should be 'yet' as far as being a professional engineer. Actually I just wanted to point out that the resistor choice affects more than just gain, and your post gave me a good opportunity to do so.

As far as low impedance phones, and having to use the pot at the very beginning of its travel, that is exactly why the new HeadRoom amps have the Gain switch. I got tired of testing the impulse response of my eardrums with the prototype units. (I know, I could learn to turn the volume control down

 

[Serow]

If you don't mind the question, what is the low gain setting that you use with the new Headroom amps? I was doing some testing tonight with a cmoy out of my home cd player with the input voltage dropped by .5 and a gain of 3.3, and the volume pot is still only between 1/4 and 1/3 of full rotation using my Sony V6. I might as well have just built a discrete buffer!