Hi all, I am building a new input switching board for my DIY amp (using a couple of CMOS and relays). I made it so that I could connect each input direct or through a voltage divider. The idea was to use it as a passive attenuator for sources too hot. The input board is going to be connected to a DSP (ADAU1701 based) with an input sensitivity of 1.0 Vrms and an input impedance of 10k. My RIAA preamp is usually outputting around 0.5 to 0.8 Vrms but my CD player and DAC are too hot at around 2.1 Vrms. So I am seeking the preferred resistor values if I would run the CD player and DAC through voltage dividers to add some -10 dB attenuation.
The values I am considering are (given R1 in series and R2 parallel to ground):
Alternative 1: R1 = 5.6k, R2 = 2.7k => -9.75 dB, Zin = 8.3k, Zout = 1.8k
Alternative 2: R1 = 6.8k, R2 = 3.3k => -9.72 dB, Zin = 10.1k, Zout = 2.2k
The DAC got a quite high output impedance of 480R so I guess alternative 2 might be preferable based on Zin but I guess it has to be balanced against the Zout. So my questions to you all, did I get the math right? Would you go with alternative 1 or 2? Does it really matter? Any other suggestions or comments?
One reason I am questioning my values is the following Goldpoint page: Balancing Amplifiers
According to the tables on that page for an amp with 10k input impedance they recommend: R1 = 19.1k, R2 = 75k. For those values my calculations gives => -1.97dB, Zin = 94.1k, Zout = 15.2k. So what am I missing here?
The values I am considering are (given R1 in series and R2 parallel to ground):
Alternative 1: R1 = 5.6k, R2 = 2.7k => -9.75 dB, Zin = 8.3k, Zout = 1.8k
Alternative 2: R1 = 6.8k, R2 = 3.3k => -9.72 dB, Zin = 10.1k, Zout = 2.2k
The DAC got a quite high output impedance of 480R so I guess alternative 2 might be preferable based on Zin but I guess it has to be balanced against the Zout. So my questions to you all, did I get the math right? Would you go with alternative 1 or 2? Does it really matter? Any other suggestions or comments?
One reason I am questioning my values is the following Goldpoint page: Balancing Amplifiers
According to the tables on that page for an amp with 10k input impedance they recommend: R1 = 19.1k, R2 = 75k. For those values my calculations gives => -1.97dB, Zin = 94.1k, Zout = 15.2k. So what am I missing here?
It seems you try to match the output Z of your network to the input Z of 10k of the DSP. That is unnecessary.
There are two somewhat conflicting issues for your attenuator network:
1 - the network should be lower impedance than the Zin of the next stage so that the next stage doesn't appreciably load down your network;
2 - the network should not be too low in impedance to avoid loading down your sources too much.
Your CD and RIAA preamps probably have output Z's in the 100 ohms region. Coupled with the 10k of the DSP, a network impedance of say 2k would work well.
Jan
There are two somewhat conflicting issues for your attenuator network:
1 - the network should be lower impedance than the Zin of the next stage so that the next stage doesn't appreciably load down your network;
2 - the network should not be too low in impedance to avoid loading down your sources too much.
Your CD and RIAA preamps probably have output Z's in the 100 ohms region. Coupled with the 10k of the DSP, a network impedance of say 2k would work well.
Jan
2k is a good choice as most opamp output stages will handle down to 2k without distortion starting to rise - some opamps can drive 500 ohms or so without distortion levels rising, but some are a bit less capable.
The lower the resistors the less noise they will inject, so 2k is a good choice - perfectly drivable by the source, no larger than they need to be (although 10k isn't a disaster).
Of course if you know the source will handle a lower impedance you can choose lower value resistors, but 2k is likely to work with any solid state source reasonably.
The lower the resistors the less noise they will inject, so 2k is a good choice - perfectly drivable by the source, no larger than they need to be (although 10k isn't a disaster).
Of course if you know the source will handle a lower impedance you can choose lower value resistors, but 2k is likely to work with any solid state source reasonably.
That was not my intention but maybe that's what the Goldpoint tables are trying to do?
Is that determined by the output impedance of the voltage divider?
Is that determined by the input impedance of the voltage divider?
The output level of my RIAA is good, I will only attenuate inputs for the CD player and DAC.
Would that 2k be the input or output impedance of the voltage divider?
I guess the source sees the input impedance of the voltage divider (R1 + R2) and that the load sees the output impedance (R1 || R2). And that I want to keep the input impedance as high as possible in regards to the output impedance of the source (e.g. 480R for my DAC) but at the same time the output impedance as low as possible in regards to the input impedance of the load (e.g. 10k for my DSP). Is that right?
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Rod Elliott got a pretty good article on the subject (as usual): Voltage Dividers and Attenuators
I will probably go with R1 = 5.6k and R2 = 2.7k...
I will probably go with R1 = 5.6k and R2 = 2.7k...