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Old 12th December 2002, 04:31 PM   #1
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Default solid state relays for switched attenuator

Has anyone tried this? What is the problem with using solid state relays (DC type) for switching resistors on a preamp output... I was thinking of doing it with my upcoming balanced line stage project.
-NS
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Old 12th December 2002, 04:34 PM   #2
tiroth is offline tiroth  United States
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problems: distortion and limited bandwidth.
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Old 12th December 2002, 04:42 PM   #3
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Thanks for the reply...
Could you explain to me why this is a problem? If it is a MOSFET SSR, and is sinking a resistor to ground, how would you get the limited bandwidth? (I'm not doubting it's true, just wondering why). Also, why the increase in distortion?
-NS
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Old 12th December 2002, 04:50 PM   #4
Jeff R is offline Jeff R  United States
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MOSFETs have a lot of capacitance and the resistance is non-linear. There are MOSFETs with some very low Rdson values (millihoms) such that the resistance non-linearies would be inconsequential, though, but you still have some capacitance and even a touch of added inductance. It may work fine for mid-fi, but I am not sure it would be good for hi-fi. Using it in the ground leg would present the fewest problems, most certainly. Using discrete MOSFETs might be a better approach, too. But, when in doubt, try it out.
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Old 12th December 2002, 04:58 PM   #5
sam9 is offline sam9  United States
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Default Silonex Devices

I've not tried these, but they may be worth investigating.


http://www.silonex.com/audiohm/softswitch.html

and

http://www.silonex.com/audiohm/levelcontrol.html

The only way one could know for sure is to compare a signal switched/attenuated by one of these with one switched/attenuated by more conventional means.

If you pursue this, I'm sure people hear would be interested to read the results.
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Old 12th December 2002, 05:03 PM   #6
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My thinking is this:
Since the MOSFET is being used as a switch, and the output of the preamp will be driving a 5K attenuator (paralleled with the input impedance of the power amp, and a 10K resistor to ground before the attenuator), won't the difference in the resistor values you are switching compared to the ohmic value of the turned off MOSFET be large, thereby making it of little effect when used in the sinking position? And when you turn it on, the Rds will go to .01 ohms or less, which is quite small compared to the resistance of the attenuator resistors. I know I have to be missing something here. The capacitance was something I was worried about.
I will be trying it out in the not-so-distant future, but I wanted to see if I was wasting my time or not. They will be built out of discrete components.
Thanks for the reply.
-NS
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Old 12th December 2002, 05:07 PM   #7
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SAM9,
I will check out the link. I was planning on using some of the siliconix or zetex mosfets, and building a similar system using relays for switching for comparison. Both will be switched (two devices at a time) using a microcontroller and optical encoder.
Thanks
-NS
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Old 12th December 2002, 05:12 PM   #8
tiroth is offline tiroth  United States
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Even though the importance of a switch/relay that is switching a resistor might seem lower than one switching the full signal, that isn't necessarily the case.

If it has limited bandwidth, it will sink current more at low freqencies than at high. Now you have a treble-boost switch, not a volume control. If it has too much capacitance, you may have a bass-boost when switched off. And if it responds in a nonlinear fashion, guess what signal - nonlinear component looks like...

Building a discrete "relay" is probably the way to pursue this as the commercial units are seldom intended for audio switching, and thus may have intentially or unintentially limited bandwidth. I still tend to think that a quality passive relay presents less opportunity for degradation.
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Old 12th December 2002, 05:28 PM   #9
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tiroth-
I see your point. What I don't understand is why there is a bandwidth limitation with a solid state relay. But, let's forget SSR's for the moment- assume that I am using discrete MOSFETs for the switching-
is a turned off MOSFET nonlinear, when looking into the drain? The only thing I can think that would cause this would be the capacitance. Now, when you turn it on, and I mean ALL the way on, what would cause the nonlinearity? The Rds will be so small (.01 ohms) compared to the resistor above it. How much distortion could this add, when so much of the voltage is dropped across the resistor? I will admit, my knowledge of MOSFETs isn't great, by any stretch of the imagination. I just don't see what the problem would be (which is why I was hoping someone could clue me in).
As far as passive switching- yeah, of course it's by far the best. It does take up a lot of space, and relays will wear out over time. Contacts won't stay clean forever.
Really, this is beside the point. We all know relay switching works well... I was just wondering about an alternative.
Thanks for the perspective.
-NS
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Old 12th December 2002, 05:59 PM   #10
BrianGT is offline BrianGT  United States
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Default Re: solid state relays for switched attenuator

Quote:
Originally posted by nobody special
Has anyone tried this? What is the problem with using solid state relays (DC type) for switching resistors on a preamp output... I was thinking of doing it with my upcoming balanced line stage project.
-NS
My roommate and I are working on a relay volume control for a preamp, and just finished the first version, and it works pretty well. Here are some pictures:
http://brian.darg.net/relay

The relays that I am using are these:
http://www.nec-tokin.net/now/english...lay_MR82_e.pdf

It seems to be quite clean, and much better then the first prototype, based on the burr brown PGA2310 digital volume control chip.

The microcontroller is a PIC18F452, on a development board for a class here at Georgia Tech. Here is info on the board. It is fairly cheap:
http://www.picbook.com

--
Brian
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