effects of gain selector switch in the feedback loop?

scone · 28th February 2005, 04:52 AM

I am going to build a preamp based on PMA's opa627/buf634 layout. It's going to drive an opa549 based amp, so the pre will have to have some amount of gain (since the max gain on opa549 is around 10, and this is not enough by itself). However, I would like to have the option of using the pre to drive different amplifiers as well, so I would like to be able to switch the gain between unity and whatever I decide for the opa549 setting.

If I put a simple SPDT switch in the feedback path of the opa627 to switch between two different resistors, will it have any negative effect on the sound? My guess is no, but perhaps others have actually tried this and know for sure?

Upupa Epops · 28th February 2005, 05:26 AM

Don't worry, it will be the same. But if you can't clicks by switching, make one resistor fixed ( in feedback to ground - lower gain ) and second one switchable with him in parallel ( higher gain ).

scone · 1st March 2005, 05:43 AM

Thanks for the suggestion... I'll implement it this way. I like the idea of having a resistor in the feedback path permanently. That way if the switch ever fails for some reason, the preamp will just revert back to unity gain.

As far as clicks go, I don't think I'll ever be changing the gain while the amplifier is on. I plan on leaving the switch directly on the circuit board (completely inside the chassis) since I think the only time I will change gain is if I move the preamp from the opa549 amp to a different amplifier. That will probably not happen often, so it won't be too much trouble to open the amp up and flip the switch manually. Plus it might be dangerous to have a gain selector on the outside... it might get pressed on accident when I'm driving an amp which expects line level input.

troystg · 3rd March 2005, 12:44 AM

Quote:

Originally posted by scone
Thanks for the suggestion... I'll implement it this way. I like the idea of having a resistor in the feedback path permanently. That way if the switch ever fails for some reason, the preamp will just revert back to unity gain.

As far as clicks go, I don't think I'll ever be changing the gain while the amplifier is on. I plan on leaving the switch directly on the circuit board (completely inside the chassis) since I think the only time I will change gain is if I move the preamp from the opa549 amp to a different amplifier. That will probably not happen often, so it won't be too much trouble to open the amp up and flip the switch manually. Plus it might be dangerous to have a gain selector on the outside... it might get pressed on accident when I'm driving an amp which expects line level input.

My JRDG Model 2 had a switch able input gain and impendence. Both switch able on the fly. It was nice to be able to do direct comparisons between the settings. Connected to my Proceed AVP the low gain and high impedance settings were best.

Upupa, as always, has the best suggestions and sound(pun intended) advice...

Evan Shultz · 3rd March 2005, 01:26 AM

Just for correctness, adding another resistor in parallel to the feedback resistor will lower gain, not increase it, because Rf will get smaller.

Reklats · 3rd March 2005, 02:29 AM

He could do something like this:

scone · 3rd March 2005, 03:47 AM

Quote:

Originally posted by rabstg
My JRDG Model 2 had a switch able input gain and impendence. Both switch able on the fly.

Studio gear sometimes has input gain selection, usually between +4 and -10dB to allow switching between different line levels. I'm actually going to have output gain selection, which I think has the potential of harming the amplifier depending on what setting is used. (gain of 3 = +9.5dB). I figure I'll hide the switch inside the amp and avert potential danger.

scone · 3rd March 2005, 03:49 AM

Quote:

Originally posted by Evan Shultz
Just for correctness, adding another resistor in parallel to the feedback resistor will lower gain, not increase it, because Rf will get smaller.

touche

troystg · 3rd March 2005, 02:46 PM

Quote:

Originally posted by Reklats
He could do something like this:

I think he was referring to a resistor to ground such as this. Parallel the Ri resistor with a second resistor seriesed with a switch.

But yours works also it just might be audible since it is in the "direct" signal path.

Upupa Epops · 3rd March 2005, 02:55 PM

I was talking about switching of parallel resistors ( from - input to ground ), not about serial ones ( from output to - input ).

28th February 2005, 04:52 AM	#1
scone diyAudio Member Join Date: Nov 2004 Location: Austin, TX	effects of gain selector switch in the feedback loop? I am going to build a preamp based on PMA's opa627/buf634 layout. It's going to drive an opa549 based amp, so the pre will have to have some amount of gain (since the max gain on opa549 is around 10, and this is not enough by itself). However, I would like to have the option of using the pre to drive different amplifiers as well, so I would like to be able to switch the gain between unity and whatever I decide for the opa549 setting. If I put a simple SPDT switch in the feedback path of the opa627 to switch between two different resistors, will it have any negative effect on the sound? My guess is no, but perhaps others have actually tried this and know for sure? __________________ audio projects

1st March 2005, 05:43 AM	#3
scone diyAudio Member Join Date: Nov 2004 Location: Austin, TX	Thanks for the suggestion... I'll implement it this way. I like the idea of having a resistor in the feedback path permanently. That way if the switch ever fails for some reason, the preamp will just revert back to unity gain. As far as clicks go, I don't think I'll ever be changing the gain while the amplifier is on. I plan on leaving the switch directly on the circuit board (completely inside the chassis) since I think the only time I will change gain is if I move the preamp from the opa549 amp to a different amplifier. That will probably not happen often, so it won't be too much trouble to open the amp up and flip the switch manually. Plus it might be dangerous to have a gain selector on the outside... it might get pressed on accident when I'm driving an amp which expects line level input. __________________ audio projects

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28th February 2005, 05:26 AM	#2
Upupa Epops diyAudio Member Join Date: Aug 2003 Location: Prague,Czech Republic	Don't worry, it will be the same. But if you can't clicks by switching, make one resistor fixed ( in feedback to ground - lower gain ) and second one switchable with him in parallel ( higher gain ).

3rd March 2005, 01:26 AM	#5
Evan Shultz diyAudio Member Join Date: Jun 2004 Location: California, USA	Just for correctness, adding another resistor in parallel to the feedback resistor will lower gain, not increase it, because Rf will get smaller.

3rd March 2005, 02:55 PM	#10
Upupa Epops diyAudio Member Join Date: Aug 2003 Location: Prague,Czech Republic	I was talking about switching of parallel resistors ( from - input to ground ), not about serial ones ( from output to - input ).

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