Stepped gainswitch with relais?

jrp · 22nd October 2007

On Micpres i find it very nice to have a stepped rotary switch to controll the gain.
You know those cheap 12step switches. What do you think about them?
Like pointed out in the other thread, i want to check out the INA103. It just needs one gainsetting resistor. What role does the distance between the chip and that resistor play? I´m talking of the distance between the chip and the switch. Propably not much, right?
Or, maybe a solution could be to have some relais switching in different resistors right on the pcb. I remember some design where a small number of relais were set up, so that a large number of different resistances could be realised, just by the combination of a handful of series and parallel resistors.

I can´t find that schematic anymore, anyone have something simmilar?
Or any oppinion on this is very welcome!

Matt Syson · 22nd October 2007

Hi
There are several aproaches to this, the easiest being a pot (cheap). A cheap switch or an expensive switch or a cheap switch with relays. FET switching could also be considered.
As long as the wiring is short (a couple of inches maximum. and you are careful not to let anything 'noisy' near it any technique could be good.
I favoured the cheap switch with relays as it allows me to be able to 'remote' switch the gains.
Matt S

JohnRoberts · 22nd October 2007

It is good to keep the path and loop area for this gain setting resistor small, to reduce noise pickup. I recall an early console design I did where the gain pot was a significant distance from the circuitry to satisfy a panel layout and packaging issue. I used shielded cable for the feed to/from the gain pot, but then I had to deal with cable capacitance which could destabilize the circuitry. I ended up driving the shield from a low impedance circuit node that caused the stray capacitance to help rather than hurt stability. In later designs I worked to keep the path short and avoid the gymnastics.

It is difficult to get an off the shelf gain pot with good law at the high gain end and good range at the low gain end. A 12 pos switch will allow accurate, repeatable, and consistent gain setting.

While this may not be a very obvious source, some large scale manufacturers can afford to tool up semi custom parts for this application. Perhaps ordering repair parts from their service department would work for small DIY projects.

JR

mpdonahue · 22nd October 2007

There are a couple of issues I see here besides the ones that Matt and John already pointed out.
The issue with relays is how do you get quiet changes in gain using relays. I haven't looked at the specs of the chip, but the issues of break before make are not insignificant here, especially when deal with high amounts of gain. Are you planning on switching in a single resistor or are you building a pot out of a series of resistors?
Also, getting usable steps with only 12 positions is virtually impossible. We are talking something like 5 or 6 dB per step. A little course for most applications. This requires a high gain position with a single gain stage (Like the Hardy M1) which makes the gain steps non linear in one or the other gain range or a multi stage preamp which has a second stage with a fixed 18 or 24 db of gain. I am of the school of thought that the 2 stage preamp has some real advantages over the single stage WRT front end headroom and amplifier linearity/noise.
All the best,
-mark

jrp · 23rd October 2007

thank you!
Yes, i should have mentioned. I actually plan to have 12 steps from say 15 to 65 db, and a second stage with around 0 to 6db of gain.
Maybe with a pot for the second stage, or somehow nicer, with a second 12 pos switch. Or spend some money on a 40pos switch.
Or maybe just one stage, sonce the ina can drive 600r loads and is most happy with gains around 100-1000.
So maybe the two switches could be combined for that first and only stage, although it might be impossible to get a usable law.
btw, here´s the datasheet. I came across this chip in some hifi forum because it´s used in some very nice phonopres.
http://focus.ti.com/lit/ds/symlink/ina103.pdf

anyone know what that "shield driver" thing is good for?

With the relais, it should be possible to do the stepping with much less than 12 relais, if you include some logic and switch in different combinations of a few resistors. Think the Emitter has a volume control that works like this.
I counted 6 or 8 relais if i remember right.
Thinking about this, i see a lot of efford, and i am not sure if it´s worth it.
But i might just as well mount the switch directly onto the pcb, so that distances are really short.
The contacts of the switch may be an issue, but i don´t belive in that too much. Don´t think i can hear the difference between a rotary switch and some relay.

The Ina103 is an instrument-amp, the basic three opamp design. It has two pins for the gainsetting resistor. If that is left open (and it will be for a moment inbetween two switching positions) the gain drops to 1.
No idea if that is going to cause any clicks.

22nd October 2007	#1
jrp Gear addict Join Date: Oct 2006 Posts: 306 Thread Starter	Stepped gainswitch with relais? On Micpres i find it very nice to have a stepped rotary switch to controll the gain. You know those cheap 12step switches. What do you think about them? Like pointed out in the other thread, i want to check out the INA103. It just needs one gainsetting resistor. What role does the distance between the chip and that resistor play? I´m talking of the distance between the chip and the switch. Propably not much, right? Or, maybe a solution could be to have some relais switching in different resistors right on the pcb. I remember some design where a small number of relais were set up, so that a large number of different resistances could be realised, just by the combination of a handful of series and parallel resistors. I can´t find that schematic anymore, anyone have something simmilar? Or any oppinion on this is very welcome!

22nd October 2007	#2
Matt Syson Lives for gear Join Date: Sep 2004 Location: UK Posts: 4,565	Hi There are several aproaches to this, the easiest being a pot (cheap). A cheap switch or an expensive switch or a cheap switch with relays. FET switching could also be considered. As long as the wiring is short (a couple of inches maximum. and you are careful not to let anything 'noisy' near it any technique could be good. I favoured the cheap switch with relays as it allows me to be able to 'remote' switch the gains. Matt S __________________ Matt S www.mseaudio.co.uk

22nd October 2007	#3
JohnRoberts Lives for gear Join Date: Nov 2006 Location: Hickory, MS Posts: 1,946	It is good to keep the path and loop area for this gain setting resistor small, to reduce noise pickup. I recall an early console design I did where the gain pot was a significant distance from the circuitry to satisfy a panel layout and packaging issue. I used shielded cable for the feed to/from the gain pot, but then I had to deal with cable capacitance which could destabilize the circuitry. I ended up driving the shield from a low impedance circuit node that caused the stray capacitance to help rather than hurt stability. In later designs I worked to keep the path short and avoid the gymnastics. It is difficult to get an off the shelf gain pot with good law at the high gain end and good range at the low gain end. A 12 pos switch will allow accurate, repeatable, and consistent gain setting. While this may not be a very obvious source, some large scale manufacturers can afford to tool up semi custom parts for this application. Perhaps ordering repair parts from their service department would work for small DIY projects. JR __________________ John Roberts www.CircularScience.com

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22nd October 2007	#4
mpdonahue Gear addict Join Date: Dec 2002 Location: Boston, MA Posts: 473	There are a couple of issues I see here besides the ones that Matt and John already pointed out. The issue with relays is how do you get quiet changes in gain using relays. I haven't looked at the specs of the chip, but the issues of break before make are not insignificant here, especially when deal with high amounts of gain. Are you planning on switching in a single resistor or are you building a pot out of a series of resistors? Also, getting usable steps with only 12 positions is virtually impossible. We are talking something like 5 or 6 dB per step. A little course for most applications. This requires a high gain position with a single gain stage (Like the Hardy M1) which makes the gain steps non linear in one or the other gain range or a multi stage preamp which has a second stage with a fixed 18 or 24 db of gain. I am of the school of thought that the 2 stage preamp has some real advantages over the single stage WRT front end headroom and amplifier linearity/noise. All the best, -mark

23rd October 2007	#5
jrp Gear addict Join Date: Oct 2006 Posts: 306 Thread Starter	thank you! Yes, i should have mentioned. I actually plan to have 12 steps from say 15 to 65 db, and a second stage with around 0 to 6db of gain. Maybe with a pot for the second stage, or somehow nicer, with a second 12 pos switch. Or spend some money on a 40pos switch. Or maybe just one stage, sonce the ina can drive 600r loads and is most happy with gains around 100-1000. So maybe the two switches could be combined for that first and only stage, although it might be impossible to get a usable law. btw, here´s the datasheet. I came across this chip in some hifi forum because it´s used in some very nice phonopres. http://focus.ti.com/lit/ds/symlink/ina103.pdf anyone know what that "shield driver" thing is good for? With the relais, it should be possible to do the stepping with much less than 12 relais, if you include some logic and switch in different combinations of a few resistors. Think the Emitter has a volume control that works like this. I counted 6 or 8 relais if i remember right. Thinking about this, i see a lot of efford, and i am not sure if it´s worth it. But i might just as well mount the switch directly onto the pcb, so that distances are really short. The contacts of the switch may be an issue, but i don´t belive in that too much. Don´t think i can hear the difference between a rotary switch and some relay. The Ina103 is an instrument-amp, the basic three opamp design. It has two pins for the gainsetting resistor. If that is left open (and it will be for a moment inbetween two switching positions) the gain drops to 1. No idea if that is going to cause any clicks.