Hi everyone - My first post here - I hope I do things OK. Thanks for reading.
I am considering purchasing a ready-made Balanced to Unbalanced Mic circuit which currently needs a dual rail supply in order to function.
I wondered if I could convert this circuit for a single-rail supply without too many problems or additional components.
(I attach the schematic of the circuit in question.)
Here are my steps.
1) The Op-amps supply rails need changing so the -18V rail now goes to ground.
2) The inputs appear to be decoupled so I don't need to worry about this.
3) I will need to decouple the output to only pass A/C so I'll put a 47uF capacitor there.
4) The circuit 'swings' around the ground point at the moment and I will need to make it 'swing' around half supply level. I think I should be able to do this using a potential divider (two 10k resistors) and feed one of the inputs to the first two op-amps with the resulting half supply.
I think that is it. Could I have your opinions of whether this would work, please, or is my thinking completely off?
As mentioned, I can get this circuit 'ready made' quite cheaply. I just wanted to use a single rail supply with it so I can power it from a single battery.
Thank you
Ken
I am considering purchasing a ready-made Balanced to Unbalanced Mic circuit which currently needs a dual rail supply in order to function.
I wondered if I could convert this circuit for a single-rail supply without too many problems or additional components.
(I attach the schematic of the circuit in question.)
Here are my steps.
1) The Op-amps supply rails need changing so the -18V rail now goes to ground.
2) The inputs appear to be decoupled so I don't need to worry about this.
3) I will need to decouple the output to only pass A/C so I'll put a 47uF capacitor there.
4) The circuit 'swings' around the ground point at the moment and I will need to make it 'swing' around half supply level. I think I should be able to do this using a potential divider (two 10k resistors) and feed one of the inputs to the first two op-amps with the resulting half supply.
I think that is it. Could I have your opinions of whether this would work, please, or is my thinking completely off?
As mentioned, I can get this circuit 'ready made' quite cheaply. I just wanted to use a single rail supply with it so I can power it from a single battery.
Thank you
Ken
Attachments
That is essentially all correct.
You connect all the ground points in the circuit (so that is the 0 volt line) to your 'virtual ground'.
You will have to see how it works with 10k's as a virtual ground. The 10k's put a limit on maximum output current to ground although for AC signals at typical signal levels it should be OK. You should add a suitable cap across the lower 10k such as a 220uF or 470uF The output will bias up to half supply voltage and so needs AC coupling as you say.
If you are thinking of using a 9 volt battery like PP3 then I would suggest the 5532 opamps are not really suitable. They draw quite a bit of current and will not work well at such a low voltage.
You connect all the ground points in the circuit (so that is the 0 volt line) to your 'virtual ground'.
You will have to see how it works with 10k's as a virtual ground. The 10k's put a limit on maximum output current to ground although for AC signals at typical signal levels it should be OK. You should add a suitable cap across the lower 10k such as a 220uF or 470uF The output will bias up to half supply voltage and so needs AC coupling as you say.
If you are thinking of using a 9 volt battery like PP3 then I would suggest the 5532 opamps are not really suitable. They draw quite a bit of current and will not work well at such a low voltage.
There are readily available battery holders that take 4xAA batteries.
There are many rail to rail opamps around that could work well off a +/- 3 V powers supply
There are many rail to rail opamps around that could work well off a +/- 3 V powers supply
That circuit is designed for low noise and requires low-voltage-noise opamps than can drive low impedance well, which is why the 5532 is a good fit. 4mA per opamp is pretty standard for high performance audio opamps. If you are happy with higher distortion go for micro-power opamps by all means, but for a mic circuit the noise performance is usually the most important figure of merit, so pick low-voltage-noise devices.
Thank you David, Mark and Mooly for your assistance.
The datasheet for the 5532 states that is takes 8ma supply current, so yes, 4ma per Op-amp as you rightly say.
All the best
The datasheet for the 5532 states that is takes 8ma supply current, so yes, 4ma per Op-amp as you rightly say.
All the best
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Hi Ken,
Welcome to the forum!
I'll offer a few comments about your project.
I understand you're buying a pre-assembled module. You might post a link so we can have "eyes on."
One issue I have with the design is opamp section IC2B. It's nominally a duplicate of section of IC2A, and connected in parallel; the two outputs are summed via R9 and R17. To the extent the offset voltages differ and the 820 ohm resistors are mismatched, the opamp sections will "fight" each other. I recommend IC2B gain resistors simply be deleted.
I think the steps you outline are reasonable, except you will have to bias both input opamps to somewhere mid-supply; if you left one (say IC1B via R12) biased to the negative rail, that section wouldn't operate since its input common range would be exceeded. There's also the dilemma of how to bias IC2A to roughly mid-supply.
To modify for single supply operation, I suggest that IC1B biased to about 2/3 of the plus rail and IC1A be biased to 1/3 of the plus rail---eg. 33k from IC1 pin 5 to B+, and 130k from IC1 pin 3 to B+. This should leave IC2A output at about mid-supply. As you mentioned, you'll need a DC block capacitor on its output. The 820 ohm resistors should have 1% or lower tolerance for best common-mode rejection in the differential stage. Make sure there's good bypassing of the supply.
One comment about noise performance. The unity gain input stages mean the noise figure is fully exposed to noise mechanisms in the differential stage. Adding some gain at the input stages would improve noise performance.
Welcome to the forum!
I'll offer a few comments about your project.
I understand you're buying a pre-assembled module. You might post a link so we can have "eyes on."
One issue I have with the design is opamp section IC2B. It's nominally a duplicate of section of IC2A, and connected in parallel; the two outputs are summed via R9 and R17. To the extent the offset voltages differ and the 820 ohm resistors are mismatched, the opamp sections will "fight" each other. I recommend IC2B gain resistors simply be deleted.
I think the steps you outline are reasonable, except you will have to bias both input opamps to somewhere mid-supply; if you left one (say IC1B via R12) biased to the negative rail, that section wouldn't operate since its input common range would be exceeded. There's also the dilemma of how to bias IC2A to roughly mid-supply.
To modify for single supply operation, I suggest that IC1B biased to about 2/3 of the plus rail and IC1A be biased to 1/3 of the plus rail---eg. 33k from IC1 pin 5 to B+, and 130k from IC1 pin 3 to B+. This should leave IC2A output at about mid-supply. As you mentioned, you'll need a DC block capacitor on its output. The 820 ohm resistors should have 1% or lower tolerance for best common-mode rejection in the differential stage. Make sure there's good bypassing of the supply.
One comment about noise performance. The unity gain input stages mean the noise figure is fully exposed to noise mechanisms in the differential stage. Adding some gain at the input stages would improve noise performance.
My take:
- Replace the GND points of R4/R12/R7/R15 with a virtual ground. Note: if the circuit is fully floating (battery supply and battery not used for anything else) then all GND points move to the virtual GND which actually becomes the true GND.
- ditch the paralleled output and use the free amp to buffer the virtual GND (derived from a voltage divider).
- use modern low power and low noise opamps with rail-to-rail output for maximum output headroom and best battery usage. OPA1644 quad is one ideal candidate for this.
- like @BSST mentioned, if you need gain (microphone signals usually do) then let the front-end opamps do the work. See this paper for details, Fig.1-3b
Most of my suggested mods were motivated by requiring minimal modifications of an already-populated module. With that philosophy in mind, I should have suggested removing either R9 or R17 to avoid the dueling differential stages; the gain setting resistors could remain in place. So the mods might be as simple as adding two bias resistors, an output blocking cap, and deletion of R9 or R17.
The instrumentation amplifier architecture is exactly what I was imagining for adding frontend gain, so KSTR's reference is great. Nor do I have any quarrel with his suggested virtual-ground approach.
Good luck!
The instrumentation amplifier architecture is exactly what I was imagining for adding frontend gain, so KSTR's reference is great. Nor do I have any quarrel with his suggested virtual-ground approach.
Good luck!