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nigeljking 8th September 2011 06:49 AM

Switchable balanced / unbalanced input buffer
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I am wanting to create a universal input buffer for a preamp that supports the following:
  • LP filter for HF noise
  • HP filter for DC block with a switchable bypass
  • Ability to switch between balanced and unbalanced behaviour
The buffer will be used as the first stage after source input selection. I will be using a uP (Arduino) to control switching of the Input and the HP and Balanced/Unbalanced behavior. The intent is that the same buffer will support multiple scenarios depending on the connected source requirements. I was wanting to avoid having dedicated balanced or unbalanced inputs as well as having to duplicate the buffer for every input.
The circuit below shows my thinking.

Relay 1 provides the balanced/unbalanced switching
Relay 2 provides the DC block bypass

For unbalanced input, R6 and R7 are in series to ground, giving a gain of 2 for the opamp, combined with the R1 + R3 and R4 voltage divider giving a gain of 1/2, resulting in overall unity gain.

For balanced input the inverting circuit is identical to the non inverting circuit and provides unity gain.

Any comments and thoughts around the suitability of the circuit would be appreciated.



AndrewT 9th September 2011 05:01 PM

balanced impedance connections rely heavily on precise matching of source and receive resistances & // capacitances, good for CMRR (interference suppression).

If you have one half of the balanced receiver switchable, then that precise matching of impedances can be lost. If you don't have or need good CMRR then there is no advantage in having nor using balanced connections.

nigeljking 12th September 2011 11:03 AM

Hi Andrew,

Thanks very much for the response. I am familiar with the concept of balanced inputs and use balanced outputs/inputs in existing preamplifier/power amp interface designs (I currently use the SSM2142/1).

In previous designs I have only used unbalanced inputs on the pre amp but was thinking it may be worthwhile exploring support for source devices that provide a balanced output. I wanted to avoid having allocated/dedicated balanced inputs which may remain unused in the short term. An increasing number of DACs seem to be providing balanced output.

I had realised about the need to match resistance but was forgetting about capacitances (slap on the head to me). As I was planning on having the capability of switching out the HP capacitor for the unbalanced input, I could forget about the capacitor on the inverting input, and short the capacitor on the non inverting input when using the balanced input.

Any further thoughts? Or am I just over complicating things and pandering to audiophile superstitions?



AndrewT 12th September 2011 11:11 AM

Pass and many others show a simple way to add unbalanced input/output next to an existing balanced socket. But, that requires an extra RCA adjacent to every XLR.

nigeljking 12th September 2011 12:03 PM

Thanks Andrew, I will check out Pass' site


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