Inverting or non-inverting for audio applications? - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: June 07, 2020, 03:41:21 pm
content: Hi,when it comes to audio applications, assuming signal polarity is not relevant for the specific case (i.e. mono signal just for signal quality analisys), what is best configuration to use?Inverting or non inverting? and Why?The only thing I am interested is the highest quality signal amplification, nothing else.Thank you

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 07, 2020, 04:13:35 pm
content: Are you specifically talking about opamp-based amplification?

author: ricko_uk
author_rank: Super Contributor
timestamp: June 07, 2020, 04:24:16 pm
content: Thank you SiliconWizard,Yes... But... What would be the alternatives you seem to imply? If you are referring to various classes (B, D etc) of audio amplifiers the yes because I want to use a op-amp specifically as power dissipation is not an issue. I want to stick to linear solution (and no switching anywhere on the entire board, not even for the PSU.Thank you

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 07, 2020, 04:26:02 pm
content: Some time ago, there was a heated argument over the DIYAudio forum.Someone cited a paper, as far as I remember one of the “Analogue Dialogue” series by ADI.There was a case presented in which the non-inverting configuration was preferred.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2020, 05:37:56 pm
content: In general, the inverting configuration, with a resistor in series between the input and summing point, has inferior noise performance due to the voltage noise of a relatively large resistor, but does not suffer from common-mode-induced distortion.The non-inverting configuration is therefore usually preferred for noise-sensitive applications, since the resistors in the feedback network can be made smaller than what one would use in the inverting configuration. The common-mode problem can be important, since the common-mode voltage equals the input signal voltage. If there be a non-linear input current due to the common-mode voltage (common with BJTs), it flows through the source impedance (including any input coupling capacitor). The voltage from that current looks exactly like an input voltage, and will not be affected by feedback.An interesting variation, almost never used, is to place the input signal between the feedback voltage divider and the inverting input, with the non-inverting input grounded, thus avoiding both problems but requiring a non-grounded source (possibly a phono pickup, microphone, or transformer secondary).

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 07, 2020, 06:14:49 pm
content: Yeah. To sum it up very briefly (and generalized), opamp-based amplification:- Non-inverting: higher input impedance, lower noise, possibly higher distortion, gains < 1 not possible;- Inverting: lower input impedance (in general), higher noise, lower distortion, gains < 1 possible.Unfortunately no one size fit all configuration. You'll have to do the maths depending on your exact configuration, gain, opamp characteristics, and then issue some measurements.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 07, 2020, 06:44:22 pm
content: Thank you SiliconWizard, SchmittTrigger and TimFoxQuote from: TimFox on June 07, 2020, 05:37:56 pm1) Would a op-amp with differential output (i.e. like an instrumentation amp where you can set the O/P ref voltage) work as a previous stage to provide a non-grounded source?2) I kept re-reading that part above but cannot understand how you would connect the input. Could you please explain "between the feedback voltage divider and..."Thank you

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 07, 2020, 07:07:12 pm
content: Some basics:https://sound-au.com/dwopa.htm

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 07, 2020, 07:16:09 pm
content: This may be a useful link to you:https://duckduckgo.com/html?q=Samuel+Groner+opamp+distortion+measurements+paper

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2020, 07:26:12 pm
content: Quote from: ricko_uk on June 07, 2020, 06:44:22 pmTo your second question, here is a simple drawing. Note that the Vin generator is totally floating.To your first, in theory that would work if the impedance at the "ref" node of the instrumentation amp is very high, but it often is only "medium", like 10k or so.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 07, 2020, 07:42:08 pm
content: Thank you all.TimFox,with reference to your schematic, could the Vin be the ground and output pins of an external floating MP3 player, like the headphone jack of a mobile phone?Thank you

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2020, 08:06:42 pm
content: You could do it if the external box were truly floating on battery power, but any conductance or capacitance from the "low" terminal of the external box to the "ground" of the amplifier would change the feedback circuit. Also, the "ground" of the external box would make an antenna for local EMFs, such as 50/60 Hz or MF broadcast stations. The only place I saw that suggested this was for an RIAA phono preamplifier, where the cartridge could float and connect through a shielded twisted pair to the input terminals. Using very low values in the feedback divider, this trick improved the noise and eliminated common-mode input voltage, since the feedback forced the inverting input to zero, agreeing with the grounded non-inverting input.If you are building the amplifier from discrete devices, you can reduce the common-mode distortion and common-mode input current problems by design techniques such as a cascode input stage. Of course, some monolithic op amps use that trick for the same reasons.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 07, 2020, 08:35:24 pm
content: Thank you Tim!