Upgrading a channel strip

[godzofguitar]

Hi all !
today is my first post in the group and I’m looking for some feedback on a mic input circuit design for upgrades or some basic clean up to drop the noise floor any help is much appreciated…

 

[JohnRoberts]

You could brute force it some with lower noise components but that preamp input stage is running open loop so will have higher distortion than preamps with negative feedback returned to the input device emitters. Most typical preamps require one or two more op amps to do that.

JR

 

[godzofguitar]

Thank you John for the input on the circuit design proposed, any suggestions for specific low noise devices based on the circuit? it is using the old work horse ne5532 except for the channel led clip indicator that is a rc4558, believe it or not it is pretty stable with a decent sound it is an aggressive forward sounding channel strip, I have a very high quality power supply powering the board thinking that could be helping the board a good bit with the noise floor although it could use some harmonic love, as far as negative feedback closed loop network it would take some rework to make that happen might have to make some circuit boards for that for a physically stable and professional build, thanks again for your time and input.

 

[JohnRoberts]

The mic preamp noise with that topology is dominated by the input bipolar transistors (PN4355). I am not familiar with those parts and expect there are lower noise modern PNPs. Back in the day I used 2sb737s extremely low noise PNPs but they went obsolete around the turn of the century. To optimize the mic pre for different devices may involve several tweaks, and you still lack the benefit of wrapping NF around those input devices to reduce distortion.

Using a 4558 for the clip indicator is ok, it is not in the audio path.

JR

 

[ruffrecords]

Have you actually measured the noise floor? Many of these designs are within a small number of dB of the theoretical maximum so there are no big gains in noise performance to be had.

Cheers

Ian

 

[gyraf]

I don't see anything here that would account for unneeded noise - as JR and Ian mentions, your improvement would most probably be maximum 1-3dB lower noise, and this only with genuine low-impedance microphones, say 50 Ohms or 150 Ohms perhaps.

That said, I find it worrying if 5532's are used in the EQ circuits pictured there: The potentiometers are not isolated from amplifier input-bias-current so these will go unnecessarily scratchy over time. For these positions, A3-4-5-6, I would recommend using low-input-current opamps i.e. JFET types like TL072.. Noise contribution would be minimal, that part is dominated entirely by the input transistor pair..

/Jakob E.

 

[Matt Syson]

Getting a good EIN figure for medium gain is pretty difficult and in days gone by only EIN at MAX gain was specified thus overlooking the gain settings that are used in practice. as it is a mixer are you sure you are measuring the most important aspects as presumably you want it as noise free as possible when recording ONE channel at a time. As Ian poiunted out the noise figures obtained were rarely more than a few dB different unless a piece of gear was very badly designed.