Looking for a single power supply balanced microphne preamp design using  5534 - Page 1
Forum section: electronics / projects

author: hafrse
author_rank: Regular Contributor
timestamp: November 06, 2021, 01:50:59 pm
content: Hello,Looking for a single power supply balanced microphne preamp. design/schematics using 5534.Thanks for any input.George

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2021, 07:08:41 pm
content: You need an AC coupled differential amplifier.Awhile ago, audioguru did schematic showing the inverting and non-inverting op-amp configurations, both DC and AC coupled. Here's my version, which includes the differential configuration. You need to use the schematic in the bottom right hand corner.RFand RINdetermine the gain.AV = RF/RG

author: jonpaul
author_rank: Super Contributor
timestamp: November 06, 2021, 07:17:30 pm
content: Hi there:1/ 5534 is a very old design, newer modern parts may be easier availability and have better performance.2/ Best mic preamps use a mic transformer to match eg 600 Ohm ><50K3/ Noise figure of the input stage is important4/ Many discrete input BJT designs are avaialble5/ The mic transformer obviats the need for a diff amp.6/ OP should state the exact ype of mic eg Low Z dynamic, ribbon, ECT, etc.Best,Jon

author: SuzyC
author_rank: Frequent Contributor
timestamp: November 06, 2021, 07:17:39 pm
content: See Revised Schematic AttachedUse 1% resistors for 10k, 100k values

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2021, 09:58:07 pm
content: Quote from: SuzyC on November 06, 2021, 07:17:39 pmThere are a couple of errors in that schematic. The pin out is for a dual op-amp such as the, NE5532, or TL072, not the NE5534 and it's not a differential amplifier. The common mode rejection is non-existent.Refer to the schematic attached to my previous post, for an example of how to properly design an AC coupled differential amplifier.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2021, 08:38:33 am
content: And here it is: fixed.If you want to vary the gain, it's better to AC couple the potentiometer, but I've not implemented it.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 07, 2021, 06:20:49 pm
content: A diff amp has the necessary common mode rejection but in practice its not that great unless you use precision resistors use a trimpot (arrgh!). Here is different take (hoho). As shown, the input impedance is near enough 600R.It's tempting to try and squeeze more gain from this stage but CM voltages may be too high and clipping will kill the Diff amp' abilty to cancel it. More noise gain too.CMRR is roughly (Gain +1)/( 4x tolerance) another reason to keep the gain low. You can leave out the input caps, but then its a dare you don't have to take.Avoid ceramic caps in the signal path. They can be "microphonic" and if your mic amp is near a power amp speaker feedback may occur.You might want to consider a sinlge ended amp fed from a microphone transformer- old school but less problematic and you can push the gain a lot higher. These XMFRs can be insanely expensive. Check the second hand market.Cut and paste error- here is the correct schematic with controlled frequency response.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2021, 08:33:55 pm
content: Quote from: Terry Bites on November 07, 2021, 06:20:49 pmThat will work, but are two op-amps really required? That may be reasonable if a dual op-amp is used, but the original poster wants to use the NE5534, which is a single op-amp IC. It also isn't unity gain stable, so would oscillate, if used for X2.R5 & R6 can be reduced to 1k, C4 increased to 100µF, X2 removed and R4 connected directly to C4.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 07, 2021, 08:43:37 pm
content: If you want high CMRR, you can also consider using an instrumentation amp. Of course not quite what the OP was asking for, but you can find single-supply instrumentation amps these days with good BW product (enough for audio) and low noise. One example would be AD8223.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 09, 2021, 09:33:45 pm
content: Thats what I would do. Aslo check out some THAT products.

author: nfmax
author_rank: Super Contributor
timestamp: November 09, 2021, 10:22:50 pm
content: We made a microphone preamp something like this back in 1992. However, we used an LT1028 and a novel circuit arrangement resulting in less than half the noise level of a 5534. It had an active common-mode cancellation circuit that required only one pair of resistors to be matched for optimum performance. The whole (stereo) thing ran off a single 9V battery. Quite proud of that design!Available online athttps://worldradiohistory.com/UK/Wireless-World/90s/Wireless-World-1992-10.pdf(page 811)

author: Zero999
author_rank: Super Contributor
timestamp: November 10, 2021, 08:05:33 am
content: Quote from: Terry Bites on November 09, 2021, 09:39:12 pmThe NE5532 is unity gain stable, but the NE5534 is only stable to gains down to 5.https://www.ti.com/lit/ds/symlink/ne5532.pdfhttps://www.ti.com/lit/ds/symlink/ne5534.pdfThe bias only has to have a low AC impedance. It can be much higher at DC, without affecting the CMRR at signal level. Using a large capacitor to bypass R6 to 0V, eliminates the potential divider's AC impedance, so a buffer op-amp isn't required.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 10, 2021, 06:07:23 pm
content: Get rid of those fat ass input caps. Use an integrator to servo the DC operating point instead. The integrator RC also sets the lf roll off. The integrator can be a general purpose opamp. Not a 741 obviously, they only exist because people buy them by accident. C1 sets the hf roll off and kills off some RFI.So this is my last effort. Who knows if I'll get any bench time for a fiddle about.The input impedance is a constant 600R up to 30kHz. Gain 30dB. The ouput coupling is for illustration only.