Looking for a decent, cheap, multiple low noise, low power opamp - Page 1
Forum section: electronics / projects

author: bd139
author_rank: Super Contributor
timestamp: February 01, 2018, 04:17:35 pm
content: I'm using 4x NE5532 in a design at the moment for a switchable Q narrow band audio filter based on cascaded R-L based gyrators and a 4066. It works nicely but I'd like to reduce the footprint and power usage somewhat as this is going in portable equipment.Can anyone suggest an alternative option to the NE5532 for this scenario. Must be low noise, much lower power and small footprint. SOT23 per opamp is acceptable or a SOIC with 4 opamps in it. Really need to get this down to a 1 by 3cm board outline if I can.

author: Audioguru
author_rank: Super Contributor
timestamp: February 01, 2018, 04:46:15 pm
content: Cheap and low power usually mean poor performance.An LM324 quad opamp is cheap, has low power and has poor performance.

author: mikerj
author_rank: Super Contributor
timestamp: February 01, 2018, 05:10:31 pm
content: Have a look at the LM837. Specs very similar to NE5532, and 14 pin SOIC package.

author: bd139
author_rank: Super Contributor
timestamp: February 01, 2018, 05:37:58 pm
content: That looks spot on. Many thanks for that. Will order a couple of samples.Edit: gah no longer manufactured

author: Zero999
author_rank: Super Contributor
timestamp: February 01, 2018, 07:16:20 pm
content: What impedances are connected to the inputs?The TL074 may have a higher voltage noise, than the NE5532, but it has much lower bias currents and associated current noise, therefore will have a much lower noise, than the NE5532, in applications where the inputs are connected to high impedances. From memory the TL07x starts to be quieter than the NE5532, when the input impedance goes about 18k, but do the maths.

author: bd139
author_rank: Super Contributor
timestamp: February 01, 2018, 07:18:10 pm
content: Had a look at that as an option and it ranks quite high. However think I found a unicorn. It’s only a quid and does the job: ST MC33079.http://www.st.com/content/ccc/resource/technical/document/datasheet/f6/03/24/8c/17/dc/4a/13/CD00000987.pdf/files/CD00000987.pdf/jcr:content/translations/en.CD00000987.pdf

author: Zero999
author_rank: Super Contributor
timestamp: February 01, 2018, 07:59:57 pm
content: Quote from: bd139 on February 01, 2018, 07:18:10 pmYou don't seem to have taken what I said about noise current on board. The MC33079 has higher noise current, than the TL074. Whether the MC33079 is less noisy, than the TL074 or not, depends on your circuit.The TL074 has in input noise current of only 0.01pA/Hz0.5, whilst the MC33079 has an input noise current of 0.5pA/Hz0.5, which is 50 times as bad. The noise current, becomes a voltage noise, when it passes through a resistor, connected to the input of the op-amp. If you build an inverting amplifier with an input impedance of 100k, the noise voltage generated by the MC33079's noise current would be 50nV/Hz0.5, whilst the TL074 would only generate 5nV/Hz0.5of noise voltage, due it's tiny noise current. Total input noise voltage would be (4.52+502)0.5= 50.2nV/Hz0.5for the MC33079 and (182+52)0.5= 18.7nV/Hz0.5for the TL074.Now if the input impedance is reduced to 10k, the MC33079 is better, than the TL074, with a total input noise voltage of 6.73nV/Hz0.5, vs 18nV/Hz0.5.So what are the input impedances? You need to conduct a proper noise analysis, rather than just looking at the figures on data sheets.

author: bd139
author_rank: Super Contributor
timestamp: February 01, 2018, 08:39:01 pm
content: I did consider that. The amp is a non inverting follower only. Input impedance is around 8k in the pass band although this is frequency dependent and that is best case and I haven’t done the math around it properly. By ear the NE5532 noise is barely audible.Please note I’m comparing this to the LM741 which was where I started not to the NE5532 directly. The design objective is an order of magnitude less shit than the 741 only which isn’t difficult. 741 was like taping sea shells to your head.

author: bd139
author_rank: Super Contributor
timestamp: February 01, 2018, 08:42:01 pm
content: Power requirement is under 15mA for the entire audio processing chain so anything fits in that which is relatively modern.Low cost is not Linear Tech £5 a go territory. £1-2 per quad.

author: Zero999
author_rank: Super Contributor
timestamp: February 02, 2018, 09:19:58 am
content: The TS524 seems to be similar to the the MC33079 and costs about the same.http://www.st.com/content/ccc/resource/technical/document/datasheet/09/15/96/b0/e9/eb/4c/e7/CD00001204.pdf/files/CD00001204.pdf/jcr:content/translations/en.CD00001204.pdfYou could also try the TL974, which has a rail-to-rail output but it's only rated to a maximum supply voltage of 15V and the input noise current isn't specified, although it's likely to be similar to the TS524 and MC33079.http://www.ic72.com/pdf_file/13pdf/TL974.pdf

author: bd139
author_rank: Super Contributor
timestamp: February 02, 2018, 09:36:57 am
content: TS524 is interesting - thanks for that. In small quantities it's 50% of the price of current selection. TL974 is even cheaperThanks for suggestions.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2018, 10:21:48 am
content: The choice of OP for low noise also depends on the impedance. The NE5534 and many other BJT OPs listed here are low noise for relatively low impedance (e.g. < 5 K) only. For higher impedance (e.g. to keep the caps small) it might be worth looking at JFET based OPs. Maybe an OPAx172 could work.A cheap BJT based version would be RC4558/9 (dual).

author: mikerj
author_rank: Super Contributor
timestamp: February 02, 2018, 11:24:13 am
content: Quote from: bd139 on February 01, 2018, 05:37:58 pmAre you sure?TIshow them as in production andMouser,DigikeyandArrowall show stock.

author: bd139
author_rank: Super Contributor
timestamp: February 02, 2018, 12:09:13 pm
content: I'm limited to Farnell / RS here for small batches and they don't stock

author: Zero999
author_rank: Super Contributor
timestamp: February 02, 2018, 12:18:26 pm
content: Quote from: Kleinstein on February 02, 2018, 10:21:48 amHe specified an input impedance of 8k, which would give a total noise voltage of 6nV/Hz0.5with the MC33079 and a J-FET op-amp with a lower noise voltage, would be considerably more expensive.

author: bd139
author_rank: Super Contributor
timestamp: February 02, 2018, 12:23:39 pm
content: Just as a point, I'm comparing to the baseline of approx 140nV/sqrt(Hz) for a good UA741CN which is still barely audible. An order of magnitude better would be a miraculously good optimisation.

author: Cerebus
author_rank: Super Contributor
timestamp: February 02, 2018, 04:54:42 pm
content: What are your voltage rails? The reason for asking is that there are a lot of nice new OPAs that won't cope with the classic +/-15V but are more in the order of +/-5V or even +/-3V.If this worked with LM741s and worked better with NE5534s I take it that you don't need more than 1.5MHz gain-bandwidth product, and a lousy 0.7 V/us is good enough as slew rate goes.

author: Marco
author_rank: Super Contributor
timestamp: February 02, 2018, 05:11:08 pm
content: I was looking for some modern high voltage opamps a while back and the OPA1679 looked nice, 2 mA per channel JFET audio opamp with up to 36V rail to rail. 4.5 nv/rtHz, 9v/us slew, 16 MHz GBW and significantly less than a $ in volume ... buy it before they realise they can charge more

author: bd139
author_rank: Super Contributor
timestamp: February 02, 2018, 06:59:38 pm
content: This isn’t golden ears territory. It’s a bandwidth narrowing filter to remove distractions close to the signal of interest which are already extremely noisy. The brain has some seriously good audio processing; merely trying to give it a little less work to do. There is already a lot of noise coming from the receiver which is the input to this which drowns the noise out anyway on all but the crappiest of opamps.What we have here is for perceptive purposes is:LM741 = seasideNE5532 = inlandGolden ears = buried in a coffin 6 feet underground.Voltage wise this is a 500mv pp signal at the very most after AGC so I’m good down to 3v opamps. Ideally I’d like to run it off a 6v single supply which is available regulated in the receiver already.

author: Cerebus
author_rank: Super Contributor
timestamp: February 02, 2018, 07:40:53 pm
content: Quote from: bd139 on February 02, 2018, 06:59:38 pmAha!ADA4692-4 quad (£3.12 1-9 off, £1.63 10+ @ RS)Low power: 180 uA typicalLow distortion: 0.003% THD + NLow noise: 16 nV/Hz0.5typical3.6 MHz bandwidthSlew rate: 1.4 V/us typicalOffset voltage: 500 uV typicalLow offset voltage drift: 4 uV/°C maximumVery low input bias currents: 0.5 pA typical2.7 V to 5 V single supply or ±1.35 V to ±2.5 V dual supplyAD8604 quad (£2.20 one off @ RS)Low Offset Voltage: 500 µV MaxSingle-Supply Operation: 2.7 V to 5.5 VLow Supply Current: 750 µA/AmplifierWide Bandwidth: 8 MHzSlew Rate: 5 V/µsLow DistortionNo Phase ReversalLow Input CurrentsUnity Gain Stable18nV/Hz0.5noise voltage50 fA/Hz0.5noise current

author: bd139
author_rank: Super Contributor
timestamp: February 02, 2018, 08:20:29 pm
content: Thanks for the suggestions - will have a look at those this evening.There are some interesting devices in the 5v-ish space. I haven't gone that low before

author: Cerebus
author_rank: Super Contributor
timestamp: February 02, 2018, 08:53:55 pm
content: Quote from: bd139 on February 02, 2018, 08:20:29 pmMe neither, more often than not I want the highest tolerable rail voltage but I'm starting to look at bootstrapping some of these new low voltage amps because some of the performance figures are almost incredible, vis:AD8009: +/-5V rails, 1GHz gain-bandwidth, 5,500 V/us slew, 1.9nV/Hz0.5voltage noiseADA4077: low cost (~£1.80), 4MHz, 7nV/Hz0.5voltage noise, 25uV offset, 0.25uV/C offset driftADA4896: 1nV/Hz0.5voltage noise, 2.8 pA/Hz0.5current noise, 120 V/us slew, 230 MHz GBW, 0.2 uV/C offset drift

author: bd139
author_rank: Super Contributor
timestamp: February 02, 2018, 09:13:06 pm
content: Crazy. Starting to wonder if I should start using opamps as RF amps at HF instead of transistors!Edit: yes I should according to the AD8009 datasheet

author: Zero999
author_rank: Super Contributor
timestamp: February 03, 2018, 06:25:56 pm
content: Quote from: bd139 on February 02, 2018, 12:23:39 pmWhere did you get such a high figure from? Did you measure it?The figure given on the uA741 data sheet is 23nV/Hz0.5http://www.st.com/content/ccc/resource/technical/document/datasheet/group1/d6/9e/4e/8a/fa/65/4c/d0/CD00001252/files/CD00001252.pdf/jcr:content/translations/en.CD00001252.pdf

author: bd139
author_rank: Super Contributor
timestamp: February 03, 2018, 07:09:26 pm
content: Data book. Think it was National Semiconductor.These aren’t new 741’s I was using as well.