Capacitance multiplier - Page 1
Forum section: electronics / projects

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 06, 2022, 02:30:22 pm
content: Hi i am going to make a audio mixer, i going to use a DC to DC converter to get the -15 and +15 volt.I seen Daves video about taking away ripple from the switching PSU, it got me intrested, thank you.What would be the schematic for on the negative rail ?Can 1 transistor handle a 20 channel mixer with about 100 opamps ?, i guess not.So i have to solder those capacitance multipliers 2 times for each channel and main mixer.How would you do such a thing ?The ripple i can not measure on my analog scope, it is smaller then the noise, does it really work ?, how much voltage drop can i expect ?thanks

author: Kleinstein
author_rank: Super Contributor
timestamp: January 06, 2022, 04:30:27 pm
content: With relatively cheap high capacitance electrolytic capacitors and a linear regulator behind it there is no real need for the capacitance multiplier as such. One may consider the extra transistor if the voltage is on the high side, more to reduce the heat for the regulators than for ripple reduction. So this would be more like a 2 stage regulator than a capacitance multiplier.100 OPs at some 2-5 mA each would be only 200-500 mA and thus not yet a lot of current. So no problem for a single regulator.With likely less than 500 mA there should be no need for anything special and a 7815 / 7915 should be fine. if the votlage is on the high side, add some resistance before the filter cap to lower the heat loss and reduce current peaks. Current peaks at the filter cap may be larger issue than residual 100 Hz ripple.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 06, 2022, 04:51:59 pm
content: So you say take a +18 -18 volt DC-DC converter and have 2 linear regulators to make 15 volt.It wont take away all the ripple and those things heat up.I really would like smooth voltage no ripple, pro audio.I just following what youtube says, i cant measure that, i would like to.Can a digital scope measure that ripple ?btw : there are no current peaks in a mixer.

author: TimFox
author_rank: Super Contributor
timestamp: January 06, 2022, 04:55:34 pm
content: "Zero ripple voltage" is not a specification, but only a hope. Values that cannot go negative cannot be forced to zero.To obtain the required noise floor on your equipment, how much ripple voltage on the power supplies can you tolerate?Start with the power-supply rejection ratio values for your op amps, and work from there.To measure extremely low AC components on a DC line, I suggest getting a low-voltage-noise amplifier (perhaps something likehttp://www.thatcorp.com/1510-1512_Audio_Preamplifier_ICs.shtml) in a shielded box powered by batteries, with high-quality matched polypropylene input-coupling capacitors to remove the DC component, followed by an appropriate voltmeter or oscilloscope.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 06, 2022, 05:19:36 pm
content: Yes, "smooth voltage no ripple"? Sounds more like marketing than electronics. And "zero" something indeed never happens, except in politician's talk.That said, there are different ways supply voltage noise can end up coupled to the signal path. The direct part from the opamps' supplies, for instance, is certainly a factor, but given the high PSRR of decent opamps (could you tell us what kind of opamps you're using?), it's likely to not matter a lot unless your supply rails are purely atrocious.Noise can also get coupled capacitively and inductively, and there, the PCB routing would matter a good deal.As to measuring output noise? Forget about using a scope. They are not made for precision amplitude measurement, we'll probably have to repeat that until the cows come home.Now unless you have access to very expensive gear (such as this:https://www.rohde-schwarz.com/products/test-and-measurement/audio-analyzers_63675.html), using a good audio interface would do the job - as long of course as your expected noise floor is already significantly above that of said audio interface... But for evaluating audio circuits, that's usually the best approach, short of using expensive gear.Using a good voltmeter in AC could work - problem is frequency. They are rarely able to give accurate measurements above a few hundreds, or thousands of Hz for the best ones. So any high-frequency noise? This isn't going to cut it.

author: TimFox
author_rank: Super Contributor
timestamp: January 06, 2022, 05:38:49 pm
content: Good AC voltmeters (e.g. the classic -hp- 3400A and Marconi 2610 digital rms voltmeters) are good to above 10 MHz.They have flat response, and can't resolve the individual frequency components (i.e. 60 vs 120 Hz ripple), but can measure down to a few mV full-scale.The AC ranges on normal DMMs are not designed for that frequency range.

author: David Hess
author_rank: Super Contributor
timestamp: January 07, 2022, 12:08:28 am
content: There is no need or advantage to be had from a capacitance multiplier, and if the noise from a common regulator circuit is too high, then something is wrong with the circuit or layout. If better performance is required, then a lower noise and ripple regulator is not difficult to implement with a reference and operational amplifier.An NE5532 dual audio operational amplifier requires 8 typical and 16 maximum milliamps, so 4(milliamps per operational amplifier. If you figure that just as much current is wasted driving loads, including feedback networks, then supply current for 100 operational amplifiers could be from 800 milliamps to 1.6 amps, so nothing outrageous.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 07, 2022, 03:28:39 pm
content: Yes i have NE5532 and like to build a mixer for my friend.I have sliders 19 piece, and found simple EQ schematics.All the potentiometers and knobs will be pricey.The converter also like 20 or 25 euro.Hobby cost money.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 07, 2022, 03:49:54 pm
content: I am trying to keep it under 1A.Trying with least opamps possible.No buffers from the channel to the main mixer saves 2 opamps per channel, maybe it works.Maybe i can even have the fader and panning passive to the main mixer ?, what do you say.

author: ogden
author_rank: Super Contributor
timestamp: January 07, 2022, 04:10:43 pm
content: Quote from: SiliconWizard on January 06, 2022, 05:19:36 pmThis sounds like sofa expert's talk. There are indeed ways to reduce supply noise by some amount. It is measurable and confirmed by actual designs. You can try it and measure as well, thou you need serious tools (The regulator’s noise is 330 nV per root-hertz at 100 hertz and the circuit reduces this noise to 20 nV):https://wenzel.com/documents/finesse.html

author: Kleinstein
author_rank: Super Contributor
timestamp: January 07, 2022, 04:13:59 pm
content: The point is not so much that reducing the regulator noise is not possible. The point is that with decent quality OPs it is not needed. If the amplifier is senstive to low levels of supply noise and hum, there is probably something wrong with the amplifier.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 07, 2022, 04:19:07 pm
content: The NE5532 is an old standard an quite cheap. However it has a hidden catch that is sometimes missed: as an BJT based OP it has relatively high input current noise. With higher impedance pots the noise can become significant and JFET based OPs (e.g. OPA1678) may give lower noise. More modern OPs also get away with less power and may save on the power supply.With a SMPS bases supply one should more worry about RF filtering (e.g. extra common mode choke, LC filtering at the output) than a capacitance multiplier that may help a little with mains hum, but hardly with RF.

author: David Hess
author_rank: Super Contributor
timestamp: January 07, 2022, 11:04:06 pm
content: Quote from: Kleinstein on January 07, 2022, 04:13:59 pmOperational amplifiers have significant power supply rejection, so even a high level of power supply noise is reduced to a level lower than the operational amplifier's own input noise. That leaves circuit design as the major source of excess noise.Quote from: Kleinstein on January 07, 2022, 04:19:07 pmThat is definitely worth doing if lower power operation is desired. The operational amplifier still needs to have dynamic performance suitable for audio, which probably means lower supply current JFET or MOSFET operational amplifiers instead of bipolar ones, (1) but a lower input current noise will allow higher impedance networks which will draw less power from the outputs also saving power. Candidate parts include:OPA1642 1.8mA 5.1nV/SqrtHz 0.8fA/SqrtHz (6.4M) $0.95 20pA JFETOPA1652 2.0mA 4.5nV/SqrtHz 3.0fA/SqrtHz (1.5M) $0.90 100pA MOSFETOPA1688 1.6mA 8.0nV/SqrtHz 1.8fA/SqrtHz (4.4M) $0.70 20pA MOSFETOPA1602 2.6mA 2.5nV/SqrtHz 1.8pA/SqrtHz (1.4k) $1.30 20nA Modern bipolar for comparisonNE5532 4.0mA 5.0nV/SqrtHz 0.7pA/SqrtHz (7.2k) $0.32 200nA Old bipolar for comparisonThe OPA1602 has input bias current cancellation explaining its low input bias current but this also increases input current noise, so it requires a lower feedback network impedance for best noise than even an NE5532. Audio bipolar operational amplifiers usually exclude input bias current cancellation for this reason.(1) For technical reasons, JFET and MOSFET input operational amplifier deliver better dynamic performance then bipolar parts for a given supply current, so a low supply current bipolar operational amplifier for audio is more difficult to design and produce.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 08, 2022, 02:48:43 pm
content: 0,32 dollar cent for a NE5532 ?, i would like to know where.The other opamps are to expensive for me.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 08, 2022, 04:37:08 pm
content: For the lowest grades and 100 quantety 35 cents range is possible at Mouser or TME. Some of the other OPs may be currently hard to get due to supply shortage.If low cost is important the RC4558 or RC4580 may be an option.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: January 08, 2022, 04:41:14 pm
content: It still needs to be for audio, no cheapos.I just looked at mouser and for 100 i see another price.100: 0,448 € exclusive tax.Glad i already have those opamps in house for old pricing at 20 cent inclusive tax.

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2022, 03:13:37 am
content: If those are too expensive, then the RC4558 mentioned by Kleinstein should be considered if you want lower power. I almost put it on the list. It will be a little slow in some circuits, so the RC4580 would likely be a better choice but draws almost as much power as an NE5532. Values in parenthesis are calculated.OPA1642 1.8mA 5.1nV/SqrtHz 0.8fA/SqrtHz (6.4M) $0.95 20pA JFETOPA1652 2.0mA 4.5nV/SqrtHz 3.0fA/SqrtHz (1.5M) $0.90 100pA MOSFETOPA1688 1.6mA 8.0nV/SqrtHz 1.8fA/SqrtHz (4.4M) $0.70 20pA MOSFETOPA1602 2.6mA 2.5nV/SqrtHz 1.8pA/SqrtHz (1.4k) $1.30 20nA Modern bipolar for comparisonNE5532 4.0mA 5.0nV/SqrtHz 0.7pA/SqrtHz (7.2k) $0.32 200nA Old bipolar for comparisonRC4558 1.25mA 8.0nV/SqrtHz (0.23pA/SqrtHz) (35k) $0.05 Bipolar 3MHz 1.7V/usRC4580 3.00mA 6.5nV/SqrtHz (0.18pA/SqrtHz) (36k) $0.06 Bipolar 12MHz 5V/usQuote from: Jan Audio on January 08, 2022, 02:48:43 pmPrices are per amplifier in large quantities to allow comparisons with singles and quads, so double them per package for duals.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 10, 2022, 09:53:52 pm
content: All good advice out there. The power routing and wiring may well foil your low noise ambitions is not carefully thought through. Good passive filtering at the DC-DC input and outputs is essential. Use common mode chokes to limit the magnetic loop to a minimum. Put the DC-DC and filters in a can and on a separate PCB if possible.

author: magic
author_rank: Super Contributor
timestamp: January 11, 2022, 12:24:10 am
content: NJM2068 is 4558 on steroids and not expensive. I recall seeing them in some entry level "pro" soundcard.Less noise than 5532 (on par with 5534), similar GBW (worse than 5534) and wimpy output stage that distorts under heavier loading.May also look for NJM4580 instead of TI's sub if you like that part. I believe the main selling point of that one is high output current combined with reasonable noise (~5532). They appear to be the go-to part for budget soundcard manufacturers who want one opamp on the BOM for everything from MIC in to HP out.LM4562 is not bad and cheaper than the OPA16xx stuff.

author: Haenk
author_rank: Super Contributor
timestamp: January 11, 2022, 02:26:50 pm
content: Just take a torodial transformer with 25-0-25 (or a bit less voltage) and 7815/7915 - so much easier to build and a lot less noise to care about.Should not cost a fortune, either.