Preamp Design  - Page 1
Forum section: electronics / beginners

author: Jbliss
author_rank: Regular Contributor
timestamp: January 21, 2018, 12:14:46 am
content: Hi All,I am looking at designing / building a Mic preamp attached is a schematic / design I'm am interested in. I understand the basics of this circuit and it operation. However it was be amazing if someone could explain, in further detail, it's operation in stages.Including Information regarding CMRR and what to look for when trimming it ?Any help would be greatly appreciated in furthering my understanding of preamp topology.ThanksBlissy

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: January 21, 2018, 12:59:25 am
content: Well its a Samuel Groner design. He is a nice guy and "hangs out" sometimes on diyaudio.com. You should post there.It looks like a fairly standard low-noise design. There are a lot of specific component recommendations. Are you willing and able to follow them?If yes, then this design is probably for you and you should post on diyaudio and get help there.If no, you should go with a simpler design. Chances are good this is too much for you. It is definitely complicated for afirstpreamp.Hope this helps,Randy

author: Jbliss
author_rank: Regular Contributor
timestamp: January 21, 2018, 01:17:08 am
content: Hi RandallMcRee, Thanks for the reply I have already put together a parts list and sourced the majority of the components. I am up for the challenge, should be a good learning experience. I have tried to get into contact with Samuel unfortunately I was unable to.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: January 21, 2018, 04:00:55 am
content: OK, good first step.Are you designing a pcb or wiring by hand?CMRR trimming is pretty far down the road--you need to get the whole circuit working first. Do you have a scope and a signal generator?The reason I ask about the construction method is that your overall quality (e.g. noise) will probably depend on your construction skills, e.g. single point grounds, low noise power and so on.Are you going for low noise?:Replace Q1-Q4 with 2SA970BL for very low noiseI would still recommend that you post at diyaudio, the folks here definitely tend more towards the EE spectrum and less the audio one. Samuel may not reply but there are many denizens happy to help. Myself, I have built pretty much everythingexcepta mic preamp (I've looked at lots of designs....).

author: Jbliss
author_rank: Regular Contributor
timestamp: January 21, 2018, 06:31:33 am
content: Designing PCB in KICAD as we speak.I have 4 channel scope and a basic Sig Genhave been doing a bunch of research regarding optimal grounding and low noise power. Taking this project slow using it as an opportunity to learn new skills.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 21, 2018, 06:53:47 am
content: That circuit has been around for over a decade. I would have thought that there were PC boards (or layout files) available long ago. Note particularly the notes in the schematic diagram and the cautions mentioned by @RandallMcRee

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2018, 10:36:24 am
content: Anyone know how this works? There's some unusual things in there. What's going on with C10 and C11? Normally to make a non-polarised capacitor, two aluminium electrolytic capacitors are connected in series back-to-back, not parallel.

author: danadak
author_rank: Super Contributor
timestamp: January 21, 2018, 11:23:35 am
content: If you need a spectrum analyzer this one meets low cost criteria -https://www.zeitnitz.eu/scope_enhttp://www.zelscope.com/http://www.ledametrix.com/oscope/http://www.virtins.com/downloads.shtmlBut first build a simple circuit to protect sound card inputs so you do notruin from transients, overvoltage. Google "protect sound card input".For examplehttp://makezine.com/projects/sound-card-oscilloscope/Regards, Dana.

author: The Soulman
author_rank: Super Contributor
timestamp: January 21, 2018, 12:02:22 pm
content: Quote from: Hero999 on January 21, 2018, 10:36:24 amC10 and C11 AC couple the gain switch, the bottom of the divider is 2 Ohms, in order to maintain that it is necessary to ad as little of esrfrom the caps as possible (as this would alter the frequency response).For any other application I wouldn't use anti parallel caps but here with very low voltage ac and minimum dc offset it is acceptable.Would I build it? No.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 21, 2018, 12:08:04 pm
content: Quote from: The Soulman on January 21, 2018, 12:02:22 pmYou wouldn't recommend this design?

author: Audioguru
author_rank: Super Contributor
timestamp: January 21, 2018, 12:17:57 pm
content: The circuit has way too many parts. It says its max gain is 66dB which is 2000 times that will pickup whispers a couple of blocks away. Why?

author: The Soulman
author_rank: Super Contributor
timestamp: January 21, 2018, 12:26:57 pm
content: To be honest I don't have any experience with this design but to me it looks designed in simulation software with perfect components.The real live implementation looks difficult and perhaps not very fault tolerant and giving the alternative's I wouldn't bother with this one, unless as a learning experience with regards to component and board parasitic's.If the goal is to build a good working microphone amplifier I'd suggest something around a INA217 that has all the difficult stuff integrated into one IC.http://www.ti.com/lit/ds/symlink/ina217.pdf

author: dmills
author_rank: Super Contributor
timestamp: January 21, 2018, 12:34:24 pm
content: 66dB is perfectly reasonable if you are potentially dealing with things like ribbon mics, you don't need it often, but there are times.Most of the complexity is in the stuff around the outside, protection against P48 accidents, RFI measures, the usual.It is a bit of an old school job and you can get similar performance out of a single chip these days (But you still need all that stuff around the edges to make a reliable design), the major upside of some of the modern parts is the use of a servo to do the DC control, makes for a much more reasonable cap value.Have a look at the THAT Corp datasheets and app notes for a single chip design that works really rather well.Regards, Dan.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 21, 2018, 03:08:02 pm
content: Quote from: Jbliss on January 21, 2018, 12:08:04 pmNot clear exactly what your goals or experience (or desired result) are here?That design is getting rather dated. It was state-of-the-art back in its day, but as others have mentioned, silicon technology and performance have improved since that day. It is also one of the more complex, fiddly, and unstable designs you could have selected.If you are looking for a challenge to occupy a few months of your spare time, it will probably fill the bill. But if you are looking for some good mic preamps for practical application in 2018, there are probably better/simpler solutions.

author: ruairi
author_rank: Frequent Contributor
timestamp: January 21, 2018, 09:52:24 pm
content: If I was building a mic pre I would go where all of the mic pre builders hang out -www.groupDIY.comThen I would decide whether I wanted something clean (SSL9K, THAT variants, GreenPre, Hardy M1 style) or something more colored (Neve ish or API ish for example).Next I would pick a project that has a PCB available and lots of support and advice available from previous builders.Then once you have completed that project and figured out power supplies etc you could consider doing a layout for your own project. Per some of the above posters I could base that first PCB around one of the single chip solutions) - something similar to what Seventh Circle Audio do with their T15 -https://www.seventhcircleaudio.com/products/T15I would suggest the T15 above as a great start for a clean pre and one of Jeff Steiger's Classic API mic pres for something more colored -http://www.capi-gear.com/catalog/

author: Jbliss
author_rank: Regular Contributor
timestamp: January 22, 2018, 09:23:36 am
content: Thanks everyone for there help! definitely considering all the options.Quote from: The Soulman on January 21, 2018, 12:26:57 pmI am Interested in the INA217 however the output in the application circuit is unbalanced, however I assume I could add a Balanced Line driver circuit using a two opamps (ne5532) ?

author: Jbliss
author_rank: Regular Contributor
timestamp: January 22, 2018, 09:46:33 am
content: Or even easier I could use DRV134PA - BALANCED LINE DRIVER possibly.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: January 22, 2018, 09:58:25 am
content: The main point to keep noise (hiss) low is to avoid high value resistors in the input circuitry. If the resistor is shunted by a much lower value it doesn't matter (as with the 22k across a 600R mic) but if it is in series with the signal or 'floating' across it, it will induce noise. That also applies to the feedback loop, and is probably why a value as low as 2R was used.All resistors in low level signal paths should be metal film, not carbon.This comes as a surprise to most people, who think that transistors hiss.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 26, 2018, 10:40:48 am
content: Hi All,Thanks for all your help so far. I have decided to start with an easier design based around the INA217.Would be great if you could have a look over my circuit. Its based off the application notes and is pretty simple.please let me know what you think and any edits.(excuse messy layout)Thanks

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 26, 2018, 03:05:44 pm
content: Good choice (IMHO). I believe that same circuit (and board layout) can also be used with That Corp. chips for the mic preamp and the balanced output.The only two tricky things with that circuit are:1) Matching the pair of 47uF NP capacitors. I plan on using one of the little $10 component testers to select matched pairs of capacitors.2) The low-value, reverse-log taper control for the gain. I have layed out a little round PC board that can mount on the back of the commodity 12-step rotary switch. You can put 12 surface-mount (0805) resistors of your choice on the board.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 27, 2018, 02:06:12 am
content: Quote from: Richard Crowley on January 26, 2018, 03:05:44 pmThanks Richard thats perfect !I am trying to work out the resistor values for a 12 step 1K series attenuator is there an easy way to do this?Thanks

author: Jbliss
author_rank: Regular Contributor
timestamp: January 28, 2018, 03:30:09 am
content: Hi All,I this is my first PCB Layout it would be great if some one could let me know what they think.Reagard the VCC and VEE nets currently I have them routed as traces daisy chaining round the board would it be more beneficial to route one as a power plane on the top layer ?Thanks any help appreciated

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 28, 2018, 05:55:17 am
content: JBliss:Just for grins, here is a calculation of 12 resistor values for the switch described by Mr. Crowley. You could have done all of the calculations by hand (and probably by now you have) but I did it using MathCad anyway. I made a couple of assumptions:1 - that you didn't need a 0 dB gain position for a mic. and2 - that gain positions were 3 dB apart. Let me know what gains you really want and I'll be happy to adjust the calculation. Any monotonic mix of positive gains can be accommodated.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 28, 2018, 08:23:07 am
content: Quote from: basinstreetdesign on January 28, 2018, 05:55:17 amThanks!!The Values I roughly Came up with for an audio taper whereR1 1R2 4.3R3 9R4 10.7R5 14.4R6 23.5R7 36.8R8 57.9R9 93.5R10 147R11 232R12 370Total = 1000.1(approx. depending on available Resistors)Would these values work ?

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 28, 2018, 08:55:42 am
content: Quote from: Jbliss on January 28, 2018, 08:23:07 amWell, theoretically they would produce gains up to 80 dB. Are you sure you want that much gain in one stage? Wiring the chip out to a 1 Ohm resistor for 80 dB gain will make an opportunity for the injection of noise into the signal via the wires. I checked only the first 3 values (starting with 1) and found that they were not consistently spaced. I think they were at 80dB, 75.5 dB, 57 dB ... or so. Gotta get to bed now. What Gains are you shooting for? I'll take another look at it tomorrow.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 28, 2018, 09:45:31 am
content: Quote from: basinstreetdesign on January 28, 2018, 08:55:42 amahhh ok, I'm looking for approx 60db of gainThanks you so much !after further research I think my best option would be a 10k stepped attenuator with a series 10 Ohm resistor to set the maximum gain at 60db and allow the minimum gain set at 6dbattached is the circuit

author: The Soulman
author_rank: Super Contributor
timestamp: January 28, 2018, 11:44:18 am
content: Correct.You want to look also at the compatible SSM2017 datasheet, it also has some info on pcb lay-out.Or a commercial design around a SSM2017 can be downloaded here: (all Crest century series consoles use ssm2017 pre-amps):http://peaveycommercialaudio.com/schematics.cfm

author: Jbliss
author_rank: Regular Contributor
timestamp: January 28, 2018, 12:53:20 pm
content: So after some calculating if the lowest value of the attenuator (gain switch) is 10ohm I have 60db of gain. Going down in 3db steps gets me down to 27db for 12 positions. What I really need is a stepped attenuator with the lowest value being 10 Ohm and highest values being around 640 ohm (approx.) for a minimum gain of 27db (approx.)So max gain 60dbMin Gain approx. 27dbjust need to work out resistor values for each 3db Drop

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 28, 2018, 10:29:44 pm
content: Quote from: Jbliss on January 28, 2018, 12:53:20 pmOK, so according to your latest numbers, here are my values. They do not take into account your 4R7 resistor on your schematic.BTW: Be wary of those switches if you go for them. I have used them. The body is made of thermo-plastic. I left the iron on a terminal for 3 seconds too long once and the plastic melted around it and the terminal flopped over. It was ruined.

author: dmills
author_rank: Super Contributor
timestamp: January 28, 2018, 11:00:12 pm
content: I am guessing there is a ground pour on the back of that board?If so, pin 1 on both XLRs (and also the earthy end of C1,C2 (Why so physically large by the way, smaller is better here) should go directly to chassis NOT the the circuit reference plane, see AES48 for the reasoning.The reference plane should be connected to the chassis at ONE point, so that shield current does not end up flowing across this plane (You need this one connection so P48 works!).Some bulk decoupling on the power rails would be a good thing, two of those 47uF caps you like would do nicely, also a pair of zenners to clip the excursion if you have a phantom accident....Input caps don't really need to be non polar (Use polar ones with the positive towards the XLR)!4148s are a bit wimpy if you ever have a real phantom power accident, 4001s are more like it (The trouble comes when a signal line with P48 on it (so cap charged) gets shorted to ground), on that subject 10R or so in series with each input cap adds little noise but does serve to limit the surge current.How are you going to mount the board? a 3.2mm diameter hole about 10mm in from each corner is something you will thank me for later.Lots of really skimpy traces on that thing, nothing wrong with putting some meat on them, especially the power lines.Have a look at your placement, especially around those input coupling caps, there are simple things that would make for much cleaner routing.Some build out resistance (maybe 47 ohms or so) right before pin 2 and 3 at the output is always a good idea, helps with capacitive loads and also improves ESD immunity (May or may not be necessary with that driver but never hurts).Regards, Dan.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 29, 2018, 09:04:11 am
content: Quote from: dmills on January 28, 2018, 11:00:12 pmThank you heaps, this is great feedback !!Have Implemented changes!Just to confirm, should the ground point for the 48v switching circuit be, circuit ground or Chassis?Regards to connecting both Pin 1 together, is the best way to do this by routing a trace across the board ?ThanksJBliss

author: Jbliss
author_rank: Regular Contributor
timestamp: January 30, 2018, 08:07:16 am
content: Quote from: Jbliss on January 28, 2018, 12:53:20 pmThanks this is great !

author: dmills
author_rank: Super Contributor
timestamp: January 30, 2018, 03:11:13 pm
content: Quote from: Jbliss on January 29, 2018, 09:04:11 amP48 switching circuit, it does not really matter.Connecting the pin 1's, gods no don't do that!Pin 1 should be bonded to chassis right at the connector (Ideally it would not enter the box at all), the objective is for the screen current to flow in the box walls (In theory over the OUTSIDE of the box walls) and not set up fields inside the box to couple to your sensitive signals.See AES48 for lots of detail on this.Regards, Dan.

author: Jbliss
author_rank: Regular Contributor
timestamp: February 07, 2018, 03:55:10 am
content: Hi All,Just wondering about adding a insert Circuit to the preamp. what would be the easiest way to do this?Thanks

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 07, 2018, 11:59:46 am
content: Quote from: Jbliss on February 07, 2018, 03:55:10 amBreak the connection between the output of the INA217 and the input of the DRV134. Use a switching TRS jack and connect the INA217 output to the Ring terminal (2), and the DRV134 input to the Tip terminal (3). That is the basic method used on virtually all audio gear.You need a "switching" connector so that when nothing is plugged in, the signal flows through from the INA217 to the DRV134. Connect the Ring switch terminal (4) to the Tip Switch terminal (5).The complications from adding this circuit comes from exposing to the (possibly hostile) outside world that "pristine" node between the INA217 and the DRV134. It would be prudent to protect that node from DC injection using DC blocking capacitors. But that rather defeats the nice DC servo circuit (the OPA2137). So, like many things in the Real World, it is a trade-off decision which is more important to you.You could argue that since you are only making a standalone preamp (vs the input circuit to a mixer) you don't really need an insert jack because you can simply route the output of the preamp to wherever you want (including whatever processing you want to use, etc.).

author: marcuswilson007
author_rank: Newbie
timestamp: February 08, 2018, 12:36:45 am
content: C4 and C5 should be 1000uF in order to preserve balance at the low frequencies and to preserve the low noise at low frequencies.At 20Hz 100uF is about 80 Ohms, electrolytics are +50% -20%, there could be an imbalance of up to 50 Ohms on the input legs. What's the base spreader resistance of a 2N4403, 5-10 Ohms? The caps would completely bugger the CMRR.Also at 20Hz the 160 Ohms of the capacitors almost doubles the microphone's 200 Ohm source impedance, increasing the noise right in the popcorn noise region.

author: Jbliss
author_rank: Regular Contributor
timestamp: February 08, 2018, 01:10:27 am
content: Quote from: Richard Crowley on February 07, 2018, 11:59:46 amHi Richard,Excellent that is what I had in mind. Thank you very much for your help.

author: Jbliss
author_rank: Regular Contributor
timestamp: February 08, 2018, 01:12:48 am
content: Quote from: marcuswilson007 on February 08, 2018, 12:36:45 amHi marcuswilson007 I believe your are referring to the original design I was planning on using I have since changed to a more simple and modern design based around the INA217.

author: marcuswilson007
author_rank: Newbie
timestamp: February 08, 2018, 01:49:57 am
content: True, I hadn't caught up with that.The logic still applies to the INA217 preamp, the unbalance due to the capacitor tolerance and the impedance at low frequencies.It's even worse as they have 47uF capacitors.1000uF non polar caps are not common but are available.Nichicon do a 470uF 50V bipolar (UEP1H471MHD), one or two in parallel will improve the low frequency performance.

author: Jbliss
author_rank: Regular Contributor
timestamp: February 08, 2018, 05:49:18 am
content: Quote from: marcuswilson007 on February 08, 2018, 01:49:57 amAs dmills mentioned in a previous post the input caps do not need to be bipolar. Would a good option be to use 1000uF polarised capacitors?

author: dmills
author_rank: Super Contributor
timestamp: February 08, 2018, 09:17:19 am
content: One nasty issue with very large input caps is that they store a lot of energy when P48 is on, and can massively increase the damage potential is you ever plug in a shorted line after powering up the P48 (1000uF with 48V across it is a whole joule give or take), if that ends up being dumped into the input stage you would definitely want low value series resistors and very butch clamp diodes to make sure you don't damage the input transistors.Personally I usually go for a corner about a decade below the audio band (so 2Hz ish) on the basis that the increased LF noise will be taken out by a subsequent highpass filter further down the chain (The single most useful form of tone control!), and that by 50Hz (which is the place we start properly caring about CMRR the caps will have become a much smaller issue), large caps are good for the noise but raise worrying issues with stored P48, trade this one carefully.Regards, Dan.

author: marcuswilson007
author_rank: Newbie
timestamp: February 09, 2018, 04:44:56 am
content: The problem is that uni-polar electrolytic capacitors need some voltage across them to keep the chemistry going and if the phantom was not turned on for a long time their characteristics can be well below spec and the worst thing is that the sound deteriorates. Bipolar ones build up a charge from the audio voltage.

author: marcuswilson007
author_rank: Newbie
timestamp: February 09, 2018, 05:29:14 am
content: Any energy from these capacitors goes through the protection diodes.By the way, 1N4001 diodes are not a good choice for op amp protection. One, they have high capacitance. Two, they are slow. Three, they turn on at the same voltage the ESD diodes in the IC, so they are not protecting the ESD diodes. (The 1N4148's have this problem also.)The diodes to use are Shottky diodes. 1N6263 is a good choice. They turn on at 200mV as opposed to 700mV for the 1N4001, they are very fast and have a tenth of the capacitance of the 1N4001.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 09, 2018, 06:29:21 am
content: Protection diode capacitance isn't really a factor in a low-impedance circuit like a modern microphone input. However the voltage-drop and the speed ARE certainly factors. Schottky diodes would be an excellent choice just for those factors.NOT recommended to put massive DC blocking capacitors on the inputs. There are heaps of disadvantages and ZERO advantages.One clever design even raised the entire circuit up to a +48VDC offset from ground when phantom power was required, just to avoid those input capacitors altogether. They are certainly the problem-child in that circuit design (along with the weird gain control requirements). Might even be worth investigating a switching circuit that bypasses those capacitors when phantom power was switched off.