Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Preamps and Effects => Topic started by: mickmad on March 30, 2013, 09:32:06 AM

Title: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on March 30, 2013, 09:32:06 AM
Hello there, I'm Mick, I study computer engineering in Rome and I'm working on my final project; the project is a 2 in 2 out digital audio USB interface tailored to guitarists, because I'm a guitarists and I've never really satisfied with the sound of commercially available, mid range digital audio interfaces.

Right now I'm facing some problems with the design of the preamplifier section, which will be probably the key point of the whole thing! The thing is: I got a 2 channel A/D, both channels are single ended, input impedance is 20k, input capacitance is 10pf, and the desired input signal level is 2 Vrms; for a normal line input, I think that connecting the line cable straight to the A/D input will work, but I know that I need some good preamplification and a much higher input impedance to connect a guitar to that input. Now, a simple adjustable gain preamp will suffice for this job, but I'd like to test out a tone controlled preamp desing too; but, all the "classic" designs I've seen around here and through the rest of the web are operating near +-15 V, that is tooooo much for me! I want a circuit that can be easily powered via USB, that means 5V maximum, and I need a single supply design! What should I do?

Note:I've seen the DRV602 from Texas Instruments which is a single supply op amp with internal charge pump that works as a 2-3Vrms line driver from a single 3.3v-5v supply source, and I'm using this as a single ended input filter and line output stage after the D/A conversion... could I possibly do something with the very same chip for the input stage??

Other note: just to clarify, I'm using a Wolfson WM8569 codec chip , and the output stage design is based around (read:copied  :) ) the WM8569 reccomended filter stage design and the DVR602 reccomended external components design, with 3.3v supply for the DVR602.

edit: corrected the topic name, it said "2 Vrms" instead of "1 V rms"
Title: Re: Single supply HiZ preamp to 2 Vrms out, HELP!!
Post by: J M Fahey on March 30, 2013, 06:31:46 PM
You can't have 2 or 3 V RMS out of a 5V DC supply, period.
You mention an Op Amp which has an internal charge pump .... meaning it generates higher voltage.
Confirming you can't really avoid it, one way or another.
Personally I'd use one of the rail voltage converters (Maxim, etc.) to get proper Op Amp approved rails ..... which allows me to use any Op Amp I like instead of tying me to a specific one.

I, for one, feel quite uncomfortable with an iron ball tied to my ankle, no matter if the chain is long or not.
Title: Re: Single supply HiZ preamp to 2 Vrms out, HELP!!
Post by: mickmad on March 30, 2013, 09:24:36 PM
Quote from: J M Fahey on March 30, 2013, 06:31:46 PM
You can't have 2 or 3 V RMS out of a 5V DC supply, period.
You mention an Op Amp which has an internal charge pump .... meaning it generates higher voltage.
Confirming you can't really avoid it, one way or another.
Personally I'd use one of the rail voltage converters (Maxim, etc.) to get proper Op Amp approved rails ..... which allows me to use any Op Amp I like instead of tying me to a specific one.

I, for one, feel quite uncomfortable with an iron ball tied to my ankle, no matter if the chain is long or not.

Well, after a whole day bashing my head on a wall, I must admit that you're totally right, I can't avoid using a double supply; I'd probably simplify my life if I decide to put in the design, say, a 9 V input with regulated 5 V and 3.3 V output, which are still necessary to power my microcontroller and the codec.

By the way, I made a mistake, I actually have both single ended ADC input and DAC output at 1 V rms; so I got to:

1)bring guitar level from 0.2 - 0.3 Vrms to 1 Vrms
2)eventually add a level input, in which case I'd need to bring 2-3 Vrms down to 1 Vrms
3)design a new output stage to bring 1 Vrms DAC output to 2-3 Vrms to match the new double supply

Actually I've seen many integrated instrumentational amplifier which could do a neat job on bringing the guitar level up to 1 Vrms, like the INA826, but I still haven't seen anything to reduce the input level signal. Probably I'll still leave out the level input for now, as I said this is going to be tailored to guitarists; I'll focus more on the guitar input part.

The INA826 looks like a very nice part, but I can't quite get the right value for the gain controlling resistor; I'd like to have a potentiometer to control the gain factor between 1 and 11; the datasheet says that the overall gain is 1 + (50 Kohm / Rg) where Rg is the gain controlling resistor; of course, the unity gain is achieved when Rg is an open circuit, and the 11 times gain is achieved when Rg=5 Kohm... do you think it is possible to achieve a similar result with just a pot? Or should I use a transistor below the saturation point and a potentiometer to bias the drain so that it works as a transresistor?

Any suggestion is well appreciated :)
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: Roly on March 31, 2013, 07:35:44 AM
Your gain control problem arises because of the op-amp you have selected.  If you use a conventional op-amp then your gain control problem becomes trivial.

The commonly available LM358 (dual op-amp) works on single rail from 3V to 32V (or dual rail), output swing from 0V to Vcc-1.5V, so on a 5V supply it would swing 0-3.5V which is more than the 2V the ADC requires.  I remember having to replace one in a stomp box a while back and they seem well suited to low supply voltage applications.

While I agree with JM, for a one-off project you can certainly use anything you can get, but when you design for production you should also design for long term repairability, and that generally means avoiding exotic and special IC's as far as possible.  In this case it is the input amplifier that is most likely to get damaged by misuse, and the LM358 has now been around long enough to consider it a "standard" device.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on March 31, 2013, 10:20:42 AM
I'm doing this project alone, at home, so I'm trying to keep the design simple, low cost, but efficient and as much professional-level as I could get it. I'm also trying to design it not to be a one-off prototype, because I'd like to maybe take it to a small run production stage, or even a big run if I get lucky :D For the first final product I'd like to make something that could be sold in a DIY kit.

Thanks Roly for the little schematic, I will try to study that a bit, and see if I can lay down a similiar approach to get a line level (2 Vrms) input interface to the ADC. For the output stage, I still got nothing new; I will probably just remove the pump-charge opamp and copy-paste the WM8569 recommended output filter circuit around the LM358, which fixes the gain to 2x to get it to line level. Should probably work fine.

Of course, I'd like to have more headroom for the ADC input signal, so that I could interface it with a more powerful preamp; when I first thought of this project, I wanted to give a potential user the power to change the preamp stage, like it was some sort of module that connects to the main board; it could have been a transistor pre, a valve pre or whatever. Maybe, to achieve this, I could design an onboard input buffer for the ADC which gets a very powerful signal, like above-line level (4 Vrms, 5 Vrms, dont really know), and then reduces it to 1 V rms, basically it should be an amplifier circuit with sub-unity gain; this way I could interface this input buffer with a variety of preamps... is it a good idea? Is it feasible?
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: Roly on March 31, 2013, 09:04:37 PM
The LM358 is a dual, so that gives you some elbow room.

If you are going to go for plug-in modules you need to be careful to select the very best connector possible, gold flashed contacts ideally for small signals, because this is where most of your failures will occur.

One of the most difficult problems with electronic design (because it requires a crystal ball) is ongoing parts availability into the future.  Many times I've encountered a situation where a particular IC, say, looks like an ideal solution to a design problem, but turns out to be so specific it has no "second source" when the manufacturer discontinues it at some unknowable time in the future.

This doesn't come up a lot in guitar amps and such but is fairly common in industrial and bio-med.  One example is the use of bucket-brigade delay lines in some older Fx pedals, and some have had to resort to daughter boards carrying a modern kludge to restore the required functionality now that supplies of the original IC's have dried up.  There is no simple answer to this problem apart from conservative design - hope for the best but be prepared for the worst.

Remember, if you are selling stuff the poor bunny who has to repair it may well be you.

As a sidebar, you have to expect people to do stupid things like connecting the output of a 100 watt amp to your low level input.  You obviously can't protect against everything but the input stage should be as robust as you can make it, then as easily repairable as you can make it.  The simplest protection is to include some series resistance to limit fault currents - diode and zener clamps are only as good as the current limiting resistance between them and the rogue source, and the fact that you need to keep the input impedance up around one meg for guitar doesn't help.

As an object example I have an old amp on the bench ATM which requires some new pots.  The problem is that that two of them are "350k tapped 70k" and apart from a weird value, tapped pots are effectively Unobtainum these days, and junking a classic amp isn't an option.

Good luck with it, and keep as posted on your progress.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on April 03, 2013, 01:46:42 PM
You know what? I'll lay down a schematic for a dual preamp, it will work with a switch to select which of the two preamp use;the first one will be an hi z simple jfet preamp, nothing fancy; the second will be a 3 Vrms line receiver to 1 Vrms out; this way I can lay down a pcb with codec,in pre and out amp, order it and start debugging for the next month (the pcb manuacturer takes about 4 weeks from order to delivery of 5 50x50mm boards @ 9.9 $, slow but cheap). The idea is to actually starting to test the whole setup rather than whine all day about which pre to use or not to use... Also, this way I could have a potentially good setup for the final project, and I will also have the opportunity to test other guitar-specific preamps if I make them to output 3 Vrms and interface them as line inputs! For me, this sounds killer! What do you think?
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: Roly on April 04, 2013, 12:56:31 AM
Sounds like a plan to me, but I keep thinking of two active inputs, one mic level, the other guitar/line level.  Think; who is going to use this, and how?  I'm thinking home studio where somebody might want to sing and play at the same time.  That might not be "best" or how you work, but I think you should "enable all options" as a friend of mine liked to say.

Straight to PCB?  I normally mock stuff up on strip or dab board first.

As a designer I tend to go around and around in circles trying to out-guess myself, "is there a better way ... is there a better way?", and it's often hard to know when to stop thinking and start doing, "throw it up against the Universe, and see what sticks".

There is something I call "publisher's pause", that moment when you have the manuscript of your award-winning novel half way into the post box - then pull it out and take it home again for a few more changes (or hover over the "send" button, and don't).

You always run into the Law of Diminishing Returns sooner than you think, and the returns from a first hardware mockup or kludge are always greater than you expected.  Those "ah HA!  THAT isn't going to fly" or wentbetterthanexpected.jpg moments.

I liken it to trying to find a gold bar hidden in a haystack with a rapier - at some point you have to take a first stab, being almost certain that you won't get lucky, but you can't take the second stab until you take the first.

tl;dr - GO FOR IT!  It will quickly tell you if you are on the right track.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on April 04, 2013, 04:44:55 PM
Ok, I got my hands on LTSpice IV, which I've never used before, to do some tesing before actually ordering PCBs, and I got the DAC output line driver and low pass filter done; I'm using the LT1413 instead of the LM358 you suggested, mainly because LTSpice only has built-in Linear Technology devices, and because I can't get the LM358 symbol to show up in the devices; but they're kinda the same chip so it's not a problem.

First, I drew the filter schematic that's on the WM8569 datasheet, and using the LT1413 in single supply mode, but a test with an input sinewave would show only half wave on the output; using that setup with a double supply eventually adjusted that.

But, I prefer using a single supply rather than deriving a negative voltage out of a positive one, so I biased the whole thing at 2.8 V (I will do that using a low dropout voltage regulator) and it works! I've attached schematic and a plot of a 1 Vrms sine signal at 10KHz in input and the resulting 2 Vrms output; the second image clearly shows that the resulting output is actually 2 times the input signal. The preamplifier section will come soon. I would be glad if anyone could do some further testing as I'm really new to SPICE simulation.

edit: added corrected schematic, removed wrong one.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: J M Fahey on April 04, 2013, 10:09:11 PM
Looks good.
Just one detail, 50pF output cap is impossibly small, use 10uF like at the input.
*Or* direct couple to DAC input or whatever.
That's not my area at all but you know what you expect there.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on April 04, 2013, 11:00:08 PM
Actually OUT refers to the line output; that final resistor and capacitor were there to simulate the input impedance of a line input, like the input of an active speaker, and the stray capacitance, like the capacitance of a cable connected.
It was flawed of course :D

I'll edit the last post with a correct schematic for it. I've also added a dc coupling cap of 10uF which works great, and fixed the offset voltage to 2.88, because I've found that the lowest output was somewhat distorted near 80mV above ground, this way the signal should be right in the range.

Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: Roly on April 04, 2013, 11:51:13 PM
Quote from: mickmadI would be glad if anyone could do some further testing as I'm really new to SPICE simulation.

You'll need to upload the .ASC circuit file.

Yahoo Groups has an LTSpice group with a huge library of models, and Googling often turns up models in text format.

I've only just figured out how to do stepped parameters myself (a wah pedal with ten different pot positions on the same plot), and there is a feature that allows you to input and write .WAV files which I haven't explored yet, but should be very useful in developing a custom fuzzbox for someone here - he sends me some clean riffs which I put through the model and send the results back for auditioning.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: J M Fahey on April 05, 2013, 04:24:01 AM
Why the huge 0.68uF C4 to ground?
It will certainly load down big way the output of the Op Amp.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on April 05, 2013, 08:12:16 AM
Ok, I got that wrong, again! XD

I've removed that faulty cap and added another stage of lowpass filtering before the 100 ohm out resistance and decoupling cap; I'm attaching .ASC circuit file, new schematic, and bode diagram of output signal just after the op amp (in RED) and after the last low pass stage (in GREEN).

The whole filter, as you can see, is actually a bandpass, with a passive high filtering stage before the op amp double pole stage; looks like a decent roll off for me near 100KHz, but still it doesn't convince me...


edit:I had some problems uploading the photos... btw I also noticed that chaining two filters like this maybe is what I was looking for, I can still get a 2 Vrms output, with straight gain from 10 Hz to 20 KHz, and significantly reduce any high frequency component. I'm also adding the two stage schematic, with corresponding bode diagram and .ASC file; this time, GREEN signal is the output of the first stage, the BLUE signal is the output of the second stage. The image clearly shows the much steeper cutoff above 10 KHz.

post-edit: I'm thinking that I should change the title of the thread because it is no longer about the preamp only, but rather it's about a single supply implementation of preamp and line driver for my audio interface.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: phatt on April 05, 2013, 10:07:28 AM
Hi Mickmad,
I'm not very sure of what is trying to be achieved here but does look much like You are trying to get some kind of cab sim up and running?
If so then frequencies of interest are 100 Hz up to around 4kHz.
Everything outside that needs to be steeply cut or it'll just cause much frustration. :'(

A simple cab sim unit after my old Quadraverb dramatically improved recorded sound even though the unit had on-board cab sim it was of little use.

Phil.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on April 05, 2013, 10:28:47 AM
I'm not doing a cab simulation, I'm doing some simulations of a guitar preamp to 1 Vrms, a line preamp (reducer) from 2 Vrms to 1 Vrms, and a line driver with low pass and high pass filter. Actually I'm working on the last one, I'm finetuning the filter to achieve an flat response filter like a C-weight filter, or something like that, cutting frequencies below 10 Hz and above 22KH, with a 2x gain in the passband region; I'm testing different solutions right now, as you can see in the posts :)

edit: after hours of twiddling, studying, changing filter topology, smoothing Q, and banging my head on a wall, I finally got it! I won't be touching this output filter anymore: first, I'm connecting the DAC output to an input buffer, which biases the signal and cuts some low frequencies; the biased hi-passed signal is fed into a unity gain Sallen-Key lo-pass, its output fed to a passive lo-pass; then another Sallen-Key lo-pass, identical to the first one, gets the first stage out and smooths it again, while boosting up the signal with a 2X gain, its output fed to a passive hi-pass; with this topology, I got a really nice bandpass, with a steep cutoff below 10 Hz and over ~22KHz. Again, I'm uploading schematic, .asc file, and Bode plot of input,first stage output ,and second stage output, which clearly shows the super steep cutoff. NEAT!
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: phatt on April 06, 2013, 05:14:55 AM
Well I looked up Weighted filters and I understand now what that means ,,,,,,
But what you just posted has gone way way past those ideas???

So what you have created is in fact the response curve of some Famous Amplifier. xP 8|

True Marshall Amp output including cab response is a 50~60 Db cut from ~100 Hz down to 10 Hz. You only show a 20 Db drop.  :o  So you still have a long way to go.

The top end roll off looks like it will work,, if that is the aim?
Sorry but I'm still confused as to what it is you are actually trying to achieve?
Phil.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on April 06, 2013, 07:28:27 AM
@Phil: if it resembles a Marshall Amp output, then I'm on a good road! :D

By the way, the output of that filter is going to be fed in a standard Line receiver, like that of a mixer input, or of an active monitor speaker.
But since the input signal of this filter/driver is an analog conversion of a digital signal, it could encounter some really harsh noise due to D/A conversion around the sampling frequency of the D/A converter, which ranges from 44KHz to 192KHz.
That's why I'm cutting a lot of high frequencies (I got -80dB/decade after 10KHz); I preferred to cut not too much around the low frequencies (I got 5 Hz at around -20dB) because I want to leave more room to the basses; if a potential user wants less basses, then he could cut them away.

In short terms, I want to achieve a somewhat powerful bass response, flat response over audible range, and total removal of very high frequencies components due to digital noise in the circuit. Hope that helps to understand better ;)

edit: I added a final gain reducer stage to the line driver; this way I can control the output volume with a potentiometer ;) I will post the revisioned schematic when I finish the preamp section, so to make a single post with all the nice stuff :D
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: Kaz Kylheku on May 19, 2013, 02:05:28 AM
Hi MickMad,

Have a look at the JFET input stage of the ADA MP-1. It's a neat circuit which places an N-channel JFET with a PNP transistor in a kind of hybrid complementary pair, which is then used as a source follower (unity gain: it's not a gain stage!)  The JFET provides the high input impedance, and the PNP delivers the current, allowing for a fuller voltage swing. The circuit runs on dual 15/15 rails.

I'm mentioning it because of the LTSpice appearance in this thread, and that circuit happens to be something I've been simulating in LTSpice recently. (The transistor part values are not the real ones.)

And, hey, use good parts in this USB interface! You will never beat the M-Audio or Behringer and whatever *s!!t* if you do what they do: use *s!!t* parts. 

I found JRC4558's in the preamp section when I opened a Tascam US-122L audio interface. Ugh!

Also read this document: http://www.rane.com/note151.html    And google "pin 1 problem".
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on May 20, 2013, 10:47:04 AM
WARNING: NECESSARY WALL OF TEXT AHEAD!

Thanks Kaz for your help, the preamp section is something that it's still driving me nuts, along with the supply problem... first of all, I'd like to clarify something more technical about this audio interface.

I'm designing this whole thing around two Wolfson chips, namely the WM8786 ADC and the WM8740 DAC, which are 16-24 bit @ 192KHz max. converters. They both have amazing specs, with 111 dB of SNR and -102 dB THD for the ADC, and 120 dB of SNR and -104 dB of THD for the DAC; also, both have differential input/output! I will interface both chips to the I2S bus of a microcontroller, the Freescale Kinetis K20 ARM Cortex-M4.

At this point of development, though, I decided to lay down a prototype board which will have a PSU, a pre section, a post section, an ADC section and a DAC section. The pre and post section are going to be, just for the sake of prototyping, the basic one that are suggested on the datasheets of the chips.

I need to filter both input and output signals, and this is done via active filtering with ultra low distortion opamps. Wolfson reccomends the MC33078 opamp for the filter section, with corner frequencies of 12.5 Hz and ~1MHz, roughly a 1MHz bandpass on both in and out. Other signal filtering is done digitally inside the chips, so that's nice.

The output filter section (the post) also has an AD797 as a differential to single-ended converter, for ultra-quality unbalanced line output.

I've seen the page you linked, and that's the subject of my sleepless nights during the whole last week! Mixed signals circuits are a pain in the a** to layout properly. And the fact that I may want to selectively test the ADC or the DAC looks like a problem; I'm currently aiming the prototype design to be as modular as possible. I want separation between ADC, DAC, preamp, postamp, and psu sections, so that I can debug any part of the whole thing alone. But this means that I have to split the pcb in multiple pcbs, which means multiple, separate, ground planes; which means possible EF antennas could create if I bridge some parts of the circuit over separate ground planes.... UGH!! My brain hurts!

In fact, I still got to finish the whole design as I want a nice, all-around, preamp, with selectable input impedance, and selectable, input-impedance dependent, gain control; I thought of something like a simple inverting buffer, with a switch to select between 1M input impedance and x20 gain, or 10K input impedance and x1 gain; this way I could plug a guitar and give it some good gain, or plug in a simple line output with no gain over it. I also want an attenuator stage; so my idea was in reality a fixed, selectable gain stage, with an attenuation stage. I also want to keep the gain-attenuation controls separate for each channel. The fact that I'm working with a balanced input ADC doesn't help either, as I might as well need to duplicate this circuit to make a balanced, fixed gain stage and attenuator... but this means that I will be using stereo pot for each channel, and big switches. This is more pain in my brain. And remember that I need to filter the input signal, so this gain-attenuation stage will and must be followed by the specified active filter. Considering the ideal setup, with balanced or unbalanced input, gain stage, attenuation, and filter, I should need 2 opamps for the gain-attenuation stage, and 2 opamps for the filter stage, for each channel; using the MC33078, which is a dual opamp on a single IC, I should need 4 ICs just for the input section. Add a pair of stereo potentiometers and a pair of something like QPDT switches (quad pole to dual throw, I don't even think they exist!), the preamp section easily became power demanding and expensive.

The output stage is somewhat simpler,; altough there is no analog gain control over it, I might use the DAC internal 256 levels attenuation circuit; I thought something like: read value with microcontroller from a random pot, map it to 8 bit and then pass it into the DAC. Simpler than it looks.  As I said, the post has already been designed as a balanced-unbalanced output, with an AD797 as a differential receiver to single ended for the unbalanced out. As for the preamp, a channel of the post section needs a dual opamp for the active balanced filtering, and a single opamp for the unbalanced output. So that's another 4 ICs.

Total: 8 opamp ICs for pre and post, two of which cost more than 7 € each (the AD797). Luckily, the MC33078 costs only ~0.5 € each, and for its specs that's way cheap, but then comes the cost of the switch and the stereo pots, which will easily outstand the cost of the opamps to which they would be connected to. Moreover, the supply I've designed, which is based around a Murata switching DC-DC converter module, with 12 V @ 0.5 A input -> +/-9V output @ +/-111mA, could not bear the whole circuit current requirements, as I have to get 5 V from it to power the microcontroller and the converters; considering that the microncontroller will consume ~30mA in idle mode, max 100mA when everything on it is used, and adding ~55 mA to power both converters, and adding the current drop from the 5V regulator and the 3.3 V regulator that is aside the microcontroller and that will drive the digital section of the converters... well it looks like I'm well out of power.

But let's forget price and power, for a moment. The main, real, important, and difficult problem is : NOISE. There's going to be huge amounts of noise into the converters, even if I suddenly become the master designer of mixed signals; I've made my math this night, and from the specs given by Wolfson, at least for the ADC, the SNR is 111 dB, with a 0dB signal of 2 Vrms. It means that the ratio of the 0dB signal amplitude in Vrms and the noise amplitude is around 10^(111/20) = 345813.389etc ; so the actual input noise to the ADC that is reckognized as zero signal is 2 V rms / 345813 = 5.63676 uVrms (u = micro). BUT, the thermal noise of a 1MOhm resistor, at 25 Celsius (which is the same temperature condition of the datasheet specs), over a bandwidth of 96Khz (assuming that the converter is working at 192KHz gives us a maximum input frequency of Fs/2 = 96K) is something like 12.5 uVrms; around 2.5 times higher than the zero input signal. Given that the usable range in Volts for the converter is ~ 5.65 Volts, the converter has an error of 1 bit if there's a difference in Volts of 5.65 / (2^24 - 1) = 0.337 uVolts. Given also that the peak to peak value of the 1MOhm noise is 35.52 uVolts, and removing the 5.63 uVolts of unreckognizable signal, it gives us around 30uVolts of error; dividing this value by the 1 bit error in volts gives us 89, which is the number that will be represented in bits when the noise of the resistor is converted. That's more than 7 bits of error! How can I possibly have a 1MOhm resistor in this circuit at all?!

I'm going so nuts about this thing, I haven't slept for more than 4 hours a day for a week straight, and then when I think that my design could be somewhat good, it is totally not! I really don't see that much need of using 24 bits converters in a circuit that easily obscures ~8 bits with white noise! I wanna use almost every bit out of it! I want it to perform! That's why I'm going to write an email to my electronics teacher and see if he can clarify something. I was thinking that I could plug in an instrumentation amplifier, as seen in all those electronic books, and use the uber high input impedance of the opamp alone. Don't know much, though. I might have been saying loads of bulls*** all the time.

The fact is: I'm 2 exams due to my graduation, the exams are in the next few weeks, my graduation admission deadline is September 24th, I still got to study for both exams (Microeconomics and Complex Calculus, btw), and I still got no design ready to send in production. Consider that I'm in a tight economic situation, so I need to use cheap chinese PCB fabbers, and the min. estimated order-to-delivery time is around 2 weeks (paying extra bucks to UPS); also, consider that I'm designing this thing in SMD technology, and I alone will be soldering it; give in that I still have to exercise on SMD soldering techinques (though those are overlymystified)... I think this is going to be a nightmare.


Link to the datasheets:
http://www.wolfsonmicro.com/documents/uploads/data_sheets/en/WM8786.pdf
http://www.wolfsonmicro.com/documents/uploads/data_sheets/en/WM8740.pdf

Btw: sorry for the wall o' text, but these questions are really driving me crazy and I could not explain my point without a deep insight of it.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: Roly on May 22, 2013, 02:50:50 PM
re: grounding - actually with mixed mode circuits it's normal to have two grounds, an analogue and a digital, but the main thing you need to avoid is impressing digital switching current signals on the analogue ground, the digital ground will take care of itself as long as the supply is well bypassed.

re: setting the DAC internal 256 levels attenuation circuit - the problem with doing this from a pot is the possibility of dither between two values, and it would be better to use a two-phase roto-pulser or even up/down buttons.

You do not need a bandwidth to 96kHz.
  Choose something much more reasonable like 20-25kHz and recalculate your noise.

Yes, you have considerable excess S/N and conversion depth, and you are discovering why mic channels are nominally 600 ohms (1k4 in actuality).  Have a look at bootstrapping; this allows you to have a very high effective input resistance while using a quite low actual resistance, and hence low thermal noise.

"You can have it good, cheap, or quick, but you can't have it all three."

You are wedged between a tight timeline, tight economics, and a 24bit converter with, what, 111dB of dynamic range?  Frankly 24 bits is galloping overkill when CD's don't even use the full 14 bit resolution available - the bottom two bits are basically crud.

60dB is good.  80dB is very good.  100dB is really outstanding.  110dB is fugging MYTHIC - 1 part in 300,000; this is the span from the threshold of hearing to the threshold of pain, a feather landing to a jackhammer at 1 metre.  It's a great ideal but well equipped labs with lots of resources and time have difficulty with this range, and you need to get real about your situation.

TIME is your real enemy here and you have to get serious about what you can realistically deliver in the available time - money you can borrow, specs you can compromise, but you have to get something with development potential running by deadline.

Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on May 23, 2013, 11:40:37 AM
Roly, you made the point. TIME is the real "threat" here; the thing that worries me most is the USB audio class firmware implementation; luckily, I've had some good news from my electronics teacher: he said that the prototype construction, from pcb to actual population of it, should be taken care by the university, along with the costs associated; so I can try to achieve a very good quality product, with the advice of another teacher of my university., who is involved in mixed signals circuits :) also, it will leave me more time to actually debug the hardware and the software, since the device will be made just outside Rome I will avoid huge delays caused by ordering the pcb in China and then having to populate it by myself :) I will keep you up with this project ;)
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: Roly on May 24, 2013, 06:33:58 AM
That you can get some skilled help is a real bonus, BUT, given the tight time a full spec result is a big ask and you need to think of what part of the spec you can relax to make sure you get something over the line.

I've seen projects like this before where there was a refusal to compromise any part for a result, and the result was no result at all, no part of the project going properly, and a fail.

Think "proof of concept", and that you can debug the digital noise floor out of your 24 bit analogue for a real world product after you demonstrate it basically working, albeit not to full spec.
Title: Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
Post by: mickmad on May 24, 2013, 11:08:48 AM
I surely will; first of all, I'm giving up on designing an over-the-top power supply, and will stick with using an ATX power supply, which already has +/- 12 V for the amps, and +5/+3.3 V for the rest of the board. Second, I will stick with the datasheets' input and output filter circuits, which are obviously tested to be within the converters' specs. I know that sticking with those filters will limit me to balanced only line input, and balanced/unbalanced line output, with no analog "volume" control for either part. And of course the input will not accept a guitar; but still, if the whole thing works as it should, then the problem reduces to adding a guitar/unbalanced frontend for the input filter, and an attenuation frontend to the output filter.