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Author Topic: Single supply HiZ preamp to 1 Vrms out, HELP!!  (Read 8704 times)

mickmad

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Single supply HiZ preamp to 1 Vrms out, HELP!!
« 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"
« Last Edit: April 04, 2013, 11:04:37 PM by mickmad »

J M Fahey

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Re: Single supply HiZ preamp to 2 Vrms out, HELP!!
« Reply #1 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.

mickmad

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Re: Single supply HiZ preamp to 2 Vrms out, HELP!!
« Reply #2 on: March 30, 2013, 09:24:36 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 :)
« Last Edit: March 30, 2013, 09:26:31 PM by mickmad »

Roly

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #3 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.
If you say theory and practice don't agree you haven't applied enough theory.

mickmad

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #4 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?

Roly

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #5 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.
If you say theory and practice don't agree you haven't applied enough theory.

mickmad

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #6 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?

Roly

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #7 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.
If you say theory and practice don't agree you haven't applied enough theory.

mickmad

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #8 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.
« Last Edit: April 04, 2013, 11:04:22 PM by mickmad »

J M Fahey

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #9 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.

mickmad

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #10 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.


Roly

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #11 on: April 04, 2013, 11:51:13 PM »
Quote from: mickmad
I 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.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #12 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.

mickmad

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #13 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.
« Last Edit: April 05, 2013, 09:04:46 AM by mickmad »

phatt

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Re: Single supply HiZ preamp to 1 Vrms out, HELP!!
« Reply #14 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.