A Direct-Coupled Input-Capacitorless Active Preamp deleted

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RogerFoote

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Wayne

Good to see you getting this far along on this! I will be following with great interest... Some innovative thinking going on during those bike rides of yours!

Congrats!
 
I didn't study things in detail but let me add two thoughts:
* it looks to me as if CMRR were pretty lousy due to the use of two 1510s instead of one.
* noise must be very high as you terminate the negative input of the 1510s with 2k ohm (remember that low voltage noise means high current noise with BJTs). And having two chips instead of one doesn't help either...

I guess it would be well worthwhile to modify the plan by using one 1510 only as this addresses the above points.

Samuel
 
I'm a learner on this stuff, so don't slap me too hard..... :roll:

Just concerns re: noise...... Looks beautifully simple. Have you listened with the pot wired in? Can you pick the difference in sound with or without a cap?
 
Very interesting, especially the floating supply servo. I do share Samuel's concerns wrt noise, though.

Do keep us informed on where this is going,

JDB
[working on a direct coupled mic pre based on a Cohen/Green topo with a modified version of Samuel's DOA. Will let y'all know if/when I get something working]
 
Is CMRR really an issue?

I mean, this is a Diff. in/out circuit......or perhaps im missing something.

It is an interesting circuit.
 
I am looking for a old TI opamp book I have I think it has a circuit with the floating gain stage.

Interesting Idea/Design.

One thing I have learned over the years is to check things in ways users use them.

Unplug and plug under power have try that again with a cord with a wire broken.

How does it power up?
 
[quote author="RogerFoote"]I ONLY saw oscillation with the inverting input grounded (0 Ohms).[/quote]

The inputs of the THAT1510 (and similar like INA217) are tied directly to the base of a pair of transistors which operate not entirely dissimilar to emitter followers. I would say the oscillation you describe match the instability in emitter followers driven by inductive low-Z sources as described by Doug Self (scroll all the way down) and this newsletter. I used to have a better link, can't find it right now.

Inserting a resistor definitely fixes the problem. Having a pair of 2k2 resistors may be a bit too much, as that puts the minimum noise figure of the entire preamp at ~15dB, assuming a Zmike of 150 Ohm. At that point you may be better off with a simple NE5534.

JDB.
 
Interesting topology, BTW.

[quote author="mediatechnology"]Please note that these resistors are not in series with the input as the "stopper" resistors you and the cites discussed.
[/quote]

As far as I can tell, all that matters for stability is the impedance between the base and RF ground. For the negative input that's the 2k resistor, for the positive input it's 4R7 in series with the reactance of the 470p cap on your input, the cable reactance and whatever impedance the microphone presents. As a wild-assed guess, I'd be surprised if the 1510 would oscillate with, say, 100 Ohm between -IN and 'ground'. On paper :wink: that would make a lot of difference noise-wise.

[quote author="mediatechnology"]In my own experience with using the IC, the noise figure drops significantly once a 150 ohm or other low-impedance source is connected differentially to the device. So I'm not sure focusing on the values of the bias resistors and their open-input noise contribution is as significant in actual use as the differential source impedance is.[/quote]

Sure, but I'm looking at your schematic where you use two 1510s. In that case, from a noise POV any resistance between IN- and 'ground' will appear in series with the impedance on IN+. The 2k resistor will also boost the effect of current noise (which is considerable in a BJT-based diff amp such as the THAT1510).

WRT my 5534-remark: it's quite a bit harder to get a good low noise differential gain block than to get the same performance from a single ended design. What you're doing is using a differential amp in a single-ended configuration, which some might consider a bit of a waste. :wink:

[quote author="mediatechnology"]I say this not based on the theoretical but what I hear. I always lean toward the empirical.[/quote]

Fair enough; whatever works best for you is alll that counts in the end (and I'd be rather interested in hearing a direct coupled amp like this one). I imagine that the self noise of a phantom-powered capacitor mic will swamp the mic pre noise anyway; the noise may be more noticeable when using dynamics or ribbon mikes.

JD 'proof of the pudding and all that' B.
[and if the noise ends up being an issue for non-phantom mics, why not use a couple of extra poles on the phantom switch to route the mike signal to a direct coupled THAT1510 in simple differential configuration ?]
 
> this circuit has up to 66 dB of differential DC (and of course AC) gain.

Why? Take 20dB (switched to 0dB) gain in your flying buffers, cap-couple to a diff-amp with a single gain control.

With 20dB DC gain, you can stand 1V of input DC offset, which should cover any healthy mike (but not some cheap Chinese models, nor of course the shorted-pin scenario).

> from a noise POV any resistance between IN- and 'ground' will appear in series with the impedance on IN+. The 2k resistor will also boost the effect of current noise

2K in parallel with the microphone. The mike will short-out the base current noise and the 2K's thermal noise. Or more accurately: it couples one input's noises into the other input as a Common Mode noise, which will be rejected.


Overall: there are many ways to shave the cat. This is one of the more expensive ways I've seen. I hope the sum of the several small advantages has some real-world benefits to justify the assembly labor (I know the parts are not too costly).

I'm also thinking that fairly simple discrete circuits ought to eat 0V-48V input without the flying-rail contraption, but might be just as much trouble.
 
[quote author="PRR"][quote author="jdbakker"]from a noise POV any resistance between IN- and 'ground' will appear in series with the impedance on IN+. The 2k resistor will also boost the effect of current noise[/quote]

2K in parallel with the microphone. The mike will short-out the base current noise and the 2K's thermal noise. Or more accurately: it couples one input's noises into the other input as a Common Mode noise, which will be rejected.[/quote]

For the 'regular' circuits linked upthread with just one THAT1510 that's true, but I was talking about the two-1510 design that was linked to the first post of this thread (http://www.tenmilecreek.net/DC_Preamp/DC_Preamp.pdf). In that circuit, the noise voltage of the 2k resistor between the negative input of each THAT1510 will be added in series with the microphone's signal.

Or am I missing something ?

[quote author="mediatechnology"][quote author="PRR"]I'm also thinking that fairly simple discrete circuits ought to eat 0V-48V input without the flying-rail contraption, but might be just as much trouble.[/quote]

Show me one.[/quote]

If you don't care much about power consumption or DC-offset on the outputs, how about a Cohen/Green front-end topology with hi-voltage discrete op-amps (like the SGA-SOA) subbed for the '5532/072 ? Have a look at everything up to and including IC1ab on http://mhumhirecords.org/DIYpics/Green/gren_pre_cct4.gif , for example; there's no real reason that can't be made to work on a higher supply with the use of a good DOA.

JDB
[working on similar, but still trying to find a way to get rid of the output coupling caps]
 
I must say that this project has some curiosity appeal for me, in that with location recording, I find it is always preferable to try and boost/preamplify the signal before it hits long lines... And these days there are more and more 'armchair producers' who want to meddle... em...er... "assist" with the recording. The first thing they grumble about... um... uh... "point out" is that 'headphones are useless' and 'You have to be in a different room', or you'll never be able to tell tell what's going on... and as a result, no matter how many mic extension cables I bring, it's never enough.

I have a few decent AC-powered mic preamps in self-contained cases, with metering. This helps a great deal, but I'm interested in alternatives.

Local metering however, is vital for me once gain starts to rise... That's more than phantom can cleanly provide, I'm certain. unity gain line driving with low source impedance however, may very well be worth employing...

Keith

Keith
 
Keith, didn't get your point--I don't think we discuss a phantom-powered mic pre here?

If you don't care much about power consumption or DC-offset on the outputs, how about a Cohen/Green front-end topology with hi-voltage discrete opamps subbed for the NE5532/TL072?
I have a 80 V DOA in preparation and I already thought about the possibility to use them e.g. with a +64/-16 V asymmetric power supply for a "cap-free" mic preamp. It is tricky (though probably a good deal less so than the approach presented in this thread), but I think it would work well if you can assume a reasonable limit to the maximum expected differential offset. What you cannot easily address without using JFETs for the frontend is the fact that there is some bias current flowing through the source impedance. Some passive mics may return with higher distortion (any data on this?).

PRR makes a valid point regarding the fact that the base current noise(s) will be cross-coupled (by the source) into each other and should be cancelled due to CMRR in the two-1510 version.
I don't think he was referring to the two-1510 version and it hardly applies to it...

When trimmed, the CMRR of this would be as good as a single 1510.
Not even in theory, let alone real world. The point is that with the standard instrumentation frontend you have CMRR (roughly equal to the differential gain) already in the first stage without differential amplifier. And your design doesn't. Sure, you can trim a null for one frequency, but drift and ageing will throw you quickly away from there. BTW, the CM input range seems to become vanishing small at high gains (or is the floating supply fast enough to follow things at 10 kHz?).

Samuel
 
[quote author="Samuel Groner"]Keith, didn't get your point--I don't think we discuss a phantom-powered mic pre here?[/quote]
[quote author="Mediatechnology"]My thinking is this: A conventional phantom power arrangement sources 48V through (2) 6K81s.[/quote]
I was thinking we were...

:?:
 
I've been following this thread with interest, but it suddenly struck me today, is this of any practical benefit? The 'swinging rail' system does what it's meant to do, sure, it gets rid of input coupling capacitors, but brings with it considerable complication in the rail generator, and some dubious effects if the microphone has offset..... But the biggest problem seems to me that it does not address the reason for having a mic amp in the first place, that is to provide a flexible and versatile amplifier with huge overload capability, huge gain range, very low transient distortion and low self noise. With respect, I'm not sure that these circuits provide any of that.
It's a delightful academic exercise, but can I suggest trying a good transformer? :guinness:
 
Wayne,
Thanks for your considered reply to my momentary comments.
I'm not sure if you haven't argued yourself into a corner when replying; regarding MCI and their swinging rail generator, most worthwhile manufacturers make gross mistakes at some time (and I don't exclude myself!).
I'm sure that the 1510 is a worthy device, it reads well in the lab, but you interpreted my 'transient distortion' as 'transient response'... not the same. Our old friend the SSM2017 performed well in the lab, but sounded sad when compared to what engineers would call a 'classic' mic amp.
But let's not go down that path right now.
The idea of an effective solid-state mic amp with no coupling capacitors IS exciting and I shall certainly put one together and have a listen.
BTW, I AM one of those grandparents who love transformers!
 
Thanks for the updated schematic--any chance for a high-resultion scan? Pretty hard to read!

One thing occurs to me is that the 10k resistor in series with the servo ouput probably should be made larger as it imbalances AC CMRR in combination with the microphone output resistance.
That was the first thing strinking me when I read the schematic. I'd just add another 10k in the other leg as already suggested by you.

Samuel
 
Just thought I'd say that I'm following this (along with many others I'm sure) and it's been quite interesting. I'd like to hear how it would sound.

[quote author="mediatechnology"]
How do I make text a hyperlink?[/quote]

Use the URL button when writing a reply. You can look at how I made this by quoting me.
 
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