Portable mic preamp project

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carlmart
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Re: Portable mic preamp project

Post by carlmart » Fri May 23, 2014 2:14 pm

The audio side is just part of this project, so I wish to keep it as by the book as possible.

The manufacturer does provide a way to do it and you have improved quite a bit on their ideas.

I have important reasons to keep this project as single supply, as my previous version made in 2000, using the SSM2017, used a 9+9 supply which worked quite well.

But this time the battery will be powering several things besides the preamp. Voltage will be less, but capacity will be larger.

The reason for not using DC-DC generator to go for +/-15v is mostly due to cost, space and noise. But I might reconsider that in the future.

For now I would like to build a working and good sounding analog supply project.

Two things I would also like to discuss is the VU-metering with LEDs and the options I might have for a limiter. Should I open another topic or we can talk about it here?

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JR.
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Re: Portable mic preamp project

Post by JR. » Fri May 23, 2014 3:04 pm

carlmart wrote:The audio side is just part of this project, so I wish to keep it as by the book as possible.

The manufacturer does provide a way to do it and you have improved quite a bit on their ideas.

I have important reasons to keep this project as single supply, as my previous version made in 2000, using the SSM2017, used a 9+9 supply which worked quite well.
Yup.. I made a commercial mixer with +/- 9V
But this time the battery will be powering several things besides the preamp. Voltage will be less, but capacity will be larger.
I've already said this but a simple charge pump could turn + battery V into +/- V, The square wave from the charge pump could also drive a doubler/tripler for low current 48V supply. All without a magnetic field.. simple charge pump using caps.
The reason for not using DC-DC generator to go for +/-15v is mostly due to cost, space and noise. But I might reconsider that in the future.

For now I would like to build a working and good sounding analog supply project.

Two things I would also like to discuss is the VU-metering with LEDs and the options I might have for a limiter. Should I open another topic or we can talk about it here?
I've already given some advice about low current LED meter. for low current limiter perhaps JFET shunt, if willing to spend more current THAT has some basic app notes.

I'll defer to Wayne about whether this needs to be split up into different threads,

JR
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carlmart
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Re: Portable mic preamp project

Post by carlmart » Fri May 23, 2014 3:32 pm

JR. wrote:Yup.. I made a commercial mixer with +/- 9V
Never got any complaints on mine, but production was very small. My greatest satisfaction was receiving a mail from a Sound Devices engineer, congratulating me on the design.

Unfortunately I do not live in the USA, which was necessary to keep such thing going.
I've already said this but a simple charge pump could turn + battery V into +/- V, The square wave from the charge pump could also drive a doubler/tripler for low current 48V supply. All without a magnetic field.. simple charge pump using caps.
My knowledge is not such as to face such an enterprise, but from similar projects I have seen, for me it would be far from simple.

ricardo
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Re: Portable mic preamp project

Post by ricardo » Sat May 24, 2014 4:13 am

carlmart wrote: Ricardo: which specific "ISOLATED P48V supply" are you referring to?
The P48V supply you show on 20may shares its GND with the 12V battery so it is NOT isolated.

A P48V SMPS using a transformer could be isolated but I have no experience with the modern chips to do this.

My suggestion to make Wayne's virtual GND, the 'real' GND, relies on the battery floating giving +/-6V. Using your P48V with this +/-6V system would refer your 48V to -6V ie P42V ... Hmm! That's ne..arly within spec for P48V ;)

But if the battery is used to power other stuff as well, you can't have the -ve battery terminal at -6V. My scheme won't fit your other stuff.

JR's scheme is best if you want to avoid as many caps as possible. You need a SMPS to generate -12V and it can do +48V at the same time.

JR, I'm not sure a Charge Pump is the quietest way to do this ... (TRANSLATION: dat beach bum buggered it up when he tried to do this last Millenium :mrgreen: )

But if you have no experience designing, building & troubleshooting SMPSs, you should stick to tried & tested stuff.

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JR.
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Re: Portable mic preamp project

Post by JR. » Sat May 24, 2014 7:20 am

ricardo wrote:
JR's scheme is best if you want to avoid as many caps as possible. You need a SMPS to generate -12V and it can do +48V at the same time.

JR, I'm not sure a Charge Pump is the quietest way to do this ... (TRANSLATION: dat beach bum buggered it up when he tried to do this last Millenium :mrgreen: )

But if you have no experience designing, building & troubleshooting SMPSs, you should stick to tried & tested stuff.
If you've ever made a cap doubler, you have used a charge pump.

Yes, you still need to manage current flows, but charging and discharging a cap does not create the same magnetic field that inductor based switcher does.

A simple (unregulated) charge pump could be made from a 555 astable. Use the square wave to alternately charge a cap with the +V battery supply, then drive the top of that cap to ground, with a diode on the - end to charge a negative reservoir cap. The higher the square wave frequency, the smaller the boost cap.. Likewise the same square wave could drive a doubler/tripler to boost the batter up to +48.

Of course if you are uncomfortable doing scratch design, do what works for you.

JR
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carlmart
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Re: Portable mic preamp project

Post by carlmart » Wed May 28, 2014 6:20 am

OK, now I'd like to discuss a different approach to this project, which I did build once as a prototype and worked quite well.

The design is based on a dual low-noise chip, which apparently does not fulfill all balanced requirements, no shining on some, but I would like your opinions on it.

Have a look at figure 45 on page 15.

http://www.analog.com/static/imported-f ... 13_413.pdf

One thing I think I might implement here is an LDR limiter controlling the gain.

Flaws, please?...
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ricardo
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Re: Portable mic preamp project

Post by ricardo » Wed May 28, 2014 8:06 pm

carlmart wrote:Have a look at figure 45 on page 15.

http://www.analog.com/static/imported-f ... 13_413.pdf

One thing I think I might implement here is an LDR limiter controlling the gain.
This is a very clever circuit. The CONS are mainly to do with small CM range.

But for gain, it is just like THAT1510 etc so a Vactrol is inappropriate. A Limiter with Vactrol across RG gives slow attack and quick release, the opposite of what you want.

carlmart
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Re: Portable mic preamp project

Post by carlmart » Fri Jun 06, 2014 7:14 am

There are some things I haven't yet made up my mind on that I would appreciate your opinion. I think I have already listed them on this or other thread, but I got no answer.

1) As multi-position rotary switch is out, because of size, weight and other reasons, my option for setting levels seem to be two:

a) An anti-log pot connected to the RG pins, suitably isolated with at least 2200uF cap
b) A three position switch, setting rough levels, and a log pot, possibly 10K, before the next stage right after the bass0cut switch and before the limiter stage.

My experience is mostly with the latter, which I used on my preamp. But on my tests I did try the "pot solution" and it's not a bad one. Problem is to find the anti-log pot, which is not a standard part everywhere. But which are the cons for this option?

2) How effective is the THAT limiter stage in real situations, in the field? Does it really save you day on most clipping situations?

emrr
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Re: Portable mic preamp project

Post by emrr » Thu Jun 12, 2014 10:14 am

carlmart wrote: How effective is the THAT limiter stage in real situations, in the field? Does it really save you day on most clipping situations?
It is quite effective within it's limitations, when used with the non-linear cap detection. More importantly, it ranks highly for transparency compared to many types. I would still operate conservatively with regards to operating levels. You might also want some sort of diode clipper after a THAT limiter, if you expect wild conditions with no tolerance for any overload whatsoever.

I've had a few recording sessions that needed a 'loud' master printed in real time, as there would be no time for a touch-up pass. In those cases I have used a low ratio RMS mode THAT comp acting as an AGC followed by a high ratio non-linear THAT comp for peak control. I could easily make worse sound with many software options in post, while not as 'loud'.
Best,

Doug Williams
Electromagnetic Radiation Recorders

carlmart
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Re: Portable mic preamp project

Post by carlmart » Thu Jun 12, 2014 10:48 am

emrr wrote:
carlmart wrote: How effective is the THAT limiter stage in real situations, in the field? Does it really save you day on most clipping situations?
It is quite effective within it's limitations, when used with the non-linear cap detection. More importantly, it ranks highly for transparency compared to many types. I would still operate conservatively with regards to operating levels. You might also want some sort of diode clipper after a THAT limiter, if you expect wild conditions with no tolerance for any overload whatsoever.

I've had a few recording sessions that needed a 'loud' master printed in real time, as there would be no time for a touch-up pass. In those cases I have used a low ratio RMS mode THAT comp acting as an AGC followed by a high ratio non-linear THAT comp for peak control. I could easily make worse sound with many software options in post, while not as 'loud'.
Can you please share which THAT limiter is the one you used?

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