New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

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mediatechnology
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Tue Sep 24, 2013 3:42 pm

The Digital Gain Controller Using a THAT5173

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Input-Capacitorless Preamp Digital Gain Controller Using a THAT5173

A DLP232 Module is used to provide a USB to SPI Interface.
Avago Opto-Isolators Provide a DC isolation barrier for the THAT5173 which floats on the flying rail power supplies.
Vcm from the flying rail generator servo serve as pseudo ground connections for the interface optos, inverters, THAT5173 and +3.3V Vdd regulator.
The 3.3V supply floats on Vcm. When Vcm is +30VDC, Vdd is +33.3V.
Low inductance Vcc, Vee and Vdd bypass capacitors to signal ground are required to maintain stability in the 1570 preamp.

The 5173 U-pad inputs are also isolated from the THAT1570 outputs by 47R resistors to provide additional stability.
The 5173 the servo capacitor connections are shunted because the internal servo is not used.
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by ilya » Wed Sep 25, 2013 3:11 am

mediatechnology wrote:Yes, you just have phantom switches for each channel.
Just like a regular preamp. Each one operates independently.
Ok, cool.
I was concerned that once there's phantom on any channel, then the whole module flys to that. But now I see that this should make no problems for other channels.

Another question. In case phantom power is different for each channel, would this work? I'm thinking of, for example, taking signal from the mic via Y-cable (split), and phantom power it from another reciever (mixer or whatever). There's no guarantee that each channel has equal phantom voltage. What difference can this preamp tolerate?

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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Wed Sep 25, 2013 4:43 am

ilya - Each channel can independently fly to whatever phantom voltage is present at the input between 0 and +48V. Every mic will be different and every channel can be different.
If the phantom voltage is supplied by another device - and phantom is switched off on this device - it will still adapt to whatever phantom voltage is at the input.
It works like a transformer does.
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by ilya » Wed Sep 25, 2013 4:50 am

mediatechnology wrote:ilya - Each channel can independently fly to whatever phantom voltage is present at the input between 0 and +48V. Every mic will be different and every channel can be different.
If the phantom voltage is supplied by another device - and phantom is switched off on this device - it will still adapt to whatever phantom voltage is at the input.
It works like a transformer does.
I see now what I was missing. I have to build a separate flying rail block for each channel and than it will work properly. I somehow thought that I'll power all channels from one flying rail generator... :oops:

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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Wed Sep 25, 2013 5:02 am

Now you got it! Yes, the flying rail circuit is in every input module.

In your scenario where some other device provides the phantom you would not want the phantom switch on this device to back-ground the 6K81 resistors since they would load the other source of phantom.
The ground connection for the switch could be made an optional link.
The choice of back-grounding or not back-grounding exists in any preamp.
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by ilya » Wed Sep 25, 2013 5:48 am

mediatechnology wrote: In your scenario where some other device provides the phantom you would not want the phantom switch on this device to back-ground the 6K81 resistors since they would load the other source of phantom.
The ground connection for the switch could be made an optional link.
The choice of back-grounding or not back-grounding exists in any preamp.
Thanks! Haven't thought about this grounding issue. Really useful tip.

Now, speaking about digital control, what are suggested ways of connecting several pres to USB? I saw that you used DLP232 (which is also used on the demo board).
Can you point me in the right direction regarding control via USB? I can code, so I believe software part would be less problematic for me. But digital electronics is, as we say, "a dark forest" for me now. Can you suggest some good reading for starters?

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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Wed Sep 25, 2013 9:34 am

I used a DLP232 from a demo board which THAT had coded for the 5173.
I'm also using their Mic Preamp Demo software.
Coding is an area where I would need help.

Although I could use a rotary gain switch with only 24 positions available I can only get 2.5 dB/step.
Keeping the gain leads quiet can be done even with longer leads but it's an extra op-amp to drive the shield.

The THAT5173 provides 3 dB steps so in terms of step size it's not too much larger than a switch but a lot cheaper than a Goldpoint.

For my application I think I want shaft-encoded knobs and buttons controlling a 5173.
Alex (promixe) developed modules and sold them from the Innersonix website: http://diy.innersonix.com/
I hope Alex can recode the front panel controller for the 5173 and build some more front panels. I could use them.

To answer your USB question the data sheet for the 5173 shows how multiple devices can be controlled from a single SPI port: http://www.thatcorp.com/datashts/THAT_5 ... pdf#page=8
One thing I found with USB is the need to ground-isolate the USB port/controller (DLP232) from audio ground.
Ground isolation is required in the floating preamp but I've found it's difficult to get quiet non-isolated USB control under any circumstances without in-band spurs due to the USB polling rate around 1 kHz.
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by ilya » Fri Oct 04, 2013 2:26 pm

Wayne,

I'm looking at 1606 line drivers. What do you think, is it sufficient to implement circuit from figures 7 and 8 in the datasheet (http://www.thatcorp.com/datashts/THAT_1 ... asheet.pdf) and put this after 1246s?

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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by mediatechnology » Fri Oct 04, 2013 3:01 pm

If you're going to use a 1606, you can eliminate the 1246's and use the differential inputs of the 1606 to realize common mode rejection.
I've used this feature of the 1606 and the CMR is quite good. It's typically 70 dB.

You can also use a pair of 1606's and cross-couple the 1606 inputs and parallel (also cross-coupled) the outputs. This doubles the drive current and has 3 dB less noise in the output stage.
I've also tried this.

Do note that the 1606 is going to provide 6 dB of Gain.

Another option is to use the Dual Class-A output board with it's 1246 inputs cross-coupled: viewtopic.php?f=7&t=509
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Re: New: A Direct-Coupled Input-Capacitorless Active Mic Pre

Post by ilya » Fri Oct 04, 2013 4:30 pm

Ok. I think I need to tell more about my project.
I'm going to put 8 channels of these pres into 1RU enclosure. I'm planning to power the units from external PSU and also be able to daisy-chain several units so they can be powered from one PSU.
Sooo...
I have several goals in mind. First: minimal component count to reduce power comsumption. Second: well, minimum component count to reduce cost 8-)

Thanks for the link to the Class-A boards. They look really cool, but are way overkill for what I'm looking for. What do you think, is it too bad f I take the output from 1246s and bring them to the real world without any drivers? Or do I have to put anything after them? If so, what would you recommend?

You said that 1606s CMRR is around 70 dB. How does that compare to 1246s CMRR in your configuration?

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