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

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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

I did a rev 2 xls for the THAT1510 THAT1512 gain. This one opens in OpenOffice Calc and is usable in both Calc and Excel.

http://www.ka-electronics.com/THAT1510/ ... n_rev2.xls
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XAUDIA
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by XAUDIA »

Hi Roger,

I bought mine from the goldpoint website. They arrived this morning and feel really nice. I may have to use them on all my future projects!

My 1% resistors are here too so I can get started with the next build of these preamps tomorrow.

Cheers!
Stewart
New music at xaudia.com
jeffrey_burr
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by jeffrey_burr »

Hi, what a cool idea this is, I'm glad that it has a home here. It's way over my head really, but I have to say how generous it is of you to regard as "very reasonable" the questions of "why not just use a transformer" or "why not just use capacitors". It's like asking a bicyclist, "why don't you just drive?" These people just don't like fun.

Anyhow, one poster above mentions applying this topology to other preamps and I had thought along the same lines, except that how does one know if any given device would survive the trip? I see some devices have "Absolute Maximum" supply voltage given differentially (Vpos + |Vneg|) which might be taken to imply that "virtual ground" schemes are going to be okay; then for other devices each rail is specified separately -- like as in these THAT corp. parts. The 1510/1512 datasheet I have specifies +20V for Vcc and -20V for Vee. Yet it lives! I guess the only thing to do is follow Mediatechnology's lead and try my damndest to destroy something. Luckily I have too many other projects awaiting completion.
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

It's like asking a bicyclist, "why don't you just drive?" These people just don't like fun.
Thanks for joining us Jeffrey. I think my original reply to the transformer question at Prodigy-Pro (posed by Ted Fletcher BTW) was: "Because it's been done already." "It's been done ad nauseum." I like your answer better...
except that how does one know if any given device would survive the trip?
The flying rails generally fake out any low voltage device by placing its local "ground," (and the rails it needs above and below ground) at some floating potential. Cable shorts at the input are a real source of interest but since the rails track the input they follow the input down to the potential of the short. Since the opposing input, not shorted, is having 48V sourced to it by a 6K81 phantom pull-up resistor and there's a fairly small termination resistor, 150R to 2K2 or so to the shorted one, the actually DC that winds up on the unshorted input is modest. There is a small amount of time for the tracking servo to follow but the TC is around 50 ms. To be really hard on the part I tried it without any protection diodes.

Still it was quite fun trying to destroy something but the maximum current IIRC was 8 mA peak or so which is considerably smaller than the 3A spike an input cap could provide.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by jeffrey_burr »

Well in fact, I was just teasing my brother with that very question, after he crashed his bike in a race last week, broke three ribs, a collarbone and a shoulderblade.

I guess electricity is just as subject to relativity as any other 'quantity', i.e., there's no real reason that what we think of as zero potential is more valid as a reference than any other. I don't know if that's really true, that's just how your idea makes me feel: post-Einsteinian. Do the batteries of a traveller going half the speed of light appear to have more charge from the standpoint of the stationary observer? Off topic, sorry.

I will eventually get to trying this out, I have a few 1510 sitting around. I thought it would also be neato to try at the front of the 9k or the popular Green pre circuits. I know you're helping develop applications for THAT so I hope this suggestion is not in bad taste.
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

I thought it would also be neato to try at the front of the 9k or the popular Green pre circuits.
Go for it. I think they'll hardly notice their rails being moved about.
clintrubber
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by clintrubber »

Wayne wrote:It hit me. Why not consider the ancient thumbwheel switch as a means of precisely setting mic preamp gain? These have gold plated contacts and have lasted 30 years in my Mom's kitchen timer. In the studio one could know exactly what gain, to the dB, was being used. Great for repeatability and pencil-based "total recall." Didn't the Sphere or Quad Eight do this on the EQ with lever switches? Since our preamp doesn't have up to 99 dB gain we might want to black out those positions above "6" on the first digit.
Great minds think alike ;) Have a bunch of Russian-made thumbwheels here, they're cheap on eBay,
am already using them for a 6-decade resistor-bank and will be using the rest for exactly this application: repeatable mic-gain setting.

But forget about all that repeatability, I just dig the retro-look 8-)

Regards,

Peter
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

Peter! Thanks for joining us.

Yes a very retro look those thumbwheels...
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JR.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by JR. »

I think we discussed this once before.. A two gang switch for 10s and 1s could give more than enough range, putting the 1s on a second gain stage would simplify managing gain interactions. I think I've seen thumb wheel switches with stops in them which could be nice as we don't need 100dB of gain.

I prefer real resistors to pots and in a few semi-custom pre's i found a 12 (13?) position rotary switch quite nice.

An old friend used some of those thumbwheel switches in a custom console for mute-solo groups (a one off design he found very useful for mixing reggae.) Being able to mute and/or solo groups was a little poor man's automation.

JR
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clintrubber
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by clintrubber »

I don't think the thumbwheels I have can be made to stop at a certain setting, so I'll be shorting the higher
10-positions to keep the same gain as the highest meaningful setting.

Bye,

Peter
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