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 »

Thanks everyone. I do hope to continue advancement of this. The burning question for me is how much microphone-induced offset is seen in the wild. My hunch is that capacitively-coupled microphone outputs with 5% phantom pick-off resistors will have the greatest offset. A transformer-coupled output mic will be near 0 and I think it was JR that noted the Schoeps PNP emitter follower type would have to have a 50% Ib error to provide 18 mV offset. (edited)

Just to let everyone know where this stands. It currently lives on a protoboard and is far from being fully-tweaked. If the offset turns out to be far less than the rather large correction it's capable of supplying, the current injection resistors (and servo noise contribution) can probably be raised considerably. You'll see in the demo board from THAT the correction resistor is two orders of magnitude larger. It's working against capacitors there. In the DC-coupled one fortunately the termination resistor will serve to equalize the offsets.
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JR.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by JR. »

I haven't thought about this for a while but mic designers make me nervous.. I've had a few conversations with them over the years and they make assumptions about mic preamps just like we do about mics.

There should be ways to deal with any potential error source as long as we can anticipate what they might be.

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

Post by mediatechnology »

I had a conversation with Neumann's engineering department about this because I had heard that the TLM series had asymmetric current draw. It doesn't.

The Behringer measurement microphone is unbalanced. That one would never work with this preamp. I'm sure that there are other strange ones as well.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by emrr »

You have to consider the Behringer mic, and I suspect you also need to think about it as you would a live rig splitter system running 1000 feet of possibly questionable wire through hazardous terrain. I've seen some very pro splitters systems fail miserably at times, and there must be some parallels to consider.
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

Well I did try just about every way possible to blow it up by producing phantom faults. Without the stored charge from the input caps it's actually more tolerant. I'm sure that there are additional faults to be considered though I'm not sure anything could survive being plugged into mains power for example. (I did entertain the use of fuses/lamps in series with the inputs to see if it could survive that though.)

Although it should be able to stand up to a hostile environment I really didn't envision it being used in live sound. If someone wanted to take a preamp without the usual transformers and caps and substitute 1000' of wire in place of them why not? Its common mode rejection should be excellent as long as the circuit impedances are fairly balanced. If not well-balanced a transformer might be a better choice. Long line applications do cry out for a good common-mode choke RFI filter.

Despite its use of a preamp IC (vs. discrete) it's not exactly a low cost circuit either: The support ICs, at least if a LT1012 is used as the servo, is more expensive than the preamp IC itself. My thinking was it would make a good dual (or M/S) rack-mount or lunchbox format unit. I don't see this circuit, with all its complexity, being built into a console or used in high channel-count applications.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by JR. »

Some of that expense is due to component choices.

I have though about applying something like this to a console where you could have common high voltage PS and perhaps other economies of scale.

In my mind it's still a little scary since it would cost more for a mostly intangible benefit, with the potential for problems. The last thing you want in a professional environment is a finger pointing exercise.

Great sound most of the time is not as good as good sound all the time.

I remain a fan of this approach and still plan to do one of my own.. I would also like to build my own A/D into it so I never have to bring analog audio back to earth. Of course my A/D won't be conventional. 1 bit, decimated down to common data rates.

Luckily I'm busy enough with other projects I don't have to deliver on my BS. :lol:

JR

PS: in live sound it's not that rare to get power problems and voltage between grounds... pin 1 and shield grounds need to be robust enough to take out fuses before too much smoke is released.
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by XAUDIA »

Hi Wayne,
Thanks for the clarification. Thought I'd get the iron out and start experimenting so I'm in the process of knocking together a pair of an extremely simple version of Wayne's circuit to see how the audio pathway sounds. I have a couple of nice new ribbon mics to play with too, and a fairly relaxed recording session at the weekend so that I can get some feel for how the preamps sound and if I want to build them up into something more complicated. For now I can skip the phantom power, flying rails etc.

I have completed the simple input (1510/1512) and output (1646) stages, and am looking to fill in some of the gaps. So I have a couple more questions.
mediatechnology wrote:On the 1510 datasheet there really should be a Cgain in series with Rgain. This keeps DC out of the pot or stepped switch.
What is the cause of the DC offset on Rg? Does it stem from an external offset at the inputs caused by the microphone, or is it something inherent to the 151x? I ask because with Rg disconnected and no mic connected, I see no DC across pins 1 and 3, (although they are about 0.6V away from 0V).
You still want a high-impedance bifet/FET follower after the 1510/1512 if you want to use a low-value film cap and 1M bias resistor. The cap needs to be at least 100V. The 1646 presents a very low input impedance ~4K ohm so a coupling cap between the 1510 and the 1646 would have to be very large (47uF/100V) and would have a lot of stored charge.
I was actually planning on using the output servo circuit to avoid using a cap at all! Is there any reason this won't work? I recall we had a discussion about this in the past, but can't recall the outcome.
mediatechnology wrote: For some OP07's it might be a good idea to use a Vos trim or the more expensive LT1012. The output offset servo shown in the datasheet does not solve the DC across the pot issue - it simply corrects the resulting Vos * Gain error.
I wanted to knock this together before the weekend, but don't have OP07s to hand. I have OP27 and OP77. OP77 seems to be a later version of the 07 with a low offset voltage so looks ideal.

Thanks for your help. I'll report back with my findings after the weekend - maybe even with sound clips.

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

Post by JR. »

Not to state the obvious but if you don't need phantom, you don't need the complicated flying PS.

KISS.

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

Post by XAUDIA »

Absolutely! I've simply got it hooked up my a test rack with a simple +/-15V power supply. Once it's working (and if I like the sound), I'll get more ambitious.
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mediatechnology
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Re: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Post by mediatechnology »

What is the cause of the DC offset on Rg?
There is a 250 uV (max) Vos in the input transistors which get multiplied by the gain. See the equation I provided earlier.
I have OP27 and OP77. OP77 seems to be a later version of the 07 with a low offset voltage so looks ideal.
The OP27 will have too much bias current IIRC for 100K servo resistors. (Input servo.) The OP77 ought to be fine.

The reference I made to the bifet follower was if flying rails were used. As JR said you don't need phantom if you intend to use these with ribbons.
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