1510 -- servo, or not ?

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chris319
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Re: 1510 -- servo, or not ?

Post by chris319 » Fri May 01, 2009 2:45 pm

OK, I will leave the center resistor out as I'm omitting those coupling capacitors.

I have considered both the 1510 and the 1512. After extensive testing I've found that below 40 dB of preamp gain I start to hear the A to D noise of the recording device. This preamp will have fixed gain of 40 dB. If more gain is needed it will be applied in the digital domain. If a signal is too hot, an attenuator will be added between the mic and the preamp input. So the low-gain noise advantage of the 1512 doesn't appeal to me. Comparing the specs at 40 dB, THD+N for the 1510 is 0.0016% for the 1512 and 0.005% for the 1512, while EIN is 1.7 for the 1510 and 1.4 for the 1512, so I'm content with the 1510 and its lower THD+N figure.

My preamp is kind of purpose-built for my use. If I were doing a preamp for commercial release I would build it more conventionally and would basically be duplicating the Rane design. I am loathe to run audio through capacitors when not needed. Last year I tried AC coupling preamp stages and the selection of cap material made a huge difference in the sound.

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mediatechnology
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Re: 1510 -- servo, or not ?

Post by mediatechnology » Fri May 01, 2009 5:09 pm

Have you looked at this one? No input or Cgain caps.

http://www.picocompressorforum.com/foru ... p?f=6&t=14

I would do the offset servo differently now and change a couple of things but it's passed the proof-of-concept stage and works. Roger sent me some AT3525s to try with it.

The servo shown here would require some modification (10K or so) to reduce the injection resistor value since there are no input caps to work against.

http://www.picocompressorforum.com/foru ... ?f=6&t=256

I'd be real curious to know what your experiences are with capacitor-coupled mics (internal to the body) and the Schoeps output topology (PNP follower with phantom resistors as emitter load) and the DC offsets they develop across pins 2 and 3.
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chris319
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Re: 1510 -- servo, or not ?

Post by chris319 » Mon Jun 22, 2009 8:36 pm

Wayne -

Sorry, I'm still a little confused. I've built my THAT 1510 preamp with no internal phantom power supply on the premise that the user will have to provide his own phantom power. Thus, the blocking electrolytics on the input have been omitted, as well as the 22k resistor to ground and the 6.8k resistors. The mic output/preamp input goes through 4.7 ohm series resistors, there is the 1N4004 diode bridge, the 470p caps and 47p cap to ground and a pair of 3.3k terminating resistors to ground. In a previous post you said:
The added 200 pF directly at the input appears in parallel with that but is after the ESL of the coupling caps and 10 ohm current-limiting resistors. It's a form of double-protection.
but in my case there are no coupling caps. Given this configuration do I want the 200pF capacitor in or out? Will it benefit the circuit in any way?

Many thanks.

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mediatechnology
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Re: 1510 -- servo, or not ?

Post by mediatechnology » Mon Jun 22, 2009 8:50 pm

Hi Chris -

I would still put a 200/220 pF across pins 2 and 3 of the 1510/12 near the device leads.

Wayne
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chris319
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Re: 1510 -- servo, or not ?

Post by chris319 » Mon Jun 22, 2009 10:29 pm

Cool, I will leave the cap in.

I am sketching out a breadboard diagram for a buddy who wants to build one. My brother (who works for a company which specializes in DIGItal DESIGN) put it on the AP at work and discovered that my breadboard construction gave it a very low level hum (like -120 dB) with a couple of harmonics to boot. The fundamental was at 60 Hz even though my preamp is battery powered. Those white plastic breadboards are so convenient but they lack a ground plane, so I went to http://www.onlinemetals.com and ordered some copper sheet cut to the size of the breadboard, soldered a wire to it, fastened it to the underside of the breadboard and connected the wire to ground. Voila! Hum gone!

They also make breadboards with metal base plates already attached but I haven't tried one yet.

Thanks again!

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