1510 -- servo, or not ?

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mediatechnology
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Re: 1510 -- servo, or not ?

Post by mediatechnology » Sun Jan 11, 2009 9:38 am

Thanks for posting here freqband.

I've spent a lot of time lately with the 1510/1512.

If you're going to be using this for instrumentation my recommendation would be to just use a Cgain and eliminate the output servo. I do recommend a stepped resistor gain control however for precise repeatability. I posted a spreadsheet to calculate gain resistors which can be used to create custom step sizes if needed: http://www.picocompressorforum.com/foru ... f=12&t=142

If it were for a recording application and one wanted to eliminate Cgain I would opt for an input servo using a very low offset op amp. We can discuss that one later if you think it applies. The output servo does reduce the 250uV (max) * gain + 5mV DC error term, but it does nothing to reduce clicki-ness during switching. An input servo helps reduce both clicki-ness and output Vos. But it requires an op amp which is several times more expensive than a THAT1510 for very low Vos applications.

Consider taking a couple of things from the single-supply drawing posted here: http://www.picocompressorforum.com/foru ... ?f=6&t=191

1) Use the T-bias scheme in which bias resistors of 1K5 to 2K meet and then connect to a 22K (or so) to ground. (Split supplies.) With a low impedance source the noise advantage is swamped by the source, but the common mode input impedance (on the right-hand side of the coupling caps) is much higher and improves LF CMR a lot because it reduces the input C matching requirement. If your application is split supply you will not need four input caps, only two.

Split the Rstop resistor as shown in the single-supply drawing. I was glad to see that in the drawing you posted.

Not shown on the single-supply drawing (or any THAT drawing to date) are added protection diodes in addition to the bridge. Add two 1N4148 diodes from the Rgain pins back to the inputs. The cathodes connect to the input, the anodes to Rgain. One diode extends from pin 8 to pin 3, the second from pin 1 to pin 2. These parallel the existing internal rev Vbe protection diodes and, along with 10 ohm series resistors make it pretty much bullet proof to phantom faults in which one input is grounded and then the second shorted. Don't ask me how I know this. ;)

Consider using a 1512 for lower noise. The maximum gain is 6 dB less but the 1646 adds 6 dB back. But, at lower gains, the 1512 will be much quieter.

Here's a split supply version:

Image

Your gain resistors and Rstop will be different. The 100R at the output isn't required if a 1646 follows.
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Re: 1510 -- servo, or not ?

Post by mediatechnology » Sun Jan 11, 2009 4:02 pm

The OP07 (or OP77) trimmed would be inexpensive. That's the option I've considered for the DC preamp. Either that or an LT1012.
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Re: 1510 -- servo, or not ?

Post by mediatechnology » Sun Jan 11, 2009 8:14 pm

The drawing looks good Randy.
Can the "pin2 to 3"..200pf cap be either a 220 or 180 ??
Yeah either one. I'm reminded of the movie "Mr. Mom" where Mr. Mom is asked "Are you rewiring for 220?" Mr. Mom answers "Yeah 220, 221."

As you point out those 1R5 stop resistors can be made bigger if you are going to use a switch that will provide a true 0 ohm connection at the highest gain. On the drawing I sent, the pot typically has a 2.5 ohm minimum resistance and a 1512 is used requiring a 5R total Rgain. With the 1R5 x2 and ~ 2 - 2.5 Ohms for the pot we get ~5 Ohms. For a 1510 and a switch I'd go with 4R7 x 2.

The NKK MRA 12 pos rotaries from Mouser worked very well as a gain pot. They have a nice detent and are only 1/2" diameter. With 1% resistors you're going to have pretty accurate gain.
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Re: 1510 -- servo, or not ?

Post by mediatechnology » Mon Jan 12, 2009 6:17 am

Randy;

The T-Bias scheme does allow the use of smaller values because of the higher CM impedance on the right-hand side of the caps. I wouldn't go much lower than 22 uF though which eliminates the use of film caps due to size.

On the single-supply preamp non-polarized capacitors are required because, due to the bias voltage at the 1510 input, the required polarization changes depending on phantom being on or off. The 100Ks in the middle do bias the caps and maintain polarization. Those capacitors however could just have easily been non-polars.

In your situation you might consider paralleling the caps with lower value films if you feel the need to do so. I'm not sure it gains you much however.

Wayne
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Re: 1510 -- servo, or not ?

Post by chris319 » Tue Jan 13, 2009 10:02 pm

What is the purpose of the 200 pF capacitor between pins 2 and 3?

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Re: 1510 -- servo, or not ?

Post by mediatechnology » Wed Jan 14, 2009 7:41 am

Randy - Your switch wiring looks exactly like mine. I opted for a "one of twelve" approach (as opposed to series-connected) since it makes the switch wiring easier and the resistor errors non-accumulative. The gain switch does open during transitions but the 1510/1512 does not go open loop - the gain just goes to 0 dB or in the case of the 1512 to - 6dB.

In Open Office to change the dB step size it's Edit|Fill|Series|Increment in column B. Av is calculated from that, then Rgain, then E96 rounded Rgain.

You might want to consider non-linear dB steps with finer resolution near the top or at the instrumentation mic's sweet spot. You can just plug in whatever value you want rather than fill the series.
One job this box will need to satisfy, is when I use a Behringer 8000 test mic, for basic acoustical tests (frequency response, amplitude levels, etc..)
Isn't the 8000 an unbalanced mic internally? I seem to recall a post somehwere where they showed the internal wiring.
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Re: 1510 -- servo, or not ?

Post by chris319 » Wed Jan 14, 2009 3:01 pm

What is the purpose of the 200 pF capacitor between pins 2 and 3?

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Re: 1510 -- servo, or not ?

Post by mediatechnology » Thu Jan 15, 2009 7:52 am

Chris - thanks for posting. Good question.

That 200 pF between the inputs reduces the differential input impedance at RF at the device pins. The differential capacitance at the XLR is 470 pF/2. 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.

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Re: 1510 -- servo, or not ?

Post by chris319 » Thu Jan 15, 2009 4:54 pm

Thanks, Wayne. I am about to rebuild my 1510 preamp and will be incorporating your modifications to the basic design from THAT Corporation.

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Re: 1510 -- servo, or not ?

Post by mediatechnology » Tue Jan 20, 2009 6:27 am

Notice the 1.5k Vishay S102 at 0.01% ...?.... I scored 100 of these for $5
Where?

Nice looking prototype.
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