High-Value Resistors for Mic Builders and Tempcos for Synths

[mediatechnology]

I'm stocking a couple of hard-to-find resistors that some members here may eventually need.

For the mic builder I currently have a few 2G ohm (for the search engines who visit us 2000 Mohm). I'm almost sold out of these but I have a few 1Gohm on the way with more backordered. These are Vishay RNX0382G00KNLB (2G) and RNX0501G00FNEL (1G). They are 3/8W and 1/2W sizes. (Doubt we'll see that dissipation in a microphone though.)

The other one I stock is the Precision Resistor PT-146 (PT146). This is a 3500 ppm temperature compensating 1K 1% ideal for use in log and anitlog amplifiers. The 1K PT-146-1K and the 2K PT-146-2K (2K available from Music from Outer Space) are often use in DIY synth building. The 2K is a better choice if it's in the feedback loop of an op amp.

Links are here:

http://www.ka-electronics.com/pt146/rnx-2G.htm 2G
http://www.ka-electronics.com/pt146/pt146-35.htm PT-146
http://www.musicfromouterspace.com/cata ... ge=CATALOG PT-146 2K

[mediatechnology]

I no longer offer the Gig Ohm GΩ resistors or the PT-146 for sale but I did have to use a PT-146 1KΩ +3500 ppm I had in stock as a replacement for a failed KRL RQ-81 (Tel Labs Q81) found in a Heathkit IG-5238 AC Voltmeter with dB.

Since I did a quick web check on both KRL and Precision Resistor I thought I would post the links here:

Precision Resistor wirewound and SMT temperature compensators: http://www.precisionresistor.com/skin/r ... sators.pdf

KRL Bantry Components: http://krlbantry.com

After replacing the 1979 date code KRL RQ-81 with a PRC PT-146 the dB scale was only about half a needle width off. Now that's precision.

See also: http://microohm.com/temperature/tempmini401.html

[mediatechnology]

Just found a couple of gems in Mouser's in-stock resistor inventory:

1K 3900 ppm/°C linear PTC "tempco" resistor PN 660-LT1/4LT52R102J39: https://www.mouser.com/ProductDetail/KO ... MEzw%3D%3D

Not 3300 ppm but in stock. 3600 ppm available from KOA Speer.

Data Sheet 1K 3900 ppm/°C linear PTC resistor PN 660-LT1/4LT52R102J39: https://www.mouser.com/datasheet/2/219/LT-16425.pdf

1GΩ 1/4W Thick Film Resistor: https://www.mouser.com/ProductDetail/TE ... LHzw%3D%3D

[mediatechnology]

Just found a couple of gems in Mouser's in-stock resistor inventory:

1K 3900 ppm/°C linear PTC "tempco" resistor PN 660-LT1/4LT52R102J39: https://www.mouser.com/ProductDetail/KO ... MEzw%3D%3D

Not 3300 ppm but in stock. 3600 ppm available from KOA Speer.

Data Sheet 1K 3900 ppm/°C linear PTC resistor PN 660-LT1/4LT52R102J39: https://www.mouser.com/datasheet/2/219/LT-16425.pdf

1GΩ 1/4W Thick Film Resistor: https://www.mouser.com/ProductDetail/TE ... LHzw%3D%3D

Got the specialty resistors in and had a look at the 1GΩ thick films.

Using the conductance measurement feature of my Fluke 8050A on the 200 nS range I read about 1.07 nS which is about 934MΩ.

I suspect that the tape parallels the other resistors and whoever packed these at Mouser may have touched them and deposited a little bit of oil. Separating the leads on the meter also improved the measurement so there are a lot of variables. Still darn close considering it's in the GΩ range.


Mouser TT Electronics 279-RGP0207CHK1G0 1GΩ Bias Resistor

I'm going to play with the 3900 ppm PTCs next.
After that I'll pair a 1GΩ resistor to an LSK170 and see if they're quiet.

[mediatechnology]

I had a chance to play with the 3900 ppm tempco resistors from KOA Speer.

The response time is really fast.
These are 5% resistors but the 25°C value was almost dead on.


KOA Speer Tempco 1K 3900 ppm Resistor LT1/4LT52R102J3900

The color-coding is kind of unique.
It's "102" (1K brown-black-red) and then "39" (3900 ppm orange-white).



I made a quick xls calculator to see what "fixed" resistor values would be required to make a 3300 or 3400 ppm composite.
I assumed a +/-50 ppm resistor and set its TCR to 0 since its TCR is de minimis compared to the large TCR of the tempco thermistor itself.

The equation I used is TCR = (R1 x TC1 + R2 x TC2) / (R1 + R2).

A 180Ω in series with the 3900 ppm works out to be 3305 ppm with a 25°C value of 1180Ω.

A 150Ω in series with the 3900 ppm works out to be 3391 ppm with a 25°C value of 1150Ω.

When the tempco is padded to lower TCR the voltage divider resistor in a log (R1 below) or expo converter also needs to be scaled by either 1.15 or 1.18.


TI AN30 Log Converters https://www.ti.com/lit/an/snoa641b/snoa641b.pdf


TI AN311 Theory and Applications of Logarithmic Amplifiers https://www.ti.com/lit/an/snoa575b/snoa575b.pdf

I also found this source for tempco resistors: https://synthcube.com/cart/tags/tempco