1 G.ohm resistors

   Does anybody have a good source for 1 G.ohm (or 2 G.ohm) resistors in Europe?
I need a couple of those for a capacitor microphone.
NOT the big ones (1 W or more), but small ones: 0.1 or 0.25 W.
'Normal' types, not SMD.

Suggestions are highly appreciated!

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Ruud Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 08-Nov-03 Location: The Netherlands Online Status: Offline Posts: 142	Post Options Post Reply Quote Ruud Report Post Quote Reply Topic: 1 G.ohm resistors Posted: 09-Nov-08 at 16:05
	Does anybody have a good source for 1 G.ohm (or 2 G.ohm) resistors in Europe? I need a couple of those for a capacitor microphone. NOT the big ones (1 W or more), but small ones: 0.1 or 0.25 W. 'Normal' types, not SMD. Suggestions are highly appreciated!
	Happiness is a warm soldering iron.

Ivan Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 08-Aug-21 Location: Praha Online Status: Offline Posts: 396	Post Options Post Reply Quote Ivan Report Post Quote Reply Posted: 09-Nov-08 at 17:59
	Hi Ruud, I have never seen what you mention. Very big values of resistors (above 10 Mohm) I saw were all about 2 W, molded in glass tubes. These tubes had to be washed by absolute alcohol after soldering to remove dampness and fat. Common PCB materials may have comparable resistance with these values, so glass or PTFE insulators are a must. Did you consider making the resistors e.g. by scraping a hard pencil on ceramics ? BR from Ivan OK1SIP

Ruud Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 08-Nov-03 Location: The Netherlands Online Status: Offline Posts: 142	Post Options Post Reply Quote Ruud Report Post Quote Reply Posted: 09-Nov-08 at 18:07
	Most capacitor microphones (Neumann, AKG, Schoeps etc) use resistors with values of 1 G.ohm or higher. The signal levels are very low ( < 1 mV. ) so you really need a stable, low noise resistor. I don't think a pencil on a isolator will be good enough for this purpose! By the way: most microphone manufacturers use a circuit on a standard epoxy PCB, covered with a thin layer of plastic.
	Happiness is a warm soldering iron.

Ivan Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 08-Aug-21 Location: Praha Online Status: Offline Posts: 396	Post Options Post Reply Quote Ivan Report Post Quote Reply Posted: 09-Nov-09 at 11:25
	Hi Ruud, I have never dealt with the internal construction of capacitor mikes. Plastic coated PCB can stop steam diffusion into the epoxy/glass insulator, improving the circuit stabitlity significantly. A stable high-value resistor is a must, of course. The thermal noise voltage on a resistor can be calculated as V_n=4kTR . The noise voltage is directly proportional to the resistor value ! So speaking about low-noise high-value ones seems a bit like searching of dry water to me. TESLA Blatná offers 1W, 3W and 6W types ranging up to 10 Gohm. Get the datasheet at http://www.tesla-blatna.cz/_soubory/rezistory-vysokoohmove-resistors-high-ohmic.pdf and see the picture http://www.tesla-blatna.cz/en/popup.htm?rezistory-02.jpg&news&undefined&undefined IMHO there is a mistake at the bottom of the datasheet - the note should be: Type 3WK 680 06 has been manufactured in B type only, i.e. with wire leads ! BR from Ivan OK1SIP

Ruud Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 08-Nov-03 Location: The Netherlands Online Status: Offline Posts: 142	Post Options Post Reply Quote Ruud Report Post Quote Reply Posted: 09-Nov-09 at 11:53
	In the meantime I have also found some 1 Watt 1 G.ohm resistors, but they have the disadvantage that they are rather big in size. Concerning the noise: Of course the thermal noise increases with the resistor value, but don't forget that the capacity of the microphone capsule is connected in parallel with the 1 G.Ohm resistor! Since the capacity of the capsule is in the order of 30 pF, this will form a low-pass filter, eliminating most of the noise. The fact that it works is proven! ALL studio microphones use this principle, with 1 or 2 G.ohm gate or grid resistors. But thanks for the info!
	Happiness is a warm soldering iron.

Ivan Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 08-Aug-21 Location: Praha Online Status: Offline Posts: 396	Post Options Post Reply Quote Ivan Report Post Quote Reply Posted: 09-Nov-10 at 12:31
	Yes, taking care about noise is not the primary task. Long-term and temperature stability are more important. BR from Ivan OK1SIP

the flying dutchman Members Profile Send Private Message Find Members Posts Add to Buddy List Guest	Post Options Post Reply Quote the flying dutchman Quote Reply Posted: 10-Mar-02 at 11:42
	RS components are a nice source for some exotic components. Usually prices are not really low but they sell only quality components, in many cases datasheets are available, and their service is excellent. They have 1G 1/4W resistors, order code 247-7856 HTH