NE5534 questions

[Wolvatron]

hey guys! i'm the newbie here, but i had a question and i wanted to come to the experts to try and get it solved.

first of all, i work in a repair shop with 4 other technicians. one of the main things that we do is align heads on high speed duplicators. the play heads can be aligned using the amp already in the machine. the copy heads are generally aligned using external amplifiers that are home-made. we have amps that were supplied to us through a vendor, but they discontinued the amp. they sent us schematics so that we can just build our own.

we're able to find all of the components required to build the amp with the exception of a 5pf 500vDC capacitor. this cap is used to connect pins 1 and 8 on an NE5534 amp. we're all banging our heads trying to figure out why they put a 5pf 500vDC capacitor between pins 1 and 8 on the amp. we don't quite know why it's there and we're not sure what would happen if we didn't include it in the circuit. so i guess we're either looking for an explaination for the existence of the capacitor, or where we can find the cap. any input would be great. thanks, everybody!

by the way: the amp is powered by a single 9v battery.

[skyraider]

Are u sure it is connected between pins 1 & 8? from the datasheets, shunt capacitance compensation is used between pins 5 & 8. to limit the bandwith and slew rate. the formula as given: c=1/(2*pi*F*R1)
R1=Rin/10

if your system doesnt need the high bandwith, you can subsitute other values. I dont think you dont need 500v, definitely. Just any value which is more than double of 9volts, and i think any ceramic caps will be able to handle that.

if youre lazy, just take the capacitor out. If only you have stability problems, put in something like 22pF.

[ashok]

I think you can use disc caps. These usually come in 50 or 63 volt rating. Anything above 20 volts should be OK. You can use two 10 pF in series to get 5pF. But in your case I don't think the value 5pF is critical. you could probably use 4.7pF or 5.6 pF or 6.8 pF. Beyond that you could use caps in series. Two 10pF in series will give 5pF , 15 pF will give 7.5pF etc.
Maybe you just stick to something close to 5pF.
Like Skyraider said , check the pin number.
Cheers.

[B.VDBOS]

Its an external compensation capacitor used to optomise freqency response for a particular application and,or gain.
I think the reson a 500volt cap is used is because they are more linear than lower voltage caps, disc ceramic caps are not known for there linearity

[Hybrid fourdoor]

What about silver dipped mica?

Dielectric
Silver dipped mica capacitors are fabricated from the finest India Ruby Muscovite Mica that is available. This particular form exhibits the very best characteristics of mica and results in a capacitor with optimum high temperature reliability and excellent stability.

Mica is a natural inorganic material, an aluminum silicate mineral, having toughness, elasticity, and adaptability to thin splitting. These physical attributes combined with excellent electrical characteristics make mica a valuable capacitor dielectric.

Electrodes
Silvered mica construction permits intimate contact between the electrodes and the dielectric enabling uniform control of capacitance and a great increase in order of stability.

The electrodes are silver which is applied to the mica dielectric by a screening process, and then fired in an oxidizing atmosphere to obtain a permanant bond. This method provides a plate which will always remain in the same relative position with the dielectric.

Digikey just happens to have 5pf 500V caps for $1.78 each (less if you buy bulk of course )

[JOE DIRT®]

Originally posted by B.VDBOS
Its an external compensation capacitor used to optomise freqency response for a particular application and,or gain.
I think the reson a 500volt cap is used is because they are more linear than lower voltage caps, disc ceramic caps are not known for there linearity

ceramic capacitors are used for that reason because of their linear in high frequency applications

[Wolvatron]

you guys have been extremely helpful. many thanks from everybody here at the shop.

you guys are the best!