High value film caps

[Kesh]

Can someone recommend me a 10-47uF no-nonsense good priced film cap/brand?

I have no experience of choosing large film caps. The market seems to be full of motor run caps (i don't even know what that is) or audiophool caps.

Not quite a stomp box, but it's going in a Hammond case. I'm making a mic preamp, the INA217 chip based one, straight from the datasheet.

It calls for two large value caps at the input (47u, though I think I could probably get away with less, even though they are effectively in series, or i could parallel smaller caps). Usual DC blocking, but to block the phantom power. And non-polarised for when the phantom power is off.

I don't believe in magic mojo caps, but do believe electros aren't perfect, especially for something as pure as the human voice. And you can't get much more in the signal chain than a mic preamp.

thanks

[MartyMart]

Hi Kesh,
Honestly I can see no reason for that value to be SO big !! .... it must be a "typo" or mistake in the schem ?

There is nothing you can buy that would even fit in an enclosure of that size, I've bought 10uf's as poly "radial" type
for output caps in a valve compressor but as "audio grade" they are perhaps 15 Euro's or more each.

Have built something very similar to your pre with 2u2 poly caps ( these are non polar anyway ) and they are easy to get
in 63volt versions - here's a 4U7 version : http://uk.farnell.com/epcos/b32560j475k/capacitor-4-7uf-63v/dp/9752315

I think that 4U7 is as high as you need to go, get WIMA or decent quality as above and you will be golden.

Marty.

[R.G.]

The nice thing about your application is that you don't need the several hundred volts of the motor run caps. If I was doing this, I would parallel up 1uF or 2.2uF 50/63V films to get to any value. 2.2uF is about as big as I've seen PCB mount low voltage films, although they keep on getting bigger as time goes on.

[amptramp]

Are you sure you need R4 and R5 to be that low?  If you go up by a factor of 10, the capacitance can come down by a factor of 10.  The INA217 is a low-noise instrumentation amplifier with a bias current of 2 microamps, giving you 4.4 millivolts bias voltage for the values of R4 and R5 as shown.  You can easily go ten or more times up on R4 and R5.  You may not need the 1.3 nV/root Hz noise performance for a microphone and the impedance would be dominated by the 6.8K phantom power resistors, R1 and R2.  You can get 1µF film capacitors that are small enough just about anywhere.  You circuit is shown in the TI datasheet:

http://www.ti.com/lit/ds/symlink/ina217.pdf

but that does not mean it is the only possible set of circuit values.

[Kesh]

Hi Kesh,
Honestly I can see no reason for that value to be SO big !! .... it must be a "typo" or mistake in the schem ?

There is nothing you can buy that would even fit in an enclosure of that size, I've bought 10uf's as poly "radial" type
for output caps in a valve compressor but as "audio grade" they are perhaps 15 Euro's or more each.

Not a typo, similar circuits on many TI datasheets. At that size it's a pole at about 1.5Hz. I'd be happy with a pole below 10Hz, which is why I'd be OK with a 10u cap. And Hammond make some biggish cases!

The nice thing about your application is that you don't need the several hundred volts of the motor run caps. If I was doing this, I would parallel up 1uF or 2.2uF 50/63V films to get to any value. 2.2uF is about as big as I've seen PCB mount low voltage films, although they keep on getting bigger as time goes on.

This is what I might well do. I've plenty of 1u, 1.8u and 2.2u films.

Are you sure you need R4 and R5 to be that low?  If you go up by a factor of 10, the capacitance can come down by a factor of 10.  The INA217 is a low-noise instrumentation amplifier with a bias current of 2 microamps, giving you 4.4 millivolts bias voltage for the values of R4 and R5 as shown.  You can easily go ten or more times up on R4 and R5.  You may not need the 1.3 nV/root Hz noise performance for a microphone and the impedance would be dominated by the 6.8K phantom power resistors, R1 and R2.  You can get 1µF film capacitors that are small enough just about anywhere.  You circuit is shown in the TI datasheet:

http://www.ti.com/lit/ds/symlink/ina217.pdf

but that does not mean it is the only possible set of circuit values.

I don't know enough about it, but mic preamps seem to always have input resitors of about 2k. Apparently mics can be fairly sensitive about how they are loaded. I have the datasheet, and several others - all seem to be quite strict about this.

[Jdansti]

I've seen some massive through hole film caps at my local surplus shop. I'll have to take a look next time I'm there.

Here's what a quick search yielded:

10uf/250v Poly: Mouser Part #: 667-ECQ-E2106KF $2.91

47uf/100v Poly: Mouser Part #: 871-B32526T1476K $12.83

33uf/10v Poly: Newark Part#: 01M7267 $3.86