Output zobel capacitor types

[ihan]

The majority of the amps I've designed (and built) have used an output zobel cap which is similar to this:
Arcotronics | Passives | Capacitors | Plastic Film | Metallised 105°C |R82EC3100DQ70J

This component type has always had the desired effect, with no failures.

But lately, I've been wondering if a 100nF 100V 1206 X7R ceramic capacitor will do the job just as well.

Does anyone have any experience using an X7R ceramic in an output zobel? Any comments?

[jaycee]

I don't know about using ceramics as the change in value with temperature might affect the working of a zobel. I have seen mylar (greencaps) used and they work fine in this application. The metallised polyester is probably overkill.

[jcx]

the Zobel C needs low esl and esr, construction/case sized to survive full output swing/full damping R current at MHz

X7R C have significant voltage coefficient - you may want 2-4x V rating to keep C modulation within reason - although exact RC time constant isn't too important and the audio frequency Z is high enough for the distortion not to matter there's no real need to build in a nonlinear load

they will also be somewhat piezoelectric and actually emit sound

many inexpensive film types should work fine

[nigelwright7557]

I have always used polyester and seen most other designs using polyester too.

[Jay]

Originally Posted by ihan

Does anyone have any experience using an X7R ceramic in an output zobel? Any comments?

May be you need to build an in-stable amp or load the amp with difficult load to see the zobel in action. For normal-working amp it is difficult to hear the difference. But when I can put the zobel off-board, I prefer to use those green resistors usually used in speaker crossover, and an Orange Drop capacitor. For on-board, I use smaller devices such as carbon-film or VTM resistor, and an MKT (those orange Philips caps that I can find in abundance in old TV/monitor PCBs)

[djk]

I've seen 0.1µF 250V Mylar caps fail in a 200W amplifier zobel network.

Caps have to be de-rated for high power, high frequency AC use.

[zebra100]

I've used Kemet CKR06 (moulded ceramic, 50v), MIL Spec (MIL-PRF-39014) with a value of 0.1uF, and a metal film resistor of 10R in my LM3875 Gainclone. I haven't had any issues with colouration in the sound output.

Chris.

[ROXR]

I ear that ceramic caps would made bad contact and crispis sound, on a previ. sansui.

I have a Yamaha AX-500 and one problem with overvolt on the bias from my amp, it should be stable to 10 mv , but I only obtain an endless volt increment and it does not seem to stop. I don't know what cause the oscillation , I be crazy with this matter. Before I fail with an cross accidentaly betwen base and collector and the power npn transistor burned and others. I changed all diodes and a lot of transistors, some capacitors electrolitic and polyester but I must ceramics capacitors to revise, 50 volts most of them, an range 5pf - 0,022 uF.
The question is, could one o some capacitors alter frequency or oscillateand increment volt bias, I think what these task is transistor exclusive work.
What type of variation could cause capacitors ceramic, mylmar and polyester, on an amplifier?
The BIAS is some type of grid who regulate to derive energy for a range power to transistor, one cathode type who do any type of absortion. It is regulate with a variable resistor from 1 omh to 2,7K or more, in my case when less omh more down the BIAS. When the bias is more to 10 mV the transistors heats in idle mode.

[Aldotin]

Originally Posted by djk

I've seen 0.1µF 250V Mylar caps fail in a 200W amplifier zobel network.

Caps have to be de-rated for high power, high frequency AC use.

What about a class X2, 0.1uF, 275Vac polypropylene capacitor?

[wintermute]

My 100W/channel amp apparently has problems with the RLC caps dying if they are not MKT-P (metalized polyester and paper) caps. After an oscillation problem cooked the resistors in the RLC I decided I should replace the caps too, couldn't find the so called MKT-P caps (the originals were Phillips). Instead I used some of these Rifa X2 metalized paper caps Haven't had any problems with them, and they look really nice too

Tony.