tank filter cap question

[dynamo]

Does the quality of the cap used for a tank filter matter (affect sound quality)?

I need to bypass the inductor on a mid with a 0.22uf cap to rectify a small blip in its response. I have on hand a 50v polyester film cap of the correct size. Is there ANY value whatsoever to going to a higher voltage and/or polypropylene film cap for this purpose? I assume 50v is enough for a mid tank circuit since I've seen 50v caps in commercial speakers before, but please correct me if I'm wrong..

Thanks!

[Jeff B.]

Re: tank filter cap question

In my opinion, the polyester cap will be fine in this application.

[dynamo]

Re: tank filter cap question

In my opinion, the polyester cap will be fine in this application.

Thanks much Jeff. Hope you are doing well!

Chris

[tyger23]

Re: tank filter cap question

I agree with Jeff - for "mortals", the 50V poly cap should be fine. This does assume however, that you're not cranking the volume to insane levels.

Also - MAKE SURE you have a low-value resistor (1-4 ohms) blocking the path to ground if you have a "cap-to-ground" after the tank capacitor. Else, you run the risk of sending some amplifiers to an unstable state. If you have any questions about this, post your crossover schematic and we can help determine if there's a possible issue.

[dynamo]

Re: tank filter cap question

I agree with Jeff - for "mortals", the 50V poly cap should be fine. This does assume however, that you're not cranking the volume to insane levels.

Also - MAKE SURE you have a low-value resistor (1-4 ohms) blocking the path to ground if you have a "cap-to-ground" after the tank capacitor. Else, you run the risk of sending some amplifiers to an unstable state. If you have any questions about this, post your crossover schematic and we can help determine if there's a possible issue.

Good thought. In this particular circuit I will have a series resistor before and after the mid crossover, so the circuit path to ground can't be completed without going thru a resistor.

On that note though, I have other builds using a tank without the cap padded with a resistor and I have never experienced an issue. What types of amps may this affect (not doubting you at all, just wondering if there are certain types of amps to avoid with non padded tanks)? I will pop them open and add one as time allows, but am curious if it affects a certain class or vintage of amp more than others. Thanks!

[Paul K.]

Re: tank filter cap question

Types of amps affected? Those that are poorly designed, built wrong or that have defective components. Any solid-state amplifier that's been properly designed should not be bothered. Don't know about tubed amplifiers, however, but I would assume a good design should not be bothered.
Paul

Good thought. In this particular circuit I will have a series resistor before and after the mid crossover, so the circuit path to ground can't be completed without going thru a resistor.

On that note though, I have other builds using a tank without the cap padded with a resistor and I have never experienced an issue. What types of amps may this affect (not doubting you at all, just wondering if there are certain types of amps to avoid with non padded tanks)? I will pop them open and add one as time allows, but am curious if it affects a certain class or vintage of amp more than others. Thanks!

[tyger23]

Re: tank filter cap question

Paul,

This issue can affect any amplifier with ultra high frequency bandwidth capabilities. Basically the issue is that if you can ever draw a straight line from the amplifier to ground through capacitors (without going through resistors or inductors), then you can short out the amplifier's output at certain frequencies. The frequencies at which the shorting would occur will depend on the value of the capacitors in the circuit. For most tank capacitor circuits, this frequency is well above the audible frequency range.

This is unlikely to affect class-D amplifiers because there is almost always an output filter that would prevent the amplifier from ever being affected by a possible short at high frequencies. However, tube amplifiers and high-end class A or class A/B amplifiers could be affected. The amplifier could be sent into an unstable oscillation state.

Basically, anytime you use a tank circuit, just make sure there's never a direct path to ground through only capacitors. Adding in a resistor is simple, easy protection.

[Paul K.]

Re: tank filter cap question

You're "preaching to the choir". While I have others design crossovers for me I understand what "might" happen, but it shouldn't happen to any amplifier if it's designed properly all things considered. OTOH it's a very good practice to add the protection resistor in the tank circuit of the crossover.
Paul

Paul,

This issue can affect any amplifier with ultra high frequency bandwidth capabilities. Basically the issue is that if you can ever draw a straight line from the amplifier to ground through capacitors (without going through resistors or inductors), then you can short out the amplifier's output at certain frequencies. The frequencies at which the shorting would occur will depend on the value of the capacitors in the circuit. For most tank capacitor circuits, this frequency is well above the audible frequency range.

This is unlikely to affect class-D amplifiers because there is almost always an output filter that would prevent the amplifier from ever being affected by a possible short at high frequencies. However, tube amplifiers and high-end class A or class A/B amplifiers could be affected. The amplifier could be sent into an unstable oscillation state.

Basically, anytime you use a tank circuit, just make sure there's never a direct path to ground through only capacitors. Adding in a resistor is simple, easy protection.

[thekorvers]

Re: tank filter cap question

paul,

this issue can affect any amplifier with ultra high frequency bandwidth capabilities. Basically the issue is that if you can ever draw a straight line from the amplifier to ground through capacitors (without going through resistors or inductors), then you can short out the amplifier's output at certain frequencies. The frequencies at which the shorting would occur will depend on the value of the capacitors in the circuit. For most tank capacitor circuits, this frequency is well above the audible frequency range.

This is unlikely to affect class-d amplifiers because there is almost always an output filter that would prevent the amplifier from ever being affected by a possible short at high frequencies. However, tube amplifiers and high-end class a or class a/b amplifiers could be affected. The amplifier could be sent into an unstable oscillation state.

Basically, anytime you use a tank circuit, just make sure there's never a direct path to ground through only capacitors. Adding in a resistor is simple, easy protection.

This!!

[dynamo]

Re: tank filter cap question

I appreciate both sides of your debate and see both of your points. I think I will put them in from this point forward to take a $.75 safety measure, although possibly not necessary. I did just add some to a design I am finishing as we speak.

On the same note, I would like your advice in this related topic. I will post the pcd snip below. I used a 3.3uf cap in parallel with the tweeter to soften the top end, and did not put a resistor on it because the signal must pass through an 8r resistor in front of the crossover or the 6ish ohm dcr of the voice coil if reversed to complete the circuit. Is this sufficient in this case to rely on the 8 ohms or voice coil dcr/inductance to negate the need for a 1ohm after the parallel cap?

For what its worth, it works and sounds fine on my old NAD 2600A.

Thanks again