Hi all,
This may sound like a silly noob question, but I hope it will be useful for others as well-
How much capacitance is required to block DC from going to the speakers on a tube preamp? Is it based on amperage or voltage?
Just concerned as I have heard of tweeters being fried from DC using a 17uF decoupling capacitor on a tube preamp. I was under the impression that small Values (less than 1uF) would be sufficient to block DC.
Any info or comments are appreciated.
Thanks
Roger
This may sound like a silly noob question, but I hope it will be useful for others as well-
How much capacitance is required to block DC from going to the speakers on a tube preamp? Is it based on amperage or voltage?
Just concerned as I have heard of tweeters being fried from DC using a 17uF decoupling capacitor on a tube preamp. I was under the impression that small Values (less than 1uF) would be sufficient to block DC.
Any info or comments are appreciated.
Thanks
Roger
Hi,
Voltage rating is what matters most. After that the value required is calculated depending on the lowest frequency you want to pass going to the next stage impedance.
Surely you're not driving speakers from a preamp?
If the cap is faulty then maybe this could happen.
What kills most tweeters though is oscillation.
Ciao, 😉
Voltage rating is what matters most. After that the value required is calculated depending on the lowest frequency you want to pass going to the next stage impedance.
Surely you're not driving speakers from a preamp?
If the cap is faulty then maybe this could happen.
What kills most tweeters though is oscillation.
Ciao, 😉
Any component used in preamp can not cause speakers getting DC.
This is only possible with faulty (SS) output amplifier.
You have now confused yourself.
Bottom line: a capacitor that passes even a little DC is "leaky" at the very least and shorted at the worst. Either way, it is pretty much due for a date with the garbage tin. A transformer that passes DC is a great way to get a nasty shock and may well be a fire waiting to happen.
Any capacitance value less than infinity will block DC. The value depends on frequency response, not current or voltage. Speakers are not normally connected to a tube preamp.rcollege said:
PS I think you meant 'coupling cap', not 'decoupling cap'.
Last edited:
Many times people choose massively inadequately sized caps in order to avoid using electrolytics because of their supposed sound impairing qualities, little realizing that they have distorted the lower frequencies and introduced significant phase shift for their pains. This is especially true of power output stages.
Allen wright advocated using 8000uf in his output bypass which would well and truly place the phase response below audible frequencies in almost all circumstances. I tend to use 1000uf because I have a big stock of them and they are dirt cheap at low voltage ratings.
Shoog
Allen wright advocated using 8000uf in his output bypass which would well and truly place the phase response below audible frequencies in almost all circumstances. I tend to use 1000uf because I have a big stock of them and they are dirt cheap at low voltage ratings.
Shoog
You're referring to the cathode bypass here? I understood the bigger the electrolytical capacitor the worse HF decoupling occurred, unless specified for audio use (or decoupled with a small foil type).
OOPS ....
We need to get the terminoloy right here. There was mention of three different functions of capacitors:
Decoupling caps (see also bypassing caps): Those are used between the d.c. rails, particularly in early stages of an amplifier, to prevent (smooth out) any common signal on the power line from contaminating such stages, in worst cases causing l.f. oscillation.
Coupling caps: Such is used in the signal line to isolate d.c. level differences between stages. As such they form pure d.c. blockers, over the signal band they (should) have no further influence on the signal.
Bypass caps: Those have the same function as decoupling caps: To 'nullify' the effect of any resistance on signal, thus effectively shorting such for signal. This term usually denotes caps across emitter/cathode resistors and others; they are essentially also doing decoupling duty.
I take it the OP meant coupling caps. Not sure which function Shoog was referring to; bypass or coupling?
Painted,
In my experience the ESR of an electro cap needs to be quite poor before introducing effects in the audio band; sometimes a little overrated. Of course, some amplifiers, particularly SS, still have response well outside the audio band in order to perform well inside. In such cases ESR mignt well enter the picture. Some advise the use of several lower capacitance value caps in parallel rather than one huge one (also more convenient space-wise). But I am OT now.
We need to get the terminoloy right here. There was mention of three different functions of capacitors:
Decoupling caps (see also bypassing caps): Those are used between the d.c. rails, particularly in early stages of an amplifier, to prevent (smooth out) any common signal on the power line from contaminating such stages, in worst cases causing l.f. oscillation.
Coupling caps: Such is used in the signal line to isolate d.c. level differences between stages. As such they form pure d.c. blockers, over the signal band they (should) have no further influence on the signal.
Bypass caps: Those have the same function as decoupling caps: To 'nullify' the effect of any resistance on signal, thus effectively shorting such for signal. This term usually denotes caps across emitter/cathode resistors and others; they are essentially also doing decoupling duty.
I take it the OP meant coupling caps. Not sure which function Shoog was referring to; bypass or coupling?
Painted,
In my experience the ESR of an electro cap needs to be quite poor before introducing effects in the audio band; sometimes a little overrated. Of course, some amplifiers, particularly SS, still have response well outside the audio band in order to perform well inside. In such cases ESR mignt well enter the picture. Some advise the use of several lower capacitance value caps in parallel rather than one huge one (also more convenient space-wise). But I am OT now.
Hi,
@Johan:
I think Shoog just replied to the wrong thread. There's another one by the same TS about the decoupling of the cathode resistor.
Ciao, 😉
@Johan:
I think Shoog just replied to the wrong thread. There's another one by the same TS about the decoupling of the cathode resistor.
Ciao, 😉
Hi,
Years ago I was looking for >1000uF caps and came across a factory sheet from Panasonic (if I recall right). It stated a considerable loss at 10KHz for some series of their bigger values which I thought would be caused by inductance. More so because they recommended "audio grade" types which I thought of being non-inductive. It would be a though job retreiving that info.
Hi,
Even the best ones have a great impact especially so at such large values. They have a highish charge/discharge delay which effect the sound.
Even the best filmcaps have an audible impact no matter what you do.
If I can avoid cathode bypass caps, I will.
Ciao, 😉
Even the best ones have a great impact especially so at such large values. They have a highish charge/discharge delay which effect the sound.
Even the best filmcaps have an audible impact no matter what you do.
If I can avoid cathode bypass caps, I will.
Ciao, 😉
- Status
- Not open for further replies.