funny thing from another forum

[pelly3s]

I was on another forum posting and two people insisted on arguing with me that an amplifiers output signal is DC. I found it very humorous. The guy also felt the need to question my intelligence when I said that DC is death to a speaker. I just thought I would share that with some people who actually know what they are talking about :applaud:

[clmrt]

Isn't there always some residual low-level DC? Maybe only if it's defective, I guess.


But yeah, DC=death. What happens, anyway - do the cones shoot across the room?

[pelly3s]

there should always be a tiny amount, no more than lets say around 100mV, I actually had a crown MA-3600 in my shop that was shooting out 25v DC and i had to recone the woofers that it destroyed, pushed so far out that the surround ripped and then it froze in that position.

[spkrman57]

DC to a voice coil sees a lower impedence than AC would which results in more current. Higher levels of DC result in flames and smoke on occasion.

Ron

[richard c.]

That is funny, BUT it is hard to prove to someone who doesn't have a clue what he is talking about. These idiots could obviously use a "basic DC theory" course, and then a "basic AC theory" course, etc. Some of these guys must have been born with an instinct for truth - no need for the time it takes to actually learn how things work. The clueless just love to argue.

Richard C.

[Hamilton]

I actually had a crown MA-3600 in my shop that was shooting out 25v DC and i had to recone the woofers that it destroyed, pushed so far out that the surround ripped and then it froze in that position.
Yep, I had an old Phase Linear 400 do the same thing, instant speaker death.

Back to the topic, in a tube amp (don't know about ss) it is impossible to have DC on the secondary of the output transformer because without frequency...there is no signal.

[Zilch]

Tell them to put a 9V battery on their woofer and see what happens.

It won't destroy it, but it'll sure move it to one end or the other of it's excursion and keep it there for the duration.... :p

[LE15-Thumper]

I am wondering how you could get any midrange or highs if the output from your amp is DC ? Unless they have found a new kind of cap that passes DC ??

Yes , the output of your amp is MODULATED DC, but there is no actual steady dc level there. Well, ok, there is a small residual voltage known as DC offset, but this should be as low as possible. Most of the "newer" (25 years and newer) solid state stuff runs with an offset of 20mV or less.

In the old days 250mV/ - 1/4 of a volt was acceptable. Now with much better semiconductors and cct designs, 100mv is a "hurrendous" amount of DC and usually signals a fault.

The ouput of your amp is true AC because it goes through a "Zero crossing point" and then changes direction (Polarity). If this isn't AC, I don't know what is.

I have been doing this for a living for 25 years now. You can cut and paste this to the other forum if you wish.

[Chas]

Any direct coupled solid state power amp can pass DC (0Hz) providing the protection circuitry (or servo, if so equipped) is disabled. No different than a simple op-amp, really. Just has the capability to deliver more power into a load.

I agree with Thumper, less than +/-10 mV is more the norm in my experience.

[Ian Mackenzie]

Chas is correct,

If an amp is indeed DC coupled it will amplify DC and it must be coupled at the input with a capacitor of the pre amp has no output capacitor.

Some DC coupled amps have a servo circuit to maintain 0 dc at the ouput to negate the use of a capacitor at the input and other places in the design however this will not stop a VFL or DC from pulse from destroying a driver.

Ian

[richard c.]

I looked up the term "modulated DC" and it seems to pertain to pulse width modulation and/or switch mode power supplies. Probably would pertain more to Class D power amps. I would say if you look at an audio waveform (analog), on a O-scope, you will see AC. For all practical purposes, one side of the waveform is a mirror image of the other. IF you look at the output of a poorly designed Class D amp, you might also be able to see remnants of the sampling frequency!

My career in the repair business started in the middle '60's. Peace!

Richard C.

[JohnK]

Apparently, lots of folks aren't hip to magnets, coils of wire and alternating current. Even though it is what provides them with electricity as well as sound from a loudspeaker.

Round wheels are probably foreign to them too.

[LE15-Thumper]

Any direct coupled solid state power amp can pass DC (0Hz) providing the protection circuitry (or servo, if so equipped) is disabled. No different than a simple op-amp, really. Just has the capability to deliver more power into a load.

I agree with Thumper, less than +/-10 mV is more the norm in my experience.

What I meant was the caps in your speakers' crossover network

[pelly3s]

the guy realized he was wrong and apologized for his mistake. I just found it humorous, that he was so confident about it he even took it upon himself to question my intellegence

[Oldmics]

Shoulda told him that its only tube amps that are DC driven.:blah: :bs:


Oldmics