Help me understand attenuators - engineering perspective

[prspastor]

Hi all,
I understand the basic premise of attenuators... yet my question is a bit deeper...

I want to understand what is happening in the output transformer with a non-reactive load like an Airbrake when playing. It is my understanding that a speaker changes impedance as music is played and that a rating, such as 8 ohms, is somewhat of an average.

This might be a wrong way to ask the question, but what kind of impedance is the OT "seeing" with an Airbrake in series with the speaker?

I've also read much discussion about how attenuators do not harm an OT. Is this true from an electrical engineering perspective?

Thanks much, I appreciate the help,
John

[diagrammatiks]

the airbrake presents the same impedance.

It's not reactive like a speaker at all.

http://www.aikenamps.com/spkrload.html

http://www.aikenamps.com/DummyLoads.htm

[prspastor]

Thanks for the reply,
Am I understanding then that the OT is seeing the reactive impedance of the speaker through the Airbrake then?

The attenuator is like a "window" that only absorbs power yet maintains a normal operating environment for the OT?

John

[Super_Reverb]

It is my understanding that a speaker changes impedance as music is played and that a rating, such as 8 ohms, is somewhat of an average.

As the referenced Aiken note indicates, speaker impedance varies as a function of frequency. The speaker is an electromechanical beast with series inductance, due to (voice) coil of wire and series resistance, due to copper voice coil copper resistance.

This might be a wrong way to ask the question, but what kind of impedance is the OT "seeing" with an Airbrake in series with the speaker?

The output tube(s) in an amp drive current from the plate(s) through the primary of the OT: the impedance or load that they "feel" is based on the impedance of the load through the turns ratio of the OT.

A simple attenuator could be an 8 Ohm resistor in series with your 8 Ohm speaker, with a 16 Ohm resistor in parallel with the 16 Ohm series comination. Half the current is divirted to the 16 Ohm parallel resistor and half the voltage is dropped across the series 8R. The circuit driving this attenuator still "sees" a similar load (with less of the total impedance coming from inductance). One issue with a resistive attenuator as described is it does not react to differing frequencies identically to the speaker, so sonic differences are possible. Remember, part of the interaction of the amp and speakers is the speaker impedance characteristics being reflected back to the output tube(s) - that is the output tube(s) 'see' a frequency dependent load through the OT.

I've also read much discussion about how attenuators do not harm an OT. Is this true from an electrical engineering perspective?

The OT should never be run with open circuit secondary. The OT won't be damaged driving a resistive load, but it is not 'real world' If the primary is driven with current into an open secondary, secondary voltage can rise to a level high enough to damage the OT. Some people insert a 220 Ohm resistor across the 16 Ohm secondary winding. This protects the OT against being run open circuit and does not steal much power from the speaker.

cheers,

rob

[prspastor]

Super_Reverb,
Thank you, this makes good sense to me now. Thank you for taking the time to answer.

I like the idea of the 220 ohm resistor across the 16 ohm tap. That is a great protective measure.

When I built my Express clone, I only utilized the 8 ohm tap. I suppose I could use the same trick on my 8 ohm tap?

John

[212Mavguy]

My $.02 on this one is different.

I really like the previous post about strategically placed resistors... simple, economical, and just efficiently brilliant IMHO.

I have a Weber 100W MASS attenuator/balanced line out with vol/tmb tonestack for that line out to the board, attenuation is footswitchable in /out. a speaker motor is inside it to react like a speaker against the power tubes through the OT. Also has headphone out. and two out jacks for the speakers it outputs to.

It has enough bells and whistles to do it's job well to compensate for percieved tone changes with reduced volume coming through the guitar amp speaker...if the operator know how to use it, that is. CAVEAT.

I'm feeling like I wouldn't have anything different. The post OT tones coming out of line out side the unit are more complex sounding than what is too fragile to survive within the crystal lattice of the cone paper.



if you take the amp speaker out of it and put the out through a more HIFI type of arrangement the sound is gorgeous, but your ear might not know that at first and need to grow into liking it. And that's where the tone controls on that stack come in. The sound man isn't the only person controlling your tone to the front of the house anymore.