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Speaker amps for headphones - Page 106

Quote:
I understand how to calculate resistance with combined parallel and series resistance but I don't understand how the Resistance In and Resistance Out in this schematic was calculated. Could someone give a brief explanation?

After playing around with my spreadsheet I know how Fulvio calculated his chart shown above. He used 0 ohms for amplifier impedance and an open circuit (infinite ohms) for headphone impedance.

I just want to verify the correct amp impedance to plug into the formula. If a tube amp has 4 and 8 ohm speaker terminals I would use 4 or 8 ohms for the speaker impedance in the formula correct?

Quote:
Originally Posted by robrob

I'm working on a spreadsheet that will calculate values for networks using 4, 3, 2 and 1 resistors.

Something wonderful is about to happen!

Quote:
Originally Posted by robrob

If a tube amp has 4 and 8 ohm speaker terminals I would use 4 or 8 ohms for the speaker impedance in the formula correct?

I so dearly wish I could help, but I'm not even going to venture a guess.

(Waiting for the gurus to return...)

Edited by zilch0md - 11/30/13 at 5:09am
Quote:
Their "attenuation" doesn't take into account the impedance of the headphone, so Attenuation is calculated wrong.  It would be close for anything 600 ohms and higher.

The "attenuation" they have calculated is just the voltage divider ratio:  Vin/Vout = (R2 + R3)/R3

What would be the correct equation to calculate attenuation incorporating the headphone impedance? (R2 + R3) || (R4 + Headphone Impedance) ?

Quote:
Originally Posted by robrob

After playing around with my spreadsheet I know how Fulvio calculated his chart shown above. He used 0 ohms for amplifier impedance and an open circuit (infinite ohms) for headphone impedance.

I just want to verify the correct amp impedance to plug into the formula. If a tube amp has 4 and 8 ohm speaker terminals I would use 4 or 8 ohms for the speaker impedance in the formula correct?

Not necessarily.  Depends on the actual output impedance of the amplifier.  If they are matching impedance for the two different loads, then yes, you are correct.  If they are using some other scheme, you'll either have to contact the amp manufacturer or measure it with an ohm meter.  Of course, that will only give you the answer at DC and may or may not be close enough across the audio spectrum.

Quote:
Originally Posted by robrob

What would be the correct equation to calculate attenuation incorporating the headphone impedance? (R2 + R3) || (R4 + Headphone Impedance) ?

DefineV1 as the voltage across V1, V3 as the voltage across R3 and VH as the voltage across the headphones (RH).

so, V3 = V1 * R3 || (R4 + RH) / (R2 + R3 || (R4 + RH))

Then

VH = V3 * RH / (R4 + RH)

The voltage divider ratio V1/VH would then be the voltage attenuation factor.

You can expand all the equations, simplify and remove the voltage references, if you prefer to stay in the impedance realm.

I'm thinking we should leave out the headphone impedance when calculating attenuation because it doesn't matter. When the headphones are directly connected to the amp that is the reference sound level with no added resistor attenuation. We then attenuate that sound level by adding resistors. Sound reasonable?

Wikipedia says this about power amplifier output impedance:

The real output impedance (Zsource) of a power amplifier is usually less than 0.1 Ω, but this is rarely specified. Instead it is "hidden" within the damping factor parameter

Do you guys agree with this? It would definitely simplify our calculations if I could just use 0.1 as a close approximation of amp output impedance.

potterma, one last question. How do you calculate attenuation (ignoring headphone impedance) in the 4 resistor network shown above--how do you account for R4?

*still scratching his head wondering what the benefit is of complicating things with a four resistor solution*

se
As I can follow the the topic here and have intention of using this topic for myself, so great info and thanks to all...I can add only this .

With respect to all here in this testing , where is the listening test,s . That is what this is really about. I think a better approach to this would be design , implement , listen , post.

Using this method we would understand the relationship to use better.

Just a thought , great reading...

Al D

What brand of resistors do you guys recommend? I've got my eyes on the Mills MRA-12 wirewound type. Any other suggestions? I'm curious to see if "upgrading" my series resistors will make any difference compared to the ones in my cheap Ebay adapter. I have no idea what kind they are, or if they are even wirewound. The Mills are only about \$5 each, so obviously I can just buy them and try them to see for myself, but I'm just wondering if there's anything else I should be looking at.

Nothing wrong with the Mills. They're of good quality and non-inductive. I used to know Jeff Mills back when their shop was located here in Sacramento. He and his partner Chris were good people.

se
Quote:
Originally Posted by manbear

What brand of resistors do you guys recommend? I've got my eyes on the Mills MRA-12 wirewound type. Any other suggestions? I'm curious to see if "upgrading" my series resistors will make any difference compared to the ones in my cheap Ebay adapter. I have no idea what kind they are, or if they are even wirewound. The Mills are only about \$5 each, so obviously I can just buy them and try them to see for myself, but I'm just wondering if there's anything else I should be looking at.

I came very close to settling on Mills wirewound resistors while researching brands yesterday.  After reading in a couple of different DIY threads discussing the "sound" of various resistors, that the Vishay Dale resistors are "silent," having already seen them ranked high in similar threads, I decided to go with Vishay Dale instead of Mills.  Truth be known, they're indistinguishable to my ears, but I'm not going to spend money to find out.

In fact, rather than follow the sensible advice to order an assortment of values that can be ganged, having more patience than a desire to end up with a pile of unused resistors, I've only ordered two 50-Ohm 10W 1% Vishay Dale wirewound resistors to start with:

http://www.amazon.com/gp/product/B005S3S94

For my quest, if this kills the hiss from my MG3 to LCD-2, I'll go to a lower value to see if I can still avoid hiss with yet a higher voltage output.  If not, I'll got to a higher value. One step at a time.  Slow but sure.

Mike

Quote:
Originally Posted by robrob

Wikipedia says this about power amplifier output impedance:

Do you guys agree with this? It would definitely simplify our calculations if I could just use 0.1 as a close approximation of amp output impedance.

After playing with the spreadsheet I have to ask, do we even care about amplifier output impedance? We're trying to match the amp's load impedance (typically 4 or 8 ohms) right?

Quote:
Originally Posted by robrob

I'm thinking we should leave out the headphone impedance when calculating attenuation because it doesn't matter. When the headphones are directly connected to the amp that is the reference sound level with no added resistor attenuation. We then attenuate that sound level by adding resistors. Sound reasonable?

Nope.  Gotta consider the headphones if you want to be close to calculating attenuation right....

Quote:
Originally Posted by robrob

Wikipedia says this about power amplifier output impedance:

The real output impedance (Zsource) of a power amplifier is usually less than 0.1 Ω, but this is rarely specified. Instead it is "hidden" within the damping factor parameter

Do you guys agree with this? It would definitely simplify our calculations if I could just use 0.1 as a close approximation of amp output impedance.

Sometimes true, sometimes not.  Bottom line, you've gotta measure it!  I have an amp with an output impedance of 80 ohms!

Quote:
Originally Posted by robrob

potterma, one last question. How do you calculate attenuation (ignoring headphone impedance) in the 4 resistor network shown above--how do you account for R4?

Ignoring headphone impedance, ignore R4....  No current flow through it, so no voltage drop across it, so no effect without the headphones attached!

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