[sunneebear]

I have a NuForce Icon that I tried with the 6 a long time ago.  The sound was mediocre at best.  The last few post got me curious because the Icon sits unused on my desktop everyday.   I opened my adapter box, put in a jumper wire to bypass the 25 ohm resistors and plugged it into the Icon.  Only the 10 ohm is in play.  The sound is no longer barely mediocre but quite good.  Relaxed full body sound, tube like almost.  I can live with this as a amp for the 6.  It is nowhere near the big 100w amps but it is quite pleasing for non critical listening.
 
This proves that getting a speaker amp to see the 6 as 8 ohm load is important to some amps and can greatly affect the sound.

 

[grokit]

^ That post of mine above completely ditzed out, I can't even put any text in it

 
Anyhow:

 
What is the difference, electrically, between the 10 dB inline attenuators that I have between my amp and source, the 10 ohm resistors across speaker binding posts in parallel, and the impedance box? I get that when the impedance box is set to 10 ohms in parallel that it's the same as the binding posts technique, but what about when it is set to 25 ohms, in series?
 
Edit: Now that I'm looking at this all laid out, I can see that logically, the in-line attenuators aren't likely to do anything regarding the output impedance of the speaker amp, all it's doing is adjusting down the voltage of the source. Mainly what I am interested in is the difference between the two settings of the impedance box.

 

[wuwhere]

I tried those Rothwells attenuators in my amp to attenuate the signal from my Oppo 95. They added a hardness that I didn't like so I returned them.
 
I don't understand the 25 ohms in series either. Now if you parallel 25 ohms with 50 ohms, its 16.6 ohms. Some speaker tube amps like my ARC has a speaker terminal for 16 ohms.

 

[sunneebear]

I think the 25 ohm in series lowers the volume to a more suitable level.  That is why I had to bypass the 25 ohm to use with the small 15w Icon amp. 

 

[wuwhere]

Adding 25 ohms in series with the 6's 50 ohms makes it a 75 ohms load to your amp.

 

[wuwhere]

BTW the Rothwell attenuates the signal by 10dB which is about 0.3 volts.

 

[grokit]

 
Quote:

I tried those Rothwells attenuators in my amp to attenuate the signal from my Oppo 95. They added a hardness that I didn't like so I returned them.

Interesting. I got the Rothwells to offset the added gain from the tube stage, which they did and then some. I don't think they really changed the dynamics, I'll have to do some more a/b though. They definitely didn't add anything.
 
I just ordered a couple of wire wound resistors from an eBay seller and my speaker amp is only 45 wpc, so I will probably end up removing the Rothwells from the chain as well:
 

 
"Ohmite 10 ohm 12 W Watt Ceramic Wire Wound Resistor"
 
Hopefully these are adequate.

 

[wuwhere]

Are they non-inductance?

 

[grokit]

Quote:

Are they non-inductance?

 
Sigh... I'm having a hard time researching that as the Ohmite pages aren't loading.
 
Digikey page:
http://search.digikey.com/scripts/dksearch/dksus.dll?PName?Name=B12J10R-ND
 
I think I may have screwed up

 
wiki:
"Because wirewound resistors are coils they have more undesirable inductance than other types of resistor, although winding the wire in sections with alternately reversed direction can minimize inductance. Other techniques employ bifilar winding, or a flat thin former (to reduce cross-section area of the coil). For most demanding circuits resistors with Ayrton-Perry winding are used.
Applications of wirewound resistors are similar to those of composition resistors with the exception of the high frequency. The high frequency of wirewound resistors is substantially worse than that of a composition resistor."
 
I thought that I was just looking for a wire wound resistor, should i have gone for a composition type instead? Or just a non-inductive wire wound? There sure are a lot of resistors out there...
 
Edit: The same seller has these, which look very similar but are only 4 ohms: "Ohmite 4 ohm 20 W Watt Ceramic WW Resistor Stereo Load"
He does describe them as being "[size=x-small]Perfect for stereo/receiver output load testing" but I think that is just because it is 4 ohms [/size]

 

[hawkhead]

Mills MRA12 series are non inductive

 

[grokit]

Thanks hawkhead, I really appreciate it. I found a couple of sources for the Mills, first I will contact that eBay seller to see if he knows about the inductance of the ones that I bought. He won't be able to substitute different ones though, he doesn't specialize in resistors and those were the only 10 ohm ones he had.
 
If the ones I bought have inductance, will the higher frequencies become distorted? More importantly, if I give them a try and they are inductive, could they cause any damage?

 

[wuwhere]

@grokit:

Mills 10 Ohm 12W Non-Inductive Resistor

"These non-inductive, wire wound resistors were designed for high-end audio applications and are perfect for use in crossover networks. Extreme measures were taken to ensure sonic purity. They utilize an Alumina ceramic core, a nickel chromium element, silver plated copper end caps and tinned copper leads. The all welded construction greatly enhances frequency response. These 1% precision resistors, offer unsurpassed performance over conventional wire wound resistors. All are rated at 12W with 1% tolerance. Resistors body measures 1-1/8" in length by 5/16" diameter. Each lead measures 1-1/2" long and .040" thick."
 
That's why I use them. Their expensive though, $4.25 each + shipping from Parts Express.
If you want to save, you can buy Dayton non-inductive resistors instead.

 

[wuwhere]

Quote:

 
 
If the ones I bought have inductance, will the higher frequencies become distorted? More importantly, if I give them a try and they are inductive, could they cause any damage?

 
No. Here's the answer to your question:
 
Because the wire is wound into a coil, it acts like an inductor causing them to have inductance as well as resistance and this affects the way the resistor behaves in AC circuits by producing a phase shift at high frequencies especially in the larger size resistors. The length of the actual resistance path in the resistor and the leads contributes inductance in series with the "apparent" DC resistance resulting in an overall impedance path Z. impedance (Z) is the combined effect of resistance (R) and inductance (X), measured in ohms and for a series AC circuit is given as, Z² = R² + X².
When used in AC circuits this inductance value changes with frequency (inductive reactance, XL = 2πƒL) and therefore, the overall value of the resistor changes. Inductive reactance increases with frequency but is zero at DC (zero frequency). Then, wirewound resistors must not be designed into AC or amplifier type circuits where the frequency across the resistor changes. However, special non-inductive wirewound resistors are also available.
 
That's why you don't want to use an inducting resistor.

 

[grokit]

Thanks for the replies wuwhere. The seller got back to me with: "[size=10pt]These will have a small inductance being wire-wound, but it should be negligible." Who knows what winding techniques were employed underneath that ceramic, I'll keep trying the manufacturer's website.[/size] If they're not going to cause any damage it could be an interesting comparison and yet another learning experience.
 
These resistors weren't cheap either, around the price of the Mills. Hopefully the high price is indicative of some inductance reducing strategy. If there is any inductance, I wonder how the sound will change. It seems that any change would be in the upper frequencies, and that the impedance might not be adjusted exactly how I want it to be?
 
Edit: I think I figured it out, looking at the data sheet for the Ohmite B12J10R that I bought. "Non-Inductive versions available. Insert “N” before tolerance code. Example - B5NJ10R". So the Ohmite I should have gotten is the B12NJ10R. Except that part number doesn't live on the internet lol.
 
Is 12 watts the ideal wattage for this application?
 
Link to the datasheet

 

[wuwhere]

I would believe that it is very low to have a perceptible effect. The big wirewound resistors are the bad ones.
 
Those look nicer than the Mills.
 
The Rothwell, I thought, also softened the overall liveness of the music besides the hardness that I heard.
I gave them 2 to 3 hours before I took them off.