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PostPosted: March 13th, 2018, 3:15 pm 
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That is an Ultrohm wirewound resistor. Vishay owns them now. These are fairly standard, high quality wirewound resistors.

I seriously doubt that they are copper wire. There are resistance alloys with a small amount of copper added which assists in termination to the leads but it is still basically NiCr based material. Maybe tha's what you are thinking about. No way 250 ohms of copper wire will fit in the volume of that resistor!

Most precision wirewounds are "bobbin" resistors. Power WWs including Ni are typically wound on ceramic formers but not a grooved bobbin like the Ayrton-Perry wound precision jobs.


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PostPosted: March 13th, 2018, 3:21 pm 
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J-ROB wrote:
That is an Ultrohm wirewound resistor. Vishay owns them now. These are fairly standard, high quality wirewound resistors.

I seriously doubt that they are copper wire. There are resistance alloys with a small amount of copper added which assists in termination to the leads but it is still basically NiCr based material. Maybe tha's what you are thinking about. No way 250 ohms of copper wire will fit in the volume of that resistor!

Most precision wirewounds are "bobbin" resistors. Power WWs including Ni are typically wound on ceramic formers but not a grooved bobbin like the Ayrton-Perry wound precision jobs.



I sacrificed one and cracked it opne to make sure it was a bobbin resistor. Two bobbins back-to-back comprised of enameled copper wire.

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PostPosted: March 13th, 2018, 3:28 pm 
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J-ROB wrote:
That is an Ultrohm wirewound resistor. Vishay owns them now. These are fairly standard, high quality wirewound resistors.

I seriously doubt that they are copper wire. There are resistance alloys with a small amount of copper added which assists in termination to the leads but it is still basically NiCr based material. Maybe tha's what you are thinking about. No way 250 ohms of copper wire will fit in the volume of that resistor!

Most precision wirewounds are "bobbin" resistors. Power WWs including Ni are typically wound on ceramic formers but not a grooved bobbin like the Ayrton-Perry wound precision jobs.


You'd only need about 250 feet of 40 gauge :mrgreen:


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PostPosted: March 13th, 2018, 4:37 pm 
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You'd only need about 250 feet of 40 gauge :mrgreen:[/quote]

Then you are only six pole pieces away from a Stratocaster pickup!

They make red poly insulating varnish, Walt. It is probably the most common color after clear/amberish. That is probably what you are seeing.

I remembered the name of the NiCrAlCu resistance alloy....Evanohm.

Maybe it looks copperish...never saw any that I am aware of.

I was looking for no Cr resistance wire at one time. Couldn't find any.*

*Oh wait, there is Manganin but it is hard to get high resistances with that wire. Manganin is mostly copper. I have some wooden spools from the 30s with double cotton covered Manganin. I used to use it to drop filament voltages, Thinner stuff was like 1 ohm/ft. This kind of wire was used for meter shunts and may still be.


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PostPosted: March 14th, 2018, 11:34 am 
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Going back to the original topic of the post about transmission lines, there is more to this than making an arbitrary choice of having the output impedance of one device be the same as the input impedance of the receiving device. The cable must also have the same characteristic impedance as well. The whole idea here is to avoid reflections when the signal propagates from the sending device to the receiving device. A reflection occurs when the cable characteristic impedance does not match either the sending device or the receiving device.

The cable characteristic impedance is determined by its structure, capacitance of the dielectric and inductance of the conductors. It is not affected by length, and has nothing to do with dc resistance. Popular RG cables include 50 ohms, 75 ohms, and a few other lessor-used ones. I think the highest common one is twin lead antenna wire at 300 ohms. This approaches the impedance of free space, which is about 377 ohms.

Cable impedance is often measured by launching a fast pulse into a cable and measuring a returning reflected pulse. The other end of the cable is terminated by a resistor, and when the resistor value matches the cable's characteristic impedance, the pulse is not reflected.

Of course these pulses are very fast to make this work, and the question of what this has to do with audio frequencies should come up. Here is where I am going to depart from established fact presented above and speculate. I think that cable reflections should not audibly affect no feedback amplifiers driving cables, and may not be an issue with the receiving device either. ( Now bulk cable capacitance will affect the sending device's apparent frequency response.) But when feedback is involved in the sending device, I could see where it may have a sonic affect. High-frequency content such as noise or other stuff outside the normal audio bandwidth is present and if this is reflected it could upset a feedback-based amplifier. This is why it is a good idea to put a resistor in series with the output, or some other network if the the losses can not be tolerated. Most SS amps have a damped inductor on the output to keep them stable. Ever explored the sonic changes with input grid-damping resistors? Could have something to do with reflections, but I don't know.

Many years ago I was curious about any advantages that might be had by impedance matching between the power amp, speaker cable, and the speaker. I had resistive 8-ohm speakers at the time, and there was this speaker wire marketed by Polk ( made by JVC?) that had a very low characteristic impedance--somewhere quite near eight ohms. It has very low inductance and high capacitance. It blew up many SS amps of the day, and it was taken off the market. But my amps worked very well with it, and at the time I was using very low feedback and could easily make the output impedance eight ohms.

Goertz makes low-impedance speaker cables; I think they have models under eight ohms as well now. I have not tried them in my system, but some others whose opinions I respect have and they tell me that they work well with my amps.

David


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PostPosted: March 14th, 2018, 12:27 pm 
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dberning wrote:
Going back to the original topic of the post about transmission lines, there is more to this than making an arbitrary choice of having the output impedance of one device be the same as the input impedance of the receiving device. The cable must also have the same characteristic impedance as well. The whole idea here is to avoid reflections when the signal propagates from the sending device to the receiving device. A reflection occurs when the cable characteristic impedance does not match either the sending device or the receiving device.

The cable characteristic impedance is determined by its structure, capacitance of the dielectric and inductance of the conductors. It is not affected by length, and has nothing to do with dc resistance. Popular RG cables include 50 ohms, 75 ohms, and a few other lessor-used ones. I think the highest common one is twin lead antenna wire at 300 ohms. This approaches the impedance of free space, which is about 377 ohms.

Cable impedance is often measured by launching a fast pulse into a cable and measuring a returning reflected pulse. The other end of the cable is terminated by a resistor, and when the resistor value matches the cable's characteristic impedance, the pulse is not reflected.

Of course these pulses are very fast to make this work, and the question of what this has to do with audio frequencies should come up. Here is where I am going to depart from established fact presented above and speculate. I think that cable reflections should not audibly affect no feedback amplifiers driving cables, and may not be an issue with the receiving device either. ( Now bulk cable capacitance will affect the sending device's apparent frequency response.) But when feedback is involved in the sending device, I could see where it may have a sonic affect. High-frequency content such as noise or other stuff outside the normal audio bandwidth is present and if this is reflected it could upset a feedback-based amplifier. This is why it is a good idea to put a resistor in series with the output, or some other network if the the losses can not be tolerated. Most SS amps have a damped inductor on the output to keep them stable. Ever explored the sonic changes with input grid-damping resistors? Could have something to do with reflections, but I don't know.

Many years ago I was curious about any advantages that might be had by impedance matching between the power amp, speaker cable, and the speaker. I had resistive 8-ohm speakers at the time, and there was this speaker wire marketed by Polk ( made by JVC?) that had a very low characteristic impedance--somewhere quite near eight ohms. It has very low inductance and high capacitance. It blew up many SS amps of the day, and it was taken off the market. But my amps worked very well with it, and at the time I was using very low feedback and could easily make the output impedance eight ohms.

Goertz makes low-impedance speaker cables; I think they have models under eight ohms as well now. I have not tried them in my system, but some others whose opinions I respect have and they tell me that they work well with my amps.

David



Of course David is correct -- however, interconnect manufacturers (audiophile) don't publish the characteristic impedance of their products. I got lucky. I had the desired Bobbins and hooked my DAC to my pre-amp with a .5-meter pair of Audioquest W.E.L. Signature cables that live up to the hipe.

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PostPosted: March 14th, 2018, 1:10 pm 
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Further to David's comments, a hidden advantage of the old school impedance-matched pro audio method is that not much RF is going to get through the lousy input and output transformers, even if common mode cancellation due to balanced wiring might fail beyond audio frequency. Repeat coils were often inserted into lines for additional isolation. There will be some random coupling due to stray capacitance and whatnot, but there always is.

The goal was rejection of hum and LF common mode interference more than RF, although twisted pair should offer some immunity to RFI.

As far as I am aware, no audio cables of the era had controlled impedance, although you can buy twisted shield pair cable today that is standardized (for ethernet and whatnot) 100 ohm being the common impedance for twisted pair. But these work at HF frequencies.

This notion of RF in high feedback solid state amps was a hot topic in the 80s, but I don't recall seeing any earlier examinations of the problem or studies involving tube amps.

Reflections in speaker cable seems a big problem if you want to introduce it as a variable. First, where is an 8 ohm speaker 8 ohms? Secondly, a transmission line model assumes that voltage and phase will vary at different points of the cable, it is an impedance transformer. Thirdly, dedicated engineering is required to come up with an amp which will have an output Z matching speaker impedances....even zero feedback output stages will only get you up to a couple ohms.

Re #2: Who needs a speaker cable that you can't cut to length? Don't we have enough problems already? :twisted:

Today's experiments with current amplifiers having a high output impedance, equal to or even higher than the speaker impedance, head in the direction of impedance matching. This might be a good approach but it throws away the high damping factor "load independent amp" scheme that has been so dear to audio engineering since the 1950s.

I got to sit through two experiments where amps were switched from voltage mode to current mode via variable current feedback schemes, one with jc and Bae at Silbatone and another by Menno van der Veen at ETF. In both cases, I greatly preferred the current drive scheme, way more psychedelic and organic, but at ETF, Jan Didden, editor of Linear Audio greatly preferred the tighter high damping factor voltage scheme.

Damn Dutch Protestants! I married one and I still can't figure them out!


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PostPosted: March 14th, 2018, 2:26 pm 
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J-ROB wrote:
Further to David's comments, a hidden advantage of the old school impedance-matched pro audio method is that not much RF is going to get through the lousy input and output transformers, even if common mode cancellation due to balanced wiring might fail beyond audio frequency. Repeat coils were often inserted into lines for additional isolation. There will be some random coupling due to stray capacitance and whatnot, but there always is.

The goal was rejection of hum and LF common mode interference more than RF, although twisted pair should offer some immunity to RFI.

As far as I am aware, no audio cables of the era had controlled impedance, although you can buy twisted shield pair cable today that is standardized (for ethernet and whatnot) 100 ohm being the common impedance for twisted pair. But these work at HF frequencies.

Belden 1800F is my go to for interconnects.

This notion of RF in high feedback solid state amps was a hot topic in the 80s, but I don't recall seeing any earlier examinations of the problem or studies involving tube amps.

Reflections in speaker cable seems a big problem if you want to introduce it as a variable. First, where is an 8 ohm speaker 8 ohms? Secondly, a transmission line model assumes that voltage and phase will vary at different points of the cable, it is an impedance transformer. Thirdly, dedicated engineering is required to come up with an amp which will have an output Z matching speaker impedances....even zero feedback output stages will only get you up to a couple ohms.

Re #2: Who needs a speaker cable that you can't cut to length? Don't we have enough problems already? :twisted:

As David mentioned, the characteristic impedance has nothing to do with distance. The cable looks like an infinite number of series inductances paralleled by an infinite number of capacitances. No many how many you cut off the combination of each inductor and capacitor still yields the same impedance. What does change with length is attenuation of the signal.

Today's experiments with current amplifiers having a high output impedance, equal to or even higher than the speaker impedance, head in the direction of impedance matching. This might be a good approach but it throws away the high damping factor "load independent amp" scheme that has been so dear to audio engineering since the 1950s.

I got to sit through two experiments where amps were switched from voltage mode to current mode via variable current feedback schemes, one with jc and Bae at Silbatone and another by Menno van der Veen at ETF. In both cases, I greatly preferred the current drive scheme, way more psychedelic and organic, but at ETF, Jan Didden, editor of Linear Audio greatly preferred the tighter high damping factor voltage scheme.

Damn Dutch Protestants! I married one and I still can't figure them out!


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PostPosted: March 14th, 2018, 2:31 pm 
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dberning wrote:
The cable characteristic impedance is determined by its structure, capacitance of the dielectric and inductance of the conductors. It is not affected by length, and has nothing to do with dc resistance. Popular RG cables include 50 ohms, 75 ohms, and a few other lessor-used ones. I think the highest common one is twin lead antenna wire at 300 ohms. This approaches the impedance of free space, which is about 377 ohms.


Wider spaced (a bit over an inch) twin-lead antenna wire with a 450 ohm characteristic impedance is widely available & commonly used by ham radio operators. Spacing the conductors at around 4" gives a 600 ohm characteristic impedance, but, while still used by a lot off hams, you generally have to build that on your own...

Roscoe


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PostPosted: March 14th, 2018, 3:18 pm 
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Of course David is correct -- however, interconnect manufacturers (audiophile) don't publish the characteristic impedance of their products. I got lucky. I had the desired Bobbins and hooked my DAC to my pre-amp with a .5-meter pair of Audioquest W.E.L. Signature cables that live up to the hipe.[/quote]


Correction -- that should have read .75-meter pair.

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