Transmission Lines as Applied to Audio
March 12th, 2018, 11:33 am
It took having a DC-coupled device and re-reading an old article in Stereophile that got me thinking about transmission lines. The article was based on audio and I thought that transmission-line theory only applied to RF (or digital). So in the past I took what was published with a "grain of salt." Before re-reading the particular Stereophile article I had months ago changed the 6-ohm output impedance of my DAC to 250-ohms and the load at the preamp end from 100k-ohms to 18k-ohms utilizing an adapter fabricated with XLR connectors with much improved performance. I chose not to modify the input impedance of my pre-amp for the obvious reasons.
Saturday was the day that I decided to waste a set of high-quality XLRs and give it a try with 250-ohms for the load to match the 250-ohm source impedance previously installed. It appears that matching load impedance to the source impedance is not only valid in terms of RF theory but seems valid in audio terms as well by eliminating interconnect issues. Of course you can’t take advantage of this source/load matching mod with capacitive-coupled tube equipment because of severe bass roll-off.
Why 250-ohms? I would not recommend a value lower than 100-ohms, but I happen to have had a cache of 250-ohm precision bobbin resistors that are superior to anything Vishay has on offer especially in terms of series inductance.
I found decades ago that solid-state amplifiers, either pre-amps or the gain blocks in a DAC or phono stage (whether discrete or based on I.C. op-amps or unity gain buffers) do not like cable reactance. Even if the circuit is supposedly designed for it -- such as my Mark Levinson no. 36S DAC with its differential FET-based 6-ohm output impedance. The late Walter Jung demonstrated to me that buffer resistance is needed between the output of a device and the “outside” world. Anytime I installed a buffer there was always improved performance in all parameters that matter for sound reproduction.
Auditioning the results of this mod on Saturday with the matched impedance (DAC to pre-Amp -- 250-ohms source/250-ohms load) offered new benefits besides improved transparency, fine detail and sweetness (breath of life) over what I was getting before -- the sound got BIGGER and more enveloping. I expected a change – even hopefully an improvement – but the extent of the improvement was much greater than what I expected. WOW!
This mod relies on an output stage that can drive a 100 to 250-ohm load (it does require a certain amount of current) and it relies on an enough gain of the piece of kit next in line. This mod in essence is an aggressive voltage divider. With those required capabilities one is rewarded with much improved sound reproduction.
Saturday was the day that I decided to waste a set of high-quality XLRs and give it a try with 250-ohms for the load to match the 250-ohm source impedance previously installed. It appears that matching load impedance to the source impedance is not only valid in terms of RF theory but seems valid in audio terms as well by eliminating interconnect issues. Of course you can’t take advantage of this source/load matching mod with capacitive-coupled tube equipment because of severe bass roll-off.
Why 250-ohms? I would not recommend a value lower than 100-ohms, but I happen to have had a cache of 250-ohm precision bobbin resistors that are superior to anything Vishay has on offer especially in terms of series inductance.
I found decades ago that solid-state amplifiers, either pre-amps or the gain blocks in a DAC or phono stage (whether discrete or based on I.C. op-amps or unity gain buffers) do not like cable reactance. Even if the circuit is supposedly designed for it -- such as my Mark Levinson no. 36S DAC with its differential FET-based 6-ohm output impedance. The late Walter Jung demonstrated to me that buffer resistance is needed between the output of a device and the “outside” world. Anytime I installed a buffer there was always improved performance in all parameters that matter for sound reproduction.
Auditioning the results of this mod on Saturday with the matched impedance (DAC to pre-Amp -- 250-ohms source/250-ohms load) offered new benefits besides improved transparency, fine detail and sweetness (breath of life) over what I was getting before -- the sound got BIGGER and more enveloping. I expected a change – even hopefully an improvement – but the extent of the improvement was much greater than what I expected. WOW!
This mod relies on an output stage that can drive a 100 to 250-ohm load (it does require a certain amount of current) and it relies on an enough gain of the piece of kit next in line. This mod in essence is an aggressive voltage divider. With those required capabilities one is rewarded with much improved sound reproduction.
Re: Transmission Lines as Applied to Audio
March 12th, 2018, 12:38 pm
What you describe is classical pro audio practice. Maximum power transfer into a matched impedance load.
200 ohms is the European standard. 600 ohms is thee US standard, based on the impedance of parallel conductors on a telephone pole, "infinitely long," an actual transmission line. (Transmission lines in RF are considered to be many wavelengths long.)
Remember, early sound engineering came out of telephone engineering. These guys were network engineers. Even the design of horns was based on impedance matching from the microphone or phono needle to the mouth of the horn. POWER TRANSFER.
See this image: http://images-mediawiki-sites.thefullwi ... 554454.png from Harrison's patent and this discussion of mechanical modeling in sound reproduction: http://www.thefullwiki.org/Mechanical_filter --halfway down the page.
Harry Olson's later work on mechanical analogies in speakers was an extension of this approach.
Another aspect of this sort of constant impedance system engineering was the quest for low system noise figure. The idea was to have excess gain at each amplification stage, 500 ohm output impedance terminated in a resistive pad which introduced loss and presented a stable termination in both directions (T or H attenuator). Everything was locked in and referenced to 600 ohms (or whatever standard).
There is also a sort of buffering aspect to this...everything sees a non-reactive termination at input and output, so instead of making an output stage that doesn't care what impedance it sees or an input stage high impedance enough to not care about the source impedance, you tell it what to see. Any hum and noise picked up by wiring was attenuated by 10dB or whatever the pad loss happened to be.
There is plenty of good info on this sort of thinking in Tremaine's "Audio Cyclopedia," to name one old text.
Basically, the impedance scheme we know in hifi, low driving impedance into a much higher input impedance is a consumer approach, allowing for most things to be plugged into most other things without thinking about it much....unless you try to plug a plate loaded 12AX7 into a 10kohm input Z amp!
6 ohms is ridiculously low output Z but "more is always more" in hifi, right?
The higher system impedance we see in hifi do make for greater sensitivity to wire characteristics, but when this was all emerging, they didn't think there were any sneaky wire characteristics. RLC, and that's it. Bandwidth wasn't even that wide, so HF rolloff wasn't a big problem.
Beyond that, I curse these ancestors for standardizing on the damn RCA plug for unbalanced connections!
And there is also the question of Miller effects with a high input impedance and high gain triodes, gone with low impedance inputs.
The drawback of the old school pro approach is that power output stages are necessary (but that's good for sonics) and the entire system has to be designed around a constant impedance topology, so you can't mix and match with consumer gear.
Also it was expensive since input and output transformers were required.
Walt, in your case, if you have enough gain to throw away, you can design attenuation pads to give you any constant impedance termination you want, in either direction. Check T attenuator design. Audio Cyclopedia is again a great resource for this.
200 ohms is the European standard. 600 ohms is thee US standard, based on the impedance of parallel conductors on a telephone pole, "infinitely long," an actual transmission line. (Transmission lines in RF are considered to be many wavelengths long.)
Remember, early sound engineering came out of telephone engineering. These guys were network engineers. Even the design of horns was based on impedance matching from the microphone or phono needle to the mouth of the horn. POWER TRANSFER.
See this image: http://images-mediawiki-sites.thefullwi ... 554454.png from Harrison's patent and this discussion of mechanical modeling in sound reproduction: http://www.thefullwiki.org/Mechanical_filter --halfway down the page.
Harry Olson's later work on mechanical analogies in speakers was an extension of this approach.
Another aspect of this sort of constant impedance system engineering was the quest for low system noise figure. The idea was to have excess gain at each amplification stage, 500 ohm output impedance terminated in a resistive pad which introduced loss and presented a stable termination in both directions (T or H attenuator). Everything was locked in and referenced to 600 ohms (or whatever standard).
There is also a sort of buffering aspect to this...everything sees a non-reactive termination at input and output, so instead of making an output stage that doesn't care what impedance it sees or an input stage high impedance enough to not care about the source impedance, you tell it what to see. Any hum and noise picked up by wiring was attenuated by 10dB or whatever the pad loss happened to be.
There is plenty of good info on this sort of thinking in Tremaine's "Audio Cyclopedia," to name one old text.
Basically, the impedance scheme we know in hifi, low driving impedance into a much higher input impedance is a consumer approach, allowing for most things to be plugged into most other things without thinking about it much....unless you try to plug a plate loaded 12AX7 into a 10kohm input Z amp!
6 ohms is ridiculously low output Z but "more is always more" in hifi, right?
The higher system impedance we see in hifi do make for greater sensitivity to wire characteristics, but when this was all emerging, they didn't think there were any sneaky wire characteristics. RLC, and that's it. Bandwidth wasn't even that wide, so HF rolloff wasn't a big problem.
Beyond that, I curse these ancestors for standardizing on the damn RCA plug for unbalanced connections!
And there is also the question of Miller effects with a high input impedance and high gain triodes, gone with low impedance inputs.
The drawback of the old school pro approach is that power output stages are necessary (but that's good for sonics) and the entire system has to be designed around a constant impedance topology, so you can't mix and match with consumer gear.
Also it was expensive since input and output transformers were required.
Walt, in your case, if you have enough gain to throw away, you can design attenuation pads to give you any constant impedance termination you want, in either direction. Check T attenuator design. Audio Cyclopedia is again a great resource for this.
Re: Transmission Lines as Applied to Audio
March 12th, 2018, 1:08 pm
J-ROB wrote:Check T attenuator design. Audio Cyclopedia is again a great resource for this.
Exactly. Wire becomes pretty much a non-issue done this way. I've got some 600ohm t-pads I used as volume controls at the output of a line stage. And you can pry my Audio Cyclopedia from my cold, dead fingers 
Roscoe
Re: Transmission Lines as Applied to Audio
March 12th, 2018, 1:38 pm
There are lots of nice pre-built pads around on ebay and junkboxes, made by Daven, Cinema Engineering, Altec, Langevin, etc.
I had some old Daven -10dB pads that were made of card wound non inductive resistors. Really nice stuff.
They don't even use this approach for pro very much any more. A lot of it employs the undefined low Z into undefined higher Z approach now, although the impedances are shifted down from the old tube days.
Basically viewing everything as an op amp. High input Z, low output Z.
Hey, if anybody wants to play with non inductive wirewound precision resistors, my bud Wiggins down in Fredricksburg has a big stash and sells on ebay ID gotham_analog Shoot him a note. He might have what you need. I have a stash too and would gladly hand some out.
I had some old Daven -10dB pads that were made of card wound non inductive resistors. Really nice stuff.
They don't even use this approach for pro very much any more. A lot of it employs the undefined low Z into undefined higher Z approach now, although the impedances are shifted down from the old tube days.
Basically viewing everything as an op amp. High input Z, low output Z.
Hey, if anybody wants to play with non inductive wirewound precision resistors, my bud Wiggins down in Fredricksburg has a big stash and sells on ebay ID gotham_analog Shoot him a note. He might have what you need. I have a stash too and would gladly hand some out.
Re: Transmission Lines as Applied to Audio
March 12th, 2018, 2:50 pm
J-ROB wrote:There are lots of nice pre-built pads around on ebay and junkboxes, made by Daven, Cinema Engineering, Altec, Langevin, etc.
I had some old Daven -10dB pads that were made of card wound non inductive resistors. Really nice stuff.
They don't even use this approach for pro very much any more. A lot of it employs the undefined low Z into undefined higher Z approach now, although the impedances are shifted down from the old tube days.
Basically viewing everything as an op amp. High input Z, low output Z.
Hey, if anybody wants to play with non inductive wirewound precision resistors, my bud Wiggins down in Fredricksburg has a big stash and sells on ebay ID gotham_analog Shoot him a note. He might have what you need. I have a stash too and would gladly hand some out.
Are they just non-inductive WW resistors or bobbins? Bobbins are like two copper-wire coils back-to-back. Look at the attached images.
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Re: Transmission Lines as Applied to Audio
March 12th, 2018, 3:22 pm
You might also want to look at some of the new thick film power resistors out there. The TO220 style case makes mounting and sinking easy. I'm using some from Arcol in the crossover for the eggs.
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Re: Transmission Lines as Applied to Audio
March 12th, 2018, 3:37 pm
tomp wrote:You might also want to look at some of the new thick film power resistors out there. The TO220 style case makes mounting and sinking easy. I'm using some from Arcol in the crossover for the eggs.
Tom -- thick film or bulk film -- whatever --- they are not even close to being competitive with bobbins. The very first time I ever tried a bobbin I got religion. If I had access to the values I would need to completely do my equipment I would do it. Vishay bulk film resistors are, as far as I'm concerned, bobbin wannabes. Same with thick film. Believe me -- my second choice are Vishay bulk films. And I have a nice selection -- may Hamfests rest in peace. To say the bobbins step out of the way is a gross understatement. Every once in a while I get lucky on eBay and can nail some that I can use. They're the audio equivalent of cocaine.
Re: Transmission Lines as Applied to Audio
March 12th, 2018, 3:40 pm
Re: Transmission Lines as Applied to Audio
March 12th, 2018, 4:10 pm
brombo wrote:You might find the Mobius resistor interesting -
https://en.wikipedia.org/wiki/M%C3%B6bius_resistor
I'm interested. So where does one buy them without a Department of Defense contract?
Re: Transmission Lines as Applied to Audio
March 12th, 2018, 4:25 pm
You make them from flexible resistor cards.