Impedance matching / bridging between tube gear and modern audio interfaces

[Cqwet Dbdfte]

The voltage for those speakers was hardly a problem in Europe, as these were built-in, and TV sets did not even have transformers, unlike the US, and running off 220-240V AC mains.

Those Philips MFB speakers are now real collector items. They were not cheap at the time, and sounded OK, but the MFB system only affected the 12" woofer, the rest was mediocre. They were not a huge success at the time, the Philips brand was sullied in audio as the brand also appeared on toasters, vacuum cleaners and light bulbs
Genius marketing folks. Their consumer audio stuff was generally cheap and cheesy plastic galore. They made very good TV sets in some of their factories.

 

[RoadrunnerOZ]

Their earlier radiograms had solid timber cabinets veneered with timbers like birds-eye maple or walnut, 2 x 12” full range speakers and sounded really good. A lot of Philips Hi-Fi drivers from the ‘70s were used in 3rd party manufactured systems - butyl surround 12” heavy paper cone bass, 8” butyl surround low mid, 4” convoluted surround upper mid and 1” dome tweeter.
I was building guitar amps in the early ‘70s virtually straight out of the Philips tube handbook application notes with tone circuits borrowed from other brand amps.

Those Philips MFB speakers are now real collector items. They were not cheap at the time, and sounded OK, but the MFB system only affected the 12" woofer, the rest was mediocre.

As I mentioned in post #89 Pink Floyd mixed 3 albums on those MFB’s but for home Hi-Fi use they weren’t in my mind the greatest.

 

[Speedskater]

Yes they do. It doesn't matter in practice, though.

Characteristic impedance is unrelated to frequency.
Telephone cables are (were) 600 ohms at vocal frequencies.

That is a common misunderstanding.
Jim Brown retired AES interference expert wrote:

Transmission Lines at Audio Frequencies, and a Bit of History
http://www.audiosystemsgroup.com/TransLines-LowFreq.pdf

 

[abbey road d enfer]

That is a common misunderstanding.
Jim Brown retired AES interference expert wrote:

Transmission Lines at Audio Frequencies, and a Bit of History
http://www.audiosystemsgroup.com/TransLines-LowFreq.pdf

There is a big difference between "the impedance of a line at audio frequencies is not the same as at high frequencies" and "Interconnect cables don't have a 'characteristic impedance'".
IINM the impedance of 110 ohms for AES3 has been chosen because it was the average of most audio cables (or maybe the other way around).

 

[JohnRoberts]

There is a big difference between "the impedance of a line at audio frequencies is not the same as at high frequencies" and "Interconnect cables don't have a 'characteristic impedance'".
IINM the impedance of 110 ohms for AES3 has been chosen because it was the average of most audio cables (or maybe the other way around).

Back in the day I standardized on using 51 ohm build out resistors for all my line level audio outputs.*****

Of course these lines were not long enough to exhibit transmission line effects but the resistor value was arbitrary so why not make it something theoretical however minuscule the possible benefit.

JR

[edit **** not all... while at Peavey many line level SKUs used FET output mutes so those SKUs had much higher output impedance to help mute kill.... /edit]

 

[moamps]

IINM the impedance of 110 ohms for AES3 has been chosen because it was the average of most audio cables (or maybe the other way around).

Based on this statement, it could be concluded that any audio cables can be used for AES3/DMX connections. From my experience, I can say that an audio cable can be used for this in an emergency, and the length must be short, under 10m. For longer lengths and correct signal transmission, designed cables with a characteristic impedance of 110 ohms should be used. I personally have not tried all audio cables in that position, but I know that a standard microphone Mogami cable has a characteristic impedance of about 50 ohms at the frequencies important for AES3 transmission. And yes, the characteristic impedance of the cable depends on the frequency.

 

[moamps]

Only power amps and long line drivers need to be impedance-matched to the load, because you want to make sure that maximum power is transferred to the load, which means maximum voltage AND maximum current.

With input audio devices, however, including outboard line-level gear, you are transferring voltage, not power (look up Ohm's law, voltage dividers). Here, the rule is that the input impedance of the next unit in chain needs to be considerably higher than the generating unit. For example our preamps (Sonic Farm Audio) have a mic input impedance switchable between 1, 2.4 and 10kΩ. There is no perceived level change when switching between them, only with the dynamic mics, whose impedance increases with frequency, there will be a little bit of a high end rolloff at 1k and 2.4k settings, which is the purpose of that switch.

Another example is an audio interface, any brand. As a rule of thumb, the line inputs on those are always solid state, no transformers, and most of them have an input impedance between 5 and 20kΩ. Engineers would go nuts if they had to impedance-match all those line-level units.

Tube stages do have a high output impedance, but the final output of those unit always has a low one, typically below 600Ω. This is achieved either by a step-down transformer or a solid-state buffer. I prefer the second solution, it's more elegant, preserves the tube sound and requires less gain at the tube stage.

One of the most important consideration of mic preamps is noise performance, which has its own impedance matching rules, but this is an issue that designers need to take care of, not the user. Typically, tube and FET stages have a much higher input impedance than transistors, and so mic input transformers in those preamps have a much higher transfer ratio (1:10 or even 1:20). Mic input transformers provide a totally noiseless gain, but they multiply the source (in this case mic) impedance by the square of the transfer ratio, so 100 to 400 times. The "source" impedance seen by the tube/FET will thus be (for a 200Ω mic) 20k or even 80k. Because you can easily achieve a tube/FET stage input impedance of 2.2MΩ or even higher, the signal transfer to the tube will still be optimum. Just don't overdo the transfer ratio, because the input capacitance of the tube/FET stage will start to roll off the high end. There you go, now you can even get into the preamp design.

Dobro došli na ovaj uglavnom zanimljiv i poučan forum.

 

[abbey road d enfer]

Based on this statement, it could be concluded that any audio cables can be used for AES3/DMX connections.

That's exactly what the AES committee suggested, within limits, since audio cables are aplenty in most facilities.
If it was not their idea, why would they have tested and measured dozens of audio cables?
Of course, there are large variations according to brands and types, which make some cables less suitable than others.
It is clear that for long lines, where problems due to mismatch and standing waves occur, the use of a certified 110 ohm cable is more stringent.
The period AES documents were clear about this.

 

[abbey road d enfer]

And yes, the characteristic impedance of the cable depends on the frequency.

Characteristic impedance is not the cable's impedance (which of course varies with frequency). It's a single figure that represents a limit for F->infinity.
Characteristic impedance is also known as "surge" impedance, or "step" impedance, because it's the impedance seen by a source when it connects to the cable. It's not a steady-state condition.
Heaviside demonstrated that sourcing and loading the cable with the same impedance resulted in minimum reflections.

 

[5v333]

Interesting

 

[moamps]

Characteristic impedance is not the cable's impedance (which of course varies with frequency). ..

Um, can you write the formula for the characteristic impedance (independent of frequency) and the formula for the impedance of the cable (dependent on frequency) so that it is clearer to me exactly what you mean, and that I can see the difference?

 

[ccaudle]

can you write the formula for the characteristic impedance (independent of frequency) and the formula for the impedance of the cable (dependent on frequency)

See the link in post #103, what you requested is exactly how the first page begins.

 

[moamps]

See the link in post #103, what you requested is exactly how the first page begins.

What you quote does not have the answer I am looking for, and I am very familiar with what is written in that article. Abbey said there is a difference between the characteristic impedance of a cable and the impedance of a cable. I want to find out what he thinks the difference is.

 

[JohnRoberts]

What you quote does not have the answer I am looking for, and I am very familiar with what is written in that article. Abbey said there is a difference between the characteristic impedance of a cable and the impedance of a cable. I want to find out what he thinks the difference is.

I can't read Abbey's mind but I first wrote about standing waves inside electronic signal lines in my 1980 "audio mythology column". As with much electronic analysis, visualizing such phenomenon assumes continuous sine waves in and out. Reality often involves random waveforms including step transients.

For audio I can't get too concerned about characteristic impedance, but this matters for video and HF signal interfaces.

JR

 

[moamps]

I can't read Abbey's mind but I first wrote about standing waves inside electronic signal lines in my 1980 "audio mythology column". As with much electronic analysis, visualizing such phenomenon assumes continuous sine waves in and out. Reality often involves random waveforms including step transients.

For audio I can't get too concerned about characteristic impedance, but this matters for video and HF signal interfaces.

JR

Sorry JR, but I honestly don't care what you did and wrote in the eighties, just like you don't care that I climbed 50-meter transmission poles in the eighties and adjusted RF cables for lamda/4 of the length.
Also, at the same time, I did a lot of work on how to adapt 600 ohm telephone lines to telephone hybrids in order to obtain the best possible isolation of the receiving and transmitting side. The fact that you weren't worried about the characteristic impedance of the cable in the audio spectrum doesn't mean that no one is.

 

[JohnRoberts]

Sorry JR, but I honestly don't care what you did and wrote in the eighties, just like you don't care that I climbed 50-meter transmission poles in the eighties and adjusted RF cables for lamda/4 of the length.
Also, at the same time, I did a lot of work on how to adapt 600 ohm telephone lines to telephone hybrids in order to obtain the best possible isolation of the receiving and transmitting side. The fact that you weren't worried about the characteristic impedance of the cable in the audio spectrum doesn't mean that no one is.

I already shared that I used 51 ohm build out resistors on audio products.

I never heard an audio difference related to that termination impedance.

JR

 

[abbey road d enfer]

Um, can you write the formula for the characteristic impedance (independent of frequency) and the formula for the impedance of the cable (dependent on frequency) so that it is clearer to me exactly what you mean, and that I can see the difference?

I must admit my answer in post #109 was incorrect; I wrote that characteristic impedance does not vary with frequency. Actually it does. What does not vary is the nominal characteristic impedance, which is the most significant single figure, since almost no one is interested in matching impedance at frequencies where the characteristic impedance is 2 or 3 orders of magnitude higher than nominal.
BTW characteristic impedance also varies with length, which is another reason for taking necessary steps to minimize the consequences, such as driving with a very low source impedance and loading as lightly as possible.

PS: why the aggro with JR? Nothing in his post justifies it.

 

[Cqwet Dbdfte]

So much misinformation.
https://www.microwaves101.com/encyclopedias/characteristic-impedance#Z0
https://en.m.wikipedia.org/wiki/Signal_reflection

 

[Speedskater]

All well made cables have a "Radio Frequency Characteristic Impedance", but that is not the cable's impedance at analog audio frequencies.
About 2 decades ago, Cyril Bateman (RIP) wrote several magazine articles on speaker cables and how their characteristic impedance changed with frequency. He even made a SPICE model with 200 frequency nodes.

 

[k brown]

This thread is concerned with interconnects, not speaker wire, no?