Impedance matching / bridging between tube gear and modern audio interfaces

[abbey road d enfer]

All well made cables have a "Radio Frequency Characteristic Impedance", but that is not the cable's impedance at analog audio frequencies.

I think we all agree with that. You call that Radio Frequency Characteristic Impedance", some call it step impedance or surge impedance. In practical terms, for frequency domains where it matters, it depends mainly on linear inductance and capacitance.
For lower frequency domains, bulk resistance or capacitance are usually the dominant parameters.
For loudspeaker interconnect, resistance is dominant, for line level, capacitance is dominant. Of course there are intersectional cases.

 

[Mr. Moscode]

These threads seem to go to "I remember when" in a hurry.

We're talking microphone levels here in the OP. Max power is not needed, max voltage is as long as it doesn't create frequency anomalies like peaks in and out of band. Max voltage helps override the noise in the next amp.

If I was doing this I'd be looking at 1k square wave from a generator with the same output impedance, 66 ohm, and going through the chain to look for roll-off or F peaks. That tells you a lot in a hurry. Also sweep a sine wave from 10 hz through 100khz and look for resonances and f response. In order to get a 66 ohm output z I would run the gen output through a wide band solid state buffer with a 66 ohm resistor in series with it.

Generally output impedance can be measured by feeding a signal into the output of the source through a variable resistor and adjusting the resistor for 1/2 the gen output at the output of the source amp under test. Then measure the resistor. However in many cases it will be frequency dependent.

The low driving impedance of a source on a short line can damp anomalies and reduce self generated noise in the receiver. As one response said, impedance matching introduces a 6db loss, which reduces s/n ratio by that much.

Solid state power amps driving speakers are not at issue here. But since it was brought up Tube OTLs have inherently hi output impedance brought down with negative feedback. Practical use of an OTL audio amplifier requires a speaker with a high enough impedance and sensitivity to make use satisfactory. Adding more output tubes lowers output impedance and delivers more power.

There are also torroid autoformers on the market for matching OTL outputs to low z speaker loads (which makes the amp no longer an OTL) but will increase power transfer.

My opinion is that too many transformers in the path creates a lot of loss and distortion from hysteresis and phase shift as they are inherrently lossy filters. In my opinion, less is more.

 

[k brown]

The OP was about line levels.

 

[Speedskater]

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

The cable doesn't care what we use it for.
How its characteristic impedance changes at frequencies below a few hundred kilohertz remains the same.
No cable no matter what type or construction has a single characteristic impedance at audio frequencies.

 

[ruffrecords]

The cable doesn't care what we use it for.
How its characteristic impedance changes at frequencies below a few hundred kilohertz remains the same.
No cable no matter what type or construction has a single characteristic impedance at audio frequencies.

​

The characteristic impedance

of an infinite transmission line at a given angular frequency

is the ratio of the voltage and current of a pure sinusoidal wave of the same frequency traveling along the line. This relation is also the case for finite transmission lines until the wave reaches the end of the line. Generally, a wave is reflected back along the line in the opposite direction. When the reflected wave reaches the source, it is reflected yet again, adding to the transmitted wave and changing the ratio of the voltage and current at the input, causing the voltage-current ratio to no longer equal the characteristic impedance. This new ratio including the reflected energy is called the input impedance.

The input impedance of an infinite line is equal to the characteristic impedance since the transmitted wave is never reflected back from the end. Equivalently: The characteristic impedance of a line is that impedance which, when terminating an arbitrary length of line at its output, produces an input impedance of equal value. This is so because there is no reflection on a line terminated in its own characteristic impedance.

Cheers

Ian

 

[CJ]

Even the best transformers require a little power to operate.

What does the waveform of this power (voltage and current) look like compared to the sine wave at the primary?

 

[Walter66]

My opinion is that too many transformers in the path creates a lot of loss and distortion from hysteresis and phase shift as they are inherrently lossy filters. In my opinion, less is more.

The philosophy of "less is more" is simple, but wrong on so many level.
It can be applied on some cases, but not on most of them.
In technology, more is most often better.

 

[drtechno]

Hi I am recording with a Warm Audio WA 67 microphone (output impedance = 200 ohms) into an Ampex 601 (input transformer impedance = 200 ohms), the output (600 ohms) is sent into a tube compressor with 600 ohm input impedance, and output impedance is also 600 ohms. This is sent into the line input of my Focusrite Clarett+ 4 pre (66k Ohm input impedance)

During mastering, I send the digitally recorded signals from my Focusrite Clarett+ 4 pre line out (output impedance 68 ohm) back into my tube gear (either the 200 Ohm or 600 ohm input impedance )

I am trying to make sure I am getting my impedances matching / bridging correct.

My questions are:

1) For recording, I am matching impedances all the way through my signal chain, then bridging the impedance going into the audio interface. Is this the correct way to do this, or should I use a transformer and match the 66k ohm input impedance of the audio interface also?

2) For mastering, should I transform the 68 ohm output to match the 200 ohm or 600 ohm input, or is bridging this ok too?

3) Is there anything else I am missing?

Thanks!

For #1 the solution I came up with is installing an attenuator in to provide the load for the 600 ohm output and the interface just samples across the load. Something I learned from a different sector that applies here and is very successful. I custom build an H pad attenuator for that situation, but you might find someone's -20 line attenuator will work ok.

#2 isn't an issue because that is just the lowest impedance it can drive and going higher in impedance will render a lower noise floor. Usually medium impedance is ok up to 5K on these low voltage powered outputs (+/-12V and below),

I do a mod for the WA67/U67 to make it work for modern interfaces either fixed at 2.2K or switchable 2.2K:200 PM me if you want that mod done by me or a quote on a custom attenuator.

 

[abbey road d enfer]

Hi I am recording with a Warm Audio WA 67 microphone (output impedance = 200 ohms) into an Ampex 601 (input transformer impedance = 200 ohms), the output (600 ohms) is sent into a tube compressor with 600 ohm input impedance, and output impedance is also 600 ohms. This is sent into the line input of my Focusrite Clarett+ 4 pre (66k Ohm input impedance)

3) Is there anything else I am missing?

The input xfmr in the 601 has a ratio of 1:15.
The secondary load is 2.2Meg, so the midrance reflected input impedance is nearly 10 000 ohms.
At LF and HF respectively, the primary inductance and the reflected Miller capacitance take over.
I woud say the mic is not impedance-matched. It is bridged.
And that's how it was meant to be.