Impedance matching / bridging between tube gear and modern audio interfaces

[5v333]

I was just making the point that power is not the same across every resistor like current.
It is just Ohms law without any reactive stuff.

Learn Kirchoff and Ohm and most things can be pulled to pieces and understood.
Its fun!

 

[abbey road d enfer]

The power delivered from the source has an impedance which also describes the optimum load impedance.

There may be a language barrier here, but this is absolute nonsense. Power has no impedance.

The source impedance is not a loss term,

Indeed, it is not a "loss term", but it's actually a very important parameter when figuring out losses (and several other things).

 

[JohnRoberts]

There may be a language barrier here, but this is absolute nonsense. Power has no impedance.

Indeed, it is not a "loss term", but it's actually a very important parameter when figuring out losses (and several other things).

Don't get lost down that rabbit hole....

JR

 

[Chandra]

The more people think they know better, the more it is going to be a mess here.
OP must have been lost.
"Read" the voltage from a source is different from "Take" the maximum power from the source.

Regards.

 

[rmburrow]

Imagine a black box with a 5 V battery in series with a 50 ohms resistor and that's the output terminal. Do the math about how much output voltage will be with various resistor loads. Remember that P = E squared divided by R. You will find that P reaches it's maximum when R is 50 ohms ( the same as the source resistance inside the black box!

Mister CMRR, agree completely and your statement on power transfer is true regardless of a DC or AC circuit.

Interface issues become more messy with added issues such as cable type and length getting a signal from one "black box" to another at audio or higher frequencies. Consider these scenarios:
1. Very high impedance sources (e.g. condenser mic capsule): Capacitance effects of cable adversely affect response of condenser mic capsule operating with impedance of 100 meg ohms or higher. This is why a impedance converter (tube or FET) to allow the mic to "talk to the outside world" is inside the microphone, connected to the capsule. There is no power transfer involved.
2. Typical high impedance sources (e.g. magnetic phono cartridges, crystal mics, etc.) Negligible power transfer involved but cable length affects the performance (i.e. 47k ohm phono cartridge cable run to preamp).
3. Typical 10 to 15 k ohm balanced bridging: 15 k across 0 dbm 600 ohm line; negligible power involved. 0.7746 v across the 600 ohm line with 0 dbm power. 0.774 volts across 15 k; .052 ma through the bridge load, negligible. Unbalanced bridging across balanced source not a good idea unless transformer used IMO...
4. Low impedance balanced line suitable for reasonable length cable runs; Belden 8412 mic cable, Belden 8451 interconnect cable; the specs can be looked up and many facilities utilize these materials. The cable equation applies, and yes Mr. CMRR, there is a CMRR factor involved with these circuits based on the source and terminating impedances, especially when dealing with audio frequencies. (CMRR a factor in these circuits with keeping RF out of audio lines..)

 

[Cqwet Dbdfte]

There may be a language barrier here, but this is absolute nonsense. Power has no impedance.

Indeed, it is not a "loss term", but it's actually a very important parameter when figuring out losses (and several other things).

The power delivered from the source has an impedance which also describes the optimum load impedance.

There may be a language barrier here, but this is absolute nonsense. Power has no impedance.
The power delivered from the source WHICH has an impedance which also describes the optimum load impedance..

Sometimes I make the mistake of assuming too much from people.

Again, "matching" for impedance in audio signalling is a moot point, as commonly sources are low impedance and loads are high, unless you are running miles of wire. I think that was the topic?

 

[abbey road d enfer]

There may be a language barrier here, but this is absolute nonsense. Power has no impedance.
The power delivered from the source WHICH has an impedance

Words are important. Saying power has an impedance is nonsense, saying a source has an impedance is correct.

which also describes the optimum load impedance..

Optimum? According to which criteria? Certainly not efficiency. Optimum is different than maximum.

Sometimes I make the mistake of assuming too much from people.

This is what happens when you use incorrect wording.

 

[Cqwet Dbdfte]

You can't let this go, huh?
Optimum for RF where matching is essential for max power transfer. Audio, as I said numerous times, not much of an issue, unless you run miles of wire.
Power efficiency you will get when the mismatched power is not reflected back to your source, this goes for the above, RF, digital transmission lines.
Power "lost" in the source is not an issue.
Your microphone cables don't need efficiency.

 

[abbey road d enfer]

You can't let this go, huh?

Can you?

 

[ruffrecords]

"... losses are 50%.." Do you mean voltage or power?

He means both - they are the same in this paricular case

Like CMMR pointed out in his example, POWER transfer efficiency is maximum when source and load are matched, i.e. equal.

When load equals source you get maximum power transfer. Half the power is lost in the source resistance so the efficiency is only 50%.

When the source is very much lower than the load, the losses are much smaller so the efficiency is much greater than 50% However, the amount of power transferred is much smaller.

It is important not to confuse maximum power and efficiency. Maximum power occurs when load and source are equal. Maximum efficiency occurs when when the source impedance is zero.

In audio we want maximum efficiency.

Cheers

Ian

 

[abbey road d enfer]

There are a few cases where loading matters (not "matching").
One is when a piece of equipment has a transformer output.
A transformer is a bandpass filter, which response depends very much on its loading. Particularly at HF, there is a resonance, governed by the leakage inductance and parasitic capacitance. Loading the output helps taming this resonance. It is often done with a simple termination resistor, or with a less simple Zobel network (C and R in series).
When designers feel the need for a termination resistor, they naturally choose 600 ohms, as it's a value that has been sanctuarized by years and years of telegraphy/telephony.
It clearly has nothing to do with efficiency nor power "transfer".

Another example is microphones, in particular dynamic and ribbon types.
Due to their inductive nature, they constitute a natural low-pass filter in conjunction with the load they are connected to.
That's the main reason why current methods recommand bridging them, with about 10x their nominal impedance. Incidentally, it also helps with sensitivity and S/N ratio.

I believe I already mentioned passive loudspeaker x-overs.

Back to the OP, except for passive sources*, matching impedance is not an issue when considering voltage/energy transfer, since any resulting attenuation can be compensated elecronically.
However, proper loading of output stages is sometimes necessary.

*Even though matching could be considered as beneficial in optimizing the connection of passive sources, it turns out that frequency response considerations result in a preferrence for bridging.
Actually, matching the impedance of a ribbon mic or a turntable cartridge is hardly feasible, due to the large variations with frequency.

 

[Gold]

I think the Maximum Power Transfer Theory definition can help here. Talking about “efficiency” can mean two opposite things. Low source impedance and high load impedance is “most efficient” transfer of power from individual balck box to black box. Meaning the least amount necessary. Matched impedance is not most efficient transfer from individual black box to black box but is the most efficient system wide as there are no resistive losses due to impedance.

 

[abbey road d enfer]

I think the Maximum Power Transfer Theory definition can help here. Talking about “efficiency” can mean two opposite things.

Efficiency in engineering is a non ambiguous term. It expresses the ratio of received energy to sent energy. that's the definition that should be used in this discussin.

Matched impedance is not most efficient transfer from individual black box to black box

That is true.

but is the most efficient system wide as there are no resistive losses due to impedance.

By definition, losses are always resistive.
In some cases, matching may be justified, which some may consider "efficient". An abuse of language.

 

[Gold]

Efficiency in engineering is a non ambiguous term. It expresses the ratio of received energy to sent energy. that's the definition that should be used.

In some cases, matching may be justified, which some may consider "efficient". An abuse of language.

The Wiki page uses that language. Not that Wiki is always right.

 

[RoadrunnerOZ]

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 input transformer on the 601 is for microphone input XLR, you can insert a dummy plug in place of the transformer for high impedance at that input. The line input on the 601 at the TS jack input is high impedance anyway.

 

[rmburrow]

The input transformer on the 601 is for microphone input XLR, you can insert a dummy plug in place of the transformer for high impedance at that input. The line input on the 601 at the TS jack input is high impedance anyway.

You are better off using the input transformer on a Ampex 601. You get around 15 dB of (almost) noise-free gain through the input transformer. The 6F5 tube isn't a noise free device; you will get increased noise using a low impedance mic with no transformer and unbalanced connection, especially at mic level.

 

[RoadrunnerOZ]

You are better off using the input transformer on a Ampex 601. You get around 15 dB of (almost) noise-free gain through the input transformer. The 6F5 tube isn't a noise free device; you will get increased noise using a low impedance mic with no transformer and unbalanced connection, especially at mic level.

It’s an EF86/6267 not a 6F5 and it would appear from the sales brochure and spec sheet the machine comes without the input transformer in place - it’s an optional extra/accessory Cat #17331-1 so it was designed with the high impedance mic input in mind - it wouldn’t hurt to use a higher input impedance transformer.
Another odd thing to be very aware of is it’s pin 3 Hot, not pin 2 - becomes important for unbalanced use as pin 2 is ground via the jumper. Pin 3 Hot was used quite a bit in the early days - I think the early Yamaha desks were pin 3 Hot - then pin 2 became the standard maybe around the ‘90s.

 

[Brian Roth]

In 1990, the Los Angeles AES section had a solution!

 

[RoadrunnerOZ]

In 1990, the Los Angeles AES section had a solution!

Seems to be what manufacturers used to help decide. Had a gig at the Byron Blues Festival some years back - desk was pin 3 hot, all the FX gear was pin 2 - some unbalanced TS inputs and outputs and all the XLR adapters had to be rewired, plus they had Ring Send and Tip Return for the inserts. Not a great day.

 

[abbey road d enfer]

Seems to be what manufacturers used to help decide. Had a gig at the Byron Blues Festival some years back - desk was pin 3 hot, all the FX gear was pin 2 - some unbalanced TS inputs and outputs and all the XLR adapters had to be rewired, plus they had Ring Send and Tip Return for the inserts. Not a great day.

Don't forget the temporary decision by German and Austrian engineers to use female sockets for outputs and male for inputs. E.G. the first AKG BX20.