Line vs Mic amps for mixdown

[JohnRoberts]

Warning this is pedantic and unlikely to be appreciated.

The decibel is a relative unit of measurement equal to one tenth of a bel. It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale.

I have learned that it is improper to describe transformer turns ratios as decibels (dB). Decibels are mainly used to describe power ratios. Transformers deliver voltage ratios between input and output, but the input and output "power" remain roughly constant (less some small internal losses).

That said it extremely convenient to use decibels when estimating the total gain of a given signal path, but don't try to buy a 10dB or 20dB transformer (they are all 0dB). They will typically be sold based on their numerical turns ratios.

Sorry

JR

 

[StarTrucker]

Warning this is pedantic and unlikely to be appreciated.

Is the term input gain in this category? Shouldn't any control adjusting the output (or input) of a stage be called a volume or level control?

 

[StarTrucker]

if you're doing anything loud you don't need so much gain, but at least whatever is required to overcome passive mixing bus losses. Those will all be 150-600 ohm passive mix busses that need identical mic input transformers for matching.

Can you do a mixing desk with all passive mix buses with no make up?

pre/line amp -->
2 to 1 passive summing bus --> Converters
pre/line amp -->

Need a line amp after, or just a buffer in order to not load down the output of the first stage?

 

[RoadrunnerOZ]

Warning this is pedantic and unlikely to be appreciated.



I have learned that it is improper to describe transformer turns ratios as decibels (dB). Decibels are mainly used to describe power ratios. Transformers deliver voltage ratios between input and output, but the input and output "power" remain roughly constant (less some small internal losses).

That said it extremely convenient to use decibels when estimating the total gain of a given signal path, but don't try to buy a 10dB or 20dB transformer (they are all 0dB). They will typically be sold based on their numerical turns ratios.

Sorry

JR

Would the impedance ratio be the way then to calculate loss or gain?

 

[StarTrucker]

Would the impedance ratio be the way then to calculate loss or gain?

No turns ratio is, but you can do it with impedance ratio, it just adds a step and isn't as accurate. 1:10 turns ratio on the input transformer will give you roughly 10x the voltage on the secondary as on the primary, given the rest of the circuit can support whatever impedance it is.

Impedance ratio is the square of the turns ratio. That's why you will see a mic input transformer say 1:20 ratio but impedances are like 100:47k ohm; it's not a plug-and-play swap between the two.

To get the gain increase or decrease you expect out of your turns ratio, impedance needs to be appropriate for your application. A tube line amp might need to deliver current into a 600ohm output load (600ohm secondary on your transformer) through a 10:1 transformer. You wouldn't get the same gain reduction out of another 10:1 transformer that has a 200ohm secondary. It would draw too much current from the tube and pull down your output (gain).

 

[RoadrunnerOZ]

No turns ratio is, but you can do it with impedance ratio, it just adds a step and isn't as accurate. 1:10 turns ratio on the input transformer will give you roughly 10x the voltage on the secondary as on the primary, given the rest of the circuit can support whatever impedance it is.

Impedance ratio is the square of the turns ratio. That's why you will see a mic input transformer say 1:20 ratio but impedances are like 100:47k ohm; it's not a plug-and-play swap between the two.

To get the gain increase or decrease you expect out of your turns ratio, impedance needs to be appropriate for your application. A tube line amp might need to deliver current into a 600ohm output load (600ohm secondary on your transformer) through a 10:1 transformer. You wouldn't get the same gain reduction out of another 10:1 transformer that has a 200ohm secondary. It would draw too much current from the tube and pull down your output (gain).

So to work out what transformer you’d need you would have to know what the source and load impedance would be as well as the source’s expected delivery voltage and the target inputs required voltage?
Sorry I’m a bit vague in this department - we were taught how to fix audio gear and I’ve been servicing for years but never got into the design of transformer driven circuits - just replacing with appropriate transformers for the application under repair.

 

[emrr]

Can you do a mixing desk with all passive mix buses with no make up?

pre/line amp -->
2 to 1 passive summing bus --> Converters
pre/line amp -->

Need a line amp after, or just a buffer in order to not load down the output of the first stage?

Maybe a 2 to 1 but after that losses are significant. But does the compressor after have make up? Does the converter input have digital input gain? 2 problem solvers.

 

[ruffrecords]

So to work out what transformer you’d need you would have to know what the source and load impedance would be as well as the source’s expected delivery voltage and the target inputs required voltage?
Sorry I’m a bit vague in this department - we were taught how to fix audio gear and I’ve been servicing for years but never got into the design of transformer driven circuits - just replacing with appropriate transformers for the application under repair.

You might find the attached useful.

Cheers

Ian

 

[RoadrunnerOZ]

You might find the attached useful.

Cheers

Ian

Thanks - seems pretty simple.

 

[StarTrucker]

Maybe a 2 to 1 but after that losses are significant. But does the compressor after have make up? Does the converter input have digital input gain? 2 problem solvers.

Exactly what I was looking for thank you. If you swap R1 for a pot between 2 outputs does that become a pan control

 

[emrr]

Final R1, I think a limited range pan, I recall at one extreme not fully isolated from the other, plus you've added the variable Z of the pan resistance. It wasn't done. Pans were rare beasts and they were a pair of T's or Ladders that went in opposite directions on each deck to preserve Z. Expensive additions you don't find stock on anything old at all. Maybe a patchable channel or 2 on some custom desks.