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Wireless Receiver Output Level

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There seems to be a bit of circuit design misunderstanding in this thread. While block diagrams can be quite helpful in understanding signal flow within a device, they seldom shed light about how the nuances of each amplifier stage operates.

 

In other words, a block diagram is intended to show you what the circuits do but not how they do it. That requires a full schematic diagram, and even then, there are times when the operating variables are not immediately obvious.

 

Also, a given amplifier stage can be multipurpose depending upon how the operating parameters of that stage are adjusted by supporting components. A device that uses the same amplifier stage for both mic and line inputs is often far more complex than simply padding the signal prior to entering that stage.

 

Here's a simple observational test: 

1) Turn off phantom powering to the input being tested.

2) Plug in a shorting plug (e.g. pins-1, 2, & 3 connected together in an XLR connector).

3) Turn up the input level until you hear noticeable circuit noise in your headphones.

4) Switch between mic and line inputs and listen for any difference in noise.

 

This should tell you more than uninformed speculation will. If you hear a large difference in the noise level, there is more going on with the circuit than just padding down a mic input.

 

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John's test is easy and solid. With a pad based circuit, you may hear a few decibels of difference (barely noticeable) due to the input short being replaced with a restive pad. If it is well designed circuit where the mic gain stage is bypassed to the next stage or the feedback loop of the input stage is drastically changed (another valid solution), you will hear 10's of decibels of difference. Putting a 50 dB pad in front of a mic preamp is cheap but not the best way changing an input to line level.

Best Regards,

Larry Fisher

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Very interesting thread - I've just made the jump to Lectro SRC's and SD's 833.
A lot of my question has been answered but there's a few things that bug me RE" 'correct gain structure'.

 

So the SRC outputs its tone with a range of -50 to +5 (going up in 1dB increments). Set at Zero, I imagine thats around -20 / -18db?.

 

So at line level out to line level in to the 833 this comes in at around -35dB. Which I guess is understandable as the Gain trim at its lowest is set to -14. So lets say I have a strong, fully modulated real world signal (dialogue) and I wanted my ISO's to say be around -20 - what's best practice here?

 

max out the SRC to +5 and have less gain on the mixer?

 

Thanks in advance

Craig.

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On 4/10/2020 at 9:14 AM, John Blankenship said:

There seems to be a bit of circuit design misunderstanding in this thread. While block diagrams can be quite helpful in understanding signal flow within a device, they seldom shed light about how the nuances of each amplifier stage operates.

 

In other words, a block diagram is intended to show you what the circuits do but not how they do it. That requires a full schematic diagram, and even then, there are times when the operating variables are not immediately obvious.

 

Also, a given amplifier stage can be multipurpose depending upon how the operating parameters of that stage are adjusted by supporting components. A device that uses the same amplifier stage for both mic and line inputs is often far more complex than simply padding the signal prior to entering that stage.

 

Here's a simple observational test: 

1) Turn off phantom powering to the input being tested.

2) Plug in a shorting plug (e.g. pins-1, 2, & 3 connected together in an XLR connector).

3) Turn up the input level until you hear noticeable circuit noise in your headphones.

4) Switch between mic and line inputs and listen for any difference in noise.

 

This should tell you more than uninformed speculation will. If you hear a large difference in the noise level, there is more going on with the circuit than just padding down a mic input.

 

 

On 4/10/2020 at 9:31 AM, LarryF said:

John's test is easy and solid. With a pad based circuit, you may hear a few decibels of difference (barely noticeable) due to the input short being replaced with a restive pad. If it is well designed circuit where the mic gain stage is bypassed to the next stage or the feedback loop of the input stage is drastically changed (another valid solution), you will hear 10's of decibels of difference. Putting a 50 dB pad in front of a mic preamp is cheap but not the best way changing an input to line level.

Best Regards,

Larry Fisher

 

 

I tested out John's suggestion above on a 302 and a 633 using a grounding plug (all 3 pins connected). Two different results.

 

On the 302 switching between mic and line there was about a 4-7 dB difference between selections not a large shift. Line had more noise.

On the 633 switching between mic and line there was also roughly the same 4-7 dB difference between the two except mic had more noise.

 

Not sure what is to be learned here. If going by what John suggested and Larry seemed to agree with then you might conclude that Sound Devices line input design is a simple pad?

 

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On most devices these days. line level signals goes through the mic preamp regardless, There are exceptions, channels 4, 5, 6 on a 633 for instance.  I also had a Tascam that hard-wire bypassed the preamps in line.

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6 hours ago, craig jackson said:

Very interesting thread - I've just made the jump to Lectro SRC's and SD's 833.
A lot of my question has been answered but there's a few things that bug me RE" 'correct gain structure'.

 

So the SRC outputs its tone with a range of -50 to +5 (going up in 1dB increments). Set at Zero, I imagine thats around -20 / -18db?.

 

So at line level out to line level in to the 833 this comes in at around -35dB. Which I guess is understandable as the Gain trim at its lowest is set to -14. So lets say I have a strong, fully modulated real world signal (dialogue) and I wanted my ISO's to say be around -20 - what's best practice here?

 

max out the SRC to +5 and have less gain on the mixer?

 

Thanks in advance

Craig.

 

As I understand it, Lectro's +5 setting is 5dBu output at full modulation, meaning that's as loud as it will get. In the case of the 833, outputting tone at +5 from your receiver into a line input at unity gain should read -15dBFS. In the US, it's common to align 0dBu with -20dBFS. So in theory, this means you would add 15dB of gain to your line input to get the max output of your receiver to match the max record level of 0dBFS. However in the real world, most people don't set the transmitter gain to utilize the full modulation (to rightly avoid the TX limiter) and you end up needing more input gain to get a healthy record level. No worries, the 833 has plenty of gain to account for this. In short, follow Larry's paper bag gain setting procedure described above and you'll be good.

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1 hour ago, Patrick Farrell said:

 

As I understand it, Lectro's +5 setting is 5dBu output at full modulation, meaning that's as loud as it will get. In the case of the 833, outputting tone at +5 from your receiver into a line input at unity gain should read -15dBFS. In the US, it's common to align 0dBu with -20dBFS. So in theory, this means you would add 15dB of gain to your line input to get the max output of your receiver to match the max record level of 0dBFS. However in the real world, most people don't set the transmitter gain to utilize the full modulation (to rightly avoid the TX limiter) and you end up needing more input gain to get a healthy record level. No worries, the 833 has plenty of gain to account for this. In short, follow Larry's paper bag gain setting procedure described above and you'll be good.

I set the RX output to +5 and my trim on the mixer at unity. I set my lectro TXs a little below what Lectro recommends (tickle the red) and find that the +5 output on the receiver makes up for that nicely.

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8 hours ago, Rick Reineke said:

On most devices these days. line level signals goes through the mic preamp regardless, There are exceptions, channels 4, 5, 6 on a 633 for instance.  I also had a Tascam that hard-wire bypassed the preamps in line.

 

However, in many cases this becomes an issue of semantics. For instance, if your input stage consists of an op-amp and its gain characteristics are set via the negative feedback loop for 50dB gain and you alter that feedback loop (can be done as simply as with one or two resistors) so the gain is, let's say, 10dB, you've now changed that input stage from a mic level pre-amp to a line level preamp. Whereas that op-amp has an open circuit gain of maybe 120dB you could never operate it successfully without negative feedback to lower the gain, as it will typically be unstable and go into oscillation. Such circuits are intended to be defined by the parameters the solid state device achieves via associated components. Also, if they're used, the gain of an input transformer, as well as its level handling capabilities, comes into the equation.

 

Of course, in a well designed preamp, there are other considerations such as slew rate, phase, hair color (just wanted to see if you're paying attention), etc. but the basic change can be done as simply as switching a different source to the input of the stage and changing the negative feedback loop.

 

If you don't change any of the operating parameters of the amplifier stage and simply pad the line input down to feed a mic-level input, that's a different case.

 

Keep in mind it's not uncommon for an input to use a combination of a pad and adjusted input amplifier characteristics. In a well designed device, such stages do not offer any significant compromise to either the mic or line signal.

 

Then there's cheap crap and also Sony cameras.

 

A block diagram doesn't reveal the design nuances, as that's not its purpose -- nor its porpoise (still paying attention?). That's one of the goals of a proper schematic diagram (the purpose, not the porpoise).

 

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