Where's the damn gain control?

This topic was inspired by complaints of "low gain" in the Tele V72 in this thread. But it applies to a number of 'plug in' preamp/boosters and line amps.

People sometimes wonder why these old (mostly tube) modular amplifiers usually have a fixed gain of +40 or +50dB. They're sometimes disappointed when they rack up that "vintage piece", plug in a mic and discover that the output level just isn't hot enough to drive the inputs of their recorders. The fact is, they were never meant to be used as stand-alone. Rather, they were intended as part of a system with carefully balanced stages of gain and loss, with the final output being a line-level signal to drive recorders and other devices on a program buss.

Just for fun during my lunch break today, I drew a diagram which shows signal levels and impedances in a typical tube mixing console. These amplifiers start to make sense when you look at them in their "native context."

Diagram

It's worth noting that preamps that were intended to be used as stand-alone units, complete with power supply, often had loads of gain. Examples include the Altec 1566A and any number of broadcast "remote amplifiers" such as the RCA OP-6, Collins 212Y, etc.

[quote author="NewYorkDave"]This topic was inspired by complaints of "low gain" in the Tele V72 in this thread. But it applies to a number of 'plug in' preamp/boosters and line amps.

People sometimes wonder why these old (mostly tube) modular amplifiers usually have a fixed gain of +40 or +50dB. They're sometimes disappointed when they rack up that "vintage piece", plug in a mic and discover that the output level just isn't hot enough to drive the inputs of their recorders. The fact is, they were never meant to be used as stand-alone. Rather, they were intended as part of a system with carefully balanced stages of gain and loss, with the final output being a line-level signal to drive recorders and other devices on a program buss.

Just for fun during my lunch break today, I drew a diagram which shows signal levels and impedances in a typical tube mixing console. These amplifiers start to make sense when you look at them in their "native context."

Diagram

It's worth noting that preamps that were intended to be used as stand-alone units, complete with power supply, often had loads of gain. Examples include the Altec 1566A and any number of broadcast "remote amplifiers" such as the RCA OP-6, Collins 212Y, etc.[/quote]
:thumb:

Dave,

Thanks for the nice summary and perspective lesson. :thumb: :thumb:

A P

A simple and good explanation. :thumb:

OTOH, with "modern music" and "hot mics", some of the vintage gain-modules are screaming into serious overload! Designing input pads thus becomes Yet Another step in re-deploying the old modules in newer applications.

Case in point...the task involved four Langevin AM-16 modules with a fixed 45 dB gain.

First of all, I warned the owner that with only 45 dB of gain, he "couldn't mic a piccolo at 30 feet with a ribbon mic" <G!>, but in this case, levels from the mics tended to be closer to line level vs. -60 dBu.

Hence, I concocted a pairing of pads...0/-15/-30 at the input, and then 3 dB steps from 0 to -15 dB at the output:

http://www.brianroth.com/projects/mgreene/langevin-front.jpg

The input pad switched in/out "O" pads while the output pad modified values in a bridged-T attenuator. I consider this to be a "classic" method when dealing with older, foxed gain modules.

But, we're still stuck with the inherent 45 dB gain of the AM-16...no magic to fix that besides cascading modules.

Bri

[quote author="NewYorkDave"].....Rather, they were intended as part of a system with carefully balanced stages of gain and loss, with the final output being a line-level signal to drive recorders and other devices on a program buss..... ..[/quote]

I'm wondering what's noise figure of those consoles (total gain= 130dB!!)

Well, here's an example of console specs, in this case a 10x2 Altec.



Pics of the console at

http://www.4sync.com/rc/picdisplay.asp?itemkey=979

Thank you, Dave.

> what's noise figure of those consoles (total gain= 130dB!!)

Noise Figure and Gain are two different problems.

The Noise Figure on classic broadcast gear is usually under 2dB. We often did record at a distance with ribbons. 2dB is not that hard with a high-ratio input transformer and a clean cathode. Even at low current, most of the medium-Mu triodes can get 2uV grid voltage noise. On the other side of a 1:10 transformer, that's 0.2uV which is near the audio self-noise of a 150Ω resistor. Grid current noise is near zero. Transformer losses into an open grid can be 1dB. Transformer winding resistance, with radiometal or better corestuff, can be under 20Ω. Specs may be 3dB NF, but most channels will beat that. Using odd-lot tubes, from 5% to 50% will have obviously excess noise (cathode dirt); when you find one, swap it out to a higher level stage. (That is one of the reasons classic gear used only two or even one tube-type throughout.)

> total gain= 130dB!! (for NYDave's first post)

PLUS 16dB+18dB+20dB+8dB= 62dB loss at normal settings. 130dB-62dB= 68dB normal gain, and note that this is for +8dBm nominal output. If the piccolo is weak, you can get 10dB more gain in the channel: 78dB weak-instrument gain. If the whole gig is weak, you can pull another 20dB in the Master: 98dB weak-gig gain. Assuming input noise near -125dBm, then 98dB gain puts noise at -27dBm, which is 35dB below nominal +8dBm output, which is bad even for AM radio. In such a case, you move the piccolo closer to the mike.

With the Master full-down, output noise is about -122dBm+50dB-8dB= -80dBm or 88dB below nominal +8dBm output. That's plenty good for any radio service, disc, or tape.

With Master at nominal setting, channels all muted, noise is about -125dBm+40-20+50-8= -63dBm or 71dB below nominal +8dBm output. Or 81dB below +18dBm, the peak level of a program showing +8dBm on a VU meter. Not theoretical CD quality, but better than most stuff on CDs.

Used as designed, with radio-studio sources, noise is just not an issue on these boards. Used with hot rock acts and hot too-close mikes, overload can be an issue; but we never had -22dBm = 28mV sources when using dynamic mikes in normal studio work. 1 or 2mV was plenty.