Different considerations for a PA System vs. a Guitar amp

[Dolmetscher007]

In a recent thread, I asked about why guitar amp speakers never have a tweeter/woofer/sub + crossover system like a home Hi-Fi would have for listening to music/movies. I received a great deal of information that made a ton of sense, and thankfully really cleared up a years-long head scratcher for me. I really appreciate that input.

This thread is not about speakers... but rather about the difference between a guitar amplifier, and a voice amplifier i.e. a P.A. System. Now, obviously a P.A. would have a mixer, or at least a multi-input 'front-end' (or so I'm calling it". That in and of itself is obviously a huge difference. But once a mixer has done all it's business, it sends out an electrical signal (or signals, for stereo) and that signal(s) is sent through an amplifier.

For the sake of this question, let's get rid of the mixer all together, and say we have just a dynamic SM-58 microphone plugged straight into a PA's amp. Within that amplifier, I imagine there must be some form of "pre-amp" to get the weak signal up to a level that can be amplified, and then the output transformer pushes the pre-amp'ed signal up to whatever level it is designed to... and out the speakers.

As I type this... I think I am starting to see that a major hole in my knowledge is around what a "Pre-amp" in a guitar amplifier really is for. In a simple amp like a Champ, that has no tone control, no reverb... nothing... just a volume pot, and that's really it. So... what DOES a pre-amp really do? I imagine that once I know that, I will probably be able to understand where a guitar amp and a PA differ.

[mozz]

You can look at some of the old Marshall schematics, they made PA heads as well as guitar. That might help to see what differences there are.

[eschertron]

I think that early amps - like the 5F1 - didn't have anything particularly 'guitar' about them other other than an input set up for a target level and impedance. No EQ, no tonal sculpting, just enough gain that the instrument could be heard. Mostly hardware choices: speaker, OT, etc., made for the band-limited sound we like to call "guitar" today.

Plug an SM-58 into one of these amps, and (if you shout loud enough into the mic!) you'll have a small, low-power PA system. My assertion is that in the early days there was no difference between PA (vocal, etc.) amp and guitar amp*. We hadn't trained ourselves to like certain guitar 'sounds' until later, after musicians abused the equipment to get novel sounds from it.

*since we're not talking about speakers, that choice is immaterial.

[J M Fahey]

Adding to that : SM58 didn´t even *exist* way back then, and if you pay attention to 40´s and 50´s "Microphone input" schematics, they were high impedance (500k or 1M) and just slightly higher gain than Guitar inputs.

The reason being that the average microphone was a very high impedance Crystal or Ceramic one, (1 M) or, best case, a variable reluctance one (50k).

Low impedance microphones (150 to 600 ohm) were completely out of reach ($$$$) for the average Musician, and would have required an also very expensive transformer balanced input.

Here we have 3 Instrument inputs, plus a high gain one, for microphone:

or check Marshall PA amplifiers



PRO low impedance microphones were used by, oh, Elvis Presley or Frank Sinatra ... and even so, once at least Elvis plugged his microphone (which must have been high impedance) into his Guitar player´s amplifier ... simply because it was "better than the house PA" ... go figure.

[Dolmetscher007]

Thanks mozz and eschertron...

So you think that ultimately a modern PA, purely from an amplification POV is just an amplifier, like any other amplfier, except with a TON more power so as to create headroom and prevent distortion and break up?

I guess there are some extra sections of the circuit for things like "preventing feedback." I just didn't (don't) know if a signal from a typical dynamic microphone or from the "Out" of an un-powered PA mixer is higher, lower, or about the same as the signal coming directly out of a guitar.

[pdf64]

...This thread is not about speakers... but rather about the difference between a guitar amplifier, and a voice amplifier i.e. a P.A...

Of what era, eg 50s, 60s, 70s, 80s etc? Design goals and what's feasible given the available technology have changed over the decades.
And I don't think it's entirely helpful to leave speakers out of it, as the technology changed so much there too; eg modern day PA speaker systems would have been useless in the 50s, as they would have been way too insensitive for the commonly available amp power levels.

[Dolmetscher007]

Of what era, eg 50s, 60s, 70s, 80s etc? Design goals and what's feasible given the available technology have changed over the decades.
And I don't think it's entirely helpful to leave speakers out of it, as the technology changed so much there too; eg modern day PA speaker systems would have been useless in the 50s, as they would have been way too insensitive for the commonly available amp power levels.

Hmmm... That is a good question.

With Guitar amps, I feel like the technolgy made a lot of strides, or... at least a lot of changes (positive or negative is subjective) in the 1980's and 90's with solid state advancements and then digital stuff. But then... sometimes in the late 90's early 2000's I def. feel like there was a return to the 1950's-60's with all tube amps making a huge comeback.

With PA's however, I can only imagine that there has not be any trend towards reverting back to some older technology. I mean , it would certainly be cool as hell to be able to hear an all tube, compleely analog P.A. System. I just don't know that there even is one out there these days.

So... I guess... I am asking about doing just that. What would an all tube P.A. system circuit look like, if one were to be build in 2019? As far as the impedence of the input, I imagine that a variable resistance potentiometer could allow for one input to be able to "dial in" an input resistance for any number of different mics/instruments. I am defrinitely out of my element here. I'm just hoping you guys catch my drift.

[eschertron]

Personally, I consider the whole system. There are tweaks that have been done to each amplifier 'lineage' over time, some have become standardized enough to be representational of "guitar amps" or "PA amps". In the final analysis, without looking at small details, an amp is an amp. If it were not so, then we would need only 1 make and model guitar amp to suffice for every player. The differences between two example guitar amps may be much greater than the differences between a (specific) guitar amp and a (specific) PA amp.

[Chuck H]

Personally, I consider the whole system. There are tweaks that have been done to each amplifier 'lineage' over time, some have become standardized enough to be representational of "guitar amps" or "PA amps". In the final analysis, without looking at small details, an amp is an amp. If it were not so, then we would need only 1 make and model guitar amp to suffice for every player. The differences between two example guitar amps may be much greater than the differences between a (specific) guitar amp and a (specific) PA amp.

THAT^^^^^^^^^^^^^^^^^

Once upon a time early guitar amplifiers were just generic amplifiers the same as what might be used in a PA. This wouldn't be adequate for sound reproduction (PA) in today's world, but still sounds just right for guitars. Today's PA systems try to get as close to accurate sound reproduction as possible while still maintaining road worthiness and a price musicians will pay. Guitar amps have actually gone in another direction with a lot of tolerance for some types of signal distortions, frequency peaks and roll offs. Which brings me to...

This being the reason why "speakers aside" cannot be part of the equation. Because it IS part of the equation. Back when guitar and PA amps were virtually the same they also used the same speakers. So differences in PA amps today reflect their use with new PA speakers and differences in guitar amps today reflect their use with (what today are) guitar speakers (which are not unlike old full range PA speakers). Deficiencies in older full range speakers were quite specifically responsible for how all amplifiers were voiced back in the day. Modern speakers allow PA design to get closer to a flat frequency response without as much need to make allowances in the amplifiers voicing. But the way those allowances built into older amplifiers sound with guitar speakers happens to sound good for guitar. Which is why you won't get satisfactory PA performance using guitar speakers or satisfactory guitar amp performance using PA speakers. And because the two tools have evolved in different directions you cannot get satisfactory PA performance using a guitar amp and speaker and you cannot get satisfactory guitar amp performance using a PA and it's intended speakers.

[nevetslab]

What would an all tube P.A. system circuit look like, if one were to be build in 2019?

Not stopping to contemplate WHAT one would have to do to replicate the current state of the art in Digital consoles commonly used these days in Tubes, my mind just shifted to the tonnage and enormous acreage required to achieve a moderate 20kW small PA System, let alone what the fuel cost and the number of semi trucks needed to haul just that around vs what is used in Hi Power SS designs, including all the Class D and others to achieve what has made systems small and enormously powerful. Let alone what the back-up tube stash to maintain all those 300-400W tube power amps in the racks would look like.

After our audio industry left the early broadcast tube mixers, like the Altec 250 series consoles with their tube tray amps.....which DO sound amazing....I can't recall any production recording consoles that were all tube, built like the vintage Neve, Trident, Helios and others in the 70's. If this 2019 current all-tube sound system were to be built, and if it couldn't be any sort of hybrid design to keep the flexibility we all have come to rely upon, it staggers the imagination!

[eschertron]

To take a step to the side ways, let me note:
There are plenty of 2-channel tube preamp 'sweeteners' available to run a mix (or individual channels) through a tube stage to get some of that tubey distortion and compression (call it warmth, or mojo, or...). Of course there are as many plugins for a digital audio workstation, just as effective at adding the tubey vibe.

If the goal of this thought experiment is to have an all-tube PA just like they had in the 60s, then just copy a design. Component quality is way up for passives (caps, resistors, etc.) and for speakers. Might be a nice nod to history. Just realize that a 200W PA that must stay within its headroom (to deliver ALWAYS clean reproduction) will be blasted out of the park by any 30W - 50W guitar amp driven into square wave territory. The whole 'music power' consideration. What Nevetslab says above - you'd need several kW to get a couple of mics up front, 10s of kW for vocals, keys and whatnot.