Vox AC30/ 6 (TBX). Add an MV ?

[Greg Robinson]

Seachief - the diagrams depict the signal flow of the amp. Each diagram shows the same amp with different settings, and the resultant signal flow that you would see with an oscilloscope at each stage with those different settings. The first 5 diagrams depict a "traditional" type design, the last three show the same with a master volume added.
Yes, the numbers are completely arbitrary, and were simply chosen for conveniance.
Av is the gain of each stage. So, if the Av of a stage is 10, then whatever you feed into it comes out 10x bigger. 1V in? 10V out. 0.1V in? 1V out. BUT... And this is fundamental to amplifiers, there is a limit to how much a stage can put out. I have described this as "headroom". The first stage, with its Av=10 and Headroom=15, means that the maximum that stage can put out is 15V, it then follows that the maximum INPUT signal it can handle is 1.5V because 1.5x10=15V. So if you were to feed 2V into that stage, it would TRY to amplify it by 10x, but "run out of headroom" at 15V, so instead of getting a clean 20V out, you would get a distorted 15V out.

So, lets start with diagram A. It shows the amp volume control set for 100% (10 on your dial). But the guitar volume control is set for 0! You've earlier stated that you didn't understand how turning down your guitar volume mutes the output of the amp. This diagram show GRAPHICALLY how that happens - the full signal of the guitar pickup is still present, but the setting of the guitar VOLUME is not passing that signal into the amplifier. So, the input stage Av=10 is still trying to multiply that signal by 10 times, but 0x10 still equals ZERO.

Next diagram B shows the same concept for a different control. The guitar volume is at max this time, so the 100mV is passed to the input stage with its Av=10, so we get 100mVx10=1V. But this time, the amp's volume control is set to 0. You also expressed how you were unable to understand the volume control being able to mute the power amp. Well, this SHOWS you. If there is 0V coming out of the volume pot, the rest of the amp is still TRYING to amplify this signal, but 20x0=0.

Diagram C shows the amp actually passing signal. The guitar volume is set to full, so the first stage Av=10 gives us 100mVx10=1V at the input to the amp's volume control. The volume control is set to 50%, so we have 1Vx0.5=0.5V, and this is what comes OUT of the volume control. The rest of the preamp has Av=20, so 0.5x20=10V. This is then passed onto the power amp which requires 10V to reach full power, so that's exactly what we get, FULL POWER.

Ignore the rest of the diagrams for now. If you can wrap your head around the first three we would be making SIGNIFICANT progress.

[pdf64]

Of course. I look at the first box, & look at the second. ...

Do you mean diagram A and diagram B?

Or are you referring to the different boxes (that represent amplification stages) within diagram A?

[pdf64]

Excellent work Greg!

Input gain stage Av = 10 means that the voltage gain of the input stage (notation 'Av') is 10.
So with an input signal of 1V it will put out 10V.

Headroom = 15V means that if the input signal is above 1.5V, the stage output will clip.

[Sea Chief]

100mv (millivolts) which is how much electricity is put out when you strike a string on a humbucker.
Input gain stage is the first input after the input jack on the amp.
AV=10 is full vol
Headroom =15v means the input stage can handle 15v of signal before distortion occurs.

If you can understand one box the rest should fall in line. now you will notice a flat line or a squiggly symbol that is the audio as it goes throught the circuit if its flat theres no sound.

nosaj

Ok although I've tried saying that distortion is to be sidelined for now (as I'm back a step trying to understand preamp vol placement) it seems I'm being ignored: so I can at least try & understand something of this facet. Try, but get absolutely nowhere, even with you explaining this first box.

15v. Still this remains an arbitrary figure, headroom having not yet been mentioned on the thread (Vol 10 isn't an arbitrary figure, because I have a reference of a dial 1 to 10 on any typical amp, so I know 10 here = full).

That's my first connundrum with your explanation of box 1. But the biggest problem I have, is what seems to be a fundamental contradiction: if the amp is full volume, I'd expect it to be at maximum distortion: so why is the word 'headroom' even mentioned, in this same box with vol 10??

Furthermore you even mention "15v means the input stage can handle 15v of signal before distortion occurs" so I'm now considering a situation of headroom, but at the same time considering the vol is at 10.

And you think 'the rest should fall into place'... when the First box, even with your explanation (which has actually added more confusing info for me) is absolutely impossibly meaningless & confusing to me. Flat squiggly lines... why on god's earth, would there ever be, a flat line, if a flat line = no sound?? (How can no sound, be of any usefulness, to anyone??).

I get more bewildered with each post- further & further away from understanding anything. Not trying?? Are you kidding me??

Thanks anyway though, SC

[nosaj]

100mv (millivolts) which is how much electricity is put out when you strike a string on a humbucker.
Input gain stage is the first input after the input jack on the amp.
AV=10 is full vol
Headroom =15v means the input stage can handle 15v of signal before distortion occurs.

If you can understand one box the rest should fall in line. now you will notice a flat line or a squiggly symbol that is the audio as it goes throught the circuit if its flat theres no sound.

nosaj

[Sea Chief]

nosaj Let me just show you, how this is impossible for me. I cannot get past even the the start of diagram 1.

The first LHS section, I understand. It has a pickup then a volume control (but immediately it has 100mV which seems very high?? Anyway..).

Then there's a box. It says "Input Gain Stage, Av=10, Headroom 15v". Almost every word in this box, plus every number.. is incomprehensible to me.

Gain stage: I've said I'm struggling with this concept.
Av=10: Audio Volume? Presumably, but why is this number 10 here? Are we here suggesting that this amp is set at full volume? On the very first diagram? That seems odd, so here I'm puzzled. No explanation as to why 10 has been chosen.
Headroom 15v: If the amp is set at 10, now the word Headroom is introduced, so there seems to be a contradiction... because one wouldn't expect headroom (which I think implies clean sound before it distorts, typically say vol 5 or so) if the vol is said to be 10. Then, the cherry on the cake, a completely arbitrary "15v" is mentioned.

So I'm just 1/3rd across the first diagram, & I'm stuck as THIS ^^ is what my head's having to cope with, from info within just THE 1ST BOX, in the diagram.

So I cannot continue to glean ANY understanding, from the diagram, especially the "explanation/ title/ label" in text @bottom. Let alone another 7 diagrams.

SC

[nosaj]

Ok if 5 is too much then try to look at just one. If it appears everyone is flustered that is so. I hate to think at this stage of the game that you cannot just twiddle the knobs and understand what is going on.
If sound is like water pouring into a cup then whatever overfills the cup is distortion.
Are you capable of taking a class on electronics? I'm sure we can suggest a few or meybe you can read Jack Darr's book Repairing Guitar amplifiers.
nosaj

[Sea Chief]

Are you even trying?

Of course. I look at the first box, & look at the second. If I cannot comprehend -why- different numbers have been added to just TWO similar looking diagrams... it all remains utterly meaningless nosaj.

Unless I have someone to say for eg ' look at box 1, it says X there'.. 'now look at box 2, why do you think the number is different?' or some form of basic help..... the diagrams, numbers, symbols even, it's all a total blank to me.

One of these diagrams, would be difficult enough for me at this stage I'm at. But presenting 5 variations of one, plus 5 variations of another, when the variations have no explanation as to why they're different between the 5/ why they're even being presented at all... I'm completely bewildered.

Easy for you, easy to throw a demeaning image at me isn't it.

SC

[nosaj]

Are you even trying?

[Sea Chief]

Greg Robinson

Much appreciated. Unfortunately I immediately don't understand why there are numerous diagrams (A to E, & from F to H). I can only really understand the box (with a label such as preamp) & a box called poweramp, the guitar before, & the speaker last. Any variations of numbers within the boxes (or around them), I simply have no comprehension of, but I think again, you might possibly be continuing down the distortion avenue &/or gain stage avenue: both of which I've said I just don't understand. So I'm not quite sure how you think I am possibly able to understand any of what's presented here within the diagrams. Or can differentiate between one diagram to any other one.

Apologies. But I did say clearly where I am. I'm trying to understand how the volume control on the preamp, affects the power amp. G1 did a simple-looking 2-box diagram, which is about all I can cope with: it didn't have numeric additions, because g1 could see this wouldn't be of use, to me, at this early level of understanding.

Distortion is irrelevant for now. Gain stages are irrelevant for now. Both of these concepts are far in the future. I keep saying this, but I'm presented with stuff a mile ahead of my understanding. I still don't know why sometimes a 'vol knob' is called Gain, & sometimes Volume. Gain 'stage'??

Thanks though. SC

[Jazz P Bass]

Nice concept drawings, Greg.

[Greg Robinson]

Images failed to load, let me try again.

[Greg Robinson]

Ok, here's my last ditch effort. I've drawn some block diagrams, that represent "an amp", both with and without a master volume control. It is NOT representative of any particular design, it is highly simplified, numbers have been chosen for easy math, controls are all linear to further simplify, the power supply is assumed to be stiffer than any typical real world design to avoid sag. Distortion non-linearities have been simplified. Waveform graphics are not to scale and do not show inversion, they are meant only to convey the concepts as simply as possible.
There are diagrams A through H, each has a description in the image. A through E show a "traditional" design with no master volume, F through H show an added master volume.

I hope this helps you to understand Seachief.

[loudthud]

sorry but we're off on the distortion avenue again. I'm not possibly able to understand this aspect. I of course know that any non-master vol tube amp will 'naturally distort' at say Vol 6. How or why it does this, I cannot possibly understand yet.

I am still trying to understand how a volume on the preamp dictates the power amp. I cannot cope with anything more than this, at this stage.

Remember the questions I asked you back in posts 112 and 118 ? I don't think you know the answers, that's why you're asking these questions. Nobody can possibly explain the answers to you unless you understand analog electronics enough to answer my questions.

Here's a new question. Which resistor in the following schematic is the Volume Pot ?

[Sea Chief]

Hi chaps,

sorry but we're off on the distortion avenue again. I'm not possibly able to understand this aspect. I of course know that any non-master vol tube amp will 'naturally distort' at say Vol 6. How or why it does this, I cannot possibly understand yet.

I am still trying to understand how a volume on the preamp dictates the power amp. I cannot cope with anything more than this, at this stage.

I am at this stage (because HH landed a bombshell on me that the preamp vol controls the power amp) just trying to get the idea of this, basic fundmental Volume placement situation. I cannot consider distortion as well: this is now an "advanced lesson" to consider, in the future.

Forgetting distortion.

That is why I am again asking about the hifi example. NO it isn't meant to distort, & a guitar amp can do. But I am not considering or needing to consider distortion with regard to thinking about the hifi preamp + poweramp combination, as a way of helping me.

There IS a similarity. Both have preamps & poweramps, no? Both have an input typically of mV's. And both AFTER the power amp, exit into speakers, no?

Forgetting distortion.

So to say there is no crossover, for me, at this basic stage of still tryng to understand the two stages of an amp & the alien concept (to me) of the volume control being within the preamp.. seems bonkers.

Thanks, Capt