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

[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]

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]

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]

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

[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.

[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?

[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.

[DrGonz78]

I just think of it as there is a noise floor. This is the non-audible level, silence and hum. So the floor represents 0 volts and that is with no signal applied. Headroom is what you have before it reaches clipping. (Did we say 15v is the max power?)Think of it as a limiter if you record or listen to music. It’s a form of compression that occurs when you run out of headroom. The truest sense of what is distortion occurs when this happens. So if have 30 volts of power supplied then I have 30 volts before clipping. Say it just that simple and think about it before thinking about it.

[Sea Chief]

Do you mean diagram A and diagram B?

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

Hi pdf, no I cannot compare diagram A to diagram B, because I cannot get past 1/3rd of the way into diagram A. I am effectively stuck part-way along diagram A.

I'm stuck specifically at: a white box, with Input Gain Stage as the first 3 words. This 15v is completely meaningless to me (& that's after two people have tried explaining it). You see 'headroom' is a concept about 2 light years ahead, if I'm still trying to figure how a preamp contains the volume control. On my amp. Both my amps in fact. I'm still stuck back here. At this ridiculously early point. HH landed this on me, last week- & I'm currently trying to get to grips with it.

So I can't possibly consider boxes containing 'headroom 15v', "noise-floors" (wtf?), oscilloscopes (I have no idea what these do), &, if I'm stuck so early on with diagram A which contains entirely new & mostly meaningless aspects to me, how I'm meant to 'compare' one diagram to another. Let alone then consider adding MV into the mix, or do sums to determine something.

I've said where I am in terms of understanding, & asked if the distortion aspect can just be sidelined for now. But with respect for the efforts, I'm then deluged not only with the whole gamut of distortion facets, but ontop, entirely new terms hitherto not once mentioned on the whole thread (headroom for eg).

I do have an extremely vague notion, that Greg is talking about the interaction between the guitar volume control, & the amp volume control. But this is light years ahead of where I've said I am. Also, I've never, once actually been in a situation wherby I can turn an amp to 10 & dial down the guitar volume, & consider headroom characteristics from simply experiencing it. Whenever I do turn an amp up towards 5-6, then dial a guitar volume down, the treble evaporates, so I have no choice but immediately dial it up again: I have asked why this is, & shown how to add a cap or something, but it's made no difference: the treble dissapears whenever I turn down the guitar vol. So in effect, I will never need to know this guitar vol + amp vol interaction, because (for another infathomable reason) the two simply do not work if the treble goes awol. Whatever the 'cleanliness' of the sound.

I think maybe Greg is possibly assuming that I can dial an amp to 10, then have a useable guitar situation whereby I can dial it down & hear different things: then possibly use a diagram to consider what is actually happening. But again, this is all to do with distortion aspects it seems. I'm still WAY BACK on the volume control placement !!

Thanks, SC

[Greg Robinson]

Ok, back to the very most basic thing.
DO YOU UNDERSTAND HOW A VOLUME CONTROL WORKS?
That was the whole thing I was trying to show you with my diagrams.
If you don't, this whole discussion has been rather pointless.

[Sea Chief]

.. INPUT signal it can handle is 1.5V because 1.5x10=15V. So if you were to feed 2V into that stage..

Hi Greg, ok 99% of your recent big post (which has this ^ section in it) I can't understand. Apologies again.

But I am sensing that perhaps you are attempting to explain the interaction with an amp vol (I'm guessing at vol 10, although av=10 is still not clear to me, because you explain this 10 figure in other ways besides amp volume 10 in this big post above).. & guitar vol.

Now, this section I've highlighted above, can you tell me: when you say "feed 2v into" are you here talking about the -guitar volume knob-?? "Feeding" hasn't been used before, so I have to consider what is feeding. To then get a picture of what your trying to put forward as an example here.

You put a figure of 100mV on the diagram, which when I asked what this arbitray (to me) figure was, someone answered that "it is the voltage of hitting the strings". But the figure of 1.5v, or 2v as mentioned above, are so far in excess of 100mv, that I cannot rationally equate it to having any connection to both your 1.5v, & 2v "input" figures above. So these two figures.... to me are a mystery not only as to their numerical figures, but also as to what they represent.

Thanks, SC

[DrGonz78]

So you’re essentially complaining about the fact that you are receiving too much help? I give up.

[Greg Robinson]

Apologies, I'm doing my best to contain my exasperation.

Seachief, do you understand how a potentiometer functions as a volume control? Do you understand that it is a voltage divider? That the guitar signal is a (varying/AC) voltage?
Ignore everything else for now. Can you just answer those three questions please.

[Sea Chief]

Ok, back to the very most basic thing.
DO YOU UNDERSTAND HOW A VOLUME CONTROL WORKS?
That was the whole thing I was trying to show you with my diagrams.
If you don't, this whole discussion has been rather pointless.

Hi Greg,

how to answer this? How do you mean? I'm tempted to answer thus..

Yes, I turn the knob on my hifi & the volume goes up. Or..
Yes, I turn the knob on my guitar valve amp, & the volume goes goes up.

Or am I to ask you: which volume are you referring to, the one on my guitar, or the the one on my amp?
Or am I to ask you: what amp Vol control are you referring to- the Gain knob, the Volume knob, or the Master Volume knob-?

If I asked this to my friend, who bizarrely has no interest in amps despite being in a band for decades & collecting electrics including a rickenbacker 360.. he'd say "how do you mean? Yes I twiddle this knob near the strings, or there's a Vol knob on my amplifier".

I'm the same, IE How do you mean-? Can you rephrase your question maybe?

SC

[Sea Chief]

So you’re essentially complaining about the fact that you are receiving too much help? I give up.

But you haven't helped gonzo!! That's your FIRST post on the thread!!

Strewth alive.

SC