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

[Greg Robinson]

Anyway if I can go back to considering my hifi preamp/ poweramp, which seems to share the fundamental principle as guitar amps, that being a volume control not on the poweramp.. but located on the preamp: I need to get to grips with this idea, which still remains kinda alien to me.

Can you accept that the design intent and function of a guitar amp and a hifi are very different, and fixating on one to try to understand the other is flawed and will only serve to further confuse you?

Please stop comparing the two, they are not the same.

[Sea Chief]

Can you accept that the design intent and function of a guitar amp and a hifi are very different, and fixating on one to try to understand the other is flawed and will only serve to further confuse you?

Please stop comparing the two, they are not the same.

Ok, then this is a lost cause. I could only accept the idea of a volume control in the preamp, by considering what I listen to & use so often/ right in front of me, my hifi. If I see it has a volume control on the preamp, & awol on the power amp, & I'm told the same of a guitar amp.... but you say this comparison is flawed, when it's the only thing keeping me on board by way of a useful learning tool: then I have no possibility of understanding.

To me, this had seemed an ideal starting point regardless of distortion characteristics, or that it's Vol knob is more akin to a MV in terms of placement: at least the Vol knob is in the preamp. If I'd had an integrated amp instead I'd not see that it's vol knob was strictly within the preamp: which is similar to a guitar amp whereby I've not been able to see that it's strictly within the preamp.. because it's a single unit, akin to a single unit integrated hifi amp.

I'll ask on my hifi forum if anyone can explain this preamp vol thing, & this poweramp interaction, just to see if it dan help me with all this. Again, the distortion aspect, is null & void, with regard to the principle of simply trying to understand how a vol in the preamp, increaces & lowers volume, in the adjacent power amp. But I can't seem to get anyone to distance themselves from distortion, & so I'm told it's like comparing chalk & cheese. But how can it be? There has to be some basic correlation between these amps if both put a mV signal into a preamp, then some association happens with a poweramp, & then it's ultimately fed out into speakers. There just has to be.

SC

[Greg Robinson]

I think part of your confusion might be due to your aversion to loud sounds - I bet you've never turned the volume control on your Twin Reverb or AC30 up so high that you can hear the amp distort, am I right? EDIT: If you had, you would notice that the amp starts distorting BEFORE it reaches 10 on the dial. Probably even below 5 on the dial...

Because the output level of guitars can vary, and not just between different pickup types, but even just by how far away they are set from the strings (which also has tonal effects), ALL guitar amps have an "excess" of gain - this is to ensure that, regardless of the guitar it is used with, it can always reach the full advertised power output. Say you have a 100W amp, and it was designed so that it will reach full output with a signal of 1V at the input, what happens if your guitar only puts out only 100mV? You will not get 100W, you will only get 10W, which would be quite dissapointing. You wouldn't want to have to buy a booster pedal in order to get the full power of the amplifier would you?
Instead, guitar amplifiers are designed to reach full power from smaller signal levels, say 10mV *note 1.
So for a "hot" guitar with 1V signal level you need only turn the volume control up to something like 1 on the dial, and your 100mV guitar you reach full power at 5 on the volume dial. The amplifier can now reach full power with any guitar you might expect to use with it.

Note 1: This is only an example, not a description of any particular amp design, the actual number varies quite a bit depending on how much distortion the amplifier is designed to generate.

HiFi on the other hand, is dealing with a KNOWN QUANTITY. This is called LINE LEVEL. Your preamp is dealing with known sources - CD player, tape deck, AUX, Phono, Bluetooth, etc, which conform to line level. The preamp does the mixing/preconditioning before sending another LINE LEVEL signal out to the poweramp. So the range of the volume control on your hifi preamp doesn't need an "excess" of gain in the same way that a guitar amp does it should reach full power at 10 on the dial regardless of which source you are listening to. So the volume control MAY be placed later in the circuit than it can be in a guitar amplifier.

Anyway, still a flawed comparison, and I still assert that your fixation with comparing one to the other is holding you back.

[Delta362]

Your hi-fi power amp is designed so that it remains clean for all of the sound reproduction, regardless of the input. The volume pot in your hi-fi preamp just controls how large of a clean sounding signal gets passed to your clean sounding power amp. Neither is designed to provide any distortion.

In your guitar amp, the preamp volume adjusts how large the signal is near the front end of the circuit. Between that point and the start of your power amp are additional stages to get the signal even larger so that your power amp can reproduce that sound louder. In guitar amps, if the amp is vintage, then the preamp signal was typically designed to give a large clean signal to the power amp, but one that is larger than your power amp can reproduce cleanly. When that large preamp signal hits your power amp, it will overdrive the power amp and give you that vintage overdriven sound. If you back off your vintage amp's volume control, the power amp will see a smaller signal from the preamp that is not larger than it's clean input capability design.

In a more modern amp that might have, say two channels, one clean and one dirty, the clean channel would typically only have a single volume control (and tone controls). This allows the user to adjust the level of this to send a large clean signal to the power amp that was designed to not clip with this now larger clean input signal. You control the volume of the clean channel only with this volume pot, and the signal stays clean regardless of setting.

In the dirty channel, you might find a gain/overdrive/distortion pot as well as a volume control (and tone controls). The gain pot, typically early in the preamp circuit, would increase the signal to overdrive the next preamp stage to give you that distorted sound. At the end of the preamp, you typically find a volume or master volume pot for this channel. This will adjust the level of the overdriven signal that's going to your power amp. If it's set lower, you'll still have the distorted sound, but the volume will be quieter.

[loudthud]

Say you have a 100W amp, and it was designed so that it will reach full output with a signal of 1V at the input, what happens if your guitar only puts out only 100mV? You will not get 100W, you will only get 10W, which would be quite dissapointing.

The OP is confused enough, he doesn't need any more misinformation. Although right in theory, there's a little mistake in the numbers. Using the formula P=E²/R and the fact that 100W equals 20VRMS at 4 Ohms, the 1 Volt (RMS) signal would need an amp with a Voltage gain of 20 to produce 100W. If the signal is reduced to 100mV, that would produce 2VRMS (the Voltage gain remains the same) at the output which would be 2²/4 = 1 Watt .

Voltage Gain, usually abbreviated A_v , (Amplification (Voltage)) is what a preamp tube does to the signal. Although the signal at the output of a tube may be at a different impedance than the input, such differences are usually ignored in most cases. Note that a Cathode Follower is a special case, the Gain is very close to 1.

[Greg Robinson]

The OP is confused enough, he doesn't need any more misinformation. Although right in theory, there's a little mistake in the numbers. Using the formula P=E²/R and the fact that 100W equals 20VRMS at 4 Ohms, the 1 Volt (RMS) signal would need an amp with a Voltage gain of 20 to produce 100W. If the signal is reduced to 100mV, that would produce 2VRMS (the Voltage gain remains the same) at the output which would be 2²/4 = 1 Watt .

Quite right, I was lazy with my numbers there. Thanks for the correction.

[Helmholtz]

The Hifi I'm familiar with has a Volume control right after the selector switch where line level signals are selected.

You're right.
I remembered wrong, sorry.

[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

[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 ?

[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.

[Greg Robinson]

Images failed to load, let me try again.

[Jazz P Bass]

Nice concept drawings, Greg.

[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

[nosaj]

Are you even trying?

[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