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

[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.

[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.

[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.

[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.

[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.

[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]

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]

Greg Robinson Thanks, again parts of this I understand.

But thing is with guitars, I don't see wildly varying inputs. I see the opposite, single coils & humbuckers, p90's in the middle lets say. 3 positions as it were, rather like the ( limited) settings on a tele pickup selector. Limited, vs, wildly different: so it seems I'm still kinda on the opposite side of the fence in terms of understanding the very basics here, too. I also don't find huge difference between a tele & humbuckers- only a slight difference, from experience.

The distortion pedal thing/ aspect.. is far in the future of comprehension for me. I didn't really want to consider this aspect (as the whole idea, of the core of the thread, was my striving to -eliminate- distortion pedals: I find the ones I can afford lifeless, tone-draining & without any natural feel).

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.

Thanks alot, SC

[Greg Robinson]

I've still got to get to grips with " As distortion depends on signal level, it makes sense that the (first or only) volume pot sits before all amp stages that contribute to distortion. " But at least I can consider the other sentences before & after this one, & these might click with me.

Different guitars have wildly different output levels - a guitar with "hot" humbuckers will need less gain/lower volume control setting to achieve the same level of distortion/volume than a guitar with weak single coils. So, the amp has the volume/gain control immediately after the first stage *note 1 where it has the most control over what signal level the rest of the preamp/poweramp "sees" *note 2.

Note 1: Which typically stays clean unless overdriven with a booster pedal or similar.
Note 2: There are amps where the volume control is before the first gain stage, right at the input, but they are an exception, and this has further consequences re loading on passive guitars and source impedance/miller capacitance effects that result in loss of highs, but that's a different discussion.

At the moment you see, I'm immediately asking why (if distortion is totally dependent on signal level) all distortion 'dials' aren't at the guitar start of proceedings. I know of booster pedals, which do this exact idea, but they're like not so frequently used. So, if a booster more commonly isn't used to affect signal, how can a guitar signal go over what the guitar vol pot stipulates without a booster?

Overdriving the first stage with a booster will have different tonal effects than the first stage overdriving the second/subsequent preamp stages. Much in the same way that overdriving the poweramp by turning up the volume control on a traditional amp design vs turning up the gain/volume control and master volume down in a master volume amp will sound different.

Chuck alluded to this earlier by mentioning "gain staging". Gain staging is the art of carefully arranging where and how distortion is achieved for the most pleasing outcome. For instance, if you have two different distortion/overdrive pedals, which order you run them in will have different results. This is like order of operations in mathematics. If we simply went left-to-right for the equation 1+2x3= the answer would 9, however if we observe the correct order of operations with multiplication taking precedence over addition, the answer is 7. The SEQUENCE in which we do things matters.

[Sea Chief]

Summary for guitar amps:

Every guitar amp has a preamp volume in the front part of the preamp.
This volume adjusts the signal level fed to all following stages.
As distortion depends on signal level, it makes sense that the (first or only) volume pot sits before all amp stages that contribute to distortion.
This wiring gives the preamp volume the best control of clean or distorted sounds.

Hifi is different.
The volume typically is at the output of the preamp.
So it's more like the master volume of a guitar amp.

Hi HH.

Ok thanks for this: this is kinda what I've needed to know since your bombshell that the Vol only sits in the preamp.

I've still got to get to grips with " As distortion depends on signal level, it makes sense that the (first or only) volume pot sits before all amp stages that contribute to distortion. " But at least I can consider the other sentences before & after this one, & these might click with me.

At the moment you see, I'm immediately asking why (if distortion is totally dependent on signal level) all distortion 'dials' aren't at the guitar start of proceedings. I know of booster pedals, which do this exact idea, but they're like not so frequently used. So, if a booster more commonly isn't used to affect signal, how can a guitar signal go over what the guitar vol pot stipulates without a booster?

Capt

[loudthud]

Hifi is different.
The volume typically is at the output of the preamp.
So it's more like the master volume of a guitar amp.

The Hifi I'm familiar with has a Volume control right after the selector switch where line level signals are selected. This allows any signals that are too loud to be attenuated. If so equipped with a phono preamp, the output of that would also be selected at the switch. Next inline would be a buffer stage and tone controls. Lastly, a stage capable of driving a cable of moderate length and a power amp with an input impedance as low as 10K Ohms.

[Helmholtz]

Summary for guitar amps:

Every guitar amp has a preamp volume in the front part of the preamp.
This volume adjusts the signal level fed to all following stages.
As distortion depends on signal level, it makes sense that the (first or only) volume pot sits before all amp stages that contribute to distortion.
This wiring gives the preamp volume the best control of clean or distorted sounds.

Hifi is different.
The volume typically is at the output of the preamp.
So it's more like the master volume of a guitar amp.

[Sea Chief]

My point was that in the same way that you can control the flow of water at your tap, or at the main shutoff valve, but if the main valve is closed, the tap no longer has control, the same applies to, in order: your guitar volume control, the volume/gain control in the amp, and finally any "master volume" - if present. If you turn the guitar volume all the way down, it doesn't matter what you set the other controls too - there's no signal to amplify, it's cut off at the guitar. Similarly, if you turn down the volume/gain control (which is traditionally immediately after the input stage of the preamp, and then is followed by the rest of the preamp), there's no signal for the power amp to amplify. And again, if you turn down the master volume, which is between the pre amp and power amp, the power amp can't amplify that now cut off signal.

Hi Greg, ok thanks. This is forming a vague, fuzzy semblence of sense to me. I do need to read it about 20x until my heads hurts though.

I'm still having trouble with this signal chain thing, presuming the signal goes from the preamp into the power amp, then out to the speakers: I still can't see any logic to not having a single volume control at the end of this chain, before the speakers. Which is, until the recent bombshell hit me 2 days ago, what I have always thought (so shifting this idea, after 40 years, is not gonna happen fast).

Again I am asking this question. My hifi has two boxes, a Preamp box & a Poweramp box. The input goes into the preamp (Turntable) & -something/ signal?- goes out to the speakers. I have one Volume control on the preamp box. I do not have a Volume control on my Poweramp box.

Putting aside the aspect of distortion (which is now sidelined as a subject, until I understand the Volume thing): is this two-box hifi a good example to use, for me to try & understand this preamp volume thing? If it is, I can ask on my hifi forum (where my modded preamp has many likes indeed, even my capacitor choices are followed by folks FAR more knowledgeable than me) about this preamp volume aspect. THEN I can come back & possibly understand distortion: 'power amp clipping before preamp' is so illogical in principle to me & impossible to understand at this point. I have no clue on earth why this should be so, or what significance it has to anything.

Thanks, SC

[Greg Robinson]

No, your only making me go more mental, Greg. I can see the analogy of a tap/ water pipe is useful for explaining electricity (current etc), but as an analogy for a far more complicated guitar amp of two distinct sections you plug something into with a vol knob on, I'm not with you (why would you need this extra one? I've no idea why: sorry I'm really not on board with your point).

SC

My point was that in the same way that you can control the flow of water at your tap, or at the main shutoff valve, but if the main valve is closed, the tap no longer has control, the same applies to, in order: your guitar volume control, the volume/gain control in the amp, and finally any "master volume" - if present. If you turn the guitar volume all the way down, it doesn't matter what you set the other controls too - there's no signal to amplify, it's cut off at the guitar. Similarly, if you turn down the volume/gain control (which is traditionally immediately after the input stage of the preamp, and then is followed by the rest of the preamp), there's no signal for the power amp to amplify. And again, if you turn down the master volume, which is between the pre amp and power amp, the power amp can't amplify that now cut off signal.

[Chuck H]

Oh I know. I just picked an amp everyone on the forum could identify with for ridiculous gain.

EDIT: In fact I'm working on a design right now that uses only 2x12ax7's and 2xel84's (four small bottles) and achieves modern tone and gain levels but morphes into a classic vintage amp tone when turned below 5. Well past what the Orange Tiny Terror did. It behaves more like a vintage amp below 5 on the volume control but ramps into metal territory as you get to 10. This required using multi gang pots to change other circuit parameters as the volume is adjusted. So far so good but it's only in a CAD right now. No idea when I'll get to a prototype but it looks REAL promising.