Hi Guys,
Building my first SE hi-fi amp using 12AU7/EL84 combination. Now I've seen different ideas of the location of the volume control, at the input or between the stages, any basic advice out there, or could I use a preset pot on the input to prevent preamp overloading and the "real" volume control between the stages?
Sorry if this shows my ignorance as a newbie, but you guys have previously been very helpfull.
Steve
Building my first SE hi-fi amp using 12AU7/EL84 combination. Now I've seen different ideas of the location of the volume control, at the input or between the stages, any basic advice out there, or could I use a preset pot on the input to prevent preamp overloading and the "real" volume control between the stages?
Sorry if this shows my ignorance as a newbie, but you guys have previously been very helpfull.
Steve
With only 2 signal stages, noise from the 12AU7 should not be a problem. Therefore put the volume control at the input so you can never be unable to manage overdriving.
Unless you are using the two triodes of the 12AU7 in cascade i.e., you actually have three stages of amplification in the one channel. In that case, you should put the volume control between the first and second triode stages, and ensure that the signal source is small enough in level to not overdrive the first triode - a preset pot at the input is a good way to do this.
Or you can do what RCA used to do in the late 1950's in their highest quality stereos - use a 4-gang pot, so each channel gets controlled at both the input and after the first stage. This has significant benefits:-
a) can use linear pots - very good tracking
b) low noise AND freedom from uncontrolled overload in early stages.
but has a significant disadvantage - 4-gang pots are rare & expensive.
Unless you are using the two triodes of the 12AU7 in cascade i.e., you actually have three stages of amplification in the one channel. In that case, you should put the volume control between the first and second triode stages, and ensure that the signal source is small enough in level to not overdrive the first triode - a preset pot at the input is a good way to do this.
Or you can do what RCA used to do in the late 1950's in their highest quality stereos - use a 4-gang pot, so each channel gets controlled at both the input and after the first stage. This has significant benefits:-
a) can use linear pots - very good tracking
b) low noise AND freedom from uncontrolled overload in early stages.
but has a significant disadvantage - 4-gang pots are rare & expensive.
Alps has the 27114 “quad” potentiometers. As far as I can see they’re difficult to get (at least over here).
Regards, Gerrit
Regards, Gerrit
Nobody else has done it like RCA, for some reason. You can always find someone who does it completely different than others. What a good role model is, how did it the majority of high quality amp manufacturers and with what kind of benefits.
Therefore, the majority doesn't use a volume control in power amps at all, because the gain always has to be attenuated with a preamp. So no need.
Some asians later began, using a pot in the input of a power amp. This has several disadvantages. So if you want to use it (for whatever reasons), it has to be build inside the circuit. Right after the first gain stage. And it has to be logarithmic.
Therefore, the majority doesn't use a volume control in power amps at all, because the gain always has to be attenuated with a preamp. So no need.
Some asians later began, using a pot in the input of a power amp. This has several disadvantages. So if you want to use it (for whatever reasons), it has to be build inside the circuit. Right after the first gain stage. And it has to be logarithmic.
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Schmittz,
It seemed that as Stevie is building his first amp, and has asked a basic question, he may be planning a unit amp (controls and power stage in one unit) or is just building a simple amp to be driven direct from a programme source such as a tuner or CD player.
That would be why he wants a volume control.
I gave RCA as an example of a manufacturer who used distributed (4-gang) volume controls, as they were well known world-wide, but they were by no means the only one. Australian manufacturer AWA was another who used 4-gangs in this way. It was only done in tube products and discrete transistor amps of the highest quality, simply because it was expensive.
In the semiconductor age now, it's not generally done because low noise low distortion op-amps let you use linear pots in a feedback configuration and get the same low noise while avoiding overload advantage.
It seemed that as Stevie is building his first amp, and has asked a basic question, he may be planning a unit amp (controls and power stage in one unit) or is just building a simple amp to be driven direct from a programme source such as a tuner or CD player.
That would be why he wants a volume control.
I gave RCA as an example of a manufacturer who used distributed (4-gang) volume controls, as they were well known world-wide, but they were by no means the only one. Australian manufacturer AWA was another who used 4-gangs in this way. It was only done in tube products and discrete transistor amps of the highest quality, simply because it was expensive.
In the semiconductor age now, it's not generally done because low noise low distortion op-amps let you use linear pots in a feedback configuration and get the same low noise while avoiding overload advantage.
What is the 12AU7 / EL84 amp configuration?
One triode stage and one EL84 per channel?
Less gain.
Two triode stages of one 12AU7 and one EL84 per channel?
More gain.
Will the EL84 be in Pentode Mode; or Ultra Linear Mode, or Triode Mode?
Those are from High Gain, to Medium Gain, to Low gain respectively.
What will the output transformer primary and secondary impedance be.
8k and 8 Ohms is a voltage loss of 31.6:1
Less total gain of the amp.
3.5k and 8 Ohms is a voltage loss of 20.9:1
More total gain of the amp.
What will the amplifier be driving?
A sensitive set of Headphones?
A 104dB/Meter loudspeaker?
A 90dB/Meter loudspeaker?
Depending on all of the above, you may, or may not, hear any noise.
Even as importantly, depending on all of the above, and other factors too, you may or may not, hear any hum.
One triode stage and one EL84 per channel?
Less gain.
Two triode stages of one 12AU7 and one EL84 per channel?
More gain.
Will the EL84 be in Pentode Mode; or Ultra Linear Mode, or Triode Mode?
Those are from High Gain, to Medium Gain, to Low gain respectively.
What will the output transformer primary and secondary impedance be.
8k and 8 Ohms is a voltage loss of 31.6:1
Less total gain of the amp.
3.5k and 8 Ohms is a voltage loss of 20.9:1
More total gain of the amp.
What will the amplifier be driving?
A sensitive set of Headphones?
A 104dB/Meter loudspeaker?
A 90dB/Meter loudspeaker?
Depending on all of the above, you may, or may not, hear any noise.
Even as importantly, depending on all of the above, and other factors too, you may or may not, hear any hum.
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> different ideas of the location of the volume control
If you do not know any better, put the VC where the sensitivity of the next stage is 100mV for full output.
This is the obvious compromise between the hiss-level of typical amplifiers corrupting weak signals, and the headroom limit of typical amplifiers corrupting strong signals.
If you were working all with 3V circuits you would re-think this. However it seems valid for 30V transistors and 300V tubes.
Pot before AND after is also known from MacIntosh tube preamps. However this is not needed in general, especially for a first-build. Even my woefully expensive and fancy Pioneer didn't go that far. And most of my DIY domestic and studio gears were gain-trimmed in prototyping then one pot set in-use gain from a whisper to a roar.
If you do not know any better, put the VC where the sensitivity of the next stage is 100mV for full output.
This is the obvious compromise between the hiss-level of typical amplifiers corrupting weak signals, and the headroom limit of typical amplifiers corrupting strong signals.
If you were working all with 3V circuits you would re-think this. However it seems valid for 30V transistors and 300V tubes.
Pot before AND after is also known from MacIntosh tube preamps. However this is not needed in general, especially for a first-build. Even my woefully expensive and fancy Pioneer didn't go that far. And most of my DIY domestic and studio gears were gain-trimmed in prototyping then one pot set in-use gain from a whisper to a roar.
For a simple one 12AU7, two EL84 in Pentode mode there is no need for neither an input pot or one inside the amp's circuit. It should harmonize with all modern line output sources and thats a good thing because no energy will be wasted.
Of course, the preamp is the right place for adjusting the signal volume. Then, no problems occur. And the preamp has this volume pot not in the front or output of the circuit but in the middle, deep inside the circuit. Thats where it belongs.
Hey thanks guys, in answer to your questions, yes this is my first tube amp, so I decided to keep it simple and decided on a low power SE stereo amp to take in line input feeds and learn as I go.
I’m initially planning/ prototyping a simple one stage of preamp using 12AU7 and an EL84 output tube with 5k/8 ohm output transformers. Hopefully I can experiment with the components to get the correct sound and then look at improving it with a tone circuit/stack which I assume I may be able to use the other half of the 12AU7’s as cathode followers because of the low input impedance of the tone circuit ?
First though I need to get a good basic amp up and running and experiment/play/learn with it . Any advice is always welcome you guys, your experience could help me a lot
Steve
I’m initially planning/ prototyping a simple one stage of preamp using 12AU7 and an EL84 output tube with 5k/8 ohm output transformers. Hopefully I can experiment with the components to get the correct sound and then look at improving it with a tone circuit/stack which I assume I may be able to use the other half of the 12AU7’s as cathode followers because of the low input impedance of the tone circuit ?
First though I need to get a good basic amp up and running and experiment/play/learn with it . Any advice is always welcome you guys, your experience could help me a lot
Steve
What do you mean by "tone circuit/stack"?
Is this an Electric Guitar Amp?
Or, is this a Hi Fi Stereo Amp?
Those are on two different planets.
What are your signal sources that you need an input switch for?
CD player?
TV?
AM/FM Tuner?
Phono Preamp?
Electric Guitar?
Microphone?
Other?
Is this an Electric Guitar Amp?
Or, is this a Hi Fi Stereo Amp?
Those are on two different planets.
What are your signal sources that you need an input switch for?
CD player?
TV?
AM/FM Tuner?
Phono Preamp?
Electric Guitar?
Microphone?
Other?
First amplifier tone circuit.
Hi 6A3sUMMER,
The term tone stack has been (incorrectly) picked up from my general reading/learning of tube amps in general. This is a simple first attempt at a hifi amp i'm trying to build with input source I'm looking at is a CD player deck. The suggestion I had in mind was a simple lo/mid/hi 3 way tone control section with 3 separate pots. just for personal choice of the overall sound nd as an addition once I get something up and running.
Hi 6A3sUMMER,
The term tone stack has been (incorrectly) picked up from my general reading/learning of tube amps in general. This is a simple first attempt at a hifi amp i'm trying to build with input source I'm looking at is a CD player deck. The suggestion I had in mind was a simple lo/mid/hi 3 way tone control section with 3 separate pots. just for personal choice of the overall sound nd as an addition once I get something up and running.
In that case, since the CD player won't have a volume control (it may have an output level preset that should not be as a volume control [its meant to be set once and left, so routine use as a volume control will wear it out], though such presets are unusual.) you should install a volume control pot right at the input. Not only will this guarantee you can avoid overdrive, it means you can try global negative feedback, should you wish to at some stage.
By "3 separate pots" I assume you mean bass, treble and volume, and not bass, presence, and treble as used in guitar amplifiers.
If I were you, when you are ready to put the unused triode stage in each 12AU7 to use, I would use it to build a Baxandall tone control circuit. Or a traditional passive tone control with the added triode stage configured as a low gain amplifier. Then you can compensate for listening room characteristics and/or compensate for any loudspeaker deficiencies.
Cathode followers are not normally needed after tone control stages when integrated with the power amp stages, and cathode followers have some traps for the inexperienced. And for those without an oscilloscope to see just what is happening.
By "3 separate pots" I assume you mean bass, treble and volume, and not bass, presence, and treble as used in guitar amplifiers.
If I were you, when you are ready to put the unused triode stage in each 12AU7 to use, I would use it to build a Baxandall tone control circuit. Or a traditional passive tone control with the added triode stage configured as a low gain amplifier. Then you can compensate for listening room characteristics and/or compensate for any loudspeaker deficiencies.
Cathode followers are not normally needed after tone control stages when integrated with the power amp stages, and cathode followers have some traps for the inexperienced. And for those without an oscilloscope to see just what is happening.
Great advice Keit, thanks for that,
I have been looking at Global feedback as something to play with as part of my design/learning process and I was thinking Volume, bass, middle and treble as the controls, and have read of the Baxandall tone control circuit so I'll look at this again and try to find a circuit as you suggested using the other triode in the 12AU7's.
At the moment I have a rather unsightly prototype single channel mock up to play with, and am lucky enough to have test gear including 'scopes, signal sources and meters available to me as an Electronics engineer. (Just lacking Tube/Valve experience !)
No doubt I'll have many more questions as I develop this, really the whole point of this is to learn as I go along with the design rather than simply copy someone else's design or just buy a kit. I think I'll have a much greater understanding at the end, even if I make a few mistakes along the way.
Steve
I have been looking at Global feedback as something to play with as part of my design/learning process and I was thinking Volume, bass, middle and treble as the controls, and have read of the Baxandall tone control circuit so I'll look at this again and try to find a circuit as you suggested using the other triode in the 12AU7's.
At the moment I have a rather unsightly prototype single channel mock up to play with, and am lucky enough to have test gear including 'scopes, signal sources and meters available to me as an Electronics engineer. (Just lacking Tube/Valve experience !)
No doubt I'll have many more questions as I develop this, really the whole point of this is to learn as I go along with the design rather than simply copy someone else's design or just buy a kit. I think I'll have a much greater understanding at the end, even if I make a few mistakes along the way.
Steve
Stevie, sounds like you have the right idea - one learns much more by building, testing, and titivating with test instruments to give you the facts, than by reading a whole lot of books before building anything. And it's more fun!
A tip:-
As I said, three tone controls are unusual in hi-fi audio, but common in guitar amps. One reason is cost, another reason is to keep audio simple, so the proverbial dumb housewife, or dumb husband, can understand what the controls do. (My elderly uncle thought the bass knob was a volume control, and couldn't understand why his music just never sounded right) That won't apply to you as you built it - you will know exactly what they do.
So, what this means is that if you read up on Baxandall circuits in audio magazines, online sources, and audio books, you'll only see circuits with bass and treble controls. Look in guitar amp references, there you will see Baxandall (and other) circuits adapted to provide bass, middle (which is usually called presence) and treble. Usually guitar amp circuits have the cross-over frequencies lower than is desirable for high quality audio playback. You can fix this by just reducing the capacitor sizes 20 to 30%.
Most audio folk regard 3 tone controls unnecessary, but it's your amp, your choice. Some audio purists don't want ANY tone controls. Providing the sources of music are impeccable, and speakers are impeccable (a very nearly impossible thing to achieve at any reasonable cost) there is some merit in their argument. But don't listen to them - do want you want and enjoy and learn. I mention this because you will get advice to the contrary.
A tip:-
As I said, three tone controls are unusual in hi-fi audio, but common in guitar amps. One reason is cost, another reason is to keep audio simple, so the proverbial dumb housewife, or dumb husband, can understand what the controls do. (My elderly uncle thought the bass knob was a volume control, and couldn't understand why his music just never sounded right) That won't apply to you as you built it - you will know exactly what they do.
So, what this means is that if you read up on Baxandall circuits in audio magazines, online sources, and audio books, you'll only see circuits with bass and treble controls. Look in guitar amp references, there you will see Baxandall (and other) circuits adapted to provide bass, middle (which is usually called presence) and treble. Usually guitar amp circuits have the cross-over frequencies lower than is desirable for high quality audio playback. You can fix this by just reducing the capacitor sizes 20 to 30%.
Most audio folk regard 3 tone controls unnecessary, but it's your amp, your choice. Some audio purists don't want ANY tone controls. Providing the sources of music are impeccable, and speakers are impeccable (a very nearly impossible thing to achieve at any reasonable cost) there is some merit in their argument. But don't listen to them - do want you want and enjoy and learn. I mention this because you will get advice to the contrary.
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Stevie,
Just one more thing: While you are not using one of the triodes in a 12AU7 (or any twin tube), ground the unused cathode, grid and anode. Strange subtle effects can occur in tubes when unused parts are not connected to anything. Not all electrons from a cathode end up in its proper destination (anode, or screen if its a pentode). A small number land on other places, and those places if not connected to anything can build up quite a charge, as electron velocity is a statistical parameter.
Also, if the heater is activated, there will be a small current circulating in the unused triode via the grounded pins, which will keep the cathode healthy. Cathodes can loose emission if the heater is in operation but no or very little cathode current flows. This was a problem that was solved in the early 1950's by using ultra-pure nickel in the cathode sleeves. But these days, a lot of new twin triodes come from China, and I just don't trust their commitment to quality.
Just one more thing: While you are not using one of the triodes in a 12AU7 (or any twin tube), ground the unused cathode, grid and anode. Strange subtle effects can occur in tubes when unused parts are not connected to anything. Not all electrons from a cathode end up in its proper destination (anode, or screen if its a pentode). A small number land on other places, and those places if not connected to anything can build up quite a charge, as electron velocity is a statistical parameter.
Also, if the heater is activated, there will be a small current circulating in the unused triode via the grounded pins, which will keep the cathode healthy. Cathodes can loose emission if the heater is in operation but no or very little cathode current flows. This was a problem that was solved in the early 1950's by using ultra-pure nickel in the cathode sleeves. But these days, a lot of new twin triodes come from China, and I just don't trust their commitment to quality.
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