I would like to mention that this schematic will be a noisy one. Because of the 12ax7 being so high in gain i recommend a different approach. There are 4 things to account for in an RIAA preamp:
1 - LOW noise.
2 - Good RIAA curve, passive equalization is better but B+ must be somewhat regulated to be more accurate if this is to be used instead of negative feedback.
3 - A reasonable output impedance.
4 - Enough gain depending on what cartridge you are using.
I have tried to build a simple 12ax7 phono preamp in the past similar to this schematic and i really recommend using parallel triodes for the first stage followed by an SRPP configuration at the end to achieve a bit more gain and to get an output impedance roughly around 10k instead of 60k in a conventional 2 stage 12ax7 design. Using parallel triodes for the first stage greatly lowers the noise floor.
Regards
Here it is, we have entered into the year 2010. And no one has answered or referred to your original question or request. A Phono PreAmp using a 12AU7.
They all speak about the 12AX7 or some other tube. And not the one that you are interested in using. The 12AU7.
I regret that I was not a part of the DIY Audio earlier to assist. I was a Aerospace Electronic Engineer for Lockheed Martin. As well, I worked for other companies designing and building military tube electronic equipment in the 1950's and 60's.
so I have a great deal of experience with tubes and their use in circuits.
So lets get started on that PreAmp, if anyone wants to know.
The 12AU7 is an excellent tube for Phono PreAmp use. In fact, it is better than the 12AX7 and any of the others that most audio expects babble about.
In my day we used the 6SN7/12AU7, but they are the same. The problem is gain. You must over come the gain issue to use it with the MM cartridge. And to do that with a 12AU7 or a 6SN7, you can not use passive RIAA. It must be active. You will lose gain in a passive circuit. You must use active.
the Phono PreAmp is made up of 2.6 to 2.85 gain stages. That is two common cathode and one cathode follower stage. The RIAA is one of the clever circuits we came up with. It covers the entire LP range. It is only three components. Two caps and a resistor.
So you will need three 12AU7's for a stereo phono preamp.
Power Supply is very simple as well. Two 12volt 2amp transformers connected reverse. and two transistors, two or three high voltage zeners and some resistors and caps. for the regulator.
I will post the drawing soon. It is a dandy.
Take Care
Ivey.
They all speak about the 12AX7 or some other tube. And not the one that you are interested in using. The 12AU7.
I regret that I was not a part of the DIY Audio earlier to assist. I was a Aerospace Electronic Engineer for Lockheed Martin. As well, I worked for other companies designing and building military tube electronic equipment in the 1950's and 60's.
so I have a great deal of experience with tubes and their use in circuits.
So lets get started on that PreAmp, if anyone wants to know.
The 12AU7 is an excellent tube for Phono PreAmp use. In fact, it is better than the 12AX7 and any of the others that most audio expects babble about.
In my day we used the 6SN7/12AU7, but they are the same. The problem is gain. You must over come the gain issue to use it with the MM cartridge. And to do that with a 12AU7 or a 6SN7, you can not use passive RIAA. It must be active. You will lose gain in a passive circuit. You must use active.
the Phono PreAmp is made up of 2.6 to 2.85 gain stages. That is two common cathode and one cathode follower stage. The RIAA is one of the clever circuits we came up with. It covers the entire LP range. It is only three components. Two caps and a resistor.
So you will need three 12AU7's for a stereo phono preamp.
Power Supply is very simple as well. Two 12volt 2amp transformers connected reverse. and two transistors, two or three high voltage zeners and some resistors and caps. for the regulator.
I will post the drawing soon. It is a dandy.
Take Care
Ivey.
Ivey:
I just joined diyAudio e few minutes ago, and found your thread. It reminds me 40 years ago when I used to work with tubes in constructing my transmitter for amateur radio (when I was in high school). Well, I am interested in Phono pre-Amp, Please send me also the diagram and inform all things needed (i.e. three 12AU7's, power Supply two 12volt 2amp transformers connected reverse (how to do it?). and two transistors (what transistors and how is the connection), two or three high voltage zeners (what is this?), and some resistors and caps (pls specify the specifications). Thanks.
I just joined diyAudio e few minutes ago, and found your thread. It reminds me 40 years ago when I used to work with tubes in constructing my transmitter for amateur radio (when I was in high school). Well, I am interested in Phono pre-Amp, Please send me also the diagram and inform all things needed (i.e. three 12AU7's, power Supply two 12volt 2amp transformers connected reverse (how to do it?). and two transistors (what transistors and how is the connection), two or three high voltage zeners (what is this?), and some resistors and caps (pls specify the specifications). Thanks.
With all respect, the 12AU7 and 6SN7 are different "animals". Yes, μ (20) and operating conditions are "the same". However, linearity is not. The 6SN7 is beautifully linear, while the 12AU7 is downright ugly. I've said it before and I'll say it again, restrict the 12AU7 to cathode follower and "concertina" phase splitter duty, in HIFI service.
It might be possible to use passive EQ and 6SN7 family tubes in a RIAA preamp. At 2.6 mA./V., gm is reasonably high. A good deal of gain could be obtained from cascoded sections DC coupled to cathode followers. A pair of such gain blocks, with the EQ network between them, seems feasible. The $64 question of "What's the S/N ratio?" hovers in the air.
It might be possible to use passive EQ and 6SN7 family tubes in a RIAA preamp. At 2.6 mA./V., gm is reasonably high. A good deal of gain could be obtained from cascoded sections DC coupled to cathode followers. A pair of such gain blocks, with the EQ network between them, seems feasible. The $64 question of "What's the S/N ratio?" hovers in the air.
Ebw
I am also a Ham Radio operator too. I do not do much hamming since I moved from Hawaii to California.
In Hawaii, I worked with the government on cold war electronic projects.
I was first licensed in 1958. But I re-licensed again in 1978, and I still have my call sign. WH6AED.
I built great ham equipment back then. Grounded Grids 811A's linears, 3 tube superhet reciever, a 12BY7/807 35 watt transmitter, and a mono quad loop antenna for 20 m. But as soon as I finish putting together my shack outside. I will be on the air again, this time as a QRP operator.
Ok., I am sending to you the 12AU7 phono preamp ckt. Its output is about 1volt. Feedng that into a line amp; should give a good smooth/warm tone output. A good 12AT7/12AX7 line amp would do handsomely.
This preamp is as simple and as clean as we could make it back then. Using the most cheapest parts. Since 12AU7's back in the 60's went for 79 to 85 cent new and far less used.
Some people used the 5814's and stated that it sounded better. But it is all up to the person listening to it.
The power supply uses two 220 volt pri. 12 volt/2amp sec. transformers, wired sec. to sec.. The transistors are MPSA42 and MJE13007. I used the MJE13007 because I used it so much in switching psu and linear converter.
But any high voltage transistors can be used. Even a TIP49/50.
Keep your wire runs tight. Use shield mic cable for signal paths and a good star ground.
Your S/N should be acceptable. And may even better if you are using a good cartridge. Good channel separation in your cartridge aids in S/N out come as well.
The ckt. is in CircuitMaker 2000 format.
Take Care
Ivey
I am also a Ham Radio operator too. I do not do much hamming since I moved from Hawaii to California.
In Hawaii, I worked with the government on cold war electronic projects.
I was first licensed in 1958. But I re-licensed again in 1978, and I still have my call sign. WH6AED.
I built great ham equipment back then. Grounded Grids 811A's linears, 3 tube superhet reciever, a 12BY7/807 35 watt transmitter, and a mono quad loop antenna for 20 m. But as soon as I finish putting together my shack outside. I will be on the air again, this time as a QRP operator.
Ok., I am sending to you the 12AU7 phono preamp ckt. Its output is about 1volt. Feedng that into a line amp; should give a good smooth/warm tone output. A good 12AT7/12AX7 line amp would do handsomely.
This preamp is as simple and as clean as we could make it back then. Using the most cheapest parts. Since 12AU7's back in the 60's went for 79 to 85 cent new and far less used.
Some people used the 5814's and stated that it sounded better. But it is all up to the person listening to it.
The power supply uses two 220 volt pri. 12 volt/2amp sec. transformers, wired sec. to sec.. The transistors are MPSA42 and MJE13007. I used the MJE13007 because I used it so much in switching psu and linear converter.
But any high voltage transistors can be used. Even a TIP49/50.
Keep your wire runs tight. Use shield mic cable for signal paths and a good star ground.
Your S/N should be acceptable. And may even better if you are using a good cartridge. Good channel separation in your cartridge aids in S/N out come as well.
The ckt. is in CircuitMaker 2000 format.
Take Care
Ivey
EF86 schematic
Chris:
The EF86 schematic of the preamp. Is it the one from Audio Mag 1958/59 or the 1961 edition.
Also there is a similar one from Quad and Mullard. Thomas and Neel, friends of mine in England, helped design those. They are High gain units and very, very good. Also a German, designed one on the same profile.
In Hawaii, in 1973, I meet a Japanese gentlemen that design one on the same profile, using 6AU6's.
If it is the same profile, I would love to view it. To see if there are any changes or improvements worth noting.
Remember: all changes.., are not always improvements. Just changes.
But if you could post it. A O.K.
Take Care
Ivey
Chris:
The EF86 schematic of the preamp. Is it the one from Audio Mag 1958/59 or the 1961 edition.
Also there is a similar one from Quad and Mullard. Thomas and Neel, friends of mine in England, helped design those. They are High gain units and very, very good. Also a German, designed one on the same profile.
In Hawaii, in 1973, I meet a Japanese gentlemen that design one on the same profile, using 6AU6's.
If it is the same profile, I would love to view it. To see if there are any changes or improvements worth noting.
Remember: all changes.., are not always improvements. Just changes.
But if you could post it. A O.K.
Take Care
Ivey
5703wa/wb uhf tube
Ed:
My god.., you have those!!!
We used those in aircraft radios, 24 volt ac /400 hz converters, gyro units, and host of other things. First built by Tung Sol. They stop making them in 1974.
They are limited to about 120-150 volts for any good use. Even thou they are rated for more. About 2.5 watts out. A good triode.
First things first. Do you have the sockets? If not, then you can clamp them to a piece of aluminum standing up and hard wire them. Isolate the aluminum stand from the chassis, with mica. But place the tube direcetly to the aluminum for cooling.
A real good schematic for that tube is the Futterman NCP-1. Just do not overdrive the tube.
Do not overdrive them by designing to close their class a cut off limits, they distort ease. Ramp up the signal using multi stages; about three. No, try two at first. Maybe even cascode. Scope and listen to your output. Then use a line amp to complete the preamp profile
Take Care
Ivey.
Ed:
My god.., you have those!!!
We used those in aircraft radios, 24 volt ac /400 hz converters, gyro units, and host of other things. First built by Tung Sol. They stop making them in 1974.
They are limited to about 120-150 volts for any good use. Even thou they are rated for more. About 2.5 watts out. A good triode.
First things first. Do you have the sockets? If not, then you can clamp them to a piece of aluminum standing up and hard wire them. Isolate the aluminum stand from the chassis, with mica. But place the tube direcetly to the aluminum for cooling.
A real good schematic for that tube is the Futterman NCP-1. Just do not overdrive the tube.
Do not overdrive them by designing to close their class a cut off limits, they distort ease. Ramp up the signal using multi stages; about three. No, try two at first. Maybe even cascode. Scope and listen to your output. Then use a line amp to complete the preamp profile
Take Care
Ivey.
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Either troll or sock puppet. Going on about 12AU7 and bipolar MC phono stages in the Tube forum is pretty spectacular.
6SN7 vs 12AU7 or 6AG7 vs 6CB6
The difference between those who know and those who don't. Is enough to fill the voids of space.
My parents were poor. But they were tough. They did not have an education pass high school, because there was no funds to pay for it in 1926.
But they taught me good lessons, of how not to judge people incorrectly. And not to say bad things about them. Because I have never and will never walk in their foot steps.
But my parents insured that I got the education that they did not. Yet one of the most important things that they taught me was that "all men are created equal".
When the normal was to treat Minorities, Polish, and Eastern Europeans like trash in the area that we lived. They taught me that every man, woman, and child. Has a value in the eyes of our Creater. And that I will do as our Creater, has taught us.
Please do not insult me. I have not done so..., to you. Respect me, and I will always respect you. Because if we were all as smart as we think we are. We would be visiting other planets.
Lessons on Tubes and nuclear science. Common sense.
As an Amateur Radio Operator. The men of the "Gold Keys" taught me simple lessons in nuclear science.
If you turn the water on full blast, and put a large pot under it to catch the water. The water splash will be small. A smaller pot will give a greater water splash. Why? Because the area is smaller and can not contain the force of the water; therefore, it will fill up faster.
That is one of the difference between the 6SN7 and 12AU7. The plate and grid in the 12AU7 are smaller than the plates in the 6SN7. So to reduce the pool ball affect (nuclear scattering). You must not drive the 12AU7, as you would the 6SN7.
The designers knew that by mini sizing the 6SN7 down to the 12AU7. There could be room available to add an additional 12AU7, to step up the required voltage or signals.
It would be an error in most cases to directly replace a 6SN7 with 12AU7, without first checking the drive and output requirements of the circuit. Plus staying within the limits of the tube itself.
But in some cases, many persons have done just that.
In Amateur Radio, when the 6CB6 (by GE) started replacing the 6AG7. We learned that lesson.
Electronics is like anything else. It is in the hands of the designer. Facts and figures do not matter at times. It is the engineer/designer that controls the overall outcome.
Take Care
Ivey
The difference between those who know and those who don't. Is enough to fill the voids of space.
My parents were poor. But they were tough. They did not have an education pass high school, because there was no funds to pay for it in 1926.
But they taught me good lessons, of how not to judge people incorrectly. And not to say bad things about them. Because I have never and will never walk in their foot steps.
But my parents insured that I got the education that they did not. Yet one of the most important things that they taught me was that "all men are created equal".
When the normal was to treat Minorities, Polish, and Eastern Europeans like trash in the area that we lived. They taught me that every man, woman, and child. Has a value in the eyes of our Creater. And that I will do as our Creater, has taught us.
Please do not insult me. I have not done so..., to you. Respect me, and I will always respect you. Because if we were all as smart as we think we are. We would be visiting other planets.
Lessons on Tubes and nuclear science. Common sense.
As an Amateur Radio Operator. The men of the "Gold Keys" taught me simple lessons in nuclear science.
If you turn the water on full blast, and put a large pot under it to catch the water. The water splash will be small. A smaller pot will give a greater water splash. Why? Because the area is smaller and can not contain the force of the water; therefore, it will fill up faster.
That is one of the difference between the 6SN7 and 12AU7. The plate and grid in the 12AU7 are smaller than the plates in the 6SN7. So to reduce the pool ball affect (nuclear scattering). You must not drive the 12AU7, as you would the 6SN7.
The designers knew that by mini sizing the 6SN7 down to the 12AU7. There could be room available to add an additional 12AU7, to step up the required voltage or signals.
It would be an error in most cases to directly replace a 6SN7 with 12AU7, without first checking the drive and output requirements of the circuit. Plus staying within the limits of the tube itself.
But in some cases, many persons have done just that.
In Amateur Radio, when the 6CB6 (by GE) started replacing the 6AG7. We learned that lesson.
Electronics is like anything else. It is in the hands of the designer. Facts and figures do not matter at times. It is the engineer/designer that controls the overall outcome.
Take Care
Ivey
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Dear Ivey, Sorry for coming back late, but wow! I never expect to meet radio hammer in this forum. It was around 1966-1969, when I was young. It was in Indonesia. You can imagine how difficult to get parts during that time in my country, so, market where they sell parts from canibalized old radio or amplifier was a place where I got my parts. I used 6V6 and 807, but I used 80m band, using only simple L half-lambda antenna. But, I am shy to say that I do not have the background theory, just try an error. My call-sign was YC2EE, but when I moved to other city in 1970, I could not install my transmitter, so never active since then, and I was realized latter that my call-sign has been used by other person, but I never bother that.
Thanks for sending me the chart and explanation. I cannot open the attachment A converters is missing in my Words program when it tried to open the attachment. I will figure it out. Thanks much for your response, really appreciate it!
Thanks for sending me the chart and explanation. I cannot open the attachment A converters is missing in my Words program when it tried to open the attachment. I will figure it out. Thanks much for your response, really appreciate it!
Circuit for the 12AU7 Phono PerAmp.
I thought that I had posted the circuit, and I did, only on the wrong thread.
How embarrassing!
I said all those words, and failed to properly post the circuit.
Well here it is, and it includes the power supply and the 12AT7 line amp.
A good passive tonal circuit (to the line amp) could be added or you could just leave it "flat".
Take Care
Ivey
I thought that I had posted the circuit, and I did, only on the wrong thread.
How embarrassing!
I said all those words, and failed to properly post the circuit.
Well here it is, and it includes the power supply and the 12AT7 line amp.
A good passive tonal circuit (to the line amp) could be added or you could just leave it "flat".
Take Care
Ivey
Last edited:
Well, forgive me for my newness to this stuff... I come from the realm of guitar amplifiers where all we use are 12AX7's and distortion is a great thing. I am taking bold steps into learning to use these tubes as they were intended so bear with me!
My grandmother passed away last year and while cleaning out the house, we discovered my mother and fathers old, albeit small collection of LP's and singles, which me as a guitarist and a recent Electrical Engineering grad was more than excited about. Anyway, I was planning on putting together a tube phono preamp, but have a few apprehensions. First, I see that RCA circuit receiving some negative press... I was planning on paralleling the first 12AX7 stage to reduce noise floor, but is the quality there? Keep in mind, I'm new to this and I have nothing to baseline it against.... sound "good enough" for someone getting into this, or is there a better design somewhere else at the same difficulty level?
Next, if I am paralleling the first triode stage, should I halve the suggested resistor value to ground on the grid of the tube, as I would with the plate and cathode resistors?
Finally: is there an aversion to a DC coupled Cathode-Follower in the hifi realm? In guitar amps/preamps, you'll rarely see an AC coupled cathode follower, although we also use 12AX7's for the job which are poorly suited to it, I know. I had planned to follow the generic topology of that RCA schemo, only paralleling the first gain stage and adding a DC coupled 12AU7 cathode follower on the output...
EDIT: Also forgot... I have never built anything without some type of level control... is it really acceptable to do that as this circuit does not? I'm guessing the gain control will be taken care of in the line stage preamp / poweramp?
Feel free to school me, and yes, I plan to buy Morgan Jones's "Valve Amplifiers" book in the very near future.
Thanks,
Joe
My grandmother passed away last year and while cleaning out the house, we discovered my mother and fathers old, albeit small collection of LP's and singles, which me as a guitarist and a recent Electrical Engineering grad was more than excited about. Anyway, I was planning on putting together a tube phono preamp, but have a few apprehensions. First, I see that RCA circuit receiving some negative press... I was planning on paralleling the first 12AX7 stage to reduce noise floor, but is the quality there? Keep in mind, I'm new to this and I have nothing to baseline it against.... sound "good enough" for someone getting into this, or is there a better design somewhere else at the same difficulty level?
Next, if I am paralleling the first triode stage, should I halve the suggested resistor value to ground on the grid of the tube, as I would with the plate and cathode resistors?
Finally: is there an aversion to a DC coupled Cathode-Follower in the hifi realm? In guitar amps/preamps, you'll rarely see an AC coupled cathode follower, although we also use 12AX7's for the job which are poorly suited to it, I know. I had planned to follow the generic topology of that RCA schemo, only paralleling the first gain stage and adding a DC coupled 12AU7 cathode follower on the output...
EDIT: Also forgot... I have never built anything without some type of level control... is it really acceptable to do that as this circuit does not? I'm guessing the gain control will be taken care of in the line stage preamp / poweramp?
Feel free to school me, and yes, I plan to buy Morgan Jones's "Valve Amplifiers" book in the very near future.
Thanks,
Joe
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