The “Universal” Preamp

4S "Universal" PreampHumble Beginnings

The basis for this amp started out as a post in a thread on the DIY Audio Projects Forum site. One of the members had suggested building a Super Simple Single Stage Preamp (“4S” Preamp for short) and there was much discussion concerning various tubes, gain, noise, etc. and several designs were presented using various dual triode tubes. Then the proverbial gauntlet was thrown down with the phrase “switch from 12AU7 to 12AX7“. My answer was to design a universal stage that would work well with an entire range of tubes. Thus was the 4S “Universal” preamp born.

The Electrical Design

The design I came up with is a single circuit that can handle the following tubes with no changes: 12AU7, 12AV7, 12AY7, 12AT7, 12AZ7, and 12AX7. Dependent on the tube used (all these have the same pinout) and whether the cathode is bypassed, the peak gain may be varied by a large margin. Here is the circuit I implemented along with the tube rectified power supply.

4S "Universal" Preamp and Power Supply SchematicThe circuit is actually very simple. It uses a reasonably sized grid stopper to control clipping and bias excursions, a pure restive load, and a high impedance grid resistor to avoid loading virtually any driving device. The volume control is on the output to help ensure that the noise figure for this preamp is as low as possible. With these components (the output potentiometer is 250kΩ audio taper) and an optional 33µf cathode bypass capacitor the calculated performance is shown in the following table.

Calculated "4S" PerformanceAs can be seen from the table, the peak gain can be varied considerably by choice of tube and the distortion (virtually all second harmonic) is very well controlled. This is a well behaved preamp.

The Build

Deciding on the physical layout and build actually took some time. Up to this point, most of my tube builds have followed a fairly predictable pattern: wood base, aluminum top plate, polished finish, etc. For this build I wanted to go for two major themes: Utility & Compactness. So I started with a stock enclosure from Pomona, the 4226. This is an “E” size (8.3″ x 4.2″ x  1.74″) die cast aluminum enclosure with a sheet aluminum cover and internal card guides. Although small, I was sure I could fit the entire PS and stereo preamp in this box. Instead of trying to run wires from the top plate to components mounted inside (which would make working on the box and sealing it up problematic) I decided to mount all of the components to the top plate. Then I could build the entire amp and when complete, just slide it into the box. Here is the layout for the major elements:

4S Control DiagramThe power components are all on the left had side of the unit and all the audio components on the right. Internally there is a metal shield between the two sections which is held in place by one of the card guides moulded into the cast chassis. One very important feature of this build is the ground lift switch. The entire chassis is grounded through the IEC connector ground pin. The signal ground is isolated from the case ground by the ground lift switch. This way, if I am driving an amp in which signal is grounded to the case, I can flip the ground lift switch and the preamp signal ground will be referenced to the amp signal ground via the audio cables. This avoids hum inducing ground loops when both chassises are grounded via the IEC connector. Here is a view of the component side with the unit all wired up and ready to go into testing.

4S Completly WiredThings are fairly cramped inside. I could have gone for a printed circuit board approach, but I really enjoy point-to-point build techniques for vacuum tube projects. Besides this, I’m not sure I could have designed a PCB that would work with all the bulky items protruding into the case (inductor, IEC connector, tube socket, potentiometer, RCA jacks, etc.). Once the build was done and all electrical connections triple checked, it was time to commence testing.

The Testing

Testing a device of this type is really fairly straight forward. The first step is to simply power up the unit and check all the major voltage points in the circuit. This is done with the unit open and sitting on a workbench. For this build all voltages were as calculated to within a reasonable margin of error. For this type of device I like to test using a signal generator with adjustable output, a dual channel AC volt meter, and an oscilloscope. Here is a picture of the unit as I was running it through its paces.

4S Test SetupThe signal generator allows me to test the unit at various frequencies and input levels, the oscilloscope shows me the input and output waveforms, and the volt meter allows me to directly calculate gain at any frequency without trying to get numbers off of the oscilloscope. So how did the unit do? Here is the gain and phase plots generated from measured data.

4S Gain-Phase PlotThis is very good performance. The single dominant high frequency pole here is due to the input capacitance of my test equipment. The tube input capacitance is at least an order of magnitude smaller and is swamped by the test equipment and the nest of wires on the table.


Of course, all this test data is nice, but it really doesn’t say much about how the preamp works and sounds. So once the unit was deemed technically acceptable, it was time to determine if it was sonically acceptable. This entailed hooking up the preamp to one of my other units. I chose the 6V6 Lacewood amp.

In the past, when driving this amp with just my iPod, I was disappointed with the overall performance. Not with the sound mind you, but with the attainable power level from the amp. The iPod output simply wasn’t enough to drive it to full power. So I inserted the preamp in front of the amp, turned the amp volume to max, and used the preamp control to adjust volume. Here is a picture of the setup.

4S In UseIn this picture, the iPod line out is driving the preamp, the preamp is directly connected to the Lacewood amp and it’s volume is at max. First, with no input and both preamp and amp turned to max, the setup was dead silent. I could hear nothing from the speakers even with my ear right next to the speaker cone.

From the first moment I started playing music I was amazed at how the pair sounded together. There was absolutely no problem with signal level or power. And the added effective dynamic range really let the amp open up.

This project was a great success. I now have a preamp that can be adapted to the task at hand simply by swapping tubes. It also allows me to do some tube rolling to compare various tubes in my stash. And it can be configured with sufficient gain to test almost all of my prototype power stages. I think this little unit is going to see a lot of use.

On one other note, like all my other tube gear, it looks pretty good in the dark as well.4S Preamp Tube Glow

56 thoughts on “The “Universal” Preamp

  1. Hi Matt,
    What is the approximate anode voltage and cathode bias voltage? Just want to be sure that I am doing the right thing.

    • The bias voltages are given in the table above under the column labeled ECo. The plate voltages run from about 65v for the 12AU7 to about 125v for the 12AX7.

    • Short answer is yes. Just be aware that if you are using the Hammond power transformer, it will increase core heating. But that transformer gets pretty hot anyway.

  2. Hello,

    Like you I am also an electrical / electronics engineer who builds audio circuits for a hobby, and sometimes a little profit. I primarily work with discrete transistors, but way back when I did build a few tube amps.

    My primary question involves how you are taking the THD measurements.
    In my case I don’t have an audio analyzer, but use “clean” recorded tones from an iPod or from Audacity on my laptop. I then use the FFT function on my oscilloscope to view and measure harmonic content, then plug the numbers into some math and come up with a %THD number.
    I also occasionally use a PC oscilloscope with a built in function generator to perform this measurement also – the one I have can give a direct %THD readout, but it is “suspect” – the function generator running in sine mode in and of itself has a THD of almost 0.5%, and I know to be truly accurate, a good audio generator with something like .001% THD or less is really required to get a good THD reading.

    I am just curious as to how others with a “basic” basement lab are getting their THD measurements.

    • First, my %THD measurements are made much like yours. In pure tone testing I use the FFT function on my digital oscilloscope to determine the dBv levels of the harmonics, I then convert to absolute, and RSS them to get a total. This is a reasonable way to generate a THD.

      Second, depending on your signal generator, 0.5% THD may well be perfectly reasonable. For one harmonic this would be about -46dBv down from carrier which would be right in the ballpark for a lot of wien-bridge oscillator signal generators. My title TENMA audio oscillator has a THD between 0.1% and 0.5% (-60dBv and -46dBv) in the audio ranges and it’s pretty typical. My Leader LAG-1208 has distortion specs between .05% and 1.5% (-66dBv and -36dBv) depending on settings, but getting down to below -60dBv is finicky on this particular unit. And my Precision E-310 vacuum tube signal generator is specified at <1%. That's only -40dBv.

      Forget 0.001% THD. You'll likely never get there and any signal generator advertising such a value is blowing smoke. That's -100dBv. For those measurements your talking conditioned lab power, Faraday cages, and high end equipment. It's the same reason that the advertised THD of most SS amplifiers is total hooey. They are theoritical numbers or very controlled lab tests under very clean conditions, not real world operation.

      • Thanks for the reply, I’m glad to hear that the FFT and RSS method is being used others. Do you do anything to subtract out the distortion of the generator when you measure THD?
        Today’s audio equipment ratings – more than “hooey” – nowadays a lot of “mainstream” audio ratings are pure bald-faced lies.
        Low distortion generators – Rohde & Schwarz has a few with THD at -120 dBv, but one problem – they cost more than a new car, a little rich for my blood.
        And, I think the reality of harmonic distortion is that it is impossible to hear <1%, most of us can't hear 5% in music or speech, and through some unscientific testing, I really can't hear anything wrong with the sound until I see clipping of music or speech on my scope.

        • As to the published specs of some of today’s amplifiers, it is important to realize how this is commonly done. What the manufacturers do is the publish a “referenced” distortion number.

          Let me explain. The normal modern audio design approach is to produce an amplifier with very high open loop gain and then feedback a significant portion of that signal gain to cancel noise and distortion. So lets say the designers need 50W from a line level signal (i.e a standard power amp design). At 8Ω, 50 watts is 20v rms. A line level signal is +4dBu which is 1.228v rms. So the actual end to end gain they need is 20*log(20/1.228) or ≈24dBv. So they design an amplifier with an open loop gain of something large; like 80dB. Then the approach is to use 56dB of negative feedback to get back to 24dBv end-to-end gain. Now in addition they want really low low distortion, like the 0.001% figure you mentioned above. Thats -100dBv in all one harmonic (i.e. the dominant harmonic). But achieving and measuring that 0.001% distortion is very difficult. So how do they accomplish this?

          Well, the theory of negative feedback says that the reduction in distortion is the same as the feedback factor. And the feedback factor (in our example) is 56dBv. So what they do is target the open loop distortion to a number they can achieve. In this case -100dBv + 56dBv or -44dBv. This is only 0.63%. Now the other thing that negative feedback does is significantly increase the bandwidth. So what they do, is design a high gain ≈80dBv but narrow band amplifier with a distortion factor of <0.6%. This is actually an achievable goal and a rather straight forward process. Then they measure the "open loop" distortion, convert to dBv, then subtract the feedback factor (in this case 56dBv) and convert back to %.

          So lets say that the amplifier as designed has an open loop distortion of 0.57% (i.e. below the target of 0.6%). This measured distortion equates to -44.9dBv. Then they subtract the feedback factor of 56dBv to get a "theoretical" distortion of -100.9dBv. This corresponds to 0.0009% THD. And THAT is the number they publish even though the real measured distortion was about 1/2%.

          The problem with this, of course, is you NEVER even come close to this theoretical distortion number in actual usage. But that doesn't keep the manufacturers from publishing it. This type of mathematical masturbation has been accepted practice in the audio equipment industry for decades. But once you understand what's actually going on, all those super small distortion numbers become just so much advertising foolishness.

          • Thanks for the interesting insight on THD measurement and calculation. Come to think of it I have seen a places on the net. That some builders do say that the “real” distortion is actually the measured number minus the calculated reduction due to NFB.
            Amazing, what you can twist with math.

            BTW, the pictures of your builds are beautiful. Your finished product looks the same or better than most of the commercially available tube gear out there!

  3. Matt,
    I have recently started utilizing Ltspice to try some modeling in order to learn how various component values may effect an amplifier response. Everything went well until I put a model of a speaker on the output of the preamp circuit during simulation. As you mentioned, your testing is done with an oscilloscope and a.c. voltmeter. I’m not sure what the final impedance is seen by the amplifier. Have you ever looked at results utilizing a speaker as a load? Is this something I should be concerned with during design and testing? As always, I greatly appreciate your time and consideration towards the DIY community.

    • In general, amplifier circuit testing or modeling with speakers is not done. This is because speakers present a frequency dependent, complex impedance which tends to mask circuit operation. Using a resistive load (i.e. a constant real impedance) elminates variations in circuit operation due to load nonlinearity. Speaker impedance is also very difficult to model correctly due to the variables involved. Models of speaker impedance seldom accurately match real world response.

      Take a look at my blog post here Sizing up a new Power Triode and you’ll see that I use a resistive load (the unit with finned heat sinks in the back) so that I can accurately determine how the tube is performing.

      About the only time I do testing with speakers is when I am measuring SPLs. And this is only for speaker testing and not amplifier testing. In rare instances it may be required to monitor speaker impedance referenced to the amplifier across frequency when employing a compensation circuit in an amplifier. This is not something I do regularly.

  4. Hi Matt,

    I built this pre and got it working over the weekend. Simple and clean sounding. I am really happy and I am sure it has some more burn in to go. I had read that the gain on this little guy can be more than some would prefer and I am afraid I might fall into that camp. Here in my office with a 12au6 in place the usable range of the volume control is between 6 and 7 o’clock. Is there any mods to this circuit which would limit the gain some.

    Thanks for the design and the knowhow. It has been a pleasure to construct this.


      • The 12AU7 provides about 20dBv of gain. If you want lower gain what I recommend is a -21dBv pad in front of the amp. It is like the diagram in your link but replace the pot with a fixed resistance. Make the series resistance 100kΩ and the resistance to ground 10kΩ. And eliminate the 500kΩ grid resistor on the first stage. Use good quality metal film resistors for minimum noise.

        • Matt,

          Thank you for the response. Please excuse my ignorance, I am a newbie. Is this even close to your instructions above?
          Modified Schematic

          Thanks Again,

          • That’s exactly what I described. This will give you a -21dBv pad prior to amplification. This means that with the 12AU7 in the preamp, you’ll get approximately unity gain with the volume at full.

  5. Hi Matt,

    I was wondering if this amp could be used as a stand alone amp if the output stage includes an impedance matching transformer to drive speakers?

    Thanks, John

    • Unfortunately, no. This is a voltage amp and not a power amp. Even with an impedance matching transformer it will not provide the power necessary to drive speakers. If you are looking for small power amp, I highly recommend the 6CY7 amp. It is inexpensive, easy to build, and sounds great.

  6. Matt,

    First of all, thanks for sharing your designs and all that stuff. Now, my question tags on the March 15, 2014 post from Arturo. My son would like me to build a mic preamp. He is using Shure SM57 and 58s. Can this circuit be used for that? Also, he may occasionally want put his electric guitar signal through it. Any help is greatly appreciated. Thank you for your consideration in this matter.

    MSgt Brian E. J. Taylor, USAF (retired)

    • Brian;

      I would like to be able to say yes but unfortunately, this preamp just doesn’t have sufficient gain to serve as a microphone preamp. Microphone preamps have two basic requirements. One is very low noise, the second is a gain of between 50 and 60dBv. While this is a good preamp design, it really isn’t low noise enough to give good results with a microphone and it definitely lacks sufficient gain by a significant margin.

      The Sure SM57 has a sensitivity of -56dBv/Pa. This means that at a sound pressure level of about 94dB it has an output of -56dBv. In order to get to a line level signal to drive a mixer or general purpose amplifier will require a preamp with a gain of abut 60dBv. For microphones, you might want to think about something like the Behringer TUBE ULTRAGAIN MIC100 Preamp for the microphone use.

      As for the guitar, with a 12AU7 this can make a really nice little boost amp for an electric guitar. Of course, you only need one channel, or you can just build a two channel unit with separate volume controls.

  7. Thanks Matt for the drawing and all the idea.

    Could I use copper chassis to build this pre-amp?

    Furthermore, I assume that the output of this pre-amp is ok for the 6sN7 pre-amp stage of my amp (6SN7-300B SE)


    • Copper makes a great choice for chassis material. It’s easy with which to work, provides great shielding, and can even be soldered. The only down side is cost.

      If you look at the calculated performance table you’ll see that the 4S has a varying output impedance with peaks ranging from ≈65kΩ to ≈80kΩ. In general the input impedance of the amp should be more than four times the driving impedance. so your amp should have an input impedance greater than about 320kΩ. So it really depends on what the input of your amp looks like. You would probably be ok if your input volume control is 250kΩ or greater. Otherwise I would abandon one of the volume controls and rework the interface.

  8. Hi!
    Do you think this pre amp will be capable of driving my power amp (LM1875 based on non-inverting design of National Semiconductor datasheet which includes a 22K resistor to ground at the (+) input of the chip)?
    I currently use a 10K potentiometer in front of the amp as my volume control and it works well instead of higher potentiometer (I tried 50K and 100K and they sounded worse).

    • This will probably not drive your LM1875 very well. The input structure of the LM1875 is such that the input impedance is simply too low for effective 4S drive. The best way to correct for this is to insert a buffer stage between the 4S and the LM1875. The buffer needs a high input impedance and a (relatively) low output impedance. This could be done with a vacuum tube cathode follower, a transistor emitter follower, or a solid state op amp properly configured for buffer operations. Just remember that the buffer input impedance should be at least 250kΩ for proper 4S operation.

  9. Hi Matt, thanks for sharing this wonderful design, the circuit is fairly simple, and
    I have a silly question about this pre-Amp.
    Is it possible to use it as a headphone amplifier?

    • Justin;

      This is a line stage preamp and the output impedance is far too high for headphone use. For headphones you need a much lower matched output impedance.

      Take a look at The Recovery Amp for a good design for headphones. I have used this amplifier (actually the “Rebuild” version) five days a week, eight hours a day for a couple of years and it still is performing flawlessly.

  10. Hey Matt, thanks for sharing that desing with the community. I love doing my own audio setup and i like to keep it simple, so your little project fit really well. I’m in the part scavenging to build one of these, but i chosen to use a 5y3GT and a 6sn7 from and old 6v6 GE monoblock, so i guess i remove the universal but keep the 4S. I think i will add an input selector to. Keep working, you got an another “suscriber”!

    Have a nice day.

      • Thanks, i build it around this schematic, i’m building a wood box and a aluminuim top plate for the retro look, will it be ok to drive a 10k ohm SS amp input?

        • The output impedance of this preamp is far too high to effectively drive a 10kΩ amplifier input. This would require a buffer stage (ss or tube) to properly match the impedances.

          • I got a little question again, when verifying the voltage, do i need to put a load resistor? Or i got a little problem where the b+ split to plate, one got 60vdc and 80vdc on the other.

          • Frank:

            As to your first question, no you do not need any output load while checking voltages. As for the other problem of varying and low B+ voltages I’ll need a little more information. If you could go over to the diy audioprojects forum thread for this project ( and post your question along with a schematic showing exactly which voltages you are measuring and what tube you are using, that would help a great deal.



  11. Hello, Thanks for adding the 12bh7 option. I am going to build this version. Also do you have any opinions on external power supplies, where the power supply is in one box and the line stage is in another box?

    • There are two conditions where I like separate power supplies. The first is in large builds where it can control the layout so there isn’t a large or unwieldy chassis housing everything. Personally if a chassis layout footprint is getting much above about 10″x16″ I begin to think about splitting the setups either in a PS/AMP layout or in mono-blocks. The other place where I like to split things sometimes is in headphone amps. This way a small amp can sit on a table next to a chair for easy volume control and the PS can sit below so there is still a place on a small side table for a mug and or a book.

      For a project like this, it is so small already, I would not even consider splitting the chassis. I just don’t think that it’s worth the extra effort.

  12. I’m thinking about building this preamp but I’m new at this. I want to use it as a phono preamp. I’m curious if this preamp would provid the necessary equalization for records. I think they call it RIAA.

    • This preammp is a line stage and, while very good, it lacks the RIAA equalization curve you’ll need for phohograph playback. There are several good solutions for building such a preamp but the one I would suggest for your first RIAA preamp is the JFET Phono/RIAA Preamplifier kit from Boozhound Labs located here ( This kit performs very well and still allows you to do your own power supply and case.

  13. Hi Matt,
    interesting little preamp and so simple.
    I’m amazed at how much can be achieved with so little in valve amps.
    I would be interested in building one of these as i have always wanted to see if I like the sound of a valve system.
    I have a turntable with a AT-95E cartridge connected to a cheap($30) ss preamp and a cd player currently connected to a pioneer elite av recvr. I take the pre outs from the pioneer and connect into a ss 2 ch yamaha pc2602 power amp. I would like to connect the cd player thru this little preamp of yours directly to the yamaha. Do you think it would sound better than thru the pioneer elite av rcvr ?. thanks in advance Phil Murray

    • Phil;
      “Better” is always a subjective assessment. However, I would hazard a guess that the CD player fed through the 4S with a 12AU7 in the preamp should sound pretty good. There is one catch however. The Yamaha PC2602 only has balanced inputs and this preamp has unbalanced outputs. An electrical translation will be required. Provided you make this translation it should sound great.

  14. Hi Matt

    i see you only used 1 side of bottle , i was thinking ,i could just duplicate the components and use the other side of the tube ,to make a push pull version.
    Could i strap a transformer on the back of it ? or is best to use the cap output.



    • Actually, I did use both sides of the dual triode because it’s a stereo preamplifier. I simply showed only one channel in the schematic.I’m not sure how duplicating the components would allow you to make a P-P version. As this is a voltage amplifier and not a power amp there really is no need to construct a push-pull stage.

  15. In your excellent write-up on power supply design you go through a calculation and arrive at a 250v DC supply output from a 275-0-275 transformer. In this preamp design you show a 250v supply being generated from a 190-0-190 transformer. The math doesn’t seem to add up. According to your analysis the 190 transformer should provide a DC supply voltage of 142 volts or so.

    • I believe what you are missing is the different loss in the rectifier and filter circuits of the two designs. The power supply design in the paper for the “Ghost Amp” has a peak rectifier current of 260mA. The rectifier drop in this design is around 28v (Er≈0.74) and the filter loss (with a 130mA dc load current) is over 40v. In the preamp design, the peak rectifier current is about 11mA with a drop of around 3.6v (Er≈0.96) and the filter loss is about a third of a volt.

      So, the ghost amp design has about 68v of loss whereas the preamp loss is only about 4v. Also the transformer secondary waveforms turn out to be quite different which affects harmonic generation and conversion efficiency. I know that at first glance it doesn’t look like it makes sense, but this is why I wrote the power supply design paper in the first place.

      • Yes, that’s clear, thanks. Makes perfect sense. Your comment about the secondary waveforms differing and their effect on harmonic generation; I’ve never heard this discussed. Can you point me to a reference? I’d like to read up on it.

        thanks for the help

        • I don’t have a reference for harmonic generation, that’s just how it turns out when you do the math.

          You can think about it this way: In a light load condition the first capacitor is going to discharge very little and the rectifier current waveform is going to approximate a square top pulsed waveform. But in a heaver load condition, the cap discharges more and the waveform starts to look more like a sawtooth waveform. When this happen, more harmonics are added. The filter is made to dump everything above the first ripple frequency so the more harmonics you have, the higher the loss in the filter. Make sense?

  16. Hi Matt. I have a realy silly question about pre amps (I am new at this):
    what would happend if I use this configuration:

    iPod or a apple laptop – your S4 tube preamp – an ADCOM 545ii (100 WATTS p/ch) AMP – and a great pmc twenty.22 speakers?

    My question, other than if this configuration is posible, is that if the final sound would be nice.
    I’m tired of seen preamp prices over thousand dollars or more, so, should I invest in little creators as yourself or some DIYers?

    Thank you Matt, realy impressed with what you have done here.

    • I think the answer to your question is that it should sound good. I checked the frequency response and you should be fine with the ADCOM 545ii’s 50kΩ input impedance. You don’t need much gain since 1.2vrms drives the amp to 93W output. So I would recommend using the 12AU7 tube and keeping the iPod output relatively low (less than 50%). Then use the preamp volume as your main volume. The 545ii is a nice amp. It should make for a very nice sounding system.

  17. Hi Matt, is there a step-by-step type instruction for this pre-amp and what would it require to crank up the gain to line level?

    • As for a step by step guide, I really don’t have one. I would however suggest that you look over the “Super simple single stage tube preamp” thread over at the DIY Audio Project vacuum tube forum. The discussion of the 4S starts here “Super Simple Single Stage Tube Preamp – Post 109

      As for your request to “crank up the gain to line level”, I’m not quite sure what you mean. The preamp has gain figures from 19.9dBv to 34.6dBv depending on configuration and output voltage swings in the range of tens of volts. As such +4dBu (1.228 vrms) is no big deal. The real question is how much gain do you need?

  18. Hi, I am looking for a Mic Preamp. Does this one delivers enough gain to get Line level (+4dBu) from mic level (-40dBu aprox)?

    • If you look at the gain table you’ll see that if you use the bypassed cathode configuration and a 12AX7 tube, you should get around 34.5dBv of gain. This will take a -40dBu level to approximately -5.5dBu. So not quite. But, whether it will be useful or not is really a function the system you are driving. If it’s going into a consumer amp, this may be just fine. If you’re driving the input of a commercial mixer board it depends on how much gain you can get in the channel off the board.

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