Unity gain line stage
This is the story of a project that mutated. It started out with some surprising measurements on a batch of ECC81s; despite their relegation to the lowest of castes by the tube fashionistas, I found that these are very low distortion tubes. Also cheap and readily available. So, thought I, let's make a new preamp. The old one is about to turn 25 and was built in the days when my sources were much different than they are now.
By the time I was done, the ECC81s were history (they're finding application in my next published circuit, a small power amp with LED biasing). And the circuit had mutated into various species, each of which had their own virtues and faults. One of them, at least, is actually pretty novel. They all perform well. And they're all suitable for pedagogical exposition and auditory enjoyment.
My format here will be a three stage proposition (with an intermission to consider power supplies), starting with the simplest, most conventional circuit, moving on to a somewhat unconventional approach, then mutating off into outer space. Each stage will anticipate the next so that this series can be followed as a set of construction/instruction projects.
Iíve gone into rather gory detail about choices of components and operating points; one great priority for me is using parts on hand whenever possible. If I specify those parts, no one else can duplicate it. But with knowledge of how the parts were chosen and how things can be juggled around to accommodate other parts, a relative novice can alter things readily with a good chance of success.
So, here we go....
First step: Define requirements
These days, a preamp is almost redundant. Signal sources in my living room include CD, DVD, satellite/cable, and MP3, not an untypical mix. All have roughly 2V output at a low source impedance. My phono will get its own dedicated step-up/equalizer designed to output a similar voltage. So, we need at least 5 inputs, all high level. My power amps are of normal sensitivity (as probably are yours), so we really only need unity gain. A10K input impedance is fine- there will be no weedy sources allowed in MY system!
There's another reason for the choice of 10K as the input impedance- I'm sold on the use of input transformers. Wait, wait, I see you starting to click to the next thread, but hear me out: given the multiplicity and uncontrollability of grounds in a multi-source system like mine, there is a decided advantage to galvanic isolation. There's also a decided advantage to common mode rejection in a noisy (electromagnetically speaking) environment. And at these signal levels and impedances, you can get good bandwidth and very low distortion. As a bonus, the transformer will do a nice job of limiting bandwidth; though some people have a religious belief that more bandwidth is better, there's precious little (ok, NO) evidence that extending bandwidth beyond 20-25kHz is audible. That's no surprise- we don't have sources that provide signal energy more than an octave above that, at best (and rarely). There is plenty of evidence that the consequences of transferring high frequency junk through the power amp ARE often audible, even with the limitations of program material. The transformer specified will take us to 90kHz or a little higher while not passing AM radio or our local CB operators. The canonical taxi drivers will not be considered. That bandwidth should give us margin to accommodate any currently available signal source and any contemplated sources for the next decade or two. See the white papers at www.jensentransformers.com for detailed and lucid discussion of transformers and grounding.
As far as output drive abilities, we can calculate what we might need. The maximum length of interconnect I am likely to use is 3 meters (say, 10 feet). That will also accommodate 99.9% of the rest of the world. With an interconnect capacitance of 150pF/m (the worst I could find), we need to drive 450pF. Call it 500pF. The power amp will have an input impedance of 10K at minimum, and might add another 500pF of input capacitance, if it's a particularly nasty design. So we have our worst case load: 10Kohm paralleled with 1000pF. The unit should also be perfectly stable into this or any other likely load- this is a restricted club and we do not take kindly to stray oscillations sneaking in the back door.
We want the unit to be quiet. Hum and hiss should be inaudible. Let the analog tape or mike preamps annoy you, not your preamp. Noise below -80 dB from full output is acceptable in real rooms, so let's go a factor of ten better and insist on -100dB.
OK, we know what we want to accomplish. How do we do it?
Second step: Basic design outline
Starting at the input, my choice of transformer is the Jensen JT11-P1. It's a 1:1 input transformer, optimized for a 10K load, with great balance and common-mode rejection, low distortion, and not terribly pricy in the US. In the UK, Sowter makes a similar-looking unit, the 3575; the specs aren't quite as nice-looking, but it appears pretty satisfactory. The CMLI-15/15C from Cinemaq is supposed to be equivalent to the Jensen, but at a lower price. If you really want to go on the cheap, Iíve heard good reports about the Edcor WSM 10K/10K, and they're under $10 a pop.
We don't want to load the transformer with capacitance, we need a low output impedance, we need unity gain- are you thinking what I'm thinking? Sure you are- we want a cathode follower. Ultra-high input impedance, ultra-low input capacitance, high power supply rejection, easy to stabilize, ultra-low distortion, and low parts count. Yes, I see you wriggling in your chair a bit- you read somewhere that cathode followers sound awful and have all kinds of performance problems. Or a "knowledgeable" buddy of yours told you that. Whatever. It's just a crock, and it's a crock that was fired in the kiln of incompetent design and filled with the ejecta of tragic ignorance. Ask your buddy if he'd turn down an immaculate pair of Marantz 9s.
We will not do an incompetent design.
Let's see what a proper design will entail. Worst case, we want to drive a 500pF load while retaining the bandwidth that the input transformer allowed us. The source impedance and the cable's shunt capacitance form a first-order low-pass filter with a 3dB down frequency of f3 = 1/(6.28*Zout*C). Rearranging terms to solve for Zout, we see that for a 90kHz bandwidth, we need a source Z of about 1.8Kohm. That's pretty doable. But we also have to consider how much current will be needed to drive the load to the maximum voltage. If we assume a 2 volt input sensitivity (that's RMS; peak will be about 2.8V) for our power amp, the signal current needed to drive the 1000pF load capacitance to the full voltage while maintaining the 90kHz bandwidth is about i = 1.6mA. Using a rule of thumb, we arbitrarily dictate that the standing cathode follower current ought to be at least several times higher, just to have margin and to minimize distortion. So let's say we need 10-15mA running through the cathode follower at minimum.
We're now in a position to select a tube. There are a lot of candidates which are happy at the desired current, two very common ones being the 6SN7 and the 6DJ8/ECC88. These also have the considerable virtue of being cheap and easy to find. Recalling that the output impedance of a cathode follower is about 1/gm, we can check the suitability. The 6SN7 has a transconductance of 2.5-3 mA/V, which translates to an output impedance of 300-400 ohm. Add a cathode stopper (more about that later) and we're up to roughly 1K, well within what we need. Similarly, a 6DJ8/ECC88 at 10mA will have a transconductance about three times higher, resulting in a base source impedance of roughly 100-150 ohm. So from this standpoint, either will work. The 6SN7 is a lower distortion tube, but with the inherent feedback of a cathode follower, both tubes are likely to have exceptionally low distortion at these signal levels.
The ECC88 is a winner in this application because of its plate voltage requirements: less than half what is needed for the 6SN7. So low, in fact, that the B+ supply can be made with a standard isolation transformer, and still have enough headroom for active regulation. As a bonus, the higher transconductance translates into a lower noise floor.
A constant current source as a cathode load will help make biasing easier. It will also ease some of the voltage requirements as we will see in the detailed design description. I have been leery of their use in the past, based mostly on unsatisfactory experiences with FET current sources that were fashionable back in the days of Jimmy Carter. Several writers convinced me to use discrete bipolar CCS and I've been delighted with the results. They're cheap, perform well, and are quite reliable, in all ways a vast improvement on those awful two-legged devils of my youth. A pentode also can make a fine current source, but it needs lots of voltage, more chassis room, more heater supply current, higher cost, higher parts count, lower reliability, separate heater supplies... but it's a perfectly valid choice. I just didn't choose it. Either way, it will be made adjustable: adjustability gives the tweaker loads and loads of fun opportunity and flexibility to play with operating points and alternative tube choices.
I think we're ready to float a preliminary design out there and start assigning component values. The basic topology looks like this:
Third step: Design details
Looking at the plate curves for the ECC88, we see that with 90V on the plate, a cathode-to-grid voltage of 1.7V will give us the desired 10mA minimum current. The significance of this choice will be clear in later chapters. Nonetheless, this is a common spot to run these tubes and for good reason: itís a low distortion point which doesnít drive the plate dissipation too high. The tradeoff in choosing a lowish plate voltage is the limitation on swing, but thatís not an issue for our requirements here.
The volume control is arbitrarily set to 100Kohm because that's an easy value to source and I happen to have some Alps Black Beauties in that value. I will not sneer if you use 50K or 250K instead. A stepped attenuator would be lovely here. Whatever you choose, you will need to parallel it with whatever resistance is necessary to load the transformer secondary with 10K.
Note that the grid is at DC ground.
The bipolar current source may be done several different ways: I've used single bipolars, ring-of-two, and cascodes. Single bipolars have mediocre performance. Ring-of-two is a good choice, but I've had one oscillate on me. So in a fit of pique, they have been sent down to the minors. And I sent them down because the cascode is a perfectly good alternative and has behaved well for me. This particular cascode will be powered by a +/-12 volt supply, a complication which benefits us slightly here and a great deal in the next couple of implementations. The -12V supply allows the tube to easily swing the required 6-ish volts peak-to-peak while allowing for sufficient voltage to keep the current source in compliance.
The easy bits are the grid stopper and cathode stopper resistors. They tend to be non-critical (sometimes even omitted entirely) and usually picked by resorting to tradition and experience. I will not omit them because I want this preamp to be rock-stable. The grid stopper can typically be between 1K and 10K. Since I have a box full of tiny 1K resistors and the input capacitance is low, that's the value I use. The output stopper is slightly more problematic since it adds to the preamp's source impedance, but I've never had a problem with 200 ohms in that spot. Half that would no doubt be fine if you've got some real drive issues to worry about. The main thing about stoppers is getting them as physically close as possible to the tube pins.
The output coupling cap is chosen to give a low frequency cutoff at least a decade below the lowest frequency of interest with the lowest anticipated load. Using the 10Kohm figure previously bruited about, for a 2 Hz cutoff the output capacitor needs to be 8uF. I have a coffee can full of 4.7uF polycarbonate caps, so a pair of those in parallel will more than suffice.
We're on the home stretch now. Filling in the details, our preamp now looks like this:
Letís look at a few of the remaining values. The cascode current source uses a red LED for a 1.7V reference; these parts have relatively low noise, low impedance, and provide a nice visual indication that all is well in their portion of the circuit. The resistors feeding it are chosen to provide about 5mA of current. The transistors can be any general purpose high beta bipolars. Double checking, the current source will be set to 10mA, so with betas on the order of 100, we will only lose 0.2mA to the transistor base. This will not appreciably disturb the LED string. With a reference voltage of 1.7V, a quick Ohmís Law check indicates that the emitter resistor will need to be about 100 ohms. A trim-pot of 250-500 ohms will allow plenty of range. If youíre the fussy type, use a 100 ohm trim-pot in series with a 47 ohm fixed resistor.
If you are building this version as a final unit and will not be pursuing the later enhancements, you can get rid of the +12V supply and run the resistor feeding the constant current source to the cathode followerís anode supply. In this case, to get 5mA flowing, the resistorís value must be increased to 20K:
The 1M resistor from the volume control wiper to ground is intended to keep things stable in the event the controlís wiper goes bad, a common occurrence. The 15K resistor shunting the control serves to load the transformer with its optimum 10K load.
The circuitry involving the MOSFET and relay is a dethumper- when the preamp is powered up, the output can have some dramatic bangs as voltages come up and the tube heats. This portion shunts all of that to ground until things have had a chance to settle themselves. The parts choices were mostly dictated by stock-on-hand; the 33uF cap is a 35V unit (a 47u will give a bit longer delay), the transistor can be any small MOSFET that will handle the relay current. In my first go, the relay was a 12V unit, so I just used the 12V rail and ground as the endpoints- that's what's indicated in this schematic. In another version, I used an IRF510, just because it was under $2 at Radio Shack. In this version, the circuit was connected between the +12 and -12 rails and a 24V relay was used. Either way is equivalent, your choice will depend on what relay you have in your own personal coffee can.
Alternatively, one could use a 555 timer or even a CMOS inverter to fire the relay. This circuitry is optional if you can guarantee that the preamp will not be powered up into a live power amp.
Power supply requirements are fairly nominal- the cathode follower runs a constant current, a lowish one at that, and sports fine power supply rejection. DC ought to be used on the heater, and the +/-12V source can take care of that duty. The series resistors are calculated on the assumption of a 300mA heater; if the tubes you use are 360mA, they will need to be changed to a pair of 100 ohm units.
Iím a crude, simple guy so used a crude, simple cabinet- a $6.50 black plastic box with an aluminum lid, sourced from Radio Shack. You may wish to do something nicer.
A note on inputs and grounding: the input transformer allows you to drive the preamp via balanced or unbalanced sources. For balanced sources, the two ends of the primary are used as the inverting and noninverting input. For unbalanced sources, the terminal marked "ground" is connected to sources' ground- the source ground is isolated from the rest of the preamp grounds. That's the beauty of input transformers. If switch contacts don't bother you, you can easily use a DPDT switch on the primary side to do polarity inversion. That will give you even more fun things to play with while listening to music.
In Part 2 , we will consider the power supply and some construction notes.
Thanks to Steve Eddy, Bas Horneman, planet 10, pinkmouse, EC8010, and the members of diyaudio.com for great feedback on this project.
Re: Unity gain line stage
In the spirit of pedagogical exposition, did you mean to condemn FET current sources generally, including cascodes, or just the "2 legged devils"?
The two legged variety like the infamous CRxxx from Siliconix. I also had tried degenerated single FETs (i.e., with a resistor in the source line so that they were running well below idss). Not happy. I abandoned CCS circuits for years until Morgan Jones convinced me to give them another try using better topologies and parts choices. He was right.
i had no idea the siliconix CR diodes were "infamous" - i though it was just me being incompetent.
great reading, thanks SY. by the way, your new avatar really freaks me out.
Nice piece, SY!
It's worth noting that CineMag's CMLI-15/15C is essentially the equivalent of Jensen's JT-11P-1. It costs about 20% less and it's available with several different mounting options.
Thanks for the link. I'll incorporate it into the post.
The drawings are the best part.
Alexis, if you knew the story behind that avatar, you'd be even more freaked out.
Come on now, you can't make a huge post with details about what you were thinking, and then make passing reference to something else! Well ok, you can, but inquiring minds want to know. What's with that pic.?
...or is it still you on a good day?
...it was disturbing to my wife also BTW. ;)
OK, but let's not get sidetracked.
I think Jesus may be reconsidering whether or not he even likes me.
I'm here to learn, so I hope you will forgive my question - what is the reason for the 1M resistor at the input pot's wiper? Regards, Bill.
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