A Heretical Unity Gain Line Stage part II
Ten Miles High
To review, in Part I (http://www.diyaudio.com/forums/showt...threadid=58757) we considered a very basic transformer-coupled input, capacitor-coupled output unity-gain preamp. That circuit is, as it stands, an excellent performer. We do need to power it and to build it, so those circuits and options will be considered before we move on to some fancier variants. But this is a fine stopping-off place if you just want a good sounding, reliable, easy-to-build preamp.
In this part, I’ll cover the raw supply, the regulators, some construction notes, and a few words on parts choices. It’s a bit unfair to have you make decisions in the dark, so I’ll preview the next variation: I hate large capacitors, so I trade the large output cap for a small input cap, direct-coupled output, and servo to clean up the offset. And the last iteration will substitute a rather novel servo for the usual method, decoupling the servo even further from the signal. This will come, as usual, with a degree of trade-off… If the idea of wrapping op-amp circuits around your preamp gives you the willies, this first version ought to be your final destination.
We need to power our circuit. To do so, we would ideally have a supply that gave us exactly the DC voltages we need with the effortless silence of a skilled servant. We have made things somewhat easy for Jeeves by perspicacious design choices: a cathode follower has excellent immunity to power line disturbances, the high impedance current source isolates the cathode from most of the noise or wobbliness remaining on the -12V rail after regulation, and the heaters can trivially be made blameless by wiring and grounding correctly. But the supply needs to be good enough to not be the limiting factor in distortion and to achieve low enough noise such that the preamp's total signal to noise reaches or betters my -100dB criterion.
Cum on feel the noiz!
Let's try to quantify our noise budget and see how that impacts power supply design. There is a school of thought that power supplies can never be too good. In a sense, that’s correct, but in my world, once the power supply is no longer the limiting factor, it’s Good Enough. I’ll try to lay out alternatives and options so that if my approach is horrifying to you (“Ohmigawd, he’s using Radio Shack parts!”), you can at least understand the rationale and make whatever changes strike your fancy.
What’s the overall target? For a 2V preamp output, -100dB equals 20 microvolts. So the RMS sum of all noise sources should be at that level or lower. We can quibble about hearing thresholds, masking, Fletchers, and Munsens, but at the end of the day, it’s unlikely that anything at the -100dB level will be audible in any reasonably normal setup. If the total contributions of the power supply and signal circuits sum up below that 20uV target, the preamp will be blameless in that sense.
As a quick aside, let’s look at noise in the signal circuitry. The principle sources will be the tube’s noise, noise from the cathode load current source, and thermal noise from the input circuit. We will start at the beginning. At full bore, source noise will depend on (naturally) the source. But if we assume a source impedance of 1K from whatever CD player we have hooked up, we can rough out a Thevenin equivalent. The 1K source is in series with the (roughly) 1K resistance of the transformer. This is shunted by the 10K worth of resistance that the volume control circuit represents. We're at 1.67K. We have the grid stopper in for another K. 2.67K. The Johnson noise is then seen to be well under a microvolt. With zip for DC across any of these resistances, excess noise is buried. Whew!
The current source might appear to be a worry, but it's not, really. The looking-in impedance at the cathode is about 150 ohms. Ignoring the favorable shunting action of whatever load the cathode follower is driving, the noise from the current source is divided down by its source impedance (on the order of 10 megohm) and the looking-in impedance. So to contribute a microvolt of noise at the output, the current source will need to produce 70,000uV (70mV) of noise. The prevalent source of noise is from the modulation of the LED reference voltage via noise on the 12-volt rails. The LED has an impedance of roughly 10 ohms at the current we're running it. It's fed by 2.4K (ignoring that pesky bipolar's base), so the divider ratio is about 240. That means we can tolerate nearly 17mV of ripple or noise on those 12V rails before exceeding a microvolt transferred to our output. This will be relatively easy- in these days of modern times, we're blessed with cheap, reliable regulators that sport noise decades lower than that. If we want to adopt a suspenders and belt approach, we could, in principle, replace the upper resistor with another constant current source, but the Escherian possibilities can quickly get out of hand. “It’s turtles all the way down.” A resistor will work just fine there.
There’s the tube, too. We’re running it in a region where the transconductance is moderately high, about 8mA/V. Using the old rule of thumb equivalent noise resistance = 2.5/gm, we can estimate the tube’s Johnson noise resistance as about 300 ohms. Considering the input resistance, we can see the noise contribution as negligible.
So, we’ve got an intrinsically quiet circuit- if we don’t muddy it up with bad power supply design, we’ll hit our design goals with room to spare. OK, let's figure out a way to power up this bad boy
Raw, raw, raw, that's the spirits we have here!
The raw supply will be in a separate box so that its emanations don't piddle all over our nice, clean circuit. Regulation will be closer at hand to where it's needed.
We begin with consideration of B+ needs. Isolation transformers are a dime a dozen, we don’t need a lot of voltage, so that is what we’ll use. There are many mansions in my father's house; my favorite mansion is the old fashioned E-I core. One can buy these new for about $16 (the Triad World Series will give us ten times more current than we need, #VPS230110). Or, if you're like me, you root around in the scrap box to find something. When you do, you'll have the heart of a 160V (or so) supply, which gives the filtering and regulation plenty of room to maneuver while fighting the noise.
There are many religions regarding how the line voltage should be filtered. Much depends on your actual power source- mine seems to be relatively clean. Do please use a fuse, and make sure that the safety (earth) ground from the power line is securely tied to the chassis. You can kill three birds with one stone by using one of the Schurter power entry modules, which combine an IEC power connector, a fuse, and an RFI filter. A suitable unit is the 5200.0123.1, stocked by Mouser at about $21. If you don’t mind doing some extra metalwork, you can use the 5110.0133.1, which has the IEC receptacle and RFI filter, and then add a separate fuse-holder. An SPST on-off switch is optional. Terminal worrywarts will use a DPST and switch both sides of the line.
Following the transformer, we need rectification. Much bird is whipped debating the merits and demerits of various schemes. My take: use high speed diodes- they're reasonably inexpensive and while I think their merits compared to old-fashioned 1N4007 types are minimal, their better performance certainly won't hurt. The Vishay UF5408/1 is ridiculously over-rated at 3A/1000V, but costs about $0.32. Buy a pile and use them for your next twenty tube projects. Bypass the diodes, if that makes you happy. I didn't bother- I'm paying for the following RC filter and by god I'm going to make it earn its keep. But a 0.1uF caps paralleled across each diode is quite customary.
What about tube rectifiers? I can grant their use for nostalgia reasons, but for me the trade-offs are too severe. High resistance, inefficiency, heat production, and unreliability are the demerits. The one strong point is the gradual warm-up, but it’s not much of an advantage here- the plate voltage is too low to induce cathode stripping and we’ve got the output solidly dethumped. Use a tube if you like, but there’s just no question that if the goal is solid and silent DC, silicon rules.
On to the filtration. We start off with a relatively honking 470u/200V cap, a common switching supply value. This isn’t an application where we need the ultimate in ESR or DA or whatever. We do, however, want that cap to be of good quality for the long haul, so my choice is a 105° C rated cap. If you’re like me, you’ll use a UCC KMH200VN471M25X30T2. That’s a mouthful of a part number, but Mouser sells them for about $2.50. If your wrist gets tired writing out that number, an equally good choice is the CDE 380LQ version, part # 471M200H022; it has a lower temperature rating (which is probably OK since there's no real heat sources in the raw supply box), but has a very high ripple current rating. It's up to you. Whichever we choose, before going any further, we've knocked our ripple down to less than a volt. A couple of RC filters and a regulator should have no problem scaling that back by another 80dB or so. We put in a 470K bleeder resistor for safety.
At this point, the raw B+ (about 160V) exits the power supply box in sufficient quantity to power two channels; mission accomplished. We’ll come back to the B+ line when we consider regulation and filtering options later.
Now, we have a heater, dethumper, and current source to power up. I used +/-12V supplies because I knew there would be a servo in my future. And even if there weren’t, the extra supply doesn’t represent any significant further investment in money or chassis space. Our current requirements are relatively modest- budget 365mA maximum for the tube heater (it will probably be 300mA, but worst case…), and another 5mA per channel for the current sources. Our servo, when it takes a bow, will only need another 5mA or so. So if we plan for a 500mA supply, we will have a comfortable margin.
I have separated out the filament and plate transformers. You can, in theory, use a transformer with both windings on the secondary, but there’s a penalty to pay- unless there’s an electrostatic shield, the interwinding capacitance will do a smashing job of coupling common mode noise onto the heater circuits. You can get a transformer with an interwinding shield and add common mode filtration; a suitable filter is shown in Morgan Jones’s “Valve Amplifiers,” figure 5.48, or you can improvise with an RF 1:1 transformer and a couple of 0.01uF caps. Hams with coffee cans full of 2.5mH chokes can use a pair of those to good effect. I didn’t use a common-mode filter in either of my units and saw no ill effect. But if you’re going to use a single transformer for high and low voltage or experience any noise coupling via the heater, spend the extra few dollars and put in a common mode filter. The transformer should be, at minimum, 30V CT (15-0-15) to allow the regulators a bit of room to breathe. 500mA current rating is more than enough. At the expense of a bit more regulator dissipation, a 36VCT unit was pressed into service; I used a scrapbox transformer, but suitable new units would include the Triad World Series VPP36560 (PC mount) or VPS36700 (chassis mount).
As with the high voltage supply, high speed diodes are a Good Trick, and a cheap one, too. If you’re a bug about power supply efficiency, Schottky rectifiers have a somewhat lower voltage drop than normal junction diodes. But, really, this is only half an amp we’re talking about. Since I was buying the UF5408/1s for the high voltage rail anyway, I just used the same thing for the heater supply. With a 3A continuous and150A surge rating, they'll do fine.
This is a good time to consider reasonably big caps for the supply. A pair of low ESR 3300uF electrolytics will knock the ripple down to about 600mV. And they’ll be cheap. An exemplar is the Nichicon UPW1V332MMH, featuring 15 milliohms of ESR for under $3.00.
And that pretty much wraps up the raw supply.
With solid raw supply in hand, we move on to the next two inter-related problems- attaining the proper DC level and getting rid of the ripple and noise. This will require some regulators, or at the very least, more filtration.
Let’s start with the anode supply. The cathode follower will have a power supply rejection of roughly mu +1, so we can tolerate at most 600-700 microvolts of noise before running afoul. Ideally, we'd like it even lower to leave room for other noise sources. There are two basic approaches: active regulation and passive filtering. I’ll present both options and let you decide.
The passive supply is quite a good option for circuits (like this one) that draw a constant current. This approach has the value of simplicity but, as we shall see, the disadvantage of size and cost. Basically, it consists of a series of RC low-pass filters. One can opt for chokes instead of the resistors for a further cost and size penalty, along with radiation issues, for a circuit which does not work better in this application. We will use resistors.
The passive RC circuit looks like this:
In this implementation, the raw supply’s ripple is knocked down to about 150uV, well within the noise budget. Separate filters should be used for each channel to prevent crosstalk. Separate raw supplies are luxurious and totally unnecessary. The last cap is bypassed with a film capacitor to ensure good decoupling at high frequencies. The filters should be placed as close to the load (the tube) as possible.
For those with a sense of adventure, active supplies are wonderful. They don’t cost much, don’t take up much room, and perform spectacularly well when properly designed. There are a lot of good options to choose from; the Maida regulator, a floating LM317 is a particularly popular choice. Fancier regulators using floating op-amp circuits can also perform superbly, albeit with greater complication. Here, I used instead a very simple two-transistor regulator, chosen for ease of construction and with an eye toward some adventures down the road.
This circuit is set up as a differential amplifier of sorts, the lower transistor acting as an error amplifier comparing a reference voltage from the zener to a divided down output voltage appearing at its base. Its collector sets the base voltage of the pass transistor and hence the output voltage of the regulator.
The divider string features a capacitor bypassing the top resistor in order to increase AC gain of the error amplifier. This cap does not need to be very large to be effective. The Thevenin equivalent of the divider string (with a little Kentucky windage to account for base current) is about 6.5K. A capacitor of 2u will then get within 3dB of ideal bypassing. I didn't have any appropriately rated capacitors of a size that would fit my little perfboard, so I stepped down to 1u. With that value, ripple voltage on the regulator output was under 100uV, so that wasn't too bad a let-down from ideal.
The zener is fed by the current flowing through the error amplifier's emitter- it may be augmented by another 5mA via a 15K resistor tied to the rail if your zener is balky when asked to wake up or it it needs to be worked harder to quiet it down a bit. Build the circuit without it, and if you experience any start-up glitches, put it in. A real stud will eschew a zener and, instead, stack a pair of LM329 references. Either way, the 220u bypass cap is your friend. The noise from the regulator I built was quite comparable to the RC passive filtered version.
The MPSU10 I used for the error amplifier is no longer available. But you can substitute any bipolar transistor with a 1W or better power rating, 200V or higher breakdown voltage, and as high an ft as you can manage. Another MJE340 can be used in a pinch.
The low voltage regulators are quite generic. Please note that there is a good reason not to separate the heater circuit from the other 12V parts of the circuit- the LM317/337 perform better regarding source impedance and noise as the current draw increases (see Errol Dietz’s paper in Electronic Design, December 14, 1989, and reprinted in “Troubleshooting Analog Circuits” by Bob Pease). Since the current sources and the (soon to be added) servo op-amps draw a piddly few 10s of milliamps, the 300-360mA heater draw really helps to make that regulator perform. The input cap is necessary when we put the raw supply in its own box; the 220uF bypass caps on the adjust pin and the output get the noise down to the sub-millivolt level, well under any reasonable need in this application. The most appropriate versions of the 317/337 are the "T" TO-220 packs. They'll dissipate 3 or 4 watts, so a small heatsink is appropriate. I used 1N4007 diodes for shutdown protection, but nearly any good medium power rectifier will work. If you want to be extra-geeky, use the same high-speed diodes that are used in the rectifier portion of the circuit.
So, with a bit of thought and some pretty conventional circuitry, we have made a power supply that is far better than our requirements. Let’s go have a drink to celebrate.
Men With Toolbelts
Given my total lack of skill in the mechanical aspects of fabrication, I feel a bit like a rabbi recommending brands of bacon. But at least I can explain my worst-case methods. As mentioned previously, I used two Radio Shack plastic boxes to house the preamp and raw power supply, with the aluminum covers providing a crude ground plane. This will probably be upgraded at some point- I think I’ll be living with this unit for quite a while. But for the moment, it works.
The circuitry was built on six small perf boards, each containing a functional block for one channel (current source, B+ regulator/servo, and signal circuitry). This was done so that I could switch various circuit chunks in and out during development. If you want to use this as an experimental base, that’s fine, but a circuit board or proper point-to-point would be a major step up. I’ve got some of those nice Tektronix ceramic/silver terminal strips that will be pressed into service when I do a rebuild. A good circuit deserves more than I’ve given it.
Input and output jacks should, logically enough, be isolated from the chassis- and from one another. Use a high-quality connector here where it counts.
The umbilical from the power supply is terminated with a 5-pin AMP female plug. The corresponding male AMP connector is mounted on the preamp chassis. There's no technical reason why audio-type connectors couldn't be used, but I highly recommend you don't do that- it's not a matter of if, it's a matter of when someone will plug the wrong thing in and let out the smoke.
I won’t specify wiring. I don’t think it’s terribly important, but if you do, use whatever flavor you like. To a great extent, I used Morgan Jones’s suggestion (in “Building Valve Amplifiers”) of solid silver wire with Teflon sleeving. I must admit that, with silver-bearing solder, it makes a lovely joint.
Grounding can be done in several ways- some prefer to use a single bus, connected to the chassis at one point. Some prefer star points. Others like to combine the two. Whichever you choose, the isolated nature of the inputs means that the usual ground point near the input jack is unsuitable. The best place to set the ground for this design is at the ground point for the input transformer secondary.
I haven’t diagrammed input switching since that will vary a bit from installation to installation. Use a good quality switch; I used a surplus Cinema Engineering 5 position two deck rotary switch with shorting between positions for optimal isolation. The input grounds are all tied together and kept completely separate from the preamp ground.
As I mentioned before, I used an Alps Black Beauty volume control. That’s a very, very good second choice. My first choice would be a stepped attenuator. If you’ve got the time and resources to build one, I’d highly recommend it.
I haven’t shown any pilot light circuitry, if you want one, I’d suggest putting a green LED in series with a 4.7K resistor, then putting that string between the +12 and -12 rails. You can also put in an indicator showing when the dethumper fires; I’d probably use a different color LED, put it in series with a resistor as before, and connect that string across the relay coil. When the dethumper unmutes the output, the light will go out.
When building this circuit, it's best to start it up without the tube in place, loading the current sources with a 1K resistor tacked between the collector and the +12 rail. Adjust the trim-pot to get 10V across the resistor. Then disconnect the resistor, warm in the knowledge that your current source is properly adjusted and may safely be used with your tube.
OK, enough for now. You have all that you need to build a good-sounding unity-gain tube preamp that will be quiet and unobtrusive. If you want to hang on for the ride, things will get a bit more novel in the next two parts.
My thanks again to all the participants at diyaudio.com whose input helped me refine and clarify a bunch of issues, Steve Eddy for hosting the illustrations (he’s even redrawn some of them!), Joel Tunnah for the cool Paint schematic symbols, and EC8010 for nagging me about details that I would have otherwise overlooked.
By the way, since you're calling this a "Unity Gain Line Stage" and since the 11P-1 has a loss of about -3dB and your buffer will have a gain slightly less than 1, a good alternative to the 11P-1 would be the 11P4-1-1. It has a slight positive voltage gain and would give you something closer to "unity gain" in the end.
The only difference between the two is that the 11P4-1-1 uses a 20k load instead of 10k.
Don't worry, I intended to resize them and only ask you to replace the old ones with the new ones. That will do until the little night-elves are finished with the cobbler and go to work on my scrawls. I needed to do that anyway to reflect a change in my circuit- the optional resistor to feed the zener reference in the B+ regulator should be included. I had an instance of a balky startup and after a lot of hair-pulling traced it to that.
Sure, the transformer can absolutely be diddled with to get gain if it's needed. The tradeoff is bandwidth, but it's not too bad of a tradeoff (80 vs 100kHz f3 in the example you gave).
FWIW, the gain of the CF part is about 0.95-0.96 with a 10K load.
The fiddly bits
Thanks for the Part II SY,
4 protection diodes are required, 2 each around the LM317 and the LM337 (due to the cap values you've used).
On the LM317 a diode from Vout to Vin (Anode at Vout) and from Vout to Vadj (Anode at Vadj)
On the LM337 diodes as for LM317 above but reversed polarity.
These can be any old power diode (1N4007 etc) as they are permanently off during operation and only operate (switch on) to shunt capacitor stored charge at power down.
I've run these regs in many circuits with and without the protection diodes. I've blown up a few regs when the protection diodes were omitted - I've never blown one when the protection diodes were incorporated. These days I just put them in routinely.
Edit: In response to below - application notes say protection diodes should be used whenever cap values exceed 10uF OR voltage exceeds 25V.
They won't hurt, but I've never seen them used in a 12 volt circuit that didn't have humungous caps. In Maida high voltage regulators, they are essential.
edit: I'll take that back. The NS datasheet does recommend them for large caps. I've never lost a regulator at 12V levels before, but better safe than sorry. I'll incorporate the change. Thanks!
Posted this in case you need it.
:D :D :D :D :D
diyaudio.com is like having the world's largest Design Review Board!
One more detail that EC8010 nagged me about: in the heater string, if you reverse the position of the heater and the dropping resistor, the heater will be at a more positive potential, a Good Thing as far as potential noise from heater-to-cathode leakage is concerned. I'll change that in my preamp and on the drawing tonight.
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