=20
Introduction
In the BA-1 and BA-2 projects we constructed = two=20 different amplifiers using very similar input and voltage gain stages = (aka the=20 =93front ends=94), but used them to drive two different Class A Mosfet = follower=20 output stages. Now we are going to flip this approach and begin = exploring some=20 different front end designs capable of driving these and other follower = output=20 stages.
Breaking it up this way allows some development focus and = speeds=20 things along. Those of you who have or plan to build an amp with these = (or=20 similar) follower stages should be able to follow along with minimal = additional=20 effort =96 you will already have the chassis, power supply, and output=20 stage.
I intend to follow this approach in future pieces =96 with = the power=20 supplies being independent from the output stages being independent from = the=20 front ends. Of necessity the output stages will be follower types =96 = this is=20 usually where we get the best performance from simple circuits anyway, = and after=20 a while I hope we will have a small library of modular pieces that can = be mixed=20 and matched.
To avoid confusion, I will be presenting the = schematics of=20 the BA-1 and BA-2 in stand-alone form here, and from now on the BA = series will=20 likely refer to interchangeable subassemblies.
So first, some=20 housekeeping. The official BA-1 and 2 front end now looks like = this:
The=20 official BA #1 single-ended output stage now looks like this:
And=20 the official BA #2 push-pull output stage looks like this:
Both=20 of these circuits now include the necessary components to bias the = output=20 stages.
The BA-3
For various rational and irrational reasons, = amplifiers=20 without negative feedback loops are of interest to many DIY amplifier = builders.=20 With the BA-1 and BA-2 we already have output stages which operated = without loop=20 correction, but they both have a front end which uses a local feedback = loop to=20 improve the performance, and in this regard their performance is pretty=20 conventional, if simpler than most.
So I think it would be fun to = make=20 the BA-3 front end also without the feedback loop and mate it to = follower output=20 stages (BA-1 and BA-2 output stages for example) to form complete = amplifiers=20 without feedback loops. Of course we have to give up the ease with which = loops=20 flatten out the response and lower the distortion. We have to make a = circuit=20 which has an intrinsically linear response - good enough for us to be = happy with=20 the quality of sound.
This begs the question, =93How small does = the=20 distortion need to be to sound good?=94
That's a very important = question to=20 which there is little common agreement. Some people think that = distortion should=20 be driven down into the =93part per million=94 region (0.0001%) to be = considered=20 adequately low, and some think that it doesn't matter at all. We are = going to=20 follow the middle path, where distortion does matter, but it is not = required to=20 be infinitesimal.
There is some good agreement that any = distortion that=20 does occur should be of =93low order=94, that is to say second and third = harmonic=20 character with very little of any higher harmonics. Is second harmonic = preferred=20 over third? Street wisdom says that you would prefer second, but in = practice=20 there is a sizable portion of the audiophile population spending money = on=20 amplifiers with a third harmonic character, and apparently they like it. = There=20 are audiophiles with single-ended tube (SET) amplifiers who think they = are=20 listening to second harmonic, but depending on the load-line specifics = of the=20 design, they often are not.
I recall Jean Hiraga wrote that the = best=20 sound was a modest mix of second and third harmonic with the second = being=20 dominant. Individual preferences vary, but I think there is considerable = truth=20 to that. This sort of thing is often achieved in only at one region of = output=20 level in amplifiers, and I have examples where they seem to deliver = their best=20 sound at only one level.
Going all the way back to Threshold, my = favorite amplifiers have tended to have a second harmonic character at = low=20 levels, gradually transitioning to a mix of second and third, and = dominantly=20 third as the output approaches the power limit. Later, we will examine a = knob on=20 the BA-3 which allows you to play with this mix.
I don't spend a = lot of=20 time getting excited about this sort of thing =96 I think it's possible = to=20 appreciate both types. The point is that low order distortion can be = tolerated=20 better, and a design without a feedback loop is going to want to use = Class A=20 operation to not only lower the absolute amount of distortion but also = to=20 concentrate that distortion into the second and third = harmonic.
Nothing=20 unusual about that =96 front end circuits are routinely Class A. The = difference=20 here will be an emphasis on high bias current, and on a balance between=20 degeneration and loading of each gain element.
Degeneration and Getting Loaded
Degeneration - reducing and=20 linearizing the gain of a transistor or tube - is easy to accomplish. = Most of=20 the time it simply means that the Source/Emitter/Cathode of the gain = device has=20 some resistance placed in series with it. This has always been a popular = technique, even before Harold Black invented the negative feedback loop, = and=20 remains so today among amplifier designers trying to avoid the dreaded = Transient=20 Intermodulation Distortion (TIM) boogeyman, a feedback loop = artifact.
You=20 should understand that degeneration is regarded by many as a form of = negative=20 feedback, and there is considerable argument as to what constitutes the=20 difference between this and feedback loops. However you want to = characterize it,=20 it remains local and not a loop.
Loading the output of a gain = element is=20 less popular among amplifier designers, at least among those whose = diagrams I=20 peer at in the midnight hours. It is not popular largely because it = reduces the=20 gain without any other apparent benefit =96 just throwing away perfectly = good gain=20 that could otherwise be used for feedback. It conjures up the image of = throwing=20 away perfectly good food, and your mother would not approve.
On = the=20 surface it makes logical sense, but of course it assumes that you are = using a=20 feedback loop. If you are not planning on doing that, then you might = consider if=20 there is some other overlooked benefit.
Certainly loading the=20 Drain/Collector/Plate with some resistance lowers the gain, as does=20 degeneration, but unlike degeneration, which raises the output = impedance,=20 loading the output lowers the output impedance. That is one potential = benefit,=20 since we often a reasonably low output impedance to drive a circuit that = might=20 follow.
I think though, there is another. We have a saying in my = house=20 (lifted from Cesar Milan) that =93A tired puppy is a happy puppy.=94 It = is my=20 experience that sometimes you get subjectively better sound from a gain = device=20 which is operated at a significant portion of its capacity =96 not = allowed to loaf=20 around with an easy job. We may find that it sounds better if we make it = do some=20 work. There's nothing magic about this. Making a gain device traveling a = load=20 line that represents real work gives it some character. Your mother = would=20 probably agree.
I know, I know - this makes the technical = cognoscenti=20 roll their eyes and/or snicker. But consider that there are quite a few=20 audiophiles who do not like the sound of loading an input differential = pair of=20 transistors with a constant current source or current mirror. They like = the=20 sound of a resistor instead, even though it doesn't measure as well. = Maybe they=20 like the lower feedback figure that this creates - or maybe they like = the sound=20 of their input transistors doing a little real work.
I'm just=20 sayin'...
Anyway, either answer suits our purpose here, so for = this=20 circuit we are going to depend on a balance between degeneration and = loading to=20 get the measurements and sound that we want.
The Meat
You probably skipped over the previous section to = get to=20 this part.
I think you'll like this front end circuit. In fact, = some of=20 you already like this circuit =96 it's a relative of the F5 amplifier, = scaled down=20 and absent the feedback loop.
This=20 is familiar enough. Q1 and Q2 are JFETs which self-bias into resistors = R3 and R4=20 at currents around 8 mA. R1 is chosen to avoid oscillatory interaction = with=20 whatever source impedance you might have, and R2 provides a DC reference = to=20 ground in the event that the source does not, and also establishes the = nominal=20 input impedance.
Q1 and Q2 are largely degenerated by R5, setting = the=20 amount of AC current which flows through them for a given input voltage. = The=20 voltage gain of this initial stage is approximately
the value of = Drain=20 load resistors R6 plus R7 divided by R5. In this case we have roughly = unity gain=20 =96 the Jfets are used as unity gain DC level shifters to Q3 and = Q4.
Coming=20 off the Drain of Q1 is the loading network of R6, C1 and P1, and there = is a=20 comparable network of R7, C2, and P2 attached to the Drain of Q2. R6 = clearly=20 sets the AC load for Q1, but the DC requirements to bias up Mosfet Q3 = are higher=20 than that, so P1 in parallel with C1 provides a higher resistance value = below=20 about 0.5 Hz, and gives the approximately 3 volt DC drop required to = bias the=20 Mosfets.
P1 and P2 are adjusted so as to set the DC bias of Q3 = and Q4.=20 You will want to set them at zero when you first fire up the circuit, = and=20 increase their resistance to achieve the correct bias voltages across R8 = and R9=20 (about 1 volt) while also keeping the output DC offset voltage at a = minimum.=20 This circuit is capacitively coupled at the output, but low offset = measured at=20 the Drains of Q3 and Q4 will maximize your output voltage = swing.
The=20 voltage appearing at the Gate of Q3 is amplified by something less than = the=20 ratio of R10 divided by R8, and with the same happening at Q4 and = considering=20 the transconductance of the Mosfets, comes out at about 15. Both of them = added=20 make a system voltage gain of about 30X, or 30 dB.
R8 and R9 help = set the=20 voltage gain, and they also help stabilize the bias of Q3 and Q4, else = it would=20 tend to drift upwards as the parts warm up. The bias current here is = about 50=20 mA, and it will deliver peaks of approximately 100 mA. Q3 and Q4 require = heat=20 sinks.
Of course you can bias this circuit higher if you wish =96 = 100 mA=20 bias is perfectly OK as long as you properly heat sink Q3 and Q4, and if = you are=20 crazy (like me) you can experiment with higher bias, remembering that = the parts=20 are rated at 25 watts, and that it costs you voltage losses across R8 = and R9. If=20 you want to play with even higher bias, you can consider lowering the = values of=20 R8 and R9 and also R13, all in proportion.
The supply voltage is = only=20 critical with respect to the voltage rating of the input JFETs, which = are=20 nominally 25 volts. In actual testing, they break down around 40 volts. = I=20 wouldn't worry about running them as high as 30V. Hot-rodding this = circuit would=20 likely involve cascoding the input Jfets to allow higher = voltages.
You=20 can also vary the bias current of Q1 and Q2. You measure the current by = reading=20 the voltage across R3 or R4. With 10 ohm, the voltage wants to be 0.08 = volts for=20 8 mA of current. You can play with this, but keep in mind that the = dissipation=20 of this part is rated at 400 mW,
and for this circuit 10 mA will=20 represent around 200 mW. These JFETs were chosen at Idss figures of 10 = mA (Idss=20 is the current that flows when the Gate and Source are at the same = voltage). If=20 you get lower values, you may want to consider higher values for P1 and = P2, say=20 1 Kohm. They are best matched, but P3 can be adjusted to compensate for = some=20 mismatch.
P3 and the Second Phase
Potentiometer P3 is provided for = those of=20 you who have distortion analyzers or want to play with the mix of second = vs=20 third harmonic. As with the F5 amplifier, you will find that you can = iteratively=20 trim P1 through P3 for minimum distortion. I do so at approximately the = 1 watt=20 output level (2.8 V). You should start out with the setting at the = mid-point.=20 The minimum distortion point will generally be where the plus and minus = halves=20 of the amplifier balance to such a degree as to null out the second = harmonic. By=20 introducing an imbalance you can variably re-introduce the second = harmonic, and=20 depending on which way you turn the pot, you can choose the absolute = phase of=20 the second harmonic.
I knew you would like a choice
There=20 is flexibility in choices of parts and you can swap these components for = others,=20 keeping in mind that you often will have to adjust resistor values to = get=20 similar bias figures. I have mentioned different Idss values for the = input=20 JFETs, but you may also find that your Q3 and Q4 have Vgs figures = different than=20 mine. If so, you may again find that P1 and P2 should be increased. =
It=20 usually is not actually necessary to run out and buy a new potentiometer = - if=20 the maximum value of the pot doesn't get you want you want, then simply = add a=20 resistor (having the same value as the pot) in series with the pot and = you can=20 increase it from there which will involve cutting traces on the circuit=20 board.
Construction
DIYAudio.com will be offering the authorized = printed=20 circuit board for sale. Anyone is welcome to cobble together their own = for=20 non-commercial purposes, that is to say not for sale.
Here is the = trace=20 diagram of this board:
Here=20 is the component placement:
Here=20 is a nice photo of the board which I assembled all by myself:
You=20 may note a few items. I have made an allowance for two different C3's on = each=20 channel, one the polypropylene capacitors in white and another an = electrolytic=20 which you don't see stuffed. You may use either or = both.
Questions always=20 arise about the quality requirements of various capacitors and resistors = and=20 such. My attitude here is that you should feel free to use what you = like.=20 Certainly the objective (measured) performance will not depend much on = the=20 quality of the passive components. On the other hand, here is a circuit = that=20 deserves nice parts.
Where you will see some dependence for = certain is on=20 the input and output transistors. Toshiba is the supplier of record for = this,=20 with the 2SJ74 and 2SK170 input Jfets and the 2SJ313 and 2SK2013 output = Mosfets.=20 The 2SJ74 and 2SK170 have become harder to get, but it's worth the = effort and=20 price. LSK170's are available, and one of these days I hope that Linear = Systems=20 will also get their P channel complement to market. Others will work = well=20 enough, usually with more noise and the need to adjust the values of the = Source=20 resistors.
The 2SJ313 and 2SK2013 are still available, and you = can use=20 them or substitute other Mosfets, such as the Fairchild FQP3N30 and = FQP3P20. The=20 Vgs of the Toshibas is about 2 volts. Some of the alternatives will be = in the=20 region of 4 volts or so, requiring a higher value for P1 and P2, = probably 1K=20 ohm.
Matching is nice, but not essential. P1-3 will cover much of = that=20 action. Keep in mind that every time you adjust one of these pots, you = will=20 probably have to re-adjust the others, so try to do every adjustment in=20 half-steps, a little bit at a time.
Performance
The data following are for a circuit with the = parts and=20 bias as shown and with P3 set for minimum distortion. Here's the = distortion=20 curve of the BA-3 front end vs output watts (as if it is driving a = perfect=20 follower output stage)
With=20 the second harmonic largely nulled by P3, we see the following = distortion=20 waveforms for the circuit at 2.8 volts output without a load (although = you will=20 note that the circuit already has its own 332 ohm load to ground). Here = it is=20 with +/-32 volts rails:
And=20 below is the same, but with +/-24V rails:
You=20 can clearly see that the distortion in this case is mostly third = harmonic.=20 Adjustment of P3 will bring the second harmonic out, and as you turn the = wiper=20 toward the Source of Q1 you favor the positive going waveform. When you = turn the=20 wiper toward the Source of Q2 you favor the negative going waveform. = Favoring=20 the positive going waveform is possibly preferred =96 with proper = speaker=20 polarity, this mimics the acoustic character of air.
Here's the = distortion vs=20 frequency at 2.8V.
Not=20 much to say here. This was taken with a 600 ohm source. It gets a little = better=20 with a 25 ohm source, but that's not to say it sounds = better.
Here's the=20 frequency response curve at 2.8V, also with a 600 ohm source:
Here's=20 the square wave response at 15 volts output:
And=20 that concludes the objective part of our show...
In=20 Conclusion...
I bet you thought that getting good Class A = performance=20 without a feedback loop was exotic, difficult, and expensive.
Not = if you=20 do it yourself.
=A9 2011 Nelson Pass
where is our downloadable pdf version
good=20 work !