OT- Solid state amp biasing

[space_ryerson]

Hi guys,

One of my Randalls is in need of re-biasing, since the last time I had it open, I slipped and moved the trimpot a smidge. I referred to the schematic, and matched the trimpot to -.6v as suggested, and it sounds really blatty. I move it one way, it sounds even more blatty (and hums a lot); and the other way it has a slow attack. I can get it almost sounding good, but I'm moving the trimpot so minutely, it's almost impossible to hit the right setting. Am I doing this right? What is the proper way to bias a SS amp? Did I do something else to my amp as well as move the trimpot?

Here is the schematic. The bias trimpot is on the lower righthand side of the schematic below the two diodes.

Thanks!

[Johnny Guitar]

I referred to the schematic, and matched the trimpot to -.6v as suggested, and it sounds really blatty.

Here is the sechematic. The bias trimpot is on the lower righthand side of the schematic below the two diodes.

My eyes aren't the best (by any means) but I was thinking it said (+.6v). Its hard to see on the monitor, but from here I'd get a magnifying glass out and make sure.

J

[space_ryerson]

It's definitely -.6v. I have the actual schematic (not the scanned one); which is sharper. When I scanned it in, I shrunk it down a little too much for the web. I should also mention that the point where the amp almost sounds right, my meter reads -.662v.

[R.G.]

Biasing a solid state amp can be simple or maddeningly complex.

The amp is an early version of the first Linn-style power amps with a differential input. The output is quasi-complementary, as the drivers Q11 and Q12 are NPN/PNP, and the outputs Q13-16 are all NPN.

In this circuit, the upper set of drivers/outputs are a darlington emitter follower, and need the two base-emitters in series to be ...just... barely... biased on, about +1.2V. The lower driver, Q12 also needs its base-emitter barely on, but the NPN outputs are not party to the biasing on the bottom.

What they're trying to get you to do is to set the base of Q12 to -0.6V, where theoretically it is just barely on, and rely on the D7 and D8 diodes with their parallel resistances to turn the upper driver and outputs on at the same time.

Fitting this scheme into the range of modern amp design, this bias circuit is something like using a stone axe with no attached handle. It's touchy, poorly thermally compensated, and difficult to get right. When it's biased too low, the amp sounds bad (from crossover distortion) and when it's biased too high, the amp can easily go into thermal runaway.

Other than that, it's a great scheme.

Modern power amp design chucks this whole thing for a single Vbe multiplier transistor and a trimmer. It's more stable and tracks better.

If it were mine, I would ignore the voltage notation, and measure the voltage across R55 or R56 or both, and diddle the trimmer pot to get 15 millivolts across either one. That point sets the standing current in Q13 and/or Q14 to 25ma, just about the ideal point for bias current for a bipolar power device in this setup. Below this and the amp will go into crossover distortion, above it and it is likely to get hot.

[space_ryerson]

Thanks RG!

We really need an emoticon of someone with their brain exploding, because you just gave me 4 different things to type into google. I very much appreciate the knowledge.

I will work with R55 and R56's voltage tomorrow. I know Randall changed their power amps in later models, but the ones I have tried lack the sonic charm of this model for me. If you have any simple power amp mods to try in order to make this more stable, I'm willing to give it a shot since I have three of these, and one of them is simply gathering dust in the corner.

Again, thanks for the detailed reply. You have given me a lot to work with.

[space_ryerson]

So, biasing using RG's method worked well, but now I'm wondering how to possibly attach a 'handle to the stone axe', in terms of biasing. Is the method this amp uses for biasing also known as 'crossover-notch biasing'?

So basically I'm wondering if there is a way that I could bias this sucker in a better way. Maybe I should find a way that the amp doesn't rely upon D7 and D8? I've been looking at some modern amps' biasing, but I'm not the most clear on power amp design yet.

[R.G.]

Biasing power amps by measuring any bias voltage is a method doomed to fail, whether the amp is solid state or tube.

The one, true way to bias an output stage is to set the standing current correctly. I gave you one way. There are others, and I'll mention something about them later.

Is the method this amp uses for biasing also known as 'crossover-notch biasing'?

No. The method used in this amp is "sudden death" biasing. There is no way to get satisfactory biasing this way; in fact, I believe that if you were privy to the Randall service literature, they would tell you some current-measuring way to do this. I think that the voltage mentioned is a typical one, just for quick checks with a voltmeter, and not actually intended for bias setting. I'd bet a significant sum of money that Randall and Randall techs actually used a current  measurement method.

Crossover notch biasing involves watching the output of the amp on an oscilloscope with a big sine wave input and looking for funny notches around the 0V signal level. Then you tweak the bias pot until the crossover notch ...just... disappears. This is almost inhumanly difficult to do right, and incredibly easy to do wrong by overshooting the notch. There is some evidence that tube amps could possibly be biased this way, but it's never been a good choice for solid state.

So basically I'm wondering if there is a way that I could bias this sucker in a better way. Maybe I should find a way that the amp doesn't rely upon D7 and D8? I've been looking at some modern amps' biasing, but I'm not the most clear on power amp design yet.

There are two and only two ways I can think of to improve on the way you're doing it now.
(1) trade out D7/8 for a Vbe multiplier, as I noted earlier, but with a 10 turn trimpot for fine adjustment; the thing you look at is still output transistor current.
(2) any form of bias adjustment, whether the trimpot you have now or the vbe multiplier, but watch the output of a distortion analyzer in real time on a scope with the signal nulled out and only the distortion showing. With this available, you can tune the bias for lowest distortion. This is the only method I know of - and I have looked for this kind of thing for decades - that is actually better than watching the output transistor current. And I have found only one amp that needs this to be biased correctly. That was the SWTPC Tigersaurus, an early 250W amplifier. The space between high crossover distortion and blazing hot heatsinks was vanishingly thin in this amp, and the distortion analyzer method was the only successful way.

[Hal]

::bows to R.G.::

[space_ryerson]

Thank you RG!

I'll talk to Randall about getting service notes for these beasts, and look into how I would put in a Vbe multiplier with a trimpot.

[R.G.]

Consider what happens when you have an NPN bipolar transistor with a resistor from its collector to base (let's call this R1) and another from base to emitter (call this one R2). We'll also assume that we have a battery and resistor in series feeding current into the collector and out the emitter, with several volts of voltage and the current limited by the external, unnamed resistor to some value that won't cause excess heating of the NPN.

Initially, there is voltage across the NPN, but no currrent through it. The voltage causes a voltage divider effect across the two resistors, so if we take the emitter as a 0V reference point, the voltage at the base tries to go up to Vcollector times R2 divided by R1+R2. At some point the voltage reaches the turn-on voltage for the NPN, 0.5 to 0.7V depending on the device, and the base starts conducting.

If the NPN gain is reasonably high, say 49, then 49 times that much current flows in the collector. The current is limited by the external resistor though, so when it reaches the current limit, the collector is eating 49 parts of the current while 1 part is going through the resistors and base. (The sharp guys are now figuring out why I hand-waved a gain of 49 instead of 50... 8-)  )

So what's the voltage on the collector where this magic point occurs?

OK... I picked a current gain high enough to ignore the base current - trust me on this one - so we have the voltage across R2 being Vbe for silicon, and to get it up to there, the voltage at the collector has to be enough to get it to Vbe. Since Vbe= Vcoll * (R2/(R1+R2)), then we can get out our high school algebra texts and solve for Vcoll.

Vcollector turns out to be Vbe*(R1+R2)/R2, or Vbe*(1+R1/R2), independent of everything else.

Looked at another way, this is a feedback amplifier, with feeedback from the collector to the base. Same equations come out.

So for your biasing you remove the diodes and paralleled resistors, etc. and substitute in a medium power transistor (we'll get to why medium power) with a couple of resistors, one from collector to base, one from base to emitter. Now let's count junctions. The Randall output stage is quasi-complementary, so there are two base-emitters in series that have to be turned on in the top half, Q11 and Q13. There is only one in the bottom half, Q12. We know that the correct bias voltage is where these base-emitterses are just barely on, so the nominally correct voltage is three times about 0.6, or 1.8V.

To get to that, you need R1 and R2 so that 1+R1/R2 is about 3, or R1=2R2. But what are the absolute resistor values? Let's calculate a bit. Get out your Ohm's law text.

How much current is going through there? Well, at no signal conditions from the schematic, there is 40V on the - power rail, and R48+R49 is 2.5K. The specified bias point is at -0.6, so there's 40- 0.6V across 2.5K, or 39.4/2500 = 15.7ma of standing bias current. Our Vbe multiplier needs to have Vce of 1.8V at 15.7ma.

So we can assign how much current we want where. We're free to have the split of current between the collector/emitter path in the Vbe multiplier and the R1/R2 path be almost anything as long as the collector can overwhelm the R1/R2 path in operation and the base current sucked out of R1/R2 does NOT overwhelm the R1/R2 current.

With a target gain of 50, let's pick the R1R2 current to be ten times the base current, so just counting fractions we have one part base current, ten parts R1/R2 current and 50 parts collector current, and a total of 61 parts. One 1/61th of 15.7ma is 259uA, so the resistor string current is 2.58ma.

Back to Ohm plate. the voltage across R1/R2 is 1.8V, the current is 2.58ma, so R1+R2 is 1.8/2.56ma = 697ohms. R2 is 1/3 of that, or 232 ohms and R1 is 2/3 or 465 ohms.

But we actually wanted to vary that a bit, maybe from 1.7 to 1.9V or even a little more.

So we need to put a pot in to vary the Vbe ratio.

First, simple choice is to use a 750 ohm (or 1K) pot, stringing the outer lugs from collector to emitter and connecting the wiper to the base.  Oly problem is range. The pot can now cause Vbe ratios from one (that is, R1/R2 is about 0) to substantially infinity by shorting the base to the emitter. Bad juju indeed.

So let's only vary one resistor. Which one? If we use the pot to vary R1, and the wiper goes open, the transistor turns off, the output transistors are turned maximally off, and the amp pours out prodigous amounts of smoke and flame. Ooops, can't do that one.
Let's use the pot for R2. Now if the pot opens, we get transistor fully on and the bias goes to one Vbe, which sounds terrible, but does not kill the map... better juju.

So we want a fixed resistor for R1, variable one for R2. Pick 470 ohms for R1. That resistor dissipates 0.00258A**2 * 470ohms, or 3.128milliwatts; a 1/4 W will do nicely.

We put all our variation into R2. Again, we could use the whole pot range by using a 500 ohm pot, but that gives us an output range of 0.6V (R2=0) to 1+470/500 = 1.94 Vbes. We can do better.

Ideally, we'd like the entire range of the pot to go from just a little too little voltage to a little too much. The output devices might start turning on at as little as 0.45V, so three of those is 1.35V. They might be hard cases up at 0.7 or 2.1V. The Vbe of our multiplier transistor may be the same range (although chances heavily favor it being close to 0.55V for a variety of reasons), so our multiplier needs to cover the case where we put out a minimum of 1.35V when our multiplier Vbe is 0.7V and where we put out at least 2.1V when our multiplier is 0.45V.

So we need a multiplier range from 1.35V/0.7V = 1.928 to 2.1/0.45 = 4.67.

We're going to use 470 for R1, and the range on R2 is that R1/R2 is 0.928 to 3.67. That makes R2 be 128 to 506 ohms.  So we use a 120 ohm resistor in series with a 380 ohm pot, and we're golden.

Errr... there aren't any 380 ohm pots...

OK, pick a 500 ohm pot. Scale up the series resistor to (500/380)*128=168 ohms and R1 to 618 ohms. So we get a 620 ohm 1/4W resistor for R1, a 160 or 170 ohm resistor in series with a 500ohm pot for R2. We connect only to one lug and the wiper of R2 pot because if the wiper fails we want that pot to go open, not to the full value of R2.

Now we need a transistor.

The transistor needs to conduct 16ma with say 2V across it. That's 32milliwatts. But there is another requirement. This transistor also does the thermal compensation for the output stage. That's why there is that thermistor on the schematic. The transistor MUST be bolted to the heat sink right where that thermistor is so it heats up like the thermistor. That's what gives you thermal compensation so as the output devices get hot, the bias keeps them out of thermal runaway.

One path is to pick a TO-126 (weenie-pack) device, another is to get a TO-220. Both of these require insulation from the heat sink because the full signal voltage is on their collector, which is tied to the heat sink. My favorite is to get full-pack to-220s which are encased in epoxy and need no insulating. If you get an uninsulated package, also get an insulator and some thermal grease or one of those sil-pad things and be sure you insulate the collector from the heat sink. Check it with an ohmmeter when you're done.

I got the following out of the Digi-key catalog.
ZTX455 - $0.63 Super E-Line -no insulation needed, bolt it on.
2SC1567or - $0.70 TO126 package, needs insulated
2SC3063  - $0.66 TO126 also needs insulated

With the range of the pot restricted, you can probably use a single turn pot instead of a ten turn.

[space_ryerson]

Thank you very much RG. I have had to re-read this a few times now, and I think I'm starting to get it. I'm sure it will start to fully make sense soon.

[R.G.]

Re-reading it myself, I think there is a lot of trees described, but maybe not much forest.

Solid state amps in general use a complementary emitter follower for their outputs. These may be darlington ( like the driver/output in the positive side of the Randall) or complementary follower ( like the negative side of the Randall.)

Those two complex emitter followers produce huge current gain from the voltage amplifier stage, but no voltage gain. In this power amp, Q10 is the voltage amplifier stage, and its collector has the full output signal voltage swing on it. The entire driver/output section does nothing but buffer the voltage amplifier output from the speakers. The Q11/Q13 side buffers positive going signal at Q10's collector, the Q12 side buffers the negative going signals. Crossover distortion happens when they don't meet exactly in the middle as the signal is crossing over from positive to negative and vice versa.

You could bias this thing by simply tying a wire between the bases of Q11 and Q12. That would turn off both the top side outputs and the bottom side outputs until the signal at Q10's collector moved high enough or low enough to start turning on Q11/Q13 or Q12 respectively. There would be a dead zone in the middle where neither the top or bottom sides were coducting. That is crossover distortion - the dead zone in the middle.

In fact, why don't you? It won't hurt the amp. Just solder a wire from Q11 base to Q12 base. This shorts out all the biasing. Then listen to it. It may sound grainy, probably will. That's crossover distortion. It would be worse, but the feedback of the amplifier reduces the crossover distortion by the open loop gain factor of the amplifier, probably about 40-60db.

Remember to pull your wire back out or you'll get major frustrated with biasing later. That's ... um... something I saw a guy do once in a labl. Yeah. That's my story and I'm sticking to it.

Biasing this amp means to ...gently... pry the bases of Q11 and Q12 apart by a little over a volt. If you pry them apart a lot, this turns both positive and negative drivers on at the same time, and that current doesn't go through the speakers. It goes in the top side outputs, through the bottom side outputs and back to the power supply. The outputs have the full DC power suppl across them, so they dissipate power equal to 42V times whatever current flows.

That's why an open is so deadly in the bias string ( the bias string being whatever is strung between Q11 and Q12 bases). It turns the outputs full on, with full power supply, and they have a race to see which one can burn its bonding wires open first, and whether the transistors will die before the power line fuse opens. And bias voltages over about 2.1V (0.7V per output transistor junction) are just as deadly as an open.

So the entire trick is to get just a ... hair... of current trickling through the outputs. That's simplest to do by watching the current going through the outputs by watching the voltage across an emitter resistor like R55. Get too little current (as measured by the voltage across that emitter resistor) and you get crossover distortion. Get too much and the amp gets... hot...

[brett]

Thanks heaps RG.  Very helpful, very understandable.

Remember to pull your wire back out or you'll get major frustrated with biasing later. That's ... um... something I saw a guy do once in a labl. Yeah. That's my story and I'm sticking to it.

:lol:

[brett]

Oh yeah.
I've just remembered how I biased an old (1970s) amp.  This might be relevant in some situations.
I knew that the bias current should be about 40mA, so I replaced the supply rail fuse with a 1 ohm resistor, and fiddled the bias until I got 40mV across it.
I suspect that the idea was borrowed from a similar cathode current biasing method in tube amps (see tube amp biasing at geofex.com for a full description of this method).
cheers

[Steben]

Well, left aside the technical thesis this is a FET sound amp after all. I wonder how it actually sounds. It looks like a swap-the-tubes-with-FET's amp, much like the ROG world.
Anyone that can testify?

[space_ryerson]

Thanks you very, very much for the help RG. Life's been very hectic lately, so I haven't had any time to look further into this yet. I'm excited to get cracking on this soon. I will post my results when I do.

Steben: This amp does sound pretty 'fet-ty'. On the disorted/lead channel, think Pantera or U2's 'the edge' on his distorted tones. They both used this amp. It sounds a little razory, but in a good way.

[space_ryerson]

Well, it has been almost 8 years since this thread, but it has been sitting in the back of my mind for the whole time. It was initially well beyond my skill level, but reading through nowadays, I can at least follow. RG's initial advice on how to bias the amp as-is has worked well for years, but one of my Randalls is pesky to get right at higher volumes, so I want to give his modification a shot. Since the link to the original schematic is long gone, you can view it here.


RG, can you verify that I'm understand this correctly? Here is the original power amp:



...and here is how I'm interpreting your suggestion:



Is this correct? I wasn't sure about C32 and C44.

[PRR]

Have not read all of R.G.'s surely-excellent old posts.

The +1.2V and -0.6V notes are clearly to show way-wrong problems, NOT for bias.

You bias amps like these by putting volt-meter across R57 R55 and adjusting for several dozen milli-Volts. <20mV will rasp small signals. >100mV will overheat at idle. 30mV-50mV is a happy-zone. Higher sounds slightly better BUT not that much better and far more risk of disaster.

(It had me fooled. Looks like Q13 Q15 are a Darlington, but Q13 carries a LOT of the load. If we biased-up Q15 to a tenth-Amp, Q13 would be at 1.1A, far too hot. So Q15 idles "off", Q13 carries the first Amp, then Q15 takes the rest. Very odd.)

> pesky to get right at higher volumes

Bias of this amp affects the soft-level (~~1Watt) tone, NOT the high power tone.

Usually.

All your electrolytic caps are old enough to be retired. C33 C34 C36 and especially C35.

> about C32 and C44.

Correct as shown. No effect on DC idle bias. Not electrolytic thus unlikely to age badly.

The quasi-comp (both-NPN finals) is a fine design.

The diode-string is not utterly wrong, but this one has clearly been bodged on the bench. Should be three diodes (to echo the Q12 Q11 Q13 loop), not a resistor-trim and two diodes shunted with resistors and thermistors.

Problem if you replace the bias affair: if you get it wrong, the amp will die at switch-on.

The proposal in red is the right idea.

Connected to chassis - no, the bias device _must_ be thermally intimate with the Q13 Q15 devices. Clamped to the transistor cases or very-close on the heatsink in good contact. (TO220 package is nice because it suits a sink; the added cost is now moot.)

ZTX455 is a 140V part, doing a 2V job.... why?

The 470:620+500 bias divider looks short to me. Was this computed? My math says the "470" needs to be 1740 to reach three junction-drops. However the lower value is safe.

[PRR]

C33 C34 C36: Polarity marks are wrong. (Q8 Q9 Bases will sit a hair Negative, not a bit positive.) Only 0.1V-0.2V, so they won't die soon.... but they have been that way long enough to maybe un-do them.

[R.G.]

@space:
That's not quite it, but close. Where you have a 470 ohm, make that 620. Where you have a 620, make that 160 or 170 ohms. Keep the 500 ohm pot as is, per this:

OK, pick a 500 ohm pot. Scale up the series resistor to (500/380)*128=168 ohms and R1 to 618 ohms. So we get a 620 ohm 1/4W resistor for R1, a 160 or 170 ohm resistor in series with a 500ohm pot for R2. We connect only to one lug and the wiper of R2 pot because if the wiper fails we want that pot to go open, not to the full value of R2.

It would not hurt to put four silicon diodes in series around that whole thing as an ultimate fail safe. It won't keep the amp from overheating if the biasing transistor/resistors decide to open, but it may give you some time to shut it down. Mostly, these will do nothing in normal operation. They're kind of a fail-less-unsafe.

Have not read all of R.G.'s surely-excellent old posts.

I find that  "surely-excellent" may depend on how much coffee I'd had that day. 

The +1.2V and -0.6V notes are clearly to show way-wrong problems, NOT for bias.

Yep!

You bias amps like these by putting volt-meter across R57 R55 and adjusting for several dozen milli-Volts. <20mV will rasp small signals. >100mV will overheat at idle. 30mV-50mV is a happy-zone. Higher sounds slightly better BUT not that much better and far more risk of disaster.

Yes.

Bias of this amp affects the soft-level (~~1Watt) tone, NOT the high power tone.
Usually.

Yep. The only exceptions I can think of is when high power drives something into unexpected behavior.

Connected to chassis - no, the bias device _must_ be thermally intimate with the Q13 Q15 devices. Clamped to the transistor cases or very-close on the heatsink in good contact. (TO220 package is nice because it suits a sink; the added cost is now moot.)

Yes. It should thermally track the output device as closely as possible.

The 470:620+500 bias divider looks short to me. Was this computed? My math says the "470" needs to be 1740 to reach three junction-drops. However the lower value is safe.

It is short, if you keep the 620+500. I actually recommended a 620: 160+500, buried in the mess I typed. It took me a while to go dig that out. 

And yes, this amp could well profit from replacement of all the electrolytics. The high voltage transistor is not necessary, but will be OK if its HFE is over 50 at 10ma.