Mystery resistor in simple audio amp

[MajorMajor]

Thanks for your suggestions. Let's for a second leave aside the possibility that the 4.7K resistor was only included to confuse the Russians; somehow I do not believe a rouge resistor in an audio design would have dealt a major blow to the might of the Soviet military forces...

It is also clear that the resistor has negligible effect on the steady state AC and DC conditions. Simulation shows that its effect on the open loop gain is less than 1%. So we are left with the possibility that it has some secondary (RF suppression) or transient protection function.

I tend to favour the argument that it is for transient protection but I still can't see why it's needed. Let's suppose the resistor it's not there (i.e. shorted out). OK, at the moment of power-up, the 47uF cap is short circuit, so the FET source will be connected to ground through a 47 Ohm resistor. Now the Idss (saturation drain current) of the BF245A is specified as 6.5 mA maximum, which for a very-very short time may occur (although it would immediately cause the source to go positive due to the voltage drop on the 47 Ohm resistor in the source, so I would expect even this 6.5 mA only to last a few nsec) but then - so what? The 6.5 mA current is far too small to cause any damage to the BC212 base-emitter junction and the FET can also handle 50 mA maximum drain current. Even at 6.5 mA and the maximum 24V supply voltage the FET dissipation is only about 150 mW - again, no problem, it can handle 360 mW continuously (and, as before, these are absolute maximum worst case transient figures). So, unless I have overlooked something, even under worst-case transient conditions all devices are operated well within limits that they are specified to handle continuously!

When it comes to the other possibility, RF suppression, again, somehow it doesn't make sense. In my experience, the BF245 series is really not very prone to this anyway and even then it's usually fixed by the usual gate-stopper resistor.

Finally, this is definitely not just a design on paper, the book shows some measurement results for quiescent current, distortion, etc, so clearly the circuit was built and tested.

Arhh, it's so frustrating because the design is so simple... Could somebody please locate the original design engineer and interrogate him...?

[Andrew Eckhardt]

The drain impedance may not be low enough for an added 4.7k to cause a dramatic shift in response above the audio band, but it is cheaper than adding a capacitor from collector to base on T2. Barely enough would have been excellent in such a circuit that may have actually wound up in someone's hybrid tube-semiconductor TV audio output at the time.

[kevinkr]

Originally Posted by SY

I don't see how. R1 is paralleled by the low Rbe of T2. It looks to me like it's just there to set DC conditions of T2, which is a current source. R2 looks like it limits the base current of T2.

Really depends on the base resistance of T2 , but R1/R2 does form a voltage divider between the drain of the fet T1 and the supply rail. Depending on that transistor the attenuation between the drain of T1 and base of that T2 might be a lot more than 6dB.

T2 is the second stage of the VAS, it's not a current source at all, and notice how its collector load is boot-strapped to the load side of the output. (That 120 resistor effectively takes the place of a current source.)

I designed amplifiers like this as a kid, using a bi-polar in the input stage, they actually work and sound much better than you would expect. Early HH Scott transistor amps and receivers also used a very similar topology and you guessed it - sounded better than you would expect, and a lot better than what subsequently followed in the consumer sphere.

I've reposted the schematic in the interest of clarity. And I will probably simulate this circuit in LTSpice soon.

[SY]

I dunno, it looks to me like a CCS of I = 0.7/4k7, which then causes the voltage across the two diodes to be constant (the Vas). The bootstrapping, as far as T2 is concerned, doesn't really change the current (except for Early effect). Base resistance is low because T2 isn't degenerated. What am I missing?

[kevinkr]

Originally Posted by kevinkr

Really depends on the base resistance of T2 , but R1/R2 does form a voltage divider between the drain of the fet T1 and the supply rail. Depending on that transistor the attenuation between the drain of T1 and base of that T2 might be a lot more than 6dB.

T2 is the second stage of the VAS, it's not a current source at all, and notice how its collector load is boot-strapped to the load side of the output. (That 120 resistor effectively takes the place of a current source.)

I designed amplifiers like this as a kid, using a bi-polar in the input stage, they actually work and sound much better than you would expect. Early HH Scott transistor amps and receivers also used a very similar topology and you guessed it - sounded better than you would expect, and a lot better than what subsequently followed in the consumer sphere.

I've reposted the schematic in the interest of clarity. And I will probably simulate this circuit in LTSpice soon.

I have created a working simulation but have to go out on an urgent errand to pick up pet medications. (Sick bunny) However my contention that R1/R2 has an effect on loop gain is clearly shown in the SIM, and it isn't too subtle either. (10dB or so with transistors chosen)

[SY]

Originally Posted by kevinkr

I have created a working simulation but have to go out on an urgent errand to pick up pet medications. (Sick bunny)

Is that a euphemism?

Thanks for doing the sim. I'm interested to see it!

[john curl]

It is probably there just to reduce the operating voltage and temperature of the input fet.

[kevinkr]

Originally Posted by SY

I have created a working simulation but have to go out on an urgent errand to pick up pet medications. (Sick bunny)

Is that a euphemism?

Thanks for doing the sim. I'm interested to see it!

OT: No, unfortunately, one of our rabbits has just started showing symptoms of e-cuniculi which killed his mate last summer. More here: Encephalitozoon Cuniculi Watching a loved pet going through this is absolutely heart wrenching.

[kevinkr]

Originally Posted by SY

I dunno, it looks to me like a CCS of I = 0.7/4k7, which then causes the voltage across the two diodes to be constant (the Vas). The bootstrapping, as far as T2 is concerned, doesn't really change the current (except for Early effect). Base resistance is low because T2 isn't degenerated. What am I missing?

That it's not a current source, but a common emitter amplifier. The base current is actually proportional to some % of the drain current. The device operates as boot-strapped common emitter amplifier - the ac current in the collector circuit is reasonably constant, the dc current is controlled indirectly via the feedback loop. (Basically whatever current is required through the 120 ohm resistor to keep the output stage centered roughly at half supply.)

[kevinkr]

Here is the first screen shot which clearly shows the gain in the first stage is roughly 8dB (blue trace) and the signal present on the base of Q2 is 33dB lower (Green Trace) The red trace is the 0dB reference or input signal, all traces shown are relative to this value. What this means is the base resistance is actually extremely low and very little of the signal current actually flows through R1.

Note that I used typical transistors which are present by default in LTSpice IV, I haven't the inclination to add more suitable models, but this will illustrate the point.

I have posted a zip file of the LTSpice circuit model file - if you are an LTSpice user you can unzip and load this file right into LTSpice and run it, play with it to your heart's content.

That said the design has a gain closed loop gain of over 40dB, probably will not operate well on voltages higher than 15 - 20V without significant changes and appears to be designed to provide really high input impedance. The linearity is not that great. I will post an FFT at a couple of watts out - note that the output transistors are 500mA types because they were the most appropriate types, and have low beta. (25) I'll try higher beta versions as well and note the difference.