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1970s Design Indulgence

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Feb 2020 at 3:47am
No, it won't look like this when it's finished!



The meters will be set in the front panel and the blue coloured pot shafts will be set back inside, with only the "screwdriver" slot visible adjacent each meter.

This design feature is no gimmick! It enables the user of this amplifier to adjust it to deliver its best, whatever the supply voltage variance.

The meters also double as "VU meters" - use them as you feel fit.

This long journey has seen all kinds of issues raised as how to obtain the best audible and measured performance out of a 6 transistors per channel power amp.

That's 6 transistors taking a sub-1 volt input signal and delivering 42 watts into 8 ohms (per channel).

Its audible performance is now pretty damn good, and as for measurements, they don't tell us much (and never have!)

Now it is time to pre-productionize the amplifier.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 21 Feb 2020 at 10:30am
The Meters and the Maths

Bob Cordell's modified SPICE models for the MJL21193 and MJL21194 output transistors gives slightly different betas (BF) and adds base resistance (RB).

This makes it possible to calculate the quiescent voltage (Vq) reasonably accurately.

The exact figure would have to be arrived at through iteration, but here we are talking about fractions of a millivolt which, even if we could set the voltage to, would be ephemeral due to minute changes in ambient temperature, and tiny mains voltage variations.

Estimating 60mA quiescent current in the output stage and betas of 70 (NPN) and 110 (PNP), the base current will be 0.86mA (NPN) and 0.55mA (PNP).

Considering the NPN upper EF2, the driver has half the resistance of the inter-emitter resistor (0.5 x 220R), and 0.625V is dropped across it, which is 5.7mA (this will be the same for the PNP lower EF2).

Now add the bias: 5.7 + 0.86 = 6.56.

The emitter resistance of the upper driver will be the thermal voltage divided by the current: 26mV/6.56 = 4 ohms.

Cordell gives MJL21194 base resistance as 3.4 ohms, to which we add 4 ohms to give a total input resistance of 7.4 ohms.

This is then divided by the beta: 7.4/70 = 0.106 ohms

This appears in series with the real emitter resistance of 0R33 making 0.436 ohms, and for 26mV to appear across it, then 59.63mA will need to flow.

This is extremely close to the estimated 60mA.

Multiplying 59.63mA by 0R33 ohms, we find the optimum voltage is 19.7 mV.

Considering the PNP lower EF2, we know from above the driver current is 5.7mA. The power transistor base current is 0.55mA, making a total quiescent driver current of 6.25mA.

The driver's thermal voltage is 26mV, so 26mV/6.25mA = 4.16 ohms.

Cordell gives base resistance as 2 ohms, to which we add 4.16 to give 6.16 ohms.

This is divided by the beta of 110 to give 0.056 ohms, which is added to the real emitter resistor value of 0R33 to give 0R386 ohms.

If 26mV is developed across 0R386, then 67.4mA must be flowing. Obviously, it cannot if the upper NPN has 59.63mA.

But continuing, 67.4mA x 0R33 = 22.23 mV.

This difference is noted by Oliver as beta difference distortion, but there is little we can do to compensate for it, except to average out the resulting voltages and adjust the meters to read the average.

(19.7 + 22.23) / 2 = 20.965 mV.

By setting the meter midway between 20 and 22 gradations we approximate the correct quiescent voltage (Vq).

It should be noted that this is the meter reading with the amplifier at a steady comfortable room temperature of say between 20 and 27 C.

The amplifier should have been left to settle for several hours before the final adjustment.

It will be noted that if ambient temperature changes by a few degrees the meters will indicate a variation of about +/- 1mV; and if the amplifier is left a long time under no-signal conditions, the meter will indicate a graduation low.

The images below show the distortion products when setting to the above approximation.

Amplifier Vq meter

Microamp meter indicating millivolts of quiescent voltage


Distortion Product Ratio Left

1/3rd Watt output channel A



Distortion Product Ratio Right

1/3rd watt channel B


FFT Trace 1 Watt 8 ohms 1kHz

1 watt 1kHz output


FFT Distortion Trace Third of a watt at 1kHz

1/3rd watt 1kHz output


FFT Trace 1 Watt 8 ohms 5kHz

1 watt 5kHz output


FFT Distortion Trace Third of a watt at 5kHz

1/3rd watt 5kHz output


Edited by Graham Slee - 21 Feb 2020 at 2:36pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 24 Feb 2020 at 3:45pm
Those are great distortion readings! Wink
Bruce
AT-14SA, Pickering XV-15/625, Technics SL-1600MK2, Reflex M, Lautus, Technics SH-8066, Dynaco ST120a, Eminence Beta 8A in custom cabs;; Using Majestic DAC
Enjoy Life Your Way!
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