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1970s Design Indulgence

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug 2018 at 7:42am
Input Switching Crosstalk

Not having the ground shorting switches which used to reduce crosstalk from adjacent inputs available to us today we need a different answer.

The line stage preamp circuit previously shown...


...can be redrawn with R11 on the input side of the switch contacts. In other words, each input has a series R11 between its input potential divider and the switch, as can be seen below (R11 to R16)...


The input to T1 base is actually a "virtual earth" which means it is low impedance to the selected input (or as low impedance as it can get using a simple transistor stage - op-amps being much better in this respect).

All deselected inputs are high impedance due to R11 to R16 per input and so any crosstalk between a deselected input and the selected input should be severely attenuated by the low impedance virtual earth junction.

C1 serves to block DC.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug 2018 at 9:06am
So here we start with the test-bed stage and the test-bed case isn't far from the expected size of the finished article.

The next thing we need before the power amplifier can be tested is the power supply, and we have everything but the board to connect the reservoir capacitors up to, so that has to be artworked and made next.




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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug 2018 at 6:58pm
It's great to see the circuit diagrams come to life in a tangible form!
Bob

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ICL1P Quote  Post ReplyReply Direct Link To This Post Posted: 10 Aug 2018 at 7:57pm
Originally posted by morris_minor morris_minor wrote:

It's great to see the circuit diagrams come to life in a tangible form!
Something we can relate to, even if don’t understand it! Smile
Ifor
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 7:10am
Discrete Voltage Regulator

Not only do the power amplifiers need the power supply reservoir capacitors mounting on a board, the preamp section will also need somewhere for its regulated supply to live.

The preamp power supply has to convert the "raw" unregulated DC voltage emerging from the reservoir capacitors to a ripple free lower voltage.

We seem to have settled on 30V for our preamp (and phono board) section so that is the voltage we need from the regulator. Its job is to take a varying 68 to 81 DC input volts and do a constant 30V DC.

The diagram below illustrates a circuit capable of the job. It is a zener stabilised series pass transistor with a few refinements.



Here I have chosen the medium power BD139 as series pass transistor, but expecting it to take the full voltage drop from as high as 81 volts to 30 volts would have it dissipating around 2 watts as heat, and that will need to be given up to the ambient air using a heat sink.

We don't have a large area available in our busy integrated amplifier, so by splitting the voltage drop between a resistor and the BD139, the BD139's dissipation can be halved and we can therefore use a smaller clip-on heat sink, hopefully saving space.

The resistor and BD139 sharing the voltage "loss" dissipate around 1 watt each so a 26 degree per watt clip-on heat sink will see a temperature rise of 26 degrees, and if we assume an inside case temperature of 35 degrees, the device will reach 61 degrees C. From its power derating curve specification it can be seen that it can do up to 10 watts at that temperature, so 1 watt will be OK.

You may also remember that the absolute maximum voltage for the BD139 is 80 volts. The series resistor also helps protect it by reducing the voltage to just under 60 volts worst case.

Further "belt and braces" protection for if the input voltage rose much above 81 volts with much less output current demand, is provided by the "if all else fails" 75V 5W zener diode between the BD139 collector and 0V. It is very doubtful such conditions could exist, but you might have heard of "Murphy's (and Sods) law"?

I established that the entire preamp section will draw 40 milliamps, which with the voltage drops shown gave rise to the power ratings of the input resistor and the BD139 transistor. But depending on the preamp options chosen by the constructor the voltage drops will differ, and so the current in the zener stabilisation chain would be different if fed by the usual resistor arrangement.

Instead, here I am using a current source to keep the zener chain (4 x 8V2) at around 5mA (5.6mA actually) which is their "sweet spot" current. The current source is: the three diodes dropping 1.8 volts; the 33k resistor; the 2SA916 transistor; and the 180 Ohm current setting resistor.

A diode labelled T.CO is included in the zener chain which has the inverse temperature coefficient to the zeners to give temperature stability and also gives us a bit of extra voltage for which to lose across the 2.7k resistor part of the "cap multiplier".

The BD139 base will draw roughly 1mA. In fact it is all approximate because of component tolerances, so currents vary somewhat, and voltages to a lesser degree because nothing is perfect in electronics - something we all ought to learn!

The "cap multiplier" is an imagined idea that if we place a capacitor (100u in our drawing) on a transistor base, the transistor's DC current gain will "amplify" it, making it a much bigger value. In this case around 4,000uF.

However there is the BD139's intrinsic emitter resistance of around 0.625 Ohms in series with it and a 4,000uF capacitor would not have an ESR of 0.625 Ohms!

The 2.7k resistor and the 100u capacitor do provide the BD139 with a very stable base voltage however, and this RC filter alone will give us a PSRR of over 40dB. The zener chain will also reduce power supply ripple substantially.

The two 1N4002 diodes protect the BD139 from reversed polarity during power down/switch off, and the 100mA fuse protects against an accidental dead-short on the regulator's output.

And the reason for the zener chain? A single higher voltage zener would dissipate more power and generate more heat (and therefore produce more noise as well). I also have hundreds of 8V2's in stock gathering dust! Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 12:40pm
if I understand this correctly you are having a pre amp power supply and power amp power supply?

If so will you have twin mains feeds to avoid one interfering with the other?



If so 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 13 Aug 2018 at 2:42pm
Originally posted by Richardl60 Richardl60 wrote:

if I understand this correctly you are having a pre amp power supply and power amp power supply?

If so will you have twin mains feeds to avoid one interfering with the other?


Richard,
In the example shown, the pre amp power supply feed is from the power amp power supply.

Ian


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