Copyright © 1998
Cadman Enterprises Ltd
Welcome to the Graham Slee Audio Products Owners Forum Open to all owners plus those contemplating the purchase of a Graham Slee HiFi System Components audio product and wishing to use this forum's loaner program: join here (Rules on posting can be found here) This website along with trade marks Graham Slee and HiFi System Components are owned by Cadman Enterprises Ltd |
1970s Design Indulgence |
Post Reply | Page <1 1011121314 345> |
Author | |
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(0)
|
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. |
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(0)
|
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. |
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
morris_minor
Moderator Group Joined: 27 Mar 2012 Location: Surrey Status: Offline Points: 6015 |
Post Options
Thanks(0)
|
It's great to see the circuit diagrams come to life in a tangible form!
|
|
Bob
Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
|
ICL1P
Senior Member Joined: 04 Aug 2012 Location: Faringdon, Oxon Status: Offline Points: 2341 |
Post Options
Thanks(0)
|
|
|
Ifor
===== Reflex M & ACCESSION M, CuSat50, Majestic DAC, a Proprius pair. |
|
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(0)
|
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!
|
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
Richardl60
Senior Member Joined: 04 Nov 2014 Location: Yorkshire Status: Offline Points: 1468 |
Post Options
Thanks(0)
|
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 |
|
BackinBlack
Senior Member Joined: 05 Feb 2012 Location: Hinton, N'hants Status: Offline Points: 2020 |
Post Options
Thanks(0)
|
Richard, In the example shown, the pre amp power supply feed is from the power amp power supply. Ian |
|
Just listen, if it sounds good to you, enjoy it.
|
|
Post Reply | Page <1 1011121314 345> |
Tweet
|
Forum Jump | Forum Permissions You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You can vote in polls in this forum |