R21 is actually a pot as I use it when testing at various voltages, the pot is used to fix the 5.5ma from the PD+ and ND- if standard value resistor of 100ohm used it will either endup at 3ma or 6 or 8ma+ as per the voltage applied.
Yes I have used heatsinks on them one heatsink connecting both the transistors. They are running a bit more than warm.
This may be one of the reasons of the idle current slipping as they warm up.
Although, normally it should settle in, say, 10 seconds or so.
Although, normally it should settle in, say, 10 seconds or so.
but why is that happening do you say its because of larger heatsink?
What is your suggestion for it. Its definitely not taking 10 seconds its taking more than 20mins.
You can simply measure the voltages for isolating the problem.
For example - connect the volt-meter over R21, switch on the amp and watch the behavior of that voltage. Does it settle relatively fast and stays constant, or it grows slowly over those 20 minutes?
If it stays constant - the issue is in the VAS stage (TO-126 transistors), if it grows slowly - we need to check the voltage drop over the LEDs in LTP collectors.
This way you find the stage where the voltage (and the current, causing that voltage) slips, then we can check suggestions why it happens.
Basic troubleshooting.
For example - connect the volt-meter over R21, switch on the amp and watch the behavior of that voltage. Does it settle relatively fast and stays constant, or it grows slowly over those 20 minutes?
If it stays constant - the issue is in the VAS stage (TO-126 transistors), if it grows slowly - we need to check the voltage drop over the LEDs in LTP collectors.
This way you find the stage where the voltage (and the current, causing that voltage) slips, then we can check suggestions why it happens.
Basic troubleshooting.
Measured across the leds its 1.6V and constant as its not variying.
voltage across R21 is slowly varying over time.
I have reduced the size of the heatsink to standard TO 220 package heatsink and now the stability time reduced to 20mins.
What else could be the problem? Just disconnected the decoupling caps for the LEDs no change so soldered it back again.
Is there anyway to sense it and fix?
What is that voltage over R21 - after power-on and after 20mins?
Does it correspond to around 2mA across R21 (I=V/R)?
We also need to know the exact value of R21 for that calculation.
It looks like those differential pairs (C1846/A992) start heating up slowly after power-on, so their bias increases accordingly.
In high-rail variants of this topology, Sansui used additional cascodes at their collectors for limiting their c-e voltage (and thus heat dissipation).
I am building my second Slewmaster with one transformer and one power supply. To avoid hum, I am curious to try the two bidges solution shown in post 45.
My transformer has three "outputs", not four.
I have connected one 55V AC to each bridge( no called bridge 1;lowest, and bridge 2; upper.
Zero (0V) is connected to the second AC point on bridge 1. From this point I have connected a wire to the second AC point on bridge 2.
Will this work?? I have connected to my variotrafo, and to me it seems to work properly.
Eivind S
My transformer has three "outputs", not four.
I have connected one 55V AC to each bridge( no called bridge 1;lowest, and bridge 2; upper.
Zero (0V) is connected to the second AC point on bridge 1. From this point I have connected a wire to the second AC point on bridge 2.
Will this work?? I have connected to my variotrafo, and to me it seems to work properly.
Eivind S
adopting a dual secondary dual rectifier to give a dual polarity supply will not cure hum brought about by a wiring error.
A centre tapped transformer with three secondary leads cannot use a dual sec/dual rectfier arrangement.
Did you bulb light up?
Yes, in case of a centre tapped transformer with three secondary leads, dual rectifier does not make sense. Single rectifier and centre tap connected to ground is the right configuration.
Thanks for the answer from both of you. Then I will go for a singel rectifier.
I did not try the bulb, only used the variotrafo on a very low level. I red +/- 5V DC on each bridge. No condenser connected.
Eivind S
I did not try the bulb, only used the variotrafo on a very low level. I red +/- 5V DC on each bridge. No condenser connected.
Eivind S
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