@Yoyoy: I am using a 100nF WIMA cap as C2 and a Cornell/Dublier 100pF as C3.
I do not use an "active" pre-amplifier.
I am using my VCPre passive pre-amp:
for my SYMASYM.
Best regards - Rudi_Ratlos
is there any sub for 2sk170?
we dont have it here?
Yoyoy, MPSA18 is a good substitute or BC550C (be aware about reversed pinout then).
Best regards - Rudi
My woes are outlined up above.... But last testing, Bias 24 mV, DC offest within spec. Amp was a massive AM radio receiver. I turned it off quickly to avoid toasting things.
Given that I had plenty of small signal transistors on hand, I decided to replace them all... it is an expedient approach to fixing things. I understand, not very sophisticated nor elegant. Regardless... now that I have all new small signals, the bias comes up solid at 24 mV as before. However, I'm now looking at about 1.2V of DC offset on the output.
Update: after 15 minutes or less, it's now to 0.9 VDC on the output.
That is with the input grounded. I DO have matched to 2SK170s on the input. Where else do I need to 'match' the small signal transistors more closely to reduce the DC offset? Or is there something else I'm missing.
Note, I also replaced the input capacitor with another MKP type.
is inching down to 0.4 VDC after 1+ hour.
check the currents through the "pairs" of devices.
The Vdc offset settled down to about 170mV, well outside of spec. So ...
I had a bunch of MPSA18 sitting around. I quickly matched a pair for Hfe and installed those (at 180degrees to the sk170).
All seems okay. Vdc is now a fraction of a mV (i.e. mostly zero.) I've set the bias to ~25.5 mV as I have larger heatsinks. I will let it settle for a few hours and see what happens. Then maybe some music.
It helps to have the output connected to the speaker terminals on the chassis... brilliant..
OFFSET is still a couple of hundred mV. So I will try to do as Andrew suggested as well as run the Sim to get voltages around the circuit. I was too anxious in hoping for a quick fix. How fussy is the "C" rating in the BC560C position?
Enough for today
My weekend is mostly over, so I've got to quit for now.
With MPSA18, 25mV bias setting, 300mV of VDC on the output:
Of the measurements I've made, there doesn't seem to be anything extraordinarily out of whack but ...
Emitter currents for the 2n5551 (measured across the 150R resistors)
Current through R12 = (.362V/150R) = 2.41 mA into emitter of Q9 (Q11 on sim)
Sim says: 2.48 mA
Current through R11 = (.343V/150R) = 2.28 mA to emitter of Q8 (Q13 on sim)
Sim says 2.44 mA
Current thru 47K resistor that connects to collector of 2N5551:31.9 uA
Simulation says: 18.2 uA ... That seems to be a problem
Current thru 47K resistor that connects to collector of 2N5401: 19.8 uA
Simulation says: 22.4 uA
Current thru 68R resistor that connects to emitters of 2N5401: 4.8 mA
Simulation says: 5 mA
Do these give a hint as to what might be amiss?
Ryan, a well-matched SYMASYM shows a DC-Offset of < 5mV. My own one has an offset of about 1mV.
Whenever I read about a SYMASYM - DC-Offset problem in the German DIY-forum and then read the
conclusion, it is one of 3 reasons:
1 - solder error (short)
2 - wrong small transistor used
3 - reversed pinout of small transistor used
Your findings (2N5551) point to this direction as well.
Best regards - Rudi
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