I'm confident now that the regulator is working OK.
Output voltage is adjustable to +/-18.1V and all the check point voltages look good. See attached (hopefully)
The chain LED's are all fine. The two that dim are on the positive side under the heat-sink and connect to the base of Q104. At this stage I can't see any connection issues and It may be just a character of the transistor?? Voltage across the two LED's is just on1.8V
I've made provision the the back of the boards for the suggested caps to control parasitic oscillation, and the transistors are a moderate 450hfe, so hopefully no problems there. I don't have the luxury of an oscilloscope, so fingers crossed.
So I think I'll press on the the amp side.
Thank you for your input, always appreciated.
voltage levels looking very good indeed. good luck with the amp section! If I remember correctly, most of the issues were with the regulator so you might be lucky 😀
The latest version R31b still has them on, where did you find that information? I do not remember, sorry .....
I saw it in one of sq's builds. Hopefully Si won't mind that I've downloaded and up loaded one of his build pictures.
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Interesting. They serve as a "weak" regulator, to keep the supplies for the cascodes centered. Maybe SQ can enlighten us?
Hi all,
I put together my Paradise Phono around a year ago and at the time had around 8V on pin 6 of the opamp on one channel. The other channel was around 0.2-0.3V. I didn't worry too much about it at the time and just enjoyed listening to music, thinking that the input transistors may be mismatched and that I'd at some point would do another matching session.
I spent some time matching transistors again, very closely, using the circuit in the assembly guide and at the operating temperature as I measured the transistors in the running preamp.
I ended up matching transistors for both channels and replaced this weekend. It seems that I'm back at the same situation I had prior to replacing the transistors...i.e., it didn't change the pin6 voltage.
Any thoughts on what might be off on the channel measuring 8V? I saw a post where someone had dropped R14 or R38 by an Ohm or two. Is that a possible solution, or could it be something else?
I followed Simon's matching guide from an earlier post and input and Calvin offset are both well below 0.1mV.
Thanks 🙂
I put together my Paradise Phono around a year ago and at the time had around 8V on pin 6 of the opamp on one channel. The other channel was around 0.2-0.3V. I didn't worry too much about it at the time and just enjoyed listening to music, thinking that the input transistors may be mismatched and that I'd at some point would do another matching session.
I spent some time matching transistors again, very closely, using the circuit in the assembly guide and at the operating temperature as I measured the transistors in the running preamp.
I ended up matching transistors for both channels and replaced this weekend. It seems that I'm back at the same situation I had prior to replacing the transistors...i.e., it didn't change the pin6 voltage.
Any thoughts on what might be off on the channel measuring 8V? I saw a post where someone had dropped R14 or R38 by an Ohm or two. Is that a possible solution, or could it be something else?
I followed Simon's matching guide from an earlier post and input and Calvin offset are both well below 0.1mV.
Thanks 🙂
How good is the matching (especially the current gain) between the NPNs and PNPs in the input stage, on the channel that has the higher voltage?
Yes, R14 / R38 might be adjusted somewhat but that would only be needed in case of a really strong (input stage) offset. Try putting a 200ohm pot in there, set to the middle and monitor the voltage on pin 6 to get it to around zero (it will drift somewhat with temperature and time, nothing to be worried about).
Yes, R14 / R38 might be adjusted somewhat but that would only be needed in case of a really strong (input stage) offset. Try putting a 200ohm pot in there, set to the middle and monitor the voltage on pin 6 to get it to around zero (it will drift somewhat with temperature and time, nothing to be worried about).
Thanks for the quick response @hesener.
The gain matching was very close, at +/- 2 (hfe between 464 and 468 to be specific) for both channels between NPN and PNPs in the input stage. The input offset was adjusted to get under 0.1mV on both channels. Both channels should be essentially the same, except that one has the ~8V on the opamp output.
I suppose I could always try to see if I can attach various testing resistors with alligator clips to R14 or R38 and see if it changes anything.
The gain matching was very close, at +/- 2 (hfe between 464 and 468 to be specific) for both channels between NPN and PNPs in the input stage. The input offset was adjusted to get under 0.1mV on both channels. Both channels should be essentially the same, except that one has the ~8V on the opamp output.
I suppose I could always try to see if I can attach various testing resistors with alligator clips to R14 or R38 and see if it changes anything.
Slight update. Have tested various resistors in parallel and a 20k resistor, with equivalent parallel resistance of around 119.28k brought me down to around -0.1V on pin 6 of the opamp. Now to find a resistor around that value.
Is it possible that there could have been a slight resistor mismatch on the +ve and -ve sides? I did use 1% resistors and measured them all to make sure they were within spec, but I suppose it's possible that the once below 120Ohm ended up on one side and the once above on the other, hence why reducing one to ~119Ohm fixed it.
Alternatively, I suppose it could still be transistor mismatch, but it would be a bit of a fluke that the issue happened on the same board even though I matched and replaced the transistors on both board. Hence why I'm leaning towards it not being the transistors. Not a statistical anomaly given it only happened twice, but both boards measured almost the same before and after changing transistors.
Alternatively, I suppose it could still be transistor mismatch, but it would be a bit of a fluke that the issue happened on the same board even though I matched and replaced the transistors on both board. Hence why I'm leaning towards it not being the transistors. Not a statistical anomaly given it only happened twice, but both boards measured almost the same before and after changing transistors.
I’m getting a buzz from one of the channels, so I hooked up the unit to a scope (and waveform gen).
I’m getting the picture as attached … indeed one channel more than the other but both have this.
I don’t really know what I’m looking at here, so any pointers?
Thanks
I’m getting the picture as attached … indeed one channel more than the other but both have this.
I don’t really know what I’m looking at here, so any pointers?
Thanks
Please show us what did you build and how is it set up.
The signal in channel A is the inverted waveform of channel B but with reduced amplitude - is this
a triggering phenomenon or correct, is one channel set to invert mode (not indicated on the LCD
screen) ? However, the problem may be in channel B, it looks like motorboating, a low frequency
oscillation.
The signal in channel A is the inverted waveform of channel B but with reduced amplitude - is this
a triggering phenomenon or correct, is one channel set to invert mode (not indicated on the LCD
screen) ? However, the problem may be in channel B, it looks like motorboating, a low frequency
oscillation.