I've got similar result when I first power it up with fast VAS transistor...
Try add 22-47pF at base to collector for each VAS transistor(b to c KSC3505 & KSA1381E).
With BD139-140 you can run without these caps, that with Shaan's PeeCeeBee
I add these caps cause I don't want to burn any 10R resistor again
Try add 22-47pF at base to collector for each VAS transistor(b to c KSC3505 & KSA1381E).
With BD139-140 you can run without these caps, that with Shaan's PeeCeeBee
I add these caps cause I don't want to burn any 10R resistor again
With BD139-140 you can run without these caps...
Depending on the producer of BD139/140 it may be even necessary to use compensation caps with them. Some BD139/140 transistors are prone to oscillation.
I've got similar result when I first power it up with fast VAS transistor...
Try add 22-47pF at base to collector for each VAS transistor(b to c KSC3505 & KSA1381E).
With BD139-140 you can run without these caps, that with Shaan's PeeCeeBee
I add these caps cause I don't want to burn any 10R resistor again
I burned a 30 watt Caddock 10R0 TO220 at $5.
With input shorted total current draw is +/- 185 mA. With input short removed current is +/- 500 mA. 8R 20 watt load resistor becomes warm quickly.
I added first 22 pf and then added another 22 pf (total 44pf) to C-B of each VAS transistor with no change in oscillation behavior.
What did stop the oscillation is to change the outer loop voltage feedback to outer loop current feedback. I have a 0R2 30 watt TO220 Caddock resistor between ground and the bottom of the 8R0 20 watt resistor. I place the feedback point at the junction of the 0R2 resistor and the 8R0 resistor and the oscillation goes away.
The only problem is that now DC offset changes from 9mV to 170 mV. I will have to balance the bias and see if I can get the offset back close to 0V.
The bias balance resistor caused asymmetric clipping in my first breadboard peeceebee.
FYI, I had no oscillation with 68pf (with KSC3503/KSA1831), or with 47pF (BD139/140-16).
For anyone else: Running with an open output may not be a good idea. With a few feet of dangling cable, I destroyed some transistors that way.
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OK, I got some more devices so I pulled the KSA's and BD's and tested them. They read OK on the hfe test but I went ahead and replaced them with matched pairs from my new supply. With baited breath, I brought it up slowly using my variac through a light bulb through a capacitance multiplier. The light never lit, a good sign. I had a DMM on the output. I see bout 1.5V on the output. I'm able to bring it all the way up to full power, which is about 38V rails and nothing is smoking but I still have that offset. As I watch, the offset starts to climb and after about 2 minutes it all the way to +29V. I shut it off and let all the voltages drain and them start it up again. Same thing, 1.5V offset and then gradually starts climbing. So I shut it off again and hook up my scope and a sine wave generator. I start it up again and check for signal at the out put and see nothing readable. I start probing and I have signal on the collectors of the 550 and 560 but not the output of the 139/140. They both have a flat spot on one end of the sine wave, opposite ends though. Then after a few moments, the signal goes away on the 550 and it starts to get hot to the touch. The offset starts climbing also. Hopefully this rings a bell with someone.
I checked very carefully for solder bridges and there are none. I double checked polarity of all devices and Caps and they are all correct. I also checked all resistors and they are all the correct values and working so I don't know where to go from here.
Thanks, Terry
I checked very carefully for solder bridges and there are none. I double checked polarity of all devices and Caps and they are all correct. I also checked all resistors and they are all the correct values and working so I don't know where to go from here.
Thanks, Terry
From post #1217 the series cap mentioned with 2,2k feedback to input should be at least 1mF as the two 1mF sitting between 15K and 100R. If using less think lows will go not needed swing. Sorry if i misunderstood circuit, but hoped to help, it is nice project.
bias and offset
Hi All.
Warning! Newbie question! My vssa bias settled on about 160ma after I adjusted offset down to about zero. I made some changes the last few days: installed the 15mm bypass caps and upgraded the 2200if/6.3v caps. When I 're-measured bias it was solid 160ma on both channels and the offset fell to near zero after a 5 minute warm-up.
But when I took measurements I left both inputs and outputs disconnected. I'm wondering if measurements are valid? The amp is truly magical, my favorite. The heat sinks are slightly warm after listening or after idling all day. No sign of oscillation but I don't have equipment to measure it.
I'm using the Mr. Evil/PMI power supply at about 34 volts. I'm running thru all my recordings to see what I've been missing. Many thanks to Shaan, PMI, Mr Evil, LC, and all other superstars here! I will try to post some photos soon. Cheers. -freeman
Hi All.
Warning! Newbie question! My vssa bias settled on about 160ma after I adjusted offset down to about zero. I made some changes the last few days: installed the 15mm bypass caps and upgraded the 2200if/6.3v caps. When I 're-measured bias it was solid 160ma on both channels and the offset fell to near zero after a 5 minute warm-up.
But when I took measurements I left both inputs and outputs disconnected. I'm wondering if measurements are valid? The amp is truly magical, my favorite. The heat sinks are slightly warm after listening or after idling all day. No sign of oscillation but I don't have equipment to measure it.
I'm using the Mr. Evil/PMI power supply at about 34 volts. I'm running thru all my recordings to see what I've been missing. Many thanks to Shaan, PMI, Mr Evil, LC, and all other superstars here! I will try to post some photos soon. Cheers. -freeman
The measurements are ok. The safer and more accurate way is to make the adjustments with inputs shorted. However, if there was a problem, the offset, and/or bias would not settle. It is not the best way to do the settings, but if successful, result will be the same, within a tiny margin of error (tested).
0.01 uf kills the oscillation. Current draw goes from 500mA to 175mA. Output offset also behaves differently with 0.01 uf at B-C of VAS.
I have to order a selection of caps between 22pf and 220 pf to see how oscillation behaves with compensation.
I will install 1mf caps at the 15k to 100 ohm junctions to see if that is an issue as well. Right now only small 10 uf cap there.
That do a huge difference in behavour, it is amps leg to reproduce AC signal linear, with 10uF it makes DC-like thumps in lows because of increasing impedance in small caps at lows. And if using your mentioned feedback 2,2k in serial with cap from 0R2/8R junction to input also do 1mF else same badness as first two caps misbehavour.
Hi Terry.
VAS transistors' metal backs should be faced to the input side, are they? Can you upload some pictures of both the parts and layout side of the board?
Hey Freeman.
Enjoy your PeeCeeBee. Happy to see you like its sound.
Hi WT.
You must have some caps as close as possible to the FET drains. Use 100uF if your PS cable is longer than 1 feet; and if less, use 10uf. This cap nulls the inductance of the cable, essential for a stable FET OP stage.
Hi Terry.
VAS transistors' metal backs should be faced to the input side, are they? Can you upload some pictures of both the parts and layout side of the board?
Hey Freeman.
Enjoy your PeeCeeBee. Happy to see you like its sound.
Hi WT.
You must have some caps as close as possible to the FET drains. Use 100uF if your PS cable is longer than 1 feet; and if less, use 10uf. This cap nulls the inductance of the cable, essential for a stable FET OP stage.
Installed 10uF OPS drain capacitors. Installed 1mf capacitors at 15k/100R junctions.
Output offset is more stable vs temperature.
Oscillation now creates 1A draw from +/- 30V supplies. Before 1mf capacitors current draw was 500 mA during oscillation.
0.01 uF compensation capacitor stops oscillation.
It only oscillates in voltage feedback mode. In current feedback mode it does not oscillate, however, in current feedback mode output offset stability is difficult. In voltage feedback mode output offset is very small and no adjustment is required from stock parts.
I have the breadboard set up so that I can move one wire to change from voltage feedback to current feedback.
Hi Pete,
I am going off the details page of Shaan's docs. They clearly show the metal plate facing the output not the input. What am I missing?
EDIT: Well of course, as I sit here with donkey ears on, I just checked the pinout of the BD139 and I see that it is reversed from the drawing. Great! So I swapped around the 139 and 140 with no happiness. Still getting 4V offset and no usable signal on the output.
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Not missing anything, as far as I can tell.
The pins of the TO-126 are marked by their initials on Shaan's silkscreen.
Look for the B C E letters on the silkscreen.
The base pin of the TO-126 should be connected to the junction of the collector of the TO-92, and a 470R.
Pete
The pins of the TO-126 are marked by their initials on Shaan's silkscreen.
Look for the B C E letters on the silkscreen.
The base pin of the TO-126 should be connected to the junction of the collector of the TO-92, and a 470R.
Pete
Hi Pete,
I am going off the details page of Shaan's docs. They clearly show the metal plate facing the output not the input. What am I missing?
EDIT: Well of course, as I sit here with donkey ears on, I just checked the pinout of the BD139 and I see that it is reversed from the drawing. Great! So I swapped around the 139 and 140 with no happiness. Still getting 4V offset and no usable signal on the output.