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Old 9th May 2012, 05:45 AM   #11
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Hi,
I did the math and went through the gain of the stages, everything seems fine.
My Output stage is biased at -60V, 120Vpp , or 41.66Vrms is the maximum I need from the driver stage. I measure a gain of 11.2x on the driver stage, so max needed Vi on the drivers is 3.72Vrms. V1 and V2 are set to a gain of about 14(open loop) .
Vin of V2 = 0.266V max, and I need Vo of V1 to be 1.62V to power the Vol pot.
Vin of V1 = 0.118V and that gives me the desired 0.2V rms (by the original design) at the Input terminals.
I increased R8 in the Powersupply and changed the Resistors in the CCS to match. i now have 285V B+ (Input Stage) .
That didnt solve anything though.
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Old 9th May 2012, 05:55 AM   #12
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OK, first things first. Does it oscillate with the loop feedback removed?

If so, does it still oscillate if removing C26?

If not, can you see the oscillation (with your scope) on the first stage's B+ supply?

Chris

Last edited by Chris Hornbeck; 9th May 2012 at 06:06 AM.
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Old 9th May 2012, 09:44 AM   #13
DF96 is offline DF96  England
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Increase the value of the 1.5uF capacitor in the CCS. With that value the CCS is only a CCS down to a few Hz. Below that it will feed through any changes in the supply rail voltage. Alternatively, use an LED to bias the CCS as that wil be good down to DC.
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Old 12th May 2012, 02:15 PM   #14
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Hello,
@Chris
there is LF Oscillation and the squeeging with the feedback loop open,
there is LF Oscillation as soon as i feed a signal <40Hz from the pc to the amp. it's visible at the bias balance leds.
the squeeging becomes audible when i feed f>2khz from the pc, and is caused by the right channel output tube. (it get more quiet when i touch the envelope of the tube with the fingernail.)
the oscillations get less when nfb is conected.
if i disconnnect c26 all oscillation stops at the output. if i measure @c26, i can still see the oscilloscope flickering at LF, so i think the oscillations are caused.in v1/v2.
the B+ doesn't look great on the oscilloscope, there is ac, maybe 5mVpp,and sometimes there are peaks of 15-20mvpp. its not a clean trace either, maybe 10000Hz on top
@df96: increasing the value of the cap to 3.5uF doest change anything
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Old 12th May 2012, 02:36 PM   #15
DF96 is offline DF96  England
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Quote:
Originally Posted by yurgs
@df96: increasing the value of the cap to 3.5uF doest change anything
Too small a change; it might change the oscillation frequency a bit but not stop it. Try 22uF or 47uF so that the LF pole introduced by the CCS is moved well away from its current frequency. Do the change to one CCS, then both. You might find that you need significantly different values in the two CCS, to separate the poles.

I note that C27 has 1M across it. This suggests that the designer knows that LF stability may be a problem, although this relates to the amp as a whole.
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Old 12th May 2012, 04:24 PM   #16
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i dont have any bigger polyprops at the moment, need to get some first.
The williamson is known to have problems but most people seem to think its not a worry with ccs. hmm... i need to get some zeners as well ,and try a simple b+ reg.
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Old 12th May 2012, 07:03 PM   #17
DF96 is offline DF96  England
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Use an electrolytic, at least for testing stability. As these caps have little signal voltage across them I would use electrolytics anyway.
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Old 12th May 2012, 07:41 PM   #18
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Quote:
Originally Posted by yurgs View Post
there is LF Oscillation and the squeeging with the feedback loop open,
if i disconnnect c26 all oscillation stops at the output. if i measure @c26, i can still see the oscilloscope flickering at LF, so i think the oscillations are caused.in v1/v2.
Excellent; you've ruled out all the difficult stuff. The first two stages share two unintended circuit paths; both B+ and "ground" are undecoupled. Without the complication of the current sources, the B+ path is short and difficult to cause anything but negative feedback. The "ground" path is unknown, and common impedance can cause a positive feedback. It appears as a magically perfect zero impedance in schematics; the real implementation is unknown to all but the builder. Might be worth exploring if the CCS changes don't do the trick.

Also, what's the value, and purpose, of R13? Seems to degrade supply impedance.

All good fortune,
Chris
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Old 12th May 2012, 09:09 PM   #19
DF96 is offline DF96  England
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Yes, R13 (value?) seems to increase the impedance of the 320V supply at subsonic frequencies - exactly not what is wanted. The CCS then pass this through with some phase shift and we then have an LF oscillator. You could try reducing the value of C22 down to around 0.22uF; alternatively increase it to 10uF. The aim in either case is to move its pole away from the CCS poles. This circuit as is seems to have lots of CR networks with rolloffs around 2-3Hz - asking for trouble as the phase shifts all add up!
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Old 13th May 2012, 03:32 PM   #20
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ok, the first step would be to take the negative feedback out of the circiut for troubleshooting.

never "fix" a motor boating problem by using negative feedback

now on your power supply schematic, Remove R18 and place a jumper wire.

take a .1 250V ploypropolene cap and apply it from point E of the heater transformer ( heater 0V ) to chassis ground.

on the mains input open the connection from FG to chassis, insert a .01 uf @ 1KV from FG of IEC connector (E) to ground.


now turn on the unit, the motor boating should be gone. now re attach the negative feedback if you need to lower the gain.
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