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Old 23rd January 2012, 04:04 AM   #531
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Default C8 and C9, decoupling caps

Quote:
Originally Posted by Elvee View Post
I missed that.
I am not sure it is a good idea: without a local decoupling, it won't be possible to achieve the transient cleanliness displayed in the oscillograms at the beginning of thread.
There is still plenty of room in the middle of your PCB, and it would certainly be possible to include at least a minimal on-board decoupling: nowadays, 10µ/63V caps are tiny, and they would make a big difference if there some length of wiring to the supply caps.

If you choose to include them, place the supply pads in close proximity, to avoid creating low impedance loops in the tracks.

I reiterate also about the respective placement of the output pad and the feedback take-up point: as they currently are, they will easily cause a doubling of the THD.
Hi there Elvee: While making a BOM, I noticed that the decoupling caps mentioned in this post (10u/63v) are diferent from the schematic in post #1 (100u/100v)...which value should they be? ...regards, Michael
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Old 23rd January 2012, 09:21 AM   #532
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Quote:
Originally Posted by kenpeter View Post
I wonder MOSFET for Q5 & Q6? Would maintain input pair at more constant current?
Even with quadrature trying to disturb this, the collector resistor drops would drive
fixed gate threshold voltages, and no base current leak. Currents can't drift far from
this mark. R6 R7 obviously must scale up as VBE becomes a slightly larger VGS.
It is a possibility, but it would have to be a matched pair: MOS have a large dispersion in the threshold voltage, and this would force Q3 and Q4 to run at different currents.
This would increase the distortion.
Quote:
Wacky drain capacitance is my only concern. I don't know how to guarantee any
simulation behave realistic in that regard. IRF510 (100V,0R5) IRF610 (200V,1R5)?
either claims about 16pF, if only that cap would stay put and not squirm... You
got 820pF sandbagging from anode to anode, it seems reasonable in that light.
It is a concern, th Miller cap there has a strong effect, and it "mixes" the modes
Quote:
Is R5 in the second tail actually helpful? Considering its position within both
closed loops, the closed loop balance and non-shutoff here is already forced...
Without it, the amp oscillates, in sim and (much more) in reality
Quote:
I was never all that clear about how one calculates loop stability to begin with?
And as we throw another loop at 90degrees effect into the blender, I get really
confused what effect the compensation is having upon each... If push and pull
are each doing nothing at some current minimum for slightly less than half a
cycle, differential compensation is clearly also active in the common mode loop.
I'm not saying that's a bad thing, it might be a good thing? Only it's an I don't
have a clue how to grasp for the math of it
Compensation is rather messy. I have tried to keep both loops as independent as possible, and each is comfortably stable in its own right, but the mode mixing at higher frequencies creates unexpected effects.
I do not have a deterministic approach to cope with these effects.
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Old 23rd January 2012, 09:39 AM   #533
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Quote:
Originally Posted by danielwritesbac View Post
Unfortunately, I had managed to connect a possibly decent predrive to a erroneously haywired output section, somehow confusing a topside photo with a trackside photo, and that smells awfully. Visible damage looks inexpensive, with 1 Tip35c and 2 resistors nonfunctional.
Be very careful not to leave a damaged component in place before making you next attempt. This includes low-values resistors, zeners, etc.

Be aware that some components might appear OK when tested with a multimeter, but could still be non-functional.
I would be very suspicious about your other TIP35: if one has gone, the chances are the other is also damaged.
I do not recommend testing the circuit without OP devices, it could do more harm than good, use the light bulb tester instead.
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Old 23rd January 2012, 10:36 AM   #534
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Quote:
Originally Posted by j.michael droke View Post
Hi there Elvee: While making a BOM, I noticed that the decoupling caps mentioned in this post (10u/63v) are diferent from the schematic in post #1 (100u/100v)...which value should they be? ...regards, Michael
The 10µ were advised in a "better than nothing" context, because of the lack of space, but the preferred value is 100µF.
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Old 23rd January 2012, 12:13 PM   #535
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Quote:
Originally Posted by kenpeter View Post
I wonder MOSFET for Q5 & Q6? Would maintain input pair at more constant current?...
I have simmed two BSS123 for Q5 and 6.
Seems to work transparently with R6 and 7 increased to 2K7, but this is a sim of course: the transistors are ideally matched, and in reality I am not sure it would be stable. It could certainly be made stable though.
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Old 23rd January 2012, 02:30 PM   #536
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An adjustible resistor for one VGS or the other then?
If the parts can't match, at least the currents can...
Trim for lowest output DC offset I suppose? I do not
think would need to measure those currents directly.

In #530, I have conspired to keep both drains > 40V.
Hopefully well above the zone where Miller tends to squirm.
That sim isn't playable yet, so I could be imagining things...

I would like if gate capacitance could save parts by being
also the dominant pole in both loops. Drain needs nailing
to some voltage constant before that becomes an option.

Last edited by kenpeter; 23rd January 2012 at 02:56 PM.
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Old 24th January 2012, 11:19 AM   #537
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Well, this thing really rocks. Only problem is not nearly enough gain, and if I set the feedback resistor up to 18k, then there is enough gain but far too much upper treble (distorts snare drum). I'm using the Circlophone pictured in post 504. Perhaps one or more values need altered to bring down that treble a bit? Otherwise, it is an impressive amplifier.
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Old 24th January 2012, 01:45 PM   #538
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Quote:
Originally Posted by danielwritesbac View Post
Well, this thing really rocks. Only problem is not nearly enough gain, and if I set the feedback resistor up to 18k, then there is enough gain but far too much upper treble (distorts snare drum). I'm using the Circlophone pictured in post 504.
In that configuration, the full power will be reached with less than 1V input.

With 18K, only 0.5V will be sufficient to reach the clipping limit.

That is probably what you're hearing. Otherwise, the amp is flat, and the characteristics are practically unaffected by the gain.
If anything, the high frequency response might just be infinitesimally reduced at a higher gain.

Quote:
Perhaps one or more values need altered to bring down that treble a bit? Otherwise, it is an impressive amplifier.
You could increase the input filter capacitor C5, but as I said the real issue is probably that you have reached the clipping limit.
You could try to fit one of the clipping indicators I have described. If you see the LED flashing when "there is too much treble", you'll know for sure the reason.
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Old 24th January 2012, 05:36 PM   #539
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Daniel, could you give your impressions about the noise of your newly-built amplifier?

I am investigating that aspect:
Unexpected noise benefits of the Circlophone topology?
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Old 24th January 2012, 08:38 PM   #540
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Default A new contender...

A new contender comes alive step by step.. This build has been almost a full "Dutch" Circlophone. I'm thinking about an elegant way for attaching heatsink for Philips BD140 drivers. Servo transistors not soldered yet.

Its companion will be the next.

Elvee: May you suggest a Philips made npn power transistor please?
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Last edited by terranigma; 24th January 2012 at 08:49 PM.
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