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Old 13th May 2016, 10:51 AM   #1
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Default Designing 200w Amplifier. Oscillations or something else? need help please.

Hi All

I have decided to finally attemt an amplifier design by myself after a few diy builds from various members here, Carlos (Destroyer) and Ostripper.
I designed and built a PCB which uses OStrippers output stage, and my own input stage, vas and thermal compensation.
Unfortunately when I power the amp through my test lamps, it seems I have some issue/s. R28 and R29 have an unsuspected 5v over them instead of millivolts and it cooks those resistors and blows a vas transistor or 2 as a result.

The design is attached below.

Simampv1.png

Simamp v1.png
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Old 13th May 2016, 12:17 PM   #2
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I would remove Q1 and Q2 as they overdrive Q3 and Q4 causing over-current on Q3 and Q4. Or remove the collectors of Q1 and Q2 and connect them to the next transistor Q3 and Q4 turning them into Darlington pairs.
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Old 13th May 2016, 03:42 PM   #3
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I'm still trying to find the links ....

But complimentary differential topologies are inherently unstable with
current mirrors. (too much loop gain , like "opposed" Blameless's)

Many of our old "guru's" and even Cordell have explained why.
I did not go this route with my "spooky leach" .
Edit - in fact .. thermally - even beta enhancement at the VAS is overkill.
Low loop gain and a good Baxandall or Hawksford will give a stable result
with 20-30ppm 20k THD.
Edit 2 - Yikes ! you are even using beta enhanced mirrors !
OS
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Last edited by ostripper; 13th May 2016 at 03:48 PM.
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Old 13th May 2016, 04:08 PM   #4
Struth is offline Struth  Canada
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Hi Guys

Have you simulated this circuit?

It is more usual to use the 1381/3503 pair for the VA, since the dissipation can be a bit higher than what a TO-98 package is good for in free air. Similarly, the reverse diodes in the hum filters are larger types like 1N4007 rather than 1N914.

There might be an oscillation causing the Rs to burn up. Self found that fully complementary circuits like this with current-mirror loads were prone to high-frequency oscillation and to idling at much lower than expected VAS currents. The helper transistors in the CMs are supposed to take this a step closer to predictable performance.

It is also beneficial to add a resistor in the collector of the EF for the VAS. This limits its current and dissipation, and thus limits to some extent how how the VAS itself might get. I found this through experience in '92 or so.

Sometimes for these kinds of problems it is good to isolate the front end and just close the loop around it. You don't want to risk damage to all the expensive output stage silicon.

Have fun
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Old 13th May 2016, 11:59 PM   #5
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Here is the original LTspice schematic.
Sim Amp mods.asc

I will modify and post the new one when I try the new methods mentioned here to try fix this amplifier. Thank you every one for your suggestions so far. 1st I will try and remove q12, q15 to get a more simple VAS that looks like this.
Sim Amp mods2.asc

Simon
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Old 14th May 2016, 12:37 AM   #6
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Oscillations is usually a sign of too much gain.
Fixes can be:
Larger cap at the VAS.
Remove extra gain stages.
Add base stopper resistors on driver and output stages.

If quasi then add BC 220pf at lower driver.
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Old 14th May 2016, 01:28 AM   #7
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Quote:
Originally Posted by niss_man View Post
Here is the original LTspice schematic.
Attachment 548949

I will modify and post the new one when I try the new methods mentioned here to try fix this amplifier. Thank you every one for your suggestions so far. 1st I will try and remove q12, q15 to get a more simple VAS that looks like this.
Attachment 548951

Simon
Nope , High Z CM's(mirrors) will always "misbehave" from the cross conduction of a symmetric VAS.
- They even do worse things at clipping.
(Below #1-3) are the ones that have worked 2 years+.
#1- 53db 1K OLG ... #2- 53db .... #3 - 60db

#3 will clip like Valve (below 4).

Why do you think professor Leach never used a CM or beta enhanced VAS ?
PS - you don't need high gain (like a blameless) with this topology to get
descent THD. The design of the VAS is more a factor.
Edit - see how in your attachments you are degenerating your differentials (470R) , and
adding that crazy miller comp to both EF's in your VAS ?
That sort of nullifies any perceived gain added by the mirrors , huh ?

OS
Attached Images
File Type: jpg spookoriginal.jpg (111.4 KB, 214 views)
File Type: jpg symetri.jpg (119.7 KB, 220 views)
File Type: jpg spookV2.jpg (126.5 KB, 206 views)
File Type: jpg Baxandall clip.jpg (50.9 KB, 204 views)
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Last edited by ostripper; 14th May 2016 at 01:33 AM.
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Old 14th May 2016, 04:01 AM   #8
JMFahey is offline JMFahey  Argentina
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Not sure how you can expect a few millivolts across R28 and R29 .

Let's see R29 (R28 is symmetrical so basic same applies):

*IF* you have a few mV across R29 , and Q2 base is 2 diode drops (~1.4V) above it, then Vce across T4 is also ~ same.

My question is: does it work properly with such a small voltage?
Considering it also has 470 ohms in series with emitter.

I'd first check voltage drop across it, then substract 2 diode drops and calculate expected voltage across R29.

As in: I'd first check DC stability or values even before worrying about AC, gain, stability, etc.

Another worry: Q3/Q4 are small TO92 transistors, how much are they dissipating?
I don't know your rail voltages, although if you aim at 200/8 I'd estimate ~65V
...
which also makes me worry at T1/2/11/12 ... I'd be happier with higher voltage transistors there ... such as 2N5401/5551 which you are already using.

OK, hope you find your problem
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Old 14th May 2016, 04:05 AM   #9
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Quote:
Originally Posted by niss_man View Post
I have decided to finally attemt an amplifier design by myself..
Really interested to know what was the motiv behind your decision.
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Old 14th May 2016, 04:44 AM   #10
nattawa is offline nattawa  Canada
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I simulated your circuit and it is supposed to be stable. You have plentiful phase margin and gain margin. There must have been something with the build, be it PCB, parts, or the assembly job that has gone wrong and smoked your VAS. The rail voltage in the schematic is +/-35V and using 2N5401/5551 for VAS is perhaps okay. Your starting post says 200w, if that is the case then better use KSC3503/A1381, like Struth suggests.

If you have double-checked all and ready for another try, perhaps try temporarily use a greater value resistor, like 1k/1W, as the VAS degeneration resistors, just in case you get an abnormal voltage there again, so that you at least have a chance to take voltage readings in the circuit long before any smoke comes out.

I changed a few components to their "usual" values when was in the LTspice,

--the emitter resistors at the EF preceding VAS changed to 1K to reduce the VAS current to 10mA from 13.7mA

--disconnected C6

--emitter resistor for predriver and dsriver transistors altered for a "healthy" standing current

--base stopper resistors for driver transistors changed to 10-ohm

I'd also recommend a CFP type bias control that works very well in my Meistersinger amp. The 2nd transistor in this circuit bleeds off "extra" VAS current and maintains a relatively constant current in the ThermalTack diodes, therefore, the OPS current is largely immune to the VAS current fluctuation that's often questioned about in a push-pull VAS without a current source defining its current. R23 is the pot for bias adjustment.

The Meistersinger, by the way, having complementary LTP input stage loaded with current mirrors and ThermalTrak output transistors, a similar but different design to yours, is a wonderful sounding amp. So I know this front end does work and works well when done right.
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