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1970s Design Indulgence

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 2019 at 4:59pm
Parting Shots: 3

10/3/19 version is given 1k6 for R7 and 150 pF (again) for C5.

(if you think I'm having problems understanding this configuration you should read Electronics For Vinyl...)

This makes the VAS "take note of what it is being asked to do" by C5. Otherwise T1 makes it roll on more just where the output stage's transition frequency approaches unity.

Amazingly the old commercial circuits used lower frequency power transistors which intuition says should have been oscillating merrily.

No wonder so many abandoned this configuration: when the going gets tough... At least I've given it my best shot.

Then you look at the highly praised Williamson (tube jobby) with zero phase and gain margins. Funny how subjectivism works...
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lucabeer Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 2019 at 5:49pm
Originally posted by Graham Slee Graham Slee wrote:

Parting Shots: 1

The bode plot according to the expert behaviour models provided by On-Semi, Central Semiconductor and Bob Cordell.

I and the computer can't lie, and can only take what they all provide and number crunch it.



An open loop bandwidth of almost 4 Mhz with such a large phase margin?????? Now I understand what you mean when you speak of "wideband design"!!!!




Edited by Lucabeer - 13 Mar 2019 at 5:53pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lucabeer Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 2019 at 5:57pm
One thing strikes my curiosity, seing such a Bode plot. Have you got a SACD player that does pure DSD (no conversion to PCM)? I think it would be very insightful to use one for testing, since SACD is infamous for its noise shaping which pushes all kind of crap to very high frequencies under the assumption that in that range it can't be heard. But I am sure it would be a naughty way to excite oscillations in equipment which doesn't have all this gain and margin... or which happily oscillates in spite of them due to "bad" components.


Edited by Lucabeer - 13 Mar 2019 at 5:59pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 Mar 2019 at 7:29am
Having had enough with trying to coax the output stage into behaving by tweaks to the voltage amplifier, I sat down and analysed what had been happening.

The sequence of events was mainly 1. fit new Cdom; 2. switch on; 3. enjoy for up to three days; 4. fit new Cdom; 5. switch on ...; 6. ...; 7 fit new Cdom ...

So once the new Cdom burned-in (or possibly out but without getting hot?) that too good to be true sound went.

And if it's too good to be true, it's got to be a lie? Hasn't it?

My life's experience thus far with amplifiers is those who can't, write (or set up in hi-fi marketing); and those who can, struggle.

Every power transistor runs into mischief more than once in a while, and it would seem the higher the voltage (to get lots of watts) the worse it gets. Vceo would seem to be part of the problem, and a trip down fake-lane tends to support that assumption (http://sound.whsites.net/fake/counterfeit-p3.htm).

Not that I have counterfeit MJL3281a transistors: when I showed mine which differ from the old MJL3281a to On-Semi, they said they'd investigate, and for back up Farnell, the distributor is doing the same.

I'm sure they'll come back with the answer that they're OK.

So it has to be parasitics. The capacitance I can live with and design round. The inductances are invisible, they exist, but are invisible because the manufacturers choose not to include the parameters in simulation models.

When you decide to make a right old mess of your simulation schematic by adding inductances to every component where inductance exists, the bode plots start to reveal what the imagination has been telling you.

SPICE is great! The models used are not!

The low-ish voltage of the Proprius must be its saving grace. The higher voltage here must be having its influence, but I'd chosen 260 volt transistors for use on a 77 volt (max.) rail!

The simulation had been showing a slight response bump at around 20MHz for a while (it has been mentioned before), but when parasitics were modelled in, it really took off.

Sometimes you just have to fight fire with fire, or in this case, inductance with inductance. I've been here before, and that was the only way of curing an always-on studio amplifier design.

According to the simulator a value of 150 - 200 nH thwarts the parasitics, and going on the number of turns and diameter used in the studio amp, isn't far away from what I have now. Did that use the MJL3281a? No, it used TIP33 and TIP34 power transistors in a DC coupled design having a LTP input stage.

This says straight away the problem is nothing to do with the voltage amp I'm using, although I've tried to get it to make the output stage behave.

Now, let's take a look at Cdom: why do we use a ceramic multilayer? What did we use before they came on the scene? Polystyrene is what was used.

The VAS transistor (as it is called) swings the entire signal which can be from 3V to 15V (42V p-p) at normal listening room volumes, and on top of the signal is the parasitics which are all the way out to 20MHz in this case.

I'll not go into the math but the capacitor needs to be fast - a pulse cap maybe? Polystyrene fits the bill, but also a polypropylene pulse cap does.

On fitting one before adding the inductors the sound was awful. A result at last! It was revealing how hard it was working to control the parasitics.

In went the inductors. These go in series with the output emitter resistors. The awful sound was no more.

The too good to be true sound had also gone. In its place was the run-of-the-mill you'd expect, which might develop into something quite palatable given time.

So are we nearly there yet? Just over the next hill... Wink
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