Patchwork Reloaded: Circuit Optimization and Board Layout.

[MJL21193]

Working at +/-30V. No signs of instability. Idle current is tracking perfectly.
Amoung the new parts from DK I have new multiturn 1K pots. The ones I have been using are from Sure Electronics on Ebay. They look like Bourns 3296W, but they are "bourts"3296W. They haven't given me trouble, but trouble I don't need so I have new Cermet ones. Also the new 47pF silver mica for the Miller cap.

Stability into the dummy load looks good, clipping looks good. Still have the slight sticking on the bottom wave, but I'm not sure that can be cured anyway.
Squares at 20k look sharp with no overshoot and no ringing. Still testing without a Zobel.

[megajocke]

I like digikey too, it takes 2 or 3 days for me to get my stuff - to Sweden! Free shipping on orders over $100 or so too. I can get 4 FJL4x15 for the same price as one TIP36C locally. (Yes TIP36C is the best output transistor in plastic package that is easily obtainable here)

[MJL21193]

Originally posted by megajocke
Free shipping on orders over $100 or so too.

I don't get that deal! But it's only $8.00 so no big expense.

I went ahead and prepped the big heatsink for mounting of this board so that I could do some full power tests.
I set the idle (which was only 17mA difference from the 30V supply) to 55mA per device and left it with a signal to warm up.
Good and warm the idle current is reset and I let it sit again for a while. After about 20 minutes of the outputs drawing 250mA, I check the idle again. It's rock solid. No more issues there.

It's warmed up, I'll do some tests. Stability is still fantastic - no sign of any ringing in spite of the open arrangement and the heatsink not being grounded.
Clipping.
Sine wave clipping at 20k is sticking again on the top. Below that, at 10k and lower it's fine.
Square waves look good and sharp at 20k, but have a small overshoot on the bottom.

So what has changed? For one, the diodes are different - I had been using the UF1003. Also the Miller cap had been reduced to 47pF, with little influence from the clamping diodes capacitance.
Could I have damaged the clamping diodes with my soldering job? I pretty much just dabbed it with my iron, very quickly. I don't know how much heat these can take though.

[Khron]

Greetings...

John, i guess it's almost "redundant" for me to congratulate you on this... genesis It's been quite a ride just popping in to your thread(s) every now and then...

One quick question though: would any major mods be necesary to make this jewel of yours work with more "earthly" active devices?
(Read: BC / BD / MJ(L) type stuff)

Yes, i'm aware there might be some performance limitations, but keep in mind not everyone has access to japanese stuff. Not to mention that some of us are still student, so...

Keep up the wonderful work... The DIY community will thank you for it, given time

[MJL21193]

Originally posted by Khron
Greetings...

John, i guess it's almost "redundant" for me to congratulate you on this... genesis It's been quite a ride just popping in to your thread(s) every now and then...

One quick question though: would any major mods be necesary to make this jewel of yours work with more "earthly" active devices?
(Read: BC / BD / MJ(L) type stuff)

Hi Khron,
No, I can never get enough or too much congratulations! Lay it on me!

The amp likes fast devices, that's for sure. You can use the BC550 for the diff pair, BC546 and 556 for the cascode and mirror.
the EF needs to be high voltage, so pick one with reasonable speed and gain.
The VAS: I would not recommend anything else for this. The 2SA1381 made such a vast difference it would be hard to find something that's comparable and more available. This is in current production from Fairchild.
The drivers can be MJE1503x and the outputs can be MJL21193/94 or others.

It's here...snow. First for the season:

[Khron]

One more thing... Is there any chance you'll be posting the pcb in a more "universal" format? (say, a pdf, for instance)

[MJL21193]

Originally posted by Khron
One more thing... Is there any chance you'll be posting the pcb in a more "universal" format? (say, a pdf, for instance)

I want to iron out all of the wrinkles first. There are a couple of spacing issues with the current layout that I'd like to fix also. When it's complete to my satisfaction I'll provide a pdf of the board.

I have bypassed D1 with a UF1003 and clipping is back to (nearly) normal. I think that maybe reducing the miller cap to 47pF is compromising clipping performance. I saw the same behavior before when I reduced it.
I will try it with the Miller cap doubled up to 100pF again, leaving the BAV103 in place.

[MJL21193]

Yes, that's it. Cdom needs to be 100pF to get that good clipping performance. The overshoot on the lower half of the 20k square wave is gone also.
This means that my soldering job on the BAV103's did not damage them.
I'm wondering what the effect would be if I were to increase the emitter resistors in the differential pair, leaving the Miller cap at 100pf? Even better clipping recovery? What does that do to my slew rate? It must be high enough anyway, there's almost no discernible slope in the vertical of the 20k square wave.

I'm not going to bother with pics of the clipping or squarewave, as they are no different from before.
Measured DC offset with the input plugged into my FG, turned on but the amplitude set to "0" is ~10mV. Maybe a lower figure could be had if the diff pair transistors were matched (I didn't do).

All in all very good. Some real life distortion tests will come soon, be those only with the RMAA. This gives a very accurate indication of what's going on anyway.

[Greg Erskine]

hi MJL21193,

Why the concern about the sticking clipping behaviour? Surely the clipping part of the curve is a bigger problem than the sticking. Is it normal to run a Hifi amp into clipping?

Increasing Cdom will probably have a negative impact on the sound.

regards

[MJL21193]

Originally posted by Greg Erskine
hi MJL21193,

Why the concern about the sticking clipping behaviour? Surely the clipping part of the curve is a bigger problem than the sticking. Is it normal to run a Hifi amp into clipping?

Increasing Cdom will probably have a negative impact on the sound.

regards

Hi Greg,
I don't want to use this amp into clipping, and at 153 watts into 8 ohms this shouldn't happen. It can happen occasionally.
It's not just sticking that I see when the Cdom is lower, it's signs of instability in the upper wave that worry me. I would like to get a picture but I don't want to leave it in this state for any more than a split second or so. Suffice to say that it turns ugly, with the upper wave shifting sideways slightly and "quivering".

I could leave cdom at 47pF and not have a problem. There are no signs of instability at all and the chances of clipping, especially at high frequency, are slim. These amps are to drive my active 3-ways, so there will be some headroom.