Randy Slone "Fig 11.6" amp, modded: will work?

[tcpip]

Originally posted by AndrewT
...low Cob alternatives ...

Found Cob: output capacitance.

Is this related to Ft, transition frequency, in any way?

[AndrewT]

Hi,
Cob is the capacitance seen at the base going to the collector of the transistor. It varies with Vcb quite markedly. Lower at high volts and much higher at low Vcb.

The LTP has a resistor or active load in parallel with the load that the VAS or it's EF provides.

The LTP works best when the load on it's collector is constant and at a value that balances the LTP.
Any extra current that escapes into the VAS robs the load and unbalaances the LTP.

The VAS reflects the downstream impedance back to it's base and the LTP sees this reflected load in // to the collector load.
However, in addition there is a third parallel load on the LTP. The Cob is zero load at DC, but since it is capacitance it's impedance(reactance) decreases with increasing frequency. At high audio frequencies the capacitance is sufficient to cause LTP unbalance. But it gets worse. At the frequencies say one decade above audio (20kHz to 200kHz) the load falls by a further factor of ten and this causes severe loading that may be heard as intermodulation (and I guess other effects). It also causes slew rate decrease since the capacitance charges at the maximum current the LTP can provide and also causes slew asymetry.
But there is more. The capacitance varies as the voltage across the VAS changes.
This capacitance change with voltage is eliminated by cascoding the VAS. But it's advantage is then almost thrown away by adding the Miller comp cap.

Adding an EF before the VAS reduces the reflected load and reduces the variable capacitance, but this is only an advantage if the Cob is deliberately selected to be low, if it were high we have that reducing impedance with increasing frequency. 669 is about 27pF and 3423 is about 2.5pF. anything below 5pF is probably OK.

[tcpip]

Sorry if I'm responding with half-unclear concepts, but your description reminded me of the effect of the compensation capacitor on the input stage loading. Doesn't the presence of the CC have a similar effect on the input stage, with its impedance reducing with frequency, etc? Will the Cob of the transistor dominate the load that the input stage sees, or will it be dominated by the CC?

Am I at all on the same page as you? If not, please say so, because I have lots of grey areas about my understanding of amp internals. I ask questions with the aim of clearing up those greys, not with the intention to find fault in anyone else's explanations.

Thanks for the note.

[darkfenriz]

It seems to me, that due to 2nd order compensation with quite a high value capacitors the whole amp may have a bit weak negative rail PSSR. A resistor in negative rail between drivers and VAS would be helpful, try 330R or so.

[tcpip]

Originally posted by darkfenriz
A resistor in negative rail between drivers and VAS would be helpful, try 330R or so.

You mean I cut the negative rail between the bottom of R22 and the bottom of R26, and insert this resistor?

On this subject, I was wondering about the entire idea of doing additional decoupling of both rails between the OPS and the VAS. Is this a good idea? So few amps seem to do it.

If I imagine that the input power supply flows into the circuit near the OPS and then flows from right to left towards the VAS and then towards the input, then the maximum surges of current are in the OPS, and the VAS and input stages get the side-effects of this. Is it a good idea to cut each supply rail between the OPS and VAS, and put in an RC there?

Taking the idea one step forward, since the OPS will occasionally sink huge amounts of current, maybe there will be momentary current flow depletion for the VAS and input stages. So, to prevent this, maybe we could fit diodes (1N4001?) before the RC filters. That way, the supply current would flow in and charge the cap, but would never flow out.

What do you think? And if yes, then what values of RC make sense? We can estimate the current draw of the VAS and input stages by looking at the CCS resistors I guess.

[AndrewT]

Hi,
a diode between r22 & r26 and similarly between r23 & r14 keep the volt amp stage running from the decoupling caps when the mains rails sag. You may need to increase the caps size slightly.
You can also fit bypass caps at the output devices.

Q.
When output device bypasses are added, do they return to the ground side of the load?
What if the ground side of the load does not return to the PCB?

[tcpip]

Originally posted by AndrewT
a diode between r22 & r26 and similarly between r23 & r14 keep the volt amp stage running from the decoupling caps when the mains rails sag. You may need to increase the caps size slightly.

Great. I'll see how I can fit them into the PCB. Will 1N4001-type things do?

You can also fit bypass caps at the output devices.

Didn't understand this. I already have bypass caps C11 and C12 near the entry points for the supply lines, on the PCB. What are bypass caps specifically near the output devices? Do you mean I should add one more pair of caps, as close to the OPS devices as possible? If I do, then what kind of cap values will be needed? Won't they need to be large and super-low-ESR to make any difference at all at those high currents?

Q.
When output device bypasses are added, do they return to the ground side of the load?
What if the ground side of the load does not return to the PCB?

What does this question mean? I'm confused. And in case it helps, my ground side of the load does not return to the PCB... it returns straight to the star-ground point.

[tcpip]

This is the new schematic. No surprises here, other than the addition of four power resistors:


I've also removed the power resistor which was in parallel with the inductor in the output rail. This does not mean I'll not put the resistor... it just means I've decided not to show it on the PCB. When I build the amp, I'll insert a 5W 10 Ohm resistor inside the inductor and solder both together in parallel.

This is the new PCB:


You can see the four power resistors. I've added one vertically (R34). This one will almost certainly have to be fitted on the underside of the PCB. (This of course implies that I'll need that much empty space below the PCB in my chassis.) The other three may either be fitted on the top surface, raised by a quarter inch, or on the bottom, as is found convenient. The inductor (marked "L1+R") will be fitted on top, raised by half an inch or even more above the PCB.

What do you think?

[AndrewT]

Hi,
move the whole Thiel network to the back of the speaker terminals. i.e. R//L & C+R, saving the space of all four components.

That Q is really for me to anybody that knows the solution. Ignore it till some expert tells us how to do it.

Bypass caps provide the very short term current into or out of a device that changes it's current. It helps attenuate the spike or notch that otherwise gets onto the supply rails as devices modulate their consumption. A local bypass works best when it is mounted right beside the device that causes the current change i.e. the source pin of the FETs and the ground return of the speaker. The closest we can get to the speaker return is the power ground on the PCB.
C11 & C12 are far too far from the FETs. Use very small, very low esr, very low inductance, very short lead=ceramic.

1N400X is OK.
Add 6V to 8V zeners to protect the FETs. // to R23 & R26.
Add 1N400x from output rail to both supply rails.

Are you making the PCB yourself? then move the mounting holes outside the PCB size limits. Similarly the Fuse holders could move to outside or put them with the smoothing caps.

[tcpip]

Originally posted by AndrewT
move the whole Thiel network to the back of the speaker terminals. i.e. R//L & C+R, saving the space of all four components.

Feeling lazy to do this change now. If I run out of space, I'll probably do it. In fact, I can make a small general-purpose PCB for this and use it for all future power amps.

C11 & C12 are far too far from the FETs. Use very small, very low esr, very low inductance, very short lead=ceramic.

How do I do this? Layout-wise, I mean... it seems so tight now. Let me see what I can do. I guess in order to do this, I'll have to move the C11 and C12 "after" the fuses, not before, right?

Add 6V to 8V zeners to protect the FETs. // to R23 & R26.

How does this work? I am somewhat confused. Does this limit the voltage drop across R23 and R26? How will this protect the OPS FETs?

Add 1N400x from output rail to both supply rails.

Is this called "clamping diodes"? I'll try to find space for them.

Are you making the PCB yourself? then move the mounting holes outside the PCB size limits. Similarly the Fuse holders could move to outside or put them with the smoothing caps.

Yes, I'll get the PCBs made myself, but I won't make them with my own hands. Therefore, I'll have to submit the Gerber files to someone "as is". I don't know whether they can do such editing for me. I'll check.

And about the fuse holders, I was initially thinking of putting them with the smoothing caps, but then I read that L-MOSFETs can actually be protected by fuses, i.e. the fuse will blow before the L-MOSFET will be damaged. Therefore, for just the L-MOSFET designs, I thought it might be a good idea to have one fuse per rail. For BJT designs (I'm also working on the Slone "Fig 11.4" design), I can move the fuses to the smoothing caps, I guess.