Floating Bias in Leach vs. Slone ??

[sam9]

I noticed this evening that the Leach "Double Barreled Amp and Slones Opti-MOS use a floating bias in the output driver cascode while Slone does is in the VAS. This is done with a voltage divider between the power rails output. Slone writes that the purpose is to achieve a soft clipping characteristic, while Leach doesn't seem to mention this. Is there any other reason for doing this? Does the Leach get a soft clip effect as well?

I also noticed that the signal from the IS is applied to the outer VAS transistor in Slone's case and the signal from VAS to driver goes to the inner one in the case of Leach. I'm surprise at the Slone approach since he usually seems to go to a lot of trouble to isolate the signal path from the rails - I would have expectedc him to do it other way around.

Anyway, I'm mostly interested in the idea of the floating bias. Is there any disadvantage? Any reason other than soft clipping?

[sam9]

I forgeot to say these are the only places where I've seen this bias method. Is it really that unusual or have I just not looked at evough schematics.

[andy_c]

Originally posted by sam9
(...)Any reason other than soft clipping?

I believe the reason Dr. Leach did this was purely to stay in the safe operating area of the output stage, halving the Vce of each output device. It was designed in the '70s, and the devices he was using had 150V breakdown, while the rails were about +/-90V. Other high-powered amps of the time, such as Bongiorno's original Ampzilla http://home.kimo.com.tw/skychutw/ampzilla/Ampzilla.htm used similar approaches, though it looks like Bongiorno didn't extend the double drive all the way back to the VAS as Dr. Leach did.

[sam9]

Ah. I'm a relative new commer to the hobby and have been used to thinking in terms on devices that have Vce's far greater than the differernce between the rails.

[5th element]

Regardless of anything else slone amps sound superb!

[sam9]

"Regardless of anything else slone amps sound superb!"

Agreed. I've built two f the smaller ones.

[djk]

"though it looks like Bongiorno didn't extend the double drive all the way back to the VAS as Dr. Leach did."

http://homelf.kimo.com.tw/skychutw/a...zilla2_sch.png

[slowhands]

Originally posted by sam9
I forgeot to say these are the only places where I've seen this bias method. Is it really that unusual or have I just not looked at evough schematics.

This is cascode connecting the output stage, so that the output transistors don't exceed their second breakdown limit. In the early days, the Vce ratings were on the order of 80-90v for the high power bipolar transistors, and they had to be stacked to handle the 150v of a 200 wpc amp. But even today, bipolars have a breakdown mechanism called second breakdown that requires cascoding if high currents and voltages are present on the device at the same time. The second breakdown limit does not exist for MOSFETs, and this is why they are typically more rugged in high powered amps. You don't have to cascode them, so they simplify the output stages dramatically.

Dynaco used this approach in the mid 70s on the ST-400 amp, one of the biggest of the era. A competing amp from Phase Linear did not use cascoding and was famous for burning out its output stage, second breakdown was not well understood then.

[lucpes]

Originally posted by slowhands
A competing amp from Phase Linear did not use cascoding and was famous for burning out its output stage, second breakdown was not well understood then.

Also known as Flame Linear for exactly the same fault.

[slowhands]

The original topic was the contrast between the Leach amp and the Slone. ..."the Leach 'Double Barreled Amp' and Slone's 'Opti-MOS' use a floating bias in the output driver cascode while Slone does his in the VAS".

I was puzzled by this myself. I could be wrong, but it seems useless in the Slone design. There is no need to cascode the VAS, and anyway this is not a cascode, the base of the second transistor is not fixed at a defined voltage with respect to the rails, which it must be to get the benefits of the cascode (no Miller effect, no Early effect). So, I don't get why it's there at all. I also think the bias voltage generator is weak, just a resistor, and the MOSFET gates need overvoltage protection, which is missing.

On balance, most of the designs in the Slone book make excellent sense. This little part of the high end design did not. I violently disagree with Douglas Self's complete dismissal of MOSFETs for output stages, so I was pleased to see Sloan using them where they work best: in higher power amps that would require cascoding with bipolars, such as the Leach "Double-Barrelled Amp". They really simplify the output stage, if you can work around the matching issues (pretty straightforward).

Anyway, I'm still wondering myself why the VAS is cascoded in the Optimos.....