New mixed-device-type symmetrical folded cascode amp

[Mr Evil]

This is something I've been slowly piecing together since I started coming to these forums. It makes use of all three common types of transistor (BJT, JFET and MOSFET), making use of the good points of each. It incorporates many features inspired by threads here, such as the symmetrical folded cascode topology from the thread of the same name, current mirrors with gain from a distortion cancellation thread, and it is through this forum that I finally managed to find somewhere in the UK that sells the Toshiba dual JFETs that I've used..

I've posted information and pictures here.

It sounds pretty good to me (as if anything I made could possibly not sound good!), and it has 10 - yes, ten - LEDs, which guarantees it must be good

[Upupa Epops]

Very interesting and very nice .

[jacco vermeulen]

Love the trace job, no idea how you do it.
Like the Roedersteins and the leds too.
Thanks for posting the Dalbani link, they do sell nice devices.


(Are the Cow speakers redone in Conolly by now, or still in the furr ? )

[Charles Hansen]

Nice job!

What are the driver MOSFETs (X1 and X2)?

[Mr Evil]

Originally posted by jacco vermeulen
Love the trace job, no idea how you do it...

I might put an article up on my PCB creation method one day. In short, I print out from the PCB layout software onto a piece of paper and use that as a template to drill holes (for SMT devices I use a small hammer and a nail to put an indent where the pads will be). Then it's just a matter of joining the dots with an etch-resist pen. It's a mix of art and science: I try to bear in mind what form might be best for a track and also draw them in a way that pleases my eye.

Originally posted by jacco vermeulen
..(Are the Cow speakers redone in Conolly by now, or still in the furr ? )

Fur forever!

Originally posted by Charles Hansen
...What are the driver MOSFETs (X1 and X2)?

I was supposed to explain that on my site, but I seem to have forgotten. They are ZVN2106A and ZVP2106A from Zetex. They seem pretty good, although not my ideal choice. What I really wanted were some rated for 2-3W so I could run them at about 50mA rather than the 20mA they are at now to better drive the input capacitance of the output FETs. However there seems to be a bit of a gap in what's available, with plenty of low-power devices and then a jump to power FETs of 20W+, which are a bit too big.

[mlloyd1]

VERY nice, mr evil :-)

looks like a neat project and great to see someone using all the free expertise that's been shared over the years here!

your board making technique brings back good memories. I used to used staedtler (sp?) markers, the black Lumicolor #317. Bought 'em buy the box at art supply stores. Those pens did a GREAT job on copper. The lumicolor pens and a couple of bubble stones on an aquarium pump in a pyrex dish made some fantastic boards very quickly!

drilling however, was ALWAYS painfully slow to me.

i used a spring load tapping device i purchased from Sears probably 30 years ago(!) to mark the holes.

heck, i even used to buy jfets from Dalbani when they were in the states. :-)

mlloyd1

Originally posted by jacco vermeulen
Love the trace job, no idea how you do it.
Like the Roedersteins and the leds too.
Thanks for posting the Dalbani link, they do sell nice devices.

[Charles Hansen]

Originally posted by Mr Evil
I was supposed to explain that on my site, but I seem to have forgotten. They are ZVN2106A and ZVP2106A from Zetex. They seem pretty good, although not my ideal choice. What I really wanted were some rated for 2-3W so I could run them at about 50mA rather than the 20mA they are at now to better drive the input capacitance of the output FETs. However there seems to be a bit of a gap in what's available, with plenty of low-power devices and then a jump to power FETs of 20W+, which are a bit too big.

Did you look at the 2SJ76 + 2SK213 from Renesas (nee Hitachi)? These lateral parts are in a TO220 package and are very easy to heatsink.

[LineSource]

Mr Evil,

Is there any significant DC offset at the output in your design? Do you think thermal drift would still generate any significant DC offset?

I ended up putting a servo on a recent symmetrical folded cascode design to make sure that 100+ db/watt speakers would not see any physical coil bias that the typical 50mv offset can cause. Naturally, there's a long story behind this........ Like you, I found the symmetrical folded cascode topology attractive for low voltage output applications.

The Ayre V3 amplifier topology includes a second symmetrical output stage to create a balanced output. It seems like a cousin to your cascode.

[AndrewT]

Hi,
I note your use of high value resistors in the NFB loop and at the input. Why?
Should r3=r11? to minimise offset?
I found Dalbani a while back but not used them yet. Thanks for the confirmation.

[Mr Evil]

Originally posted by Charles Hansen


Did you look at the 2SJ76 + 2SK213 from Renesas (nee Hitachi)? These lateral parts are in a TO220 package and are very easy to heatsink.

They look quite interesting. Lower capacitance than other similarly rated devices is good. That was a particular consideration, since any increase in the speed of driving the output stage by choosing a power FET run at high current tends to be offset by them being slower devices. It's too late to change now, but I'll keep them in mind for future projects.

Originally posted by LineSource
...Is there any significant DC offset at the output in your design? Do you think thermal drift would still generate any significant DC offset?..

Offset is about 3.5mV (which implies that it would be ~38mV without the DC feedback cap) and it's the same immediately after being turned on as after heavy use, so it doesn't seem to change much with temperature. Offset depends a lot on the current mirrors being well matched in gain.

Originally posted by LineSource
...The Ayre V3 amplifier topology includes a second symmetrical output stage to create a balanced output. It seems like a cousin to your cascode.

Indeed I looked at the V3, and one of my earlier designs was very close to it. I did want to use a balanced output like that at first (for insanely high PSRR), but in the end I couldn't because both sides of the LTP are used just for one.

Originally posted by AndrewT
...I note your use of high value resistors in the NFB loop and at the input. Why?..

I used high value resistors so that I could use small capacitors. That allowed me to use the polypropylene ones I had spare, otherwise I would have had to use electrolytics.

Originally posted by AndrewT
...Should r3=r11? to minimise offset?

Matching the impedance at both inputs is not necessary, since a JFET input stage has extremely low input bias current.