Pearl transistor choice for Cascode

[Petter]

Hi there,

I am building the Pearl and noted that the Ono uses different cascode transistors. Since the pinout is different, I have checked out what Zetex has which might be more appropriate and come to the following conclusions which I would like to have verified:

1. I am not quite sure whether the ZTX450 is optimal in cascode position -- I guess it would be optimal to go with a lower current unit. Should I also consider going for a high Hfe unit such as the ZTX692B? (http://www.zetex.com/3.0/pdf/ZTX692B.pdf) (The unit used in the Pass Ono is 2SC1844)

(((2. The capacitance multiplier circuit benefits from high beta transistor. I am going with a high Hfe Zetex unit (ZTX692B) of that I am certain)))

Your insight into would be valued.

Petter

 

[mlloyd1]

Hi Petter:

I wonder if the lower junction capacitance part would be best in the cascode position. Are the capacitances of the Zetex parts you're evaluating similar? I'm not fmiliar with the 2SC1844; I'll have to take a look.

mlloyd1

 

[Petter]

Exactly

My brain says high Hfe, but my gut feeling says that a less radical unit might be better.

Regardless, from an input capacitance point of view alone -- it does not seem to make much difference.

Petter

 

[HarryHaller]

pearl cascode

You might want to try a Zetex ZTX3310 mosfet! An increasing number of designers are turning to all fet designs. I have been doing cascodes on amplifier front ends with it and it sounds great. You might want to put a 1k gate resistor connected directly to the gate. I note that the votage dividers are not real close to the ZTX450 cascode transistor in the pearl layout. I also wonder if bypassing the bottom resistor with a small cap might not be worth doing as I have seen it done in many bipolar cascodes. The Zetex are pretty nice transistors in my experience.

H.H.

 

[Nelson Pass]

I like the 3310's a lot. I'm building a small computer
with 20,000 of them. Gates biased single-ended with
resistors.

Not kidding.

 

[hifi]

why?!

 

[mlloyd1]

re: why???

Ooooh, oooooh, let me:

Because he can

mlloyd1

 

[hifi]

yeh yeh.....but anyway....why not use tubes...that would be waaay cooler hmm or maybe not... ;-)

but then with 20 000 fets that not a hel of a lot of logic circuits...but then it was a smal computer..hmmm when i come to think about might be fun after all......once i builded a extreamly small computer of logic cicuits in school...





/micke

 

[Dave]

Could that be the ZEN CPU?

 

[Karen]

Or..ZPU, maybe?

 

[Won]

Perhaps he's building a simple digital signal processer using emitter-coupled logic*. I wonder if a ECL buffer could be used to "clean up" digital signals for audio...

-Won

* ECL is the class-A of digital circuits. Basically, it involves using differntial BJTs in biased so that they never shut off (hence, class-A). A simple exmaple would be using an LTP as an inverter. Because they never shut off, they can be "switched" exceedingly fast. The original Cray used them.

 

[Sud]

...small computer with 20000 fets

Small in gate count, maybe.

Small in size...? 20000 is a lot of FETs!

A 141 x 141 array of FETs... or 27 x 27 x 27 if you want to go 3D...

Hey Nelson, are you point-to-point wiring the FETs?

 

[hifi]

to WUN: yep i seen ECL logic beeing used in re-clockers i believe that Audio crafters Guide have a project online about that... it works quite well ;-)

 

[Nelson Pass]

No point to point. I'm not totally crazy.

The bulk of the devices ae going into a small amount of
memory. I haven't finalized it yet, but probably about
128 bytes.

Processors themselves don't consume nearly as many
devices, and that's how they managed tubed computers -
the memory was in mercury delay lines or on wire spinning
on bicycle hubs. I'm not that sophisticated, though, and
I'm simply building transistorized registers.

 

[HarryHaller]

CPU diversion

So...... You really don't want to tell us how they sound in a cascode.......eh? Clever diversion! Maybe we have stumbled on to something......

Maybe it makes sense that you build a computer as hobby since you build amps for a living. I guess since I built digital telephony equipment for a living I can build audio for a hobby.....


H.H.