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cadaverdog 17th September 2006 07:26 PM

Need help with discrete op amps.
I really want to build a set of these for my Rotel RCD-990- especially since I saw all of the circuitry the AD-817 has in it. A few questions:

1. Will the discrete devices have enough drive?

2. If I go with all jfets, which are best.

3. What's with all the extra circuitry in most of these chips?

Thanks for your replies in advance

poobah 17th September 2006 07:37 PM

What do you want to build?

1) drive what?

2) depends...

3) Assuming you are tlking about the schem for the 817... most of them are current sources and mirrors, pseudo-resistors & capacitors made from transistors.

cadaverdog 17th September 2006 08:05 PM

Preferably the output will produce the standard +-2Volts and be able to drive a somewhat low input impedance such as a tube amp. I want to use jfets because they are self-biasing- and are low noise. I want to use them both as gain stages and current sources, yet keep the parts count low. I will probably custom-make my own pcb's for the modules should I choose to build them.

poobah 17th September 2006 08:18 PM

search for "folded cascode" & "Blowtorch"

lineup 17th September 2006 09:36 PM

Re: Need help with discrete op amps.

Originally posted by cadaverdog

Need help with discrete op amps.

...... especially since I saw all of the circuitry the AD-817 has in it.

Yes, in an IC op-amp can be 20-30 transistors! even in the less advanced
and signal may have to pass 7-8 transistors from input to output.

When I build a discrete preamp / op-amp
I not often endup with more than 8-10 T
and signal wont pass more than 3, maybe 4 devices.

You know there are 100 and 1.000 Op-Amps for sale and they are all different in some details, and in what they are intended to do, be used for.
It is same for building with discretes. 1.000 of possibilities.

So member poobah question is valid:
What do you want to build?

One of the first questions we need find an answer to:
What is the environment your discrete op-amp will live in?

Between a source and a load is where every amplifier is found.
A good description of those 2 factors, is what we start with.
This can be several different sources / loads to make your amp more generally useful.

But can also be a tailor made custom preamp for a very specific job.
Between for example CD-player Brand1-xxxx and Power Amplifier Brand2-xxxxA3

lineup ;) one generally useful diyAudio member

lineup 17th September 2006 10:00 PM


A good example of what I wanted to tell about,
is in my topic about a preamplifier design for another Power amplifier.

I this post a member [ MIKET ] suggests Design Goals.
What we would try to work towards.
What specifications we wish for a resulting amplifier to have.

Design Goals. Pre Amplifier for SEWA 7 Watt ClassA

lineup :cool: starts a topic, once in a while

johndiy 17th September 2006 10:03 PM

in your case i would recommend you use a good op ic
you could use a simple class-a discrete op amp


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