Simplest discrete op-amps

[Martin B. Kantola]

Much good advice, thanks! Wasn't able to find the TubeTech circuit anywhere, but on this page the Borbely hybrid design is presented (as in Glass Audio):

http://www.headwize.com/projects/opamp_prj.htm

As soon as I've figured out a good way to control DC-offset, I think I might have enough to start building and testing some crude circuits. Really interested in what they sound like compared to the usual black little bugs.

OK, so distortion will be higher in magnitude and lower in order, but minimizing NFB and running at higher voltages might give an audible difference too. Better? Not too sure about that. Hearing is believing...

Martin

 

[CJ]

Uh oh, looks like a feedback winding that nobody sells.

 

[Larrchild]

Looks like my 407a mic pre, feedback-wise.
Hows that iron CJ?

 

[Kit]

a simple current source at the differential pair.
helps a lot for cmrr and the gain goes up i think

Gain has nothing to do with it........

CMRR?

Thats only an issue in instrumentation/diff. circuits.

Were dropping half the supply voltage over the tail R so a simple resistor is fine for most application. But yes, a true CS is better and sooooo cheap so why not.

 

[bcarso]

[quote author="Kit"][CMRR?

Thats only an issue in instrumentation/diff. circuits.

[/quote]

Beg to differ. Common-mode distortion is a big problem in noninverting configurations. Often it is as much or more to do with the variable capacitances, admittedly, but it's to be ignored at one's peril.

 

[Kit]

Common-mode distortion is a big problem in noninverting configurations.

Sorry, im not sure what you mean.

Feedback related?

 

[Samuel Groner]

Sorry, im not sure what you mean.

www.groupdiy.com/index.php?topic=10865

Samuel

 

[JohnRoberts]

[quote author="Kit"]

Common-mode distortion is a big problem in noninverting configurations.

Sorry, im not sure what you mean.

Feedback related?[/quote]

This may be getting tangled up in terminology. The non-inverting topology will certainly be much more sensitive to input DC operating point when using a simple resistor bias vs. current source bias.

While the differential gain doesn't (significantly) change between resistor and current source bias an internal operating point shift with common mode voltage will. It is not uncommon to use resistive bias for an application specific discrete design such as an inverting summer where little or no common mode range is expected (and higher gain non-inverting apps). For low gain non-inverting applications a resistor bias will probably make it's presence felt in limited input dynamic range and to a lesser extent reduced linearity due to parameter shift with changing bias current.

I would vote current source bias for any general purpose solution.

JR

 

[Martin B. Kantola]

CJ, thanks for posting the TT schematic.

Here's an OTL tube headphone amp from a Swedish guy that I also found interesting in this discussion. Not an OP, but pretty close if you ask me...

Martin

 

[Kit]

Samuel,

thanks for the link. Good reading.

Interesting about the C modulation distortion of TLO7x devices, it has been discussed in some other threads, and its something that i think is definitly audible.

JR,

Thanks for clearing that up.

 

[clintrubber]

FWIW let's add another tube-circuit with an opamp-like topology.
Tek won't have meant it that way, but OK, some compensation etc.
OL-gain is ~60dB.

http://home.hetnet.nl/~chickennerdpig/FILES/Tek122/Tek_Type122_circuit_ampsection_rt.jpg

 

[LazyOne]

that reminds me i have an old scope from tek full with tubes and it has a plugin unit that is a differential amplifier. i think i might have the schematic.
practically that is an opamp but probably with reasonable good specs.

 

[bcarso]

[quote author="LazyOne"]that reminds me i have an old scope from tek full with tubes and it has a plugin unit that is a differential amplifier. i think i might have the schematic.
practically that is an opamp but probably with reasonable good specs.[/quote]

Almost always, scope amps are not using global feedback like opamp circuits. This is due to trying to get the maximum bandwidth out of things, which feedback always compromises.

 

[ruffrecords]

[quote author="bcarso"]Almost always, scope amps are not using global feedback like opamp circuits. This is due to trying to get the maximum bandwidth out of things, which feedback always compromises.[/quote]
This is completely wrong. Global feedback does not compromise bandwidth.

Ian

 

[clintrubber]

[quote author="ruffrecords"][quote author="bcarso"]Almost always, scope amps are not using global feedback like opamp circuits. This is due to trying to get the maximum bandwidth out of things, which feedback always compromises.[/quote]
This is completely wrong. Global feedback does not compromise bandwidth.

Ian[/quote]
Initially my reaction as well (in somewhat lesser strong terms), but since it was Brad who wrote it I kept thinking and now think I agree.

As in: how about the open circuit not having to 'prepare' for feedback, hence no safety margin required w.r.t. the BW-limiting (dom. pole).
Maybe not the perfect wording, but possibly what Bcarso meant ? Brad chime in please.

Regards,

Peter

 

[JohnRoberts]

[quote author="ruffrecords"]
This is completely wrong. Global feedback does not compromise bandwidth.

Ian[/quote]


No, but the internal compensation required for stability with negative feedback does indeed.

I'm no student of scope schematics so the following is mostly WAG. They probably use less overall NFB and apply it locally rather than globally to minimize this constraint. They may also favor phase and amplitude response preferentially over extreme linearity as we are not going to eyeball nonlinearities more than say 30-40 dB down. or not...YMMV

JR

 

[clintrubber]

[quote author="JohnRoberts"]No, but the internal compensation required for stability with negative feedback does indeed.[/quote]
Right, that's the wording that expresses it better :thumb:

 

[LazyOne]

ok i found the schematic again
it's basically quite simple but not the standard way of making an opamp.

at the input there is a diferential cascode.
basicly the same as a diff amp but you use an extra valve (see the cascode stage for that). you can set the current with that extra valve so it's easy to balance the circuit almost perfect. to do that u need an extra voltage at the grid of the tubes that act as a current source en this is done by a resistor divider followed by a cathode follower with an 12au7! i think thats overkill but it works. after that to balnce the cricuit there is a pot ant two resistors (10k) to ground
the valves are 5814 triodes (close to an ecc82 or 12au7)

after that stage there is an amplification stage with 5879 penthodes
they are also balnced.
the cathodes are connected by two resistor that are variable (=variable gain)
and they are balanced to ground with two 39k resistors
then the second grid is set at voltage also with a cathode follower that suplies 190v. and it is also balanced with a pot and referenced at ground by 82k resistors.
after that there are two cathode followers constructed with 12au7's and then you hav ethe output.
i think it could drive 10k out easy.
and the gain is good enough i think (they state 1mv/div and at that point the bandwith is 350kHz!) at full gain there is some 80db i think.
gain is reduced by attenuation with a resistor at the balanced output of the two amplification stages.

if you see it as a gain bloack you have 2 inputs (+/-) and 2 outputs (also+/-).
and the BW at full gain is like 350kHz and with lesser gain goes to 2MHz

may be i could try something.
use the thing in combination with the lundahl transformer at the input and see what i get on that tek scope. and maybe try it as a mic amp
or just try some basic opamp applications with it.
the tek scope btw is full of those circuits. it is nice to see how they managed to get a BW of 30MHz! (the scope, this thing was just a plug in)
and basicly the most of the circuits contain basic opamps build up with tubes.
if someone really wants i have the schematics of it. maybe interesting to see?


btw cmr is not so good at 60 Hz they specified 55db (ac coppled low frequency's)
at DC it is 80db (dc coppled al frequency's i think).

 

[ruffrecords]

[quote author="JohnRoberts"][quote author="ruffrecords"]
This is completely wrong. Global feedback does not compromise bandwidth.

Ian[/quote]


No, but the internal compensation required for stability with negative feedback does indeed. [/quote]

Except internal compensation is not required for stability with global feedback. It is true many op amps use a dominant pole internaly so the op amp is unconditionaly stable with feedback but is is not required. There are plenty of designs around without a dominant pole.

Ian

 

[JohnRoberts]

[quote author="ruffrecords"]

Except internal compensation is not required for stability with global feedback. It is true many op amps use a dominant pole internaly so the op amp is unconditionaly stable with feedback but is is not required. There are plenty of designs around without a dominant pole.

Ian[/quote]

Sure, if the closed loop gain is high enough stability criteria may be met without additional internal roll-offs.

If we are going to wrestle over semantics indeed "internal compensation" is not a specific constraint for stability, just a common technique to roll-off HF gain before it becomes problematic when using lots of negative feedback.

GBW or more specifically open loop gain factored by the percentage of negative global feedback applied must be less that unity by the frequency that the total accumulated inter stage delay/phase shifts approach 180' to prevent instability (Bode).

This could also be managed by local negative feedback in the sundry gain stages, and/or not having much open loop to closed loop gain margin in the first place, etc.

While none of the posts in this thread are models of precision in language (mine included), I believe I follow what bcarso is saying and don't agree with your position that he is "completely wrong".

Global negative feedback does indeed impose a constraint on how much HF open loop gain can be used while maintaining stability.


JR