Cornish P-2 guts

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Post by aionios »

JBJ wrote:
analogguru wrote:Normally I have no intention to waste my time by responding to paranoid false accusations without any substrate. But since this is a theme of general interest I will respond in this special case.

The basic technology of the "mythical Cornish Buffer" is well known since the early 60´s an was already used in the germanium era as you can see here:

[ Image ]

The technology is called "bootstrapping" whereby a portion of the signal is fed back to the bias divider to increase the input impedance. The darlington configuration shown is only necessary with low-gain (hfe ~ 80) transistors as the 2N1309.

So where is the difference to the Roland AP-2 buffer I mentioned ? First, here is a redrawn version:

Analogguru AP-2-style Buffer schematic

So what is obvious is that the bias for the first stage is not taken from V+ instead it is taken from the collector of Q1 via R2. This leads to an additional (AC and DC) feedback resulting in more temperature stability, lower distortion/increased headroom and lower collector output impedance.

Then you can see that the signal is not taken from the emitter instead it is taken from the collector. You can ommit Q2 and R7 if you only want to drive "high impedance" stages.

Because the signal is taken from the collector, the first stage even has a (small) gain of 2 (6 db). The second transistor doesn´t amplify, it is only a buffer to lower the output impedance (< 100 Ohm). That´s important when driving long cables.

Overall it can be expected that the "AP-2-Buffer" has:

a higher input impedance,
a lower output impedance,
less noise,
less distortion,
and even a small gain

compared with the "mythical Cornish Buffer" - with only 2 additional components.

Does anyone have the schematic of the AP-2 buffer redrawn by Analogguru?
Seconding the request... did anyone happen to save it? It's not pulling up on either.
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Post by Cub »

Not by Analogguru, but perhaps this is of any help until the file that he drew surfaces.
Roland AP-2 Phase II Buffer - Bridge Fx layout.gif
Roland AP-2 Phase II Buffer - Bridge Fx layout.gif (11.38 KiB) Viewed 207 times
Roland AP-2 Phase II Buffer - Bridge Fx traces.gif
Roland AP-2 Phase II Buffer - Bridge Fx traces.gif (1.84 KiB) Viewed 207 times
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Post by JBJ »

Cub wrote:Not by Analogguru, but perhaps this is of any help until the file that he drew surfaces.
Hi. Thank you for the layout. But I'd like to see the Analogguru's schem... :cry:
Does anyone?

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