hifizen
Well-known member
Hi folks,
This is my maiden post here... I've been lurking for the last two or three months, and am normally found on the hi-fi forums.
On to the topic at hand - I've been eyeing the GainBloak circuits, and have decided to build a version of the BigBloak (FET input) for my current project, a stereo preamplifier (darlington output will drive headphones nicely). My goals for these mods are to allow unity gain, and to provide very low offset for DC coupling the input and/or output. I also don't want to use a DC blocking cap in the feedback resistor network.
To accomplish these goals, I've added a current source for the input FET (as seen elsewhere), and a DC servo (maybe not needed if I can massage the tempcos) integrated into the input current source, rather than dangling off the negative input node as is usually seen. Two other changes: a Vbe multiplier for output bias instead of the diodes, and discrete darlingtons instead of the integrated ones (should be a little more linear, plus I can play with the bias).
I realize this is a bit of a departure from the spirit of simplicity embodied in the original design, but I think these additions are justified for the intended application, and with some luck I'll pare down the complexity some as the design evolves toward it's final implementation. Since it's picked up quite a few frills, I've been calling it the FancyBloak.
Here's the schematic so far:
[deleted]
I haven't simmed any of this yet, so some of the component values are missing, others (such as the servo capacitors) are just a temporary value which I'll correct later.
Opinions and suggestions welcome...
- Chad.
<EDIT> This thread is no longer active, but lives on in the Lab area: "DCBloak on the bench..."
This is my maiden post here... I've been lurking for the last two or three months, and am normally found on the hi-fi forums.
On to the topic at hand - I've been eyeing the GainBloak circuits, and have decided to build a version of the BigBloak (FET input) for my current project, a stereo preamplifier (darlington output will drive headphones nicely). My goals for these mods are to allow unity gain, and to provide very low offset for DC coupling the input and/or output. I also don't want to use a DC blocking cap in the feedback resistor network.
To accomplish these goals, I've added a current source for the input FET (as seen elsewhere), and a DC servo (maybe not needed if I can massage the tempcos) integrated into the input current source, rather than dangling off the negative input node as is usually seen. Two other changes: a Vbe multiplier for output bias instead of the diodes, and discrete darlingtons instead of the integrated ones (should be a little more linear, plus I can play with the bias).
I realize this is a bit of a departure from the spirit of simplicity embodied in the original design, but I think these additions are justified for the intended application, and with some luck I'll pare down the complexity some as the design evolves toward it's final implementation. Since it's picked up quite a few frills, I've been calling it the FancyBloak.
Here's the schematic so far:
[deleted]
I haven't simmed any of this yet, so some of the component values are missing, others (such as the servo capacitors) are just a temporary value which I'll correct later.
Opinions and suggestions welcome...
- Chad.
<EDIT> This thread is no longer active, but lives on in the Lab area: "DCBloak on the bench..."