The John Hardy 990C+ Op Amp

If you’ve seen the preamp project, you’ll understand. I wanted to see if an active preamp could beat a passive preamp, and I think I
succeeded with the previous preamp. But in the last two years, I’ve had something gnawing at me off and on. The way to make an active preamp better is to make it as much
like a passive preamp as possible. To that end, I took another pass at the preamp, and came up with rev 4.

This time, it is nothing but the discreet op-amps and their supporting parts. I took the skinny sounding, low contrast, OPA2134 out, and the offset protection that hangs
on the outputs. It’s down to the DOA and the volume pot. Does it sound better? I can’t tell. But I feel better about it. Added benefits: It is smaller, cheaper, and less complex than it was.

The beauty of this preamp is the 990C+ discrete op amp. Its distortion spec of 0.005% is at 12.23VRMS output (+24dBu) at 20kHz into a 75Ω load.
If that isn’t “worst case”, I don’t know what is. It gets better as the load and gain are reduced.

Preamp Board

Preamp board
Preamp Board


This is the assembled board with the 990C+ discrete op amps. There are more pictures on the build page.

Preamp schematic
Preamp Board Schematic


The schematic of the preamp board. The 990C+ is unity gain stable, but they don’t recommend running it that way. I’m running it at a gain of 3.
There is a 50k pot on the 990s non-inverting input, which provides a decent input impedance and allows me to adjust the overall gain of
the circuit to set maximum output and channel matching. That comes in handy when you first set it up for a power amp. The preamp can be
adjusted so that the power amp doesn’t clip when the volume pot is at maximum.

Power Supply

Power supply board assembled
Power Supply Board

Preamp power supply schematic
Power Supply Schematic

The power supply provides ±24V for the two DOAs. It also sends +15V to the input relay board. Each main supply is good for just under 1A, with 1.1A total for both.
The transformer is an Antek AS-0522. It provides ±31.3VDC with no load after
rectification and filtering. I chose the LM317/337 regulators so I could tweak the output voltage if I needed to change op amps. This is the power supply from version 3.