Hello folks! I’m currently working and testing a new mic preamp kit as successor to our SK-1DI 500 Series Mic Preamp Kit. I’m calling it our NK-72DI. This new mic preamp will feature Carnhill transformers, imported from the UK – the same kind of transformers used in Neve preamps and consoles, i.e. it’s not a reproduction or a clone, but rather the real deal Carnhill transformers.
The NK-72DI Mic Preamp kit will have the same front panel layout as our SK-1DI Mic Preamp, and will also have an instrument DI input. But that’s where the similarity ends.
The new preamp will have a different topology, and different internals from our SK-1DI. It will use both input and output audio transformers to interface to the outside world. It will also be using a Discrete OpAmp (featuring our FiveFish DOA-12), made up of discrete transistors, resistors and capacitors. The amplifier topology is similar to an API-type preamp, which is basically a non-inverting amplifier topology… but with Carnhill transformers for the input and output stage. 🙂
For this prototype, I’m using my vintage St. Ives 10468 for the Input Transformer. On the kit versions, it will be a Carnhill/UK type.
And here’s a photo of the new prototype under construction…
As you can see, it’s got a big output transformer, and room for a Discrete OpAmp. You can use whatever Discrete OpAmp, but hopefully you’ll be using our FiveFish DOA-12. Note: The DOA-12 will be discounted generously for buyers of this mic preamp kit.
What else? The mic preamp will have +48Volt, relay controlled -20dB Pad, Polarity Reverse, and Mic/Instrument input selector switch. A 1/4″ instrument jack is also located on the front panel for easy plugging of your electric guitar/bass/acoustic guitar.
There is an on-board trimmer to adjust for DC offset, so you can get as close as possible to 0Volts DC offset, within +/- 1 millivolt or less. By removing the slightest DC offset, we can shoot for the maximum output signal swing with equal clipping on both the positive and negative waveform peaks.
Talking about the output signal swing, preliminary testing shows a maximum output of +26.6dBu (16.6Volts rms, or 48.0Volts peak-to-peak output signal) on a supply voltage of +/-16Volts DC (voltage typically found in 500 Series lunchbox/rack).
P.S: The above figure is the maximum output signal level. The maximum GAIN is about +72dB. Yeah, I know… crazy!
If you have the option of using +/-18Volts DC for the power supply, the preamp can even achieve a maximum output of +27.5dBu (18.4Volts rms, 53.2 Volts peak-to-peak)!
Note: The Discrete OpAmp DOA-12 can operate up to +/-22Volts DC supply voltage. When I get the chance, I’ll measure output voltage swing on +/-22Volts! It will be interesting.
The above readings were taken with the preamp output loaded with 600-ohms. Even under loaded conditions, the waveforms are looking beautiful with just a crazy amount of output voltage swing. This is one “beefy” preamp, both in terms of performance and weight. These input and output transformers are huge and heavy — with lots of iron. But it’s not a problem for our Discrete OpAmp DOA-12 to drive them.
Hopefully, this new kit will be ready by December. Pricing TBD but it will be very competitive and attractive. This new mic preamp is also going to be a great complement to the SK-1DI Mic Preamp in terms of it’s sound characteristic.
A great winter project for musicians and studio owners. Build this mic preamp and keep warm with your soldering iron this coming winter!
I’ll update this blog with more pictures the coming days. Please LIKE, SHARE and follow our Facebook page so you won’t miss any news and announcements from FiveFish Audio. Thanks!
