WORK STILL IN
Yep... A new preamp; again... Well, almost. This project is for a preamp, but not just for line-level use as I have always built. Recently I have joined the realm of analogue playback, yeah, I got a record player. I wouldn't be myself if I did not want to make my own RIAA correction for it. I'm new to RIAA, so I don't claim to know what I am talking about. Yet, even here the idea of using inductors is very appealing to me. I'm still figuring out the math and looking if my gain will be sufficient to still drive the rest of the system.
I don't want to re-invent the wheel, so I did what most people do... I ripped the schematic for the Tango EQ600 unit. It has gotten some rave reviews and the one-stage idea is very interesting. I put the circuit to the test (in theory) using MicroCap to simulate the response in combination to an inverse RIAA filter. The result is shown below.
The line may look wobbly, but this is over a 0.5dB deviation from 1Hz to 40kHz, after that it start to drop off slowly. Not really bad I think. The nice thing about a simulation is that you can change the parts within a few seconds and see the results, without ever heating up your soldering iron. Yes, this is purely software and there is no telling what reality may look like. Even so, I tinkered a bit with the values and got an even flatter response, as seen below...
This line is pretty much straight from 1Hz to 100kHz, about 0.1dB deviation here and there. But... how will it test in real life? I don't know... I hope to have some parts here soon to try it all out. The changes are minimal to the original values, as you can see below.
I noticed that the load is crucial to LCR corrections. Removing the 600 ohm load completely destroyed the correction in simulations. Even raising it to 620 ohms has enormous effects. The circuit will need to be tuned in real life because of this. The 600 ohm load also requires that the stage in front be able to handle this load, hence a stage is needed that itself has 600 or even lower output impedance. Without resorting to cathode followers the only solution left is using a step-down transformer. There are no tubes that have both a high gain and a plate impedance of 300 ohms or less.
This raises the question why the filter has to be 600 ohms. This low impedance means the values that are used in the filter are very low, but it also puts some strain on the choice of driver circuits (either step-down or cathode follower). I would like the load to be more in the area of 5K, this would mean I can drive with a lot less step-down or even directly with some tubes.
A 5K filter opens up the bag of tubes, but also poses another problem. Using a tube directly means DC into the filter. The LCR will not handle this very well as the chokes are made for AC only (saturation) and it's filtering will be destroyed (specification wise). Adding a capacitor in front of the LCR does the same as you are adding another part to the whole filter, so this will not work either. Adding an RC filter behind the LCR offers a different load, which is crucial to the LCR. In other words, the only way to do it right is to use a step-down inter-stage or a cathode follower.
Sticking with the inter-stage, I revised the circuit to be used on a 5K load. I will probably use a 6K:2K inter-stage to drive the LCR. This will only pose a 1:0.6 step-down in signal while still driving the circuit properly. The values of the chokes in the LCR filter have increased quite a bit, yet the capacitors have only decreased. I will place the circuit once it has been tested.
The candidates and their specs
© Jim de Kort