Last post by dimkasta - June 24, 2022, 08:13:50 PM
Slow day at work today, so I had some time to experiment.
There are two interesting parts in the first gain stage.
The first is the capacitor C5 and how it interacts with the gain pot. At first glance it looks like a coupling cap. But it's in the feedback loop, and interacts with the pot. As the gain increases, the upper part of the pot divider decreases, and the cut off frequency of the high pass filter is increased (less bass passes the filter). But because the filter is in the negative feedback loop, this means that low frequencies are increased at the output. This works exactly like a resonance control. But the problem is that bass increases with gain. Which is quite counter intuitive, especially at higher gains, unless you like fuzzy distortion. Removing C5 removes that weird interaction and high gain sounds tighter.
The second area is C4, R4, R5 and the switch. The switch acts exactly like turning up a presence control at high gain. R4 sets a minimum resistance so that things do not go too far. Ideally, we would replace R5 with a 100K or 50K pot wired as a variable resistor, and then remove this pole of the switch so that we have full control both at high and low gain. This would give us a very versatile presence-like control.
Last post by dimkasta - June 23, 2022, 06:56:22 AM
Short update about my progress. I am currently going too far. I have designed my own PCBs and face plate. I will probably open a new thread when I have some more to show. Unfortunately PCB fabrication takes too long and design changes hurt both in time and money
Most likely the pot has failed, you might need to replace it. The wiper which is the center pin can grind away the carbon track or just get dirty and looses contact with the track. Also look for hairline solder cracks around the pins on the PCB. You could try circuit cleaner spray into the pot but a new pot might be cheaper than the can of spray. Phil.
One schematic with 3886 output with +/- 31 volts supply has 130 ohms in the PSU circuit. The other schematic with 2050 output with +/- 22 volts supply has 10 ohms in PSU circuit No mention about the output power in both the schematics.