If you don't need more gain, then maybe you don't need a preamp. After this one is done, I do plan on attempting a power amp with this topology. In fact, I did start playing with it. That might be more appropriate for you.
There are many source follower power amps available, such as Mike's MoFo, the store VFET amps, and other VFET follower designs. This pre goes really well with those type of amps that don't have any gain of their own.
There are many source follower power amps available, such as Mike's MoFo, the store VFET amps, and other VFET follower designs. This pre goes really well with those type of amps that don't have any gain of their own.
Here's the update so far. All schematic changes done at this point.
| # | Change | Location: Schematic or PCB | Complete | Comment |
| 1 | Bypass cap for C102 | Sch | Yes | |
| 2 | Simpler CCS with one FET | Sch | Yes | |
| 3 | Buffer on the output | Sch | No | Likely to not include buffer. |
| 4 | Flexible, improved regulator. Fewer zeners. | Sch | Yes | Use trimmer pot to Zener reference. |
| 5 | Power LED indicators for each channel. | Sch | Yes | |
| 6 | Cap across input FET bias resistor for better bypassing. | Sch | Yes | |
| 7 | Snubber across rectifier | Sch | Yes | CRC snubber based on Mark Johnson’s Quasimodo application note. https://www.diyaudio.com/community/...rmer-snubber-using-quasimodo-test-jig.243100/ |
| 8 | Onboard 10R NTC//22nF cap GLB output to chassis GND | Sch | Yes | |
| 9 | Various pitches for I/O caps. Bigger cap sizes. | PCB | ||
| 10 | PCB mounting on store chassis/baseplate pattern. | PCB | ||
| 11 | TO-220 for rectifiers | PCB | Yes | |
| 12 | Gate resistors closer to FETs, or add SMD pads as option for builders | PCB | ||
| 13 | Bigger pads where appropriate | PCB | ||
| 14 | Smaller overall PCB size | PCB |
Yes for #6. For #7, I have never used a snubber before but clearly they seem important given their use in commercial designs as documented by Mark. The snubber network will be optional but it will be there on the PCB.
I would be using a follower amp with this - the DSP box that precedes the current Singing Bush is set for enough attenuation that one could use a follower amp with gains close to 0 in the DSP using your stage as a buffer for the op amps which are in the DSP box. It would seem redundant to have gain in the line stage only to have to use lots of attenuation in the DSP box. Typical pop music levels. Attenuation would still be needed for most orchestral recordings.
I think you mentioned that the goodness of this circuit would be retained with no gain. Just wanted to be sure I understood correctly,
Looking forward to the boards being ready.
Thanks
I think you mentioned that the goodness of this circuit would be retained with no gain. Just wanted to be sure I understood correctly,
Looking forward to the boards being ready.
Thanks
I have the SCG built, but need to track down why one of the channels is not working. I'm able to dial all the settings in with no problem. I was thinking that the issue could be the wire, but initial testing is showing that is not the case. I think that I will remove the LBB110 and see if that is the problem. Hopefully I will be able to get everything working this weekend. The one channel does sound nice.
Is it possible a wire from the selector switch is not correct? Did you confirm there is no signal from the SCG output header?
That is another possibility I tried, but when I switched the input on the board the sound did not follow. I was planning to test again by removing the selector switch for the signal path.
I like to try the simplest things first.
I like to try the simplest things first.
Yup, try bypassing the selector switch altogether and taking output directly from your screw terminals on the SCG board.
All good suggestions above. You can hook up a signal generator directly to the input (bypassing everything else) and put your voltmeter on the output to see if you are getting an AC signal. I’d test at 60 Hz coz that’s where ordinary multimeters will be most accurate. There are several free signal generators on the app store. I use one called F Generator. Good for quick testing so you don’t have to break out the complex gear. This will help eliminate the board/LBB110 as a problem.
I finally figured out my problem. It was the input wiring. I had the ground and signal wires on the right channel reversed. Now I need to test it in my main system.
Thanks everyone for the help.
Thanks everyone for the help.
Good point. Will do. I started laying out the PCB. Will keep that in mind.
If you have it working, then you can just adjust it directly without shorting Rlim. For initial power up, I recommend setting the CCS separately and doing it before anything else, otherwise you can get into odd places where you’ll keep turning pots and nothing will happen. That’s because the Vgs of the middle FET depends on the Vd on top of it and in certain pot positions it will keep the FET off. Can be mildly frustrating.