Oh, good grief. Anyone spot the mistake I made in the above photo? Can't wait to try and fix that tomorrow. Hopefully I can salvage the part.
Looks like the DIP switch is in backwards, but of course it will work like it is, just have to reverse the numbering on the switch functions.
Hmmm. Guess I will have to look at that too. The trimmer....how do I remove it to measure it if it is soldered in? Dumb...thats what I get for trying to do this after a long day at work right before bed.
Well, I'm not sure after checking it again. Please feel free to chime in here. I referenced the build guide (post #103) of the original Quasimodo thread by Mark Johnson on DIYAudio. His orientation matches mine. I'm not the expert so please let me know if you are in the know..lol. Also can anyone point me to where the dip switches are talked about. I just went through the PDF and I didn't see a reference to them. Thanks.
I just quickly compared yours to a couple loaded boards online, I don't have any personal experience with it, yours on top, couple others below, but yea, looks like you were supposed to install pins for the pot so it could be measured ...
After a period of involuntary downsizing, starting to put together a quality sound system again. This is my DIY First Watt F8 inspired clone. Single ended power JFET output stage. Around 25 watts per channel. Solid and reliable since I first powered it up a year ago. Sounds very good in my Class A biased opinion. imgur.com
Looking at the forum post schematic, the 1k VR just connects between ground and a C3 pin. Clip to ground, and jam a wire into the C3 side closest to the trimmer. "Oscilloscope required", if you really want to see and tune the effect, although not pointed out is that the overshoot shown on the first page diagram has a frequency of about 300kHz, well above audio or any audio interface's ability to even measure it, and unlike most critical damping, the period is not increased by adjustment. The easiest way to desolder through-hole is with a desolder pump "pen". Heat the joint, then quickly place the desolderer over the thru-hole and press the "suck" button to release the spring. Sucks all the solder out of the hole, maybe just needing a wiggle to crack any that remains. (Thru-holes where every pin is bent over by the placer are a bit more annoying.)
Sure was! I unsoldered it this morning and fixed it. I was a bit worried I would damage the trimmer from all the heat it took to remove it. I installed the pins and am good to go now. I also measured the trimmer and it seems fine.
Looks like you did a very nice job! Well, except for those trimmer pots taped to the capacitors, that's a bit tacky
Good eyes Sir! Still playing with standing bias to fine tune distortion levels. In search of system Sweet Spot.
Was previously in a deep STAX headphone rabbit hole. Last amp used for that was a DIY KGST with SR-009 electrostatics. Differential EL34 output with solid state driver.
I've been toying with the idea of a small phono loading board. Four caps and four resistors per channel, each switched in/out via jumper. My current thinking is each channel would have a 100pf, 220pf, and two 470pf capacitors. I'd select some resistors that would give me a range of 100k down to maybe 10k or 20k, I'd have to swap the resistor in my preamp to 100k, but that is no big deal. But as I've been playing with these old cartridges, getting them loaded for their best performance right now requires shoving capacitors and resistors into a socket, and the legs only last so long before they start bending, etc. It would just be nice having something on which I can set jumpers to adjust. This is the first time I'll have made traces this fine on my CNC router. The jumpers are spaced at .1" so everything is based on that. The board is only about 2" wide and 1" high.
Looks good. I built something similar, at least functionally, for measuring phono preamp input capacitance, you may recall the thread I posted ... Measuring phono preamp input capacitance In my case, it just uses a DIP switch and standard proto board, with wired connections on the bottom, but I've done dozens of circuits like that, usually much more complex, so it's the easiest for me. In this case, it is both capacitors and resistors, and a couple pins for custom values ...
Yep pretty similarI may change to a DIP switch. Given the small size I think I can modify the shape a bit and with a couple of unthreaded spacers sort of turn this into a daughter board on my TC-750, and then due to the placement of my gear a DIP switch may be easier to adjust (than jumpers). My only concern with DIP switches is picking out a decent make/model rated for lots of operations before wearing out, I guess. I could order from Digikey so I don't get any fake garbage.
Here is one of the last pieces I need to really go down the rabbit hole and start this amp build. Cant wait to use it! Also all of the transformers and chokes have been ordered from Toroidy.
I'd suggest you get an external probe calibrator since that doesn't have one built-in, specifically if judging high-frequency overshoot. Here's one on sale - whoops, for twice the price of the oscilloscope. Never mind. I suppose you can crib the circuit out of a Tek 2246 service manual, using two op-amps, +15/-7.5 and a +2.5V voltage standard.
I think I can do it this way. I have no idea how accurate this way is though. fnirsi 1014d probe calibration - Google Search
I looked into this scope a bit, which goes by four different brand names besides just a "Oscilloscope" badge, and found more from those who had hacked into the thing. First, hope you got one that doesn't have an opto-isolator installed backwards, making DC coupling on one channel not work. The signal generator doesn't have a DAC IC, it has an 8-bit R2R parallel ladder of arbitrary grade resistors on the circuit board, to a voltage follower op-amp. A square wave is about the only thing I would trust it with. Then to measure your square wave and probe response, use a BNC banana adapter instead of a cable, or just jam a wire into the center pin hole. It has some capacitors floating the BNC ground between DC voltages (which also means you can't use the output's ring as the missing ground terminal), but the traced schematic someone made doesn't have the values. Then read the manual. Besides some entertaining English, they give you some warnings about the input capacitance being "100-300pF", how much they don't know. A far cry from the standards-traced 20.0pF input capacitance that is set with an external calibrator to a fraction of a percent, like you'd find on typical instrumentation (good enough to then use your oscilloscope's input and a bridge as a capacitance calibration source). And also a forum user (that has disassembled the source code) pointing that they use software averaging and smoothing because of the input noise and ringing. Hopefully it will give an accurate enough answer when tuning the snubber. I worked up a schematic and sim for both 1kHz and 4-25kHz square wave oscillators modding Tektronik/HP calibrators from simple parts. About the only thing I wasn't able to arrive at was a 50% duty signal both clean and not slew-bound at higher frequencies, while simultaneously able to be buffered by 5V TTL with bench supply voltages.
Here is something I have been working on for a while now. It is one board that houses relay switching for four inputs (hot and ground), relay switching for dual selectable outputs, a relay switched volume attenuator. It will also have PCB mounted RCA jacks (like seen here) . These jacks will actually mount the board a few millimeters from the back panel. I would like to give a shout out to a few people: @fully_articulated for some inspiration from one of his latest projects doing a relay switched attenuator @Jaytor for his never ending technical assistance @Pars for his assistance in helping to find a technical drawing of the PCB mounted RCA so I could create a footprint first the schematic and the board and the 3D version unfortunately there is no 3D model for the 18 pin jumpers or the PCB mounted RCA jacks. This one was a long time coming......
