Before everything else let me introduce myself. I'm a french audio enthusiast, and i've been hooked by the DIY virus some years ago. Since then I've already build a large number of gears (Pass F5/M2/Aleph/B1, Tubelab SSE, 300B se, some lm3886 stereo amp, some AB amps, a shigaraki MKIII cd player and repaired quite a bit of vintage gears like Onkyo M504, Hitachi HMA8500MkII, Marantz PM-55, Sharp SM-511H) as well as some speaker designs.
Recently I've built SL LXmini speakers and I think I got here one of the finest speakers I've ever listened to.
Right now they are driven by a Hitachi HMA8500mkII on the woofers and a Quad405 clone on the Fullranges and as I plan to add the subs (LXmini+2) I wanted to build a multichannel amp to drive the whole thing (with at least 6 identical channels), I then tumbled upon the 8PAKAMP kit witch seems to fill the bill nicely.
Now for the questions/problems/trouble
I am planning a full unbalanced build (so -6db of gain) and since I need a gain of at least 27 I will have to change the gain somehow, so here is what I've done:
-Built the board with kit parts except for R11 witch has 220R instead of 1K (+12db of gain)
-Chips mounted below the board and fixed to a huge Heatsink.
-I use 300VA 2x24V trafo (a little less than 300VA for instance but it's for test purpose, will use a proper 500VA 2x24V for final build)
-I'm using two 200V 25A bridges for pos and neg rails (outputs wired backward on the negative rail bridge)
The wiring is made as follow:
-one return to starground from on of the ground terminal in the middle of the board (currently it's taken from on of the speaker return but will be changed for final assembly)
-return for each bridges to starground
-Mains earth to starground
-Speaker return to starground (may be soldered directly to the board in the final assembly)
-RCA hot to +in (pin 2)
-RCA cold to -in (pin1)
-Board shield (pin3) to ground (on board)
Now for the problems:
If a signal is present on the input (music or sin from a generator) or if the input is shorted, everything works fine, but if there are no input or an input without signal, the output give 28V AC, speakers do not like that at all
I'm surely doing something wrong with input wiring but Before trying every possible combinations I prefer to ask for your enlightenment.
Also, It seems like shorting R1 and R4 and installing a fixed 10K res in place of the trimmer is a cleanest way of getting back 12db of gain, but as my board is already populated I don't know where are R3 and R4, can you show me on a board picture?
I made some changes:
I Changed R3&R4 for pieces of wire, replaced R11 for a 1K res, and put a 10k res in place of the trimmer, and that for all channels.
I relocated the pcb ground return to the terminal witch is between filter caps.
I wired the speaker return directly to the PCB (next to the speaker output terminal)
I wired the rca hot to +in (Pin 2) and rca cold to -in (Pin 3) no bridge from pin3 to ground
I wired the two 24V secondaries of my transformer in parallel (2x24V 130va became 1x24V 260VA) so to only use one 25A 200V bridge.
Voltages after bridge are +-35,4 VDC.
Now for the measurments:
I use a Shanling M2s DAP connected to the two wired channels (channels 1 and 3) with a jack to 2x RCA cable (common ground)
-With no connections at all on both rcas I got 0V AC and 10mV DC offset on each speaker outputs.
-With one RCA connected (but not connected to the DAP) on channel 1, I got 0,1V AC and 10mV DC offset on each speaker outputs.
-With two RCA connected (but not connected to the DAP) on channel 1 and 3, I got 8vAC witch collapse abruptly to 0v AC after less than a minute, and 200mV DC offset witch rise abruptly to -4,15v DC after less than a minute. This is exactly the same when the DAP is connected and music signal is playing (except AC voltage is on range with the music signal)
Same behavior whenever pin 3 is bridged to ground or not
Somethings weird here