I did it on another similar type with a film capacitor bridging the two inputs.....Do not intend to steal the thread but since my question relates to this project that im trying to complete...What would be the best way to implement Mono-Stereo Switch ?
Short the resistors that go to the non-inverting input of the second opamp together at the input terminal of the opamp is how I would convert to mono.....Do not intend to steal the thread but since my question relates to this project that im trying to complete...What would be the best way to implement Mono-Stereo Switch ?
There are, albeit theoretical, reasons why you would not wish to do it this way. The series coil resistance for a LOMC cartridge is in the c.3-20 ohms region. To get true mono the impedance at all audio frequencies has to be significantly lower than this. If the cap is sufficiently low impedance to allow for true mono down to LF then a very large cap will need to be used.
Hi Sachin- what value are the resistors? I assume that the switch is applied directly at the output of the amps, and some care needs to be taken to avoid either overloading the output opamps or increasing the output impedance too much that the HF is rolled off with the cap load that the next stage/cables represent.
Makes sense, as does your suggestion that leaves the front ends as gain buffers to prevent the unintended consequences.
The reason I'm asking is that the picture seems to have brown black black red- then brown as the color codes. I thought that was 10k ohms 1%, but it's been a while since I've been completely facile with the color codes. If so then the mono output resistance is 5k to each of the two outputs in parallel, which is pretty high.
Hi Sachin, I see now, I was trying to find pics of other peoples builds to check that the power wiring was correct, so I should have a wire coming out of the power board -v marked as number 1which is the negative and this will go to my negative on the phono stage, a wire from the ground on the power board marked as number 2 going to the ground on the phono board and a wire going from +v marked number 3 going to the plus on the power board? Then I should set up the voltages as per the original instructions to get plus 12 volts on the plus side measuring between +v and ground and minus 12 volts measuring between -v and ground? Is this correct?
Didn't get a chance to try it out with correct wiring of power supply till this morning but it's still the same, do you the think i might of fried something when I connected it up incorrectly before? When it was wrong I had the ground from the power supply to the minus connection on the.phono stage and the +v in the power supply going to the plus on the phono stage?
Alas, it's possible that the opamps are dead. In particular the 797s have input protection diodes that could be toast. If you have a DC meter it's easy to check if the opamps are OK. Probe the + and - supply pins- they should be no more than about a couple of hundred mv lower than the board edge supplies, then probe the + and - input terminals and the output terminals of the opamps. The input terminals should be equal and the output terminals should not be railed. The actual output voltage value for the input opamp should be pretty small- tens of mv at most, but the output opamp value is quite variable- up to 2.5v or so depending on the opamp trimmed offset values and whether or not the circuit resistors were modified to give the best output offset as I described about 10 pages ago.
Hi Wyn
Yes, black probe on ground. The supplies have 10-20 ohms of series resistance to the opamps. For there to be 2v of drop you need to have 100mA of current or more. The opamps have c. 12mA of quiescent current combined. This also assumes that the electrolytic caps are undamaged.
