AMB Laboratories DIY Audio

[Telstar]

Hi Ti Kan and the others,

I have got a fully populated PCB with sigma22 psu (electrolytics on the ck2iii board have been replaced with small mkp), which was used as preamp. I plan to drive my 40-Ohm HPs instead and I have a couple questions:

1) On my board gain is set at 6x -which is the lowest i could go without increased distortion? I think 4.1 with R12, R13, R36, R37 @825ohm as in the parts list page. Can it be further lowered (2x-3x)?
2) I have a good heatsink and a few pairs of output transistors that I plan to try*. Keeping the actual drivers, Which is the maximum bias? I'd like the amp to stay in class A to at least 300mW. Bias is currently set at 30mA. I can replace the drivers if needed.

I plan to keep the voltage at 15v. My transformer is 30VA rated for 2x 0.8A, which gives some headroom.

*2SC2238Y/2SA968Y; 2SB1186A/2SD1763A; MJE243/MJE253, eventually Hitachi Lateral Mosfets.

[amb]

You can reduce the gain by changing the value of those resistors further, and you may also need to change the compensation capacitors too. But it is not a tested configuration, and your amp may turn into an oscillator (may not be outright oscillations, but could be parasitic). If you want to make these changes you should be equipped with a wideband function generator and oscilloscope to check its behavior.

Same goes for the transistors, only BD139/BD140 and MJE243/MJE253 were tested.

[Telstar]

The standard version (BD output transistors at 30mA bias) is tested and works flawlessly, but as I thought gain needs to be lowered to better use the pot. The sound is very clean and detailed. It requires 15+mins of warm-up. I didnt do any comparison with my other headamps yet.

The MJE will be the first alternative devices to be tested (i'm waiting to the Rohm ones and the molex sockets).
So my 100mhz Tek scope will be put to work in the next weekend.
I think it is the ideal circuit to evaluate the output stage due to the lack of global feedback.

[Telstar]

PS: in the sigma22 psu my partner added a diode between R1 and Q1: I'm not sure of the exact implications of this change.

[Telstar]

It took much longer than expected to have all 8 output devices pairs, and setup a A/B-able circuit to compare them on the fly. I used six 6-way swtiches instead of sockets. Even with several cms of cables and traces (where should be only few mms) the amp is dead quiet.

I think it is the ideal circuit to evaluate the output stage due to the lack of global feedback.

While i still think this to be true, the CFP configuration used in the C2KIII works towards minimizing such differences. Therefore evaluating the output transistors was harder than I expected. Moreover, the faster ones required a diode trick to keep them in check (it doesn't seem to worsen sound, since i put it on all pairs including the original BDs.)

The actual results of my auditions wasn't exactly what i expected, with the BD sounding much better than the MJE, and in a higher class, the Toshibas were very close to the Rohm, the first providing a more coherent sound and the second extracting an amazing amount of detail. In this setup, low gain devices seem to fare better.
My next move will be to find a matched quad of toshiba 2SC2238/2SA968 and try and EF configuration, and find a way to raise the bias to keep it in real class A up to 2W.

PS: there seem to be something wrong with the bias setting, so I had to leave it at 30mA as was set.

[Telstar]

I'm not able to change the bias, any advice? i want to put it to 150mA, but whatever I do, the current in the output transistors does not change... so I left at 30mA.
I'll soon close the box and declare the amp finished and i would like to get this bit of performance.
PS: my heatsinks are well specced.

[amb]

You're using non-stock output transistors, correct? If so, the bias adjustment network (Vbe multiplier) may need to have its resistor values changed.

[Telstar]

You're using non-stock output transistors, correct? If so, the bias adjustment network (Vbe multiplier) may need to have its resistor values changed.

Actually, there is a derived pcb on breadboard with 8 output pairs (mounted on big heatsink) with 4 switches to select each pair.
Now that I unmount that and solder only the 4 i prefer, i'll try again to change resistor values, but I have a suspicion that the CFP or something the previous owner mounted on the board wasn't the most correct. *puzzled*

Therefore I have ordered another C2K3 pcb (you shipped last friday so 1-2 weeks to come here), and I'll check on the clean board. As long as the bias DO change with the default output transistors, I should be able to do the same with proper values with the other ones, right?
There is a complete BOM? I need to order caps and resistors while the PCB arrives.
Ofc i'll use my sigma22 psu in place of the onboard one.

[amb]

Have you looked at the CK²III website?

[Telstar]

Ofc, there is everything written there, thanks