QSXM2: Start preparing!

[goldkenn]

While waiting for the chassis to really finish building the QSXM3, I am going to start working on the preparations of QSXM2.

I am going to order SSM-2210/SSM-2220 (actually found and placed order with Digi-Key online this morning), BC847BS/BC857BS along with BC850C/BC860C, but when I read through the article, I notice that there should be something things need to change value such as R25, R31 and C7, but without mentioned what value should be exactly, pleae kindly tell which and what I should change according to my use of transistors.

Meanwhile, I also want to add a dip switch with serveral resisitors, hard wire solder right on top of the PCB for shortest length, making an adjustable gain, specially for MC cartridges, is that either R35 or R37 I could touch for doing the change? What value of resistors, or should say what range of gain changing will be safe from distortions and oscillations?

Thanks and best regards.

[peranders]

If you plan to use the SSM2210/20 you should change the R17/R16 to 15/4 kohms = 3.9k and only one pair is sufficient.

R25, R31 C7 requires some measurements but you can leave it.

If you want to change the gain of the pre amp you may only change the resistance to ground. The amp is stable in MM mode which means that you can use any gain for MC.

R35 and R37 sets the gain of this particular VAS stage and you should not change it unless you have knowledge about this. R50 + R51 (R52/53) sets the gain of the amp.

[goldkenn]

Thanks so much!

So all I need is only one pair of SSM at Y1/Y2?Y3/4/5/6/7/8 leave for either BC847BS/BC857BS or BC850C/BC860C will be fine?

If all Y1 to Y8 use SSM is just a waste, or will cause the circuit unstable? Since I ordered 8 pairs, if no use will just throw in drawer.....

Change the gain of the pre amp may only change the resistance to ground is means R51, lower value higher gain, or vice versa, am I right?

As the pre amp gain now is set to 43dB, how high at maximum I can go? I have serveral cartridges, the lowest one is at 0.15mV, since I use passive line stage (even the new coming one will be B1 which still as passive have no gain), therefore I can only and must get enoguh gain from the phono.

Meanwhile, other then somewhere critical like RIAA or feedback path, any further advantage to use high precision components, such as resistors at all emitter? I am considering to order 0.1% Welwyn, but it is really costy, so I want your comments to aviod spend money for nothing, since right now I already seems spent for nothing at the 8 pairs SSM...... :-(

Thanks again.

[peranders]

Connecting transistors in parallel is a way to lower the noise (from ordinary transistors) but the SSM2210 has lower noise than 16 BCxxx in parallel.

You can of course use all SSM's you've got but keep in mind that you won't get any lower noise.

I think you can wait with gain adjustments until you know how much you'll need.

The choice of resistors is non critical in most locations but the feedback resistors of the preamp must be metal film for the noise but 0.1% will add nothing. Remember that 1% metal film has a tolerance of 0.3% in real life.

[goldkenn]

Thanks peranders,

The reason I most concern about resistor is actually the ppm rather then the tolerance, since only 0.1% resistor have lower ppm (mostly 15ppm)  then 1% (mostly 50ppm), what I understand is lower ppm = lower noise, am I right?

Any further suggestions to have a one stop non-compromise (base on real technical issue, not brands or luxury materials, such as I never belive those hundred dollars silver foil caps) configurations without stupid spending?

I will start ordering  parts next week when I get my salary, I must make everyone shock for the QSXM2 due to wife issue, she already said what I spent on QSXM3 enough for a resonable good AV system but still acceptable, since she was a pianist before marry me, she know the performance of QSXM3 worth the cost, but one more phono stage and spending money again, she start complain, so if the QSXM2 can't convince her? She may take over the finance control...... :-P

Regards.

[peranders]

Tempco isn't particularly important either because the feedback will take care of the temperature stability. Plain metal film resistors have 50 ppm and that is sufficient.

[goldkenn]

Understand! Thank you! Then I syuck with Philips resistors  :-D