orangechili said:
Alright, I knocked together a bipolar 24v supply, built up the spectra sonics 110 vero and it works well
It is verified as well.
Here's my observations, keep in mind I've only built 1 board of each so I'm basing this off a small sample size….
-The 110 is quitter in operation, dead quiet to be exact. Love it! I used Edcor wsm 600:600 transformers for the ins and outs on both pres cause I have a bunch of these lying around. Eventually want to try the Triad a67-J wired as 150:600 for the inputs as people seem to like that best.
-The scratchiness of the pot caused by the dc isn't as noticeable on the 110, much less offensive
I notice the Q4 and Q5 transistors can kick off some heat at times on both the 101 and 110(hot if you touch them). I would recommend the bigger bodied original 2n3566 transistors because of this instead of smaller bodied modern replacements. I did match my pairs to well under 2mv as well and tested every resistor before going on the board. I could have used higher quality resistors and caps for sure but wanted to prototype these and not make a custom order for these parts just yet. I have been running these for an hour straight and no smoke.
If any owners of racked up 101 or 110's can comment on how hot the last 2n3566 pair runs for them that would be helpful for sure! I'm wondering if this is normal for their operation to just run a little hot or if even more careful component selection and matching is the key.
Nice one.
Just to let you know that neither the 101 or the 110 were design to be used with the pot. The feedback resistor is supposed to be fixed, set and forget. Gain should be changed by the use of input Pad or output pad.
Here are the recommendations of Richard Guy. Richard worked in Spectrasonics and has 50 years of experience with this gear. He is probably the most knowledgeable person on Spectrasonics designs in this planet, much similar to what Geoff Tanner is for Neve stuff, here it is:
"Gain Adjustment
The Spectra Sonics pre-amplifiers are optimum at 40 dB of gain. It
is possible to adjust the gain by changing the feedback resistor according
to the following chart:
35 dB gain = 5.6 k resistor
40 dB gain = 10.7 k resistor
45 dB gain = 20 K resistor
55 dB gain = 42.2 k resistor
Resistors should be low noise types, preferable mil-spec metal film
types. RN60D is a good designation to look for.
Installation of a gain-adjust potentiometer is not recommended. It
will compromise stability of the amplifier as well as adding noise.
Switching feedback resistors is also not a good idea. At the very
least, it would require a system of constant value feedback resistor
bypassed by higher value parallel resistors to ensure that the feedback
loop never became open. Furthermore, changing the resistor
will be noisey, so switching while "on" would not be recommended.
I suggest you find a gain setting that works for your lowest output
microphone, then utilize input pads to make the adjustment. This is
superior in performance and stability, and does not add noise or distortion.
"
