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Mixer mod
Old 17th July 2019
  #61
10k is a standard input load for those chip preamps. Most designers lessen that via a resistor across the input leads to lower that to 1.2k ohms. A 2.2k resistor across the inputs is commonly used to do that.

Here I remove those and set the input impedance higher via the 10k and 6.81k phantom resistors on the load. Then you get around 4k input impedance, better for some mics.

Most SSM 2017 layouts use a single cap across the input leads to roll off rf frequencies. 470 pf is a common value. It does not help stabilize the chip's inputs as they are running full bandwidth. Here I use a pair of 470 pf caps from each input pin to ground, that will stabilize those parts especially with longer mic cable runs.

Finally, the noise specs are not that great with these chips. If you need better the Audio Upgrades PIP modules will lower it to -132 db EIN at 50 ohms. At lessor gains the opamp noise is also lower than the chips. Distortion is also improved down to .00015% CCIF IMD, the residuals of my Audio Precision analyzer.
Old 18th July 2019
  #62
Gear Maniac
Quote:
Originally Posted by Jim Williams View Post
10k is a standard input load for those chip preamps. Most designers lessen that via a resistor across the input leads to lower that to 1.2k ohms. A 2.2k resistor across the inputs is commonly used to do that.

Here I remove those and set the input impedance higher via the 10k and 6.81k phantom resistors on the load. Then you get around 4k input impedance, better for some mics.

Most SSM 2017 layouts use a single cap across the input leads to roll off rf frequencies. 470 pf is a common value. It does not help stabilize the chip's inputs as they are running full bandwidth. Here I use a pair of 470 pf caps from each input pin to ground, that will stabilize those parts especially with longer mic cable runs.

Finally, the noise specs are not that great with these chips. If you need better the Audio Upgrades PIP modules will lower it to -132 db EIN at 50 ohms. At lessor gains the opamp noise is also lower than the chips. Distortion is also improved down to .00015% CCIF IMD, the residuals of my Audio Precision analyzer.
Thanks for the great answer. That PIP module seems cool. The only issue is the size. Might not fit on the 8 channel pcb in Soundtracs.
Old 13th October 2019
  #63
Gear Maniac
I decided to quit this mod and use the transformers in other projects. No matter what changes I did to the Soundtracs, the fingerprint of SSM2017 was there lurking. Sounded unispiring to record, dull highs and over all blunt. I might be using the mixer for playback only, as is.

I got 2pcs of YAMAHA MLA8 and a D&R stereo preamp, those seemed to be a better match for the transformers in initial tests.
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