Pacific Recorders & Engineering ABX control room monitor upgrades?

[plumsolly]

I am in the final stages of refurbishing a PR&E ABX-18, which is a pretty neat beast - more to follow.

I would like to use it's CRM in place of my current outboard one - It will save me some money and the console's solo functionality only works in the context of monitor path - It is not destructive solo, but has a separate solo bus.

I have attached the specs and the schematic. The specs are nothing to sneeze at: 0.5db down at 20khz, and .004% THD.

However, I would like to make this path even cleaner. I realize that the improvements will most likely be inaudible, especially in the context of my amp/speaker/room system. Consider it an academic exercise.

I plan to replace the volume pot with a rotary switch to maintain L&R matching.

I am going to experiment with replacing the CMOS switches with DIP plugin relay cards made from these: http://capi-gear.com/catalog/product_info.php?cPath=102&products_id=195

I ordered some fancier chips to replace the 5532/4s -LM4562 and LME49710.

I think the bandwidth is set by the feedback caps - (C6, 9 14 & 20 for the left channel) - I calculate 0.5db down at 20khz for the current values, which is in line with the specs. I'd like to shoot for something like -0.1db at 100khz which would mean replacing the 100pf with 5pf. 

Will I be able to get away with this? Is this the sort of thing that depends on layout? RF present? Specific opamp?

I see here: http://www.soundskulptor.com/docs/mc624-schematic-02.pdf, no feedback c.

Lastly, I am wondering whether it would be worthwhile to try to change the instrumentation input amps with THAT chips - I'm thinking/hoping that with the fancier opamps, the distortion of this stage as is will be vanishingly low.

Thanks for your input,

Ben

 

[plumsolly]

Specs

 

[plumsolly]

CMOS switches are Harris HI1-307-5. See attached datasheet.

 

[PRR]

> C6, 9 14 & 20

Consider C2 C3.

The 6 9 14 20 set is at 160KHz, 2 pole, so will be -2dB at 80Hz, -1dB at 40KHz.

The C2 C3 set is a third pole, still near 160KHz. I'm too tired to work that out.

It smells like they expected large supersonic signals. Was this a broadcast machine that might set right under an AM transmitter? You could probably use much smaller caps.

 

[plumsolly]

Thanks PRR

It smells like they expected large supersonic signals. Was this a broadcast machine that might set right under an AM transmitter?

Yessir, exactly.

I'll have a look at C2, C3 and  will experiment with some smaller caps.

Ben