Pro Audio Design Forum

Posted: **Fri Feb 03, 2012 3:02 pm**

Yes, Dennis Bohn, Chief Technology Officer.
It was good to hear from him.

Posted: **Tue Feb 07, 2012 10:16 am**

Hi Wayne,
We've finally had a chance to look this over and get back to you, but first a little history:
The HC 6 was Rane's first product originally designed and first manufactured in in 1982. Amazingly it is still a good seller -- 30 years later.
Over this time the circuit has been revised several times due to many reasons. Some of these are due to vendors discontinuing parts, due to going from single sided through-hole components to surface mount, etc.
Each revision comes with its own tweaks.
In this case it is one of circuit stability due to layout. Adding R8 close to the positive input works with the input capacitance to create a high frequency pole that creates extra stability. Also over the years we have changed op amp types. Some are more stable than others. It is just a simple way to make things less squirrelly at high frequencies, plus adds a bit of RFI filtering as well.
Hope this helps explain the mystery.
Thanks for the inquiry.
Dennis Bohn
CTO
Rane Corporation

I sure appreciate Dennis' response and his allowing me to publish it.

So there you have it: A HF pole formed by R8, package and 4580 input capacitance.

Posted: **Tue Feb 07, 2012 10:41 am**

Good to hear from the horses mouth...

Interesting but still a little obscure... we know the R but the internal C is not to ground but IC package or chip substrate? likely just one rail? Not like the good old days with grounded metal cans surrounding ICs.

Whatever... It may have worked at some time in the past 30 years and stayed in the design JIC.

A real C to ground would be more conservative and reliable RC pole.

JR

Posted: **Tue Feb 14, 2012 6:54 am**

I realize that this thread has veered OT, but:

Check out the 1K input series resistor in the NE5532 test circuit.
This figure is from ON Semiconductor. My 1981 Signetics data sheet also shows the same circuit. (No surprise.)

NE5532 Voltage Follower test Circuit.

With the 5532 and 5534 having back-to-back diodes across the input I suppose that the 1K in the above test circuit would serve to limit input current during large common mode errors and power-up.

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