John Roberts' audio articles

[JR.]

I don't recall adding those inductors myself or why there are two of them. Perhaps one at each transistor emitter. Surely to reduce RF gain and rectification susceptibility. (I had a junior engineer doing the detail work).

The R is never 0 Ohms but 22 ohms minimum/25k max, the L is 5 uH (two 10uH in series) the C is 470 Uf. There is probably some ESL inside that electrolytic cap.

Are you sure of your resonance calculation? 1.6kHz seems uncomfortably located in the middle of the audio frequency band.

Which gain setting 22 Ohm (max gain) or 25k Ohm (min gain)?

JR

PS: This console was developed in parallel with Peavey's Mark VIII sound reinforcement console. Several sub circuits were otherwise very similar. Those inductors may have been borrowed from the SR board, and crudely incorporated both together instead of separately (one at each device). This was a pretty massive undertaking including developing several new potentiometer series. I was very pleased with the wien-bridge channel strip EQ topology, which incorporated one of the custom potentiometers to control the sweep EQ. The wien-bridge EQ is very low noise and low distortion. I tuned the Qs and center frequencies differently for recording vs the live sound version.

[mediatechnology]

It's 20 uH 22 Ohm min or 25K max and yes its 1.6 kHz but as I point out the Q is nada at both extremes.

It introduces a nice HF gain rolloff. Joe Neil in Waynes mic pre used them IIRC at max gain the 3 dB point we used was around 80 kHz.

I would have split the two inductors and placed each at their respective emitters for balance which may be the way it was laid out but not drawn.

[JR.]

It's 20 uH 22 Ohm min or 25K max and yes its 1.6 kHz but as I point out the Q is nada at both extremes.

my bad, I don't do much work with inductors, [brain fart] I used the relationship for caps in series.

It introduces a nice HF gain rolloff. Joe Neil in Waynes mic pre used them IIRC at max gain the 3 dB point we used was around 80 kHz.

hopefully above 20khz

I would have split the two inductors and placed each at their respective emitters for balance which may be the way it was laid out but not drawn.

I suspect the two inductors were copied from the live sound version, and my junior engineer didn't grok what they were there for.

I looked for the parts placement drawing and those pages were missing from the "short" version of the PDF.

JR

[AnalogJoe]

Q = 0.009 with VR1 at 0Ω.
Q = 8.25E-6 at 25KΩ.

I've put inductors in Rg to rolloff RFI.

Yes, the Q is ridiculously low, but I was wondering about their purpose. At first I thought they might've been chokes, but the clear "L" and uH notation ruled out that theory. RFI rolloff makes sense.

Joe Neil in Waynes mic pre used them IIRC at max gain the 3 dB point we used was around 80 kHz.

10 uH is large enough to require a core, unless you like big air core coils; did you ever measure a noticeable increase in distortion when you guys used them?

[AnalogJoe]

PS: This console was developed in parallel with Peavey's Mark VIII sound reinforcement console. Several sub circuits were otherwise very similar. Those inductors may have been borrowed from the SR board, and crudely incorporated both together instead of separately (one at each device). This was a pretty massive undertaking including developing several new potentiometer series. I was very pleased with the wien-bridge channel strip EQ topology, which incorporated one of the custom potentiometers to control the sweep EQ. The wien-bridge EQ is very low noise and low distortion. I tuned the Qs and center frequencies differently for recording vs the live sound version.

I was looking at the Wien (Wien-Robinson?) style EQ you used, I don't particularly like how it's drawn, but I do like it. I will take a closer look later..

[mediatechnology]

10 uH is large enough to require a core, unless you like big air core coils; did you ever measure a noticeable increase in distortion when you guys used them?

Are you referring to saturation from DC currents or signal?
Joe had an AP and ran lots of tests on the circuit before he started using them in the truck. I have no idea if he looked for it.

I could see the possibility of DC saturation occurring in a true "Cohen" topology since the inductors are having to carry emitter current sinked/sourced from the op amp output. Joe, in Wayne's Mic Pre, used THAT1510s or 1512s which don't have emitter current in the feedback network.

500 to 1000 ft mic lines were not uncommon for Sam's "Praise the Lord Press Record" Gospel Remote Truck.
RFI rejection was excellent under those conditions.

What I saw in my prototypes was an increased susceptibility to LF hum fields due to lack of shielding.
If they are oriented to each other well and are actually in opposite emitters (not lumped into one as was drawn) then the hum can be made mostly common-mode.
I have some of the shielded MCI inductors with their mu-metal tubes.
Those things must be worth a fortune now.

[AnalogJoe]

10 uH is large enough to require a core, unless you like big air core coils; did you ever measure a noticeable increase in distortion when you guys used them?

Are you referring to saturation from DC currents or signal?
Joe had an AP and ran lots of tests on the circuit before he started using them in the truck. I have no idea if he looked for it.

I could see the possibility of DC saturation occurring in a true "Cohen" topology since the inductors are having to carry emitter current sinked/sourced from the op amp output. Joe, in Wayne's Mic Pre, used THAT1510s or 1512s which don't have emitter current in the feedback network.

500 to 1000 ft mic lines were not uncommon for Sam's "Praise the Lord Press Record" Gospel Remote Truck.
RFI rejection was excellent under those conditions.

What I saw in my prototypes was an increased susceptibility to LF hum fields due to lack of shielding.
If they are oriented to each other well and are actually in opposite emitters (not lumped into one as was drawn) then the hum can be made mostly common-mode.
I have some of the shielded MCI inductors with their mu-metal tubes.
Those things must be worth a fortune now.

Nevermind, with 500 to 1000 ft mic lines the least of your problems is a tiny bit of distortion (if any). In general I tend to frown upon inductors with cores, particularly those tiny axial inductors, they tend to be quite non-linear. At the emitters of a common differential stage, after the degeneration resistors, the signal ought to be so tiny that they are probably harmless, but I've never experimented with them before in a Cohen pre, so I was wondering what your experience with them might be.

Thanks to both of you for the great info.