[quote author="Val_r"]Hi,
Here's another hybrid mic preamp.
Please post your comments and suggestions.
Thank you,
Val.
:idea:
[/quote]
Looks pretty familiar. We used a version very similar to that in the AMR recording consoles (late '80s), Peavey Mark VIII, and sundry other Peavey models. Major difference (from memory) we used single 2SD786 low noise input devices per input (now obsolete), TL07x opamps, Rs for upper current sources, and polarity switch when provided was at mic input not pre output.
A few comments on stability... Since the second inverting opamp is effectively inside the overall feedback loop for the first, it’s delay needs to be compensated for. The approach that I am familiar with is a RC network between + and - inputs of the first opamp to reduce open loop gain at very HF. I have never seen any capacitance (like the 470 pF) between the emitters. Perhaps it is used to roll off feedback factor for stability but it looks like it will add closed loop gain at very HF which could be an issue if input becomes unbalanced in presences of RF. It is also pretty common to use a shunt C and some Cs to ground at mic input. C7 across the second inverting stage will have some impact on stability and too much roll off there will lead to asymmetrical differential output at HF.
While less important, the 100uF input caps may be problematic for fault currents if inadvertently shorted (as discussed in recent threads). Also the 1k input termination is a little low for my tastes. I typically target 2k nominal (10x 150-200 ohms). With a minimum gain of 26dB+ it might also benefit from a switchable front end pad. I also routinely use anti-avalanche clamp diodes across input b-e junctions, precision Rs for phantom, etc.
Based on my experience with a similar version this topology is a solid performer and somewhat more cost effective than Transamp/Cohen topology as it can generate SE output with one less opamp (useful in large consoles and cost sensitive smaller designs). The 470P makes me a little nervous and I would instead suggest a compensation RC at opamp 1 input.
JR
PS: Kit- If by CCS you mean "cap coupled" the reason for using a coupling cap in gain leg at emitters is mainly to prevent scratchy pot noise (or switch clicks) due to Vb-e differences. With larger offsets and max gain, the input current density of + and - inputs could also be impacted but this is usually secondary to the perceptual (noise) issue. For commercial use the scratchy pot is problematic, for personal use I have no problem with DC coupled at that point.