Balanced feedback preamp topology questions

[L´Andratté]

A few questions just out of curiosity:
this topology (see below*) is well known from the Cohen/Benchmark/a lot other mic pres as well.
It seems to be one of the most clean (low thd+noise) preamps.

1. The variant of the Sony MXP3000 has resistors in a T-arrangement going from both emitters to -V. What is this going to achieve?
Schem:https://groupdiy.com/index.php?topic=44790.msg563629#msg563629

2. The Cohen preamp use two LM194 dual transistors (transistor banks really), and arranges them crossed (one of each duals is paralleled with one of the other).
This is for excellent CMRR. But the seventh circle clone leaves that out, just parallels the twin resistors on each LM194 package. Why? Is the former overkill? Speculation welcome!
7O schem(link is a download):https://88804d15-9e02-44d8-a87b-402652deb5ad.filesusr.com/ugd/7ed8a6_ba3972caf1a241518344e40557d9532a.pdf

3. There´s also the Soundcraft-version using only two opamps, also featured in Self´s
"Small Signal Audio Design". Is there an obvious down side to it?
Schem:https://groupdiy.com/index.php?topic=44786.msg909247#msg909247

Hope someone is in the right mood to answer  ;D

*taken from "Microphone Engineering Handbook, Chapter 8, by Peter Baxandall"

 

[abbey road d enfer]

A few questions just out of curiosity:
this topology (see below*) is well known from the Cohen/Benchmark/a lot other mic pres as well.
It seems to be one of the most clean (low thd+noise) preamps.

1. The variant of the Sony MXP3000 has resistors in a T-arrangement going from both emitters to -V. What is this going to achieve?
Schem:https://groupdiy.com/index.php?topic=44790.msg563629#msg563629

Nothing extraordinary. R9 goes to the negative supply and R6&7 distribute current to the emitters.

2. The Cohen preamp use two LM194 dual transistors (transistor banks really), and arranges them crossed (one of each duals is paralleled with one of the other).
This is for excellent CMRR. But the seventh circle clone leaves that out, just parallels the twin resistors on each LM194 package. Why? Is the former overkill? Speculation welcome!
7O schem(link is a download):https://88804d15-9e02-44d8-a87b-402652deb5ad.filesusr.com/ugd/7ed8a6_ba3972caf1a241518344e40557d9532a.pdf

Wrong schemo!
Anyway, Graham Cohen was aiming at leaving  no unexplored avenue. He spoke a lot about that. It offers superior input offset control.

3. There´s also the Soundcraft-version using only two opamps, also featured in Self´s
"Small Signal Audio Design". Is there an obvious down side to it?
Schem:https://groupdiy.com/index.php?topic=44786.msg909247#msg909247

It saves one opamp over the Cohen topology. Soundcraft have moved a lot since this, since the cost of opamps is close to nothing now. I believe now they use the Cohen topology with Szlikai pairs at the front.

 

[JohnRoberts]

A few questions just out of curiosity:
this topology (see below*) is well known from the Cohen/Benchmark/a lot other mic pres as well.
It seems to be one of the most clean (low thd+noise) preamps.

This has been a widely used topology. This particular variant shown is providing the operating current for the input devices through the feedback resistors. This works but forfeits a few volts of peak signal swing. I used this approach in a MC phono preamp kit design I published in early 80s. 

1. The variant of the Sony MXP3000 has resistors in a T-arrangement going from both emitters to -V. What is this going to achieve?
Schem:https://groupdiy.com/index.php?topic=44790.msg563629#msg563629

2. The Cohen preamp use two LM194 dual transistors (transistor banks really), and arranges them crossed (one of each duals is paralleled with one of the other).
This is for excellent CMRR. But the seventh circle clone leaves that out, just parallels the twin resistors on each LM194 package. Why? Is the former overkill? Speculation welcome!
7O schem(link is a download):https://88804d15-9e02-44d8-a87b-402652deb5ad.filesusr.com/ugd/7ed8a6_ba3972caf1a241518344e40557d9532a.pdf

3. There´s also the Soundcraft-version using only two opamps, also featured in Self´s
"Small Signal Audio Design". Is there an obvious down side to it?
Schem:https://groupdiy.com/index.php?topic=44786.msg909247#msg909247

When I started working at Peavey in the mid 80s, they were already using the two opamp approach (and the same 2sd786 low noise NPN). I was using the complement 2sb737 (slightly quieter PNP and 3 op amp approach). I could not argue a significant benefit from using the extra op amp or my preferred device so I started using the two opamp approach and their already in system device for my preamp designs while at Peavey.

JR

Hope someone is in the right mood to answer  ;D

*taken from "Microphone Engineering Handbook, Chapter 8, by Peter Baxandall"

 

[user 37518]

I like the SSL mic pre which uses two NE5534 stages to raise the gain, the first stage is a conventional Cohen stage whilst the second stage is a 3 Opamp IA, the gain setting resistors in both stages are controlled by a dual gang pot. The NE5534/2 doesn't have enough GBP to amplify 60dB in a single stage, however, modern opamps have much higher GBP.

 

[abbey road d enfer]

I like the SSL mic pre which uses two NE5534 stages to raise the gain, the first stage is a conventional Cohen stage whilst the second stage is a 3 Opamp IA, the gain setting resistors in both stages are controlled by a dual gang pot. The NE5534/2 doesn't have enough GBP to amplify 60dB in a single stage, however, modern opamps have much higher GBP.

I'm not so sure about it. Only the designer has the right answer.
In a transconductance amp, which the Cohen is based on, the presence of the bipolar transistor extends considerably the GBW product. A 5534 coupled with a half-decent bipolar could have a GBW of 100's MHz, if there were not other issues (stability, layout, reverse admittance...)
I would think the most important advantage is that the gain pot's control law becomes very smooth, without resorting to custom taper pots.

 

[user 37518]

A 5534 coupled with a half-decent bipolar could have a GBW of 100's MHz, if there were not other issues (stability, layout, reverse admittance...)

I see, that is interesting, so the  gain of the bipolar multiplies the open loop gain of the opamp and raises its GBP?

 

[abbey road d enfer]

I see, that is interesting, so the  gain of the bipolar multiplies the open loop gain of the opamp and raises its GBP?

Somewhat...the mechanism is altered by the presence of additional resistors; I guess they could be replaced with current sources, but it's probably not worth the complication.
I would think the dominant limit is open-loop phase-shift due to parasitics and reverse transconductance of transistors.