Small Signal Circuits vs Discrete Opamp

[abbey road d enfer]

*** there may be more than one "soundcraft" mic preamp.

Exactly! there is the original 2-transistors+1 opamp, the 2-transistor +2 opamps (both without NFB to the emitters), then the one with 2-transistors and two opamps and NFB to the emitters, the one with Szlikai inputs and 1 opamp, the one with 2-transistors and 3 opamps, and probably a couple others, not mentioning the early xfmr-based ones.

 

[k brown]

Exactly! there is the original 2-transistors+1 opamp, the 2-transistor +2 opamps (both without NFB to the emitters), then the one with 2-transistors and two opamps and NFB to the emitters, the one with Szlikai inputs and 1 opamp, the one with 2-transistors and 3 opamps, and probably a couple others, not mentioning the early xfmr-based ones.

Well, there goes that theory!

The Folio Notepad and Mackie 1202 had similar topologies.

 

[abbey road d enfer]

I was directly comparing Folio Notepad with 1st gen Mackie 1202;

The preamp in the Notepad is the one with Szlikai inputs and one opamp. The Szlikai arrangement provides NFB to the input emitters, not really as good as real global FB from the 2 or 3 opamps types, but still quite good.
I don't know for sure for the original 1202, but seeing the original 8-bus had a similar arrangement (who copied whom?), they should not be too different.
The VLZ preamp is a different animal where the transistors are current loaded.

It's been claimed all Soundcraft mixers used same pres.

Whoever said that is ill-informed.

 

[k brown]

I sort of assumed the remark about Soundcraft pres referred to their lower-end products contemporary with the Notepad.

 

[abbey road d enfer]

I sort of assumed the remark about Soundcraft pres referred to their lower-end products contemporary with the Notepad.

Even in the Folio range there are at least 3 different preamps.

 

[k brown]

Thanks.

(shock) . . . can't believe everything you read.

 

[JohnRoberts]

Thanks.

(shock) . . . can't believe everything you read.

true but you can believe Abbey...

JR

 

[ruffrecords]

Thoughts on this? Seems rather different from the common 'ring of three'.

Advent MPR-1 accessory mic pre.

At first glance it loooks like a ring of three with the input modified into a diff pair.

Cheers

ian

 

[k brown]

At first glance it loooks like a ring of three with the input modified into a diff pair.

Cheers

ian

Could maybe be modded to a transformerless balanced input (ungrounding the other input)? I doubt the little stock trafos are much good.

 

[ruffrecords]

Could maybe be modded to a transformerless balanced input (ungrounding the other input)? I doubt the little stock trafos are much good.

Possibly. The only problem with that is the input you want to use for the second of the balanced input lines is loaded by the (variable) negative feedback gain setting network.

Cheers

Ian

 

[k brown]

Possibly. The only problem with that is the input you want to use for the second of the balanced input lines is loaded by the (variable) negative feedback gain setting network.

Cheers

Ian

Ah - good point.

Care to speculate what they were trying to achieve with that symmetrical input, rather than simply trafo followed by ring of three? Isolating the transformer from the feedback?

 

[abbey road d enfer]

Ah - good point.

Care to speculate what they were trying to achieve with that symmetrical input, rather than simply trafo followed by ring of three?

Distortion reduction. At the cost of impairing stability and slew-rate.
IMO this additional transistor would have been much better used in the output, because the drive capability of this preamp is appallingly low.

 

[k brown]

Distortion reduction. At the cost of impairing stability and slew-rate.
IMO this additional transistor would have been much better used in the output, because the drive capability of this preamp is appallingly low.

Not that surprising - it was meant to sit alongside the Advent 201 cassette deck; the pre had hardwired RCA cables that were only about a 18" long; I imagine the deck's line in was probably fairly high impedance.