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[zmix]

I thought this topic needed a separate thread to do justice to the concepts discussed here.....

cerberus wrote on Wed, 22 February 2006 08:19

thanks for that info zmix.  can you comment on feedback in preamps such as the chandler geranium which has a feedback control in lieu of an output pot?jeff dinces

You've gotta admire the efficacy of marketing hype...

Feedback is important as a design device to eliminate certain types of nonlinearity in an amplifier. In the early days of hi-fi, negative feedback was used to linearize the output of a poweramp by feeding the signal at the output transformer's secondary (or sometimes tertiary) winding in antiphase back to the input. This so-called correction signal could actually cancel out frequency response aberrations of the transformer itself and eliminate crossover distortion in push-pull circuits. Because the signal you feed from the output  is in anti phase, it will cancel out the input signal, so 20dB of negative feedback will reduce the gain of the amplifier circuit by 20dB, broadly speaking.

Most designers of power amps use a fixed amount of feedback, generally just enough to correct the distortion and frequency response problems.

There are several preamps on the market which use the circuit feedback to adjust the gain of the amplifier.

The API mic pre uses the 2520 opamp with a feedback circuit comprised of a voltage divider from the output to the inverting input of the opamp, and a pot forms the lower leg of the divider. As the resistance of this pot is decreased, less signal from the output is fed to the inverting input; less feedback, more gain.

The gain adjustment on several Neve amplifier boards is set by emitter degeneration (aka negative feedback) and the entire line of Neve preamps use a combination of feedback and input attenuation to control circuit gain.

The Chandler Germanium pre simply puts the feedback on a pot seperately from the input attenuator.  I'm certain that it has some great tones available that other preamps cannot produce.

The question you are no doubt asking at this point is likely:

"Why has nobody advertised this feature, if all these preamps already have it?"

Well, the answer lies in the difference between corporate and boutique audio. Several aspects of the performance of the circuit are severely affected by changes in the amount of negative feedback. Most major manufacturers have learned that in order to compete in the spec-sheet driven market they need to ensure a consistent level of performance across the entire gain range of the preamp. These manufacurers often incorporate more complex feedback adjustment for this reason. You could pretty reliably count on getting the same sound from a Neve or SSL at 20dB as you would at 50dB, not so with the chandler, but then that's why we love it on some things more than we do on other things....!

-CZ

[malice]

The Requisite PAL+ has this features as well, it's an amazing tool.

malice

[cerberus]

this is a very clear explanation for me and i hope for others who were taken aback when i suggested that feedback loops could be employed by an audio engineer as a production technique in mixing or mastering; and it's not as radical or uncommon in signal processing as it might sound to a lay person.

thanks again for such a detailed yet accessible primer on the topic, and for separating the application of feedback to an envelope detection circuit from it's common application in amplification.

jeff dinces

[maxdimario]

Perhaps what they are stressing is that they have EXTREME settings over neg. feedback.
...to the point where you have so little that you can really hear the grain of the germanium transistors.

but of course you are right, most mic preamps change gain by changing the amount of neg. feedback.