Miller Audio

Miller Audio LLC

Negative Feedback Selector (NFS) Upgrade

Introducing our new Negative Feedback Selector (NFS) upgrade for Dyna-70 and Dyna-120 Amplifiers. Questions such as: “What is Negative Feedback?” and “Why do I need a Negative Feedback Selector?” often follow. Therefore, some background on what Negative Feedback (NFB) is, how it’s used, and why you should consider this upgrade.

The Dynaco ST-70 is designed with 4-, 8- & 16-Ohm speaker outputs. As with most audio amplifiers, the ST-70 uses a signal called Negative Feedback (NFB), taken from the audio output, which is then fed back into the audio input circuit of each channel to correct for distortion. NFB is a “control loop,” and is always working to correct the audio signal, to minimize distortion. 

A compromise Dynaco made was to use the 16-Ohm output for the NFB signal for all speaker impedances. This is fine if one is using 16-Ohm speakers, but what if you are using 4- or 8- ohm speakers? The 16-Ohm NFB circuit is not optimized for the lower speaker impedances, meaning the distortion products are not properly compensated for, which can lead to distortion in the amplifier and in extreme cases can impact overall frequency response and bandwidth performance.

The NFB Selector corrects this by using a correction circuit that has all 3 speaker impedances available to properly compensate for NFB, providing the proper feedback signal correction for any 4-, 8-, or 16-Ohm speaker. One simply sets the selector for the desired speaker impedance and your Dyna Ultimate Amplifier will provide the lowest distortion signal correction for the speaker impedance used. It’s simple, easy, safe, can be done in a matter of minutes, and does not require one to work inside a powered amplifier; installation and NFB change can be made with no power to the amplifier.

Does it matter? / Does it really work? We promise, if you are using the 16-Ohm Output from the A-470 as your NFB signal source in your Dyna-70, you will hear & sense an immediate improvement in sound output if you install the NFS system in your amplifier and your audio system speakers are 4-or 8-Ohms.

If you use a 16-Ohm speaker system, continue to use the 16-Ohm terminal for both NFB and to drive your 16-Ohm speakers. However, remember to use the NFS to select the proper NFB signal should you change from 16-Ohm speakers and set for the speaker impedance you will be using, whether 4- or 8-Ohms.

The NFS upgrade kit comes with 2 PCB’s for left & right channels, as well as all components to assemble and install the NFS. We have the NFS upgrade kit in stock and ready for installation in your Dyna amplifier- only $39.95 + S&H.

NOTE: The NFS Upgrade Kit is not compatible with a standard Dynaco ST-70. It is designed for use with the Dyna-70 & Dyna-120 Ultimate audio systems. It is not compatible with other 3rd Party audio-upgrade circuit cards.

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