Sound City 50 plus Mk4 - active tone controls?

[Chuck H]

The threag has morphed a little, but... It's been concluded here (ampage/mef) in previous threads that: passive tone controls only subtract gain @ frequency and active tone controls can add or subtract gain @ frequency. This may seem like grey area, but it's not. Of course with all the knobs at fifty percent an increase in the "bass" control does add bass qith passive or active tone controls. But it's how the circuit affects the voltage of the input signal that I think determines passive or active. An "active" circuit can add gain and a passive cannot.

[jrfrond]

For me, the jury is still out on whether these are active tone controls or not. The signal after the volume control is resistively divided for the bass control, feeding a gain stage, then by a high-pass ladder network to feed the mid and high gain stages, and ultimately to a summing amp, which also provides gain. Gain is provided at for each band, and there is no output with all controls turned down. Because each band operates in gain mode, I maintain that they ARE active tone controls, but not in the convention sense that we see with feedback networks and gyrator circuits.

[imaradiostar]

I also fail to see how they're active. As mentioned previously there isn't a feedback loop. Think of an SVT preamp, most console EQ circuits of the past or even the tone controls on an HH Scott tube stereo. All relied on some sort of frequency selective filter placed between a gain stage and a buffer stage.

Wikipedia (always a reliable source, right?) makes it sound like any circuit that relies on an amplifier to function is considered active. This Sound City circuit wouldn't function without the three 12ax7's relating to the three pots. Most popular amps with a FMV tone circuit don't require an active element for the circuit to do its job as they are "loss only" controls.

In other words I don't know.

jamie

[log1982]

Responding to sgelectric's question. I have a SC with "active" tone, and ... well... it sounded crappy. Hum. Hiss. Not particularly well articulated high end. But the real downer was that darned tone circuit. It seemed to me that the channels (I liken this circuit to a channelized amp with low, mid, and high "channels") interfered with one another at the band breaks, and not in a good way. I believe it was wacky phase interference. Best way to describe it is a slurring at the crossover frequencies. As a side note, I fixed everything, more like rebuilt the amp, and it sounds glorious. Put the Partidges and Mullards to good use! Joe.