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Some Discussion on Amp EQs

#103
Bass Boost just switches in another a cap in the coupling of the first stage to the volume pot resulting in lowering the HPF that is formed. Thus, cutting less bass.

Contour on the old 1200s tube amp was just the Mids control of a FMV. I cant quit tell what is the Mid Boost switch from the scheme but there is a 3PDT switch listed which adds varying gain to parts of the amp which would have an apparent boost to upper mids and high end.
 
#107
dunwichamps said:
2. The Ampeg SVT Mid Range EQ

The Ampeg SVT Mid control (and Mid Selector) is one of the few true active mid range EQs and is the ideal (IMO) mid range control for bass and guitar. It offers approximately plus and minus 20dB of control on a specific mid range frequency. It is an active bandpass filter which involves 3 triode gain stages, and a tapped toroid transformer. The tapped toroid provides different inductance values for the filter which allows one to change the center frequency of the bandpass from 220 hz to 800 hz to 3 khz. The only say disadvantage of this EQ is that is requires a lot of parts, a lot of space, and design work to get it to work. I do not think its often worth the effort given how much real estate you need to take up. It has dozen plus parts in it, would require a weird 3 triode NOS tube and or combinations of 12ax7s, 12au7s, or 12dw7s.

Here is the schematic:

Link Removed

Suffice to say its a bit 2 much to explain here. Just understand it requires a lot of work to achieve.

Now the 3 way switch works to ground different parts of the tapped toroid which I am showing at the bottom of the schematic, represent as different inductors with different inductance values. I was able to reverse engineer their approx values since the schematics on the internet do not show their values.
Click to expand...

I am curious how the mid range circuit affects the output impedance. Is it possible to drive a signal to a mixer from the output? Would it need a transformer, like the [Invalid or Expired Link Removed]? Where exactly would the output be taken, between R38 and R40?
 
#108
I would not drive very low impedance signals off this but if you could drive into a 100k load that would be good. If you needed <10k impedance then a transformer might be better.

Someone has mentioned that the Ampeg bax treble sweep is down

Link Removed
 
#111
Very informative and useful. Thanks everyone. Read the whole thread again just now. Some was over my head, but plenty wasn't. I have a V4B and appreciate the info that applies. A lot of TB users, like I, use the SVT-CL. Is it safe to generalize the EQ as working the same way but with the addition of two center frequencies in the mid selector?
 
#112
wighat said:
Very informative and useful. Thanks everyone. Read the whole thread again just now. Some was over my head, but plenty wasn't. I have a V4B and appreciate the info that applies. A lot of TB users, like I, use the SVT-CL. Is it safe to generalize the EQ as working the same way but with the addition of two center frequencies in the mid selector?
Click to expand...

Possibly but I could need to see a schematic to confirm
 
#120
It might be completely unrelated, but since the Aguilar DB680 is long discontinued now I was curious as to whether it fell into one of these categories (minus the 2 bands of parametric, obviously).

Great informative thread!
 

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