Active Tone Controls

[Theashe]

Hi all,

I might get my membership revoked for suggesting this, but does anyone have experience putting op amp-based active equalizers into tube amps? I've been working on a solid state desktop headphone amp, and the 8-band graphic EQ I put into that works so damn nice that I rejigged it into three controls with wider bands for a guitar amp (see attached).

I don't often see solid state tone controls in tube amps, and I'm wondering why (reasons other than heavy loading, since I can buffer the tone stack with another op amp). I figure as long as I make sure the input signal can never overdrive the op-amps, this'll make for a really powerful set of tone controls.

I know right at turn on, the HV in my current tube amp spikes up to about 450V before settling to 400V. That makes me think that the transient spike at turn on would might the op-amps erupt, but I've never attempted it before to find out.

[John_P_WI]

Active tube controls have been around for a very long time. Playing with them some 20 years ago it was apparent that the bass could be over whelming easily.

What signal level are you expecting to handle with your op amps? Expect to attenuate the signal 40 db or more depending on the amp / location and rail to rail abilities of the op amp.

Should be easy to actively clamp the power rail to avoid start up transients. Sounds fun.

[Theashe]

I made a +/- 15V split rail power supply for the op amps, and I have a whole pile of NE5532s sitting around. The maximum gain you can get is about 10dB (I may lower that to 6dB), so I'd play it safe around 5 Vpp input.

I've been wanting to experiment with the sound of a DC coupled cathode follower, so in my simulations I've got:

1. Gain stage (12AX7 right now)
2. DC coupled cathode follower
3. Op-amp active EQ.

The simulation shows that the turn-on transient is only about 24V at the point where the op amps are connected in the circuit. However, it's just a simulation and I'm conservative, so I'd rather play it safe.

Very likely I'll attenuate the gain stage with negative feedback or switch to a lower mu triode like the 12AU7. Any idea if these effects will still work if I take either of those routes?

[TUBEDUDE]

You can adopt Tha tube driven active tone controls found in RDH4.

[JazzGuitarGimp]

If I'm not mistaken, Mesa Boogie's 5-Band graphic EQ is active. Though I am pretty sure they are using discrete transistors (as well as inductors) in their design.

[Reeltarded]

Great bass amp!

Boost is evil. TS loss is a game I love to lose!

[Theashe]

I have one of these for playing acoustic:
https://secure.schertler.com/en_US/shop ... rs/jam-100

It has similar active boost/cut tone controls. Absolutely devastating when cranked. I love it.

[JMFahey]

Hi all,

I might get my membership revoked for suggesting this, but does anyone have experience putting op amp-based active equalizers into tube amps?

Why woulod anybody?
Mesa Boogie has been doing so since the early 70`s.
Inductorsv are coils but the EQ stage itself is a discrete Op Amp equivalent.
And since signal peaks can´t go beyond SS rated voltage rails anyway, replacing them with gyrators won´t change the game at all.