Bass preamp - inductor driven midrange - help!

[JGR]

Hi all. I'm going to build an SVT style bass preamp. Ampeg commonly uses an active midrange control with a frequency selector switch that selects various LC combinations. I would like to use this design but simply do two dedicated midrange controls at different frequencies.

So how do I add the second midrange control - in series or parallel with the first? Or is there something else I am missing.

Thanks

JR

[f.leghorn]

LC tone controls

There is an excellent LC tonestack design by Steve Bench that will allow you to do exactly what you want to do, without having a custom inductor built as in the SVT.

Go to www.duncanamps.com and look in available downloads. There is a piece of free tonestack software there that includes not only the LC stack but also Fender, Vox, Marshall and some others. It allows you to model your own stack!

I am using the LC stack in my next guitar amp. In order to get two midrange boost & cut frequencies, all you need to do is play with the .022 uF (and possibly the 6H) midrange LC to get two boost & cut points, which you would then connect in parallel with an SPDT switch. I guess you could do it in series too if you prefer. For example, I am using 6H and either .022 or .068 uf cap to get 450 and 250 Hz boost/cut points respectively. I changed the 51K/5.1K resistors to 68k/6.8k in order to accentuate the boost/cut amplitude.

The inductors used are actually tiny audio PCB mount audio transformers, wired Pri. & Sec. in series, which
you can get from Mouser. Details on Steve Bench's webpage, or PM me and I'll dig up the part nos. for you.

[Ray Ivers]

JR,

I'm using a variant of the Bench TS in my own amp, along with inductor-implemented loudness compensation, harmonic enhancement, deep-bass boost, etc., so I guess you could say I'm a fan of inductors.

That being said - here's an alternative circuit I stole from the Marshall 9001 preamp and simplified, which uses no inductors:



This circuit offers far more flexibility that you probably need, so you can just 'prune' it as needed. NOTE: Neither the driving nor the feedback stage should be a 12AX7; a 12AU7 should work OK, although I would use a 12FV7 myself.

I had a 9001 on the bench a few weeks back, and I thought the EQ worked fine - it sounded to me like it's got a bandwidth of maybe 1/2 octave or so for each of the three bandpass controls (bass, lo-mid, and mid).

Ray

[Steve Conner]

That is neat Ray! But IMO it won't sound like an inductor- or gyrator- driven EQ because it only has half the slope. (Gotta love those off-axis poles.)

Looking at this schematic for the SVT preamp: http://www.freeinfosociety.com/elect...view.php?id=42

I'd duplicate R34, R38, R39, the pot, midrange inductor and everything else, and connect the new circuit in parallel with the old one at the top ends of R34, R38 and R39. But the two bands may interact.

I once designed an EQ with two sweepable midrange bands:

http://scopeboy.com/peq.html

It replaces the inductor with a bunch of op-amps in a gyrator type circuit. Unfortunately it's a bit noisy and I never quite figured out why.

[Ray Ivers]

Steve,

Yeah, the 9100 EQ doesn't sound like an inductor circuit, but I thought I'd throw it out just in case.

Here's another schematic theft, from the Ampeg V9 preamp:



This is easier to implement than the SVT circuit, although it doesn't sound quite the same (I've compared both side-by-side). The gain stage is the 12AU7 half of a 12DW7.

Ray

[JGR]

Thanks for the replies everyone!

[Enzo]

RAy, can you clarify the MID switch? Is the diagonal dotted line a wire or the mechanical connection between the two halves? The former makes sense, and the latter does not.

[Ray Ivers]

Enzo,

Is the diagonal dotted line a wire or the mechanical connection between the two halves?

It's a wire - it's on the genuine Ampeg V9 preamp print too.

Ray

[rocket]

Are the inductors really Micro (µ/u) Henry or rather milli (m) Henry?