new preamp for 200w amp- tone stack question

[black_labb]

ive been using a holden/wasp 200w (4x6550) amp for bass, as ive started playing bass in the band im in, and want to make a new preamp for the amp, as the preamp has alot of hum in it, and i just like making things (and getting trannies for such an amp suitable for bass wouldnt be cheap).

im thinking that it would be kind of cool to use a preamp idea ive had runnign through my head. the concept is to have 2 or 3 volume controls feeding into rc filters. one for bass(lowpass), one for treble(highpass) and maybe one for mids (band pass). id also have pots to adjust the upper and lower bounds of the bandpass for the mids, or if i didnt have the mid adjustment i'd adjust the break frequency of the low pass and high pass to determine how much low-mid and high-mids are allowed to pass through. if i had the mid, id adjust the break frequency of the low and high pass that make up the bandpass.

i know that id have to use a lower value pot for the mid control if it is there, as the mids will be covered by the low and high pass filters to an extent as well.

my questions are

-would this preamp have alot of losses compared to most systems? shunting alot of the signal to ground in each of the filters would be alot of losses in my oppinion. would a LC filter arrangement solve this problem? im planning on using a ef86 instead of a 12ax7 at the input, would the extra gain obtainable from it make up for the extra signal loss? i assume the mid control would add aot of losses to the preamp.

- would it be best to put the volume controls before or after the filters? i think it would be best to go before, but im not quite sure.

- would the output of the rc filters cause the signal coming through the other filters to be drained? or do they only effect the signal passing through them? if they do, it might be suitable to add a 12ax7 in there, to amplify the separate signals in a triode each after going through the filter, before they go into the cathode follower. this could also aleviate the increased losses of this system.

- and now a simpler question, what are the bounderies of what one calls mids, where does it become highs, and where does it become lows? seems simple, but im not sure? is 350hz considered low mids, or would that be 200, or 700?

thanks for any help and comments

[Steve Conner]

Just fix the one you have now and get it working properly, before you start messing with it. You should be able to get rid of the hum without changing it from stock.

And don't judge it by its performance at bedroom levels either. I bet once you take your repaired amp to a jam or a gig, and let it rip with a loud drummer, you'll be happy. If you're not, then you can start modding it. But I warn you there is a lot of "mojo" in the traditional Fender/Marshall type preamps and tone stacks that you should understand before you start making your own. (Hint: what knob settings are needed to give a flat frequency response? And what knob settings do musicians normally use?)

[black_labb]

there is nothing wrong with it. there is actually less hum than i suspected from the amp, but im still toying with the idea.

i played the amp at some decent levels today through a homemade 1x15 that sounded good, maybe a bit brighter than nessisary, but that would be due to the bass (the bass is also homemade and i havent put the neck pup in there yet either, and its also missing the optional preamp to boost the underwound pickups)

it sounds good at the moment , but the motivationis for experementation. the current preamp is on a circuit board, so i would just remove the board and put my own in there. im just looking for a bit of guidance on predicting the downsides to the design

[Steve Conner]

OK, well the main downside I can see is that RC filters just aren't very good filters. If you go to all the trouble of fitting controls to change the break frequencies, you probably will struggle to hear the difference between the frequency controls and the level controls.

LC filters have slopes twice as steep (in layman's speak, they're twice as good at filtering stuff, got to love those off-axis poles!) so may give a more interesting effect. The Ampeg SVT used a LC filter for its midrange control, and I've built amps with parametric EQs that work well on bass. I used gyrators for mine to save the hassle of real inductors.

You ask about outputs causing other things to be "drained", I guess this would come under the heading of interaction. You can reduce interaction by driving your three filters off a cathode follower and summing their outputs with an anode follower.

Also, you asked about signal loss. It's hard to tell from your description, but it sounds like you want to put the three filters in parallel rather than in a chain. This approach would potentially have less losses than a classic tone stack. I think Sound City did something similar in the 70s, but can't remember, maybe another forum member can help out.

Finally, the tone circuit from the Big Muff stompbox is a simple 2-band version of what you described. A low-pass RC, a high-pass RC, and a pot to "pan" between them.

[black_labb]

thats a very helpful post. i didnt think much about the low gradient in the rc filter, glad you mentioned it. that would make quite an uninteresting tone stack.

the filters were going to be run in paralell as you understood.

part of the idea was to use this system as a volume control, i dont think i mentioned that above.


after going through that, the idea doesnt seem that great. the adjustable break frequencies was something i really liked, but not really being useful, as too little of a slope makes it a silly idea.

maybe ill run the 2 channels together (just a separate triode for the first gain stage, with very different values on the cathode caps and resistors and different coupling cap values) so that i can adjust the volume of each to get a bit more versatility in the sound.