Baxandall EQ questions

[rock soderstrom]

I want to create a tube pre-amp with a baxandall EQ. I've searched bits and pieces and came up with this design.

You can use this proven design for inspiration - an Ampeg B15n Portaflex bass amp.

If you want to feed an existing high-impedance instrument amp, the preamp is sufficient, if you want to play into a console or interface (or even lower impedances), you need a driver output, possibly a tube cathode follower with or without output transformer. Solid state is of course also possible.


Edit: Ampeg, not Apogee

 

[PermO]

How about an Alembic copy ?

1M input impedance, 3 band EQ ... add Edcor for balanced out.

 

[abbey road d enfer]

I looks like it's a passive instrument with some form of pickups to me;

Looks (and sounds) very close to Hohner Pianet. Low level output, so yes you need the gain of the input stage. And probably another gain stage if you want"grit".

 

[BGroen]

Thanks all!! This is great information. I can carry on with my project with confidence. When I'm finished I will share

It has indeed an internal little pre-amp already, powered by a 9V battery. It's a single transistor stage with output transformer.
I was planning to keep that the way it is. But now you mention it, I will experiment with higher gain and removing this stage.

The schematic of the Apogee B15n Portaflex bass amp looks very similar to my plans. I will definitely use it as a reference (or copy it completely). Thanks!

I'm going to build it from a Hammond percussion amp, so I would like to keep it simple so that everything fits in there.
I am also a great fan of single knob things. So I'm going to combine the bass and treble control in one stepped rotary switch + a by-pass switch. So although the 3 band would be more flexible, I'm not going to use it. Thanks for the idea anyway!

 

[rock soderstrom]

The schematic of the Apogee B15n Portaflex bass amp looks very similar to my plans. I will definitely use it as a reference (or copy it completely). Thanks!

The manufacturer is of course Ampeg and not Apogee. I have corrected my original (stupid) mistake. Sorry!

 

[abbey road d enfer]

I will experiment with higher gain and removing this stage.

You may be disappointed because the pickups are low impedance and thus the solid-state preamp is probably best in terms of noise than a tube stage that is definitely better for high-impedance sources.

 

[fobi]

You may be disappointed because the pickups are low impedance and thus the solid-state preamp is probably best in terms of noise than a tube stage that is definitely better for high-impedance sources.

Yes.
But there is a trick - the pot installed on the guitar.
If is linear, the amp must have relatively low input impedance. Around 1/4 to 1/10 of the pot impedance will convert the linear pot to logarithmic.


If the pot is logarithmic, the amp must have high input impedance.

Here is an example:

 

[abbey road d enfer]

Yes.
But there is a trick - the pot installed on the guitar.
If is linear, the amp must have relatively low input impedance.

1Megohm input impedance has a very limited effect on the actual taper of a typical 250 or 500k volume pot..

If the pot is logarithmic, the amp must have high input impedance.

Which is the case here.