[xfmr]

I have been working on this new design for a unit that has an RCA BC console style preamp using two 12AY7/6072 preamp tubes along with an EQ circuit based on the Baxandall layout using a 12AX7 and 12AU7. I am not so much looking for a neutral EQ, rather I want a heavy handed type similar to those from mono PA mixers from the 60's. This circuit started as the EQ and line amp from a Bogen mixer as a matter of fact. The output of the preamp is wired directly to the EQ input via the switches on the 1/4" jacks, but the two circuits can be separated of course. I have built this preamp circuit before, have one of these Bogen mixers already, and I have used the two together. So I have a good idea of how the final product will sound. This week I will be building a pair of integrated units with the modified circuits into some Akai / Roberts monoblock chassis. Anyway, I am just looking for opinions on what I've got going on here before I put them together.

[ruffrecords]

Just a couple of points to ponder.

1. The passive Baxandall EQ really needs to be driven from a relatively low impedance source so I would recommend changing the 12AX7 that drives it to a cathode follower. There's enough gain make up in the other half of the 12AX7 so you don't need the gain.

2. For a preamp you don't really need to go to the expense of an inductor in the power supply. With semiconductor rectifiers you can simply you three sections of RC filtering to reduce the ripple to extremely small values. I use 470uF caps they make for switched mode power supplies. Rated at 400V and made in quantity so not that expensive.

Cheers

Ian

[alexc]

Hi Xfmr

Nice design ideas there, along with Ruffrecords comments regarding driving impedances.
I'd be fine with choke psu  personally, given that they are not so expensive.

Makings of a very nice preamp + eq there.

I'm a fan of tubes with baxandall eqs  

And I really love the Bogen products. Including the aesthetics!

The Akai Roberts stuff is for sure a worthwhile place to spend some time and money.

Like to see the results of your efforts some time.

Cheers and happy diy'ing

[pstamler]

Just one nitpick of nomenclature: This isn't a Baxandall EQ. His design used feedback around an inverting stage, while this is a strictly passive EQ plus a makeup gain stage. That's not to say this isn't a good design, just that it's not a Baxandall.

Peace,
Paul

[pyjaman]

There's no cap in the feedback loop of the output stage ?

Laurent.

[ruffrecords]

There's no cap in the feedback loop of the output stage ?

Laurent.

In itself, that any not be an issue. If the output plate is at say 80 volts there's less than half a mA through the 180k feedback resistor which is just added to the cathode current flowing through the 2.7k. It does mean the NFB works right down to DC and he loop will unconditionally stable at low frequencies.

Cheers

Ian

[xfmr]

The choke in the power supply is already there when I use the Akai Roberts monoblock as a donor, so its not costing me a thing extra.

The feedback loop layout in the line amp was one fo the things I hoped you guys would bring up. When I first saw it in the schematic before I had one fo the actual Bogen mixers I thought it might be a mistake as well, but it turns out that it is correct. Now, thanks to you guys I know why it is set up like that.

As for the nomenclature, 'Baxandall' for this type of layout is just the generic term that I've seen on guitar building sites. Maybe 'PA Mixer EQ' is more appropriate because I like the simplified versions of this style for its exaggerations.

[PRR]

> As for the nomenclature

Apparently a guy named James wrote a good summary of tone networks including this one. "James" is an appropriate name for this type.

[pyjaman]

> As for the nomenclature

Apparently a guy named James wrote a good summary of tone networks including this one. "James" is an appropriate name for this type.

 Yep, this circuit is also  named "james" in the Duncan Amps' tonestack simulator (in wich you could find it fun to play with caps values for your purpose - I did, and had the idea to make a switchable freq "james" eq. somedays)

Laurent.

[xfmr]

I've always liked the 'James' in guitar amps, so when I saw it again in this Bogen I thought it might be a fun thing to tweak out into a color EQ for recording.

[xfmr]

I've built a few of these now and they are freaking awesome. I need to change the schematic because I made a few tweaks including a 600:600 output transformer for the preamp stage and the "beef" setting is now a DPDT that switches in a cathode bypass cap on the first and second 12AX7 triodes in the EQ/line amp section, the second effectively erasing the feedback loop. The result is more profound and very cool sounding: a completely different animal in each mode.

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