Albert Kreuzer Bass preamp

[gtrmac]

Well I finally finished one of these and I thought I'd post something about it here. I made mine on a PC board that I laid out with Protel 99SE. Everything worked out OK with the layout and I made some additions to the circuit. I put a balanced output on the unit using an Analog Devices SSM2142 IC. I really wanted to use the preamp in the studio as a DI Preamp so this seemed like a good idea and it worked out OK.

My criticisms of the design are as follows:

-Not enough clean headroom. You can get enough level out of the unit but the gain pot starts to clip the second stage at about 30% of rotation. For a bass preamp it would seem more useful to have more clean headroom before the input starts to clip. I'm probably going to try a different design for the gain stages and see if I can get something that can stay cleaner.

-Active EQ is noisy. Well I have to qualify this since I am using a switching PSU which I suspect is contributing to the overall noise level. I ordered a linear supply PCB from ESP and I'll try this and see what happens. The Active EQ is sort of a nuisance in the first place since the rest of the preamp can run off a single ended power supply but the EQ needs +/-15V. I think my next attempt may use a passive tonestack and single ended supply. I might also bump it up to 24volts and see if the headroom is better.

-Too many switches. I think I'll just put a bright switch on the next one.

Overall though it is a good project and I have to say thanks to Albert Kreuzer for his work. I haven't seen any other DIY Bass Preamps except for Elliot Sound Products Opamp based project and Albert's discrete JFET design is a lot more interesting to me. I got the JFET bug now anyway.

I'll update this if I make any new versions. If I get something I really like i could get some PCBs made if anyone's interested.

[DavidS]

Hey, have you checked out the Cafe Walter? http://www.cafewalter.com/cafewalter/rackpre/rackpre.htm

[gtrmac]

Hey, have you checked out the Cafe Walter? http://www.cafewalter.com/cafewalter/rackpre/rackpre.htm

No I hadn't seen that one, thanks

I dunno, I'm kinda stuck on this JFET track at the moment. Seems like everyone is doing something with them so I guess I'm into a fad.

It looks like a good project though. He's using OPA2134 amps so it probably sounds nice and clean.

[seanm]

Hey, have you checked out the Cafe Walter? http://www.cafewalter.com/cafewalter/rackpre/rackpre.htm

Now you show me

I have been working on a bass preamp based on the mu-amp/minibooster. It has no switches... no tone controls active or otherwise.... or any pots  Not generally useful. I want something for the practice room that is literally plug and play. I am having a lot of problems taming the gain! I posted it below just in case anybody is interested.

I am actually still using a FET for the output buffer in the "real" version. But if you do replace the bipolar with a FET, keep the 220k resistors. They are there to tame the gain or you *will* distort. Also, Rload is not part of the preamp, it is just there for simulation reasons.

[DavidS]

The Cafe Walter was no big secret, I found it and the Kreuzer by typing "bass preamp schematic" into google.

[RDV]

Hey, have you checked out the Cafe Walter? http://www.cafewalter.com/cafewalter/rackpre/rackpre.htm

Now you show me

I have been working on a bass preamp based on the mu-amp/minibooster. It has no switches... no tone controls active or otherwise.... or any pots  Not generally useful. I want something for the practice room that is literally plug and play. I am having a lot of problems taming the gain! I posted it below just in case anybody is interested.

I am actually still using a FET for the output buffer in the "real" version. But if you do replace the bipolar with a FET, keep the 220k resistors. They are there to tame the gain or you *will* distort. Also, Rload is not part of the preamp, it is just there for simulation reasons.

Try a 10k trimpot in the place of that 5.6k and adjust to taste, though I think you need a volume control at the end, maybe another trimpot(100k or so).

[seanm]

Try a 10k trimpot in the place of that 5.6k and adjust to taste, though I think you need a volume control at the end, maybe another trimpot(100k or so).

Thanks for the suggestions. And in "real" life I would probably make R2 a 10k pot.

The biggest problem is headroom. In another thread I mentioned that at 24V (3 x 8V batteries) the distortion goes away. At 30V this would probably sound really really good.

[Nikolay]

what about this article
http://home.comcast.net/~markfuksman/hamptone.jpg

[seanm]

what about this article
http://home.comcast.net/~markfuksman/hamptone.jpg

Very interesting! I breadboarded the JFP and got very little distortion on the E string with 24dB of gain @ 100Hz  And that was with only 12V. I bet at 24V it would be very clean. Looks promising.

I am a bit worried about the output impedance. I need it very low (preferably < 1k). I do not have the power amp here to test against.

[gtrmac]

Just an update to this topic.

I replaced the crappy switching PSU with a regulated linear supply that I built with a PCB from Elliot Souind Products and it completely changed the way the preamp was acting. It now has a lot of clean headroom and it's nice and quet. This was a lesson for me in how a good PSU is a necessity. Even though I was reading +/-15V on the rails I think the switching supply was not delivering enough current for the preamp.

Anyway, I'm working on a new version now with a different biasing scheme on the FETs and a passive Baxandall tone stack. I also stripped off the tuner output, the LED Clip indication and all the filter switches because I didn't use that stuff anyway.

The basic amp has a nice tone now that I can actually hear it. The FETs go into clipping very nicely and the distortion is soft and sounds like compression until you push it harder. I'll update when i get the new version finished.

[seanm]

gtrmac, good news. Keep us updated!

I am working on a Baxandall tone stack pedal. The current thinking is to keep the tone stack passive but I plan to add a +20dB gain recovery stage and an input buffer. I want to build it as a pedal to play around with the tone stack without affecting the preamp. When I am happy, I may design a new preamp that includes the tone stack, or I may leave it modular.

Any idea of the frequency centers you may use? My current thinking is 80Hz and 4kHz or maybe 6kHz but I have not breadboarded it yet.

Any thoughts on a mid control?