"Fender Blackface" FET preamp - anyone done this?

[threefish]

Hi everyone,

I'm looking for a new preamp for my lm1875-powered guitar combo - I'm after a fairly clean sound that I can put my stompboxes in front of. I've considered the Thomas Vox preamp and limiter that has appeared in discussion on this forum here (and where I first saw it diystompboxes) , but stumbled across this preamp circuit today : http://www.redcircuits.com/Page120.htm.
Anyone done it? Know anything about it? Thoughts? Impressions?

Thanks

[teemuk]

Haven't build it but it seems to include SRPP gain stages that have quite round clipping characters when compared to single-jfet gain circuits. The tonestack is of course the heart of the circuit so I think the "quality" of this circuit relies to fact whether it has a suitable frequency response or not. I have been pretty dissapointed to tonestack circuits on that page (Red circuits) so far... The circuit is ok (straight copy from Fender design) but IMO you'd better simulate the frequency response (for example with this: http://www.duncanamps.com/tsc/) and figure out whether you need to alter component values.

[turbolx5oh]

Can't help you but I've got an 1875 project that needs to be put together as well - after my 3875 project of course.  Post up what you find or use.  I'm interested!

[Stompin_Tom]

Well, the "blackface" looks quite a bit like ROG's tonemender (http://www.runoffgroove.com/tonemender.html) but, ironically, they use fets instead of the tlc2272 chip. You could just stick the tonemender tonestack in the blackface and have more options (marshall and fender) with the added 'bonus' of fet tube tones. Does it sound better then the tonemender? Seems like the blackface would have lots more headroom and output since it runs at 18 volts or more.

I'd be interested to know what impediance and input signal level the chip power amp wants since redcircuits actually tells you that info about their circuit...

[turbolx5oh]

I wonder if there is a plus/minus power supply version of either of these two?

[threefish]

Thanks to you all for your input.
Yes Teemuk and Tom, good points about the tonestack. I'll be experimenting with thos options you've suggested. 
I'll be interested to hear the headroom offered by the 20v I have to work with - the power supply for my LM1875 poweramp is +/-20v, so I'll be running it off of the + supply rail. I know that running it off one supply rail may unbalance things a little, but I'm anticipating a really low current draw....which brings me to a +/- supply for this. My electronics knowledge doesn't extend as far as to design this conversion, but I know it would be fairly straight forward. If anyone wants to give me some clues how to do this I'd love to hear from you. I could then run it as +/- and not just +, and be able to use it all the way up to the 30v swing it appears to be capable of, not just 20v.

[Stompin_Tom]

Well, I definately don't know enough to convert a circuit to +/- rails, but there might be some hints in the six band eq project over at general guitar gadgets (http://www.generalguitargadgets.com/index.php?option=content&task=view&id=216&Itemid=241). They say you can run it off either, but I don't exactly know how they're converting it. I agree, though, it does seem like it would be fairly straight forward.

I don't remember what the max volt for the tlc2272 (could look at data sheet again) in the tonemender, but I'm pretty sure it can't take too much (10v or so?). I suppose you could sub something else in there...

[threefish]

I haven't really researched it, but I've got a few hints that converting a +v discrete circuit to +/- isn't really that straight forward as doing it with a chip circuit. I think I'll stick to my +22.5v.
Getting the parts for the circuit today. Hope to assemble next week or so. I'll post results.

[didier]

yo

i made a pcb layout for this ss Blackface... it was done in a hurry so some thing are not ideal. (eg. pot connections...)
sometimes i feel like this software is kind of slow, but you gotta love it though.

also, this isn't verified... (also not planning to verify in the near future.)

http://www.gigafiles.co.uk/files/1414/brol/Blackface.rar

[elberto]

anybody build this one yet?

I'm ordering the parts now for a LM1875 amp with this as the preamp.  I'll probably change the circuit somewhat--a one knob tonestack and I think I'll add the buffer further down the page.

[Robin755]

How funny, I'm actually building the same preamp with the same LM chip,

I've only tested it on an oscilloscoop and it seems to work,
just waiting for my LM chip that i've ordered, then I can fit it in my case,
I cant wait until I hear the sound coming out of it,

Has anyone build it by now?

I'm also curious what I have to do, to supply it with a +/- 20V ?

[Robin755]

Finished, just needs a lick of paint, it really sounds great with my tele,

[armstrom]

Did you end up running the circuit as-is from the positive rail of your power supply? Or were you able to convert it to use a bipolar supply?

Chassis looks great BTW. Kudos.
-Matt

[Robin755]

Just feeding it with a positive voltage, I don't know how to let it work on bipolair supply

[armstrom]

Great. I may give this circuit a try once my LM3886 kit from chipamps.com arrives. Do you find the tone stack adequate? The values are pretty much identical to the "default" component values listed for fender in Tone Stack Calculator.

Regarding running this off just one side of your bipolar power supply... I'm assuming there are no problems with this approach? I can't imagine what would happen, but I'm no expert!

-Matt