2 ch. rack preamp

[Zozobra]

Redesigned the MOSFET output buffer and made a tagboard layout for it.

NB: I reduced the output coupling to 100n as it will be easier to fit on the board and the bass roll off is neglibile compared to 330n. I also have a big stock of 100n caps. Less so of 330n caps

[Zozobra]

Also a more pressing issue is what colour LEDs to use for each channel
I have all sorts of mad shit in my LED draw, which are helpfully unlabelled

EDIT: white for the D60 and orange for the Nitro

I only have 3mm orange LEDs so it's back to Maplin again for me on Monday...

[shep]

looks awesome. i'm going to have to get a few of those boards to play around with. especially after looking at what your doing.

great job

[Zozobra]

A hungover afternoon's work while checking the new carpenter brut album.

The tagboard has the MOSFET output buffer and regulated supply to drive the relays and LEDs.

[Zozobra]

After a brief foray into considerably higher voltages I'll be back on this tonight. All being well it should be making sound tonight

[Zozobra]

Thwarted by responsibility but I did manage to hook up the PSU board to the output buffer and switching sub-board

[Zozobra]

Tonestack and inter-board wiring.

[Zozobra]

Everything on the board is wired up. I just need to connect the input jack, B+/GND and connect the output to the send buffer and this is ready to fire up

[neilrocks25]

That looks cool, I hope you got the LED's before maplins goes bust.

[Zozobra]

I'm waiting for the closing down sale then I'm going to clean them out of magnet wire for making pickups and Telsa coils haha

[Zozobra]

First full power up report: It makes sound and is pretty much there. I need to add the treble shunt cap across the tonestack to kill some fizz and then debug the output buffer as it's lacking signal swing and is cutting too much bass. Nothing half an hour with a scope wont find. A job for saturday

[Zozobra]

Super quick and shitty clip of the first power up of my nitro pre into my speedtwin fx return. The mosfet output buffer isn't swinging enough signal as it and is cutting bass so the output is taken straight from the volume pot here, which isn't great tonally. I should get that ironed out over the weekend then I'll do some proper clips.

[video]https://youtu.be/ZILOf3_T_OU[/video]

[shep]

[facebookpost][/facebookpost]sounds pretty sick!!! what did you do with the output buffer?

[Zozobra]

Ta, it's not sounding quite right at the moment as the nitro is a hella bass heavy preamp.
The op buffer is the IRF820 circuit I posted above. I'm going to scope it to see where the signal is going and I'm gonna replace the output coupling cap with a large electro to decouple all the way down to 0 Hz. The low signal swing could just be a duff mosfet.

[trey85stang2]

thats freaking sweet, did you make the boards? Ive wanted to build a second slowclone and actually make the board but it seems like a pita.

[ajaxlepinski]

Awesome thread and that nitro pre sounds BRUTAL!!!

Downloaded the zip file with the PI Theory docs.
And I just ordered Merlin Blencowe's book on Guitar/Bass Preamps... hopefully, these two resources will help me understand what is going on here!


Did you add the treble shunt cap? If so, where did you put it? Between pin 1 (of the treble pot) and ground?

[Zozobra]

Trey
The boards are from here: http://valvewizard.co.uk/universalpcbs.html
They're reasonably straight forward to work with. I made a superbass-esque amp with them too.

I do want to make my own PCB for a more complicated build in the future. Check out EasyEDA if you want to make some boards. They have an online schematic and board editor and the PCB costs are ludicrously cheap. You can export the gerbers if you want to use another service too.


ajax
Good stuff. It's a great book and it contains (especially the 2nd edition) pretty much most of the stuff you'd ever want to know outside of the more esoteric high gainers. It takes a long time for it to sink in though Once you've read through the book and gotten to grips with at least what part of the book explains what, grab yourself a schematic of a relatively simple amp, say a JTM45 or a bassman, and then set yourself the task of knowing/understanding what each component is doing.

I mounted it on the PCB as it was easier, but it goes from the treble input (pin 3) to the mid pot ground (pin 1). It really does shelve a lot of the treble and makes the treble control usable, as otherwise it needs to be set low to cut enough and then it sounds a bit like a cocked wah. Some of the Engl preamps use the same trick but with a pot in series as the treble control; the usual FMV treble control is labelled high mid and uses a larger than usual cap value to lower its corner frequency.

[trey85stang2]

Zozo, thanks for the information. I will for sure check that out for my next build whenever I get around to it.

[Zozobra]

Had a day off and revisited this. Got everything working as it should and I think I've made poor choices in preamps for this rack as they both want different things from the poweramp to get their sound and a SLO poweramp isn't either of those things and that's basically what I'm using here. Back to the drawing board I go.

[ajaxlepinski]

ajax
Good stuff. It's a great book and it contains (especially the 2nd edition) pretty much most of the stuff you'd ever want to know outside of the more esoteric high gainers. It takes a long time for it to sink in though Once you've read through the book and gotten to grips with at least what part of the book explains what, grab yourself a schematic of a relatively simple amp, say a JTM45 or a bassman, and then set yourself the task of knowing/understanding what each component is doing.

I mounted it on the PCB as it was easier, but it goes from the treble input (pin 3) to the mid pot ground (pin 1). It really does shelve a lot of the treble and makes the treble control usable, as otherwise it needs to be set low to cut enough and then it sounds a bit like a cocked wah. Some of the Engl preamps use the same trick but with a pot in series as the treble control; the usual FMV treble control is labelled high mid and uses a larger than usual cap value to lower its corner frequency.

Amen! It does take a long time to sink in.
I understand the math behind arranging resistors, caps, diodes, to make a circuit but, I still don't get why groups of circuits are hooked up the way they are, with leads connecting them in the weirdest of ways.
... and ground layouts are beyond comprehension!
Thanks for the treble shunt explanation - I cut-and-pasted it into my notes.

[Zozobra]

I've decided to go back to how I was planning on doing this rack a few years back when I first bought the chassis.
That is basically a big old bus for the tubes to go on, similar to how it's done in the x88r. I can then use a turret board or terminal boards to wire the preamp on and have greater flexibility with my layout. Those little pcbs are great as a one shot, you know what you want deal. I need to tweak and they aren't ideal for that. I'll probably roll with a tube output buffer for this too as that will save some headache and I have heaps of power spare in the Px so fuck it

Heaters are elevated to fuck so there shouldn't by any annoying noise seepage from the heaters into the cathodes (hopefully).

[Zozobra]

I have enough tag board to do this

[Zozobra]

Chassis drilled and tag board mounted.
I have a love/hate thing with tag board; it's quicker than turret but mounting the filter caps is a pain, thus the parallel boards. The board nearest the front of the chassis will have the filters on and probably the tone stack. All connections to the tubes will be on the top board.


Simple single stage output send buffer. This is pretty much the same as an engl and gives an appropriate signal swing and output impedance in a very simple and elegant design.
I may use the spare stage to drive a pseudo depth/presence a la CAE 3+ if I need more tonal tweakage.


Above populated. I'll wire up it up when I've finished one of the preamp channels. I'm thinking bassy and dirty for channel 1 and tight and dry for 2.

[Zozobra]

Back on this tonight. I'm thinking of starting with a Mig100H preamp as I've always like the Migs and then tweaking it into taste. Kranks are based on these to put it mildly
I've gone with a second gain pot on the 2nd stage divider for subtle tones or for when I want to use pedals for dirt.

Main differences are:
no 10p cap from plate to grid at the input. This can be done better and safer if I need it.
3rd to 4th stage divider goes to ground and not 3rd stage cathode. This was a kludge to make it work on the same PCB as the other Migs. The bias shift and feedback should be pretty negligible.

I may play with smaller plate loads on stages 2 and 3 for more clarity.

[Zozobra]

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