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2 ch. rack preamp 
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For my next project I was going to convert a busted Bugera 1960 into something using a spare OT and a pair of KT88s. That project is on ice for the time being while I take a more considered approach to the layout as squeezing a high gainer in will be a bit of a dick. This leads me to plan B: restart a rack pre that I started building for a friend a few years ago. This was supposed to be the preamp test bed for a run of amps but we couldn't be dicked with it in the end. We both had full time jobs so it wasn't the time for this particular endeavour.

Anyway, my plan here is to build two high gain pres in a 2 U chassis that I had already started the basic machining on. I have a custom wound toroidal PT (from toroidy.pl - check them out, great custom winds and super low prices) which is conservatively spec'd to run 6 preamp tubes and a whole bunch of relays without breaking a sweat. B+ at the filter will be around 400VDC so I can build some very modern sounding preamps, which is the plan.

Here is the chassis.

Image Image

Preamp A is going to be loosely based on the VHT D60. The schem here doesn't show the less/more switch which bypasses the 3rd stage or the local NFB loop switch to change the mids/feel.

Image

Preamp B is more or less a Splawn Nitro but with a post tonestack MV. I'll likely add options to switch the 2nd stage cathode resistor from 1k5 to 3k to also to reduce the gain a bit by adding an 82k//1n in parallel to the 220k in the 2nd stage attenuation network.

Image


The above also shows an idea for a transparent high voltage MOSFET output buffer using an IRF820. This still needs a bit more work in LTSpice to get right. I'm not super confident designing with MOSFETs yet, but there is only one way to learn and if they they the magic smoke out then who cares? I may well go with a tube output buffer as below though.

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Down the line when I'm happy with the circuits I may tap off the 12VDC regulated supply for the LED and relays to power a small solid state poweramp board so I can run into a cab. A 1W class D board is tiny, although I may opt to stick with a more conventional AB MOSFET power amp board depending on what sounds best.


Fri Feb 16, 2018 2:39 am
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After some dicking about in LTSpice I think this output buffer should work pretty well. It can handle 30Vpp in (which is more than it'll ever see) and scale the output swing down to 10's of millivolts to up to 5Vpp.

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Fri Feb 16, 2018 6:25 am
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Neat!!

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Fri Feb 16, 2018 6:35 am
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Check out Merlin's cathode follower section in his preamp book. I've simmed them with an IRF820, with his circuit protection, and it's almost identical.

Also, the bootstrapped variation can be placed on a switch for a mid-thickening effect. Just tailor the capacitor to whatever floor frequency you want to boost.


Fri Feb 16, 2018 7:11 am
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jgreenwd wrote:
Check out Merlin's cathode follower section in his preamp book. I've simmed them with an IRF820, with his circuit protection, and it's almost identical.

Also, the bootstrapped variation can be placed on a switch for a mid-thickening effect. Just tailor the capacitor to whatever floor frequency you want to boost.


Yeah, I've simmed them to step in as cathode followers in circuits. Same additional sillicooties can make them behave more like the real thin too:
http://milas.spb.ru/~kmg/irf_en.html


Fri Feb 16, 2018 8:56 am
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That's bad to the bone Zozo. PM me how much you'd want for a 4 channel pre with tweed bassman, super lead, gh50 and bogner xtc. No channel switching.

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Fri Feb 16, 2018 9:10 am
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Zozobra wrote:
sillicooties
:lol: :rofl:


Fri Feb 16, 2018 9:46 am
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Ry Manchu wrote:
That's bad to the bone Zozo. PM me how much you'd want for a 4 channel pre with tweed bassman, super lead, gh50 and bogner xtc. No channel switching.


Thanks :) I can't help with that really I'm afraid. I couldn't commit to anything being built in a half way reasonable time frame and shipping to the states would be a fairly daunting prospect!

Have you ever thought of building a preamp yourself? They're a lot less complicated than full amps and those little dev PCBs I'm using are great and easy to work with. I'm happy to help with that if you do.


Sat Feb 17, 2018 3:16 am
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Well I will be having some spare time available coming up. I can screw, solder and test, but I can't understand a circuit diagram. Are you sure I can do it?

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Sat Feb 17, 2018 4:34 am
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Start small and simple. Maybe a 1 or 2 channel preamp. TBH, unless you're designing something yourself then everything you need is already out there in terms of layout and I can help you layout a circuit you're interested in.

Have a read of this to get your head around reading circuits:
http://ax84.com/p1/P1_Theory_Document.zip

This is a great resource too:
http://www.valvewizard.co.uk/


Last edited by Zozobra on Sat Feb 17, 2018 7:30 am, edited 1 time in total.



Sat Feb 17, 2018 5:02 am
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Thanks Zo. I'll check back in in half a week after I've better educated myself.

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Sat Feb 17, 2018 5:05 am
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Did some more metal work today.


Sat Feb 17, 2018 9:26 am
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test fitting passed - checking out different knobs. I may actually leave it with different ones for each channel :lol:


Sat Feb 17, 2018 11:03 am
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Power supply all wired up. Cables will be cable tied to adhesive bases once everything is in.
I'm getting 430VDC on the main filter under no load so I'm set for around 400 under load.

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Sun Feb 18, 2018 12:51 pm
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Zozobra wrote:
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test fitting passed - checking out different knobs. I may actually leave it with different ones for each channel :lol:

Damn man! Looks awesome!

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Mon Feb 19, 2018 7:42 am
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Looks mean as heck.

:fap:

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Mon Feb 19, 2018 11:01 pm
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Thanks guys. I think I'll probably leave the front unlabelled. I know what all the knobs and switches do anyway :lol:

I should have a few hours tonight to work on this. The initial test of the build will just be the 2nd channel and the output buffer. I'll be ordering the bits for the switching circuit and fresh pots for the 1st channel later.


Tue Feb 20, 2018 3:01 am
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There is part of me which is think that the big empty space in the top left could be filled with an ecc99 power amp to turn this into a full amp. I could also then also add a dummy load and run the whole thing into a big motherfucking solid state power amp for playing out :lol: :cop:


Tue Feb 20, 2018 8:55 am
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that looks awesome

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Tue Feb 20, 2018 9:08 am
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Thanks. I started stripping the components from one of the boards that I built a butterslax pre with. It's slow work :(


Wed Feb 21, 2018 2:12 am
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Nitro channel input board populated.

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Wed Feb 21, 2018 12:03 pm
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Zozobra wrote:
Nitro channel input board populated.

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Sick!!! This is so damn cool!

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Wed Feb 21, 2018 12:08 pm
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I wanna hear the one based on the D60 Zo!


Wed Feb 21, 2018 9:08 pm
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Thanks dudes. These boards are great and if anyone has ever wanted to try making a preamp then they do take a lot of the ballache out of it. That said they aren't too great to rework and stripping out one of the old boards if proving a bit of a nightmare, although that is largely because I need a new desoldering pump.

I will hopefully get the 2nd nitro board populated tonight. From there I need to design and populate the MOSFET output buffer and hook up all the wires. May be finished with all that by the weekend. Once that's done I'll start on the D60 boards ;)


Thu Feb 22, 2018 2:42 am
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2nd board populated. This has the 3rd and 4th stages and the EQ on. I didn't have any silver micas for the treble cap in the tonestack so I've used a ceramic for now, which might have a slightly harsher top end if you believe everything you read on the internet.

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Thu Feb 22, 2018 3:28 pm
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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 :lol:

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Fri Feb 23, 2018 4:01 am
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Also a more pressing issue is what colour LEDs to use for each channel :lol:
I have all sorts of mad shit in my LED draw, which are helpfully unlabelled :facepalm:

EDIT: white for the D60 and orange for the Nitro :lol:

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


Fri Feb 23, 2018 5:10 am
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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


Fri Feb 23, 2018 6:49 pm
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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.

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Sat Feb 24, 2018 8:42 am
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After a brief foray into considerably higher voltages I'll be back on this tonight. All being well it should be making sound tonight :rawk:


Thu Mar 01, 2018 2:44 am
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Thwarted by responsibility but I did manage to hook up the PSU board to the output buffer and switching sub-board :)

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Thu Mar 01, 2018 2:05 pm
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Tonestack and inter-board wiring.


Wed Mar 07, 2018 12:57 pm
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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 :D


Fri Mar 09, 2018 3:02 am
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That looks cool, I hope you got the LED's before maplins goes bust.

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Fri Mar 09, 2018 7:37 am
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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 :cop:


Fri Mar 09, 2018 8:14 am
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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 :)


Wed Mar 28, 2018 11:01 am
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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.

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Thu Mar 29, 2018 12:05 pm
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[facebookpost][/facebookpost]sounds pretty sick!!! what did you do with the output buffer?


Tue Apr 10, 2018 12:06 pm
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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.


Fri Apr 13, 2018 2:40 am
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