DIY Stompboxes => Building your own stompbox => Topic started by: craigmillard on January 17, 2013, 09:36:03 AM

Title: Supertex LR8 + SMPS
Post by: craigmillard on January 17, 2013, 09:36:03 AM
Hi All,

Anyone used a Supertex LR8 regulator on there amp/pedal builds?

Thinking it might be good on my SMPS's that i use for preamp builds as they are low current draw?

Think it would make much difference? I use a 12v secondary transformer that is then boosted using an smps, but alot of the crap from the transformer still gets through!

Banica has used one here to much success:

Thoughts guys? :)


ps if anyone has one/two they dont need ill be willing to take them off your hands! Mouser shipping is a lot to the UK for 2 parts!

Title: Re: Supertex LR8 + SMPS
Post by: defaced on January 17, 2013, 11:45:03 AM
Looks pretty interesting.  I have not used that exact part but have achieved similar goals with a few resistors, hand full of zeners and a largeish power MOSFET.  Q1 in this schematic: (
I think I used an IRF820 in that build, I'd have to open up the chassis to check. 

That said, the SMPS should regulate pretty well.  I say "should" because SMPSs are complicated little buggers that can get very temperamental for no obvious reason (stray properties be damned). 

Banicas' build uses the LR8 to drop voltage off the rectified/filtered transformer secondary, no SMPS needed, so his application is much different than yours. 
Title: Re: Supertex LR8 + SMPS
Post by: craigmillard on January 17, 2013, 12:03:24 PM
hey defaced! I loved that project!!

i use this layout and design for my SMPS

Its heavily based on Nick de Smiths version but instead of using a laptop power supply i thought id use a cheap 12v transformer, its just the noise from this leaks through to the HV line the other side of the SMPS  on the HV rail and makes for a very unhappy amp with lots of buzz and hum!
Tried a few resistor cap networks but no matter how much i put in its still bad.

Thinking a high voltage reg might sort it out once and for all.  :icon_biggrin:

How would you attach your way onto my schematic?
Title: Re: Supertex LR8 + SMPS
Post by: craigmillard on January 17, 2013, 12:30:23 PM
I take it everything to the right of c6 up until c2 in your schematic is for the voltage regulation.
You have sent me this before:)
D2, 5, 6, and 7 are zeners being used to set the gate voltage of the follower MOSFET (Q1, I'm using an IRF820 because I have it on hand).  They are each 100v zeners, and since the drain of the FET follows the gate, it provides a low impedance path for the 400v what's used to supply the preamp.  Or in other words, it's a brute force high voltage series voltage regulator.  This whole circuit was added in an attempt to kill any noise in the HV rail from the switcher.  In my very basic trials with the prototype SMPS it seemed to decrease noise, but I only tested it long enough to know the SMPS worked (made +400v) and could run a tube preamp.

Are D3 and 4 also 100v zeners?

Sounds good but never got round to trying it! might have to give it a go at some point soon, see if it helps!:)

Really want to use the traffo as it has 2 secondaries so can elevate the heaters! Just need to iron out the last bits...
Title: Re: Supertex LR8 + SMPS
Post by: defaced on January 17, 2013, 01:24:45 PM
Thanks for the compliments.  I wondered if that was you I sent that info to.  Yes, everything to the right of C6 and up until C2 is the regulator circuit.  If you print the schematic and compare it to Nick's, you'll notice it's verbatim his work.  I just used different parts to support the through hole design, adjusted a resistor to get higher voltage, and a larger inductor from your work. 

Sounds like you are having the same problem I was with that design.  In my work with the circuit, the addition of the regulator circuit did not appear to cure this little issue, but I am still waiting to really confirm that (I haven't touched that project since that competition, too many other irons in the fire).  I mainly added the regulator so that I could crank up the SMPS voltage (I was able to get 550v out of it no problem), which also raised the switching frequency, and use the regulator circuit to cut the hash off and leave me with steady DC.  In thinking about this problem since then, I think the ability for the inductor to source current is too great which has allowed lower (audio range) switching frequencies.  I plan to look into this possibility by evaluating inductors with lower inductance and/or higher series resistance.  I think I bought down to 10uH or 15uH for this project, and series resistance is pretty easy, I just need to get unlazy, buy some more 1771s and switching MOSFETs, and get to it.  This approach, if it's successful, would eliminate the need for the regulator circuit entirely.  

D2, 5, 6 and 7 are 100v zeners.  D3 and 4 are 12v zeners for gate protection.  C9 and C5 are the same type of low ESR filter cap.
In theory, you can also use a voltage divider to set the voltage at the gate, but I suspect that any noise on the rail will also end up present on the gate unless you do adequate filtering in parallel with the gate/ground resistor (it would take the place of the series string of 100v zeners).  

Do you have a scope?
Title: Re: Supertex LR8 + SMPS
Post by: bancika on January 18, 2013, 04:33:04 AM
* It's bancika...banica in serbian means