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Rg100es preamp red channel

Started by Kizzlecake, April 13, 2014, 06:07:08 PM

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Kizzlecake

anyone have a circuitboard layout for one of these? I have a schemamtic made up on express PCb but havent been able to do the pcb layout yet. I was looking to make a stompbox version of this. And I need 100 posts to be able to sell in the classifieds for some randall stuff haha.

JHow

Do a search on "preamp from hell". I think a board layout was done.   I am fussing with a nine volt SMP version but its not ready .


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Kizzlecake

I found that and used it as a base to make the schematic but need a fike I guess to load into express pc to have the board cut

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Kizzlecake

and the nine volt thing was something i was gonna tackle later on.  need some more time.

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Roly

Quote from: Kizzlecake on April 14, 2014, 07:21:27 PM
and the nine volt thing was something i was gonna tackle later on.

Can I just observe from designing and building preamps that a nine volt supply rail is a bit too confined.  It generally works best with line level mixers and where the sources are already well compressed e.g. dubbing from disk/CD.  Live sources have a much larger dynamic range.

You can certainly get it to work but higher supply voltage(s) keep you well away from unintended clipping.  You might find that as some commercial mixers do, using two 9V batteries to give you either +18V monopole (for transistors and FET's) or +/-9V split-rail (for op-amps) supplies, will give you better overall results.

HTH
If you say theory and practice don't agree you haven't applied enough theory.

Kizzlecake

good info,  thanks.  Have you ever made a "voltage multiplier" (probably not the right name).  to step up from 9 to 24 volts? That would make it possible run a regular 9 volt adapter

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Roly

Yes, quite a bit actually.

with charge pump multiplier supply
http://www.fastpic.jp/images.php?file=2654398803.jpg

A standard 555 timer chip can also be made to work like this, however in all multiplier cases the 'no free lunch' rule operates, watts in = watts out + losses.  Whatever factor you multiply the output voltage by, you also multiply the input current, an 9V layer batteries are much less happy about this than. say, a stack of AA cells.

If you are going to run from an AC adapter then you may as well pick one with more voltage.
If you say theory and practice don't agree you haven't applied enough theory.

Roly

Just ran an LTSpice on the circuit posted below.

Using MPF102's, J201's, and 2N3819's the FET's won't bias properly on 9Vsup or even 18Vsup.  With Vsup at 18V I still had to increase the Source resistors R3 and R8 to 15k.


The two zener diodes seem to be redundant because they are already bridged by anti-parallel diodes - they can never conduct.

The circuit also seems to need some additional supply decoupling, and I would suggest a 100uF and 0.1uF (not shown) in parallel, on the preamp board near the +18V infeed.  The feed to the first two stages, J1-3, should be via a 1k series resistor with a 47uF or 100uF to ground on the input side.

attach:
rg100es preamp schematic - vulgar.png
RG100es preamp 140415 mods and bandpass.jpg
If you say theory and practice don't agree you haven't applied enough theory.

Kizzlecake

anything wrong with just running it at 24 volts per the original print?

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J M Fahey

No, in fact it is *recommended* to run it from original 24V.
Roly talked about 9V and 18V simply because that's what you can get straight from batteries, no conversion.

Kizzlecake

I'm not an expert at this stuff so now i ask,  can you build a board,  etc that will jump 9v to 24v so someone could use a standard 9v adapter

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phatt

Rather that trying to go up you may find it easier to go down. 8|

As most pedals run 9VDC it's understandable that folks look for fancy things to boost up voltage for one particular pedal but you may find it a lot more rewarding to build a 24VDC supply (to run the one off pedal) and then step it down to 9VDC to run the rest of those pedals.
With regulator chips this is quite simple to implement and can be built into one supply unit with 2 separate voltage supplies.

Use of 2 different connector types for different voltages guaranties no burnt out pedals.
I've found this idea to be far less hassle. :tu:
Phil.

Kizzlecake

very good point.  i wanted to keep the pedal as simple as possible.  only thing i was gonna add to it was the effects loop.  I live the red channel on those amps and would really like it in a pedal form.  pocket preamp so to speak lol

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Roly

The circuit I have doesn't show a supply voltage, but at 24V it does rather demonstrate the point I was making about having enough headroom.  I've just checked in LTSpice and it does now bias with Vsup = 24V, however the Drain voltages are only about 5-6V so I'd still be looking at bumping the Source resistors up a bit to aim for more like 12V on the Drains.

Even altering several resistor values you are still going to have an interesting time getting this to work on only 9V.

Yes you can build a simple charge pump voltage tripler or quadrupler around say a 555 timer running at a supersonic frequency so it is inaudible.

It's late here now so I'll model a DC-DC inverter like this tomorrow and get back to you.
If you say theory and practice don't agree you haven't applied enough theory.

Roly

9V-24V DC-DC inverter

Notes:

This is a charge-pump inverter consisting of a power oscillator running about 17kHz followed by a voltage tripler stack.  The design is optimised for cheapness and parts availability, not efficiency or voltage output.

C3 (10V or greater) is required because the circuit draws fairly large current impulses and C3 provides a local storage to reduce any possible noise injection to other equipment (pedals) running on the 9V supply.  It should be fitted as close as possible to the inverter.

The operating frequency is set by R1, C1 and R2.  The frequency may be changed by changing R1 and/or C1, smaller values giving faster oscillation.  R2 must be no more than one-third R1 or the circuit won't oscillate.  17kHz was selected to be well above the audio band, yet low enough to allow the use of common diodes for D1-6.

The NE555 timer must be the normal bipolar type, not the CMOS version.  Note that it is shown here from the underside, pin view, not the more normal top view.

The reset pin RST may be left open or tied to +9V Vcc.  If it is grounded the circuit won't oscillate.

The green trace is point "CR", the blue trace "out" is the output pin of the 555, and the red trace "Voutput" is the voltage developed at "Voutput", supply to the preamp.  The plot is for the first 5mS after startup, but note that capacitors charging in the preamp will cause the Voutput to rise much more slowly and may take a second or two to "warm up" and stabilise.

C5-9 should be 10V rating or higher.  If there is a choice then the physically largest should be used for best ripple current rating.  It may be worth using a higher voltage rating, e.g. 25V if they are physically larger.

D1-6 are any 1 amp power diodes from the 1N400x-family but some advantage may be had by using fast diodes such as the UF4000-series <http://www.datasheetarchive.com/dlmain/Datasheets-30/DSA-591698.pdf>.  Any of the types 1N4001-1N4007 or UF4001-UF4007 will suit.

"R3" is the effective load of the preamp, not an actual resistor.

Because the inverter contains high level square waves it should be kept as far away from low level audio signal paths as possible.

The circuit should produce 23.5V off load and 21.6V loaded by the preamp.

I would still increase the values of R3 and R8 in the preamp to obtain half the supply voltage at the respective FET Drains for best clipping headroom.
If you say theory and practice don't agree you haven't applied enough theory.