DIY
Factory
Green
Mic-pre Project
A project brought to you by PeteC.
Updated 22 December
2004
The is a complete reprint with very few changes of the original Group DIY Green-Pre as it appeared at Recording .Org.
yeah ... it's green.
General Introduction
Just for the record the PCB is at Peter's home site and so to the circuit but here is a PDF of the same thing Mozart
As I said, I made a prototype (can I call it a prototype? Pete's already done
all the hard work).
In order to do more than just a few test it was time to put this into a box
and get it into a couple of real life situations and recording sessions. This
is coinciding with the designating or a couple of general purpose power supplies
for Group DIY so I decided I'd use one of ours.
Note: Pete does have a power supply for this project at his site.
Since I am going to make a PCB for the power supply so it was decided to make
two new Green PCB's as this will be a Dual Green-Pre. Even though there may
be some things I might do differently to Pete in the layout and connections,
I saw no particular reason to change anything.
This is my prototype of Pete's Green-Pre
As you can see above the PCB is widely spaced and has mounting holes. Under
the PCB the tracks are wide enough for the DIY'er to get a good etch result
at home. The 3 pin XLs I use can be mounted on the inside of the box so you
could solder everything on the bench and get the whole thing operational and
clean and spray the underside of the PCB before mounting it all in the box.
I generally spray a lacquer on the track-side of the PCB after thoroughly cleaning
and polishing the copper and solder joints. I use paper towel and an acrylic
thinners to moisten the towel. It make a bit of a mess as the paper disintegrates
but the results are sensational. Don't touch the underside with your sweaty
oily fingers and then once it is lacquered it will stay shiny and tarnish and
oxide free.
After a lazy afternoon
shopping at JAYCAR.
Sure the PCB's didn't come from JAYCAR and neither did the 5532's but the rest
of what you see did.
Power Supply
The power supply for this project was a modified version of the KDMP as it was my feeling that the shape of the original board wasn't going to suit the box layout for two Green boards so a rethink was in order. Before leaping in a quick look at requirements might be sensible. I tested the Green PCBs at idle and found that the current draw on the positive rail was 18.4mA and the negative rail was 18.1mA. I do plan to do a max current draw once I have it all working. This will mean running the outputs into a 600ohm load with a signal near to clipping and perhaps near max gain at the input. perhaps later. So in principal the above supply will be fine and the same design ideas will stay.
The above power
supply is form the Group DIY KDMP.
I think I'll use a very similar supply but will shuffle the components about
and
make a better shape to suit the two Green PCBs in my usual 200mm rack box.
The main balanced power supply is constructed with the usual suspects, 7815
(pos 15 volt regulator) and 7915 (neg 15 volt regulator). These are fed from
rectified AC via a PCB mount 6VA power transformer. I have use four diodes in
a bridge rectifier configuration rather than a dedicated bridge rectifier as
people have had trouble finding P04 and W04 rectifiers and it allows you to
get esoteric with low noise switching diodes. I used the simple and easy to
get IN4004 but some at Tech Talk
have suggested more exotic devices.
note:- Tech Talk was a popular forum at recording.org
but alas things change and it is now defunct.
A google search will probably find most of the people that did frequent Tech
Talk.
Link to a CM2000
file of the above to come.
not quite right but here is a balanced supply to simulate.
CM2000 schematic (right
click - save as)
Run the Analog Simulation and get to know the circuit.
Bridge rectifier followed by the main caps of 2200uF at 25 to 35 volts. This
smoothed DC is fed to the fixed voltage regulators. I use 10uF 25 to 35 volt
Tants either side of the regulators to help keep thing a little more stable.
There are some variable regulators that may offer less noise and in the future
we may look into this and a couple of other fancy bits but for this project
I think this simple approach will do just fine.
The phantom section is borrowed from the Gyraf G9 Tube Mic-pre. It takes 15 V AC from one of the secondaries and then applies the trick of voltage multiplying. Can be difficult to explain and does ' mess with your head ' somewhat but really it is quite simple. At first thought you might think that by using a single diode to charge a cap and then successively stack multiple caps until the desired voltage is reached you would have your voltage multiplier. Trust me , that doesn't work. Difficult to explain why , but it is to do with voltage potential and relative ground. The way we have achieved the voltage multiply here is to stack the diodes and use the stacked Voltages to charge the last cap.
CM2000 schematic
(right click - save as)
Run the Analog Simulation and get to know the circuit.
Using Circuit Maker to help explain things we have the circuit layout above with the addition of a Signal Generator of 21 V at 60Hz. Below you can see a series of sine waves which depict 6 points on the above circuit. My web graphics are not that good but I hope this does shed some light on this little piece of magic. The first sine wave swings +21V to -21V and represents the output of a 15V (RMS) AC power transformer. The second is very hard to see, it starts the same as the first but narrows as the first cap begins to charge and pegs out at about 21V.
You may have noticed that I have labeled the circuit diagram with voltages at the junction of each diode cap pair. These voltages of 20V,40V,60V,80V and 100V are actual measurements of my prototype. Sure they have been rounded off slightly to make it easier to explain. These voltages can vary a little and they don't agree with the graph. You get that ! - welcome to simulators. Perhaps Circuit Maker can be tweaked to tell the story a little closer to the real thing but for our purposes it's not worth the time. The second thing to be weary of is the small 6VA PCB mount power transformer. Like plug packs these very small transformers will vary at the output quite a lot depending on how much load they are under. The voltage multiplier will accent this variation. The first time I tested my prototype it was unloaded and the voltage at the last cap was 107volts. The caps are being used at near or above there operation voltage and one was not in good condition and had to be replaced. Once loaded the volts came down a little and things were better. The phantom section will only be required to pass low current but beware these high voltages may still cause 63V and 100V caps some distress.
Please take note of the above.
Some of these caps will be near there voltage limit.
However I hope that things will be OK as the current is quite low through the
phantom circuit.
I mentioned earlier than I put the power supply under load and
the transformer voltage came down. I should point out here that is was not the
phantom circuit that was loaded but the +15V and -15V circuits that were put
under load. I used 100 ohm resistors for the job and they did get hot very quickly
so make your measurements fast and set the phantom volts to around 40V to start
with. We can up the volts later once the project is mounted in a box.
Link to power supply details including CM files and PDFs to come.
Check Plot for the above as a PDF (right click - save as)
Bottom Layer for the above as PDF (right click - save as)
Assembly of the
pre-amp cards
Although all the details you need for the Pre-amp itself can be found at Pete's
site:
Pete's home site
.... we also have a few details for you here too.
CM2000 schematic
for the Green-Pre. (right click - save as)
Run the Analog Simulation and get to know the circuit.
Then compare that to measurements of the real thing.
The above is what I call a Check Plot , as it is the term used by both, Protel and Circuit Maker. It is a parts overlay on top of a track layout viewed from above. Very useful for identifying the placement and orientation of parts. I usually start with the resistors and the other components that lay flat on the board, diodes, ferrites and perhaps the low flat trim pots. (However, I do tend to use the upright 25 turn trim pots).
A short note about
matching
I suggest you check ALL resistors with a Multimeter before soldering them into the PCB. Start with the 6.8k phantom resistors. Try to find pairs that very closely match each other. Don't worry so much that they are exactly 6.8k but group what you have into pairs. For example out of a pack of 8 resistors you might find 3 at 6.7k and 3 at 6.8k and 2 at 6.9. Now this may seem strange but I suggest you use the 6.9k pair for the first pre-amp and 2 of the 6.7k resistor for the second pre-amp. Keep the single 6.7 and the 3 6.8k resistor for another pre-amp project.
I am about to make another strange suggestion. Since we are going to check
the values of ALL other resistors it won't take much time to group the other
values into pairs. But why Kev ! Bare with me and I'll try to explain. As this
is a Dual Mic-pre it is likely that one day you will want to use it in a stereo
configuration so it might help if both sides behave as close as possible to
one another. Choosing matching pairs to fit to the left and right gain switches
makes obvious sense but how to make best use of the 10k resistors associated
with the unity gain op-amps at the output is not so clear. I'm sure I will get
some dissenters among you. If you feel you have enough knowledge to make your
own matching choices or you feel you want to match more than two pre-amps then
go for it and make your own choices. For those that find all this a bit beyond
their knowledge here is my suggestion. Using the last op-amp as an example,
you need four 10k resistors to set both left and right to unity gain. Say you
have two 9.8k and two 9.9k , I suggest you use both 9.8k resistors in the R12
position on left and right and the 9.9k resistors in the R21 position on left
and right.
Oh I can hear all the experts now ... " BUT Kev, that's not unity gain
! "
Correct, it is not unity gain and if we used both 9.8k or the left and both
9.9k on the right the two op-amps would be set to unity gain. As I said before
if you have the knowledge and skills to make your own choices then go for it.
So why have I suggested the above. This is a blanket suggest for all components
on the two boards. My thinking is that a sure fire way to get both boards behaving
the same way is to attempt to mirror one to the other. By using the paired 9.8k
in R12 then what ever error you have will be duplicated on both boards. This
method will make best sense when it comes to the Stepped Gain Switch where you
should make a special effort to pair up the resistors and both sides of the
pre-amp will have a good chance of tracking together the various settings.
I have chosen not to use connectors on these boards and this will keep the cost down and perhaps makes things a little more reliable. Select a colour code for you wiring ... or just copy mine. Once you have all the components on the boards, solder your hook up wires directly to the PCBs and clean the under side of the boards. Make the hook up wire longer than you need so you can trim them to length once you have things mounted inside the box. This is now a good opportunity to test the boards and check the operation of the CMR trim pot. Individually connect the pre-amps to the power supply but leave the phantom disconnected for now and check for +/-15 volts on each op-amp. This is a good start. Now apply a balanced signal to the input via a dynamic mic or a signal generator. I won't get into a lengthy 'first time fire up' discussion here now, perhaps Group DIY can look at this in the near future. Once you have confirmed a clean signal flow through the circuit you can probably relax a bit and take the time to feel pleased with yourself. Now apply a common signal to both pos and neg input and trim the CMR pot for the minimum ... that is ... zero signal on the output. Now do the same for the second pre-amp. If you have arrived at this point without letting the smoke out of any of the components .. then you have done well. :-) congratulations. It is now time to clean the underside of both boards and the power supply. As clean and shiny as possible and without putting your greasy fingers on the tracks spry the undersides with a PCB lacquer.
> Pete suggested that it might be nice to include his method of setting
the CMRR and so here it is :
To trim the CMRR, find two .1% matched 100R resistors. Solder one end of each
together to make a "V". Attach the free ends to the + & - inputs.
Set the gain to the mid point & set your signal gen to 100Hz. Put this signal
into the circuit at the join of the 2 100R resistors. Trim R10b until the signal
is as low as possible at either of the outputs as seen on a scope.
Parts Lists
Group DIY Dual Green Pre parts list
This is a list of parts for just the pre-amp stages, JAYCAR parts numbers and
Australian dollars.
Although I have made this as close as possible to Pete's notes, I did make a couple of parts changes. C15 was increased to 100uf which happens to be the same as the two input caps. Why - I felt like it. I though 10uf might be a little low and the hole spacing suited the cap I had in my hand at the time. Typical here might have been 47uf - refer to John Klett's thoughts on phantom on Group DIY's Power Supplies page. I also substituted two 1ohm resistors for the 2 ohm resistor at R32 on the Gain switch. I just couldn't find a 2 ohm resistor ?
Power Supply parts list
be wary of the caps upstream of the voltage multiplier as the voltage does get
up around 100 volts depending on the power trafo and the load you have at the
time. 63 volt caps may expire even with very little current so if you can find
100 volt , do use them.
Mounting it all in a rack box
I have a method and a style for all my rack boxes so I find this very easy. I'm sure in time you will develop a method that satisfies you. Until then take things slowly and think before the drill or cut. I take the base of the box and lay it out with the PCBs in their approximate positions and then place the extra hardware like switches and XL connectors in their relative positions.
_
Once you have decided on the basic position of the PCBs then mark the base with a pencil. I usually use a rule to check and round off a few measurements and[perhaps draw a few squared lines and double check it all again. Once I have the four corners of the PCBs aligned the way I want I'll then use a Pop Punch to mark the base when I then use a vertical drill to drill the M3 holes for the PCB Stand Offs The Pop Punch helps to keep the drill centered as you begin to drill the holes.
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M3 Stand Offs and shake proof washes and screws are available and the countersink screws are available from Farnell. In fact all the mounting hardware is available from Farnell and perhaps it is better quality. So if you plan to make a purchase buy more than you need and you will be covered for many of the next DIY projects to come.
_
All that is needed to do now is to drill a few holes in the front panel. I have obtained some anodized front panels in some basic colours and obviously the green was always going to be for this project. I use masking tape on the front panel to protect the anodizing while working on it and it also provides a good surface to draw out your options and mark down some basic dimensions etc.
A very fuzzy picture of the completed unit. I'll try to do better and replace this one. You can see I'm still experimenting with the Brother Label Maker with Black on Clear or White on Clear label strip. I am leaning to the White on Clear as it should be easier to read on the Green background in a dark studio environment.
_
A closer view of the back of the Lorlin switch. Lorlin have these switches with solder lugs but I choose to use the PCB mount switches for all jobs. It keeps things a little smaller and I can stock just one type of switch. However, it does require a little more effort and perhaps skill when soldering two resitor legs to one short pin but It works for me. It will definately aid in the development of your soldering skills.
Comments
This project as probably as simple as a complete rack project with AC power
supply is going to get and so I think this should be high on any beginners list.
The bulk of this project was done on the kitchen table at home. The PCB's can
be etched in a small tank in the sink even the Aluminium box could have been
drilled with a battery drill in the kitchen even though I chose to do it in
the garage. The squared hole at the rear for the IEC power inlet can be be made
using a Nibbler ( I have a punch for this job ).
Sound-wise the Green is quiet, clean and neutral. A good all round workhorse. Many of the budget mic-pres and the ones found in budget Channel Strips may have the SSM2017 chip as the basis of the mic-pre section. This includes some of the Digital interfaces like the Digidesign 001 which has a surface mount version. Others units from the Joe Meek VC1Q through to complete desks like the Euphonics CS2000 and some SSL's ( I think) have used the SSM2016 SSM2017 chips. With this in mind I think that the Green-Pre is an excellent choice as I can't see the advantage in making another SSM2017 box when it is likely you already have access to that sound.
If you want to make a simple, cheap Mic-pre, then this one is it. Even if you work in a top flight recording studio this unit will not be out of place and is likely to find a job to do. As a battery driven unit the Video boys may find this one useful for folly, effects and dialog recording.
For those that still want to know more about the SSM2017 style of doing things
then check out:
Kev's Dual Mic-pre. KDMP
perhaps the KDMP will move to Kev's home site
Downloads
(right click - save as)
Mozart PDF
sim circuit for the power supply
sim circuit for the Motzart
Dual Green Pre parts list
Power Supply parts list
Check Plot for the power supply
Bottom Layer for the power supply
perhaps we can add the check plot and bottom layer for the Green-Pre when there is time or if there is a request or two.
ERRATA
You probably won't see anymore development on this one, as the Green MkII is now the current project.
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Contact :
Feel free to email me if you think you have found a mistake or
if you need some of the files I have mentioned above.
brought to you by Pete and the DIY Team at :
www.diyfactory.com