IC micpre schematic check

Hi guys,

I've drawn a mic preamp based on the SSM2019, and OP275 (based on their application notes). I wanted to run it by all the gurus here before I build it however. Pls let me know if you see anything wrong, or something that could be improved:

http://www.musiciansgig.com/micpre/schematic.pdf

Many thanks as always!! :thumb:

Martin

hi martin,

if you are going to alter your gain setting RG (pin 1/8) with a pot (10k log-), don't forget to put a resistor (10R) in series with your pot. This will limit this first stage to 60 dB (AV=1000). Though AVmax for ssm2019 is 3500, the bandwidth will decrease rapidly above 60dB.
At your output section I would exchange R17+R18 with 4k7 resistors and use a 1k trimmer for P1 if required anyway. I don't think, imballance will be that large and this way may be more easy to catch.
Have a look at dc offset, may need compensation/servo/coupling-caps.

All the best
Harpo

What is the intended function of the pot P1?

In addition to what was mentioned allready:
* I'd set R5 to 470 ohm, as this will greatly increase ripple suppression
* add a large resistor (~100k) in parallel with C3 in order to provide discharge of this capacitor if no microphone is connected and 48 V inadvertently switched on
* for foolproof phantom powering add a 4.9 ohm resistors in series with C1 and C2 each and use 5 W zeners or 1N4004 diodes to the supply rails (this has just recently been discussed here in at least two threads)
* RFI protection is somwhat mediocre--I'd go with a pair of 1 nF capacitors (preferably 1%) to chassis

I'd skip the discrete output stage and use a suitable line driver, as this will be much easier to implement and provides about the same (or likely even better) performance.

Samuel

[quote author="mediatechnology"]... You might consider using an SSM2142 or THAT1646 for the output just to keep the build simpler. [/quote]
the page is getting a little old now but it may help
http://www.celestial.com.au/%7Erosswood/diy/KDMP/kdmp.htm

Mikkel also has a good PCB kit for this

yeah
there all sorts of incomplete links associated with my home site, the Factory and the defunct Group DIY pages

one day
I may get the inspiration to rework the lot

WOW......... Thanks guys!!!!!!!! :thumb:

I'm going to redraw and post a new schematic with all the suggested changes - hopefully this weekend.

I was thinking of using a 1x12 position switch with the following resistors: 13k, 4k7, 2k2, 1k1, 604r, 330r, 180r, 100r, 57r6, 32r4, 18r2, 10r. This should give me 5-60dB of gain in 5db steps

Many Thanks,

Martin

[quote author="mediatechnology"]Just curious, why did you use a ~6dB gain stage between the 2017 and the 2142? I've been wondering about this very thing.

BTW I think the +/- inputs on the LM833 need reversing.[/quote]
could be just the schematic as I think LM833 wasn't a library piece at the time

even so a quick change to the way you wire the outputs can fix

it was never meant as Golden Mic pre and was built for a purpose ... originally a Battery Driven Mic-pre and then later as a partner to the AM3 card

'twas a long time ago


the LM833 buffer and +6dB of gain was just to help things along
I had a very long write up at the OLD Tech Talk pages

it was the added feature of the Kev Mic-pre ... that and the 10mF tants on each op-amp ... including the 2017 and 2142 ... through the 33 ohm resistor

If you look deep at the original SSM2016, SSM2017 ... and I thnk the 2015 notes
all part of a BOOKLET that was put out when these things first were released
at least it was when I first became aware of SSM and later Analog Devices

it discussed a few things and the current capability of the 2016 ... 2017 output suggested that an impedance/current buffer could help
so we did
we also added a little gain at the LM833 to help with noise and headroom of the 2017

it also helped with providing a buffered unbalanced output that could be used for a second output OR a meter driver

This was all part of the application notes and nothing earth shattering

A good PCB layout for home etching and the 10uF Tants with the 33 ohm resistors were the key to a reliable and good presentation of the 2017

lots of gear from Euphonics to Digi001 and many Talkback systems have used this one
no secrets

[quote author="mediatechnology"]I do have that booklet and should scan some of the older datasheets for the 2015/2016.[/quote]aahh
now there's the rub
:shock:
if I had ... it would be up
and would havew been in the first days of Group DIY

but I loaned it to someone before we all had scanners and cameras etc
AND
so stupid of me to loan the original and not a photo copy

it was this event that changed the way I deal with and save ... and loan ... data and reference material

it has been discussed on the forums before and we have been looking for it ever since

The Analog Devices data sheet for the 2017 ... with obsolete as a water mark is close
but I do remember the original to be a little different.
need to find an original to see if my memory is correct ... :roll:

PMI ... yep
that was it

a couple of booklets
some where just data sheets, and that 2016 sheet does look like it's from the right era
this sheet was revised 1988

this ones suggests the 2016 can drive a Jack Feild directly ... however it does refer to .. taking care with package dissapation limits
it also does refer a little to the 2015 ... which later sheets don't

There was also a booklet that was devoted to Application Notes ... or had a large application section at the back.
It was in that stuff that had circuit suggestions and here was the buffer idea and the discussions on supply smoothing.

Allrighty, so I went back to the drawing board and added most of the suggested changes, see:

http://www.musiciansgig.com/micpre/schematic2.pdf

Build or no build??? :grin:

Also, I should have explained my goal for this preamp. I want something clean and quiet that can stand its ground with heavies like Mil-Media, Grace and DAV yet isn't too hard on the wallet and/or time consuming to build. Am I on the right track with this pre?

Thanks again for all your GREAT help!!

Martin

I'm hoping to design something similar.

Can you not use a THAT 1600 line driver instead of the two opamp output? (edit: I just noticed that mediatechnology had already suggested this).

Also, the THAT1500 Demo board schematic has a servo configuration I haven't seen before (page 3 on http://www.thatcorp.com/datashts/1500demo.pdf ) . What's going on there? I notice that you can reduce the size of C1 and C2 significantly by using this servo. Will this improve things?

Just thinking out loud....

Looks fine now. I don't think grounding of R3/R4 is necessary when phantom power is switched off though (in fact it is allready AC-grounded through C3). As shown the output will likely show a few mV output offset. Nothing to bad about that, just you should be aware that this might happen and that you could address this with output coupling capacitors.

Samuel

Excellent!! Would it be better to make C5 a 3300uF/10V cap or should I just keep it a 220uF/25V.

I ask because on this drawing:

http://www.thatcorp.com/datashts/1500demo.pdf

they used a biiig honkin' 3300u.

Thanks,

M.

Thanks for the info Wayne!! When I get ready to design the PCB I'll incorporate a space for the servo, so I can go back and forth between using caps and the servo.

What formula are you using to calculate the values of C5 (and its paralleled film cap)?

So if I decide to stick with C5 (vs a servo) and I'm using a rotary switch, should I add an extra deck - allowing me to switch cap values that are matched to Rgain?

All the best,

Martin

Gotcha!

I'll redraw and incorporate the .1 and .01 uF's in parallel with the 3300uF.

Also, will the 5W zeners create problems with distortion because of their non-linear input C?

Thanks,

M.

Hmmm.... Why would I need all this whistles'n bells with transformer and EF86?

[quote author="mediatechnology"]He had a post somewhere with more pictures but I can't find it.[/quote]
The pics are at the bottom here: http://www.groupdiy.com/index.php?topic=10417&postdays=0&postorder=asc&start=30

And yes, I could be persuaded to sell PCBs :grin:

[edit]PCBs added now...[/edit]

Best regards,

Mikkel C. Simonsen

Mikkel,

Does the kit include the switch?

Sorry for the off topic.

Thanks,

Brian

[quote author="skipwave"]Does the kit include the switch?[/quote]
Yes, all the parts are included.

Best regards,

Mikkel C. Simonsen