Dual/Quad 1081 Mic Preamp

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justinheronmusic

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Jan 13, 2015
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I am looking to build a dual or quad 1081 mic preamp with DI, Phase, and +48v phantom power from scratch. I am still a newb, but I have ambitions to do this - I want to do the PCB layout, the OPA layout, the front panel, and everything in between. I'd like to do this in a 1U or 3U 19" chassis (depending on how many I can fit in one project).

I know very little about mic preamp circuits and design, but to my understanding the 1081 mic preamp section is an Input transformer, the BA338 and BA340 op amps, a gain switch, and an output transformer. How hard could it be?

I've got a few resources - the Ten81 links, the JLM audio 1081 pre schematic, and original Neve schematics, and I downloaded Eagle this morning and have been watching the videos.

If I can pull this off, I would be looking to order PCBs for this build. Unless of course someone beat me to it, then I would have no problem going that direction.

So I will need some resources for teaching myself about mic preamps, I'll need help understanding the 1081 mic preamp design. The schematics I have found seem to only be the basic circuit, but don't have +48, DI, or the output section. Any help with this would be much appreciated!  ;)
 
Its a fair undertaking but surely doable.

For someone new to the 'great game' of diy audio, it can be a bit challenging, though.

If you have the motivation and patience to persevere, there's no reason why not.

On the other hand, there are plenty of other, already available mic-pre schemes which may suit you with less time and effort.

As they say 'it's all good'  :) 

I'm doing some ba-438a based mic pre's myself at present.

Other options with proven results, for example, are the 'capi' mic-pre kits - most of the challenge of diy without the difficulties of pcb design, fabrication, component choices and debugging effort  !

But if you see it thru, there is no thrill better than tracking (successfully) thru your own full custom  kit  :)

Good luck with it.
 
alexc said:
Its a fair undertaking but surely doable.

I'm doing some ba-438a based mic pre's myself at present.

Thank you! Yes, I'm working on my 5th build at the moment; a dual SSL 4000E EQ. The CAPI stuff is really intriguing to me, but I've never really been into the API sound - to me it sounds very hard in the mids and soft up top, with a thinner bottom than Neve inspired flavors. I would love to mix something on an API console, but it's not my preferred sound for tracking.

- What is the best way to mount Neve style Op Amps to the pcb? Should I use the 2520 pins? What are those pins called?
 
I too have been playing with the ba338 and ba340.  I have actually redrawn in eagle the 3415 circuit which uses the ba338 and ba340 and to my reckoning is very similar to the preamp section of the 1081.
I'd love to get some feedback on the pcb design i.e. comparing the schematic, ground pouring, trace size, etc... and I'd gladly share my work via email.
The ten81 project was epic and hard but I think much of the problems people had were with the eq .
I have a bunch of stuffed but untested ba338s and ba340s from that project.
Let's do this!
 
Rocinante said:
I too have been playing with the ba338 and ba340.  I have actually redrawn in eagle the 3415 circuit which uses the ba338 and ba340 and to my reckoning is very similar to the preamp section of the 1081.
I'd love to get some feedback on the pcb design i.e. comparing the schematic, ground pouring, trace size, etc... and I'd gladly share my work via email.
The ten81 project was epic and hard but I think much of the problems people had were with the eq .
I have a bunch of stuffed but untested ba338s and ba340s from that project.
Let's do this!

Haha! I wish I would have seen this post earlier. I just got done doing my first ever schematic in Eagle. I have little to NO idea what I am doing as far as case sizes/component selection within Eagle. I subbed out some of the components for stuff that is readily available or the US versions of EUR parts. Here is what I have so far - the BA338 Op Amp.

 
Hey its good practice and making them fit the 2520 format is still pretty useful. As far as symbols there are cheat sheets online  which have all the symbols needed to write schematics as well. After a short bit it becomes easy.  I have the eagle lib. 2520 footprint designed by someone (Harpo?) from here which if you go that route will prove useful for you.  I redrew the 3415 circuits to fit the original circuit but its not hard to make them for 2520 type (I had already played with this idea).
Iirc someone has already done it. I think its an Australian dude who also redesigned an API compressor.
 
I am wrong about the 3415 being the preamp section to the 1081.  Quoting from a similar claim Geoff responded:


"I hate claims like that because, minus EQ, the 1081 is 10468 input transformer, a B338 pre amp with a ton of wiring to the sensitivity switch, a B340 output amplifier and a chunky LO2567 output transformer.

The 3405 is 10468 input transformer, B338 pre amp with simple gain boost option, B340 output amp and a skinny LO1173 output transformer.

They won't sound exactly the same... but that aside, the 3405 and 3415 make nice preamps if wired correctly.

Neve made a non-EQ 35 series module called the 3117 that's a 3114/5 less EQ and similar to the 3415 with a stepped gain switch for mic/line inputs."

I stand very corrected.


 
Your schemo is a good start!  The ba338/ba438 discrete (op) amp  is an intriguing little circuit - I still like messing about with it  :)

As far as a pcb layout, I would say there is merit in keeping it simple  ie. no pins and sockets for the gain blocks.

As for the ba340/440 line amp, the historic form factor is a pia with the smallness of it , and those heatsink attachment cans for the output transistors are even more of a pain.

Unless you are going for historic stylings, a simple all soldered layout with the minimum of wiring  has a lot going for it  :)


Would you be using single-ended psu or bi-polar? Either can be used. (with some adjustments to bias arrangements)

How do you want to wire in gain control ?  - the historic way is pretty painful although there is merit in the way it's done.

Using something like the 'jlm interpretation'  is also just fine! Or a hydrid of the old way and 'api' using 'ganged' control over both the first gain stage and second (line amp) stage ?

If you want so see how something similar can be laid out, try searching out the 'bauman 312' pcb layout - it is very well done with respect to phantom power, mic/line/di input switching, various-input-traffo options and all of the other important 'system level' things that make up a worthwhile mic pre implementation.

Just sub the 'api doa' section for a 'ba338/438 and ba340/440' section as you wish.

In the end, I wouldn't expect a 'mongrel' build based on the ba338/438 and ba340/440 gain blocks would sound much different to an opamp circuit  or any other number of similar implementations.

Sure, the class-a neve circuits are fairly unique, but class-ab output + ungapped transformer schemes are pretty generic imho.  I could be wrong, however!

 
alexc said:
Your schemo is a good start!  The ba338/ba438 discrete (op) amp  is an intriguing little circuit - I still like messing about with it  :)

As far as a pcb layout, I would say there is merit in keeping it simple  ie. no pins and sockets for the gain blocks.

As for the ba340/440 line amp, the historic form factor is a pia with the smallness of it , and those heatsink attachment cans for the output transistors are even more of a pain.

Unless you are going for historic stylings, a simple all soldered layout with the minimum of wiring  has a lot going for it  :)


Would you be using single-ended psu or bi-polar? Either can be used. (with some adjustments to bias arrangements)

How do you want to wire in gain control ?  - the historic way is pretty painful although there is merit in the way it's done.

Using something like the 'jlm interpretation'  is also just fine! Or a hydrid of the old way and 'api' using 'ganged' control over both the first gain stage and second (line amp) stage ?

If you want so see how something similar can be laid out, try searching out the 'bauman 312' pcb layout - it is very well done with respect to phantom power, mic/line/di input switching, various-input-traffo options and all of the other important 'system level' things that make up a worthwhile mic pre implementation.

Just sub the 'api doa' section for a 'ba338/438 and ba340/440' section as you wish.

I will only go as historic as I need to for the sound of the 1081 with stepped gain switching - I was thinking Grayhill?

But I think I am keen to use 2520 style pins for the sake of organization. This being my first go at this, if I keep things compartmentalized it may be easier? I don't know. I've never done this before. Would it indeed be easier to work the op amps into the whole pcb?

There is a part of me that thought about using JLM GoBetween and OpAmp DI pcbs and just adding them to this circuit, as it would be the easiest way to get this sound without too much work while still maintaining basic Mic Pre utilities.

Thank you for the resource. I will check out the Bauman 312 PCB layout and see what I come across. Are you saying that I can use that as kind of a template and then just sub out he DOA for both the BA338 and BA340? That almost seems too easy.

As far as power supply goes, I am not sure what the benefits of each one would be. At this point I would say whatever gets the job done at a minimal cost and is easiest to source. Does JLM have anything that would work?
 
justinheronmusic said:
I will only go as historic as I need to for the sound of the 1081 with stepped gain switching - I was thinking Grayhill?

But I think I am keen to use 2520 style pins for the sake of organization. This being my first go at this, if I keep things compartmentalized it may be easier? I don't know. I've never done this before. Would it indeed be easier to work the op amps into the whole pcb?


Thank you for the resource. I will check out the Bauman 312 PCB layout and see what I come across. Are you saying that I can use that as kind of a template and then just sub out he DOA for both the BA338 and BA340? That almost seems too easy.

As far as power supply goes, I am not sure what the benefits of each one would be. At this point I would say whatever gets the job done at a minimal cost and is easiest to source. Does JLM have anything that would work?

Well for troubleshooting purposes the 2520 or any op amp pinned circuit is a smart move.  Especially the B340 which can get hot prior to being trimmed.
As far as the Bauman, I believe he is saying take a look at how the phantom, phase switch, xfmr mounting, etc... is being implemented as its an easy layout to trace.  Its an Api circuit otherwise.
 
Everyone has their own opinion on these things - I dislike pins and sockets and small , over crowded doa pcbs with poor heatsinking  :)  Spread out a bit makes it a lot easier to work with in case of maintenance.

API is a special case, in that it is an industry standard format and sure, if one places value on keeping in line with that, a 2520 footprint is a plus, as is the bipolar psu they typically use.

Neve historical is also a special case, just because it is. At Neve they made their own reality as they went!

And then there is the issue of what power supply. Single polarity +24V or something else. 2520 footprint pcbs are usually associated with bipolar +/- 17V supplies. Again, there are a number of ways to go.

So it's a case of defining what appeals to your build - if you like the 2520 footprint that's all well and good. But since the neve discrete amp module  isn't interchangable with  2520 doas (on the power supply issue if nothing else!) what's the point?

Similarly with the historical layout - if you want to make it compatible with that, then fine. But for what purpose ?

So - decisions. What kind of 'mongrel' do you want to 'create' ?

My prefs are , ( and others clearly have their own :)

-  spread the design out a bit and dispense with pins and sockets
- plan for an off-board power supply connected by say a 5 pin cable-socket connector
- +24V psu keeping with the historical
- similar parts to the historical, as far as you can get them (some components are not so common anymore
- modern-ised gain setting  ie.  something like a Greyhill  3 deck  for a historical take or a simplified 2 deck version

The Bauman thing I mentioned is simply a layout that I found very nice, but again, their are plenty of ways to go.
It was from a long time ago ..  he used to hang out here ...

Here  the schematic of his 'do-over' of the mic pre people would make from Neve pcb modules - '1272' or 'n72' as such things are commonly known.  Its similar, but different  :)  to what you're working at.

He posted it here  as a project  for a long time , years back, so I don't think there is a problem putting it up here

He did a 'motherboard' layout for the mic-pre 'system' things keeping historical in some ways, modern in others.

Kept all pinouts, block diagram of the historical, as well as shifting to modern for the gain switch.

As well as providing ....

- phase and phantom switches
- gain switch and/or output level pot
- di/line/mic input connectors and selection (if applicable)
- line output connectors

The DI circuit sounds very good too!
 
And for the discrete amp blocks, he did them in his own format, keeping with the historicals' pin outs but not physical form factor.

Here is the schemo for his layout
 
Here's a pic of Bauman's ba283av module  with voltage amp section and line amp for his n72. In this case, they are used together, like in a 1272  but they can also 'snap' apart for use seperately like in a 1073 and others.

The very different 2520 doa, of course, also has a voltage amp part and a line amp part together on the small pcb.

And then again, the 1081 uses a ba 338/438 module for voltage amp and a seperate ba340/440 module for line amp duties.

It's all good - one just needs to decide :)
 
Here is the main board for one channel  .....    psu is fully seperate from the main board.

Its similar in size to a 500 series module but not compatible.

I put 2 of these plus a JLM psu module and psu toroid into a 2RU box.
If I were doing again, I would put the psu external.

With transformers and gain switches, it all piles up pretty quick - so if you are thinking to jam 4 of these kinds of mic pre's into a single rack, then that takes some extra skill in cramming it all together somewhat.

 
Fabio also did his take on the 312 - it's out there .. similar to the above,  but done modern with 2520 footprint, bipolar supplies, single deck grayhill and a seperate level pot.

He kept the flexible input routing which allows seperate mic pre, line and di inputs - that's what makes it a really useful box :)

As the make up box for a voltage sum bus, they are hard to beat  :)

And finally, there are lots of others who do and have done similar things! So some research into how they package their 'takes' on the 'classics' can be very instructive.

 
alexc said:
So it's a case of defining what appeals to your build - if you like the 2520 footprint that's all well and good. But since the neve discrete amp module  isn't interchangable with  2520 doas (on the power supply issue if nothing else!) what's the point?

Thank you for your input Alexc. The more I think about it, the more I think it would be more cost effective to just use a single PCB instead of the two op amps and motherboard concept. This may simplify the layout and schematic and save some time. Transformer mounting wouldn't be on board though, as the VTB9057 that I was planning to use for this project is best mounted to the chassis. That would save board space as well. The VTB9045 and it's equivalents may be PCB mounted though (though I was planning on using the Ed Anderson from Hairball Audio).
 
So I will place the Greyhill switch before the BA338. Would I be able to use the input section of the 1272 schematic up to the point of the input transformer? Then after the transformer and Grayhill, I will follow the 1081 schematic until the BA338. I am fairly lost as to what to do next. I am not sure how to do the output section or where the output section starts on the 1081 schematic.
 
A simple mic pre would have a mic input transformer - the primary side connects to an xlr.

To add phantom powering at 48V requires a decent supply, a couple of 6K81 tight tolerance resistors and usually an electro cap + indicator led.

Then one can add further options, into the primary side - commonly seen is are

-  phase reverse switch, which is simply a dpdt switching polarity of the balanced signal

- 20dB attenuation switch which is simply a dpdt switch with a resistive divinder for each polarity of the balanced signal
 
- input switching :  for a line input and for the fancy persons among us, some kind of a di input

Now all that is one the mic transformer primary side.

The historic 1081 does better and has a seperate Line input transformer, optimised for the job, as well.

The switching between the mic input transformer and the line input  transformer is done with a complicated and expensive 3 pole, 12 position (I think!) rotary switch, make-before-break.

So  the key decisions are :

1. what sources do you want to have ?  eg.  mic, mic +48V, line, di

2.  how many input transformers ? historical, with 2 input transformers or just one, like more modern interpretations  ?

2. what rotary gain switch arrangement do you want ?  full historical with elmas or part historical with grayhill or full modern like JLM and api ?

Once you've answered those questions, you will narrow the solutions to what suits you (for now!).

The other thing, is what kind of overall gain and output section do you have in mind?

A historical 1081 has the ba338/438  as a voltage amp and a class AB discrete b340/440 module as a line amp.

The overall gain is something like (indicative values only) :

+12dB at the mic traffo secondary, +40dB in the ba338, +0dB in the output amp, +6dB in the output transformer

for an overall max gain of +58dB  or so.  The actual figure might be +65 .. +70 dB  - its in the 1081 docs.

For the neve stuff, the gain switch is arranged over both the ba338 and b340 modules, so it's a little complicated and therefore usually means 2 pole 12 position for a modern interpretation  or 3 pole 12 position for a historical.

Once you have that cleared up in your reqirements, you can think about 'cut-n-paste' from some existing design, even if it's just for illustrative purposes. You'd most likely be doing your own pcb layouts on CAD.

To truly do a cut-n-paste of someone else's contributions, you need to get access to the CAD files and all that.

You can also do a search for the '1290 mic pre'  that at least one member here did a nice renditon of - there's probably all you need to know in that design, except that it is the class A historical line amp rather than the class AB from the 1081.

Here's a link to the 1290 project - it's 95% on the info you want - the differences lie in back end
https://groupdiy.com/index.php?topic=22828

and the build doc ..
http://www.diyrecordingequipment.com/products/ez1290-preamp-pcb

Hope this helps rather than confuses you  :)
 
alexc said:
So  the key decisions are :

1. what sources do you want to have ?  eg.  mic, mic +48V, line, di

2.  how many input transformers ? historical, with 2 input transformers or just one, like more modern interpretations  ?

2. what rotary gain switch arrangement do you want ?  full historical with elmas or part historical with grayhill or full modern like JLM and api ?

Once you've answered those questions, you will narrow the solutions to what suits you (for now!).

The other thing, is what kind of overall gain and output section do you have in mind?

A historical 1081 has the ba338/438  as a voltage amp and a class AB discrete b340/440 module as a line amp.

Once you have that cleared up in your reqirements, you can think about 'cut-n-paste' from some existing design, even if it's just for illustrative purposes. You'd most likely be doing your own pcb layouts on CAD.

You can also do a search for the '1290 mic pre'  that at least one member here did a nice renditon of - there's probably all you need to know in that design, except that it is the class A historical line amp rather than the class AB from the 1081.

Hope this helps rather than confuses you  :)

That's so crazy, I was just looking at the EZ1290 schematic this morning! To keep cost down, I think that basing my PCB layout around this design would be a great first "from the bottom up" project. Bells & Whistles can be added after the fact.

1. Mic & +48v
2. A single VTB9045 or EA10468 on the input
3. Grayhill for Gain.


I would like to add an output knob as well, but not stepped. I believe I can do this after the BA338 and before the BA340? Is that right? It would be even better to make it optional.

Otherwise I would want the output to be identical to the 1081 - which I am still not sure where the beginning of the output starts on that schematic. The JLM schematic doesn't even show the output, nor does the "1081 preamp only" schematic. I'm assuming it's after the BA440, but I am not sure what the "boost" pad is, or how it connects from there. This is where my ignorance starts to shine. My apologies.

At this point the only part of the schematic I understand is the input onto the BA338, I can probably get started on that part of the schematic. It's after that that I get lost. I really want to understand the anatomy of it, but I feel like I am hitting a brick wall.
 
So you've narrowed it down some, which is good.

Just to try to be clear, there is no actual historical '1081 mic preamp' as a stand alone product - the 1081 product was a mic/line pre with 4 band eq, using a ba338/438 'discrete amp' module front end and a b340/440 'discrete class ab line amp' module  backend  (it also uses several filter gain modules in between)

So whichever way you put it together, a stand alone mic-pre only will be a somewhat different animal from the venerated classic.

Now the reason I point that out is the product 1081 gain switch is a complicated affair, more so because it does switching between mic & line input transformers as well as providing attenuation of both the ba338 and b340 modules together.

IF you only have a mic input, you can simplify down to a 2 deck rotary or even 1 deck, as done in the JLM by sacrificing the line amp altogther.

ie. the JLM '1081 mic pre' circuit takes the '1081 pre+eq' front end ba338 module and utilises it in a modern, 1 deck gain switch configuration - a simple  inverting amplifier with attenuation being in the -ve leg ....  'api style'  :)

However, it's  current drive capacity is not really up to the job of a line amp - the one I'm playing with now is not as strong an output as even an api doa. And it does require heatsinking if it's trying to drive heavy (low Z) loads.

The '1081 mic-pre+eq' and others have the b340 for that reason.

Just to be clear !

So the design question now is 'do you want to keep the historic gain switch attenuation OR simplify and use the 'non inverting op amp' configuration  (jlm and api style).

The answer dictates how 'close' to the historical you will be.  If you want closer to original, then you need a 3 decker.

If you can stand to deviate, then a 2 deck or even 1 deck can be used, bringing you closer to a pretty generic config.

If you can leave out the line amp as well, then you are in jlm's circuit territory, where a 1 decker is all that's needed.

What that means to the sound is another question.


I think I would be hard pressed to tell the difference between an 'old school' ba338/b340 config or a 'modern' ba338/b340' config, if they were using the same parts.

Still further, I doubt I could tell apart a 'modern' 'ba338/b340' config from say an 'api style mic pre'  made from same parts.

However, it is all good - quality in will surely give quality out whichever way you go.

SO - did you get all that? 

...

Also, you mention an output level control - the original product of course used the channel strip fader for that.

So, to add that has some considerations.

In the historic way, the output fader is connected between the the ba338 and the b340 (just before the b340).

SO, if you are going historic, meaning you will have a b340 amp section, that's no problem and a simple affair.

If you are not planning on using a b340 line amp module, and go for the  'jlm 1081' then you need to make other arrangements.

The decision therefore is 'do I want a proper mic pre with a good drive capability that the use of a line amp+traffo brings'.

If so, then the output fader is simple to implement.  :)


NOTE - it's been a while since I looked at the original 1081 gain switch detail and fader arrangement. I could be a little mixed up between models, so please fact check :)

...

My own 'mongrel 1081 pre+eq' uses the b338 amp module in the modern api inverting amp config, with a single deck rotary attenuator  (ie. like the jlm 1081)  and unipolar +24V rail.

The line amp is a 2520 doa with bipolar +/- 18V rails,  driving a 1:1.73 step up transformer which can drive 600ohm loads and higher to a very respectable voltage

The output fader is a 10K reverse audio log pot attenuator on the -ve leg of the 2520, again 'api style' or  'modern inverting opamp config' .

Luckily the hard yards of pcb design/fab/test were done by others ... members here over the years  ;)

Mine  uses a 'bauman simple 1081' pcb implementation of the 'jlm 1081 mic pre'  and Pier Paulo's APP Studio 'I/O board'.

The APP Studio pcb is possibly the best preamp board design I've come across  - beautifully implemented with di, twin 2520 blocks and transformer options  etc. All round class act and worthy of high praise  :)

Perfect for a preamp with eq courtesy of the 2nd 2520 used as eq-gain-make-up. 

Add some 'poormans eq' blocks from Ruffrecords in the 'between' and you have a fab '1081 micpre+eq' mongrel build.

Actually ->  1081 mic/line/DI preamp + pultec-ish eq + fixed api makeup + variable api  makeup/line amp + traffo

Ok then.  Coolio!

 
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