Dual/Quad 1081 Mic Preamp

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
The Boost pin, in all the schematics and in the Ten81, does not seem to be utilized. I am a bit lost with that. Being that it is just a tap, couldn't I just leave it out, being that it doesn't seem to be utilized on the original schematic and it's not joining to things together?
 
alexc said:
Just having a looksy now ...

Referring to your designations  for the ba338 amp module and jlm mic pre  scheme

- C5 470uF 25V is wrong way around. The electro + should go to pin 2
- the top junction of C4 100p and R108 2K2 should not connect to the back of D2 NTE519
- pot P1 10K pin 3 should go to rail B- (ground)
- feedback cap-to-ground C7 is wrong way around -ve leg should go to rail B- (ground)

And

-  One can use a non-polar electrolytic  in your C7 position.  If you can get one use it, otherwise a polarised will do
-  One can use a non-polarised electrolytic in your C5 position, although a polarised will do

and also

JLM micv pre schematic uses the ba438 module, the later iteration of the ba338 and shows no 'Boost' pin 4  that the ba338 and your version is showing.

I would need to check how that is handled in this hybrid implementation - I think it's left floating but can be connected up for various modified outcomes.

Other than that, looks good :)

...

These make very nice sum amps and can be used as 'virtual earth' (also known as 'current' summing) or 'passive' (also known as 'voltage' summing) mixing.

It's pretty amazing the performance one can get with so few active devices, and even with the less-than-perfect build values inherent in my efforts :)

> 100dB of signal-hum margin, THD can had in the 0.002 % range with a boat load of headroom to boot.

Add it to your 'to do' list  to make a passive sum bus, not too many channels, to keep it simple -  and hook it up to this mic pre.

Awe inspiring sound. The 'minimal' approach really shines through.

My 'opinion' is that too many opamps in-a-row , like in most mixers, really does detract from the subjective sound quality.

In my music setup, I use a small virtual earth mixer, built around these modules, as the master 'mixer of mixers'.

My diy stuff sounds far and away mixed thru a neve styled sum bus than thru the digital console alone. Much better.

That's why I like building sum bus units  :)  Can be done fairly cheap with heavy weight sound.

I feed the 'master sum bus'  my DAW  'ITB mix' stems,    the Yamaha digital desk stems (for keyboards, edrums and fx)  and feed in yet another mixer (y.a.m  :)  with the sum of all my analog diy channels (for monitoring).

I feel I get the best of all worlds  :)

I dont mean to sidetrack but its relevant as its very much the future im seriously considering for my studio; which yammie do you use and how are you connecting it to the summing mixer?
 
I'm using a Yamaha 03D - for keys, edrums and fx and I take the Main Out balanced analog to the 'mixer of mixers'.

Its very hard to beat in a small package - some things it does with ease that few other can :

- fx with sidechains sourced from any channel
- effective compander on any channel  (gate-compress-limit dynamics)

which makes it killer for complex fx mixes, including pedals - which sometimes needs 'noise management' :)

Also good for it's digital I/O - my digital stuff is centered around Motu 2408 Mk3 units and a Finalizer. I have an adat interconnects going - some spdif and aes-ebu between them.

A whole bunch of Motu analog outputs connect into the 'mixer of mixers for 'OTB summing'.

As I said, all that is brilliant and no doubt *but* for the diy stuff I build, it does tend to 'homogenize' the magic or uniqueness out of them.

I'm talking about preamps, eqs and limiters here, both discrete and solid state, mostly with transformer I/O.

The best way to hear the things that are special about the classics, is as close to the amp-speakers as possible.

Hence, building a 'mixer of mixers' with a low-ish channel count, like 4 or 5 stereo balanced inputs, and a very 'minimalist' summing 'make up' amps can be a simple and cheap route to go.

Of course, when one talks minimal discrete op amp structures, there is no shortage! A couple of names surely stand out and our friend here's ba338 and b340 are certainly in that category.

SO - having done the 'hard' work of designing a pcb ba338-b340, it would be crazy not to also do the 'sum bus' thing.

Which is what I'm currently testing  .....    a few that I've been working on for a few months. :) In the home stretch now  :)

...

Regarding pin 4 'boost' I would just take the trace to a pad somewhere convenient and leave it floating in the build.

 
One of my go-to mic preamps is the m581 by Vintage Design that's based on the 1081 preamp.  So if you're looking for some ideas on how its laid out, I'd look there as it is one of my favorite preamps and comparable to Avedis and/or Great River. Its also super colored but in a very good way.  I eventually hope to build something similar. Its worth mentioning Chameleon Labs also has a 1081 preamp that is well laid out as well.
*edited
 
Rocinante said:
One of my go-to mic preamps is the m581 by Vintage Design that's based on the 1081 preamp.  So if you're looking for some ideas on how its laid out, I'd look there as it is one of my favorite preamps and comparable to Avedis and/or Great River. Its also super colored but in a very good way.  I eventually hope to build something similar. Its worth mentioning Chameleon Labs also has a 1081 preamp that is well laid out as well.
*edited

I've been looking for that module for the last 4 years! Can you share some photos?
 
alexc said:
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.

Alexc,

I am finished with the PCB design and I am ready to order it, but I am second guessing it due to the heat sink situation. I was thinking of buying individual heat sinks, but I am not certain which transistors will need them. I am torn between individual heatsinks, or just putting the transistors really close together and then using a plate style heat sink. I am leaning toward individual heat sinks to help with dissipation.

Do you think these heat sinks will work?

http://www.mouser.com/ProductDetail/Aavid-Thermalloy/578305B00000G/?qs=%2fha2pyFaduiMQYwNm%2fGgA9W%252bhokrO9oAqMSzPA3TOfCsb9vmk7Kr2g%3d%3d
 
Those avid thermalloy ones you linked to might work ... they shouldn't touch each other or  any exposed board components ..  generally speaking ...

You can also use the 'star shaped' cookie cutter style ones ..  with the circular center

The actual amount  of heatsinking required for the output devices is pretty low

You can also substitute for 'flat package' output transistors and use a rectangular heatsink that mounts well to the pcb.
Of course, the pcb would have to cater accordingly.
 
Which 1081 schematic did you use?  Its hard to tell where the ba338 and 340s are, I can somewhat make them out.  Have you tested it in spice?
 
justinheronmusic said:
Alexc,

I am finished with the PCB design and I am ready to order it, but I am second guessing it due to the heat sink situation.

Try looking for SKK-15 from Fischer Elektronik: http://de.farnell.com/fischer-elektronik/skk-510/k-hlk-rper/dp/1892347
 
Rocinante said:
Which 1081 schematic did you use?  Its hard to tell where the ba338 and 340s are, I can somewhat make them out.  Have you tested it in spice?

The 338 parts are labeled in 100 (C101, C102) the 340 parts are 200 (C202, C203). The surrounding parts are single and double digits. It was difficult to lay them out in a traditional way, space is a concern, I aim to keep the cost low. I used the schematics available on the Technical Documents section of GroupDIY - There is the BA312, BA338, and BA340 schemes there.

I have never heard of spice, so naturally it would not have been tested in spice. Please advise!  :)

 
After looking into Spice, I am seeing that Eagle has a simulator as well, however, I am not familiar with testing procedures....
 
It will save you in cost as it will detect mistakes and cant hurt to run a simulation. 
I too used a colamoration of schematics for the 1081 pcb i have made in eagle. I thought to use the input circuit of the 1081 coming from the xfmr and then the output using the 340 to the output xfmr.
The 'interstage' have several component differences at the output of the 1st 338 stage when compared last 338 output that connects to the 340 , and so i was wondering how you decided which to use?
Did you just use the 1081 output?
 
Rocinante said:
It will save you in cost as it will detect mistakes and cant hurt to run a simulation. 
I too used a colamoration of schematics for the 1081 pcb i have made in eagle. I thought to use the input circuit of the 1081 coming from the xfmr and then the output using the 340 to the output xfmr.
The 'interstage' have several component differences at the output of the 1st 338 stage when compared last 338 output that connects to the 340 , and so i was wondering how you decided which to use?
Did you just use the 1081 output?

Good plan. Are you saying that there are discrepancies in the original schematics?

I am using the JLM schematic into the BA340, then the original Neve BA312 schematic for the BA340 to the output transformer.
 
justinheronmusic said:
Good plan. Are you saying that there are discrepancies in the original schematics?

I am using the JLM schematic into the BA340, then the original Neve BA312 schematic for the BA340 to the output transformer.

No no no. Sorry bad typing on my part.
Okay on the 1081 going from the input tranformer to the ba338 and afterwards is a bit different than the ba338 in the output section.
Im trying to figure out for instance if pin "7" on the ba338 is used like it is in the 338 following the input transformers? Theres also the 180pf cap.  Also is the ferrite bead needed?

 
Rocinante said:
No no no. Sorry bad typing on my part.
Okay on the 1081 going from the input tranformer to the ba338 and afterwards is a bit different than the ba338 in the output section.
Im trying to figure out for instance if pin "7" on the ba338 is used like it is in the 338 following the input transformers? Theres also the 180pf cap.  Also is the ferrite bead needed?

The 1081 doesn't utilize a zobel on the input transformer like the 1073 - so the 180pf isn't in the 1081 scheme. It seems to have a LPF network of the 1200pf capactitor, and a 1k5 resistor that is blocking excess highs from reaching the BA338 from the input trans, which I have included in my schematic (so I guess that is sort of a similar thing?). So I guess I was wrong - I use the JLM schematic for the gain switch, leading into the 10K pot/fader, then the BA312 scheme for the the 10K pot to the BA340 onto the output. The input section from input transformer into the BA338 is totally original BA312 scheme, but with the JLM gain switch.

In my findings, pin 7 of the BA338 just goes to ground. That is what it looks like to me, at least, and how I have implemented it into my schematic. I cross referenced pin 7 of the BA338 with the original BA312 scheme and the JLM scheme.

I may make one change to my scheme, which is noted in the JLM scheme which is that the BA338 and the BA438 differ in that the input transistor in the 438 is the BC214KC instead of the BC461. That would indeed cut cost quite a bit.
 
So on the below version of the 1081 amp there is a ferrite bead going to pin one of the ba338 and a 82v diode? after the ba338 stage along with another resistor; R6 which goes to BN?  I had used this schematic which is different then others to design my pcb (hence the confusion).  Are these components unnecessary then?

https://postimage.org/
 
Back
Top