TillM

Neve BA440 with 2520 pinout
« on: April 07, 2021, 07:00:15 AM »
Hi Guys,
I was thinking about convert the BA440 to 2520 pinout.
But now I’ve some questions.
The BA440 works with 24v.
Is it possible to drive them with 16v?
Which components I have to look at?
Second question.
The 441 and 461 and really big transistors. I’m sure, I’ve to substitute them.
Any help is really appreciated.
Best
Till


abbey road d enfer

Re: Neve BA440 with 2520 pinout
« Reply #1 on: April 07, 2021, 07:22:56 AM »
Hi Guys,
I was thinking about convert the BA440 to 2520 pinout.
But now I’ve some questions.
The BA440 works with 24v.
Is it possible to drive them with 16v?
Yes, understanding that headroom will decrease sihnificantly.

Quote
Which components I have to look at?
You may have to adjust R1 slightly, for symmetric clipping.

Quote
Second question.
The 441 and 461 and really big transistors. I’m sure, I’ve to substitute them.
In the decades that have passed since the days this circuit has been designed, transistors have evolved considerably. However you must take into account that there is not overcurrent protection apart from the 1.5r emitter resistors.
ZTX651/751 are good candidates.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Neve BA440 with 2520 pinout
« Reply #2 on: April 07, 2021, 12:34:37 PM »
Do you really need or aim to run it at 16V?  Or did you mean + & - 16V as is usual with the 2520?

With the BA340/BA440, in order to achieve decent headroom in the desks using a 24V single sided supply, the amp was usually run into a transformer with a primary strapped for 50 or 75 ohms, and the usual secondary Z of 600 ohms. 
This means that if you like to cook your signals on the hot side of things, the amp is running class B a lot of the time.   
Nothing at all wrong with that, just pointing it out.

 :)

D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

TillM

Re: Neve BA440 with 2520 pinout
« Reply #3 on: April 09, 2021, 05:31:46 AM »
Yes I mean for +/- 16v.
My idea was to use it as a OP Amp in my API style preamps or for the Hairball Lola.

Problem is, that I’ve to learn a lot about electronics in this case.
So I need a lot of help to realize it.

abbey road d enfer

Re: Neve BA440 with 2520 pinout
« Reply #4 on: April 09, 2021, 05:40:50 AM »
Yes I mean for +/- 16v.
My idea was to use it as a OP Amp in my API style preamps or for the Hairball Lola.

Problem is, that I’ve to learn a lot about electronics in this case.
So I need a lot of help to realize it.
The BA440, like most of the vintage Neve designs, does not have true differential inputs. The input impedance of the inverting input is much lower than that of the non-inverting. It makes it unsuitable for most EQ's and requires adaptation for other stages.
The main issue being the output offset that is not stable and requires individual adjustment.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Neve BA440 with 2520 pinout
« Reply #5 on: April 09, 2021, 07:27:59 AM »
Abbey nailed it. 
If you keep in mind everything he said and tailor your surrounding stuff accordingly, then it's certainly possible to use it as you want.

For a Neve amplifier that's much more 'plug-and-play' in an API type environment, you could look at the BBC designed BA512 which has a more typical 3 stage 1/diff-amp input, 2/transconductance amp, 3/low Z output stage topology

The BA 512 is neat in that it was a little ahead of its time and had two pole compensation, although the stock miller cap might be a little too severe for higher gain applications. 

Edit: corrected mistake with amp number.
« Last Edit: April 09, 2021, 05:57:44 PM by Winston O'Boogie »
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

mjrippe

Re: Neve BA440 with 2520 pinout
« Reply #6 on: April 09, 2021, 04:54:05 PM »
For a Neve amplifier that's much more 'plug-and-play' in an API type environment, you could look at the BBC designed BA540 which has a more typical 3 stage 1/diff-amp input, 2/transconductance amp, 3/low Z output stage topology

The BA 540 is neat in that it was a little ahead of its time and had two pole compensation, although the stock miller cap might be a little too severe for higher gain applications.   

Hi Winston, a quick google did not bring up anything at all for the BA540.  Do you have a schematic you could post?

Re: Neve BA440 with 2520 pinout
« Reply #7 on: April 09, 2021, 05:33:18 PM »
Hi Winston, a quick google did not bring up anything at all for the BA540.  Do you have a schematic you could post?

That's because I'm an idiot  :-[ 
I wrote the wrong number, I meant BA512 not 540.  Sorry!.

The details are attached:
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

Re: Neve BA440 with 2520 pinout New
« Reply #8 on: April 09, 2021, 05:43:38 PM »
C3 & C4 at 1nF each, together with R12 of 1K are the two pole compensation scheme.  The effective dominant pole miller cap value would be the series value of these two, so 500pF.
This gives enough stability for a closed loop gain down to about 10dB.

If you wanted/needed a closed loop gain that was somewhat higher, then you could lower the values of the caps.  As a practical example, I used values of 470pF each (235pF series value) for a closed loop gain of approx 24dB and the circuit was quite stable.

An easy rule of thumb I use for checking stability is a margin of 12dB, so if you can lower the gain by 12dB less than you want without instability or seeing anything strange happening on your 'scope, you're OK.  It's worked for me anyway.

Of course, you could just leave things as they are regardless of the gain you want, but there are benefits in terms of having more feedback at higher frequencies when using the lowest practical values there.
 
« Last Edit: April 09, 2021, 06:02:43 PM by Winston O'Boogie »
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

mjrippe

Re: Neve BA440 with 2520 pinout
« Reply #9 on: April 09, 2021, 05:49:46 PM »
Thanks, Winston.  I do have a schematic for the 512 but as always, your insight and explanation are invaluable.


Re: Neve BA440 with 2520 pinout
« Reply #10 on: April 09, 2021, 05:54:46 PM »
Thanks, Winston.  I do have a schematic for the 512 but as always, your insight and explanation are invaluable.

Cheers dude.  Sorry again about sending you off on a wee goose chase  :)
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.


 

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