[atavacron]

I like where this is going.

I had been wondering about that primary resistor, and why it was different in different schematics. Great to know!

All the outputs and inserts on this patchbay have 1K2 across the normals, which are out of circuit when feeding anything in addition to their normal loads in the next row.

What is the purpose of the 47p cap? It is a 470uF when it comes from the tertiary of the TO129. Would it need to change based on the value of the NFB resistor?

The safety value with parallel pads for R16 sounds good, as does the LED trick. The series resistor on the output would be more Neve, the inductor and resistor more Hardy I guess.

Making that output really flexible is a good call. Maybe someone’s got eight transformers in a box wired in to their patchbay or whatever, and they want to run the output of these amps to hi/lo and be able to patch to different transformers or something. That would require a robust approach. The instructions would read to jumper that if everything was in the same box.

Here is a version with labeling for board layout, attached.

[Winston O'Boogie]

I bet the 47pF cap is compensating for that particular output transformer's response.  What you're using doesn't need it.

We know the amps are stable at the gain you want, and we also know those beefy output transformers are flat with the zobel etc. 
Keep it simple.   

OK, I'm off for a nap...

[Winston O'Boogie]

Ltnt.  Columbo:  "Just one more thing..."

After you've thought about what you're doing, if you need me to lay out a PCB, no problem.  I need a project to get up to speed on this cloud based software for Mac OS X I just downloaded.   Maybe I can get a free board out of whatever you buy but, whatever really, it's all good ✌️

Which side of the World are you?  Guessing Stateside.   I'm over the pond in the UK.

Later...

[atavacron]

I have no experience with laying out traces. I can draw size, positions, rough pad placement, I/O, silkscreen. Illustrator. DXF. Same for a schematic.

What software are you using? Maybe I will learn Eagle someday.

Uh, yeah of course you’d get a free board out of it. I’m in California. I’ll do a quick dimensioned drawing for a starting point.

[Winston O'Boogie]

Easy EDA is what I downloaded, can't say if it's good or not yet.  I'm used to Protel but don't have a Windows machine now and not really interested in messing with Bootcamp on the Mac Pro. 
Believe it or not, I grabbed a thing called PCB Droid for Android and am messing about with it right now on a cheap Samsung tablet.  I was expecting it to be a joke but it really doesn't seem to be so for simple stuff like audio.  And I like not being tethered to a desk or a laptop.

OK yep, thought you may be in California, my old neck of the woods.

Regarding dimensions etc - You said 35mm per amp to line up with the channel widths which is good.  But what centre spacing are the transformers?  Coupling between them (crosstalk) is something I haven't checked as they're usually inside individual cases.  Anyway, might not be an issue but just a thought

[Winston O'Boogie]

Messy drawing:

[atavacron]

Lemme get a clearer pic of the messy drawing?

Here’s my version with a layout, 35x70 is tight but possible. The top left and bottom right are mounting holes, the pad stands up next to the trimmer, and the caps are BC 138.

Transformers are remotely mounted (1-1.5m of FST) with plenty of space around them - I don’t think they’re close enough to interact. I will ensure leads are telescoped.

[Winston O'Boogie]

OK got it, thanks, I'll work to your dimensions. 
What I was thinking regarding the feedback and gain setting of the amp was to use the exact configuration of the 1081 output stage since that's the closest as far as staying 100% true to Neve with you using the same transformer 'n' all. 
The Neve document shows a 15K feedback resistor where as my scrawl showed a 7.5K.  That's just because I was adding in the internal to the amp 15K feedback resistor in parallel with the external 15K.  On the layout I'll do it the same as the Neve doc. 
You OK with using  2n3053 / 2n2905 instead of hunting down BC441/461?

[atavacron]

I like that you have a solid idea for my specific application and want to see it through. Thank you so much. There are some things to bear in mind to minimize revisions.

While a perfect accommodation of my application is welcome, I want to be clear that the supporting circuitry has to work for other applications, as a standalone gain stage. This needs to be usable with not only the LO2567/LO1166 and modern knockoffs, but also the Carnhill and Sowter LO1173 equivalents, and even the API 2503 and 2623 and their equivalents. Basically any step-up between 4dB and 10dB gain whose secondary is set up for 600Ω. As I mentioned above, the values of components that have a user option associated should be indicated by their function and not their value (these being Rfext., Rg or RVg, the series output resistor, and Rp1/Rp2/RVp should they be a good choice)

The 150µF on the inverting input is a size advantage over the 470µF, so I'm glad that's possible. No BA489 version uses a 1000µF smoothing cap, and because of its size (12.5x30 as opposed to 10x25), I'd like to understand if it is completely necessary in the 1081 version. Also I am curious how running series/series will change the output choices as opposed to the 1081 parallel/series.

Does it change the operation of the amp to bring back NFB into the inverting input at the node between the fixed gain resistor and the blocking cap (1081), versus bringin it back in to the node between the blocking cap and the inverting input? Regardless of values, this won't be a user-selectable option.

I don't think we need a separate set of gain boost pads in parallel with Rg, because its value will be set by the user prior to assembly. It may however be smart to give Rg additional pads for a trimmer (RVg), which potentially could negate the need for a pad. A trimmer in this position would need a range fix in series.

I see the advantages of a slugged rotary fader. Are you thinking of putting this onboard as a trimmer, and providing hookup points for folks to offboard it if they wish (i.e. a pin at the non-inverting input as opposed to just a solder pad)?

Any difference in function within the 10Ω-47Ω range of the series output resistor, or is that just a way of saying "anything within this range will do"?

Maybe more detailed thoughts than necessary in advance of any actual layout, but it'll be nice to be on the same page. Not trying to look a gift horse in the mouth. Again, I really appreciate the interest.

[Winston O'Boogie]

While a perfect accommodation of my application is welcome, I want to be clear that the supporting circuitry has to work for other applications, as a standalone gain stage. This needs to be usable with not only the LO2567/LO1166 and modern knockoffs, but also the Carnhill and Sowter LO1173 equivalents, and even the API 2503 and 2623 and their equivalents. Basically any step-up between 4dB and 10dB gain whose secondary is set up for 600Ω.

All those transformers are known types with good response so no reason it wouldn't work.   We already know the amp can drive 50 ohms.  The other original configurations of the Neve gain block with transformer use lesser known types with a tertiary feedback winding etc.   That's why I thought this the best way.
I'm not sure I see this as a group project for widespread use though.  Maybe I'm wrong?  It's a lot of output amps on a single board, perfect for you but not many folks have a Neve desk with spare transformers inside 😃

As I mentioned above, the values of components that have a user option associated should be indicated by their function and not their value (these being Rfext., Rg or RVg, the series output resistor, and Rp1/Rp2/RVp should they be a good choice)


The parts can be indicated with whatever system you want or think best, doesn't matter to me. 


The 150µF on the inverting input is a size advantage over the 470µF, so I'm glad that's possible. No BA489 version uses a 1000µF smoothing cap, and because of its size (12.5x30 as opposed to 10x25), I'd like to understand if it is completely necessary in the 1081 version. Also I am curious how running series/series will change the output choices as opposed to the 1081 parallel/series.

Force of habit on upsizing the 470uF to 1000.  My bad.   
Not sure what you mean about the output choices, transistors or transformers?  If transistors, then using 2N3053 over BC441 etc has no bearing on how the transformer is strapped. 

Does it change the operation of the amp to bring back NFB into the inverting input at the node between the fixed gain resistor and the blocking cap (1081), versus bringin it back in to the node between the blocking cap and the inverting input? Regardless of values, this won't be a user-selectable option.

If the cap is at the inverting input junction (1081), the cap is within the loop.   Non linearities of it are then equalised.


I don't think we need a separate set of gain boost pads in parallel with Rg, because its value will be set by the user prior to assembly. It may however be smart to give Rg additional pads for a trimmer (RVg), which potentially could negate the need for a pad. A trimmer in this position would need a range fix in series.


Having a place for another gain boost  resistor in parallel was just replicating the function that was brought out to the rear connector on Neve amplifiers.  A trimmer is fine too if that's best for you.


I see the advantages of a slugged rotary fader. Are you thinking of putting this onboard as a trimmer, and providing hookup points for folks to offboard it if they wish (i.e. a pin at the non-inverting input as opposed to just a solder pad)?


Yes to both.

Any difference in function within the 10Ω-47Ω range of the series output resistor, or is that just a way of saying "anything within this range will do"?

It's not known if you will need that isolation resistor in your situation given the distance to the transfirmers   Ideally you don't as anything there will raise the output impedance of the amplifier.   If you do need it then start at 10 r and move up if necessary.  Stop as early as possible. 

Hope that helps.

[Winston O'Boogie]

You mentioned revisions and I forgot to answer: How about  If I lay out a single channel and post it here or pm it to you  - Take a look, pass it around, look for any problems or screw ups.  If you're happy then buy a single channel and a couple or three spares for a few dollars.  After you've built it and it works for you then I can duplicate it as many times as necessary on one board with power distribution etc.   

[abbey road d enfer]

I don't see the need for such a big cap in the C2 position. With a typical value a a few kiloohms for Rg, the -3dB point is <0.1Hz. Considering the LF response is dominated by the xfmr, I would set this cap at about 10-22uF. Will save some space, at the benefit of the output or decouping cap.
You don't want the output stage striving to drive the xfmr into something it can't handle anyway.

[Winston O'Boogie]

I don't see the need for such a big cap in the C2 position. With a typical value a a few kiloohms for Rg, the -3dB point is <0.1Hz. Considering the LF response is dominated by the xfmr, I would set this cap at about 10-22uF. Will save some space, at the benefit of the output or decouping cap.
You don't want the output stage striving to drive the xfmr into something it can't handle anyway.

Thanks Abbey.  The scheme I'm working to is the one attached here, the output from the 1081 amplifier.  Which C2 are you referring to?
Cheers

[abbey road d enfer]

Which C2 are you referring to?

C35 here.

[Winston O'Boogie]

OK gotchya.  I do understand your reasoning there.
I'm quite comfortable with a -3dB of 1Hz or thereabouts in these spots myself.   Douglas Self shows that distortions from electrolytics is quite measurable and best reduced by placing them a decade + below audio.  Although I'd still want to use the tantalum 10uF cap on the +ve input to retain that particularly Nevesque distortion 😀
I'm also thinking that since the amp is now not driving 50 ohms but, at the lowest, 200 ohms, the feedback network can be reduced in impedance to lower noise.
Make sense or you think I'm off base?  It was good enough for Rupert but that doesn't mean we can't better it

Edit:. Also, I don't think these spots on the 1081 were the old Philips axials, I think they were better types that were radial.  Which would suit me as the Philips BC series have quite high ESR.  I'd use a better radial and save board space myself.
Just use the Philips for B+ filtering etc.

[abbey road d enfer]

Although I'd still want to use the tantalum 10uF cap on the +ve input to retain that particularly Nevesque distortion 😀

Considering the very low level across this cap, I very much doubt there's any resulting distortion.

I'm also thinking that since the amp is now not driving 50 ohms but, at the lowest, 200 ohms, the feedback network can be reduced in impedance to lower noise.
Make sense or you think I'm off base?  It was good enough for Rupert but that doesn't mean we can't better it

Since the input transistor runs at about 200uA, the OSI is likely to be about 1-2kohms. Indeed, decreasing the NFB elements by a factor 2 could reduce noise, although so tinily it may not be perceptible. Anything below that will meet with the law of diminishing returns.

Edit:. Also, I don't think these spots on the 1081 were the old Philips axials, I think they were better types that were radial.  Which would suit me as the Philips BC series have quite high ESR.  I'd use a better radial and save board space myself.
Just use the Philips for B+ filtering etc.

Current 'lytics are so much better than they were then, almost any decent type will be an improvement.

[mjrippe]

Following this thread with much interest.  We have a Neve 53 series board here.  Also a "Green Phone" SABC 54 type board with 4 discrete 34127 channels that we would like to add direct outs to.  Definitely into buying some boards.

[Winston O'Boogie]

Considering the very low level across this cap, I very much doubt there's any resulting distortion


The Neve guys will tell you differently.  Changing the tantalums for something else is a big no-no on any Neve gear.   Myself, I will say that there's a definite difference that is measurable with standard test gear.  Not sure what Cyril Bateman said about the distortion through tants but there is some at these signal voltages. 


Since the input transistor runs at about 200uA, the OSI is likely to be about 1-2kohms. Indeed, decreasing the NFB elements by a factor 2 could reduce noise, although so tinily it may not be perceptible. Anything below that will meet with the law of diminishing returns.
 Current 'lytics are so much better than they were then, almost any decent type will be an improvement.



I got more than 200uA for the input transistor myself (although not as much as the Neve doc said) but even a factor 2 reduction in fb values for a smidge better noise is worth it I think.

Yes, anything made now in electrolytics is better - my point being that I would use something in radial footprint for -tve in and output coupling caps to keep footprint on board size down. 

[Winston O'Boogie]

Following this thread with much interest.  We have a Neve 53 series board here.  Also a "Green Phone" SABC 54 type board with 4 discrete 34127 channels that we would like to add direct outs to.  Definitely into buying some boards.

Hey, OK cool 👍. 
Well, tell us what would work for you.   I've got no vested interest in this, just felt that a real Neve desk with spare Neve transformers deserved some Neve amps.  Of course there are easier ways to do this as you know. 

[atavacron]

{Abbey:] I don't see the need for such a big cap in the C2 position [1081=C35]. With a typical value a a few kiloohms for Rg, the -3dB point is <0.1Hz. Considering the LF response is dominated by the xfmr, I would set this cap at about 10-22uF. Will save some space, at the benefit of the output or decouping cap.

Smart. The 3405 uses a 22µF/16V in this position. Image attached. Still can't figure out how to do inline images.

[Winston:] If the cap is at the inverting input junction (1081), the cap is within the loop.   Non linearities of it are then equalised.

Good. That's the external IN– point then. Might want to make the pads to accommodate a 150µF, even if we go with 22µF in this application.

[Winston:] I'm not sure I see this as a group project for widespread use though.  Maybe I'm wrong?

This wants to be a 12-channel board at 35mm per channel in a v-groove snap-off form factor, which will make for easier mounting in the desk (though each channel does need to have its own mounting option). If I'm going to have 'em made, I want to provide an inexpensive way for people to use just a few channels for projects.

Also I have a 16-into-4 unit that I'm developing that could use four of these channels – rather than mounting BA489s to proto boards with card edge connectors, as was the previous plan. In that application I will need to voltage sum into the inverting input, so I realize we need to have both an IN+ and IN– on the header that also has the 24V and 0V inputs.

I don't think the power wants to be bussed across the boards, as B– impedance rules are so strictly tied to signal path length, and the central B– reference in these desks is the buss channel. Each channel should be considered as part of the module it is wired to.
 In my 16-into-4 application I can jump power across. Others can too.

[Winston:] Although I'd still want to use the tantalum 10uF cap on the +ve input

I'd agree that we want to keep the signal path the same within the amp layout itself, save for necessary substitutions (your transistor sub for instance) and that temperature protection with the LEDs, and whichever bias spreader trim technique you feel is easiest at setup time.

[Winston:] How about  If I lay out a single channel and post it here or pm it to you

If you want to go minimal at first, four channels on a board would be smart, as I could use the prototypes for the other project I mentioned above, and would have some spares.

[Winston:] Having a place for another gain boost  resistor in parallel was just replicating the function that was brought out to the rear connector on Neve amplifiers.  A trimmer is fine too if that's best for you.

Pads for either a single fixed Rg or a 3/8" top adjust trimmer please, in the same board position.

[mjrippe:] Definitely into buying some boards.

That's the idea! DM me with channel count and config. I hope a 12-channel snap-off form factor will work for you too.

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