Collins Radio Co. 356A-1 preamp gain mod and rackup

[emrr]

Here's a process log on a recent Collins 356A-1 preamp restoration, mod, and rack-up.  These were for a client, he had them already and sent them to me.  He wanted all the modern conveniences, along with padding for line use, so the input pad is -20/30/40.  These are fixed gain as stock, I sorted out a feedback path mod to increase gain without giving up the tertiary winding path entirely; there’s another thread that addresses that.  Max output level is high enough to justify an output attenuator, so it’s on this one.  The output load switch is kinda pointless using a modern CAPI/Hairball 600 ohm T, as it’s minimum loading is 1K, unlike a correct T which goes open at the off position, so the load resistor is a 2K2 for a minimum load of 710R.  I found a real Collins logo for the front panel.  PSU's are all DC and off-the-shelf.  Linear B+ and filament, switching 48V. 

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PSU.  Required iso diodes to avoid latch-up through shared A-/B-.  Added second DC output jack for use with theoretical additional preamp rack.
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[musika]

Very nice!!!

 

[mjrippe]

Beautiful work, as usual, Doug!

 

[My3gger]

Very nice and it is always interesting to get well informed about American approach to pro audio. Why is input cathode cap C202 of so low value? They could easily pick foil type instead what looks lytic. C 203 seems coupling cap, i'm not sure if C204 makes filter with R205. Interesting circuit which could help making layouts because HT is below. Maybe CJ's joke about German audio and amphetamine paranoia wasn't just joke Wish we had modules from them or any other good European for cheap, American are not well known at all. Those who know them like their sound a lot, like that "sound glow" from BA-2, very exciting.
I'm just finishing modifications with very similar controls as yours here. Added IT primary switching for ribbons and typical 150-200 ohms mics, might not be needed today with active ribbons. I'm still not sure if better new models have ~200 ohms impedance and higher gain than passive. No one replied. Output control will be 1k pot, works well with NYD schematic having resistors before it.

 

[emrr]

Look again.  That's not the cathode cap. 

But if it was, it'd be for treble boost. 

C204/R205 is addressed in the link about the gain mod. 

 

[audiogeeks1]

Would you be willing to post a diagram of the gain mod? It's not 100% clear from the description in the thread linked.  thanks

 

[skippo]

PSU's are all DC and off-the-shelf.  Linear B+ and filament, switching 48V. 

Hi Doug,
Really beautiful work on this, I've been looking at some of your other posts too on these preamps. I going to be racking up some 356-As soon and I was wondering if you had a link to the power supply you used?

Also the plug in connectors on the modules look new, do you have a source for those?

Thanks for your help!
best
Asher

 

[emrr]

International Power for B+ and filament, both from Mouser.  The 48V is a Meanwell, I've mentioned it in a thread about phantom somewhere here.  I put another RC filter on it's output.

https://www.mouser.com/ProductDetail/Mean-Well/RS-25-48/?qs=sGAEpiMZZMuEhFEw3BeJbc0V%2fVK7k4iujqnPf2qQDII%3d

The connectors are intermittently available from several different manufacturers, it's different every time I look.  If they aren't at Mouser today, they will be in 6 months.  Or from Allied.  Waldom sometimes. 

http://www.mouser.com/catalog/catalogusd/645/2570.pdf

 

[skippo]

International Power for B+ and filament, both from Mouser.  The 48V is a Meanwell, I've mentioned it in a thread about phantom somewhere here.  I put another RC filter on it's output.

https://www.mouser.com/ProductDetail/Mean-Well/RS-25-48/?qs=sGAEpiMZZMuEhFEw3BeJbc0V%2fVK7k4iujqnPf2qQDII%3d

The connectors are intermittently available from several different manufacturers, it's different every time I look.  If they aren't at Mouser today, they will be in 6 months.  Or from Allied.  Waldom sometimes. 

http://www.mouser.com/catalog/catalogusd/645/2570.pdf

Thanks!!

 

[rjb5191]

Here's a process log on a recent Collins 356A-1 preamp restoration, mod, and rack-up.  These were for a client, he had them already and sent them to me.  He wanted all the modern conveniences, along with padding for line use, so the input pad is -20/30/40.  These are fixed gain as stock, I sorted out a feedback path mod to increase gain without giving up the tertiary winding path entirely; there’s another thread that addresses that.  Max output level is high enough to justify an output attenuator, so it’s on this one.  The output load switch is kinda pointless using a modern CAPI/Hairball 600 ohm T, as it’s minimum loading is 1K, unlike a correct T which goes open at the off position, so the load resistor is a 2K2 for a minimum load of 710R.  I found a real Collins logo for the front panel.  PSU's are all DC and off-the-shelf.  Linear B+ and filament, switching 48V. 

Click this link for larger pic

Click this link for larger pic

Click this link for larger pic

Click this link for larger pic

Click this link for larger pic

Click this link for larger pic

Click this link for larger pic

PSU.  Required iso diodes to avoid latch-up through shared A-/B-.  Added second DC output jack for use with theoretical additional preamp rack.
Click this link for larger pic

What is the purpose of the "out" terminals of the phantom switch being strapped together.  It also looks like there's a resistor and cap between the two switch terminals as well.  This confuses me a bit.  Also,  how and where did you implement the isolation diodes for the power supply lines?

thanks,
Ry

 

[emrr]

What is the purpose of the "out" terminals of the phantom switch being strapped together.  It also looks like there's a resistor and cap between the two switch terminals as well.  This confuses me a bit.  Also,  how and where did you implement the isolation diodes for the power supply lines?

Look at the Jensen phantom schematic AS016.  Ground bus to drain the local cap. 

The resistor is a load for the switcher when no phantom is active.  The cap is local filtering, there's also a small resistance between the switcher and the local filter. 

The iso diodes were not added at the time of the pics, they are in series with B+ and filament if I recall correctly.  Seems confusing now, possible it's only the B+, unless I modified the filament supply for higher voltage, which I may have. 

 

[rjb5191]

Look at the Jensen phantom schematic AS016.  Ground bus to drain the local cap. 

The resistor is a load for the switcher when no phantom is active.  The cap is local filtering, there's also a small resistance between the switcher and the local filter. 

The iso diodes were not added at the time of the pics, they are in series with B+ and filament if I recall correctly.  Seems confusing now, possible it's only the B+, unless I modified the filament supply for higher voltage, which I may have.

Thanks Doug,

I guess I found it confusing because it looks like a nSPDT switch is drawn in the Jensen schematic and the switch in your pictures look like an SPST, so did I miss that?  If one just used an SPST switch for the +48 VDC line, then the caps would not drain, correct?

So do you have an additional resistor in your design that's not in the Jensen schematic? Is R1 in your case a part of a separate RC filter at the Input XLR connecter on the back of the unit as seen in the photos or is it the resistor that is the load at switcher?

I think I'm close to wrapping my head around this. 

thanks,
Ry

 

[emrr]

I think you’re looking at the load switch. 

 

[donivan]

what is the b+ and filament values?

 

[mjrippe]

what is the b+ and filament values?

Page one of the manual: 250-300VDC @ 6.5 to 7.5mA, 6.3VAC or DC @ 0.3A

 

[rmburrow]

International Power for B+ and filament, both from Mouser. The 48V is a Meanwell, I've mentioned it in a thread about phantom somewhere here. I put another RC filter on it's output.

https://www.mouser.com/ProductDetai...=sGAEpiMZZMuEhFEw3BeJbc0V/VK7k4iujqnPf2qQDII=
The connectors are intermittently available from several different manufacturers, it's different every time I look. If they aren't at Mouser today, they will be in 6 months. Or from Allied. Waldom sometimes.

http://www.mouser.com/catalog/catalogusd/645/2570.pdf

Be careful about phantom power; the 48 v phantom power must be brought up very slowly, no voltage spike! Voltage spikes can puncture insulation on the (step-up) input transformer. The manufacturer of the 356A transformers has been out of business for decades.

 

[emrr]

Be careful about phantom power; the 48 v phantom power must be brought up very slowly, no voltage spike! Voltage spikes can puncture insulation on the (step-up) input transformer. The manufacturer of the 356A transformers has been out of business for decades.

Nonsense. Pure and simple bullshit with zero proof, just rumor. I’ve had a set of 8 of these and another 20+ vintage American channels in use more than 20 years using the Jensen suggested phantom values.

Except all the channels in use with phantom fed to the primary center tap, as was originally the fashion, when primaries actually had center taps.

Never once in hundreds of channels built has one come back with a failed input.

That V series eurotrash made with silk is what fails.

What the paranoid can do is turn on the pad before the phantom, add some series resistance in between. You’re gonna need the pad anyway with a condenser.

 

[rmburrow]

Nonsense. Pure and simple bullshit with zero proof, just rumor. I’ve had a set of 8 of these and another 20+ vintage American channels in use more than 20 years using the Jensen suggested phantom values.

Except all the channels in use with phantom fed to the primary center tap, as was originally the fashion, when primaries actually had center taps.

Never once in hundreds of channels built has one come back with a failed input.

That V series eurotrash made with silk is what fails.

What the paranoid can do is turn on the pad before the phantom, add some series resistance in between. You’re gonna need the pad anyway with a condenser.

My earlier comment was based on a V76 type input transformer . Jensen product is good and I have read some of their technical bulletins. You are correct about padding down a 356A, Gates M5215, or similar preamp when using a condenser mic. I choose to be careful working on equipment using mission critical components made by long-gone manufacturers. I won't attempt to guess CJ's bill to rewind an ADC transformer. While the cost of NOS EF86 tubes soars, finding quiet 5879's for the 356A should not be difficult and expensive.

 

[mjrippe]

CJ does it for love, not money. Although he is now "retired", he once rewound a Langevin output for me for much less than the cost of a modern replacement.

 

[rackmonkey]

Ditto for me on a Peerless input. Too bad he’s out of that racket.