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

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emrr

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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.
Click this link for larger pic
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My3gger

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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

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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

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Jan 20, 2017
Messages
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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

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Jan 31, 2019
Messages
5
EmRR said:
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

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Messages
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Location
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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

Member
Joined
Jan 31, 2019
Messages
5
EmRR said:
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

Well-known member
Joined
Oct 26, 2017
Messages
71
EmRR said:
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
33726035625_96c1c64ea9_c.jpg


32883563004_a04b74f809_c.jpg


33685335056_3abd428159_c.jpg


32883564634_4464d79b42_c.jpg


32912735043_aab8f87e79_c.jpg


Click this link for larger pic
32883566814_9a7e680841_c.jpg


Click this link for larger pic
33341726520_646cbc6a43_c.jpg


Click this link for larger pic
33341727000_dd88012db7_c.jpg


Click this link for larger pic
33341731510_7dfa2ea1c3_c.jpg


Click this link for larger pic
32912743073_cf44671973_c.jpg


33685345376_446643d4cf_c.jpg


Click this link for larger pic
32912738983_b99caefc36_c.jpg


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
32273721660_90a8fa0a70_c.jpg

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

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Messages
7,849
Location
NC, USA
skippo said:
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

Well-known member
Joined
Oct 26, 2017
Messages
71
EmRR said:
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
 

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