RTS-1400 Battery Powered Preamps

[bruce0]

Moved from Portable Preamp Thread.

With respect to RTS-1400 (schematic attached) I was really just looking for specs on the frost output transformer.
There is nothing online.

They all seem to have Beyer or Jensen input trafo's and they ALL seem to have Frost output transformers.

I put the full Technical Manual in the Technical Documents section: http://www.groupdiy.com/index.php?topic=53073.msg677365#msg677365

Anyway, I bought a couple of dozen of those RTS preamps a few years ago for $10 each.  The frost magnetics transformer in them is unspec'ed on the internet.  I bought them for the parts (nice little box, and two nice transformers in each).

I use 1 that I keep in my portable toolbox, but rarely need it. The rest I use as a source of transformers, connectors, etc for other projects (I built a couple of ssl 9K's with all the 5532's).  Most of them had Beyerdynamic 1:10 7 pin PCB mount shielded studio transformers for inputs and Frost for outputs.  A couple had Jensen 1:8 input transformers (JE number not JT whatever that means).

Anyway in my test the Frost outputs seem to work well (quadfilar wound #32 wire about 400+ turns on a EI lam ) but only with low impedance transistor drive (too much interwinding capacitance).  It is wound similar to a JT-123slpc but has much lower inductance (jensen 9.43 henries, frost I think a little less than 1)  I assume the core is steel vs 50% nickel for the jensen.

The beyer I have used on tube preamps with success.

Anyway I was looking for projects (ideas welcome)  that could consume these many frost trafo's (I have made a reamp box, some transistor preamps, and tried re-winding a couple with little success.

 

[mjrippe]

[Copied from the previous thread]

The JE/JT designation is simply a matter of when they were manufactured.  I forget the year they switched from one to the other, but that is all it means.

Interesting, I have seen a couple with Jensen inputs only and Frost outputs.  The single one I have is the only Jensen In/Out that I have seen so far.  It is a later version with a darker grey (anodized?) case. 

[snip]

I used four of the Frost 600:600 in a transformer/attenuator box along with four Triad A-67J and four Western Electric 111C.  The idea is you send a hot level into the box, saturating the input xfmr, then go through a stepped attenuator to drop the level and out through another xfmr.  It's a "flavor" thing and you can switch between the different xfmrs with the Frost being the most "lo-fi".

 

[bruce0]

The idea is you send a hot level into the box, saturating the input xfmr, then go through a stepped attenuator to drop the level and out through another xfmr.  It's a "flavor" thing and you can switch between the different xfmrs with the Frost being the most "lo-fi".

So which are you using for the input transformers?

I get the idea... ( jack - switched transformer choice - attenuator - output transformer - jack ) but what do you use for the output transformer?  In fact why is there an output transformer ?

 

[mjrippe]

The whole crazy thing is 600 ohms. 

It goes XLR -> Input Xfmr (600:600) -> Daven Attenuator (600/600) -> Output Xfmr (600:600) -> XLR

The Output Xfmr is there to properly load the Daven and give it one more dose of "that transformer sound".  The first one I built with UTC transformers is used in an SSL room.  The owner wanted to run his signal through some iron.  http://analogaudiorepair.com/2013/06/29/transformer-saturation-box-wutc-davens/

I would be interested to see some of your projects too!

 

[bruce0]

..snip..
I would be interested to see some of your projects too!

I don't have any pictures of the ones that used the Frost.  Here are some pics of projects I have done (I am new to this stuff.. so some of it is a bit crude).


A Scott Hampton Low Cost Mic Preamp Tube preamp , 4 channel (this used 4 of the Beyer 1:10 input trafo's)

Some Royer Mod mic's. The little short LDC one ended up with a Beyer running reversed as the step down, works great

A G9 from AudioKitchen Boards, modded to have an external PSU.

Some 51X projects...
- PSU #2
- PSU #1
- TASCAM Connector Mod for the 51X rack
- Meter PPM or VU for the 51x Rack or anything really
- GIX51X Rodney's great G9 for the 51X rack project

The only thing I have used the Frost successfully are 2 Scott Hampton JFet Preamps, and a couple of 51x modules of my own design and I don't have pictures of those online (actually you can see 1 in the PSU#1 picture above).  Beyers I have used in a direct box too i think.

 

[mjrippe]

Wow, great stuff!  Will have a bunch of these parts kicking around...

Also, the client with the Frost/Triad/W.E. saturation box contacted me today and said "That attenuation box with those trafos from the RTS modules added the missing link to the album I'm mixing.  The band is through the roof right now!"

So I guess they're good for something ;-)

 

[bruce0]

So you used the Frost trafo as an input transformer (so it is the one getting overloaded, right?)

Some other projects...

Trying to Build a copy of an Peerless S-217-D
Messing about with building test equipment
A coil winding counter
A kit based 51X 1073 without the EQ section
An panasonic electret capsule mic biasing head amplifier
A rebuild with mods of an Oktava MK-012
Bruno's 51X SSL 9K5 preamp

 

[mjrippe]

The Frost are both input and output.  There are four total as it is a stereo unit.  You can switch so it uses Frost, Triad, or W.E. but it switches both in and out together so you are only going through one type at any time.  You could do different pairs on left and right for two mono signals.

 

[mjrippe]

Hmmm, was just reading the Technical Manual (thanks again) and at one point it says the limiter is 4:1, then later it says 5:1.  I thought that when I measured one of mine it was 10:1, might have to re-check.

 

[bruce0]

The Frost are both input and output.  There are four total as it is a stereo unit.  You can switch so it uses Frost, Triad, or W.E. but it switches both in and out together so you are only going through one type at any time.  You could do different pairs on left and right for two mono signals.

Oh... I get it.  Well that would consume a few... Maybe I will try that. 

Point to point I assume?

 

[mjrippe]

Yes, all hand wired ;-)  But the Frost xfmrs are PC mount so I used some perfboard to mount them.

 

[hereforever]

I just picked up 4 of these for cheap and I'm planning to rack them up for phantom with a power supply

I was hoping someone could suggest a practical design for adding a instrument/hi-Z input after the input transformer?  I could I just wire a shorting jack in between R1 / R2?

 

[Bo Deadly]

I just bought one of these. In addition to the very cool switch guard nuts, this is actually a really nice circuit. The 5534 sips power. The vactrol looks like it only needs ~650uA to get to 2K3 which would yield ~-20dB attenuation with a 300R source (like SM57) but it would probably never even get to that because of the limiter feedback mech. So it should last a looong time on battery. At high gain settings it's probably the quietest pre one could ever make.

I'm thinking about some mods though:

1) To get the limiter to kick in sooner to use as an effect, I'm thinking of wiring the OT secondary in parallel. I guestimate at around +1.5dBu instead of +7.5dBu. Should be great for micing a guitar speaker with an SM57 and pushing the gain to make it limit. With 3.5ms rise and 500ms decay it might sound pretty nice. Then ...

2) Increasing C9 across the vactrol LED should lengthen the rise / decay and stretch into borderline compressor territory.

3) Reduce the size of C2 in the gain network to 10u or maybe even less to get low-cut in the highest gain position. The higher the gain, the higher the Fc. That increases dynamic range which dovetails perfectly with higher gain settings. And it pushes the limiter / compressor. Low cut is kinda important IMO.

4) Add a proper gain pot w/ knob. Where can I find a decent C25K reverse log? Or maybe C10K? Otherwise, I'll have to go with a stepped switch provided there's room.

5) Power indicator LED so you remember to turn it off. Super dim red (2mA maybe) to conserve that battery.

What do you think? Problems? Better ideas?

 

[mjrippe]

Hi Squarewave, you've stumbled on one of my favorite little secrets!  I have bought, modded and sold dozens of these little guys.  https://analogaudiorepair.com/2013/12/31/4-channel-mic-pre-limiter/  Also easy to make into a 500 series module, I have some of those for sale.

To address your questions:

1) Replace R9 with a 10k pot for ratio/threshold sort of control.

2) Yes, changing C9 changes timing

3) Interesting idea.

4) Try Small Bear Electronics for the rev log pot.

5) Sure, LED is a good idea.

As for better ideas, if you don't need the portability, go with racking or 500 series.  They will run happily off of +/-15 or 16VDC, just replace the electrolytics with 25V rated types.  I may try breadboarding one with a 2520 style DOA to see if that works. 

And of course swapping different optos may be interesting.  Some of the early ones I have use CLM6000 optos instead of VTL5C2.

 

[Bo Deadly]

Nice work. Racking is a great idea because then you can buy the really ugly ones for cheap and just toss the enclosures. The one I got has a pretty nice enclosure and it probably runs for literally days on battery so I'll definitely keep it as is.

It's hard to fault these. It looks like some commercial anti-mojo thing you would find at a TV station but in reality it's basically an API 312. The only knocks are no phantom and the input transformer isn't magnetically shielded. It could be quite nice for micing a cab with an SM57. Presumably it has an interwinding shield.

I'm really interested in how the vactrol sounds when pushed. I suppose it could turn out to be a little too slow for many things. But then maybe reducing C9 would be in order.

 

[hereforever]

The RTS 465 is basically the same circuit with a built in PSU + phantom (switches on the rear)

There is a gain mod listed here- http://www.watamix.com/rtstw/#Q%2020

For the 465: change R13 from 300 to 75 ohms for a 10db boost.  That corresponds to R4 in the 1400 schematic, however the value shown there is already 75 ohms.  And fwiw, there were already 75 ohm resistors at R13 in the pair of 465's I have

At some point I hope to get around to changing the slotted gain control & moving phantom to the front