McCurdy AT320A module-any information?

[P.O.]

Hello there!

Just got a rack of 8 McCurdy AT320A modules from a local radio...
Googled all over the place, can't find any reference to those. Does anybody here have some info as to what is their function/pinout/etc?
It looks like they are input stages for console, discrete 11 transistors design, one transfo.
thanks!

 

[mjrippe]

No actual docs, but my guess would be a distribution amplifier.  The AT305 is a program amp, but the 310, 313, 317 are all distribution amps.  Eleven transistors would make sense because they would be driving multiple outputs.  If you can post a photo or two maybe we can be sure.

 

[P.O.]

Thanks for your response!
here is a photo of the card...
Since I could not find any info, I decided yesterday evening to tackle the problem myself.
It appears the thing is some sort of buffer/line amplifier/driver, not unlike the Ward-Beck M124.
Also not unlike those Ward-Beck cards, it seems to be 48 Volt powered.
I have studied the wiring on the card cage they came in and the cards themselves, and came to the following pinout:

1 input, transformer coil 1A. coil 1 18.8 Ohms DC
2 input, transformer coil 2A  coil 2 18.8 Ohms DC
3 input, transformer coil 1B
4 input, transformer coil 2B
5 connection to some capacitors
6 Traces on PCB with no components...
7 GND
8 goes to big capacitors on card. Maybe part of "gain control" or another output
9 GND to cover
10 Output +
11 Output -
12 traces to inner circuit. connected to 8 via 1.8K resistor on some card sockets. "gain control"?
13 GND
14 Supply voltage, +48V
15 GND

I started with lower supply voltage, it gave distorted sound. 48V seems right, but I still get distortion on peaks.
Transformer is marked "Hammond", "2015" and "97879". some are just marked "2015".

As a quick and dirty test I wired it with a microphone at the input and Beyer 100 Ohm headphone on the output, and I confirm it has enough gain to drive the headphones to "loud" level (about 1.5V). On this test I connected dynamic microphone to 1 & 4, and strapped 2 & 3. I don't know what effective impedance it gives, but it measures 37 Ohms DC.
I still have to investigate what this Pin 8 and pin 12 circuit is. And to assess the sound...

 

[mjrippe]

Good work!  The 2015 is the same transformer on the AT317 DA cards I have.  Here is my drawing of it.

I also posted the AT242 schematic in the Tech Docs section.  It is a ten transistor "universal" amplifier and may offer some insight. 
https://groupdiy.com/index.php?topic=45018.msg972545#new

 

[P.O.]

Thanks!!
I'll really have to investigate what happens between pins 8 and 12, I have a feeling it's some sort of feedback path. The Ward-Beck M124 has something like that to control gain. Similarities like that lead me to think they may have been designed by the same guy...
I'll be curious to know if the sound is OK. I do think this card has way too many components for its own good... 11 transistors, 15 electrolytic capacitors, not exactly a "purist" approach.

 

[mjrippe]

Referencing the AT242 schematic again, there is a place between the first and second stages where a gain control can be wired to the connector.  Attached is a pic of how I think it would be wired.  In this case it is not a feedback resistor, but a simple pot to ground.

 

[P.O.]

I just conducted some more tests, and unlike the AT242 that has an "insert" point between front-end and driver, points 8 and 12 of the AT320A really appear to be a feedback path. I can change the gain (which appear considerable, at least 50dB open-loop) when I insert resistance between those points (less resistance=less gain), and if I put a capacitor between this resistance the high frequencies dissapear. It sorta looks like a discrete op-amp...

 

[mjrippe]

That definitely sounds like a feedback loop!  50dB gain is plenty for preamp use, what is the output level before clipping?

 

[P.O.]

I am not there yet, I would need the oscilloscope set up for that and frankly, my bench at the moment is far from an adequate testbed. I will though eventually get back to these modules and do a full report: it intrigues me greatly!
In the test I had done frequency response appears good, reasonnably flat from approx 20Hz to some 40KHz.
There may be a future for them...

 

[P.O.]

Output level could be frightening if you think that it's powered by 48Volts and the output is balanced!
between +20 and +30dB?!!

I find it peculiar that so much information is available on Ward-Beck stuff, and yet so little is for McCurdy...

 

[mjrippe]

Until a few years ago you could email McCurdy and pay to get a copy of any of their schematics.  Unfortunately the gentleman running the company has since passed away.  There is another company that was in negotiations to take over the brand and related IP, but now they have to deal with his estate which has slowed things down.

 

[P.O.]

Some more hasty testing with the Ferrograph test set. Onset of clipping was assessed by ear, so it is not very precise...
Arbitrary position of my "gain" pot, I get an output level before distortion of 16V (+26 dB) for an input of 30mV (-28dB), which gives me a gain of 54dB, .33% distortion. Re-checked open-loop gain, which now looks like 80dBs!
I'm putting this project on the shelf now, I'll be back when my test setup is straightened up ans I have more time.

 

[mjrippe]

Excellent, sounds like you are pretty close already!  Will be watching for future news.

 

[P.O.]

Just a little progress: I have lent one AT320 and a M124 to a friend to assess the sound, and he says they are on par regarding the sound (hint: these two preamps use a quite similar Hammond input transformer). That's good news. Like I observed, he says the McCurdy has a tremendous amount of gain. That sort of reinforces my theory of a "discrete opamp". I really wish I could find some schematic or information on this card. I just hope I won't have to recap those babies, cause they are full of axial electrolytics...

 

[mjrippe]

That is good to hear!  I've picked up a few more McCurdy docs lately, but nothing that would help.  Will keep looking 

 

[P.O.]

Thanks!
it's good to know I have at least one ally...