Spectra Sonics 110A build - Student in Dire Need of Help

Hey all,

I'm a pretty new DIYer and am working on building a Spectra Sonics 110A from scratch based on the schematic.  I have the circuit breadboarded and have been testing it to get it functioning before I put it on a PCB and chassis, etc.  However, the output is very distorted.  I have a 600:600 transformer on the output but none on the input (I plan on acquiring the appropriate input/output transformers when I get the circuit working).  As far as things within the circuit, some of the transistors and resistors heat up excessively (I've had a few resistors fail).  Essentially, the main problem seems to be that the circuit is drawing way too much current.  I'm not totally sure if I have all the inputs/outputs hooked up right or not.

Does anybody have any ideas or suggestions as to how to get the circuit working properly?

Thanks!

Schematic. Voltages.

search for the 110 it's been discussed before it runs off +/-24VDC, where the 101 it's predecessor runs off a single 24VDC. if you do some searching, you will find the pdf's on the data sheets for the 110

oh and here is a pdf scan of spectra sonics info regarding the 110

http://www.technicalaudio.com/pdf/SpectraSonics/SpectraSonics_110_amplifier_manual.pdf

You have to make sure you ground pin E & F together along with the input and output transformers, otherwise it will distort. Check the schematic. I've just racked 4 units based on that and it all works just fine, but not grounding either pin E or F will lead to distortion. Also remember that it is a bi-polar pre unlike the earlier single ended 101A.

Hope that helps!

Cheers

Matt

Sorry to bring this thread back to life, but I am racking a pair of Spectra Sonics 110 cards. I have a pair of Triad A-67 line transformers for the inputs and am going to use them as 600:600. I've tied the appropriate wires together, but am unsure on which wires go to the XLR ins (primary) and which go to the card (secondary). I see Joe Malone's wiring diagram, but need to know which color wire goes where. And does the ground wire on the transformer go to just the XLR in?

Thanks for any help,
Adam

It should say it right on the transformer - this is from memory so verify the colors with the legend on the a67j...

The best thing would be to run a piece of 2 conductor shielded cable from the XLR to the transformer - pin 2 conductor to blue and pin 3 conductor to red/white...

Connect the shield to pin 1 and HF ground pin 1 to the chassis via a .47uf ceramic cap right there at the connector.

Now run a piece of shielded 2 conductor from the secondary to the 110. One conductor from green to input +, the other from black/white to input -....

Now connect the shield of the XLR/Primary cable to the shield of the Secondary/110 cable so the shield is continuos from the input pin 1 to the 110.

Now here is the most important/tricky part.

At the card edge connector, connect the shield to input - and put a big blob of solder from input - to output - and then ground that node to the PSU 0V point. That provides a ground reference for the microphone via the shield without ohmically referencing the chassis.

Finally tie the uninsulated lead from the a67j to the PSU 0V....

And that should do it...

If you need, I make some PCB's for the a67's and HS66's that make these connections, and mounting a snap...

Tell me, are you doing a 2 amp stage preamp? Or just a single 110 per channel?

How are you dealing with input padding?

Using and output transformer?

Let me know if you need any other help with this, and good luck - you're going to love it!!!

Timothy

Sorry for a late reply. Busy with KIDS!! Thanks for the wonderful info. I was able to decipher the wiring of the A-67s but have them, wired as 600:600. Now I am reading that a lot of people prefer the sound of them loaded 150:600? Maybe I should switch them around. As far as input padding, I bought these boards from a forum member and he included the trannies, a 1u rack with a wonderful Front Panel Express engraved panel, and also two custom etched input padding boards, each having the in and out and 8 separate resistors for the gain settings, and some 12 position 1pole Alpha switches. I'm still trying to figure out if I should just use the standard 3 position gain/pad settings and a 10k or 20k linear pot for output trim.

And I was considering some Jensen JT-11 output transformers.

Oh yeah - i remember seeing that for sale - congratulations!

the early Spectra 1020's were done 600:600 into 101 cards but with the switch to the 110's they began to go 150:600… The noise specs are slightly better for a step up input, but you'd just have to see - there will be some performance loss in the a67 implemented that way… I would probably try it out as you have it and see if you think it needs it...

as for switching the gain resistors - i would strongly recommend not doing it - the cards are just not made to be used like that - with a 10k7 resistor there you get 40dBm of gain, which gives the best performance, if you want more gain you can put a 42k2 and get 50dBm but introducing all that extra stuff into the feedback loop is just asking for trouble - you are going to increase the risk of HF oscillation and instability, and ultimately degrade the performance of the amplifier.

If I were you I'd do the input padding the way he has it labeled, which would require a 3 position 2 pole switch and a few resistors and then instead of adjusting the "gain", just put a 2k5 log pot across the output of the 110 and either get another set of 110's for output drivers, or build a small output driver with a 553*… The later 1024's were done this way, 110a's on the inputs and IC drivers.

that way you can set the 110 to do the heavy lifting -a 10k7 feedback resistor for 40dBm of gain and the best performance, and then an output attenuator to adjust the output…

the Jensen's look great, but i have never heard them - you could also use any 600:600 transformer, Triad HS66's were the traditional ones, the Stax console had UTC A20's I have some Hammond 3002's that are nice - there are countless options...

let me know if i can help!
T.

I think I'm going to use the 10.7k fixed resistor. I've got some input gain switches from an unused Green Pre that I may use for input, but would I wire those directly before or after the input transformer? And where would I add my output trim pot?

Thanks,
Adam

Hey Adam,

you can put the pad after the a67j, before the 110 card,  but from what i can find on the gain switch for the green Pre it looks like a 1 pole 12 position switch which I can't really think of a way to make work - in general you want 2 poles so you can switch the the series and the shunt resistor…

the output pot will go across pin H (+) and F (0V) and then the wiper will feed into output driver amp, which could be another 110, or a lo gain opamp circuit, which would drive the output transformer…

I think its a good idea to use the fixed gain tho - you'll be happy with it once its up and running….

not sure if you are still working on this, but here are some switches that might work for the variable pad.

http://groupdiy.com/index.php?topic=60205.0