> add a high turns ratio output transformer like 6:1 or 8:1 for lower gain and lower output impedance. So the low current output of the opamps will see a high impedance on the output transformer secondary.
Well, yes, but what is the max output voltage now? 11v peak from this amp, step-down 6:1, is 2V peak or just about +6dBm, and we want like +18 to +28dBm.
The 5K rated load and the 600 ohm pro-audio load suggest a 2.9:1 transformer. That gets us 4V peak or +12dBm. I've got stuff that level, but the whole system is designed around it.
We really want at least 14V peak and over 24mA peak, maybe twice that much current if feeding iron. That's just beyond this module's ability no matter how you transform it.
> Hows about something like this?
That is the most direct way to get pro line level out. The drawback of I.45 is that its power efficiency is around 5%: big heat for reasonable output. A practical compromise for 600 ohm load is Re=300 ohms, which gives 15V/300= 50mA standing current. Heat in transistor and in Re is about 0.75W each, suggesting a 2W resistor and a 2"x2" heatsink. 2N3055 would be suitable (and an appropriate vintage). For many reasons (including blow-up protection), add 50 or 100 ohms in front of the 600 ohm transformer.
+/-15V supplies are a bit low for high line levels; that's why all that gear that runs at +/-24V or with step-up transformers. You could go to a 150:600 transformer. It adds 6dB of gain you have to lose somewhere else. Your Re must be lowered to 75 ohms, maybe 100 ohms, idle current 150mA-200mA, 3W dissipation in transistor and in resistor, over 12W dissipation in a 2-channel box. Will be warm. Will need more than a 24VA power tranny to feed this.
The "problem" with this is that you already have a lovely package of vintage parts, and need to tack-on this booster. However it is just two/three parts, and nothing that could not have existed back in The Day.
The text promises better performance with I.46 below. You have not posted that but I can guess. And shudder. I fear it is the second coming of crossover distortion. If not, then it is a major project in itself and will clutter your neat package.
I said it elsewhere and I'll say it again: you could hide a 5534 under the Phil Brick and get modern perfomance with vintage smell.
Go back. What gain do you need? Lo-output mikes on female choir direct to disk, I sometimes need +57dB. But hot mikes 6 feet away from large percussion, I've run less than +6dB and still clipped. If you work with hot mikes in a small room with loud drums, you hardly need gain: AKG 414 will deliver around a volt directly. Use a 150:600 or 2K:10K transformer for a little gain and a place to put phantom, and run right into a Line Input.
And what parts do you have? The 1:2.8 input iron is actually pretty good for an op-amp of this class. I'd go as high as 1:5 for ab-fab noise performance, but 1:10 would be too much, and the difference with 1:2.8 isn't any problem unless both your room and your performers can be ultra-quiet. The 20:600 output iron is just wrong for this use. The 20 ohm side can't be fed more than about 3V to 5V peak before you get too-high output level on the 600 ohm side, and probably significant distortion. And the cascade of 1:2.8 input iron and 1:5.5 output iron gives gain of 1:15 or 24dB, -without- any voltage gain in the amplifier (but it must have enough current gain to buffer 10K into 20 ohms). Many many hot-close mike situations need less than 24dB gain. While they are very pretty things, a 600:600 is more what you need here.
