Another RCA BA-2 / OP-6 hybrid

[vinyvamos]

So now that I had so much fun building a BA-6A I decided to tackle a pre-amp based on Chris' (Pres. of Sound) idea of building an OP-6 with a first stage from a BA-2, due to the lack of info on the bespoke attenuator. Very good discussion on that subject right here in fact!:

https://groupdiy.com/index.php?topic=50018.0

I did try Ian's idea (from the thread above) of the 1meg pot with resistors to GND either side, and V1 wired in pentode as per OP-6 schematic, but that only gave me 56 - 79dB of gain range (250 ohm input). Frequency response was fine. Then I tried it with the normal 100K pot and no NFB, and I got similar results for gain, F.R. and noise as I had with the triode (BA-2) configuration. Just food for thought, and fun to try it I guess!

So I have essentially built two pre's into one 4U chassis, I have original 900849 input iron, am using 6SJ7 as it's what I have on stock, plus it's a bit easier than using 6J7 or the original 1620 tubes with grid caps. Spec should be the same bar them not being long-life/special quality blah blah... yes? / no?

For the output I am using the same as Chris; an EDCOR WSM 15K:600 cap-coupled to the plate along with the Hammond 156C choke (note that for all tests so far I have been using a 44H choke from an old AM rig, so LF is suffering until the hammonds arrive!). Note that I have managed to improve the LF to be -3dB at 20Hz with some phase advance, by increasing the coupling cap from 250nF to 1uF. I need to learn more on calculating this value...

I have been running many tests and have learnt quite a few things. Firstly, that input trafo rings like hell around 16KHz if it isn't loaded with circa 50K (nominal imp.). To load it or not to load it, that is one question... would you put this on a switch at the back maybe? Then I can try it when finally in a recording situation to hear the differences...

The other question I have for you good folks is regarding the impedances of the input trafo. I am currently designing the front panel for this thing and am trying to decide what and what not to have at the front. Would you put a switch for 30/250 ohm impedance? Or am I never going to use that 30 ohm tap? Bear in mind that with the secondary not loaded with the 50K I have a lot higher than the nominal impedance, measuring at about 400 ohms on the 30 tap and 3K ohm on the 250 tap (1KHz)... I have never used a mic pre design of this vintage, so I am uneducated right now in how I can or cannot use these particularly low impedances...

The other interesting fact is that with the BA-2 first stage, which is a pentode in triode mode, I am still getting close to the stated 88dB of gain when using the 30 ohm taps on the input trafo. 85dB in fact. Now I had assumed that I would get far less gain with this stage being a triode format, but no...

Any input you might have on what I should be putting at the front panel would be much appreciated. I definitely want a 20dB mic pad, a Hi-Z DI input, maybe a line input, and I have also toyed with the idea of adding phantom power. Oh and I will have a basic stepped output attenuator for driving the beast hot hot hot.... One big question right now is whether I should have switches there for the input impedance...

Happy New Year from the cold Eastern edge of Iceland!

 

[emrr]

Unless you have 30 ohm mics, almost no value, unless you just want an option to load the mics down too heavily so they bandpass. 

Original input iron measures somewhere around 26dB gain going into the 250Ω input with an unloaded sec.  Ratio math tells us 50K is way too low a load even to arrive at a MATCHING condition, which you never want with mics.  Let it ring, let it fly, that's the classic sound everyone is chasing, not the 'fixed' one.  Or if you gotta 'fix' it, you want a Zobel, which you'll have to figure out.  I've never seen the upper range resonance tamed that didn't totally kill everything above it, so it's a trade off, no matter what.  Consider RCA's predominate theory is use of those resonances and the occasional designed-in EQ boost to counteract the lack of top end in ribbons. 

 

[vinyvamos]

This is exactly what I needed to be told, in order to stop pondering these options, thanks! Yes I was just passing some music through it there and the response change between loaded and unloaded secondary is negligable, plus I lose gain too (not that there aint piles of it in this thing already!). Will leave the sec. unloaded then .

As for the Zobel, I did try the approach with a variable cap and as you say, it totally killed the top end so I ditched that plan.

I will give more details and outcomes when I get the Hammond chokes and fit them. Hopefully she's gonna be a sweet sounding beast

 

[emrr]

good luck!  I want to try doubling up those chokes in  a hum bucking fashion for greater L and better hum rejection too. 

 

[vinyvamos]

good luck!  I want to try doubling up those chokes in  a hum bucking fashion for greater L and better hum rejection too.

Thanks Doug . So do I need to be aware of inductive pickup issues when placing these chokes? Just as I do with input and output trafos. This is my first time building a choke fed output stage. I guess I can do the listening test on the choke while moving it around the chassis. I usually use the scope and headphones. Had great fun with the input trafos as I got it wrong the first time and had them all nicely mounted!

I am actually thinking to mount the EDCOR and this choke within one screened can, just for elegance on the chassis top really, but I guess I should avoid orienting their cores in line, yes? This could cause various issues depending on the phasing...?

 

[emrr]

https://groupdiy.com/index.php?topic=54109.msg818282#msg818282