BA-2 build issues

Hi all,

So I built a BA-2C using a 5u4 and an unknown power transformer that had a secondary of 330-0-330 (ended up raising the R-8 and R-9 values a LOT). Made that decision cause I found a nice chassis with all this crap already on it.

Anyway, got V1 plate to 69V and V2 plate to about 239V. Current measured correctly too. I used a cinemag 3440A in 150 loaded with 360k across the secondaries and a gapped CM-27101. Surprisingly, I'm getting no audible hum at full fader volume on my console with the volume all the way up.

The problems i'm encountering are:

1. The first stage is so hot that the MK47 I made clips it so long as i'm less than 4 inches from the mic. It doesn't clip with quiet dynamic mics or ribbons. If I use an EV-20, full volume is comparable to around 70 db gain on my other preamps.

2. I'm getting a lot of both low and high end loss. I don't know how to use a real spectrum analyzer, but when compared to my supposedly totally flat preamps the analyzer in logic shows about a -8 db difference at 100 hz and about the same difference at 12 khz when fed a sine wave. As I go further, the difference just gets greater.

I would've just posted in the BA-2 thread that already exists, but I got a scary warning message saying not to revive a thread that was last posted in in 2015. Anyone have any ideas?

Sounds like the negative feedback around the first stage is not working. How did you implement the gain switch?  Can you post your schematic?

CHeers

Ian

Hi,

I used the schematic here:

http://www.vintagewindings.com/gen%20pop/8299543VW8335/RCA-Consoles/RCA%20BA-2A%20BoosterAmp.pdf

Only real modifications I made were making a switch to activate the EQ section, raising the values of R8 and R-9, and using different audio transformers (but ones that supposedly have worked for this build before). Used a log type 100k pot for the gain as it is in the schematic.

The only thing I can think of is that I hit the 10 uf cap (C-7) pretty good while drilling xformer holes. Took a bit of coating off the cap, but I presumed the damage was primarily cosmetic.

That 120 day warning message is bullsh*t and shouldn't be there, nor deter you.  It's an entropy invitation, and here we are in another thread when it should be in the original. 

The front end gain sounds normal.  There's almost no way you can put a condenser mic into any old style circuit like this without using a 20dB pad in front of the input transformer.  It's designed for speech with a ribbon.  I can get away with a ribbon on a fairly loud 15W tube amp without a pad, but may see waveform clipping in the DAW, and the gain control will be way down.  I had an MK47 on a fairly quiet singer a foot and a half away the other day, and it required a 20dB pad. 

The EQ bypass switch is also a gain boost, couple of dB. 

If you took out C7 with the drill it'd still work if open, not at all if shorted.  Disconnect it and see if you hear a difference. 

Can't address loss at extremities without seeing a plot.  Sounds like loading issues with the particular transformers. 

emrr said:

That 120 day warning message is bullsh*t and shouldn't there, nor deter you.  It's an entropy invitation, and here we are in another thread when it should be in the original. 

The front end gain sounds normal.  There's almost no way you can put a condenser mic into any old style circuit like this without using a 20dB pad in front of the input transformer.  It's designed for speech with a ribbon.  I can get away with a ribbon on a fairly loud 15W tube amp without a pad, but may see waveform clipping in the DAW, and the gain control will be way down.  I had an MK47 on a fairly quiet singer a foot and a half away the other day, and it required a 20dB pad. 

The EQ bypass switch is also a gain boost, couple of dB. 

If you took out C7 with the drill it'd still work if open, not at all if shorted.  Disconnect it and see if you hear a difference. 

Can't address loss at extremities without seeing a plot.  Sounds like loading issues with the particular transformers.

Click to expand...

Well, it's really good to know that the pad is necessary I thought when you were saying you used it on Gearslutz, all you meant was the signal was a bit high without it or something. I'd presume a lot of louder dynamics probably need it as well. Someone gave me the values for one that supposedly went well with the BA-6 according to RCA so i'll give it that try tomorrow.

As for the response, i'll try to set something up tomorrow and get a chart over here.

Use a standard 20dB U pad.