Rca BA-2c input pad and impedance

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Purplenoise

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Sep 19, 2019
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Hey guys. I am building a ba2c and I am going with a Sowter 1960 ( v72 style 200/47k) input tranny.
First off, which value resistor you believe would work best for termination on he secondaries of the input? Sowter suggests 47k yet I have a feeling people use much higher values.
Also I am thinking of using an input pad. Is it better to have it before the transformer or after in the form of a variable 50 or 100k pot? ( secondary -pot- grid of first tube).
Thanks in advance.
 
I padded mine with a switch before the input transformer with two 750 ohm and one 160 ohm resistors and added an output pot on the output transformer secondaries (i dont remember the value but i can look if it can help you!)
 
I padded mine with a switch before the input transformer with two 750 ohm and one 160 ohm resistors and added an output pot on the output transformer secondaries (i dont remember the value but i can look if it can help you!)
Thanks. I am wondering if I could do it at the secondaries…kind like a gain control to the first stage. Would Adding a 50k pot mess with the transformer termination? It’s a variable resistor after all right?
 
These go into full overload before you could ever need a pad on the output. An original doesn’t even need a secondary load resistor.

No input sec load needed, this is a Fender Champ. Lipstick on a pig. Seriously, try it without as the original first. A 47k would load a mic too heavily.
 
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These go into full overload before you could ever need a pad on the output. An original doesn’t even need a secondary load resistor.

No input sec load needed, this is a Fender Champ. Lipstick on a pig. Seriously, try it without as the original first. A 47k would load a mic too heavily.
Thank you so much. Right now I have a Jensen 115 and I use the 150k termination recommendation by Jensen. So I could try it without the 150k without…strange behavior?
 
Try it. Do what your ears like.
My ears make me tweak things to no end lol. What I was wondering is that when I plug in a hot mic like a u47 the sound is pretty “hairy”so I am trying to find a way to “clean it up” a little. Is that saturation coming from overloading the input transformer or the first tube stage? I was wondering about pading after the transformer to keep the slight overload of the transformer but tame somewhat the overload of the first tube. If that is the case of course…
 
It's possibly overloading both, condensers into mic to tube grid transformers pretty much always need an input pad. This particular circuit was meant for turntable outputs and ribbon mics.
 
It's possibly overloading both, condensers into mic to tube grid transformers pretty much always need an input pad. This particular circuit was meant for turntable outputs and ribbon mics.
20db U pad it is then.Would you mind giving recommendations for the resistor values?
By the way I tried the 115 without the 150k loading and it sounds nice. A little more “rock&roll”.
Thank you again for all the great help.
 
I padded mine with a switch before the input transformer with two 750 ohm and one 160 ohm resistors and added an output pot on the output transformer secondaries (i dont remember the value but i can look if it can help you!)
Hey alphasnk, how much attenuation are you getting at the input with those values? I am thinking of using two 1k and one 220r. Thanks
 
20db! Try and report back how it works for you!
Great thank you so much. Since I haven’t build a pad before using a dpdt switch I got another question…
The transformer primaries go to the middle 3 and 4 pins of the switch and then two sets of wires leave the xlr ( 2-3), one with the u pad on 1 and 2 of the switch and the other set straight to 5 and 6 of the switch right?
 
Great thank you so much. Since I haven’t build a pad before using a dpdt switch I got another question…
The transformer primaries go to the middle 3 and 4 pins of the switch and then two sets of wires leave the xlr ( 2-3), one with the u pad on 1 and 2 of the switch and the other set straight to 5 and 6 of the switch right?
exactly as you said it!

dpdt:
12 xlr/trs
34 transformer pri
56 / 220r

1k resistor from 1 to 3, and the other one from 2 to 4
 
It's possibly overloading both, condensers into mic to tube grid transformers pretty much always need an input pad. This particular circuit was meant for turntable outputs and ribbon mics.
Hey emrr, as I am playing around with this build I thought I’d ask you about the negative feedback option since you know this circuit so well.
Is it possible to introduce nfb via a switch? What is the best way to do it in this build? Thanks
 
You can only do a local loop because of the gain control. Either front end or output stage. You’d have to measure and design.

You can switch the cathode bypass caps on and out, try that first.
 
You can only do a local loop because of the gain control. Either front end or output stage. You’d have to measure and design.

You can switch the cathode bypass caps on and out, try that first.
I thought about disconnecting the cathode bypass caps. I could do both? That shouldn’t introduce too much negative feedback though right?
 
I haven't tried it myself, you'll have to see. Usually a 6dB change per cathode. Changes the tube loading also.
Great…I expected less but that’s good. If I try the output to first cathode route could you give me a starting/ballpark value for cap and resistor to get approximately 20db of nfb?
 
I could do both?
Yes, you can make both caps switchable. This gives you relative flexibility to adjust the gain and distortion structure. Together with an input pad you can then react to almost any recording situation and microphone.

As Doug wrote, the BA2C has Champ genes and tends to overdrive with increasing input levels. That is an absolute plus of this negative feedbackless design.

Sometimes, especially with hot signals, this is too much of a good thing, so switchable caps can make the difference here. Listen to it and decide for yourself if it makes sense for you.

Of course, you will also lose output level, which is best compensated with a compressor / limiter.

The cathode caps can be switched almost poppless, if you connect a high value resistor to the capacitor in series.
 
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