davemascera
Active member
- Joined
- Mar 22, 2017
- Messages
- 42
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?
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?