Symetrix 528E MIc Preamp with input-capacitroless-front end.
By using a PNP front-end the common mode range includes ground intrinsically.
I like it. So obvious but not.
It suffers from the same lack of input stage feedback as Doug's Yamaha preamp.JR. wrote: ↑Tue Feb 12, 2019 9:33 pmIn my old late night doodling on this subject I used pnp (737s) but not like this.
Somewhere I have the old high voltage capacitors I bought to breadboard one (decades ago), but never did... similar +60V rail
BTW the NF does not wrap around the input devices so this is a variant on open loop soundcraft preamp. Not uber-linear performance.
note: they could cap couple some NF up to the device emitters and make this right, or not. They have both polarities available in the dual op amp.![]()
JR
I'd have to look at (find) my old notebooks, but imagine two current sources instead of one, and decent sized electrolytic caps connecting to the high sides of R10 and R11. The caps would also provide a first order filtering of noise from the current sources.mediatechnology wrote: ↑Wed Feb 13, 2019 7:47 amIt suffers from the same lack of input stage feedback as Doug's Yamaha preamp.JR. wrote: ↑Tue Feb 12, 2019 9:33 pmIn my old late night doodling on this subject I used pnp (737s) but not like this.
Somewhere I have the old high voltage capacitors I bought to breadboard one (decades ago), but never did... similar +60V rail
BTW the NF does not wrap around the input devices so this is a variant on open loop soundcraft preamp. Not uber-linear performance.
note: they could cap couple some NF up to the device emitters and make this right, or not. They have both polarities available in the dual op amp.![]()
JR
But the input-capacitorless technique looks solid.
Looks like a job for the ZTX951.
The problem with this is which A/D to use... the A/D I would have used 20 years ago would be a joke today.
I don't like the cap coupled gain either, while I have used it more times than not...mediatechnology wrote: ↑Fri Feb 15, 2019 7:03 amThe Symetrix design lends itself well to a simple AC-coupled Rgain without overall NFB.
That's what I was trying to describe a couple posts ago.I was thinking about the Symetrix circuit and how one would add feedback to it.
Since the op amp output isn't floating up to Ve, the (current) feedback would have to be AC-coupled into the emitters.
Keeping the resulting 2 pole response from peaking might be difficult.
no more IMO than needed to perform the task well... the low impedance buffer stages could be removed and replaced with op amps but that would involve far more devices. The all discrete nature of this might appeal so some market segments, who buy with their eyes.@JR
I do remember that drawing.
There sure are a lot of transistors in that level-shifting circuit.*
I should not have to defend my design cred (and for the record I have been more than an engineering manager, that was just one of my several positions along the way), but I am glad you are so easily amused. If only 30 years old that part was not even in production, when I started working at Peavey(Statement coming from a guy chastised by the engineering manager for using an 8 cent multi-sourced part in production for 30+ years. TL431 cough.)