High voltage pre amp design

Archived from groups: rec.audio.tech (More info?)

Need opinion on design,

I have amps that have an inexpensive op amp creating gain on the input
stage. I have an extremely high quality Preamp. My thoughts were to
by pass this input stage and use the pre amp straight.

Not enough gain...

Ok, how can I build a TUBE Preamp that would have adjustable gain to
power the amp and eliminate the opamp?

I do realize the voltage would be 40 to 70 volts, but is that a
problem?

Ed Houston
Cincinnati, Ohio

Archived from groups: rec.audio.tech (More info?)

Joe Kesselman wrote:
> Why do you want to do this? "An inexpensive op amp", in the right
> circuit, may be a perfectly decent gain element. Op amps are popular
> precisely because they are surprisingly effective component for their cost.
>
> (If you have to ask how to build a tube preamp, you probably don't know
> enough to meaningfully criticise the existing circuit.)

You have a point and and I should have expected such a response...

I feel that the Input stage is a weak link and would like to try tubes
in this area. I have built many circuits in the past and see this as
my next attempt at creativity.

To define my reasoning, the amps have a forward and reverse polarity
output and I felt that if I eliminated the op amp inverting the signal
and used the Jeff Rowland Balanced out as the inverter it would
eliminate or make better. I have already dupicated the input circuits
(less Gain stage) in the amps and tested them, hoping the Preamps gain
(8 to 12 volts) would be enough. Once I looked into a seperate gain
stage I began to think the Preamp should do this once. Why have a
duplicate circuit?

Thanks

Ed

Archived from groups: rec.audio.tech (More info?)

NEC 859, The amps are Acoustat servo, the quad op amp controls first
stage gain, inverting, and auto on off.

I have tried a hand full of other op amps, None sound as well as the
NEC.

The modified circuits being performed by others use a Burr Brown 627 i
think, I heard that and it was very dark sounding.

Thanks

Ed