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Old November 28th 19, 10:16 PM posted to rec.audio.pro
Rick Ruskin[_2_]
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Default Input transformer for Spectra Sonics 101 preamp

On 28 Nov 2019 11:07:08 -0500, (Scott Dorsey) wrote:

>Rick Ruskin > wrote:
>>On 27 Nov 2019 09:28:21 -0500,
(Scott Dorsey) wrote:
>>>Rick Ruskin > wrote:
>>>>I have a trio of racked 101 preamps with 502 eq's. Can anyone
>>>>recommend an input transformer with a reasonable step-up to increase
>>>>the gain up from the 40db max they deliver now?
>>>
>>>No, because the input impedance on those is already 600 ohms. It is
>>>dominated by the emitter resistor on Q1. If you put a step-up transformer
>>>on there, you get an even lower input impedance which is probably not
>>>what you want.
>>>
>>>There's no free gain to be had here.
>>>Not that a 1:1 transformer might not help the miserable CMRR on those things.

>>
>>The current version of these units state 64db of gain with the same
>>101 card. Where is the extra gain coming from?

>
>Who knows? Folks butcher perfectly good consoles, stick the modules into
>boxes and God only knows what they do from that point. There are all kinds
>of ways you might modify them to get more gain out of them.
>
>The 101 was a general purpose gain module... you could use it as make up
>gain on a mix buss, or make-up after a passive equalizer. Using it as
>a mike preamp is a bit of stretch but certainly Spectrasonics did on some
>models.
>
>These were intended for 600 ohm inputs and 600 ohm outputs since they
>would likely be buried in the middle of a long console signal chain. So
>if you want to get a little more voltage gain out and you're not driving
>a 600 ohm load, you might get away with a step-up transformer on the
>-output-, maybe stepping it up to 5kohms which woould be fine to drive a
>lot of modern gear.
>
>You might be able to tweak some resistor values on the board too, and get
>a little more gain out of the design at the expense of linearity. 40dB
>is an awful lot for this design already, though. And it wouldn't be a
>Spectrasonics 101 any longer, it would be something else.
>
>If it were me, I'd put a balanced input stage in front of the thing,
>two nice large area transistors with a few dB of gain, maybe stick a
>little global feedback around it. But of course then it wouldn't be just
>a Spectrasonics 101 anymore either.
>--scott





Are you saying that a step-up transformer on the input simply will not
work or just that it will alter the performance of the card?

Would adding a resistor to the feedback loop along with the 502 eq
that is already there do anything to increase gain?

As far as I know, the 101 was used as the front end on all
Spectra sonics consoles until the 110 came along.

RR
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