Dumb gain calculation question. (solid state pultec)

Forgive me for the stupid question but my math just isnt working out. I'm looking at figure 2 on the API 2520 spec sheet:

http://www.danalexanderaudio.com/ApiInfo/api2520_02T.jpg

I'm trying to figure out how they arrive at 49db gain for the line amp circuit and the formula on the page.

I figured out the transformer in it's 1:3 config is giving 9 db gain using 20log√(secondary/primary)

but my calculation for rs+rf/rs is giving me 101 instead of 40

I know this is a rudimentary question and i must be missing something simple.. HELP!   Thanks!

-jay

fallout said:

but my calculation for rs+rf/rs is giving me 101 instead of 40

Click to expand...

101 is voltage gain from your calculation. Log10(voltage gain) x 20 is the dB value of this voltage gain, giving 40.0864dB for Av 101.

Thanks, i knew i was missing something simple..

My plan was to use this line amp as a gain stage for a pultec style EQ (using Frank's enhanced pultec filter PCB)

I've read that the pultec EQ needs approx 20 db of makeup gain.

So my assumption is a 12k resistor for rs would give approx 20db of gain if i wired the output transformer 1:2.

Is this correct? Thanks!

-jay

So my assumption is a 12k resistor for rs would give approx 20db of gain if i wired the output transformer 1:2.

Click to expand...

Well, what's the feedback resistor..? Don't tell me to just look at the pic you linked, it's *a bit* on the low res side .

20log√(secondary/primary)

Click to expand...

20*log(ratio) is simpler.

Samuel

The transformer adds +6dB, so you'll need to make up +14dB with your line amp to get +20dB out.
+14dB is voltage gain of 5.01187. With feedback resistor 47K, the shunt arm resistor will be 11,715 ohm on paper, so your 12K seems to match your plan. The cap in series with this 12K could be decreased to 10uF, giving a Hpf @1.33Hz.

I'll give it a whirl. Thanks for the response guys.. Haha, sorry about the low res pic!

-jay

Here's the picture in readable. At least ist seems to be the same.

Chris

+14 dB is voltage gain of 5.01187. With feedback resistor 47K, the shunt arm resistor will be 11,715 ohm on paper, so your 12K seems to match your plan. The cap in series with this 12K could be decreased to 10 uF, giving a Hpf @ 1.33Hz.

Click to expand...

To keep noise low you'll want to scale down the feedback network impedance by 10. That makes 4k7, 1k2 and 100 uF. If you want something fancy use two anti-series connected 220 uF caps to keep their distortion contribution low.

Samuel

Thanks Samuel for the suggestion! Would using the values you suggested be better than my original plan which is more or less the same as an API 325? It'd be easy enough to try both.

I'm also considering trying to build the gain stage using the schematics for a solid state pultec drpat posted a while back. However, if my calculations are correct the gain stage is making up approx 26.5 db of gain? Is this too much?

I'm getting 23.3 db from the 2520 and 3 db from the transformer (250:600)



I was also wondering if i could use the sowter 3603 as a replacement for both the input and output as i dont have an hs56v or hs66. I'm sure it'd be fine for the input but my concern is for using one on the output. Can i wire the 3603 using it's center tap to get 300:600 and adjust the gain of the 2520 as needed?

http://www.sowter.co.uk/specs/3603.htm

Thanks,

-jay

about the gain calculations - another option is just stick a pot or trimer and adjust for unity gain....the easy way