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previous: Bjorn Looong post on DR mods -- 4/23/2000 9:25 PM

Inductor-based Feedback Circuit Mods

4/25/2000 7:08 PM
Ray Ivers	Inductor-based Feedback Circuit Mods Bjorn, I'm answering this here in case anyone else might benefit from this stuff. If you connect a cap in series with an inductor across the 47 ohm feedback divider resistor in your BFDR, you will get a mid boost without affecting the frequency extremes much, if at all (your existing 1 uF cap will have to come out, though). Here are some values: 390 mH inductor, .47 uF cap = 370 Hz 390 mH (.390 H) .22 uF cap = 543 Hz 390 mH .1 uF cap = 800 Hz The formulae I used, from Boscorelli's 'The Stomp Box Cookbook', were (L = inductance): F (frequency) = 1 / (6.28 x [sq/rt of L x C) C (in farads) = (1 / [39.4 x F squared x L) L (in henries) = (1 / [39.4 x F squared x C) There's another issue, though, regarding the Q, or bandwidth, of this boost. With that 47 ohm resistor in parallel with the LC circuit, the boost is going to be relatively mild and spread out over a wide range of frequencies centered around the frequencies above. In order to make the gain boost larger and more focused, the feedback resistors are going to have to be changed. I would multiply them both by a factor of ten - this will give 470 ohms and 12K ohms for your current, non-stock configuration. If this doesn't give you enough, I think you might be able to double it again before you start affecting the bias of the phase inverter using the stock (22K) value of tail resistor. You're using a 12AX7 driver, though, so the whole feedback and bias situation changes - you could probably go up to a 2.2K / 56K feedback setup with no problems, and your feedback will be greater with the higher output stage gain. You're going to have to just plug this stuff in and try, though - you've got 6L6's in there and are running a 4 ohm speaker load, and the schematic is really only of limited help to me. Also, try putting back in your presence cap when you're done (you'll have to scale it for whatever value of bottom feedback resistor you end up with). I'm waiting for Boogie to get back to me about ordering inductors - I'll let you know what they say. I paid from 5 to 7 bucks each for them back in '94. Ray

4/25/2000 7:08 PM

Ray Ivers

Inductor-based Feedback Circuit Mods

Bjorn,

I'm answering this here in case anyone else might benefit from this stuff.

If you connect a cap in series with an inductor across the 47 ohm feedback divider resistor in your BFDR, you will get a mid boost without affecting the frequency extremes much, if at all (your existing 1 uF cap will have to come out, though). Here are some values:

390 mH inductor, .47 uF cap = 370 Hz
390 mH (.390 H) .22 uF cap = 543 Hz
390 mH .1 uF cap = 800 Hz

The formulae I used, from Boscorelli's 'The Stomp Box Cookbook', were (L = inductance):

F (frequency) = 1 / (6.28 x [sq/rt of L x C)
C (in farads) = (1 / [39.4 x F squared x L)
L (in henries) = (1 / [39.4 x F squared x C)

There's another issue, though, regarding the Q, or bandwidth, of this boost. With that 47 ohm resistor in parallel with the LC circuit, the boost is going to be relatively mild and spread out over a wide range of frequencies centered around the frequencies above. In order to make the gain boost larger and more focused, the feedback resistors are going to have to be changed. I would multiply them both by a factor of ten - this will give 470 ohms and 12K ohms for your current, non-stock configuration. If this doesn't give you enough, I think you might be able to double it again before you start affecting the bias of the phase inverter using the stock (22K) value of tail resistor. You're using a 12AX7 driver, though, so the whole feedback and bias situation changes - you could probably go up to a 2.2K / 56K feedback setup with no problems, and your feedback will be greater with the higher output stage gain. You're going to have to just plug this stuff in and try, though - you've got 6L6's in there and are running a 4 ohm speaker load, and the schematic is really only of limited help to me. Also, try putting back in your presence cap when you're done (you'll have to scale it for whatever value of bottom feedback resistor you end up with).

I'm waiting for Boogie to get back to me about ordering inductors - I'll let you know what they say. I paid from 5 to 7 bucks each for them back in '94.

Ray

Replies:

jason Nice tip ray,Thanks! -- 4/26/2000 12:25 AM
Bjorn Ray, Wow! I’ll have to try ... -- 4/26/2000 8:29 AM