Functions of C1 and C8 - AstroSonic - ANK AudioKits Forum

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Functions of C1 and C8

In Reply to: RE: Bg in series and bypassing Mtube Mundorfs, anyone? posted by Tubeheadz on April 16, 2010 at 20:49:20:

Tubeheadz,

The pentode in the 6BM8 is a series pass regulator. The triode is a high speed (wide band) error amplifier. C1 appears to bypass C3 (a power supply reservoir cap), but the regulator pentode is between them (C3 and C1). The pentode functions as a dynamic dropping resistor in a pi CRC filter.

The B+ (stock) is taken off the top of the resistor string on the pentode's cathode. The error signal and the grid bias for the regulators error amplifier (the triode) come from the voltage dividers in this resistor string.

C1 acts to correct the fastest and earliest portion of the ‘error’ (really just the voltage drop across the power supplies impedance) caused by the analog stages drawing power. This cap needs to present a low impedance across the audio range, but only supplies energy during the short time between when the error voltage occurs and when the regulator responds. It covers (corrects) the leading edge of the error and decreases the error. If C1 does its job well, the regulator sees an error that it is fast enough to correct. A higher value cap in this position makes the regulator work harder, as it must correct the error and fill the droop in the cap.

C8 is a coupling capacitor that passes the remaining error (stripped of the fast leading edge by C1) to the grid of the error amplifier (the 6BM8 triode). The signal that has passed through C8 is amplified by the triode and direct coupled to the grid of the series pass element (pentode), where it changes the series resistance (of the pentode) to compensate for the error. Any alteration of the error signal by C8 is amplified by the error correction amplifier (6BM8 triode). This causes the Pentode to produce a change in resistance that over or under corrects the actual error, resulting in coloration or worse.

The functions of C1 (initial error correction/reduction) and C8 (coupling the remaining error to the error amplifier) place them in a position to strongly influence the sound quality of the analog stages, as you have observed.

Regards,

Bob

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Follow Ups Full Thread

Topic - Sonic Signature of Caps??? - The Mad Hatter 08:52:12 04/11/10 (12)

RE: Sonic Signature of Caps??? - Tubeheadz 21:46:40 04/16/10 (0)

I don't hear a big difference between - Frihed89 12:35:22 04/16/10 (2)

RE: I don't hear a big difference between - Wojciech 12:46:43 04/16/10 (1)

RE: I don't hear a big difference between - Frihed89 13:46:33 04/16/10 (0)

RE: Sonic Signature of Caps??? - DougE 18:48:05 04/15/10 (5)

RE: Sonic Signature of Caps??? - AstroSonic 08:16:29 04/16/10 (4)

Bg in series and bypassing Mtube Mundorfs, anyone? - DougE 12:20:51 04/16/10 (3)

RE: Bg in series and bypassing Mtube Mundorfs, anyone? - Tubeheadz 20:49:20 04/16/10 (2)

Functions of C1 and C8 - AstroSonic 19:35:58 04/17/10 (0)

RE: Bg in series and bypassing Mtube Mundorfs, anyone? - The Mad Hatter 08:09:57 04/17/10 (0)

RE: Sonic Signature of Caps??? - beto1 15:33:53 04/11/10 (1)

RE: Sonic Signature of Caps??? - IkeH 18:27:58 04/21/10 (0)

Follow Ups

Functions of C1 and C8 - AstroSonic 19:35:58 04/17/10 (0)

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