Automatic Volume control (AVC or, AGC), timing ?

[Ron Rich]

Hi All,
Anyone know what the timing should be on an AVC circuit from needle set down, to full volume, when playing a 45 rpm record ?
Thanks, Ron Rich

[robnyc]

Ron, you knew full-well who would responded here..didn't you .....

You left out whether it was a Seeburg or other and S-S or tube.

For the tube Seeb's "full" gain recovery from squelch occurs at approx 14 seconds. Full gain here is at the point where there is still about 2db of gain reduction in the system but this would not be audible. You won't be able to measure voltage accurately due to very high impedance at the control points. A 10 megohm meter will load the circuit at the time constant cap and reduce the gain reduction by about 2 db.

HFMA1 (V-VL and the R-J versions) 16 vdc= -1 db 40vdc= -17db (approx depending on 6SK7 tube condition. These amps show slight nonlinerity on sine waves when AGC voltages are in the 21-32 vdc ranges. This is independent of tube selected. I don't know why this happens. I do hear it very slightly in some music.

HFMA2 (also L and K models w 6BJ6 reactance tube) 6vdc= 0.5 db reduction. 21vdc= -19db reduction.

Two points here; Even though there is substantial gain reduction from low to max AGc voltage this is a compression circuit -not a limiter so volume still goes up at the output. the ratio is about 3-1. For every 3 db of measured gain reduction the output level still rises by 1 db. Also, it is critical in these tube units that the voltage on the Grid-1 of the reactance tube not exceed that of the cathode. So a max of around 40v on G1 for an SK7 and 25v on G1 for a BJ6.

For Diode based Seeburgs there seems to be a slightly shorter recovery from squelch at about 12 sec. to reach the point where only about 1 db reduction is still occurring.

Some test results taken in the early 1990's with freshly rebuilt amps:

TSA6 W/stabistors: 0.6 v =1db reduction. 1.7v = 16 db reduction. Note that in an un-modded amp only about 1.2 v for -9 db reduction will ever be attained.

TSA8 Three diodes replacing each stabistor: 0.9v = -1db reduction 1.8v = -16db reduction. Again, this is a modded AGC drive to increase gain reduction for a given input level. Normal maximum reduction (un-modded amp) with a "loud" record will be in the 8-10db region.

SHP1 W/original stabistors. 0.4v = -1 db reduction. 1.6v = -14 db reduction. As above, drive was increased to give higher reductions for a given input level.

--Note that the mods to the AGC drive only increase the amount of AGC voltage and thus reduction possible from a given input from the pickup. They do not directly affect the recovery time from squelch, but since the added AGC compression (reduction) takes away overall gain, the volume control lives at a higher point on average so the apparent squelch-recovery time would appear shorter even-though the actual squelch voltage (around 5vdc for S-S amps) decay at the time constant won't be different.

i did detailed AGC volt-gain curves for these amps at that time but can't show them here..and this has gone on long enough.

Sorry about the long tome.

Rob

[Ron Rich]

Hi Rob,
I yield to your far superior knowledge ! Thanks- did not really want to be so specific, as this was an "in general" question about AVC, rather than any one brand of anything --Ron Rich

[babycat]

12-14 seconds is such a long time. i notice on my LS2 that many records end up 'fading in' during the start of audio. i always just figured that mine is old and something's out of spec. but maybe it's normal? why would they design such along ramp?

cheers,
eric