Carvin Belair Presence Control/NFB Loop

[bobloblaws]

Greetings,
I've have this Carvin Belair 2x12 combo here and the "Acoustic Presence" control doesn't seem to be affecting the tone of the amp at all, or if it is it is imperceptible to me. I've done some troubleshooting but kind of at a dead end. The control is meant to work on the clean channel only. I verified that the relay is grounding the one pot lug when switching to the dirty channel. So the default would be the clean channel with presence/negative feedback. I cleaned the pot and it seems OK but it doesn't go quite down to zero ohms, the range is something like 40 ohm to 28 Kohm. C70, R70, and R71 are in spec and I can scope a test signal on either side of C70. On the pot side of C70 the amplitude of the signal is affected by turning the presence control, however, on the R70 side the signal barely changes. I'm kind of iffy on which points in the circuit will have negative feedback signal show up only or original signal only or some combination of both.

Can anyone suggest next steps?

- Bobby

Carvin-Belair-Schematic.pdf

[The Dude]

From the manual:

"The rear ACOUSTIC PRESENCE control adds a sibilance to the high frequencies of your guitar.
Most presence controls work in the 3k to 4k range. However, the VINTAGE TUBE’s presence starts
at a very high 8k Hz delivering 10 dB at 12k Hz and continues to 20k Hz which extends all the upper
harmonics of your guitar. The amount of sibilance will depend on the speakers used. To keep both
channels totally independent, the ACOUSTIC PRESENCE is switched by relay only into clean channel
1. The effect of the ACOUSTIC PRESENCE will seem ever so slight, however, the result is added
sibilance only to the ultra-high frequencies."

I suspect it's working and, as the description says, ever so slight. If you're running a typical guitar speaker, frequency response may not even include 8K or higher and you would hardly notice it at all since we're in tweeter range.

[bobloblaws]

From the manual:

"The rear ACOUSTIC PRESENCE control adds a sibilance to the high frequencies of your guitar.
Most presence controls work in the 3k to 4k range. However, the VINTAGE TUBE’s presence starts
at a very high 8k Hz delivering 10 dB at 12k Hz and continues to 20k Hz which extends all the upper
harmonics of your guitar. The amount of sibilance will depend on the speakers used. To keep both
channels totally independent, the ACOUSTIC PRESENCE is switched by relay only into clean channel
1. The effect of the ACOUSTIC PRESENCE will seem ever so slight, however, the result is added
sibilance only to the ultra-high frequencies."

I suspect it's working and, as the description says, ever so slight. If you're running a typical guitar speaker, frequency response may not even include 8K or higher and you would hardly notice it at all since we're in tweeter range.

OK, that is most likely the case. Thank you!

[Chuck H]

If you have a higher fidelity speaker available you could try it. But I'm sure you've thought of that now.

I'm interested... Does that presence control change the "feel" of the amp in any useful way even when using speakers that don't allow you to hear the frequency changes??? Maybe you could play test the amp when it's back together and report, please.