Presence controll: Doesen't do anything!

[syndromet]

Hi, I'm back to bother you with my Hiwatt amp sim once again!! It's almost complete, except from the presence controll, which doesn't do anything. If some of the smart guys here (and there is a lot of them, for sure) would care to explain to me the theory of how the presence controll is supposed to work, and suggest what I have to do to make it work, I would be extremely happy.

My schem:


The original Hiwatt schem:


Here is a few soundclipps that does it a little more justice than the earlier ones... Backing is with gain and vol at 12, while the solos are with the knobs dimed. The playing sucks, as everything you hear is first take. It's recorded Guitar -> Hiawatha -> SB Audigy SE -> Cab sim in Guitar rig.

Single coil
Humbuckers

[scaesic]

isn't presence post gain treble control?

[Seljer]

all I know about the prescence control on the regular Marshall type amps is that its got something to do with controlling engative feedback, and ends up controlling the treble past the range of the treble control (and really is, presence, turning it down is like throwing a blanket over your amp...)

[JHS]

The Hiwatt-amp has a 100k + a neg. FB-loop (from the output-trannie connected to the presence control circuit), in your clone there's no FB-Loop.

Drop the Hiwatt type presence control and use a presence circuit similar to the Egnater TOL 100.
The Barber DD has a good presence control, you can build somthing similar.

JHS

[JHS]

BTW:

Your circuit won't sound anything like a HIWATT amp. You should use 30V or more as supply voltage to get the headroom and the very light crunch of the original preamp, at 9V it will only sound like a cheap compressed distortion pedal. 90% of the HIWATT sound comes from the power tubes and the output trannie + the Fane Speaker.

JHS

[syndromet]

The Hiwatt-amp has a 100k + a neg. FB-loop (from the output-trannie connected to the presence control circuit), in your clone there's no FB-Loop.

Drop the Hiwatt type presence control and use a presence circuit similar to the Egnater TOL 100.
The Barber DD has a good presence control, you can build somthing similar.

JHS

Thanks, mate.
That was what I suspected. After reading up on presence-controlls, I found that it was a controlled feedbackloop, however I couldn't find out where the loop was on the Hiwatt schem. Looks like I totally overlooked the 16 ohm feedback connection, and if I had seen it, I wouldn't have known where to connect it anyway.. I'll look into the controlls you mentioned.

BTW:

Your circuit won't sound anything like a HIWATT amp. You should use 30V or more as supply voltage to get the headroom and the very light crunch of the original preamp, at 9V it will only sound like a cheap compressed distortion pedal. 90% of the HIWATT sound comes from the power tubes and the output trannie + the Fane Speaker.

JHS

I was afraid of this too, and I didn't really draw up this schem to make a good distortion. The main reason for playing with this was to learn how jfets work, and get into the basics of electronic design. Having heard that the Hiwatts are super-clean, with an almost HiFi preamp, I suspected that this would sound like sh*t, actually. I built it anyway, and I'm really surprised by the way it sounds. It's not that compressed, has a descent amount of gain and a really nice overdrive sound at least in my ears. Kind of bluesy. I actually like it enough to replace my Danelectro Fab overdrive, which is basically a TS, with it. It would be fun to try it at something like 30V, though.

[Mark Hammer]

On a great many amps, the presence control provides degrees of negative feedback from the speaker side of the output transformer.  The output transformer, if pushed hard (along with the power section), will produce additional harmonic content that wasn't there at the phase-splitter stage.  When that harmonic content is fed back to an earlier point in the circuit, it can be used to cancel out the "unwanted" added harmonic content ( ). You will note that Presence controls limit the bandwidth of such feedback (only high end is fed back) and that more series resistance yields more treble by reducing the amount of treble-cancelling feedback. 

To have a presence control work in that manner on a FET-based simulator, you would need to have a transformer stage that a) isolates the output from the semiconductor preceding it, and b) generates its own harmonic content.  I don't know that a $3 Mouser interstage transformer is going to necessarily produce the same sort of added harmonic content that a Partridge transformer will, but it may be worth trying a 10K:10k on the output, and feeding back some of that to an earlier point in the circuit.