fake cabinet voicing

[dbw]

Okay, so I built myself a 1/2 watt solid state "Little Gem" amp (http://runoffgroove.com/littlegem.html).  My goal was to get something I could plug headphones into and practice through in the middle of the night while being considerate to my neighbors.

So I wired it up and built it in a plastic box I got a pickup in.  It definitely works!  However I was astonished to hear the distortion when I plugged my headphones in... it's extremely harsh and unmusical, not at all like the sound clip on runoffgroove.  After rechecking the circuit to make sure I didn't screw up, I tried plugging it into my Mesa 4x12 (not expecting much to come out), and lo and behold, nice smooth distortion came out (at an incredible volume considering it's being powered by a 9V battery).  I also hooked it up to the line in on my Peavey 1x8 combo and that sounded quite nice too.

Okay, so how can make my headphones sound more like a real cab?  Doesn't need to be great, it's just for practice, but that distortion seriously gives me a headache!  I just need to mellow it up a bit.

Oh and I tried putting a 1nF cap across the volume pot... it cut the highs in the guitar's tone some, but the distortion is just a square wave and it doesn't get filtered

The clean sounds are fine, by the way.

[Aren]

Well, you need a speaker simulator before the power amp. You can build the ROG "Condor", and add a 386 power amp after it.
http://runoffgroove.com/condor.html

[dbw]

Sounds good... thanks!  But shouldn't the speaker sim go AFTER the amp, between the amp and the output? Better still I could turn it off with a switch... though I don't know why I'd want to power a cab with this thing.... I guess I could be in too much of a rush to let tubes warm up

[Aren]

Speaker simulators are designed for direct recording and headphone amps, so they accept instrument-level signals.
What you can do, is insert a bypass switch between the booster circuit and the power amp, so you can use the same box to drive headphones or a cabinet.
If it's not clear I may draw something up later.

[dbw]

Nope I got it, thanks   I'm still skeptical about adding the cab voicing before the gain and clipping... but I will try it and find out for myself

[Aren]

No-no-no-no-no.... The way it goes is:
Gain stage (clipping, etc.) -> Speaker Sim -> Power amp
See the picture:


Connect a bypass switch (DPDT) between the two red dots, and add a 386 stage at unity gain afterwards.

[dschwartz]

Aren..The clipping from the little gem comes from the power amp (386), so, as dbw stated, is futile to put the condor cab sim before the clipping stage..specially when the condor adds more gain to the circuit.. The tone will change but the harsh clipping will be still there, and definetly will add more noise...

i recommend two ways.. the cheap, easy, and not so good, and the hard, proffessional one...

the easy one: put a presence control after the 386...just use a 10nF (or adjust to taste)  cap to a 50KB pot connected to ground,,check dr boogie presence control..this will cut the highs as you regulate the pot.. Or you can replace the pot for a switch to ground, and select a cap that sounds the best for you..the label the switch "phones/cab"

the hard one: put a condor after the little gem, then use other 386 power amp, but with a really clean sound (hard to do with a 386 at 9V though)

[dbw]

Thanks dschwartz.  I've tried a variety of caps and it can mellow the sound somewhat... but I think I'm just going to rewire this thing so it's always clean.

[Mark Hammer]

Your average 10-12" speaker shaves off an enormous amount of top end.  Frequency response drops off dramatically over 5khz in a great many instances.  Consequently, a big part of what speaker simulators do is not just trying to recreate the assorted resonances of speakers and their cabs, but a certain amount of lowpass filtering so as to mimic what the speaker "refuses" to reproduce.

Now, you could insert some of that filtering before the power amp stage.  Of course, when a hot signal reaches the power amp stage and generates additional harmonic content at that point, the speaker-sim circuit placed before the power stage can't do anything about that.  While it may be difficult to mimic the resonances of speakers in a totally passive circuit placed between power stage and headphones, the place for that lowpass filtering is between power stage and headphones.

If you look at the assorted JFET-based amp emulators at runoffgroove.com, you'll see that the vast majority, if not all, have a two-pole lowpass filter on the output.  While the specific values would be inappropriate for driving headphones from a 386, the basic principle is solid.   So, for instance, consider sticking a 10R resistor between the 220uf output cap and the output volume control.  Then, run a 2u2 or even 3u3 cap to ground from the junction of the 10R resistor and output pot.  Not perfect, but somewhere in the ballpark of what you want.

[dbw]

Mark, sounds like you understand my problem, but I don't totally understand your notation... 10R = 10 ohms?  2u2 = .22 uF?

Also, someone on another forum suggested it could be the difference in load that's causing the tone difference.  My headphones are about 16 ohms in mono, so If I put a 15 ohm resistor parallel to them will that help get a similar sound I get with my 8 ohm cab?

Thanks

[petemoore]

10R = 10 ohms?    
  yupp
   2u2 = .22 uF?
  nope, 'u' = 'uf' and many other abbrs. so:
  2u2 = 2.2uf

[suprleed]

Mark and dshwartz, I believe what you are describing is a zobel network (correct me if I'm wrong)...  There is a great wiring diagram on GGG for a zobel network on the ruby amp which may be very helpful in helping smooth out your headphone jack.

[Mark Hammer]

The Zobel network is already part of the textbook/appnotes circuit for most low-power headphone amp chips.  It essentially controls for the fact of a variable impedance load, in the form of a speaker/headphone-driver, on the chip's performance and stability.  What I suggested was something to mimic the tone of the speaker.

And just to confirm, what pete noted is exactly what I meant.

Given that the chip is capable of putting out 1/2w, you may have some reticence about using a 1/4w resistor in series with the output (in addition to the notable absence of a 1/2w 10R resistor in your parts bin).  That's fair.  Realistically, though, the chip only rarely is asked by the input signal to provide that much current output for more than an instant at a time, so the normal 1/4w parts that most of us use should be able to dissipate and cumulative heat just fine.  If you are stil concerned, simply use a pair of 22R 1/4w resistors in parallel.  That'll give you 11R and slightly less output, but it will dissipate all the heat your little circuit can throw at it.  Maybe even moreso, since the two smaller resistors provide more surface area for heat loss than a 1/2w resistor.