LM3886/Vox SS Amp Thread(w/ Samples!)

[PenPen]

Just remember that all the distortion came from the pedal.

RDV

Of course. I've just found that some amps sound great clean, but as soon as you stomp on a box, it sounds bad. Conversely, I've had many amps that sounded fantastic distorted with my Rat or DS-1, but sounded horrible the moment I switched them off. I've only heard a couple of amps I like the sound both ways, and I think that Vox there is one of them. I'm going to try it to see, it could end up not being what I'm after, but I'll never know unless I try. What I'm really after is an extremely clean base, but gets gritty and darker when pushed with a boost, and then can sound great in full gain Rat/Big Muff distortion. I'm hoping the combo I mentioned can be tweaked to give me something close to that.

[R.G.]

This is kind of fun. I've told people how good the Thomas Vox solid state stuff sounds for a couple of decades off and on, and got reactions varying from incredulity to laughter.

I still think they sound good.

[p1_ind]

I agree with R.G.
I repaired a solid state Thomas Vox Cambridge Reverb last year for my brother and loved the sound of the amp.

They are not fun to work on (wiring nightmare), but great sounding.

By the way I don't remember if I thanked you R.G. for the schematic and help fixing it.  But thanks!  My brother was very happy to hear it working properly.

Adam

[RDV]

UgLyVoX
(Uglyface + Vox/LM3886) = 
RDV

[RDV]

Well I got my 10uF 50v caps in yesterday and replaced the little 16v stompbox ones in the preamp and ran the positive power supply rail(27.5v) to the preamp hoping for increased performance, less ground loop noise etc.

It is a bit louder, but still has a hum. I suppose my tranny is too close to the rest of the amp.

I don't think the tone is quite the same either.

More testing is needed though.

I think perhaps an emitter folllower before the power amp would help things also, though I'm not quite sure which one to use.

RDV

[PenPen]

Well I got my 10uF 50v caps in yesterday and replaced the little 16v stompbox ones in the preamp and ran the positive power supply rail(27.5v) to the preamp hoping for increased performance, less ground loop noise etc.

It is a bit louder, but still has a hum. I suppose my tranny is too close to the rest of the amp.

I don't think the tone is quite the same either.

More testing is needed though.

I think perhaps an emitter folllower before the power amp would help things also, though I'm not quite sure which one to use.

RDV

I always wondered about the noise caused by a transformer. In my preliminary sketches for an amp I planned to maybe shield the coils of the transformer with some kind of steel/alu hood run to ground to try to block the noise of it from getting into the circuit. I noticed on my friend's Bassman 135 head Fender mounted the transformers upside down so that the coils where outside the chassis, with just the leads coming through the inside. Now I wonder if they did this to prevent noise. You could try just putting some kind of shroud around the transformer coils, not touching the transformer itself of course, making sure the shroud also touches the chassis, to see if it clears up any of the noise. Thats what I'd try, anyway.

Note, I am a total newb at amps, so maybe R.G or someone else with more knowledge will chime in too.

[RDV]

I mounted the whole shebang on a piece of crappy brown Radio Shack perfboard so the tranny is right there just a few inches from the chip-amp. Tonight I'm going to separate it and just run wires to the rest of the circuit.

RDV

[Eric H]

I mounted the whole shebang on a piece of crappy brown Radio Shack perfboard so the tranny is right there just a few inches from the chip-amp. Tonight I'm going to separate it and just run wires to the rest of the circuit.

RDV

If you leave enough slack in the wires, try rotating it while it's on, as well. You can "aim" the magnetic field for least hum.

-Eric

[RDV]

Well it's not the proximity of the tranny, nor is it filtering. I moved the tranny and it still hummed. I added a couple more 4700uF for a total of about 10,000uF of filtering on each rail.

I guess it's gonna hum a bit.

It's not a real loud hum, just a bit irritating when I read how the guys make their Gainclone amps virtually hum-free.

RDV

[Eric H]

It's not a real loud hum, just a bit irritating when I read how the guys make their Gainclone amps virtually hum-free.

RDV

Star grounding.

-Eric

[R.G.]

Eric is very close to the truth.

To do a ripple and hum-free class AB amp is very demanding on the wiring. The( +) and (-) power supply leads both conduct full-peak-current half-wave-rectified signal current on alternate signal polarities. The ground for the speaker must NOT be returned anywhere but to the power supply ground (perferably that's the star ground). The rectifier current pulses must not share any length of wire with either signal ground or speaker return.

The only way to make current go where you want it to is to leave it no option - make only one wire for it to flow through.

[RDV]

I've gotten my 1st significant reduction in the hum  by establishing a central "Star" ground. I really did a bang-up job of screwing this thing up thinking I could wire it like a pedal. Uh Uh. Gotta wire like an amp. My next amp project will go much differently. I've still got to come up with a booster/buffer after the volume control.

Jeez

I've still got further to go but I'm worn out.

RDV

[Elektrojänis]

After star grounding, how about making a separate RC-filter for powering the preamp part... Or even an LC-filter.

Power amps need a lot of current, so they need to be powered straight from the rectifier/filter caps.

Preamps on the other hand usually don't need that much current but can use some extra filtering. Even if this one eats more current than a typical spomp box, it can probably live with 1-10 ohms (the R-part of the filter) in series with the power supply (and after that a cap to gruond ofcourse, the C-part of the filter).

That's what I'd try anyway...

[Eric H]

After star grounding, how about making a separate RC-filter for powering the preamp part... Or even an LC-filter.

Power amps need a lot of current, so they need to be powered straight from the rectifier/filter caps.

Preamps on the other hand usually don't need that much current but can use some extra filtering. Even if this one eats more current than a typical spomp box, it can probably live with 1-10 ohms (the R-part of the filter) in series with the power supply (and after that a cap to gruond ofcourse, the C-part of the filter).

That's what I'd try anyway...

That's an excellent suggestion.

-Eric

[RDV]

About all I hear now is my little cooling fan, so I'm pretty happy with it now.

I'm going to do another one soon with a much better transformer and heat sink, and all the lessons I've learned here will be applied.

Thanks all.

RDV

[octafish]

Hey Ricky I hope you check this. I just want to clarify this one. The cap selector rotary switch is the the amount of mid boost selector and the switch between the inductor and the 10K resistor is a mid boost on/off switch. Is this correct?

[RDV]

The cap selector rotary switch is the the amount of mid boost selector and the switch between the inductor and the 10K resistor is a mid boost on/off switch. Is this correct?

No, and yes. The cap selector just switches between different caps for different mid frequencies to be boosted. If you go back to the samples and play the one where I'm twisting the cap selector you can hear it changing the peak of the mid boost up and down. The switch between the inductor and the 10k is the on/off switch for the MRB.

HTH

RDV

[Ge_Whiz]

Nice one, Ricky - I'll buy the album. "RDV - The Thomas Vox Sessions". 

Those non-polarized electrolytic capacitors in the preamp - does the labelling indicate a pair of capacitors, each 2uF or 1uF, or a non-polarised arrangement of actual capacitance 2uF or 1uF, please?

Phil

[RDV]

Nice one, Ricky - I'll buy the album. "RDV - The Thomas Vox Sessions". 

Those non-polarized electrolytic capacitors in the preamp - does the labelling indicate a pair of capacitors, each 2uF or 1uF, or a non-polarised arrangement of actual capacitance 2uF or 1uF, please?

Phil

Thanks Phil, I'll send you a free copy when it gets pressed!

I used 1uF Phillips MKT film caps, though I would imagine you could use anything in there that would equal the right value. I just prefer films if I can fit them. I believe they're indicating a non-polar electrolytic on the schem though.

Cheers

RDV

[R.G.]

Those non-polarized electrolytic capacitors in the preamp - does the labelling indicate a pair  of capacitors, each  2uF or 1uF, or a non-polarised arrangement of actual capacitance 2uF or 1uF, please?

They varied from model to model. If you have room, put in 2.2uF polyester/mylar or polypropylene. If you must, use 2.2uF NP/bipolar electrolytic. The caps do need to be nonpolar in some fashion, because there are signal conditions when they are polarized both ways.