Classic Guitar Amp project (approx. 0.3w)

[chptunes]

PRR is generous.  This thread is in reference to a design that he created during a "small amp" discussion.. it is a 0.3w amplifier with Transistor Gain, Darlington Power, and an Output Transformer.

Reference this thread for even more background:  http://www.diystompboxes.com/smfforum/index.php?topic=100791.0

Here's the accelerated history:

..just thought it'd be cool to get a simple low powered transformer amp working.

> get a simple low powered transformer amp working.

I have cooked-up a scheme for a low-power transistor transformer amp.

It should perform excellently, within its roughly 0.3 Watt ability.

But it isn't particularly simple. Two small transistors, a TIP120 (or similar TO220 Darlington). Chip-amps are much simpler.

And the transformer. A *good* class A SE OT is not common. I thought of a hack using a part which used to be common, and it seems to have gone out of style. (25V 5W speaker transformer.) I'm finding like just 22 or 8 in stock at prices like $8 and $16, plus $5-$10 shipping.



So the bare parts cost nearly $100 per Watt! ($1/watt has been the mark for decades, at least in larger utility amps.)

And it is NOT a battery amp. Scaled for 12V 120mA. OK, it will run all week on a car battery, all day on eight D-size NiCads, but small batteries will fade fast.

Notes:

Stuff in dotted boxes is simulator fluff. Far left is just a signal source. In the middle, R9 R10 is a 100K Volume control.

Transformer must be rated for _25V_ line at 5 (or 4) Watts. The typical speaker line transformer is covered with taps. Be sure you know which ones are 25V at 5 Watts.

As a check: feed 6VAC 60Hz (heater transformer) to the suspected "25V 5W" taps. At the 8 ohm winding you should get 1.5VAC. (1.0VAC at a 4 ohm winding.)

Supply should be 11V-12V DC. Don't go higher, it'll melt. Lower, it will suck.

The TIP120 in TO220 package can run without a heatsink, but don't lean a finger on it.

Target bias is about 1V at the 10 ohm power transistor emitter (about 100mA). The first stage should show 4V to 6V at collector.

> Which component would overdrive first

Power is precious. You paid for your mighty 0.4 Watts, you want to get ALL that you paid for. Power amps are normally designed so the power stage overloads first.

The first stage is designed clean up to 500mV signal, a hot guitar.

The power output stage will or will not overload depending on the Volume control.

Classic guitar amp. Somewhat scaled from the better Fender Champs. Hot guitar wall barely strain the first stage, then a volume control lets you select the level of strain in the output stage.

If you truly find it "too loud", we can re-tap the transformer and increase the emitter resistor for lower power output.

Questions:

> What is the arrow symbol near the 8 Ohm speaker load?

Simulated "voltmeter probe" so I can see what comes out.

> A single TIP120 Darlington can be used instead of the "Q2N6059" pair, right?

Yes. My sim doesn't have TIP120. The '6059 lacks the '120's base resistors, but I did simulate with more '120-like values and no real difference (because the driver stage current is ample-enough).

> The Negative side of the output is connected to Common Ground.

Why not?

(One exception: BTL transformerless outputs have neither end grounded.)

Yes, it will work if the secondary is left floating.

Historically, we did NOT, because the grunt came from a 105 Volt - 300 Volt vacuum tube. If the OT developed an internal short, there could be high voltage on the speaker wires. Bad for humans. If the speaker line is grounded, something else will blow, humans are safe.

Yes, this is only 12V.

Another thing: for "best fidelity" we would run negative feedback from the speaker side of the OT back to an earlier stage. Since that earlier stage is ground-referenced, we need one end of that winding referenced to ground.

However I don't know the phase of your transformer. (They are not marked.) If you phase it wrong, it howls full power. (Less ear-splitting here than on a Fender Twin.) So I took NFB from the primary. This was common before the hi-fi damping-wars of the 1950s. The polarity is unambiguous. The distortion is reduced (>26% toward 10%). There's some speaker damping. We don't want hi-fi infinite damping in a stage amp (dresser amp) because the speaker's liveliness is part of the flavor.

And the exact reason is that my sim complains about nodes with *no* path to zero volts reference.

-Corey

[chptunes]

So, I have wired this circuit up.  Physically, it will fit into a 1590B or 1590BB comfortably.  After searching Transformers, I've decided to use Hammond part 117K4.. mine is in transit now.

Hammond 117 series Data Sheet:  http://www.mouser.com/ds/2/177/5c0056-48788.pdf

I'm anxious to get this amp completed.. I have my Regulated 12vDC supply in hand.. here's some pics of my loaded Perfboard:





-Corey

[PRR]

May not be clear on my schematic: the Negative end of C2 is grounded (it's drawn upside down).

[chptunes]

Made some progress.. I like how it's coming along.  My first time to utilize Roll-Bars.  This project could fit into a 1590B, but I stepped up to the BB size.

The Hammond 117K4 Line Transformer is a real chunk [read "heavy"].  For balance sake, I centered it on the surface.

I'm hoping to have it powered up in the next few days.

Some thoughts.. I used sockets for each Gain stage, so different Transistors can be tested.. I wonder if switching the Negative Feedback Loop would yield interesting tones...

Here's some pictures:










-Corey

[chptunes]

Hey Paul.. It needs a cool name.  Fired right up without any troubleshooting!  Whoa!

Sounds sweet.. I used an A100k Pot for the Volume, and it feels great!  Soft conversation-level output around 11 o'clock, through my 12" Weber Sig.  Past 12 o'clock, the volume exceeds normal conversations and sounds progressively more saturated and ..urgent.

For the two preamp gain stages, I have 2N2222 Transistors installed.  Maybe I'll try different parts there tomorrow.

A clean boost pedal should drive the amp to fairly loud volumes, for the living room anyway.

Thanks again PRR.  I am very happy with your design.  More pictures tomorrow...

-Corey

[Tony Forestiere]

No sound-clips = Didn't happen. 

[chptunes]

No sound-clips = Didn't happen. 

Hi Tony.. hehe.. I'm not set up to record anything.. I suppose that I should find an easy way to record sound clips from time to time.

Meanwhile, here's some more pictures.













-Corey

[tca]

I like it! How does it sound? How do you describe it comparing to a Ruby, or a PUNCH amp?

Cheers.

[chptunes]

I like it! How does it sound? How do you describe it comparing to a Ruby, or a PUNCH amp?

Cheers.

It's output level is comparable to the PUNCH amp that I built.. but, the PUNCH amp has a brighter clean tone.. and my PUNCH stays clean [great for auditioning effects, just as designed].  This little Transformer amp has a warmer clean tone, when set below 50% Volume.. as the Volume is turned up, I hear more harmonics.  At full Volume, there's added brightness and overdrive.  The 100k Audio Taper Volume control really has the right feel.

Compared to the small DIY amps that I've built and my cheap-o practice amp (modern 15w/1x8 solid state), this Transformer amp feels more like a traditional guitar amplifier.

PRR has shared a cool project here, indeed.

-Corey

[chptunes]

I'm a dummy.

I accidentally sent -12vDC into the circuit, instead of +12vDC.. for about 15 seconds.

Do you think any components were harmed?

[gcme93]

Does it still work? That'll tell you if anything was harmed

Did anything go bang? That's also another good indicator

[chptunes]

Does it still work? That'll tell you if anything was harmed

Did anything go bang? That's also another good indicator

Haha.. No bang.

I replaced the plug on my adapter and accidentally reversed the polarity.  I didn't have time to fix it yet.  But even if it works properly tonight, I'll be wondering if some of my components have been compromised...

-Corey

[chptunes]

Fixed my adapter.  All is fine.

Actually, after another session, the amp is quite dirty at full tilt.  I like it.. very much.

-Corey

[PRR]

> accidentally sent -12vDC into the circuit

C6 (main power bypass) is the obvious firecracker. Even if it works now, I'd be inclined to replace it.

The rest of it.... I'd have to think too hard to guess what could go wrong. Small transistors probably OK at 12V reverse. TIP120 "should not" turn-on, if it does it can probably swallow more current than most supplies can deliver. Actually the 10 ohm will limit current to a large Amp.

Don't do it again.

[chptunes]

> accidentally sent -12vDC into the circuit

Don't do it again.

Already installed a 1N4001 for protection.. dropped my supply voltage to 11.4v.. the amp is happy.

Tried some BC107B TO-5 transistors.. they're a little dirtier than the 2N2222.  I'm wondering how the Negative Feedback Loop effects tone.. may try bypassing it.

-Corey

[chptunes]

Installed a "half-power" toggle switch (SPDT).. an Impedance selector, really.  It just places an 8 Ohm Resistor [rated for 3 Watts] in Series with the output jack.. if I connect an 8 Ohm cabinet, the total load becomes 16 Ohms.  This mismatch with the Output Transformer cuts the wattage by half, and yields a warmer tone with less sensitivity.  The difference is obvious and useful.

Also, I've added a B50k Pot which will be used to adjust the amount of Negative Feedback.  It'll be in Series with R8 (15k, which will be replaced with 1k).  This should leave a range that's near 1k thru 51k.

Added an LED too.






-Corey

[PRR]

I really don't think you want 1K NFB. It messes the DC point, and will probably squeal, perhaps supersonic. Also you can't dial-OUT the NFB, which you probably do want for ab-max gain and brashness/nastiness.

Keep the 15K or drop it to 10K. Put a 10K or 50K pot right across the OT primary. Take the NFB there. When the wiper is at the Collector it is max NFB, low gain, max fidelity. When the wiper goes to the B+ end of the winding there is zero NFB, high gain (perhaps enough to be unstable on your layout), and 26% THD just before an ill-defined clipping.

An alternative "power" control, which actually reduces power drain and waste, is to switch-up the 10 ohm resistor to 22, 47, or 100 ohms. You will feel the power transistor run cooler, and it will overload at lesser room volume.

[chptunes]

Thanks for the improved Feedback control.  So, using a Pot in this fashion allows balance between the paths on each outside lug.. so, the larger a Pot's resistance, the more definate each extreme setting would become?  I'll use the B50k.

Switching the TIP120's Emitter resistance.. I do have some nice Make-Before-Break Rotary Switches [1, 2, and 4 Pole].. I could set-up a Power Scaling switch.  Should I switch Taps on the OT at the same time?

The designed output of 0.3-0.4w is superb for my purposes.  Strong clean tones at conversation level, and just enough grunt to sound "loud" when cranked.  But, "Impedance Selector" and "Power Scaling" controls are nice-to-have and fun to install and use.  I have wife/kids/babies

[chptunes]

Keep the 15K or drop it to 10K. Put a 10K or 50K pot right across the OT primary. Take the NFB there. When the wiper is at the Collector it is max NFB, low gain, max fidelity. When the wiper goes to the B+ end of the winding there is zero NFB, high gain (perhaps enough to be unstable on your layout), and 26% THD just before an ill-defined clipping.

I wired in the improved Feeback Control.. thumbs up.  A distinct "crackle" occurs as the Pot is rotated near the zero NFB end [like when adjusting the Volume control of a SHO].. adds a fun raunchy attitude.  This control gives a lot of personality to the amp.

[chptunes]

Update:

This little 12vDC amp is getting regular use in the corner of my living room.  As far as tweaking/fine tuning.. I've changed C4 from 0.1uf to 0.01uf, which rolls some bass and reduces the overall gain.  I've substituted 2N2369 trannies for Q3 and Q2, lowering the gain further.  With my Esquire, and a 1x12 Weber Ceramic Sig, the tone is balanced and controlled.

Thanks again Paul R.