I built this little amp on Perfboard last night, but the output is not right.. it's too low. I know that it's supposed to have low output, but even with a cranked Fuzzbox in front, I have to put my ear down by the speaker to hear the whisper output.
I only made a couple of deviations from the schematic. I added a 100R Resistor and a 220µf Cap to filter the 9v supply (Alkaline battery).. and, I used a 2N5088 Transistor instead of the 2N3904. My battery measures about 9.2v and I'm getting a strong 8.9v after the 100R/220µf Filter. ..I've been tweaking the Bias trimmer...
The enclosure is a 1590B.. Input Jack is a Switchcraft 12B, with the Battery snap's Ground connected to the 'switch' lug.. Output Jack is an insulated (black plastic thread) type, so the Transformer's Output does not contact the circuit's Common Ground.. no other controls or external parts.
It's connected to a 1x12 cabinet loaded with an 8 Ohm Weber Signature Ceramic speaker.
Any thoughts, personal experiences, or ideas are greatly appreciated.
(http://s6.postimage.org/7u7myz27l/Ultra_Class_A_Superdrive_Power_Amp.gif) (http://postimage.org/)
pic upload (http://postimage.org/)
I referenced the redrawn schematic at Beavis Audio too.. ensuring that I made all the right connections with the color-coded Transformer wires.
(http://s6.postimage.org/5jrydrkz5/Class_A_Superdrive_Power_Amp_schem_ALT.jpg) (http://postimage.org/)
free image hosting (http://postimage.org/)
-Corey
Debugging page (http://www.diystompboxes.com/wiki/index.php?title=Debugging)
Transistor voltages (there's only 3).
Quote from: PRR on January 10, 2013, 05:03:00 PM
Debugging page (http://www.diystompboxes.com/wiki/index.php?title=Debugging)
Transistor voltages (there's only 3).
Hi PRR.. Fresh readings:
Battery- 9.2v
Collector- 9.0v
Base- 6.7v
Emitter- 6.2v
I've build it. I has a very low output, less than a LM386, lower than 1/4W for sure (0.0405W ???). I've used the MPSA18 and the TL013 transformer from musikding. Not amazing and did not made a permanent version out of it. If you use it as a preamp it has a very Fenderish sound.
Cheers.
I tried it as a headphone amp, with a preamp the volume would be perfect.
Quote from: psychedelicfish on January 10, 2013, 08:14:06 PM
I tried it as a headphone amp...
I also tried it as a headphone amp, but it was to sparky for my ears.
Quote from: tca on January 11, 2013, 04:23:23 AM
Quote from: psychedelicfish on January 10, 2013, 08:14:06 PM
I tried it as a headphone amp...
I also tried it as a headphone amp, but it was to sparky for my ears.
Hmm.. really.. it's
that low in output? Well, mine *might* be working properly.. I just thought that it would be louder than a whisper when driven by a cranked Fuzz into a 12" loudspeaker.
If mine is working correctly, then it's just not useful. My plucked strings are louder.
I'm gonna scrap the Ultra Class A and put a 1w Punch amp (TDA7052) in this enclosure instead.
Thanks for the help...
-Corey
Somewhat unrelated, but how do you like that weber speaker? I have two in an old Showman cabinet and I absolutely love them.
> 1x12 cabinet ... Weber
Yes I was glad of that, because it sure needs a good-size guitar-type speaker to have any hope of audibility.
> Battery- 9.2v
> Collector- 9.0v
> Base- 6.7v
> Emitter- 6.2v
The MAXimum (ha!) power output would be with Emitter closer to 3V.
That maximum looks like 0.0176 Watts. (20 times less power than a lowly LM386.)
However putting E up near 6V will increase Gain, And with just one transistor, it sure is short on gain. It maybe can NOT approach MAX power with a typical guitar unless you put another stage between (such as gain/boost pedal, even a buffer would help).
> even with a cranked Fuzzbox in front
OK, tried that.
Hmmmm..... the input sensitivity is near 20mV, but the input impedance is... 1K!! Compare to the 5K-500K of guitar or most guitar-cord accessories. It just sucks (signal). Kinda hasta to get anything done with just one transistor. But we may just be asking too much.
Got any Darlingtons? Put one in. Throw-out that bias pot. Instead bias with 1Meg from 9V to Base, 680K (or 470K) from Base to Ground. That should put Emitter at 3 or 4 Volts. Input impedance will be over 300K, suitable for guitar-work.
On paper the emitter cap should be like 300uFd for full guitar bass; and with Darling and big bias resistors the input cap only needs to be 0.01uFd. 0.05u is fine for input, and guitar often "likes" a little bass-slope (especially when straining for scream) so 100u or 50u may be fine at emitter.
Up the Vcc?
> Up the Vcc?
Up the Irish!!
If gain is the problem (as I suspect it is), then higher supply is a brutal way to get very little improvement. In fact the input impedance _drops_, so we may get nowhere at all.
For Max Power: yes, but. There is now about 6mA of DC current in the OT winding. This OT is made for _zero_ DC current.... the intended use is push-pull where the DC of two transistors cancels. With too-much DC it stops working as a transformer, it shorts the load and output is really poor. Although the OT design is "no" DC current, there's some leeway. I figure it won't suck with 4 or 8 mA of DC. We are already right in that zone.
However, to a point, nothing will smoke. If mis-biased, 12V may be applied. If Base is held close to 1/3rd of supply, it will live at 24V supply. The transistor will be too hot to touch, but it can take that for months, and it's only/under a buck if it does croak.
IMHO the Darlington is the simple direct path to something that may play guitar usefully.
0.017 Watts is puny, but in a good guitar speaker (95dB/W) it plays louder than casual conversation.
If we get away from "simple direct", there are better choices.
As-is, you may re-label the "Loudspeaker" (ha!) output as a guitar-cord output and try it as a tone-shaper feeding some other (real) amplifier.
OK PRR.. I built your Darlington idea, from scratch instead of picking apart the other circuit.
Slightly louder output! But, the signal does not decay smoothly.. As the guitar signal fades, the output cuts out. If I strum hard, the volume is conversation level, but if I pick delicately, there's no output.
We are on the right track, I think.
Readings on the Darlington version:
Battery 9.1v
Collector 9.1v
Base 1.6v
Emitter 1.6v
Should I have omitted the 470 Ohm on the Emitter? I used all the values that you suggested, except my Emitter Cap is 150uf. My Darlington is a TIP120 from Radio Shack.
I'd forgotten the OT sat between Vcc and the collector, thought it was between the collector and ground (with a cap).
..another thing.. Since I'm anxious to get this going, I was just measuring my voltages again. With only a guitar lead plugged into the input (and the Weber on the Output of course), the Collector is getting 9.1v but the voltage at the Base and Emitter fall steadily while I'm measuring them. Also, if I plug a cranked boost pedal (Mini-Booster) in front, I hear a steady high frequency tone through the speaker while measuring the voltage at the Base.
> Readings on the Darlington version:
> Battery 9.1v
> Collector 9.1v
> Base 1.6v
> Emitter 1.6v
Base and Emitter should not be the same voltage...
> signal does not decay smoothly.. cuts out. ....My Darlington is a TIP120
Argk! My fault. I should have been more explicit.
I was thinking a jellybean Darlington, same-size as your '5088 except Darlington. Typical popular parts are MPSA12 MPSA13 MPSA14. Or two simple jellybeans wired together.
The TIP120 is a great part, except intended for use at 100X to 1,000X the current we need here. We can often get away with a generous transistor, sure. But the TIP120 has the "convenience" of bleeder resistors. When a high!-gain transistor gets HOT its own leakage can turn it on unexpectedly; the TIP120 does not do that easily. But for LOW currents these bleeder resistors suck-off the small bias we need for low current.
Can you find or rig a *small* Darlington? A couple 2N5088 '3904 '2222 etc.
http://en.wikipedia.org/wiki/Darlington_transistor
I note that your top plan (http://s6.postimage.org/7u7myz27l/Ultra_Class_A_Superdrive_Power_Amp.gif) shows MPSA14 (Darlington), but also MPSA18 (non-Darl) and other simple transistors. The difference should be very obvious, so I wonder why that is not noted.
Why bother with a Darlington? why not just get yourself something like a BD139, stick a heatsink on it, and that should give you about 1 watt... ;)
> BD139, stick a heatsink on it, and that should give you about 1 watt...
With guitar-level input? With 1K load and 9V supply?
In this circuit, BD139 is likely to suck the guitar more, not less.
And with 1K transformer load and 9V supply the maximum power (given very strong drive, far more than guitar-cord level) is 0.040 Watts, dissipation 0.081 Watts, so the fat tab on BD139 is wasted effort.
Yes, I've seen plans with 12V and choke-coupled directly to 8 ohms delivering 7 Watts (dissipating 18 Watts!). This needs anOther "power amplifier" to drive it.
You can only do so much in one stage. And I don't expect miracles from 2 stages.
Let's do math. Say we want output of 0.05 Watts (pocket transistor radios always had at least this much). How much power can we extract from guitar? Say hard strum is 0.2V. But load less than 100K will clobber the highs. So we can extract 0.2V^2/100K. This is 0.000,000,4 Watts. We need a power gain of 125,000 times. 50dB. Turning to GE 1964 I find that the best class A power amps at 9V and 0.05W give power gain of 38dB. And even that is hard to reach. So a 2-stage amp is a minimum.
Copy that. I changed the tranny to a MPSA12.
The result is about the same.. Conversation level with hard strum, and cut output as guitar signal decays.
C 9.1v
B 3.5v
E 2.8v
Thanks for sticking with me.. What do you recommend next? My Bias resistors are 1M and 680k.
Quote from: chptunes on January 13, 2013, 01:20:36 PM
My Bias resistors are 1M and 680k.
Instead of the 100k pot?
Quote from: tca on January 13, 2013, 01:30:51 PM
Quote from: chptunes on January 13, 2013, 01:20:36 PM
My Bias resistors are 1M and 680k.
Instead of the 100k pot?
Yep TCA.. Mr. PRR has suggested some changes with a MPSA12. Read a few posts above...
If I'm seeing this right the MPSA12 can dissipate about 612mW, in a class a configuration you can only get at most half of it, 306mW. For 9V you will need a transformer of 135Ohm for the primary. This is, somewhat, the value (100Ohm) that appears in some class b push-pull late 50-60's transistors amplifiers. There is no way around it.
If you look at Escobedo's circuit he says that the current draw is about 4-6mA, and so, for 9V this gives the amazing 22.5mW! Unless your speakers are really, really, sensitive >=100dB it is better to build another amp (that is what I've did).
P.S.
Although it is always fun to play with this circuit.
Time to re-draw.
http://i.imgur.com/0Wa9R.gif
> B 3.5v
> E 2.8v
I'd expected B to be more than 1V above E; but the meter may be loading-down the B point a whole lot more than the E so this may be OK.
The E number should be trustworthy (no meter loading) and is in the OK ballpark.
It may be good-as-it-gets. However it is odd that two respected gurus have drawn and posted this.
Yea, I may be "barking up the wrong tree". ..just thought it'd be cool to get a simple low powered transformer amp working.
I brought the 680k up to 890k and now I'm reading 3.5v on the Emitter.. still, the output cuts as signal weakens.
Enough is enough, I guess. Thanks for all of your help.
-Corey
> the output cuts as signal weakens.
It may be enlightening to meter that point while you play.
We may have run into another problem. The emitter cap can charge-up a whole lot better than it will bleed-down. So bias is probably changing all the time.
I can think of so many ways to modify this, each opening new problems, and no good reason to complicate.
There's lots of other transistor+transformer power amps. Some REAL power, such as 1956 Cadillac car radio. Delco transistor on heatsink the size of two packs of Parlaments (not the funkadelic kind). But it's hard to expect to be convincing without three stages. (IIRC, that's about the minimum I've seen in commercial practice amps.)
Mispost.
> 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.
(http://i.imgur.com/tZNzUqf.gif)
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.
Wow! Sweet design PRR.. I'm tempted to search out parts and get started. Do you think this amp will remain clean with a boosted guitar signal driving it? Which component would overdrive first, if overdriven by a boosted guitar signal?
.. ...
-Corey
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.
Quote from: PRR on January 23, 2013, 01:59:53 AM
...$1/watt has been the mark for decades, at least in larger utility amps...
Nice benchmark... in my case that would be 0.75€/watt.
I remember seeing a half a watt TL431 (adjustable shunt regulator) in the datasheet made by Motorola (http://pdf1.alldatasheet.com/datasheet-pdf/view/5774/MOTOROLA/TL431.html)
Check page 9 Figure 29.
But never built it.
Cheers.
Thanks for the link tca. :)
To be sure.. I am enjoying my 1w Punch Amp very much. Cool clean little power amp.
Quote from: PRR on January 23, 2013, 03:10:31 PM
> 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.
Yeah! I'm grateful that you shared this cool design PRR. In order to show my gratitude (and for fun), I'm gonna build it. I'll lend you my impressions and thoughts.
I have everything in stock except for the 10 Ohm Resistor and the Transformer. I'll order the Bogen T725.. Data Sheet (http://www.bogen.com/products/pdfs/specialelectronicspdfs/WMT1As.pdf)
Several stores sell this Bogen device on-line, including Amazon and Best Buy: http://www.bestbuy.com/site/Bogen---T725-Step-Down-Transformer/4472885.p?id=1218477791887&skuId=4472885 (http://www.bestbuy.com/site/Bogen---T725-Step-Down-Transformer/4472885.p?id=1218477791887&skuId=4472885)
-Corey
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).
Cool.. thanks..
One more.. The Negative side of the output is connected to Common Ground. This is not the case with other small amp circuits I've been seeing. Is there a clear explanation for this?
-Corey
Quote from: chptunes on January 25, 2013, 09:10:49 AM
Cool.. thanks..
One more.. The Negative side of the output is connected to Common Ground. This is not the case with other small amp circuits I've been seeing. Is there a clear explanation for this?
-Corey
I think it's to tie the lower end of the output to "something" stable, so you can "measure" the output on the upper end. You can connect any side (in simulation, or in the real amp) to ground with no difference in function.
> 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.