'Ultra Class A Superdrive Power Amp' :: Questions

[chptunes]

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.


pic upload

I referenced the redrawn schematic at Beavis Audio too.. ensuring that I made all the right connections with the color-coded Transformer wires.


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-Corey

[PRR]

Debugging page

Transistor voltages (there's only 3).

[chptunes]

Debugging page

Transistor voltages (there's only 3).

Hi PRR.. Fresh readings:

Battery- 9.2v
Collector- 9.0v
Base- 6.7v
Emitter- 6.2v

[tca]

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.

[psychedelicfish]

I tried it as a headphone amp, with a preamp the volume would be perfect.

[tca]

I tried it as a headphone amp...

I also tried it as a headphone amp, but it was to sparky for my ears.

[chptunes]

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

[wavley]

Somewhat unrelated, but how do you like that weber speaker?  I have two in an old Showman cabinet and I absolutely love them.

[PRR]

> 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.

[Perrow]

Up the Vcc?

[PRR]

> 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.

[chptunes]

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.

[Perrow]

I'd forgotten the OT sat between Vcc and the collector, thought it was between the collector and ground (with a cap).

[chptunes]

..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.

[PRR]

> 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 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.

[psychedelicfish]

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...

[PRR]

> 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.

[chptunes]

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.

[tca]

My Bias resistors are 1M and 680k.

Instead of the 100k pot?

[chptunes]

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...