9v Guitar Amp w/out LM386

[Audioguru]

Don't electric guitars need the severe distortion caused by crossover distortion?
The speaker might rectify audio if the output coupling capacitor is removed since the DC will force speaker cone one way either in or out then it cannot swing normally in and out. Another way to increase the distortion.

 

[Don Perry]

Why would the need that distortion?

 

[AnalogKid]

Don't electric guitars need the severe distortion caused by crossover distortion?

Not sure "need" is the right term, but ...

I've seen three kinds of distortion mechanisms for guitar effects:
Diode clipping - sharp transition corners, lotsa high freq harmonics
Overdrive - soft transition corners for a fat sound without the harshness
Crossover - some people just like it.

LM301's still are used in new designs, and uA709s are prized.

ak

 

[Don Perry]

Alrighty so I've to order a few parts, resistors and transistors but here's the first draft.

No output yet but that's not surprising. Do the caps have to be that large?

 

[AnalogKid]

The output coupling capacitor is calculated from the speaker impedance and the lowest frequency you want in the flat passband range. C = 1 / (2 pi f R). 8 ohms and 100 Hz works out to 200 uF. The power supply filter cap wants to be at least as large.

Whatever the output power is, around 40% of that could be dissipated in the output transistors.

ak

 

[Audioguru]

Your transistors appear to be small and might not conduct the high current needed or they might overheat.
Please post the schematic showing resistor values and transistor part numbers.

 

[Don Perry]

Hey so I'm waiting for new parts. In the meantime I've assembled this circuit:
It sounds great but it's a bit quiet. First off, THANK YOU GUYS SO MUCH! I'm a dolt with analog circuits but you guys really sent me in the right direction!


What can I do to get more volume out of this speaker? Also, is an 8Ω speaker ideal for this circuit? What would you guys do?

• It's pretty loud but I want it a touch louder. (not really important, it's pretty loud)
• Q2 is getting pretty hot, what can I do about this?
• Which transistors would be best?
• Which speaker impedance would be best?

Here's what it looks like



Here's what it sounds like

 

[davenn]

What can I do to get more volume out of this speaker? Also, is an 8Ω speaker ideal for this circuit? What would you guys do?

• It's pretty loud but I want it a touch louder. (not really important, it's pretty loud)
• Q2 is getting pretty hot, what can I do about this?
• Which transistors would be best?
• Which speaker impedance would be best?

you have the speaker connected incorrectly .... probably why Q2 is getting hot
you also have C1 and C2 reversed

be more careful following a schematic when building the actual circuit

go back and look at the schematic I supplied

 

[Don Perry]

you also have C1 and C2 reversed

One day my dyslexia is going to get me killed.

you have the speaker connected incorrectly

Do you mean that physically I have it connected wrong or that the above diagram is incorrect? I've looked it over a few times and other than the caps, I think it's right... I'll keep looking!

 

[73's de Edd]

Sir Don Perry . . .

You say . . .
One day my dyslexia is going to get me killed.
Alas . . . afraid to tell you . . . . but you have digressed even further, all the way to full blown LYS-DEXIA.

4 VARIANTS OF SCHEMA REFERENCING . . . .versus . . . . YOUR ACTUAL END BUILD . . .

REF A is being the submitted schematic, excluding a disagreement with C1 being a 470 ufd unit, as I don’t that you can ever get a long enough E string or be able to hit for whipping out a hot lick of decimal 026 cycle note. Even the suggested 4.7 ufd is being excessively liberal.


REF B has the RED markups where you swung the speaker up to the 9V supply buss vice ground. With an additional unknown gain factor by the R1 not going to 9V.

Your C1 polarity lucked out in that arrangement but would be reversed when the speaker normally goes to ground.

You left out any current limiting and balancing due to the omission of the two emitter’s current limiting resistors.

You omitted the R5 resistor on Q3 base BUT try its sound quality and gain effect in BOTH ways in trials / evaluations of the final amp.

You have both a power loop thru the Q1 transistor and the speaker, so I can see why ole Q2 down there is getting pissed off and hot under the collar by having to pass both their loadings thru himself.

REF C has the YELLOW areas of involved changes to be made.


REF D has an amp stage in the front to bring your overall volume up a bit more . . . . as your audio clip demonstrated, I would classify that as “strumming” volume level.

In the units final testing see if the overall pleasing result is being better with or without R5 in circuit.

I think that you have bled out this dedicated circuits limits with a 9 V supply level and those

bipolar outputs transistors that are being utilized.
If having any propensity for abnormal effects, consider the shown 1k of filtering / bypassing across the 9 V battery supply . . . referenced at amp D circuitry.

Tube circuitry typically uses the push pull circuitry designation while this transistor circuitry is usually specified as complementary symmetry output.


73’s de Edd

 

[duke37]

The emitters of Q1 and Q2 should be sitting at 4.5V. If Q2 is too hot, then this voltage is too high.
R2 is the feedback and could be adjusted to achieve this but the voltage will be temperature dependant.

 

[Don Perry]

4 VARIANTS OF SCHEMA REFERENCING

Wow, thank you! I can't wait to get in and try this out.

 

[AnalogKid]

In schematic B, the Q2 emitter is grounded so that transistor is running wide open no matter what its collector load is. I like using the output capacitor and speaker as part of the bootstrap. I saw that a loooong time ago, then forgot about it. Cute.

ak

 

[Don Perry]

In schematic B, the Q2 emitter is grounded so that transistor is running wide open no matter what its collector load is.

This is bad, yes? Sorry I'm really new to analog circuits... I took digital logic in uni but that was years ago, and doesn't help.

 

[AnalogKid]

No, it's the way it is supposed to work. One problem with schematic B is that Q1 is "dangle-biased", where the base current is kept so low that the circuit's operating point is set by Q1's gain. This value varies with changes in collector current and temperature, making the operating point drift. In schematics A and D there is an additional resistor from the base to GND, and both base resistors are lower in value. This forces the base to a specific static current that is semi-independent of the transistor's gain.

ak

 

[Don Perry]

Thanks I understand that, very well stated!

 

[Don Perry]

REF C has the YELLOW areas of involved changes to be made.

Hi again, quick update before the weekend sets in.

I've completed REF C with the exception that R5 is 1K (waiting for 1.5 in the mail) . I'm not sure why but it's not outputting at all. I've been digging into and going over it for a few hours with no success. I'm sure it's something silly. I'll start probing it tomorrow morning with my new scope (yay!) I'm very optimistic and will be spending the rest of the evening gormandizing education content on youtube. I'll check back in this weekend with my progress. As always, thanks guys!


EDIT: The input leads look backwards but they are connected correctly.

 

[Audioguru]

The loudness is low because the maximum power in the speaker is puny, about 0.25W like a cheap clock radio.
It will be a little louder if the output transistors are replaced and the supply voltage is increased. The 3.3 ohm values of the emitter resistors for the output transistors are much too high and are throwing away almost half of the puny output power. Bootstrapping would increase the gain and the maximum output power.

When R5 is 1k then the voltage divider of 100k and the 1k produces a base voltage for Q3 of almost nothing which turns it off. 1.5k will also turn it off. If you want to use an R5 then a reduced value of the 100k resistor must be re-calculated (maybe 8.2k when R5 is 1.5k) which will reduce the amplifier sensitivity a lot.

The new schematic shows two R1's and two R2's which causes confusion.

 

[73's de Edd]

Sir Don Perry . . .

Your last breadboard rebuild fault shown here, seems to be the use of 33 ohms ( Orange-Orange-Black + Gold + 5% tolerance) for RE1 and RE2 emitter resistors, now resulting in current availability being even worse than AG’s comment.

IF eventually being used, up the wattage ratings from the presently shown units.

If you want to proceed forward NOW and ride bareback instead of with a saddle.

Just try omitting the emitter resistors as you did before. No problem in using two series 1K in place of R5 to then see if the overall performance / voicing result is being better for you with or totally without R5.

Then, wiring in the additional frontal preamp xstr circuitry will see if your slight extra volume need is then met .


BREADBOARD WIRING AND COMPONENTS CONFIRMATION:



73’s de Edd


.

 

[Don Perry]

I'll try it first thing in the morning!