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Simple Pre-amp problem

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Alex M

Sun Nov 27 2011, 12:13AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Hello, I am trying to put together a very simple pre-amp circuit that I plan to use to boost the audio signal that will be feed to my plasma speaker driver.

However I cannot seem to get it working correctly. My parts for the new plasma speaker driver have not arrived yet so I connected the output to a small speaker and found that the audio is very quiet and distorted. It sounds much better and louder just directly connecting the MP3 player to the speaker which is not what I want.

All connections have been triple checked and the circuit is how it should be.

Can anyone with knowledge please advise on what I am doing wrong. I would greatly appreciate it.

Thanks.

BTW this circuit is not my own design, it is just one I found on the web but it appears to be a very common design.

Sulaiman

Sun Nov 27 2011, 03:23AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

This looks like an amplifier for an electret microphone,
not a power amplifier as it requires a load >= 10 kOhm
so it's probably working as designed
it's just that it's not for driving a loudspeaker.

Proud Mary

Sun Nov 27 2011, 08:42PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Alex, your R2 looks much too low. As a rule of thumb your R2 should be equal to R3*hfe.

So stick one or two meg in the R2 position, and see where that gets you.

Conundrum

Sun Nov 27 2011, 08:59PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Seem to recall a very simple circuit using a single 2N7000 with ridiculous gain at frequencies up to 10 kHz.
From memory it used 2*10M resistors to bias the gate and a 0.1 uF capacitor for input coupling.

-A

Newton Brawn

Tue Nov 29 2011, 03:25AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

HI

The output impedance (5~10Kohms) is no matched with the loudspeaker impedance (4 ohms)..

Try to use a small audio tranformer between the autput and the loudspeaker. Zp=5~10 Kohms, Zs=4 ohms
Or use small 230/5V power supply trnsformer

better sound and higher volume
Regards

Alex M

Tue Nov 29 2011, 10:30AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Thanks for the replies guys, I am going to try your suggestions and see how it fairs.

Regards.

Alex M

Wed Nov 30 2011, 08:02AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Just tried it with a 1mohm resistor and it sounds even worse.

It is really scratchy and hard to hear now.

Proud Mary

Wed Nov 30 2011, 01:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If you are connecting this circuit directly to a loudspeaker, rather than to another stage - as shown in your circuit diagram - there is no way it can work properly. As Sulaiman and Newton Brawn have said, the impedance mismatch is too large - indeed, an 8Î© impedance is not far off being a short across the output.

Alex M

Wed Nov 30 2011, 02:52PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Proud Mary wrote ...

If you are connecting this circuit directly to a loudspeaker, rather than to another stage - as shown in your circuit diagram - there is no way it can work properly. As Sulaiman and Newton Brawn have said, the impedance mismatch is too large - indeed, an 8Î© impedance is not far off being a short across the output.

ok thanks anyway.

BTW what does "impedance mismatch" mean? And doesn't the DC blocking capacitor stop it (the speaker) from being a direct short?

Proud Mary

Wed Nov 30 2011, 04:12PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Alex1M6 wrote ...

ok thanks anyway.

BTW what does "impedance mismatch" mean? And doesn't the DC blocking capacitor stop it (the speaker) from being a direct short?

impedance mismatch, analogy: imagine the flow of water through a very small diameter hose (your output) coupled to one of very large diameter (your loudspeaker) Consider how the velocity of the water will change as it moves from the narrow hose into the large one.

Blocking capacitor: yes the capacitor does block DC, but it is the AC component - your audio signal - which is getting shunted to earth through your low impedance loudspeaker for very little effect.

This circuit would work with high impedance headphones - say 2000Î© - but these are usually expensive, and not easy to come by.

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