Weird noises coming from amp

[SJF]

Hi,
I joined here a while back but haven't done much electronics since then. I've got the bug again and breadboarded up a small amp based on a AN7511 chip. I read about in a silicon chip mag - "The Champion" is what they called. Its a 1-3w amp depending on voltage. Its their update to the long running "Champ" kit which was based on lm386.

Anyways I just breadboarded the stock standard amp as per the panasonic technical data sheet http://www.semicon.panasonic.co.jp/ds4/AN7511_BEB_discon.pdf
I bought a couple chips at Jaycar.

The stock amp sounds fairly nice on the breadboard but clips a bit harshly when you dig into the strings a bit.
My plan was hook up a runoffgroove fetzer valve as a preamp and see how it would improve things.

The Fetzer is working nicely on its own bread board and the An7511 amp is working nicely on its own breadboard. But when i take the output from the fetzer and insert into the Amp - I get weird oscillations only - kind of like a synth. When I turn the volume pot on the fetter i get a frequency/pitch change of the noise.

Anyone have any idea of what is going on - i,m stumped. My brain has fried. Thought i would ask you guys.
cheers
SJF

[phatt]

Do you have a common psu or does each unit run from a separate supply?

Have you got ground connection between units?

Is the output of 1st unit decoupled from 2nd unit? (i.e a cap)

If you are still stuck others here will likely know more. :tu:
Hey at least it makes a noise which is better than no noise.  :tu:
Phil.

[Roly]

Give up guitar and take up synth? 

Agree with @phatt, this sort of instability commonly arises from the power supply arrangement.  If the preamp is running from the same supply as the power amp then you will need a small resistor, say around 220 ohms, in series with the preamps supply, then a bypass cap across the preamp, say around 100uF.  The power amp of course must have its own good supply bypassing as close as possible to the chip.

Informal lashups can often have stability problems, particularly due to the power supply, but also due to general layout and proximity between stages with high gains.

[phatt]

Give up guitar and take up synth? 

Agree with @phatt, this sort of instability commonly arises from the power supply arrangement.  If the preamp is running from the same supply as the power amp then you will need a small resistor, say around 220 ohms, in series with the preamps supply, then a bypass cap across the preamp, say around 100uF.  The power amp of course must have its own good supply bypassing as close as possible to the chip.

Informal lashups can often have stability problems, particularly due to the power supply, but also due to general layout and proximity between stages with high gains.

Did someone mention SYNTH!!!! 8|

Psst,, I know you play keys Roly. 
Go check out the Roland VR09,, I'm Very Very tempted at only ~$1200 Aus gets you one.
Roland have obviously finally got the plot,, We need live perfomance gear not 10,000 patches of swooshy sounds that are friggin useless on stage.
OK back on topic.
Phil.

[SJF]

Hi and thanks..
Yeah I originally had 2 breadboards, but a single 9v battery clip hooked to the preamp and jumpered to the other. So maybe problems there.

I ended up re-doing the whole thing on a larger multi -lane bread board that allows for separate power supplies, but again I have jumpered it all the way around so in effect it's one supply. With this setup I now get silence when the preamp is attached to the power amp???

Reason I have been attempting it that way is I now have no 9v batteries so I  rigged up an old dick smith brand multi power adapter- that puts out 3.5, 5, 9 and 12v. Chopped off the end split the lead and hooked it into the power rails via this circuit snippet http://beavisaudio.com/bboard/projects/bbp_FilteredPSU.pdf. It's working we'll and there are no noise issues with the amp. As I say it works but clips, like a lot of amps not designed for guitar input.

Will have a look and try your suggestions and report back. I think you guys may have the answer. This one has just about done my head in. But I'm here to learn  :dbtu: so it's all good.
Cheers
Steve

[phatt]

Hi Steve ,Great to hear you got it sorted. :tu:
Maybe just don't expect a 1 watt chip to produce clean sound especially when driven by an electric guitar and extra preamp.
Phil.

[Roly]

Well point the first, the bench power supply may have a lower source impedance (better filtering) than the battery you were using, particularly if the battery wasn't brand new.

Point the second is that the Beavis circuit you linked contains series resistances (isolation) and shunt caps (bypassing) so this should also provide a power supply line that is better grounded to AC and perhaps even better isolated between power and preamps.

{DC supplies have filter caps which means that for AC signals they are (or should be) effectively ground.}

[Kaz Kylheku]

First of all, about wattage. If you look at the ambient-temp versus dissipation graph, it gives 850 as the maximum without a heatsink, which derates above 25 degrees at 6.75 mW per degree celsius. If you want anything more than 850 mW out of these chips, you must stick a heatsink on them.

The thing is also quite sensitive for the amount of headroom. An input well below 100 mV drives to saturation with THD above 10% (grossly audible). See the Po, THD - Vin graph on  P. 6. Basically, you can easily clip this thing out by directly plugging a guitar pickup into it, and one that isn't even hot. Something like a DiMarzio PAF will put out 300 mV, supposedly.

It's not clear to what extent if at all this lack of headroom will improve with a heat sink.

If all you want is to amplify the output of a preamp into a small wattage, you basically do not need any voltage gain beyond what the preamp is capable of. You just need an output stage that can drive the speaker (e.g. class B push-pull emitter follower, with a buffer in front of it so the preamp doesn't have to deal with the low-ish impedance of that stage).

About the noise that you get when you connect the circuits together, from the description it sounds as if it might be motorboating: a positive feedback loop through the shared power supply. Motorboating is a common plague of single-supply circuits with multiple AC-coupled stages. The circuits have poor power supply rejection, and so parasitic feedback loops form through the power supply connections. These AC couplings introduce phase shift at their cutoff frequencies, and so you can end up with a low frequency phase shift oscillator.

I see the that the Fetzer is just a JFET based common-source stage. But it is an inverting stage. So if there is feedback into its input from the output currents of the amp, it will be inverted, and so 180 degrees of phase shift is enough, which could be achieved by just two capacitive couplings (90 degrees each).

Can you post the complete circuit diagram: Fetzer plus your AN7511 amp?

[SJF]

sorry for the late reply. Thanks guys. 

Phatt and Roly. Yes seperating the circuits and providing each with a power supply and being more carefull with my grounding has solved the oscillation issue. Thanks for that..

Kaz Kylheku - yes you are right, it completely slipped my mind. The silicon chip circuit has a heatsink. I just found a basic buffer as supplied here on the Beavis site



Would this be suitable? Will hunt down a small sink and try that this afternoon? Its a lovely cold, damp day to do electronics here

Here is a schematic I have done showing my version of the amp. Same as what is in the silicon chip mag but I have stuck to the stock values in the chip spec sheet for the standby and mute switches on pins 1 and 4. The silicon chip values differ because they add a transistor and switches. I just wanted the standby and mute to be off all the time. The panasonic spec sheet shows the 47k resistor going +vcc but i couldn't get it to work, it does work if grounded. May be an error??? The values on pin 2 are from the Silicon chip schematic - and they have made a vast difference to the stock amp in the spec sheet.

I have to say I'm pretty impressed with how clean and loud this amp is on just 9v supply. When I did the Silicon chip kit it came with their preamp which was tailored to dynamic mics. It didn't sound great with an electric guitar with hum buckers.

The stock amp without the preamp sounds really clean. Tried it with an EHLPB-1 clean boost. Pretty playable with the pot in the 1st quarter of a turn - anything Above that it gets a bit too overdriven/distorted - loud and feeding back.





If anyone can suggest any other changes. Will try them and report them back as well.
Happy to be making some progress - cheers all

I,m not sure if I would be allowed to post a copy of the silcon chip schematic??? Are there copyright issues here. Sure would be great if you guys could see that??
SJF

[Roly]

For direct input from a passive guitar you would be better off using the TL071 op-amp circuit, changing R1 and R2 to 2.2Meg to get 1Meg input resistance, and if you really need more gain than the chip amp's 30-odd dB, then altering the TL071 feedback from a voltage follower to a degree of gain.

[SJF]

Thanks Roly - what do you suggest changing the voltage divider on TL071 buffer for piezo/acoustic guitar?

[SJF]

For direct input from a passive guitar you would be better off using the TL071 op-amp circuit, changing R1 and R2 to 2.2Meg to get 1Meg input resistance, and if you really need more gain than the chip amp's 30-odd dB, then altering the TL071 feedback from a voltage follower to a degree of gain.

Thanks Roly yhat has made a pretty good improvement - its starting to sound like a guitar amp. The problem I seem to be getting now is that after a few minutes of pay the cip starts to get overloaded and gets noticably distirted sound creeps in. I think Kaz mentioned that i need to get a heat sink. How does one place a heatsink on 8pin DIP on a breadboard. I have a couple small heatsinks but they require a screw/nut/washer and generally go through a pcb. Found this on the jay car website. would this be suitable  - its a type of adhesive heatsink for different package - but ... http://www.jaycar.com.au/productView.asp?ID=HH8580

[Roly]

The main problem with the transistor amp shown in your post #8 is its low input resistance.

what do you suggest changing the voltage divider on TL071 buffer for piezo/acoustic guitar?

Piezo pickups require very high input impedance/resistance or they suffer serious loss of bottom end, sound thin and brittle.

A fairly typical value is 2.7Megs.

The TL07x series are FET input and they will tolerate quite high bias resistor values, so if you made R1 and R2 = 4.7Megs this would produce 4.7/2 = 2.35Megs input impedance.  You can experiment with higher values if you like, but eventually you will start to see the op-amp output being offset from the mid point because the leakage resistance of whatever board you build it on will start to become significant.  Fiberglass board (green) is better than phenolic (brown), and Teflon-loaded (blue) should be even better - but try whatever you can get and see how you go.

Even cheaper; find a junked computer and snarfle the heatsink on the CPU.  Even without the fan it will provide some serious cooling if you can arrange the chip hard against it (and even more serious cooling with the fan going).   :dbtu:

[SJF]

I have some 4.7M and 10M resistors to try in my TL072 buffer  - will give it a try over weekend, and do some experiments with an acoustic with piezo. cheers again for the great assistance...
SJF