Preamp Gain

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OK, so that means we're talking line level throughout. If a passive preamp (just a volume control) isn't quite enough then you need modest gain.

This is based on the Radford HD250 and gives around 7db gain.
 

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No, but I do have the full board layout its lifted from. You would have to add R1 and C1 to the design. Or you could build the full version with tone controls.
 

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I couldn't endorse that as a being a good design based on the information given on the diagram. The description of the gain being R2/R1 is just wrong, it makes no mention of the fact that R1 is not really intended to be varied much, and a supply range of maybe 9 to possibly 40 volts shows the design is compromised and trying to be all thing to all situations... and its not.

Nothing wrong with the basic topology (which is pretty much a text book implementation), its just the 'one size fits all' I don't like.
 
What do you think a cap from output to ground will do for the signal ? It will shunt it to ground and you'll get nothing useful.

Likely the issue is feedback via the power rail. You probably used the power supply of the power amp for the pre amp without any decoupling because Mooly didn't show any abd you copied it blindly. You need to place a resistor in the power rail between the pwr amp and pre amp along with that 220 uF capacitor of yours between the preamp power rail and ground. The resistor can be arounf 100Ohms, more is better if the preamp will work with a bit of voltage drop. This forms an RC low pass filter which keeps the preamp power rail free of signals riding on the power amp power rail caused by the large current flows along it when music is playing through the speaker.

Just a guess from reading this thread.
 
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I never suspect bad parts behind simple beginners project problems.

I *always* suspect lack of experience/skill.
I *clearly* remember my own beginner projects, none seemed to work properly or work at all.

WHO will fake BC-something 5 cent transistors and 1 cent resistors?

Transistor brand and model is so irrelevant in these simple designs that respected Elektor Magazine ceased posting specific numbers and for most such projects only specifies TUN , TUP , DUS , DUG and so on.
As in: TUN means Transistor Universal NPN, *any* transistor meeting certain (very easy) parameters will work there.

dugdustuptun_03_tuptun_caracteristicas.gif


so in this particular project I´d rather worry about poor layout/grounding/powering/shielding/soldering/etc. rather than mythical "bad parts".

some transistors (out of thousands) which meet such specs
dugdustuptun_06_tuptun_caracteristicas_2.gif


an extended table showing usual pinouts so you don´t need to downlod individual datasheets:
dugdustuptun_05_tuptunes_cases.gif


I build amps commercially, and even so only stock BC547 and BC557 for 99% of small signal transistor duty, plus 2N5401/5551 for up to 120V duty on my highest power amplifiers.
Have a few MPSA42/52 ones because they can stand even higher voltages, but almost never use them, specially because high voltage specs are paid (every design is a compromise) with very poor Hfe .
 
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Yes, I remember the TUN and TUP from years ago.

I believe this preamp is being used on 35 volts and so its possible there could be transistor breakdown if using unsuitably rated devices.

Knowing the voltages on all pins of the two active devices might help us gauge a little better what might be happening.

Also, does the preamp work OK (with music) when running on lower supply voltages.
 
Yes, I remember the TUN and TUP from years ago.

I believe this preamp is being used on 35 volts and so its possible there could be transistor breakdown if using unsuitably rated devices.

Knowing the voltages on all pins of the two active devices might help us gauge a little better what might be happening.

Also, does the preamp work OK (with music) when running on lower supply voltages.

I highly suspect the transistors are fake! The printed transistor numbers looks as if a 2 yr old has written it with a crayon.

Didn't try with music. Will do that tomorrow.
 
Tried the decoupling. Didnt work. Seems like the oscillation frequency has probably halved but it's still there.

Here is what I did. Connected a 1.5 k resistor in series with the power rail and a 220uf cap across it to the ground.

I hope the capacitor was on the preamp side of the 1.5k resistor?
By the way, this is quite a large resistor, maybe you lowered the rail voltage to the preamp too much with this value, but then again, if it didn't help then the problem is elsewhere.

Mooly, my concern was not the transistor stability but the fact that the base junction of the input device is dc-biassed from the power rail with a simple pair of resistors and usually I see that most designers like not to have that without some decoupling because any noise on the rail is coupled to the base of the input device - you can get feedback via the rail - am I wrong in this case though ??
 
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It may help to know what frequency it is oscillating. I have found that the cause is different in different frequency ranges. For example, I've had MOSFETs oscillate locally at the output stage only at rf frequency (MHz) and so fast and sensitive to parasitics that I could never find it on a scope. But oscillations at lower frequencies, say up to 100kHz were usually some local feedback around the whole amplifier due to wiring issues, could be wires all tangled up on the experimental bench top, or I had some feedback due to poor grounding with the way it was wired up for experiments.
 
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