Amplifier issue - Velleman

Thread Starter

CosmicOrderMachines

Joined Oct 31, 2019
76
I was I guess stressing the amp too much. The fuse went first. Put a new fuse in and then R4 went second and so did the new fuse. Some smoke and dark. I am glad to learn more about this, can anyone help me?

Capture+_2021-02-06-19-29-33.png
 

MrChips

Joined Oct 2, 2009
23,524
What you have is a complementary pair class B push-pull output audio amplifier.
Usually the cause of a blown fuse is one or two bad output transistors, T7 and T8.

Unsolder both T7 and T8, making a note of where each transistor belongs.
Using an ohmmeter at the lowest resistance range, measure for conductivity between collector and emitter of the transistor with the meter leads one way and again with the leads reversed. You may find one or both of the transistors to be shorting.

With both T7 and T8 still off the board, disconnect the loudspeaker from the amplifier. Replace the fuse.
Apply power measure the +40V and -40V supply voltages at C12 and C13.
Measure the voltage at the loudspeaker output LS+ and at R5, R6, R16, R17, R19, R20.
 

Thread Starter

CosmicOrderMachines

Joined Oct 31, 2019
76
What you have is a complementary pair class B push-pull output audio amplifier.
Usually the cause of a blown fuse is one or two bad output transistors, T7 and T8.

Unsolder both T7 and T8, making a note of where each transistor belongs.
Using an ohmmeter at the lowest resistance range, measure for conductivity between collector and emitter of the transistor with the meter leads one way and again with the leads reversed. You may find one or both of the transistors to be shorting.

With both T7 and T8 still off the board, disconnect the loudspeaker from the amplifier. Replace the fuse.
Apply power measure the +40V and -40V supply voltages at C12 and C13.
Measure the voltage at the loudspeaker output LS+ and at R5, R6, R16, R17, R19, R20.
You're right they're both shorting

C12 and C13 are +-40 with the transistors removed

0V from LS+ and the resistors
 
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Ian0

Joined Aug 7, 2020
2,206
Much more likely that it is to do with slow output transistors that can't switch off fast enough, leading to a shoot-through condition. Don't try that with an amplifier with a darlington output stage. A MOSFET amplifier might manage it.
Although lag-compensation (C4, C5) on the VAS in addition to dominant-pole compensation does suggest a designer struggling with a bit of instability!
 

Thread Starter

CosmicOrderMachines

Joined Oct 31, 2019
76
Much more likely that it is to do with slow output transistors that can't switch off fast enough, leading to a shoot-through condition. Don't try that with an amplifier with a darlington output stage. A MOSFET amplifier might manage it.
Although lag-compensation (C4, C5) on the VAS in addition to dominant-pole compensation does suggest a designer struggling with a bit of instability!
The datasheet says up to 200khz. I'm using this to find at what frequency I can drive plasma tubes and a large air transformer at optimally.. From there can get a more specific power amp designed.. I'm open to any amp designs you know about around the 100khz 200watt range. It'll come in handy. Alternatively I'm not even sure I need 200 watts. Point was to use something like this to narrow the power requirements and frequency.
 
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Ian0

Joined Aug 7, 2020
2,206
If SPICE is to believed, then it can just about manage full output at 12kHz, before it goes into slew-rate limiting.
 

Thread Starter

CosmicOrderMachines

Joined Oct 31, 2019
76
If SPICE is to believed, then it can just about manage full output at 12kHz, before it goes into slew-rate limiting.
Well so much for that. But yea I did notice huge drops as I increased frequency. Literally every documentation I found on this amp before buying said 3-200khz. But I'm sure it wasnt claiming full power. Can you tell what power to safely expect at 75khz from this thing?
 

Ian0

Joined Aug 7, 2020
2,206
Well so much for that. But yea I did notice huge drops as I increased frequency. Literally every documentation I found on this amp before buying said 3-200khz. But I'm sure it wasnt claiming full power. Can you tell what power to safely expect at 75khz from this thing?
This is its attempt at 75kHz. Note the huge DC offset due to unequal slewrates.velleman75k.png
 

Thread Starter

CosmicOrderMachines

Joined Oct 31, 2019
76
Why don't you just drive it with a squarewave, using something like a IR2153?
The application requires a specific signal. Kind of a cut and paste waveform, but that waveform to my knowledge is only made by an arbitrary waveform generator. I guess thats definitely the easiest way.. So I have to amplify a arb signal generator.

I dont know the inductance or capacitance between the plasma tubes, so the point of this amp was to find the right frequency. Then use that info to build a more specialized amp. Or if I got lucky to use this amp to do the whole thing. But seems thats more and more of a bust.

I haven't bought the plasma tubes yet, I wanted to get a proof of concept of achieving very high voltages with this waveform first.

Since its esencially a tesla coil look alike, lots of people assumed reaonance and said my arbitrary waveform would just turn into a sine wave by natural laws. However I'm not actually using a tesla coil configuration for resonance and sparks, just trying to use it as a transformer.
 

Thread Starter

CosmicOrderMachines

Joined Oct 31, 2019
76
I've wrote down everything so far,

- Using an asymmetrical bipolar tesla coil
- Has its own capacitance and natural high frequency resonance.
- I have two 1 liter 50/50 Neon Helium tubes
- The tesla coil will be a transformer, not a resonate system.
- Trying to drive the coils primary with a high current waveform (specific to this project - generated by a siglent sdg2042x)
- The waveform will essentially modulate the coils natural high frequency resonance (which will be what excites the tubes)
- An asymmetrical bipolar tesla coil is used instead of a transformer because the area of charge is essential to the application.

This should be my requirements:

- Find out the capacitance between the leads that go to the tubes and the gas inside the tubes.
- Find the inductance of my primary and secondary.

All this for the purpose of finding the frequency that reduces the reactive impedance. Or as my approach, get a broadband amplifier and tune into it.
I don't have the tubes though. I am testing one step at a time. This test was supposed to be generating high voltages with the signal gen.
 
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