Amplifier low gain

[jheatac]

I have an car audio amplifier that has low gain.

At about 1v 1khz in, max output is 4v at 2/3 gain. Any higher doesn't clip but looks like a phase shift on the scope. Amplitude and frequency don't change but it looks like a double trace.

Rail voltage is + and - 53v

If I run the input up to 2v, output won't go any higher than 4v

Can anyone give me a direction?

[CaptDon]

What is the load on the amp? Be careful because more than likely the amplifier is a full H bridge amplifier and you must not ground the outputs!! In other words the speaker is floating and not ground or chassis referenced. It sounds like the amplifier is blown up with the output devices ruined. You are still seeing a bit of the drive voltage or current leaking through. Did you blow it up or was it given to you in AS-IS previously blown up condition? Your test setup may be placing an unintended ground on one side of the output and the amp is folding back for self protection.

[jheatac]

Thank you for taking the time to respond. I know no history. In trying to learn, I bought a stack of 6 broken amplifiers from facebook marketplace for 50 bucks. So if it goes up in smoke, it isn't the end of the world.

Power supply load was 2.5 to 2.8 amps at 14v when the 4v output was into a 4 ohm resistor, if I recall correctly.

I believe this is a class AB amp. SMPS ran by a TL494C producing + & - about 53vdc. Output transistors are ST TIP35C and TIP36C

I'm going to go look at it a little closer in a few minutes but nothing jumped out at me earlier.

[TimFox]

How are you measuring the output?
As stated above, with a (very common) H-bridge output, you must not connect one side of the output terminals to the grounded input of an oscilloscope.
If you use a DVM (floating), you will not ground the output (be careful for personal safety), but you still need a load resistor to simulate the loudspeaker.