Ampeg BA210 - Lacking volume past 4 on the dial

[SteveJo]

Hi All,
This bass amp doesn't get any louder past about 4 on the Master output. As I turn the amp up, it gets louder until this point (just under half way) and then there is no extra level from there until max. I've checked the pot and all is well. I've checked the amp against another BA210 which has continuous gain until maximum and gets a lot louder. Any ideas?
http://diagramasde.com/diagramas/aud...A210-BA115.pdf
Not sure if this is the correct version, my amp has version 1 on the preamp board.
-Steve

[MarkusBass]

The amp has a limiter circuit. Is it possible that it failed and it limits the signal in an incorrect way?

Mark

[SteveJo]

Are you referring to the vtl5c7 on the power amp board? Any tips how / what I should test?

[patlaw]

Are you referring to the vtl5c7 on the power amp board? Any tips how / what I should test?

The easiest check would be to look for about 2 volts DC drop across the LED in the VTL5C7. If there is a voltage drop, the LED is conducting, the resistance on the photocell is low, and the gain of that stage is low. You could pop one leg of the LED out of the circuit, the LED is not conducting, the resistance on the photocell is high, and the gain of that stage should be high.

Guys who have serviced that amp will have a better suggestion, but that's how the photocell works.

[52 Bill]

There are many reasons that your amp could have reduced output. Guessing at reasons why are just that, guesses. Are you equipped and qualified to run some tests?

If you are, then check the power supply voltages. How do they compare to the voltages listed on the schematic? There are a number of test points on the schematic that list the ac signal voltages that you should find at different points in the circuit. Can you test these voltages?

While the headphone jack is not driven by the main speaker output, there is a mute to the input of the power amp circuit that is controlled by the headphone jack. Have you tried cleaning the switch contacts on the headphone jack?

[SteveJo]

Hi,
Thank you for your responses. Here are some measurements under test conditions of a 1K sine input at 170mV RMS as specified on the schematic. I'm in Salford, UK. mains voltage is 240VAC.

The voltage drop across the LED of OC-1 (VTL-5C7) at different Master Volumes (MV) is:
MV 0 = 0.2V
MV 3 = 1.2V
MV 5 = 1.24V
MV10 = 1.25V

Here are the RMS Test Point readings:
TP1 (700mV) = 166mV - Is the 700mV listed on the schematic an error / typo?
TP2 (3.8V) = 3.94V
TP3 (450mV) = 512mV
TP4 (450mV) = 493mV
TP5 (250mV) = 289mV
TP6 (1.15V) = 1.35V
TP7 (1.15V) = 1.35V
TP8 (1.1V) = 1.14V
TP9 - I couldn't find this one on the schematic
TP10 (2.0V) = 1V This starts low at minimum master volume and increases to 1V at around 4 on the master vol, but does not increase from there, stays at 1V.
TP11 (26.3V) = 30V
TP12 (800mV) = 908mV

Some I could test with the headphones plugged in, some only gave good readings through the speaker - I think I've annoyed the neighbors...

[SteveJo]

Also,
the +50V is at 49.6V
the -50V is at -49.6V
the +16V is at +16.58V
the -16V is at -16.86V

[MarkusBass]

Here are some measurements under test conditions of a 1K sine input at 170mV RMS as specified on the schematic.
....
Here are the RMS Test Point readings:
TP1 (700mV) = 166mV - Is the 700mV listed on the schematic an error / typo?

Yes, it's a typo. As you already stated, the test signal is 170 mV. The voltage at TP1 is/should be the same.

TP11 (26.3V) = 30V

It seems that the amp has expected power. Isn't the only problem that the volume pot is changed to incorrect taper (compared to the other amp)?

Mark

[SteveJo]

I've pulled and tested the master volume pot. It's 19K end to end, 16K at half way. Reverse log?

[g1]

It seems that the amp has expected power.

Isn't this supposed to be a 450W amp? It has so far not been tested for full power.

[MarkusBass]

Isn't this supposed to be a 450W amp? It has so far not been tested for full power.

With +/-50V power supply and two pairs of IRFP9140/140? I think that with some luck you may get 200W. Unless it has 2 Ohms speaker but I don't think that two pairs of IRFP can stand it.
On the schematic the expected voltage on the output is 26V RMS. Here you have 30V. With 2 Ohms you would get 30x30/2 = 450W but I don't think that speakers are 2 Ohms.
PS: I assume that the schematic posted in post #1 is the correct one, isn't it?

Mark

[g1]

Maybe there is a new version with higher power? Anyway, the spec for the current version is 450W RMS.
I guess he could compare full power at clipping on his unit and compare to the other unit that sounded louder.

[52 Bill]

With +/-50V power supply and two pairs of IRFP9140/140? I think that with some luck you may get 200W. Unless it has 2 Ohms speaker but I don't think that two pairs of IRFP can stand it.

According to the owner's manual 220 Watts into a 4 ohm load.

[g1]

Well that would line up better with the 30V he measured at the output.
The new owners manual I got (BA_OM.pdf) says 450W RMS to the woofer, 50W to the tweet. So maybe the new one is a ClassD module?

[Jazz P Bass]

Well that would line up better with the 30V he measured at the output.
The new owners manual I got (BA_OM.pdf) says 450W RMS to the woofer, 50W to the tweet. So maybe the new one is a ClassD module?

Or they are lying, I mean quoting Peak Power.

Note: I have a Behringer B615D on the bench.
Switch mode power supply, switch mode output (class D)

Big badge on the speaker grill says "1500 Watts"

Installed is a 250 watt speaker & a 30 watt tweeter.

Not only do they quote 'Peak Power' but they also ADD the two speakers wattages together.