Ampeg BA210 - Lacking volume past 4 on the dial

[Randall]

Note: I have a Behringer B615D on the bench.

There's your first mistake right there.

[The Dude]

I had to chuckle at this quote from their ad slick.

"The EUROLIVE B615D 1500-Watt `active loudspeaker is exactly what you’ve come to expect from BEHRINGER."

I'm not sure they're aware of just how accurate they are.

[g1]

If it is putting out full power now, perhaps the OP has inadvertently "fixed" it during the process of dis-assembly and testing?

[J M Fahey]

That limiter triggers when speaker output waveform is within 5 volts of rail voltage.

So start with your input signal as suggested but with volume at 0 , then rise it slowly, amp loaded (speaker or resistor), monitoring voltage across VTL LED.

Or you may tack solder a LED across it (proper polarity of course) and look at it.

At some point the visible Led will light or if you clipped a meter across it voltage will jump from a very low value ( a few mV) to around 2V (Led on).

Without changing any setting, measure voltage across load.

If around 20/24V , it's what's expected, calculate actual power into load.

If much lower, say 10/15V you have a limiter problem.

Or dismiss the whole silly "Tech" measuring and just trust those wondeful measuring instruments you have on both sides of your head.

[SteveJo]

Thank you everyone for your suggestions - it certainly seems like the Master vol pot is the wrong type, giving all the volume at the beginning. This was my gut feeling before I started looking in to it. And when I've swapped it for a normal log pot (instead of the reverse log pot that's standard) it certainly responds as I would expect. i.e. volume steadily increasing over the entire range of the Master Volume Pot. But...

It took me a while to get back to testing the amp - and I've now measured the voltage across VTL LED.
Under the same test conditions described previously and stated on the schematic with the input gain on max I get this:
With the Master Vol on 0 I get -200 mV across the LED
With the Master Vol on 1 I get +400 mV across the LED
With the Master Vol on 2 I get 1V across the LED
With the Master Vol on 3 I get 1.1V across the LED
With the Master Vol on 4 I get 1.2V across the LED
From there upwards with the master volume the LED voltage peaks a little above 1.2V and stays there.

So that's not right, is it? There doesn't seem to be a point at which it jumps from low to high as I can get just about any voltage from -200mV up to +1.2V by careful adjustment of the master volume pot and the input gain in combination. But no higher than 1.21V.

Also, what's the best / recommended way to test this without sending it through the speaker?

[J M Fahey]

Hate to self quote, but here we go again, with emphasis on the main points, without which no real answer is possible:

That limiter triggers when speaker output waveform is within 5 volts of rail voltage.

So start with your input signal as suggested but with volume at 0 , then rise it slowly, amp loaded (speaker or resistor), monitoring voltage across VTL LED.

Or you may tack solder a LED across it (proper polarity of course) and look at it.

At some point the visible Led will light or if you clipped a meter across it voltage will jump from a very low value ( a few mV) to around 2V (Led on).

Without changing any setting, measure voltage across load. <-- this includes leaving the 1kHz signal injected steady, not cutting it or modifying its level at all.
Post what you measure and state master level indication.
Then rise it step by step, I guess output voltage across the load will rise somewhat but not much (limiter doing its thing)
At some high level, you might hear some distortion, but if so, it will probably come from an overloaded preamp

If around 20/24V , it's what's expected, calculate actual power into load.

If much lower, say 10/15V you have a limiter problem.

Just need the boldfaced results.

[SteveJo]

Hi,
Thanks for your patience I appreciate your help.
I was going to measure the voltage across the load as you said, but could not find a distinct point at which the voltage across the LED jumped from a few mV to 2V. Instead it varies continuously from master volume zero to master volume 4/5 in the range -200mv to 1.2V, with no distinct "jump" and never more than 1.21 volts. So I thought this might be significant in itself.

I'll swap the master volume pot for a regular log pot as maybe this is making it difficult to get good readings. Then I'll repeat the testing with a meter and again with another LED soldered across as per your instructions. If I can't do it today, it will be a couple of weeks as I'm on a break from tomorrow.

I don't have any particularly heavy duty resistors suitable for the load, so I'll order some in to save my ears. What specs would you recommend?

-Steve

[J M Fahey]

The actual limiter is the LDR inside the optocoupler and it only "knows" whether the Led is ON or not, not what voltage it's getting, that's why I suggested soldering another red Led in parallel with the internal one (which you can't see) and simply looking at it.

Do the test on a not too bright room (turn the bench lamp off or something) because the Led won't shine very bright, the LDR is sensitive and requires only a little light to act.

Just seeing it shine, even if weakly, is enough.
I had suggested 1kHz tone, maybe you'd prefer to use 400 or 440Hz .
You can download free MP3 from:
Download Audio Tone Files

As of resistors, you really need a big 4 ohm 200 or 250W one.

I use a 50W one but drop it in a bucket of water, no kidding.

Plastic of course, a metal one may short it.