Review and Measurements of Behringer A500 Amplifier

[sergeauckland]

My amp is SS. So the A500 has a much lower output impedance than the manual claims...

One thing that has confused me is the input sensitivity of the A500. The manual claims it is 1.64V which fits with what Peter Aczel measured back in 2005: ‘the maximum output is a little over 33 volts at the test limit of 1% distortion’ (1.64 x gain of 20). That would make the sensitivity +6.5dBu, according to the online sengepielaudio calculator, but the balanced inputs are labelled as +4dBu. So my confusion is which figure to use (+4dBu or +6.5dBu) when working out how much attenuation to use given max output of the source – in your case +22dBu (9.75V), I believe, and in mine +8.7dBu (CD- level of 2.1 V)? I realise that some A500 users would not bother with attenuation for CD-level max voltage.

My measurements indicated that the A500 has a higher output impedance than the spec, but still low enough for it not to matter.

As to levels, the differences you're concerned about don't seem to me to be that important, a few dBs is neither here nor there, considering that you will pretty much never be driving the amp that hard. Also, I measured mine not at 1% distortion, but at 0.02% distortion, i.e. before any clipping, not 1% THD into clipping, so that will account for some of the difference.

For a CD-level output of around 2V, I wouldn't bother with any attenuation, as depending on what sort of music you listen to, not all CDs are mastered to 0dBFS anyway. Classical and Jazz CDs, especially those originally issued in the 1980s and 1990s still leave some headroom. Even the original Dire Straits CDs have something like 6dB of headroom, so won't put out more than 1V. Some classical CDs have even more than 6dBs headroom, so maximum output will vary between masterings.

Considering how volume control is set entirely subjectively when listening, a few dBs more or less seems to me to be unimportant, as long as nothing is being overloaded at one end, and is quite loud enough at the other.



S.

 

[Specialcause]

I suspect he meant to say "balanced" and not "unbalanced."

Regardless, as I've mentioned previously, any operation of the A500 with the controls at less than maximum compromises the measured distortion performance of the amplifier.

Dave.

You were right, I did mean to say ‘balanced’.

Before buying the A500 I had seen the video where the recording studio had measured high levels of distortion at less than full setting and I stuck with controls at maximum for a couple of years. Then I saw on this thread that Serge had contested this saying that distortion was short-lived after re-setting the controls. As I was looking for a way to reduce gain I followed this advice. Now I would say to a domestic user leave them at full, certainly seems best from a channel balance point of view.

As regards controls at less than maximum compromising the measured distortion performance of the amplifier I am reminded of what Mary Douglas wrote about the bitter disagreement between the advocates of nuclear medicine and their opponents amongst the general public. This illustrated to her ‘the selective deafness in which neither of two parties to a debate can hear what the other is saying’. Or are we in the happier position where there is some audio science that settles the matter one way or the other?

Phil

 

[Pulkass]

There have been numerous reports of serious defects in new A500 amps. Someone did a test and found lowering the volume controls on the front of the amp cause distortion to rise to .4% from under .1%. My understanding is there is no speaker protection. Thank you Amir.

I have been owning an A 500 for some years now, been using it occasionally, now I note a distortion in some frequencies, both channels, changed loudspeakers too, so the problem is in the Behringer, before having it repaired, unlikely because a repair could cost 100 euros, !!!! I d like to know what could it be??? Many factors, ok.....but

 

[Ron Texas]

I have been owning an A 500 for some years now, been using it occasionally, now I note a distortion in some frequencies, both channels, changed loudspeakers too, so the problem is in the Behringer, before having it repaired, unlikely because a repair could cost 100 euros, !!!! I d like to know what could it be??? Many factors, ok.....but

Check for loose connections as that was a reported problem. If it were mine, I would not spend 100 Euros on a repair. Just get something else.

 

[phoenixdogfan]

I have been owning an A 500 for some years now, been using it occasionally, now I note a distortion in some frequencies, both channels, changed loudspeakers too, so the problem is in the Behringer, before having it repaired, unlikely because a repair could cost 100 euros, !!!! I d like to know what could it be??? Many factors, ok.....but

Do you have the gain turned all the way up? The gain controls on the front panel introduce ultra high levels of distortion if used as volume controls. They must be set maximum for the amp to perform at its optimum.

 

[sergeauckland]

Do you have the gain turned all the way up? The gain controls on the front panel introduce ultra high levels of distortion if used as volume controls. They must be set maximum for the amp to perform at its optimum.

I think this is a myth. Looking at the circuit diagram there's absolutely no reason for it, and my three A500s don't do that. The amplifier has a very long time constant servo loop, and when the volume controls are moved, the DC conditions can change slightly, and indeed it's possible that whilst the amplifier is rebalancing, the distortion is higher, but not after a few seconds.

In any event, those controls are NOT volume controls. They are sensitivity controls to match the amplifier's gain to the overall structure including sources, mixers, pre-amps or whatever. They are adjust and leave controls, not operational volume controls.

I have mine at maximum, not for any idea of added distortion, but because in my active set-up I must have a fixed and repeatable level of gain, to ensure that the six channels all track correctly, and full on is repeatable. In normal stereo use, just set them such that the system volume control works fully, and isn't cramped up at one end.

S.

 

[phoenixdogfan]

I think this is a myth. Looking at the circuit diagram there's absolutely no reason for it, and my three A500s don't do that. The amplifier has a very long time constant servo loop, and when the volume controls are moved, the DC conditions can change slightly, and indeed it's possible that whilst the amplifier is rebalancing, the distortion is higher, but not after a few seconds.

In any event, those controls are NOT volume controls. They are sensitivity controls to match the amplifier's gain to the overall structure including sources, mixers, pre-amps or whatever. They are adjust and leave controls, not operational volume controls.

I have mine at maximum, not for any idea of added distortion, but because in my active set-up I must have a fixed and repeatable level of gain, to ensure that the six channels all track correctly, and full on is repeatable. In normal stereo use, just set them such that the system volume control works fully, and isn't cramped up at one end.

S.

 

[Deleted member 2944]

Yeah, it's all a "myth." Hilarious.
Try doing some actual testing vice speculating.

Dave.

 

[sergeauckland]

Well aware of that video, but firstly he doesn't wait long enough for the amp to stabilise after moving the control, and secondly he says that the amp has a variable gain stage. It doesn't. The gain control is an ordinary pot between the fixed gain (actually 14dB or so attenuated) input balanced receiver and the power amp proper.

I also don't know how long that auto-balancing distortion meter takes to rebalance, that may have something to do with it. I'm not saying the A500 couldn't have been better designed, for example, the +4dBu balanced input is actually attenuated to -10dBu whereas I think it would have been better if the -10dBu input had an extra 14dB of gain, but that would have cost one more opamp per channel and switching more difficult. The volume pot is also a very cheap and fragile control that won't take a lot of abuse, so clearly not an operational volume control.

Next time I get my power amps out of the rack, I'll check the distortion at lower volume levels and post results. Until then, not convinced.

S

 

[Specialcause]

Is the gain of the ‘power amp proper‘ 40dB? So the figure of 26dB given in manual is the net gain for balanced input after taking account the 14dB attenuation.

 

[sergeauckland]

Is the gain of the ‘power amp proper‘ 40dB? So the figure of 26dB given in manual is the net gain for balanced input after taking account the 14dB attenuation.

That's what I would expect. 225mV (-10dBu) input to the power amp to around 33 volts out (+32dBu) so total gain of power amp is 42dB or thereabouts.

S.

 

[Deleted member 2944]

With the front panel controls at maximum, the gain of the amplifier is 39db via the RCA inputs and 27db via the XLR inputs.
Setting the control to 2 o'clock yields 29db gain via the RCA inputs.
The level control is NOT configured as a simple pad attenuator. It is actually interfaced as the input resistor of an inverting op-amp input stage via a shunted attenuator. (More complicated than it needs to be, which is part of the problem.)

A500 distortion with level control at maximum
A500 distortion with level control at 2 o'clock
A500 distortion with level control at maximum and external attenuator at input to achieve equivalent.

 

[Sal1950]

Do you have the gain turned all the way up? The gain controls on the front panel introduce ultra high levels of distortion if used as volume controls. They must be set maximum for the amp to perform at its optimum.

Maybe it's the internal connection rather than the drive level. Are the controls noisey at all, maybe the commutators are becoming microphonic?
Try cleaning the controls.

 

[Deleted member 2944]

It's not noisy controls. It's the design of the circuit.
I'm surprised this is news (or a myth) to some folks. This is a known characteristic of this amplifier that was identified many years ago.

Dave.

 

[Sal1950]

It's not noisy controls. It's the design of the circuit.
I'm surprised this is news (or a myth) to some folks. This is a known characteristic of this amplifier that was identified many years ago.

Dave.

OK, not the first time I've seen this behavior in audio products.
If you don't have one or havn't done homework in the interest of buying there's really no reason for others to be aware.
Maybe time to break out the ole soldering iron and take the control out of the system, replaced by resistors of appropriate value to keep the drive below the distortion level.

 

[Deleted member 2944]

A modification to alleviate the issue by disabling the level controls has been posted elsewhere. That's easy to do, but a modification to keep the level control operation and not increase the distortion is the preferable solution. However, that involves some circuit modification.
I've prototyped a fix on one channel of my A500, but set aside the project awhile back because I had more important stuff to work on.
Considering how easy it is to just add a standard attenuator/pad external to the amplifier, I don't think it makes much sense to go to the trouble of modifying.

Behringer considers the A500 schematic proprietary and I'm not sure if the ASR moderator would appreciate it being posted here. Regardless, it's easily findable by Googling.

Dave.

 

[sergeauckland]

Well aware of that video, but firstly he doesn't wait long enough for the amp to stabilise after moving the control, and secondly he says that the amp has a variable gain stage. It doesn't. The gain control is an ordinary pot between the fixed gain (actually 14dB or so attenuated) input balanced receiver and the power amp proper.

I also don't know how long that auto-balancing distortion meter takes to rebalance, that may have something to do with it. I'm not saying the A500 couldn't have been better designed, for example, the +4dBu balanced input is actually attenuated to -10dBu whereas I think it would have been better if the -10dBu input had an extra 14dB of gain, but that would have cost one more opamp per channel and switching more difficult. The volume pot is also a very cheap and fragile control that won't take a lot of abuse, so clearly not an operational volume control.

Next time I get my power amps out of the rack, I'll check the distortion at lower volume levels and post results. Until then, not convinced.

S

I've been doing some rearranging of my rack, so pulled one of my three A500s onto the bench and measured distortion.

This amplifier was purchased new in October 2011 so is now some 9 years old.

All measurements were made at an output of 28.3v into an 8 ohm resistive load (100 watts into 8 ohms) Balanced inputs, tone generator REW into a Lexicon I-Onix U22. THD meter - Ferrograph RTS2

Left Channel, Volume at Full THD 0.018%
Right Channel, Volume at Full THD 0.018%

Left Channel, Volume at 2/3 of Full THD 0.02%
Right Channel, Volume at 2/3 of Full THD 0.02%

I trust this puts to bed the nonsense that an A500's distortion goes up rapidly if not at Full volume.

S.

 

[Deleted member 2944]

It doesn't put to bed anything.
Get yourself a contemporary piece of test equipment and test the amplifier in a nominal operating area vice way up at 28 volts and you will see the issue.
I've posted three plots above that clearly show the characteristic.

Dave.

 

[tvrgeek]

I used a cheap Berhringer as a sub amp to good extent. Slightly modified as I added much larger heat sinks and an ultra low speed fan as the original fan I could hear. Not a HI-FI amp, but was excellent on a pair of Peerless XLSS subs. For kicks I tried it on my mains. NOPE. Helped hold up my dislike for class D. Terrible actually. If I was a garage band, I would keep an extra one of these in the van as they are so cheap as a PA backup.

 

[Pulkass]

I need a 20 Hz downward cut for my 18 incher and then a 120 Hz upward cut . What do you suggest ? Thanks