Digital Designs Z1 amp down on power? rails are there? output isn't?

[ThyDntWntMusic]

Just going to copy and paste a few posts and comments I've made on other forums to give you guys an idea of whats going on

Two amps "strapped" but subs only connected to one amp or the other. (for testing) so only one gain control to set both amps.

One amp has been blown numerous times, I've just finished fixing it... the other has never been blown or repaired...

Repaired amp as slave did 148.1dB, Original amp as master did 146.4dB

same volume on headunit.

original amp clips sooner than repaired amp?

unloaded output voltages:
when original is master, the voltage is higher till clipping, where the repaired gets higher? (6volts maybe?)

when repaired is master, voltage is higher across the entire volume range, and at clipping the repaired ends up ~20volts higher... (may have been more clipped than the first test)

Its just doing my head in?! I'm thinking the "Original" amp has had issues since new!

voltage rails on the "low" amp are +-131volts with the engine running (13.8volt input) (about the same as the amp I've just repaired)

if I turn it up so it's badly clipped, I can get ~100volts AC output... the other amp did the same voltage with the clipping light just solidly lit...

but this now makes me wonder, perhaps there's nothing wrong with the amp, but the input preamp stage is less sensitive, so when strapped, it's down ~20volts on the output, and the clipping light is also more sensitive....

I'm going to match the output voltages setting the gains to whatever they need to be on each amp, then compare the output waveforms of each amp to see if they appear to be equally clipping...

I matched the gains using a multimeter, ~98volt output from both amps...

then I put the oscilloscope on each amp...

louder amp:


clearly clipping, but acceptable...

quieter amp:


basically a square wave, while not even doing the same voltage....

Why would an amplifier not bring the output to within ~10volts of the rail voltages, like the functional amp does?

[ThyDntWntMusic]

Thinking I may have been clipping the preamp stage for some reason, I've just checked the preamp rail voltages, and they're +-15volts...

so, going to try find the output from the whole preamp, and see if it's clipped anyway

[ThyDntWntMusic]

confirmed, I am pushing the preamp stage to the point of outputting a squarewave..... ah, why?

the amp can't make full power because the preamp doesn't give enough voltage....

[ThyDntWntMusic]

entire preamp section is clean, till the last half of the final opamp...



as per my diagram, it's had a 4.7meg resistor added, but I don't think that'll have any effect...

Thing is though, I'm having to set the gain on this amp higher than the other amp... which makes me think it's not an issue with the preamp, but rather the class D controller board... which I don't have a schematic for...

I get the impression that if I simply gave this controller board a higher signal, that the amplifier would reach full power...

[ThyDntWntMusic]

Pin 4 of the X1 connector (in the middle of the pic) is the audio input to the amplifier controller board...

The first thing the signal does is pass through a 68k resistor (R3).... except on the "louder" amplifier, the resistor is 56k? it's been changed on the "quieter" amp...

[ThyDntWntMusic]

Swapped the 68k resistor for a 56k....
now the amplifier does 250volts peak to peak with the same amount of clipping as the other amp?!?!

why was the resistor ever changed?!?!?!?

[Perry Babin]

On the 'quieter amp', with all controls set precisely the same and with the exact same input, did it have less output (before either amp was driven to clipping)?

I would have guessed that the amp with the 68k resistor would have had greater overall gain.

[ThyDntWntMusic]

As far as I'm aware, the entire preamp section of the two amps matched.

So if the gain controls were set the same, the quiet amp was down on output, as mentioned in the first post, about 1.5dB down.

The schematic I posted above had the "strapping" part of the circuit edited out, as it wasn't relevant to the issue, but when the amps were run as master and slave, none of the controls on the slave amp did anything, gain/filters were all set on the master amp.

I'd tested it like this, so the gains were matched between the two amps... that's where the 1.5dB difference was discovered.

When attempting to match the output voltages between the two amps, that was when I came up with the first two graphs in the above post... the amp could only do 206volts peak to peak, no matter what the levels were set at.

edit: I realised after posting this, that it didn't really answer your question

[ThyDntWntMusic]

The most power I'd ever seen from the quiet amp was about 2900watts, and that was into 0.5ohm rising to about 1ohm.... with an acceptable amount of clipping.

the loud amp tested at 3200watts into 1ohm, rising to 2.66ohm!?!?

edit: basically, at the point of clipping, this amp never did the kind of power people claimed they could.... it'd do rated... so I simply assumed people were full of poo

[Perry Babin]

Did you check the maximum unclipped output with the amp driven normally?

When the op-amp clipped, what was the peak voltage that the output signal reached (on the op-amp output pin)?