AMEK 9098 Dual problem, schematic / help wanted

Hello!

The studio´s amek 9098 has channel 1 with an ugly distortion at the output, both on DI and mic modes, channel 2 is OK. The pre works for a while then the sound is completely broken...but the metering seems OK on both channels, i mean, the metering doesn´t show signs of low signal or reduced dynamic range or headroom or something, it responds exactly as the channel 2 meter with the same signal applied....

The pre had this exact failure a year ago and was fixed by RME in the florida, but this time is quite expensive to send it again so any help would be GREATLY appreciated!

Any info on this pre such as topology and design details would be very useful too, thanks!

Hi Silvas,

A copy of the mic pre schematic can be found here:

http://users.pandora.be/Rogy/Amek%20System%209098/amek9098-a.jpg

However I believe after the mic pre there is a trim control and an output buffer with transfo which are not shown on the schematic.

Hope this helps,

Regards,

Rogy

it is the same topology of the mozart and the green pre exept for the balanced output stage. did you check the rail voltages? the distortion change with the gain control?
the distortion is present on the mic out? the line input is not part of the preamp.

Voltages ok, Channel 2 OK. distortion on channel 1 is the same regardless of the gain. Distortion is present on mic and line signals. Both are going to the out xlr. Distortion is like when you blow hard on an old u87 and the sound goes then comes it again....when i hit the preamp hard enough, the sound tries to come back fine for a little while, then distorts again.

Thanks rafa for the reply!

maybe the phase or mic/line switch needs cleaning, or some issue with U5, U6, U4 area.

with a signal tracer should be easy to find where is the problem.

i thought that the problem was the switches as they were noisy, and i cleaned all of them....the i fired the pre and it worked ok, for a while...then, when the pre was warmed up, then the distortion shows up.

i´ll trace it with an scope and the schematics.

Scope the output around the CMRR trim stage op-amp.

You never said if this is a console or a piece of outboard gear, but I'm assuming the latter.

Schematics or not, you'll need a scope.

Does this (assumed) piece of outboard gear have TWO outputs per channel? -the first electronic or differential, and the second transformer balanced? (A model number might help reduce the number of questions) If so, are BOTH distorting?

If the transformer output is distorting, you should look at the transistor pair which drives it... and check to make sure that you don't have DC going into the transformer primary... if so, you should also replace the transformer because it might very probably be magnetized by now...

Anyhow, look for sn op-amp output sitting a LONG way from 0V, but compare with the good channel at ALL times, because some outputs are [intentionally[/i] biased 1V or so away from 0V.

Keith

Thanks Keith for the reply

It is the Amek system 9098 dual preamp, the one with blue/white face. i see 2 transformers onboard, these are for the output right?

Only one output for channel.

And yes...i remember a pair of transistors near the transformer on TO? package, i don´t remember the name for this package, the one who is smaller than a to220 with a hole for heastsink...

I think you might be able to get the schematic from Soundcraft.

Samuel

The output driver (from memory) is a 5534 or perhaps a 33078(?) into a complementary pair of fairly warmly-biased TO-220 (or whatever) transistors. The transformer should be a toroid "pumpkin" and has a tertiary winding. I don't remember if the Primary has a DC-blocking capacitor or not, but the secondary feeds the output XLR with a Zobel network and a light load resistor, and the tertiary is used as negative feedback. There's also a heavily-lowpass-filtered direct feedback loop which gives DC feedback, but should be largely dead at Audio frequencies... that way the Op-amp is DC-stable, but still responsive to load change in the AF band across the transformer core.

...That at least is how I remember reading it. -I may be way off. -See if you see a 5534 (or similar) near a pair of big transistors feeding the transformer, and if there are six leads from the 'pumpkin' then it seems feasible that my memory is not too far off.

Keith