Xtant X1001 (MTX 81000D)

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Dr Zeus

Dr Zeus

Member
Joined 2010
#1
This amp is similar to MTX 81000D style of amps. Its using A3120 devices near outputs. Are these optocouplers? Or IGBT output mosfet drivers?

Referencing ground;
While this particular amp has +-75vDC rail, Im unsure about the regulated side of things.

The amp has -7vDC on RCA input shields, and -9v on RCA output shields
+2.5vDC on positive speaker terminal.

Audio is not passing to terminals.

Where are the positive and negative regulators in this amp?
 
#2
I don't understand the 3120 question. Look up the HCPL3120 datasheet. The FOD3120 is a bit different but I've used them interchangeably.

Do you have 0v on one end of R199?

The regulated and driver voltages are generated by the small square transformer.

Did you find the MTX81000 page in the tutorial?
 
#3
Yes Ive been looking at the 81000D and 1501D pages. The difference with this amp is its using sync'd transistors for driving into the small square transformer. After which, the rectifiers appear to be providing -28v and +9vDC. I'm not sure if that is correct or not. The square transformer gets a tiny bit warm and the core is slightly loose but no other sign of issue

I'll locate R199
 
#8
Here is a photo which may shed some light. I think D471/D474 are units equivalent to D198/D199 in mtx amps.

I have a parts amp 81000d I’ve been looking at here.

D471/474 has -9vDC on one side, and +9.8 on the other.

I’m not sure which is equivalent to r199.

rca shields and amp b- as well as secondary/ terminal negative is 5M ohms
 

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#12
Making SOME progress.

Grounding RCA shields actually helps, and so does actually plugging in a source. As long as I have the amp set to Balanced input I can measure about +-18vDC on main opamps

While I do see audio on all opamps, the last opamp which I believe drives the opt-drivers has quite a bit of DC offset on it.

NE5532 - in the photo near the small cap with yellow glue on it
1. 8.8v
2. 4.7
3. 4.1
4. 4.3
5. 4.2
6. 3.6
7. 6.0
8. 15.5v
 

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#15
Amp passed audio, but now under load I get a strange +-1vDC floating across speaker terminals. The amp also draws between 2A-6A as it 'wobbles' up and down on the scope. Audio passes. The more input signal I give, the flatter the wobble goes.

When I increase the Amp-GAIN knob, things get worse. BTW this amp has a GAIN knob, and it ALSO has the classic XTant style pre-amp gain jumpers so the end user can really boost things in their car (Lovely). If I leave the AMP gain CCW, and only use the XTant gain, things seem mostly OK.

This amp does not really have a remote gain control. No U141 2180 device that I can see.

I replaced the U201(I think) opamp discussed in post #12. No difference

Any clues?
 
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#16
Im thinking about this for a second. Since this amp does not have the traditional MTX U141, I'm thinking this amp is using the Xtant input pre-amp circuits instead or in-place of the MTX traditional U141 external gain control, and then the normal amp-gain afterward. Is that a correct assumption?

This may explain why RCA shields have DC on them when floating or so Im thinking.

If I set the Xtant balance line input jumpers to ENABLED, the output terminal DC float and RCA shield DC readings go completely away. Amp seems to be fixed in this situation.

However,

If I momentarily ground RCA shields, the amp will COMPLETELY freak out and bounce off the PS current limiter (20A) and 'reboot' no matter the balanced jumper setting.
 
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