Crown D-150A II input sensitivity 1.4v ? Soundcraft MH3 can't drive it

Crown D-150A II input sensitivity 1.4v ? Soundcraft MH3 can't drive it to full output. I think some bad luck on my part . I repaired the amp and set DC and Bias. All good but on my version of the PCB there is no way to set the input to .774v, Only the later version's have that. Maybe someone has some ideas? ?
My pcb has only one dual opamp and the layout is totally not the same.



  • Schematic.jpg
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  • Jumpers.jpg
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R104 and R204 are part of offset adjustment, not what you need for adjustment of gain.

If I understand, the attached schematic is not correct for you amp? Can you identify your exact model? There's a good chance of finding the appropriate schematic.
Mister Audio might have something? ill check it out in the morning.
BSST- I have searched high and low for the schematic of my model and no luck Figuring out how to change the input sensitivity is beyond me, this is why I decided to post here. I will attach two pics. one of my PCB and the one with the jumpers found in the service manual. I think you will see a big difference.
What I can tell you is I hooked the Crown amp to my console and it sounds great but i could not drive it very hard towards full output.
D-150A II PCB.jpg

mine above
service manual below

opamp below
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Well, look up the datasheet of the 739 and see which op amps connect to the two inputs. Then increase the feedback resistor on those. I don't agree with BSST post 4, yes the feedback resistors do connect to DC adjust pot, but I don't see any later local feedback. There is global feedback from output, but messing with that messes with phase compensation. Pinouts of pin 8 and 13 as op amp inputs point to a 14 pin op amp, not an 8 pin one. Hint, the feedback resistor the third band will be yellow.
Agree for 1981 op amp production date, this is not a DC-150a. See schematic pdf on D150A, note says schematic is for serial # 140001 and above only.
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Thanks, Chrisng! Your provided link is certainly a much better match to Soundtec's photos.


Assuming that the D-150A schematic is a reasonable match to your amp, I agree with your original assessment that it has no easy provision for modifying gain. Its gain is established as (1+R115/R109) or 20.6, or 26.3dB. There are two feedback loops in this design, so trying to modify their gains would be perilous.

Since you have repaired the amp, I suggest that that you confirm that it is generating the expected gain of 26dB. Do you have instruments to do that?
I have no idea what Crown was up to but the Chassis clearly says D-150A Series II. As far as the PCB the foil version is M43197-8 and that is not the Crown D150 with the M1261A foil.
From what I can find out the D-150A series II had three board revisions and only the last one that the service manual was based on can be modified to .775v.

Today ill make a new cable balanced to unbalanced and see what I get. I will then put it back on the bench and do some more tests (26db gain)
This is the most useful link I've discovered.
I tried to upload, but file was too big for the forum; I hope the link works, but you may have this data already.

It's interesting that Crown seems to have been be very consistent in maintaining the same Ref Des for the gain setting resistors, R115 and R109. You could mimic the increased gain mod option by paralleling R109 with 825 Ohm 1%. Be cautious and test carefully before connecting valued speakers. In later versions, it's apparent that the amp was stable with this feedback revision, but I can't speak your variation. Most conservative course would stick with the lower gain, but the change may be perfectly OK.

Good luck!
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I tested the Crown on my bench I measured the output wattage. At 24.439Vrms into 8 ohms I reach clipping 74.6 watts per channel. So no problem with the amp. i was using 1k at 1.4v output from my generator.
Problem is the Soundcraft MH3. Originally i was using a Hafler T1100 with an input of .774v I guess this is why i did not see the problem with the MH3 output.
Again thanks everyone.. As for myself a lot of good lessons learned.