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Old 8th July 2012, 03:26 PM   #1
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Default Studiomaster 16-4

hello guys,

im new here in this forum and i wanna say hello to all of you!

I have bought an old Studiomaster 16-4 Mixing Desk. And i started to recapp and mod the Mixing Desk.

I recapped 10 of the 20 Channels and i changed the opamp in the mic pre section of these Channels for IC-Sockets so i can change the opamps when i want. The replacement opamps are ne 5534 pa, i used an 22pf cap to compensate the ne 5534. Sounds good so far.

Yesterday i started recapping and changing the opamps in the master section, i also bypassed the Caps with 0,1 uf Wima MKS 2. Now to my problem, when i rise the volume fader of the buss the vu meter rises with the fader... and i dont know why this happens. do you guys have an idea what is going wrong? the replacement opamps are the bb opa 134.

Thanks and regards

Last edited by suchenderxxx; 8th July 2012 at 06:25 PM.
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Old 10th July 2012, 02:52 PM   #2
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You propably, through some of the changes you made, introduced oscillation (>18 kHz) before the fader. The easiest way to find it is tracing the circuit with an oscilloscope. If you don't own one use your computer and soundcard with one of the many free programs. The sampling rate on your soundcard should be 96 kHz (or higher) so you can capture things above 20 kHz. E
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Old 10th July 2012, 02:59 PM   #3
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Or look for a hot chip
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Old 10th July 2012, 07:00 PM   #4
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i solved it with an cap between inverted input and the output. now the opa 134 is stabilized.
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Old 10th July 2012, 10:09 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

The NE5534 only need compensation capacitors if used at gains of less
than 3 which is highly unlikely in mic preamps. Remove the capacitors.
The chip is internally compensated for any gain over 3.

rgds, sreten.
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Old 10th July 2012, 10:22 PM   #6
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sreten thank you for your input. but the opamp used in the buss section is an opa 134. the ne 5534 are used in the channel mic pres and with the compensation caps it works really good, so no problem with them at all.

But i did a mistake when i soldered the bypass caps on the buss elkos. i soldered 1 bypass cap on the wrong solderpoint, i soldered an 0.1 uf filmcap to 1 pin on the elko and to 1 pin on an resistor. i got a bad smell and 2 of the resistores are gone to the other world....

when i have no signal on the desk and i push the faders on the modded 2 channels of the busses. 1 channel is quiet and the other gets noise which rises with the volume fader of the buss. this is the channel were the 2 resistors burned... after realizing this i changed the opamps, cause i thought that they could be damaged. but still the same noise. then i changed the elkos and desoldered the bypass caps. but still the noise. maybe the transistors are damaged? or the other resistors? the other resistors looks good to me but maybe not?

so any tip or advice is welcome. I wanna finish this project cause this makes me headaches now, it is very difficult to find the problem. which part could be damaged?


maybe i should get an multimeter to check everything...

thanks and regards
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Old 10th July 2012, 11:01 PM   #7
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I have a unit similar or same as this, which i have given up using, as half the pots are worn/loose.
If you crank up the gains on all the inputs, the white noise is considerable. But its just noise. and each channel is never the same noise level either.
Rattle each pot to see if it changes the noise level: it could eliminate bad pots first.
I think it uses 2SA970 transistors on the mic gain cct. worth changing.
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Old 10th July 2012, 11:02 PM   #8
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Forgot to say, do the grounding mods first. google will help.
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Old 10th July 2012, 11:18 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by suchenderxxx View Post
sreten thank you for your input. but the opamp used in the buss section is
an opa 134. the ne 5534 are used in the channel mic pres and with the
compensation caps it works really good, so no problem with them at all.

thanks and regards
Hi,

They don't need the compensation capacitors. Bus op-amp is irrelevant.
Your just chucking away extra performance with unity gain compensation.

rgds, sreten.
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Old 10th July 2012, 11:50 PM   #10
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before the i connected the inverted input with an 33pf cap to the output. the opa 134 was oscilating. after soldering that cap the problem is gone.

so it is better to dont compensate the ne 5534? i read in an thread that this will give me an sonically better output?
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