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Direct Out adding? Help
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Old 22nd March 2020, 09:30 PM   #1
DSurridge is offline DSurridge  United Kingdom
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Default Direct Out adding? Help

Hi All!

New to this forum, thought Id start my hello with a project!
I have a Yamaha MQ1202 (1980) and Im trying to add direct outs to each channel. I managed to get hold of the schematic and found a point I though was good; post an electrolyte cap which was post IC2 (4558) and that was all post Channel fader & eq.

I took the sig and a near by earth to a jack socket that was previously being use for HI - Z in, when I plugged it into my interface it was pretty noisey, quiet and bit crap sounding when using a dynamic microphone. Id like to use my interface as AD/DA and not for gaining and utilise the preamp in this desk.

Any help as to where Im going wrong and troubleshooting would be great as Im really new to audio electrics. Electrician by trade but this is a different world!

Many thanks!
Dan
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Old 22nd March 2020, 09:36 PM   #2
DSurridge is offline DSurridge  United Kingdom
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Old 23rd March 2020, 01:11 PM   #3
Mark Tillotson is offline Mark Tillotson
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You can't just route an opamp output direct to a cable, its a capacitive load and can easily
send the opamp into supersonic oscillations. Try at least 100 ohms in series to isolate the
cable from the opamp output. 220 ohms might be a good compromize value.
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Old 23rd March 2020, 06:28 PM   #4
DSurridge is offline DSurridge  United Kingdom
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Hi Mark,
Will add the 220 resistor and report back.
It’s so noisey and quiet any advice on what to do with the earth/ground. This is an unbalanced output so I assumed it’s very important in how noisey it is
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Old 23rd March 2020, 06:54 PM   #5
turk 182 is offline turk 182  Canada
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buffers are your friend!!
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Old 24th March 2020, 02:40 PM   #6
DSurridge is offline DSurridge  United Kingdom
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Alot of the noise I was getting was because the Hi-z input must have a chassis ground and I was carrying a ground from the channel back so I guess inducing a ground loop? I removed the ground that I took and keep the original in place &
With the 200ohm resistor in series I've got a pretty clean un distorted signal!

Although looking at the level diagram in the PDF I have, at the point i've taken off Im about -20db, I can gain it a bit at the pre amp but its not ideal...

I've been looking at non-inverting buffers with negative feedback loops, which i think is what IC2 is, is it possible to change the resistor in the Feedback loop to essentially get more gain?
If so how do i work this out?
I have some 5532 op amps arriving tomorrow, would these be better suited for the job then the JCR4558s?

Thanks for you advice!
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Old 24th March 2020, 07:22 PM   #7
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by DSurridge View Post
Alot of the noise I was getting was because the Hi-z input must have a chassis ground and I was carrying a ground from the channel back so I guess inducing a ground loop? I removed the ground that I took and keep the original in place &
With the 200ohm resistor in series I've got a pretty clean un distorted signal!
Good!
Quote:
Although looking at the level diagram in the PDF I have, at the point i've taken off Im about -20db, I can gain it a bit at the pre amp but its not ideal...

I've been looking at non-inverting buffers with negative feedback loops, which i think is what IC2 is, is it possible to change the resistor in the Feedback loop to essentially get more gain?
IC2 has a gain stage and there are 3 sections acting as gyrators I think using IC3 and the other half of IC2. They act as synthesized tuned circuits that can be faded into the feedback network to modify the response - i.e. do the EQ. Well I think that's how it works.

The top 15k resistor is setting the overall gain, perhaps try doubling it to 33k for about 6 dB more gain. The capacitor across it should keep things stable. You may lose a little bandwidth by increasing the gain, but that's probably well above 20kHz still.


Increasing that resistor more will give more gain, I'd not go crazy with it though as this will reduce bandwidth more and start to affect distortion levels.

Quote:
If so how do i work this out?
I have some 5532 op amps arriving tomorrow, would these be better suited for the job then the JCR4558s?

Thanks for you advice!
You can try the 5532 if you want, but they need 100nF ceramic decoupling between V+ and V- close to each chip for best performance. This prevents RF internal oscillations which degrade distortion slightly, according to some experts.

5532's have good overall performance and can drive quite heavy loads easily (down to 600 ohms), so generally they will work in most circuits. Their best noise performance is in the 3k to 15k impedance range and should be somewhat quieter than the 4558's if that's an issue.
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Old 24th March 2020, 10:36 PM   #8
MarsBravo is offline MarsBravo  Netherlands
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Quote:
Originally Posted by Mark Tillotson View Post
Well I think that's how it works.
You are right here.

Quote:
Originally Posted by Mark Tillotson View Post
The top 15k resistor is setting the overall gain, perhaps try doubling it to 33k for about 6 dB more gain. The capacitor across it should keep things stable. You may lose a little bandwidth by increasing the gain, but that's probably well above 20kHz still.
Increasing that resistor more will give more gain, I'd not go crazy with it though as this will reduce bandwidth more and start to affect distortion levels.
Instead of increasing the gain of this stage, you can try to lower the input resistor of 15k just a but (but not to much!) down to 10k with a 33k resistor parallel with it.
Those double function circuits (gain, tone, buffer) has a lot work to do, so maybe already running close to the limits (dc, power, stability, name it).
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