Mixer mods for increased headroom on the mix bus?
lobit
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29th February 2012
Old 29th February 2012
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Mixer mods for increased headroom on the mix bus?

I have a ramsa wrs 8112 mixer, I like the way it sounds but would like to have more headroom on the mix bus.. Is this something that can be improved with mods? if so, what components should I be looking at? what regulates the quality of a mix bus?
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29th February 2012
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Quote:
Originally Posted by lobit View Post
I have a ramsa wrs 8112 mixer, I like the way it sounds but would like to have more headroom on the mix bus.. Is this something that can be improved with mods? if so, what components should I be looking at? what regulates the quality of a mix bus?
The quality of a mix bus is a rather complex consideration.

Simple headroom can be easily added to most virtual earth sum buses by dropping down to a lower value feedback resistor... This will shift your nominal zero VU level lower by X dB and give you that same X dB more headroom.

If your master section output driver is single ended (not both output legs driven), you could add a differential output driver that gives you +6dB more output in connection with a -6 dB pad on the sum bus from lower value feedback resistor, for 6dB more headroom with same nominal 0VU.

Whether this gives you better quality is IMO very suspect. All noise sources will pretty much scale back to same as before, and since final result will often get dropped to only a couple volts for A/D conversion, having 60Vp-p swing seems academic.

Headroom seems to be a poorly understood and often misused term in comparison of sundry audio paths.

JR
lobit
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29th February 2012
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I might be out of my league.. I thought If there were some component values or op amp swaps I could do to let me turn the channel faders up a little more without distortion in the mix bus, I would do it. probably would require looking at the specific components of this particular board.
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Hi
John has just about 'nailed it' with his description and it may be easier and cheaper to review your use of the existing circuitry in terms of levels and simply turn the input channels 'down' a touch. While it would be possible to mess about with it as John points out there will be other limiting factors so you may have to do a full 'rework' to make significant differences.
Matt S
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1st March 2012
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The best way to allow higher fader levels without distortion is by reducing the signal level going into the mixer. That may sound dumb but it's true.
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1st March 2012
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It's a Ramsa, don't expect miracles. Most opamps will run to within 1.5 volts of the power rails, = 13.5 volts for a + - 15 volt supply. Increase that to + - 18 volts and you buy back that 1.5 volts.

The best alternative is to use the new "rail to rail" opamp designs. The BurrBrown OPA1611/12 are such devices. They will run to the power rails effectivly giving + - 18 volt headroom on a + - 15 power supply compared to standard opamps. The BB parts are also very low THD, noise (1.1 nv) and high open loop gain at 10 khz offering a clearer, quieter sum amp.

The other benefit is the lower noise and THD. That will offer an increase in dynamic range, a couple more db's at the top and a lower noise floor on the bottom = greater dynamic range without a complete re-design. Lower the signal levels a tad and that equals more headroom.
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Hi
Although rail to rail op amps are a move in the right direction the 'effect' of increased headroom only amounts to a little less than about 2dB so it is not a 'magic bullet' in that respect. Less noise and distortion would be added improvements but again to reap a full benefit you will probably need to be changing quite a lot of 'stuff' in there. Simply swapping a chip or two will not do it.
Matt S
lobit
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2nd March 2012
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thanks for the advise...
Ill do the bb op amps, taking it further than that is probably not worth the investment for what im using the board for.

I also notice the main out are +4 unbalanced.. would balancing inprove the signal quality to my converter? (balanced inputs as well)



now I just need to find a service manual somewhere.
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2nd March 2012
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I would like to see a comprehensive benchmarking of the audio path, to determine what needs fixing? Then confirm that you actually made an improvement with your efforts.

Throwing money and better parts at it could improve the path but may not. Could actually make things worse in some hypothetical situations.

JW has done a lot of this so his specific advice should be good.

measure twice, cut once.

JR
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2nd March 2012
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Quote:
Originally Posted by lobit View Post
I might be out of my league.. I thought If there were some component values or op amp swaps I could do to let me turn the channel faders up a little more without distortion in the mix bus, I would do it. probably would require looking at the specific components of this particular board.
I have never understood why people want to do this. You set the channel fader to 0 then alter the channel gain so the signal peaks at 0VU. If the signal is later too loud you turn down the fader. As most (decent) mixer channel faders have 10dB in hand then if the signal gets less you just bring the fader up. When you get a full mix going you will need to pull down the faders on all channels anyway so the output peaks to 0VU so there is even more room to play with if you need to bring up a channel.

Cheers

Ian
lobit
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2nd March 2012
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Quote:
I have never understood why people want to do this
for sure. I would be thinking the same thing reading this thread..
its just that with this particular board, to me anyway, it just sounds nicer when you turn the gain up. It gets a bit of natural compression or something going on.
This is just my opinion of course, someone else might tell me I'm doing it wrong. what sounds 'good' to me might sound terrible to someone else.
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2nd March 2012
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Hi
Unless it is under specified transformers you are sending your signal through most high feedback amplifier circuits will not 'create' significant distortion until you start to approach 'clipping'. Most will get quite nasty as this happens.
I have been looking at a 'cheap' mixer recently with regard to 'improvements'. It has TL081 chips in it. Replacing them with some supposedly good BB types does not actually improve things much as a direct 'swap' as there are significant 'errors' in the circuit layout and correcting these improves things somewhat with the TL0 chips, then fitting 'better' chips improves things considerably. Having chips capable of 0.0005 percent distortion is a waste if the circuit layout drags it back to 0.03 percent.
Matt S
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13th March 2012
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Quote:
Originally Posted by lobit View Post
its just that with this particular board, to me anyway, it just sounds nicer when you turn the gain up. It gets a bit of natural compression or something going on.
This is just my opinion of course, someone else might tell me I'm doing it wrong. what sounds 'good' to me might sound terrible to someone else.
No, you're doing it right. You like the sound you get when you come close to running out of headroom. But then you push it a little further, and you don't like the sound when you actually do run out of headroom. If you performed modifications to increase the headroom, you could turn things up more before you ran out of headroom, but you would also have to turn things up more before you almost run out of headroom to get the sound you like. So it won't help you.
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13th March 2012
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Hi
Running out of headroom can have a number of effects.
Well designed circuits will 'flatten off' the peaks and in a way can be 'driven' in that it simply chops the peak and that is the end of it. Drive it harder and you chop more off.
However, some amplifiers do not simply 'stop getting louder' but exhibit strange responses such as wild HF oscillation (SSM2015) or output inversion (it's output suddenly flips phase) or various other versions depending on the devices used and the exact circuit conditions.
Class A circuits often exhibit an asymmetric response to 'clipping' which is a little more progressive, giving a feeling that it can be 'driven' unlike other designs.
Matt S
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13th March 2012
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I've measured some new stuff (mic preamps) I'm working on here and I have seen a different looking clipping curve. These don't clip with a hard line across the waveform like most other designs, but rather a half smile curve that is flatter than the un-clipped waveform but it does not flatten out. This seems to add more even harmonics than odd, a good thing. The design is differential all the way through using transconductance opamps, they sound and work different than traditional voltage feedback opamps.
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13th March 2012
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I actually looked into some alternates to hard clipping back in the '70s and some I tried sounded surprisingly bad (much worse than simple clipping).

I settled on a design approach that all clipping, or slew limiting, or... whatever nonlinearity was undesirable and should be avoided. Of course any clipping that does occur should recover very quickly after the overload is removed and not generate audible artifacts besides the simple clipping.

JR
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13th March 2012
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Agreed, but it is nice to know that a mic pre will not 'splat" when an occassion peak is passed. Bandwidth is well beyond 20 mhz with 2k slew rates so it's not a frequency dependent reaction, it behaves the same, 20 hz to 20k hz and beyond. So far I haven't seen this type of overload in a bipolar transistor design, a symetrical rounding of the tops of the waveform. It's intriguing but I realize it cannot be a selling point, clipping is generally not a good thing for recording.
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Perhaps you are seeing some anti-saturation clamps to prevent output stage from hard clipping and loss of negative feedback that can charge some internal nodes and slow down recovery time after clipping. Used to do this inside power amplifiers.

Never a bad thing to recover very fast.

JR
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14th March 2012
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Hi
On the basis that power supplies have a finite value, everything will clip at some point unless some form of 'clamping' is installed.
Back to back zener diodes could be a cheap and dirty method, or various other permutations. Not saying it would sound better than fast recovery from occasional peaks though.
The SSM2015 as a mic amp would burst into a short but constant period of VHF oscillation if 'triggered' by a peak. It didn't seem to matter how long the 'trigger' was.
Matt S
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14th March 2012
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Quote:
Originally Posted by Matt Syson View Post
Hi
On the basis that power supplies have a finite value, everything will clip at some point unless some form of 'clamping' is installed.
Back to back zener diodes could be a cheap and dirty method, or various other permutations. Not saying it would sound better than fast recovery from occasional peaks though.
The SSM2015 as a mic amp would burst into a short but constant period of VHF oscillation if 'triggered' by a peak. It didn't seem to matter how long the 'trigger' was.
Matt S
Clamping is pretty much controlled clipping.

it is a generally unavoidable fact that when an output stage saturates any overall negative feedback is no longer closing the loop, so internal circuitry will keep trying to drive the output further. Multiple internal clamps are routinely used to prevent these internal nodes from getting too far out of whack and requiring a bunch of return to return to stable operation after the overload event is removed.

Clamps in the NF to prevent saturating the device may be useful, if the IC design is not already managing it's recovery time. Generally easier to do complex anti-sat circuits inside an IC than externally with discrete design.

Zeners in the feedback have their own issues, and I suspect a very high speed part, like JW seems to favor, may recover faster than a slow zener.

Note: Instability around clipping could be evidence of inadequate power supply decoupling, or not...ymmv

JR
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14th March 2012
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This design uses all transconductance opamps, no standard voltage feedback stuff. Speed and recovery are super fast as bandwidth is set by the feedback resistor values, not an RC feedback network. Perhaps it's just the nature of a saturating current feedback loop doing this, or maybe the front end transistors that are in the current feedback loop? I'll probe on further...
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