Continuously Variable Mic Input Pad

[ruffrecords]

Most mic pres include a switch-able pad to enable them to cope with high input levels from, for example, condenser mics. Often these are just a two position switch offering 20dB attenuation of none. I have seen more complex ones offering 15dB and 30dB pads but that is about it. On another group, someone asked if it was possible to have a continuously variable input pad. You can certainly do it with very special pots but there does not seem to be a simple low cost way of achieving it...... So I gave this some though and came up with the attached schematic. It is by no means perfect but it is simple and uses readily available components.  As is often the case, in making it so simple there are a couple of compromises.

First, with the control fully off, the mic is connected straight to the mic transformer input. The compromise is that that there is a total resistance of just over 2K in parallel with the input. As most mic pres tend to make their inputs 'look like' about 1K5 then this is in parallel with the 2K making the actual input impedance about 857 ohms. This could be bad news for a mic with a true 600 ohm source impedance but should be no problem for 150 ohm mics or 50 ohm condensers.

Secondly, with the control at maximum attenuation. the source impedance seen by the mic input transform is pretty much the 68 ohm resistor. This is usually not a problem in fact it it not much different from a 50 ohm source condenser. The mic sees the two 1K pots i.e. 2K so basically there is no compromise here.

In intermediate positions it is a bit of both. For 6dB attenuation the pot wiper is roughly at the 500 ohm position. In this case the mic sees about 1K6 which is fine and, with a 150 ohm mic attached, the transformer sees about 530 ohms - perhaps a little high than usual but it should not cause any real problems.

At 20dB attenuation (pot at about 900/100) the impedance seen by the transformer is about 200 ohms, which is fine and the impedance seen by the mic is over 1800 ohms so again no compromise here.

I have not built or tested this circuit nor have I simulated it so at the moment it is just theory. It is so simple I am surprised no one has thought of it before; either that or there is some fatal flaw I have missed.

Comments welcome.

Cheers

Ian

 

[emrr]

Look up the JLM solution that's floated around here for a decade.  I think he sells a version of it. 

 

[ruffrecords]

Look up the JLM solution that's floated around here for a decade.  I think he sells a version of it.

Thanks Doug. Looks identical except he uses a 100R resistor for up to 25dB of attenuation and mine uses 68R for up to 30dB. Anyway, the fact it already exist at least proves I was not completely bonkers. I am just surprised it is not more widely known.

Cheers

Ian

 

[MHanson]

Ian,

I love the simplicity of this. 

As I see it, this could wreak havoc on CMRR, right?  That's not the end of the world, but .... 

This must be placed after a phantom power source, right?  I ask this only because my first thought was to build this into an external box for floating purposes.  Am I right to assume that an external box wouldn't be a good idea?

Thanks,
Michael

 

[ruffrecords]

Ian,

I love the simplicity of this. 

As I see it, this could wreak havoc on CMRR, right?  That's not the end of the world, but .... 

In theory it is perfectly balanced at all times. In practice the CMRR will be determined by how close the two halves of the dual gang pot track each other. I have specified a log pot and these do not track very well. A linear pot might improve matters but it will alter the gain versus rotation law.

This must be placed after a phantom power source, right?  I ask this only because my first thought was to build this into an external box for floating purposes.  Am I right to assume that an external box wouldn't be a good idea?

Thanks,
Michael

it probably can be used in an external  box with most phantom powered devices.  Phantom power is usually supplied via a pair of 6K8 resistors, one to the hot and one to the cold input. This circuit only adds 1K in series with each one. The phantom power to a device will therefore drop an additional 0.5V for every mA it draws. I suspect most modern mics draw leass then 5mA in which case the voltage drop will only be another 2.5V.

Worth trying.

Cheers


Ian

 

[moamps]

...it probably can be used in an external  box with most phantom powered devices.  Phantom power is usually supplied via a pair of 6K8 resistors, one to the hot and one to the cold input. This circuit only adds 1K in series with each one. The phantom power to a device will therefore drop an additional 0.5V for every mA it draws. I suspect most modern mics draw leass then 5mA in which case the voltage drop will only be another 2.5V.
Worth trying.

The main problem of stereo log pots is poor tracking. So, I will not use it in the front of the microphone amp, especially with the phantom power engaged.  The thru stepped balanced pi attenuator is much better solution, IMO.

 

[gyraf]

Good idea

As this is a dual pot with both halves participating in common attenuation, I think it would work well with a Linear potentiometer - and these are in general MUCH better at tracking than log ones.

Jakob E.

 

[JohnRoberts]

I would not expect very good tracking from a dual pot, in the context of good CMRR.

I am having a hard time getting excited about adding yet one more adjustment.

JR

PS: using a "H" pad topology with single pot in the shunt leg should deliver decent CMRR using precision Rs in the other legs. While adding fixed Rs in series with mic could hurt S/N.. The nice thing about a pad switch is you can alternately change several values with one switch throw.

 

[ruffrecords]

Good idea

As this is a dual pot with both halves participating in common attenuation, I think it would work well with a Linear potentiometer - and these are in general MUCH better at tracking than log ones.

Jakob E.

Agreed. However, the scale becomes a bit cramped in the second half. With a LIN pot there would be about 6dB attenuation at the mid point with the remaining 24dB occurring over the last 50% rotation. With a LOG pot you get about 1dB at the mid point with the remaining 13dB occurring over the last 50% rotation. The usual approach to modifying the law of a LIN pot is to slug it. In many applications this is not an issue but in this instance the reduction of the input resistance at zero attenuation  that this causes may be an issue.

The other concern I have is that linear pot tracking is usually specified in ratio-metric terms, not in terms of absolute resistance which is what matters for CMRR. This means that the actual total resistance of the two LIN tracks are probably different. The fact that both pots are at exactly 50% of this value at the half way point does not help us with CMRR. Since pot tolerances are typically 20% I am not sure if LOG pot tracking is in fact less of an issue than absolute value.

Having said all that there is a tacit assumption that a really good CMMR is essential. Certainly in some situations this is true but in most home recording situations it probably isn't.

If you are really concerned about CMRR then a simple 2 pole 6 way switch version with a 0 to 30dB range in 6dB steps can be made using fixed resistors.

Cheers

Ian

 

[emrr]

Somewhere there's a post in which JLM argues CMRR isn't an issue, and that the attenuator could be made successfully with a single pot section for microphone use.  I recall it's in a thread about the AM-16, and I argued about it.  Could be 8-9 years ago. 

Ian, I see you talked about the JLM option within the year in another thread. 

This solution would appear to diminish the treble of old ribbon mics even further, given that they are expected to work into an input transformer with open grid secondary, and this type input typically measures a very high input impedance, 5K-20K.  I would still want option to switch it out completely. 

As I believe I argued in the historical thread, the mid position is problematic too.  With many open grid secondary situations, the higher source Z will diminish bandwidth.  Maximum attenuation position can push an input transformer into a smiley face curve, extending response with boosts at the extremities, if the shunt is small enough.    I'd want to take a critical look at effects for any specific preamp before implementing. 

 

[ruffrecords]

Ian, I see you talked about the JLM option within the year in another thread.

Ah, the joys of a 65 year old brain!

Cheers

Ian

 

[emrr]

I'm probably not far behind.....more added to previous post....

 

[ruffrecords]

I'm probably not far behind.....more added to previous post....

I am not sure this is likely to be necessary for old ribbon mics. I was interested in it mainly from the point of view of the my Classic mic pre design which has a first stage with a fixed gain of about 48dB. Given that it clips at over +30dBu  output means it will take an unattenuated input of as much as -18dBu. A condenser mic might well exceed this input level but it is a lot less likely with a ribbon. Condenser mics with their much lower output impedance will not have their frequency response curtailed and at this sort of signal level CMRR is much less of an issue.

Cheers

Ian

 

[emrr]

As I say all the time, I regularly pad ribbon mics feeding fixed gain 40dB preamps that are overloading +22dBu converter inputs. 

 

[ruffrecords]

As I say all the time, I regularly pad ribbon mics feeding fixed gain 40dB preamps that are overloading +22dBu converter inputs.

Interesting - what instruments at what distance produce these levels from a ribbon mic?

Are you really worried about CMRR at such high input levels?

Cheers

Ian

 

[Samuel Groner]

Somewhere there's a post in which JLM argues CMRR isn't an issue, and that the attenuator could be made successfully with a single pot section for microphone use.

Indeed CMRR is only a minor issue here. The pad attenuates differential-mode signals only; common-mode signals are not attenuated, and thus there's no chance of common-mode to differential-mode conversion from pot tracking errors (which would show up as degraded CMRR). CMRR get's worse by the amount of differential-mode attenuation, but that is a modest amount that is surely tolerable in practice.

If the 68r resistor were split in half and the center tab grounded, common-mode signals would see attenuation as well, and pot tracking would be extremely critical.

Samuel

PS: If you implement this, make sure to place the phantom power resistors ahead of the pad.

 

[emrr]

As I say all the time, I regularly pad ribbon mics feeding fixed gain 40dB preamps that are overloading +22dBu converter inputs.

Interesting - what instruments at what distance produce these levels from a ribbon mic?

I work with a few singers who can do this when 2-3 feet back from a ribbon, and they can do it for hours on end.  It hurts to be in the room with them.    Besides that, drums and guitar amps at concert volume while tracking basics.  I would guess more people than not do not have an environment in which such SPL's can occur on a regular basis, and it wouldn't be an issue.  Indeed, most people seem to think they need a 70+dB preamp for a ribbon of any type, but I do not find this to be the case. 

 

[ruffrecords]

The only ribbons I ever owned were a pair of Grampian GR2 that I bought in the mid 60s. My first recording using them was af a school mate's band consisting of drums, guitar and Hammond organ. One mic was split across the guitar and organ and the other was on the the drums. I had no mic pres. The mics were fed through Grampian 600:50K transformers straight into my Brenell HI Fi tape link and onto tape. The tape link input sensitivity was, 75mV. I had no trouble fully modulating tape so I guess the mic must have been outputting about 7.5mV which is -40dBu. From the published sensitivity of the GR2 (-76.5dB ref 1V/dyne/cm2) I calculate this equates to about 105dB SPL.

The track recorded was mostly instrumental but at one pint the lead guitarist grabbed one mic and screamed into it.

I still have one of these mics and it still works.

Cheers

Ian

 

[abbey road d enfer]

Comments welcome.

Cheers

Ian

The issue there is that microphones expect to see a load of about 1-2k and preamps expect to see a source impedance of about 200r for proper noise performance.
It is quite easy to show that less than 15db attenuation does not fulfill these constraints; anyway, what's the use of a 10dB pad?