[Mondy]

Hi All

I’m new to the forum, but have been lurking around some time, a very interesting place indeed!
I am a gigging guitarist and singer with an electronics tinkering obsession and I built the Bo Hansen DI-box with the OEP transformer for my acoustic guitar. This classic little circuit’s sound quality instantly retired my old DI-Box to spare status and I was a happy bunny. However my old one had a muting function on a stomp switch and, as those who use similar devices I am sure will tell you,  once you have this feature there is no going back!  I am planning to upgrade and build another Bo Hansen and this time I am going to go with the Lundhal transformer, however I am looking to try and implement a “stompable” mute switch in the circuit but I do not how to go about this.

Can anyone advise the best place to put a mute switch/circuit in the Bo-Hansen so that it is a “pop” free as possible? I
was thinking maybe I could put this in the transformer secondary similar to these scenarios...

http://shure.custhelp.com/app/answers/detail/a_id/2066/~/mute-switch-with-phantom-power

http://www.crownaudio.com/mic_web/popless.htm

However I am not sure if this is OK. My main concern is that I do not cause any damage to phantom supplies in mixers etc and also I don’t want to do anything that will harm the wonderful sound that this little DI produces. I know there are many boys and girls here who are very familiar with Bo’s circuit and I wondered if you could help me in my quest!

Many Thanks

Ray

[PRR]

> Topic: Silencing Bo Hansen!! 

I thought this was going to be political??

[Mondy]

Thanks PRR! It’s reassuring and good to see you here also, I always enjoy your posts.

Damn is it that easy!? I am about 2000 light years from home right now but will get this bread boarded when I get back and see how it works. The LED is also a handy addition.

There is one thing though, this solution looks like it will mute both the txfrmr out and the line out. Is there any way of just muting the trxfmr out as I can then keep the line out for my tuner connection as this is how I use it, I never play through any back line?

Thanks again for your help
Ray

[abbey road d enfer]

> Topic: Silencing Bo Hansen!! 

I thought this was going to be political??

How silent is it gonna be? I can predict a pop when the operating point changes.

[Jean Clochet]

And surely one of the switch position should be wired widdershins?  As is, when the signal is not muted, the LED is in the collector circuit.

[PRR]

> keep the line out for my tuner

You didn't say that.

Note that those tricks of shorting the XLR also short the transformer secondary, thus its primary, and the direct-out signal is severely loaded.

> the LED is in the collector circuit

So? There was 12V emitter-to-collector, now 10V e-to-c, signal levels are hardly 1V. Still tons of headroom.

> The LED is also a handy addition.

IMHO, essential for live performance. There's too many ways signals get lost. Adding another signal-kill, you really need a go/no-go indicator.

And the LED can't really go anywhere else because current is tight.

> when the signal is not muted, the LED is {on}

Matter of taste. I figure ON is ON (un-muted). This also confirms that Phantom is ON. After the first screw-up, Ray will always look for the red light before starting to play. It could be wired "Red = STOP! you are muted".

> I can predict a pop when the operating point changes.

Yes, but you can work it out. Mu of a BJT is near 1,000. We induce a 2V change at the collector. There's 2V/1000 or 2mV change at the emitter. I think the pop/tik will be 20db-40db down from normal signal. If it was a $40 trick, I'd think about it more. Since it is a $3 experiment, try it.

A selected resistor (330-470) or two diodes in the LED-shorting line can reduce this to a few mV.

Nobody objects to "huge peak currents" on a shorted emitter-follower? That was my main fear.

[Jean Clochet]

> the LED is in the collector circuit

So? There was 12V emitter-to-collector, now 10V e-to-c, signal levels are hardly 1V. Still tons of headroom.

   OK, fair enough. 

Nobody objects to "huge peak currents" on a shorted emitter-follower? That was my main fear.

I just ASSumed that this would get fixed when the revision for not muting the direct out was implemented     

[Mondy]

Thanks for your help here PRR!

I bread boarded the latest circuit mod and it provides a mute on the XLR whilst still outputting enough signal for my tuners to work well. There is a slight pop, but I think I can live with it as in reality I do not think any audience is going to object to it.  I opted for LED “on when muted” as this is the format with my old DI and I would also keep like to keep the circuit unaltered during sound “on” as the quality is exceptional. PRR is spot on when he says you need all the help you can get in a live situation as even if your gear is working correctly, your mind is not in a rational state when in front of your audience and indicators are a boon!

I tried the resistor between output cap and transformer primary starting at 100R, then tried 220R, 330R and 470R. There was a definite change in sound quality above 100R and therefore I stuck with this value as the pop was pretty much at the same level with all the resistor values tried. The sound if anything seemed to improve a tiny bit with 100R and the OEP transformer. I will try this experiment with the resistors again when I have the Lundahl transformer

Nobody objects to "huge peak currents" on a shorted emitter-follower? That was my main fear.

Could you guys clarify this issue for me as I am uncertain exactly what this means? Will a reduction to 100R make this situation worse! I really want to keep the sound quality and reliability of this circuit as high as it can be and therefore if there is any degradation in either sound quality or reliability, then I would rather do without the muting facility even though it is really useful when gigging!

Thanks all for your time!

Ray

[PRR]

> clarify this issue

It's safe, though I had to walk the driveway to convince myself.

For empirical proof: mute the box, put a booster in front, gulp an energy drink, and beat your axe. That's worse abuse than you'd ever apply on stage.

[zebra50]

Ray emailed to ask me how I had done this. I took a different route.

I can't find my notes, but it was something like this.

I had an extra output jack, just for the tuner, which was just a straight connection (i.e. parallel) from the instrument input. So the guitar always sees the tuner.

One pole of the footswitch disconnected the guitar from the input to the circuit (i.e. before the 10K resistor), and
show the circuit a dummy load (10K?) when in the mute position. The other pole turned an LED on and off.

To reduce pops I had to add a pull down resistor (started at 1 meg and experiment) before the 10K.

It was still not totally silent.

Hope that helps!

Stewart

[Mondy]

Hi Stewart

Thanks for your help with this, your time is much appreciated!

Having the tuner always in parallel with the input would not be ideal with my pickup as it is quite high impedance and the shunting effect of the tuner takes the top end slightly off the sound. The difference is only very slight with the OEP transformer, but I now have the Lundhal 1538 to play with and with this transformer the difference is quite noticeable. Even with a pull down resistor the pop is only slightly less than PRR’s quick and dirty mute circuit and here, the input impedance stays constant and so I think I will stick with this solution if I go ahead with the mute function. To be honest, the sound is so good with the Lundhal, I am thinking of leaving the circuit alone and sticking to my old DI where I feel that the need to mute is more important than obsessive sound quality. Decisions, decisions...!!

I have not bread boarded the input Zener protection scheme(no diodes to hand) and so I  do not know if these diodes have any effect on the sound quality. I would like to leave them off really and only add them if absolutely necessary. Would it be OK just to increase the value of the 10K input resistor as a measure of input protection, such as used on FET inputs etc, or will that not provide suitable protection with the bi-polar transistor topology of this circuit? Expert opinions would be appreciated!

Thanks again to you and PRR for all your help

Ray

[pablobolche]

Hi there ... I just wired PPR's solution for a mute on bo's di (1st schematic)  ... and though it works great no pops...it doesn't kill the signal.. I get a -40/50 db signal on my metric halo..
Is this solution work this way or am I doing something wrong..
Thanks

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