MPP vers E, need help making an excellent volume control, any ideas?

[Polymer]

Anyone who has made the MPP knows it's LOUD.

I have been playing around with the following volume control possibilities:

1.) Instead of R33 10k I have inserted a 5k pot to adjust the bias on the transistor.
It works quite well, BUT the control-ability in the low volume area is very short on the pot - zero ohms (no sound) to about 500 Ohms (loud enough).

I then inserted a 470 Ohm pot, the resistance range for comfortable listening being 190 to 270 Ohms. Only a short-turn-range again.

What is going on here? Suggestions?

2.) Tried various pots 1k/5k/10k in-line to the headphones. It works over a wider spread-of-turn than the above method, but I can't get it really quiet.

I have measured 5-15 mA current flowing in this setup (when vco is in low frequency range up to a few 100 Hz). Not sure if I measured right - RMS mA on fluke.
I wonder how well the pot wiper can handle this over time. Is anyone here knowledgeable about this? Is this an issue?

My goal here is to have an excellent volume control, especially in the lower loudness region.

Maybe I've gone about this totally wrong and there are better ways of doing this - Ideas?

Thanks!

[waltr]

Maybe try a resistive divider on the output with a pot and two fixed resistors.
This would divide the Voltage out of Q6.
Vary R38 (larger value) then a pot end and on the other end of the pot a resistor to -VB.
Connect the head phone to the pot wiper.
The total series resistance from Q6 collector can be set to limit current to a few mA.

Also, different head phone require different Voltage input due to having different impedance.
Matching is needed.
Which head phones?

[Polymer]

Thanks waltr.

Headphones are round 64 Ohm @ DC (two 32 Ohm speakers in series), they're from koss.

[Polymer]

... Maybe try a resistive divider on the output with a pot and two fixed resistors.
This would divide the Voltage out of Q6.
Vary R38 (larger value) then a pot end and on the other end of the pot a resistor to -VB.
Connect the head phone to the pot wiper.
The total series resistance from Q6 collector can be set to limit current to a few mA....

I have tried your suggestion on a breadboard to detector with 3 pots to vary the pot-end resistors too.
I found no combination that really works well in the low volume range.
I tried yet another circuit to vary the transistor bias and that works better than what I had done previously.

Read up on sound stuff, headphones etc ... learnt a bit - mine have 97 db mW.

[Polymer]

Hmm, not many replies ...

waltr ... I have now tried bigger resistors at the output and can get a good adjustment range.
The pots I received were antilog instead of log, that caused some confusion until I noticed.
I am up to 150k on the output to get good results with my headphones now.

Is the following calculation roughly correct?

Voltage 12v. I=12v/150k=80uA

Power through headphones at lowest perceivable volume:
P=I*I*R=80uA*80uA*64Ohms=0.38uW

[waltr]

Hmm, not many replies ...

waltr ... I have now tried bigger resistors at the output and can get a good adjustment range.
The pots I received were antilog instead of log, that caused some confusion until I noticed.
I am up to 150k on the output to get good results with my headphones now.

Is the following calculation roughly correct?

Voltage 12v. I=12v/150k=80uA

Power through headphones at lowest perceivable volume:
P=I*I*R=80uA*80uA*64Ohms=0.38uW

Calcs look correct.

Seems you have a very low power head phones or the impedance spec is not correct.
Either way good you keep experimenting with R values to get what you wanted.

Antilog pots would not help. Easy to not know what they are.
Is this with the resistive divider I suggested?
Or series R like how you started?

[Polymer]

Calcs look correct.

Seems you have a very low power head phones or the impedance spec is not correct.
Either way good you keep experimenting with R values to get what you wanted.

Antilog pots would not help. Easy to not know what they are.
Is this with the resistive divider I suggested?
Or series R like how you started?

Thank you waltr.
Indeed, now I have found out what antilog pots are and that they even exist. Learnt a lot reading "The Secret Life of Pots".
I dunno if its a quirk of mine, but I often find older articles about technical stuff much clearer/better understandable than modern ones.

I have used the series version not the divider. One fixed resistor 47k instead of 100 (R3 and 100k pot after in-line to headphone.
Putting in only a 10k as R38 is still very loud when pot set to zero. That would be 1.2mA or 89.6uW.
I figure the pot-wiper would not get annoyed with this.

The headphones are bog-standard Koss Ur 20's. Maybe impedance is much higher than DC ohms. My hearing is very acute though, when I want

I have another issue with the sound, VCO related, the frequency is already maxed out or fails at 6 inches away from the coil when testing for example a nickel.
Not sure if it makes sense to make an extra thread as this one is about sound stuff too. Maybe this behavior is normal for the MPP?

[waltr]

Good idea to draw up a schematic of the connections and values and post in this thread as a reference for others.

Another thread on MPP VCO might be good and easier to find later.

I never could get the 555 VCO working the way I liked on my HH2 build (the MPP is similar) so replaced with a PIC processor.

[Polymer]

Good idea to draw up a schematic of the connections and values and post in this thread as a reference for others.

Another thread on MPP VCO might be good and easier to find later.

I never could get the 555 VCO working the way I liked on my HH2 build (the MPP is similar) so replaced with a PIC processor.

OK. Thanks for mentioning the not being content with the vco. I thought I was the only one whining about it ... haven't found much on this here.
An interesting question for me is whether log or antilog frequency rise. The second one being better for more easily hearing faint targets but perhaps
more annoying with noise ... are you happy with your PIC sound generation?

[Polymer]

Here's my solution to the sound being way too loud in my headphones.

Note: test your detector with whatever you will be listening with out in the field.
I just hooked up a simple speaker for testing inside and didn't notice the loudness. My ears nearly fell off with the headphones on.

These values are for my headphones - Koss UR20. Others may vary. Play around with the resistors/pot to get the sound comfortable/practical to you.

If you have better solutions, do show them.

[waltr]

more annoying with noise ... are you happy with your PIC sound generation?

Yes, it does a constant volume but +- frequency relative to SAT stage output Voltage.
How the frequency varies as passing coil over a target has lots of info.

[Polymer]

Yes, it does a constant volume but +- frequency relative to SAT stage output Voltage.
How the frequency varies as passing coil over a target has lots of info.

I can imagine that. Sounds good. I assume a new learning curve comes with it.

[Polymer]

I played with the volume control some more. The 555 has more than enough power capabilities, so I
hooked that up directly to the headphones via a 2k resistor and 10k pot to GND with headphones in between.
Only 5v to deal with, instead of the 12v before. Leaving out the "tone shaping" parts doesn't make much difference to me.
This is even simpler now.