"WHAMMY" Pass DIY headphone amp guide

[locky]

Does it get louder if you turn the potentiometer clockwise?
If it gets quieter then reverse the wires.
It gets louder (too loud!) as there's a lot of volume when it's fully off...

 

[2 picoDumbs]

Ok reduce gain of amplifier and retest.

I am kind of suspicious something is not quite right.

 

[locky]

Having checked the potentiometer it is showing 195K fully open to 115k fully closed between input and output on both channels.

 

[2 picoDumbs]

Measure resistance across each pin and wiper.
You should get 0 Ohms on one of the measurements with pot fully turned counter clockwise

 

[locky]

Ok reduce gain of amplifier and retest.

Will see if I have any 3K reistors or something thereabouts. Might need to order some. Thanks for the assistance btw... I may try using the CD player as a source and see how things are although I guess the issue may be more the sensitivity of the phones?

 

[locky]

Input to wiper is 97.5k fully closed
Output to wiper is 16k fully closed
On both channels
Both input and output to wiper is 97k when fully open

 

[2 picoDumbs]

Ok that seems better but not what I would expect.

Reduce gain of amp.

I think I have the same pot I will measure mine

 

[2 picoDumbs]

I have ALPS 531G 100KAx2

When rotated completely counter clockwise I get 3 Ohms and 96 kOhms across each pin and wiper.

Are you sure output to wiper is 16 kOhms and not 16 Ohms?

Well that would explain a big part of the problem if true

 

[2 picoDumbs]

Both input and output to wiper is 97k when fully open

What?
That is bizarre.

If that is true there is something wrong with the pot?

If you take the pot out of the circuit and measure it, is it the same?

 

[locky]

Input to wiper with pot fully counter clockwise

Output to wiper with pot full counter clockwise

 

[2 picoDumbs]

This does not match my Alps 100k pot.

This would certainly explain why it is so bloody loud

Maybe take it out of the circuit and measure again,

If it still measures weird, the pot is either faulty or incorrectly labelled as an Audio taper pot.
In that case I would ask for a replacement

 

[locky]

Input to output when fully open (clockwise)

Inpit to output when fully closed (counter clockwise)

 

[locky]

Mmmm. Looks like a problem with the pot. It was one I had left over from a previous project so has been knocking about for a while. I think I'll replace it. Would I be better off going for a 27k one?

 

[2 picoDumbs]

Depends 25k or 47k.
100k can be good to reduce the load on the source equipment but you might possibly roll off top end frequency.
I like to go high as possible provided it doesn't roll off high frequency.
Do you know what the input capacitance of the opamp is?

 

[locky]

I currently have an OPA 1612AID (SOIC8) mounted on a DIP 8 adaptor

 

[2 picoDumbs]

Can't find the spec on the data sheet.
47k will be a good compromise

 

[locky]

https://www.ti.com/product/OPA1612#product-details##params

 

[2 picoDumbs]

Couldn't find the input capacitance spec, but if you set the gain so you're using around 3/4 (or more) of the total signal for your requirements, the output impedance of the pot will be minimised (ie avoid -6dB of attenuation on the pot), and this won't significantly affect bandwidth, in which case a 100k pot will be fine.

I would probably buy a 100k and a 47k pot and try both, as well as setting gain to an appropriate level for your headphones.

 

[locky]

Thanks. I have a pretty rudimentary knowledge of the physics behind this. What's the formula for deciding which resistors to replace the stock ones for my Shure 1840 cans? I've ordered a 50k pot so will install that and see how it goes...

 

[2 picoDumbs]

(R1 + R2)/R2

Eg (5k + 5K)/5K = 2 (ie 6dB)

R2 is the resistor connected directly to ground

Keep the impedance close to the stock values eg between 4k and 5.5K

You could use 2 x 4.75k resistors