Volume Pot Extension? Why?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Member
Joined 2018
Paid Member
Hello there,

I will soon be building a LM3886DR amplifier and I have been gorging on as much info as I can.

One thing I have seen in several amp builds is where there is a volume pot on the line level in that is placed at the "back" of the amp and an extension has been used to extend the control to a knob at the front of the amp.

I can only guess that that has been done to make the line-in signal path as short as possible inside the chassis.

Does this really matter when the external line-in signal patch cord is longer than the amount of "path" saved inside the amp by doing this?

Am I missing something?

Thank you,

David.
 
Member
Joined 2018
Paid Member
"Hi, Yes that is something I have always wondered. But one thing I would never do is use a patch cord. Just use quality hook up wire from the pot mounted on the case to the input terminals.

Cheers"

I meant the patch cord connecting the source to the amplifier, not that a patch cord would be used internally...
 

PRR

Member
Joined 2003
Paid Member
The signal coming off the wiper is much weaker than the signal in the external interconnect. It may be wise to minimize this run.

An extreme case: I rebuilt a 1937 oscilloscope with 1979 chips. Used a high value "volume" (vertical gain) pot, and ran the signal 18 inches back to the sweep amplifier on the end of the CRT. Then did my math and realized at half-gain the pot and wiring would be down 3dB at 15kHz!

Putting the amp in front and long wires to the back of the CRT gave about the same trouble. I was using very small 300V transistors for sweep and they barely did the job mounted on the neck support.

I ended up with a buffer on the front pot to drive the run to the amplifier.
 
As PRR inferred, if mounting the pot at the front it is wise to run the source cable from rear to front then keep the next stage close to the volume pot as this will carry the attenuated signal.
Many pre-amps have this arrangement.
If the pot is mounted at the rear then the next stage must also be at the rear otherwise you are making matters worse not better.
 
The last few preamp chassis I build have the pots and source selector in the rear. Next to the input and output jacks.
The knobs on the front. Use shaft extenders to connect the knobs to the controls.
I have seen many commercial products that run cabling from the rear to the front panel, and then back to the circuitry. Asking for noise.
This is why I build my own.
 
One thing I have seen in several amp builds is where there is a volume pot on the line level in that is placed at the "back" of the amp and an extension has been used to extend the control to a knob at the front of the amp.

I can only guess that that has been done to make the line-in signal path as short as possible inside the chassis.

Does this really matter when the external line-in signal patch cord is longer than the amount of "path" saved inside the amp by doing this?
It depends on pot value used and driving impedances involved.

If, say, external source is a phone or player earphone out, source impedance is a few ohm and practically any cable can be used.
Out of lazyness/hurry sometimes I wire a mini plug to a large guitar type one to test an amp using *literally* desk lamp quality zip cord: parallel 2 x 0.75mm cable, not even twisted, say 1 or 2 meters long: no hum or noise. :eek:
Now if internally, say in a Tube amp, I use a 1M volume pot, when set halfway (electrically) source impedance become 1/4 of full pot resistance: 250k :yikes:
It will DEFINITELY be a problem, both capturing ambient buzz/noise AND killing treble response. What PRR mentions by the way.

So I think given the lower resistence pot values involved (50k?) you *can* mount the pot on the front panel (what 99% of people does) but it´s not ridiculous, just an abbundance of caution to use the shaft extensions and save signal path at a critical point.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.