How preserve loudness circuit with new volume pot?

[mfaughn]

I am monkeying with an old Allied 935 (rebranded Pioneer SA-400). I actually have two of them because I kinda like them. The volume pots in them are concentric pots with extra lugs for the loudness circuit. They track HORRIBLY. I really like the loudness circuit on the amp and pretty much leave it on all the time. Is there a way I can preserve the loudness when using a standard pot?

Other stuff: The original pot is 1M. I don't use the phone section so it is essentially the only thing between the input and driver stage. I replaced one pot already with a 100K stepped "DACT type" pot. Sounds good but the usable range of the pot is small. It is basically at full volume within about 6-7 steps. Whats going on here? What do I need to do to get this right? Higher value pot, change circuit?

Thanks for any help.

-Michael

 

[Tom Bavis]

The loudness circuit makes the volume pot's taper much deeper - 47K across the tap. if you can get to the solder joints for the resistors in the attenuator, you could wire in the loudness switch... but existing values would not be ideal for 100K.

If you just want to improve the range of the attenuator, add a resistor in series at the top - obviously any range past clipping is wasted.

 

[mfaughn]

The loudness circuit makes the volume pot's taper much deeper - 47K across the tap. if you can get to the solder joints for the resistors in the attenuator, you could wire in the loudness switch... but existing values would not be ideal for 100K.

I don't think I want to fool with trying to solder to the smd pads of the attenuator. I can see them but getting at them without making a mess is probably beyond me.

If you just want to improve the range of the attenuator, add a resistor in series at the top - obviously any range past clipping is wasted.

...and here is where I demonstrate my probable ignorance...
If I have a 100K pot and I put a 500K resistor in series then will it mimic approximately the lower 50% of the range of the 1M pot? If so that is fine since I rarely had the old volume at much more than 12 o'clock.

 

[LinuksGuru]

Hi, mfaughn,

If you don't have tapped potentiometer you can use schematic like this one (attached).

I'm also have various articles covering this subject (with both potentiometers and stepped attenuators), tell me if you wish me to upload them.

 

[Tom Bavis]

I don't think I want to fool with trying to solder to the smd pads of the attenuator. I can see them but getting at them without making a mess is probably beyond me.

...and here is where I demonstrate my probable ignorance...
If I have a 100K pot and I put a 500K resistor in series then will it mimic approximately the lower 50% of the range of the 1M pot? If so that is fine since I rarely had the old volume at much more than 12 o'clock.

Add 100K to double the usable range of the pot.

 

[mfaughn]

LinuksGuru (hurray for Linux)--I would appreciate more material on loudness circuits. It is the particular loudness circuit on this amp that I like. I've heard other loudness circuits on other amps and they don't all seem to change the sound in the same way. I understand what they are *supposed* to do. In fact, this particular one seems to be more of a "fatness" boost and, at least to my ears, doesn't do what a loudness circuit is "supposed" to do. In contrast, my friend has Voice of Music 1448 and 1428 with loudness compensation and they do seem to create a scooped sound, pulling the mids down at lower levels, and it is totally different than what my amp does.

As for the range of usability of the pot...right now I have about 1/3 of the pot usable. If I add 300-400K won't I be closer to getting all the steps usable. I don't mind if I toss away the last bit of maximum volume out of the amp. I also measured the original pot and it was ~1.35M

 

[LinuksGuru]

Hi, mfaughn,

Here are another schematics for volume control with loudness, without using tapped pots.

Frequency response shown next. Please note third schematic doesn't have "0" (mute) level.

If you wish to change RC values, no problem. For example, if you increase R values by a factor of 5, you will need to decrease all C values by the same 5.

 

[Thx1326]

How To Add This

I have a tube preamp that has room for this 21 stepped VC with loudness tap and circuit board. It currently has a standard stereo VC with no loudness. I do have an open un-used on-off button on the front panel. Can I just pull out the old volume control and replace it with this one? How would I wire it to the switch so I can turn the loudness function on and off?

These are readily available on ebay.

Picture is attached.

Thanks in advance.

 

[Wavebourn]

Initially loudness pot used to extend low end boosting lows above Fs using excess power. It was a misinterpretation, as if it has anything to do with F-M curves, making loudness pots useless.

Aim for 12 dB/Oct below Fs on minimal loudness, if your speakers have limited bandwidth.