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Old 19th March 2013, 08:45 AM   #1
dgta is offline dgta  United States
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Default Distributed volume pot

I've just run across a Heathkit integrated amp that has a dual-volume pot (per channel) and I vaguely remember seeing it in an old Bogen tube amp as well. There is one pot at the front end of the preamp and one at the front of the power amp. The pots are ganged together as a single control.

Not quite sure I see any good reason for it, other than perhaps to allow the pre and the amp to be usable separately with their own gain control. In normal operation the volume control does feel a bit unusual, as you need to crank it up higher for mid-level operation than you would for a normal control.

I see several distinct downsides to this strategy, first and foremost throwing away gain you've already made. Does anyone see any advantages?
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Old 19th March 2013, 09:35 AM   #2
JMFahey is online now JMFahey  Argentina
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Advantages?
Yes, of course.
As you correctly labelled it, it's distributed gain control.
The conventional options are:

1) volume pot at the power amp input.
Pro: you can zero preamp noise/hum/hiss.
Excellent for home use, quiet surroundings, where residual noise can and will be annoying.
Con: if you use any signal source appreciably higher than expected, as in connecting a CD or Digital Audio source with, say, 2V RMS output instead of the 100/200mV expected at the "Aux" input , or use a higher output cartridge , if Phono is your thing, etc., you can easily overdrive the preamp and create *horrible* distortion.
Or at least make it work in an appreciably less linear area.
And you have no way to correct that
I mean, within that amplifier.

2) volume pot after an early gain stage, but still with one or 2 gain stages between it and Power Amp.
Pro: it's much "safer".
Con: even with volume on 0 , you still hear some hiss/hum.

3) the split solution you mention, addresses most concerns posted above.

As of: "throwing away gain you've already made." , well, that's exactly what passive volume controls do
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Old 19th March 2013, 11:53 AM   #3
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Since I don't know 'which' Heath integrated amp you are referring to, here's a diagram of the level and loudness control on a Heath SP2 preamp. You could achieve a variable loudness effect using these two controls. The Bell Carillon 6060 integrated amp used a similar setup, but with the tone control section between the level and loudness controls.
sp2_level-loudness.gif

Last edited by cogsncogs; 19th March 2013 at 12:12 PM. Reason: made things a little clearer...
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Old 19th March 2013, 12:08 PM   #4
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The Bell 6060 has the level and balance controls before the tone controls and the loudness control after the TC amp and before the pwr amp section.
Attached Images
File Type: gif Bell6060_level-balance.gif (55.7 KB, 176 views)
File Type: gif Bell6060_loudness.gif (22.0 KB, 166 views)
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Old 19th March 2013, 01:06 PM   #5
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Yes, Stromberg and Sherwood stereo amps also used two separate gain / loudness controls, allowing variable loudness compensation. But the original poster was talking about two ganged controls (Stromberg also used this in the mono days...). One further feature this could give is more choices for control taper - two linear pots won't give a linear response, since level will be 1/4 at 1/2 setting etc.
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Old 19th March 2013, 04:08 PM   #6
dgta is offline dgta  United States
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Tom, correct, we're NOT talking loudness or gain, we're talking about a dual volume pot. The actual amp is a Heath AA1515 (ss) integrated with 4 pots on one shaft (stereo) operated together. As said I've seen this arrangement on some mono tube amps as well.
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Old 19th March 2013, 04:17 PM   #7
dgta is offline dgta  United States
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Quote:
Originally Posted by JMFahey View Post
Advantages?
Yes, of course.
As you correctly labelled it, it's distributed gain control.
The conventional options are:
1) volume pot at the power amp input.
I've never seen an integrated with the volume pot at the power amp input. Can you show me one?

Quote:
Originally Posted by JMFahey View Post
Pro: you can zero preamp noise/hum/hiss.
Please explain how a volume pot can "zero" preamp noise/hiss. And what exactly is "zeroing".


Quote:
Originally Posted by JMFahey View Post
As of: "throwing away gain you've already made." , well, that's exactly what passive volume controls do
Yes, that's my point exactly, why do it twice? What's your point?
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Old 19th March 2013, 07:04 PM   #8
JMFahey is online now JMFahey  Argentina
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Quote:
Originally Posted by dgta View Post
I've never seen an integrated with the volume pot at the power amp input. Can you show me one?
There's *tons* of them , it was the classic topology in 70's and 80's consumer Hi Fi (Akai/Sansui/Technics/Aiwa//Philips, etc.)
Unfortunately all schematics and service manuals seem to be on sale.
Here's 3 free ones, by Sansui and HH Scott:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Quote:
Originally Posted by dgta View Post
Please explain how a volume pot can "zero" preamp noise/hiss. And what exactly is "zeroing".
That's 2 questions for the price of one
Google is your friend for any term not in your vocabulary, but to save you some time, here are accepted definitions, straight out of Dictionary, Encyclopedia and Thesaurus - The Free Dictionary

Quote:
....
b. A cardinal number indicating the absence of any or all units under consideration.
....
d. An argument at which the value of a function vanishes.
....
5. Informal One having no influence or importance
....
6. The lowest point
....
8. Informal Nothing; nil
....
adj.
1. Of, relating to, or being zero.
2.
a. Having no measurable or otherwise determinable value.
b. Informal Absent, inoperative, or irrelevant in specified circumstances
....
1. To eliminate
2. To reduce to zero.
Apply any or all of the underlined meanings to noise/hum/hiss and you'll get the point of having a volume control as late as possible in the signal chain.

Quote:
Originally Posted by dgta View Post
Yes, that's my point exactly, why do it twice? What's your point?
Again "two for one"?
OK.
a) reread my original explanation, it's all there.
b) *MY* point?
I'm not pushing any point, I'm answering your question and explaining what "the point of the original designer was"
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Old 21st March 2013, 05:03 AM   #9
dgta is offline dgta  United States
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OK, I give up Click the image to open in full size.
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