Upsides/downsides of running B22 without volume control?
Nov 10, 2008 at 4:00 AM Thread Starter Post #1 of 18

Fryguy8

100+ Head-Fier
Joined
Jan 18, 2005
Posts
148
Likes
10
I have a passive volume control sitting around with balanced ins and outs, so could I wire up a B22 to act more like a standard power amplifier and get rid of all of the volume pot and use a pre-amp (in my case, a fully active preamp), instead?
 
Nov 10, 2008 at 4:12 AM Post #2 of 18

Navyblue

Headphoneus Supremus
Joined
Apr 2, 2008
Posts
1,673
Likes
14
I guess the downside is you have to run the connection between the volume knob and Beta 22 outside of casing and that section of cable is particularly susceptible to RFI? Though it is probably ok if you use heavily shielded and short interconnect.
 
Nov 10, 2008 at 4:48 AM Post #3 of 18

amb

Member of the Trade: AMB Laboratories
Joined
Apr 1, 2004
Posts
4,933
Likes
41
Yes, the impedance after the pot is high, so the signal is much more susceptible to interference. What's worse, it's also more susceptible to capacitive coupling, and since interconnect cables are usually quite capacitive, this setup will promote high frequency rolloff (capacitive coupling between signal and ground) and stereo crosstalk (capacitive coupling between the two stereo channels).

Passive preamps without an active output buffer is a bad idea in my book for this very reason. The wiring between the volume pot/attenuator and the input of the next active amplifier stage should be kept as short as possible.
 
Nov 10, 2008 at 4:54 AM Post #4 of 18

Fryguy8

100+ Head-Fier
Joined
Jan 18, 2005
Posts
148
Likes
10
Since my volume control is passive, could I connect it to the output stage of the amp and gain any benefits that way? Or should I really just spend the extra on getting a pot and making the amp integrated rather than build separates?

Why does it seem like a lot/most higher end setups have separates for pre-amps and power amps if it's so bad to have the volume control far away from the amplification stage?
 
Nov 10, 2008 at 5:00 AM Post #5 of 18

amb

Member of the Trade: AMB Laboratories
Joined
Apr 1, 2004
Posts
4,933
Likes
41
Quote:

Originally Posted by Fryguy8 /img/forum/go_quote.gif
Since my volume control is passive, could I connect it to the output stage of the amp and gain any benefits that way?


Which output stage?

Quote:

Or should I really just spend the extra on getting a pot and making the amp integrated rather than build separates?


A headphone amp is somewhat akin to an integrated speaker amp only in that it's got a volume control built-in, and sometimes has multiple switchable inputs. But it is also somewhat like a preamp except that it could drive much lower impedance loads...

Quote:

Why does it seem like a lot/most higher end setups have separates for pre-amps and power amps if it's so bad to have the volume control far away from the amplification stage?


Because most preamps aren't passive. There is an active amplification stage built-in, immediately after the volume control. The output impedance of the active stage is usually low enough to render all of the issues I mentioned (interference, capacitive coupling) moot.
 
Nov 10, 2008 at 5:19 AM Post #6 of 18

Fryguy8

100+ Head-Fier
Joined
Jan 18, 2005
Posts
148
Likes
10
Quote:

Originally Posted by amb /img/forum/go_quote.gif
Which output stage?


Output of the amp, between amp and headphones.

Quote:

Originally Posted by amb /img/forum/go_quote.gif
Because most preamps aren't passive. There is an active amplification stage built-in, immediately after the volume control. The output impedance of the active stage is usually low enough to render all of the issues I mentioned (interference, capacitive coupling) moot.


How does active amplification on a pre-amp help? If the problem is because of impedance on the pre-amp output (because of increased resistance to lower output voltage), wouldn't an active amp suffer the same problems because you eventually need to get the same output voltage to the amp? I'm missing something here, and I'm not sure what.
 
Nov 10, 2008 at 5:32 AM Post #7 of 18

nikongod

DIY-ku
Joined
Jan 24, 2005
Posts
8,882
Likes
124
Quote:

Originally Posted by Fryguy8 /img/forum/go_quote.gif
Output of the amp, between amp and headphones.


baddddd news.

You are creating variable source impedance to drive the load and have not eliminated the attenuator from the circuit. There are quite a few headphones that dont deal well with source impedances more than 0.3ohms (slightly exaggerated), and certainly anything that varies. Some others do deal well with (and seem to like) a 120-ohm source impedance but are pretty particular about 120-ohms, this is a good reason for 2 output jacks.
Quote:

Originally Posted by Fryguy8 /img/forum/go_quote.gif
wouldn't an active amp suffer the same problems because you eventually need to get the same output voltage to the amp? I'm missing something here, and I'm not sure what.


Source impedance, its important. preamps with gain typically have buffers at the outputs so their source impedances are very low.

Are you 100% sure you want to undertake a build this complicated?
 
Nov 10, 2008 at 5:33 AM Post #8 of 18

Navyblue

Headphoneus Supremus
Joined
Apr 2, 2008
Posts
1,673
Likes
14
The output impedance of an active preamp, or a passive preamp with a buffer, is typically close to zero. It is the reason of adding a buffer to a passive preamp. Whereas the output of a 50k ohms pot is, well, 50k ohms.
 
Nov 10, 2008 at 5:34 AM Post #9 of 18

Fryguy8

100+ Head-Fier
Joined
Jan 18, 2005
Posts
148
Likes
10
Quote:

Originally Posted by nikongod /img/forum/go_quote.gif
baddddd news.

You are creating variable source impedance to drive the load and have not eliminated the attenuator from the circuit. There are quite a few headphones that dont deal well with source impedances more than 0.3ohms (slightly exaggerated), and certainly anything that varies. Some others do deal well with (and seem to like) a 120-ohm source impedance but are pretty particular about 120-ohms, this is a good reason for 2 output jacks.

Are you 100% sure you want to undertake a build this complicated?



I'm not building. I'm contracting a builder, I'm just trying to consider some of my options.
 
Nov 10, 2008 at 8:17 AM Post #10 of 18

amb

Member of the Trade: AMB Laboratories
Joined
Apr 1, 2004
Posts
4,933
Likes
41
Fryguy8, to help you understand the issues better, I think some pictures would help.

First, consider what you propose. Here is a passive preamp (volume control only, no active buffer or gain stage) connected to a headphone amp without a volume control at its input. The bold lines are the interconnect cables. Otherwise, the picture, along with the explanations above, should hopefully be self-explanatory.

attachment.php


In contrast, here is a headphone amplifier with a volume control at its input, fed either by an active preamp or directly from the source.

attachment.php


Does it make my point clear now?

 
Nov 10, 2008 at 9:29 AM Post #11 of 18

JamesL

1000+ Head-Fier
Joined
Dec 5, 2007
Posts
1,214
Likes
16
(on a slightly related.. but off topic note)..
How about autoformers as a passive preamp? I've read about it somewhere, but I can't seem to find any detailed information on them.
It attenuates the signal through variable inductance, right? And so what becomes of the input impedance of the proceeding stage.
I'm sorry if it's a stupid question. I haven't yet taken any proper physics classes covering basic electrical theory. =/
 
Nov 10, 2008 at 9:34 AM Post #12 of 18

amb

Member of the Trade: AMB Laboratories
Joined
Apr 1, 2004
Posts
4,933
Likes
41
An autoformer is just a variant of a transformer. Depending on the specs of the unit being used, it may or may not exhibit a problem over a long interconnect cable. I still think that an active buffer or gain stage would go a long way to eliminating any potential issues.
 
Nov 10, 2008 at 9:58 AM Post #13 of 18

JamesL

1000+ Head-Fier
Joined
Dec 5, 2007
Posts
1,214
Likes
16
Quote:

Originally Posted by amb /img/forum/go_quote.gif
An autoformer is just a variant of a transformer. Depending on the specs of the unit being used, it may or may not exhibit a problem over a long interconnect cable. I still think that an active buffer or gain stage would go a long way to eliminating any potential issues.


mhmm..
I know it is a transformer, and do not plan to use any kind of passive/active preamp myself. I was just wondering what impedance the source sees when a transformer is placed after it... just for curiousity's sake.
tongue_smile.gif

I think I've found the answer though.
Thanks though =)
 
Nov 10, 2008 at 2:23 PM Post #14 of 18

luvdunhill

Headphoneus Supremus
Joined
Mar 22, 2006
Posts
2,304
Likes
12
Quote:

Originally Posted by JamesL /img/forum/go_quote.gif
(on a slightly related.. but off topic note)..
How about autoformers as a passive preamp? I've read about it somewhere, but I can't seem to find any detailed information on them.
It attenuates the signal through variable inductance, right? And so what becomes of the input impedance of the proceeding stage.
I'm sorry if it's a stupid question. I haven't yet taken any proper physics classes covering basic electrical theory. =/



I'm actually testing this exact setup now. Autoformers are after the preamp. Here's some more information:

ecp.cc
 
Nov 10, 2008 at 2:55 PM Post #15 of 18

Fryguy8

100+ Head-Fier
Joined
Jan 18, 2005
Posts
148
Likes
10
I got it the first time
smily_headphones1.gif


Thanks for the pictures though.

Realistically I'm just looking at ways to trim on the parts count (especially given that the pot is one of the most expensive parts of this), so I can keep the build of the entire thing down. Would like to come in under $1500 (ideally closer to $1000) for a 4-board setup, including the cost of contracting a builder. That might be tough with the cost of some pots around.

So, just to clarify, as long as I have an active pre-amp (or a buffered output stage, like a variable output from a DAC), I CAN do this?
 

Users who are viewing this thread

Top