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Attenuator between pre- and power amplifier?, Moved from General Audio
AudioKitten
post Apr 11 2012, 06:19
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My receiver sounds beautiful but has far too much gain for my relatively efficient speakers. This causes me to listen to all of my music within the first 10-15% of the volume pot (20% if I'm blasting it). Because my potentiometer has rather apparent channel balance problems below 5% that don't fully go away until 10% or so I'm considering some way of lowering the amplifier's gain.

My question is this: Is there any real pitfalls to adding an attenuator between the preamp out and poweramp input in my receiver? I'm thinking anywhere -15dB up to -20dB fixed. My other options are to perform surgery and maybe replace a resistor or two or possibly replace the pot for one that has better tracking. Replacing the pot was my first plan but the replacement pots for my amplifier cost 70 bucks a piece and there's no guarantee the replacement will track appreciably better. My understanding is that at <5% pretty much all pots have tracking problems.

So... should I go ahead and add a fixed level attenuator between my pre- and power- stages or should I refrain and what are the problems that I should be looking out for?

Thanks!
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woody_woodward
post Apr 11 2012, 08:14
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Friend, if you can solve your problem with a couple of resistors don't spend the $70.

Woody
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Glenn Gundlach
post Apr 11 2012, 08:35
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Well an attenuator is certainly an inexpensive experiment. Personally I'd go for about half the dBs you're talking. Also, keep the resistor values on the low side. A 10 dB unbalanced pad consisting of series 6810 ohms with 3160 to ground will be very close to 10 dB attenuation at a little less than 10 K load. Lower values will help keep the noise at a minimum. Solid state preamps should have no trouble with 10K but some tube gear might not like it that low. Simply scale the resistor values. The 1% values I gave you should have very good channel balance.

G

This post has been edited by db1989: Apr 11 2012, 20:53
Reason for edit: removing unnecessary full quote of first post
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AudioKitten
post Apr 11 2012, 09:46
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I don't quite understand. I was under the impression that adding the attenuators increased the source impidence and not the load impidence. Does it depend on the attenuator topology? I was considering a pi network.

Anyway, to throw some numbers out there. I pulled the gangs out of the back of my receiver and sent a 440Hz test tone through it and then proceeded to jam the probes of my multimeter into the pre-out jacks. At full blast the meter reads 4.7V, at three-oh-clock it shows 3.3V, at noon it shows 1.0V, at nine-oh-clock it shows 0.4V and at halway between that and zero it shows 0.3V. Right now I'm operating it somewhere between null and "nine-oh-clock" but I'd really like to be operating it at MINIMUM nine-oh-clock because halfway below that starts to get noticable channel skew.

So, you're right about 15-20dB being too high. How about -12dB? That's roughly a 4:1 ratio. It would bring the dial at 50% down to what used to be the mid of my listening range with a hot source while still giving me the headroom for turn even the most feeble sources up to respectable volumes?

PS: My receiver is fully solid state. Also, I'm going to assume those numbers are SMD resistor codes because they don't make sense any other way.

This post has been edited by db1989: Apr 11 2012, 17:33
Reason for edit: removing unnecessary full quote of above post
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AudioKitten
post Apr 11 2012, 09:56
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QUOTE (woody_woodward @ Apr 11 2012, 00:14) *
Friend, if you can solve your problem with a couple of resistors don't spend the $70.

Woody

Well... to be fair the schematic I was reading was a rather poor quality reproduction. I can't really tell if the resistors I was looking at were for gain or if they were part of the motor control mechanism. Stupid motor making parts expensive mad.gif . I think the most desirable solution would be to replace the pot entirely in favor of a stepped attenuator but the volume pot is mounted on a circuit board instead of case mounted which makes those kinds of mods a *lot* harder.

This post has been edited by db1989: Apr 11 2012, 20:55
Reason for edit: Please quote above, not below, your reply.
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soulsearchingsun
post Apr 11 2012, 13:35
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QUOTE (AudioKitten @ Apr 11 2012, 09:46) *
I don't quite understand. I was under the impression that adding the attenuators increased the source impidence and not the load impidence. Does it depend on the attenuator topology? I was considering a pi network.

Line-level connections are not matched in terms of source and load impedance. So you won't need a pi network, as differences in load impedance don't matter for the source. Whether you say you change source or load impedance is just a matter of perspective IMHO.
Usually you attenuate by creating a voltage divider that has a resistance high enough to not short the output and low enough to be independent from different loads.

CODE
INPUT --- R1 --- OUTPUT --- R2 --- GND

QUOTE (AudioKitten @ Apr 11 2012, 09:46) *
I'm going to assume those numbers are SMD resistor codes because they don't make sense any other way.

Those are resistance values. In OHM, as stated.
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stephan_g
post Apr 11 2012, 17:21
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QUOTE (woody_woodward @ Apr 11 2012, 08:14) *
Friend, if you can solve your problem with a couple of resistors don't spend the $70.

Seconded.

Besides, you may be amazed by how quiet the background noise floor can really get! (Many amplifiers have less than ideal gain structure from a noise perspective, as I discussed here. In ones with pre-out/main-in connections, you usually get around 16 dB + 29 dB for pre and power amp, respectively, adding up to a total ~45 dB. Noise levels are usually acceptable to OK on average-sensitivity speakers, but more sensitive ones tend to be more demanding.)

For starters, I'd suggest something like 10k in series and 3k3 in parallel for -12 dB (3k3 goes to power amp input side). Ideally one would consult the schematic to determine whether lower-impedance loads are feasible, and if so, scale down resistor values accordingly (until output noise is no longer dominated by power amp source impedance noise - which may already be the case with the values given, depending on preceding circuitry).

Incidentally, T-type attenuators are virtually never used in audio. Unlike in RF which uses power matching, you don't want to present the same impedance on both sides, but rather a high input impedance and low output impedance (short/open system). Hence the good ol' L-type.

This post has been edited by stephan_g: Apr 11 2012, 17:42
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DVDdoug
post Apr 11 2012, 19:24
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Parts Express has these attenuators in 1, 3, and 6dB for about $5 USD each., or these in 3, 6, and 12dB, sold in pairs for $26 USD.

I have some of the cheaper ones, and one bad thing is that they are not marked with the attenuation.
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pdq
post Apr 11 2012, 19:35
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If you don't mind minor modification to your amplifier, there is a very simple change that you can make. Add a resistor from the wiper connection of the volume control pot (the center connection) to the ground side. Make the value of this resistor something like one fifth the resistance of the pot itself. Do the same on both channels. This makes the volume control non-linear, in that most of the volume change is in the upper part of its rotation, and you get much finer control in the lower end.

The only down side is that the input impedance is now volume-control dependent, and it places a significantly greater load on the pre-amp at high volume settings.
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AudioKitten
post Apr 11 2012, 19:49
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Volume control dependent input impedance seems like quite the downside. Also, the pot isn't a six-pin model. It's a strange 10-pin model and the service manual doesn't identify the pins. I'd have to reverse engineer what each pin does from the schematics. Nah, much easier to add an attenuator.

Thanks, though!

This post has been edited by db1989: Apr 11 2012, 20:55
Reason for edit: as in posts #4 and #5 :P
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pdq
post Apr 11 2012, 20:00
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That sounds like the pots have one or more taps, which is commonly used to achieve non-linear gain with rotation. Given your description of the imbalance at low volumes, I suspect there is either a bad component or a bad connection related to this function. Without a schematic I am only guessing, but if you can find and fix this problem then you probably won't need the attenuator.

By any chance does your amplifier have what is known as a "loudness switch"?

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AudioKitten
post Apr 11 2012, 20:08
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QUOTE (DVDdoug @ Apr 11 2012, 11:24) *
Parts Express has these attenuators in 1, 3, and 6dB for about $5 USD each., or these in 3, 6, and 12dB, sold in pairs for $26 USD.

I have some of the cheaper ones, and one bad thing is that they are not marked with the attenuation.


Some quotes from the amazon reviews of those Parts Express attenuators:

QUOTE
These are of poor design. The ground connection is not carried directly through and that can cause hum and static if the two pieces of equipment are not well grounded to each other by other means. I have done audio engineering for many years. I bought 4 of these and then put them in the trash after testing. The internal attenuators are of "balanced" design but are installed in these unbalanced units, this can cause the mentioned problems.


QUOTE
I should have listened to the one other review -- I bought two of these and one was dead on arrival. Take your business elsewhere, this part is worthless.


I don't like the Harrison Labs attenuators because they're too long and because Harrison Labs links to some pretty extreme right-wing religious literature which, as a queer person, makes me very much not want to do business with them. I was, however, looking at these attenuating cables that I could have custom built for the same price as the Harrison plugs.

I'm pretty sure they can't beat what I can do with some resistors and electrical tape for a third of the cost but I'm considering them just for the fit-and-finish. They are pi network attenuators though, which according to other people in this thread aren't generally used in audio circuits and aren't necessary. I think this builder uses them just so that they don't care which way they're plugged in, which is nice.

My question: Is there any reason *not* to use a pi network attenuator for this purpose?
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AudioKitten
post Apr 11 2012, 20:11
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QUOTE (pdq @ Apr 11 2012, 12:00) *
That sounds like the pots have one or more taps, which is commonly used to achieve non-linear gain with rotation. Given your description of the imbalance at low volumes, I suspect there is either a bad component or a bad connection related to this function. Without a schematic I am only guessing, but if you can find and fix this problem then you probably won't need the attenuator.

By any chance does your amplifier have what is known as a "loudness switch"?


Yes, yes it does... dry.gif
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pdq
post Apr 11 2012, 20:36
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Are the channels balanced at low volume with/without the loudness switch on? If one position keeps them balanced and the other does not then we may be able to determine the problem.
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Glenn Gundlach
post Apr 11 2012, 20:56
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QUOTE (AudioKitten @ Apr 11 2012, 00:46) *
I don't quite understand. I was under the impression that adding the attenuators increased the source impidence and not the load impidence. Does it depend on the attenuator topology? I was considering a pi network.

Anyway, to throw some numbers out there. I pulled the gangs out of the back of my receiver and sent a 440Hz test tone through it and then proceeded to jam the probes of my multimeter into the pre-out jacks. At full blast the meter reads 4.7V, at three-oh-clock it shows 3.3V, at noon it shows 1.0V, at nine-oh-clock it shows 0.4V and at halway between that and zero it shows 0.3V. Right now I'm operating it somewhere between null and "nine-oh-clock" but I'd really like to be operating it at MINIMUM nine-oh-clock because halfway below that starts to get noticable channel skew.

So, you're right about 15-20dB being too high. How about -12dB? That's roughly a 4:1 ratio. It would bring the dial at 50% down to what used to be the mid of my listening range with a hot source while still giving me the headroom for turn even the most feeble sources up to respectable volumes?

PS: My receiver is fully solid state. Also, I'm going to assume those numbers are SMD resistor codes because they don't make sense any other way.


The resistor values I gave you are standard 1% values from the E96 table. I calculated for 10dB but I predict you'll end up with 6-8 dB. (I've had to do this too)

The circuit would be an unbalanced L pad shown here

http://www.uneeda-audio.com/pads/

1% resistors will give a very presentable channel balance that I don't think you'll need to 'tweak'.

G
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AudioKitten
post Apr 11 2012, 21:07
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QUOTE (pdq @ Apr 11 2012, 12:36) *
Are the channels balanced at low volume with/without the loudness switch on? If one position keeps them balanced and the other does not then we may be able to determine the problem.


Nope... I tried turning on "loudness" and the channels are still significantly out of balance at low volumes.
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pdq
post Apr 11 2012, 21:19
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Well, the simplest fix would be if one switch contact is dirty and not making a connection. Start by switching it on and off many times, then see if the problem is fixed. If not then get yourself a can of contact cleaner and spray that into the switch, again working the switch on and off.

If that still doesn't do it then we are looking at either a bad solder connection, a bad switch, a bad component (rare), or an incorrect component value (even rarer).

Something that is often used to look for bad connections or components is to squirt freeze spray in various places to see if that makes a difference.

Edit: By the way, it is the channel that is louder than the other that has the problem.

This post has been edited by pdq: Apr 11 2012, 21:20
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AudioKitten
post Apr 11 2012, 21:28
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QUOTE (pdq @ Apr 11 2012, 13:19) *
Well, the simplest fix would be if one switch contact is dirty and not making a connection. Start by switching it on and off many times, then see if the problem is fixed. If not then get yourself a can of contact cleaner and spray that into the switch, again working the switch on and off.

If that still doesn't do it then we are looking at either a bad solder connection, a bad switch, a bad component (rare), or an incorrect component value (even rarer).

Something that is often used to look for bad connections or components is to squirt freeze spray in various places to see if that makes a difference.

Edit: By the way, it is the channel that is louder than the other that has the problem.


Doesn't it make more sense that the problem is the potentiometer itself? Even six post pots show channel balance problems at very low volumes.
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