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Gain Matching


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After reading through the Manley Euro thread I found this link by russ69 intriguing:

http://www.upscaleaudio.com/a-discussion-about-idle-hiss/

By way of background, I bought a used Audio Horizons TP2.0 tube preamp more than a year ago but haven't been able to run it due to noise. Standard CD player source (tried several) or any other line level source into the TP2.0 into little T-amp, VTL TT monoblocks, or Welborne Laurel 300b monos into KHorns all produced an intolerable steady state HUMMM. In addition the volume control range of the TP2.0 was hard to use manualIy and almost unuseable with the remote. The tiniest press of the button produced big changes in volume.

I read up about hum here on the forums and tried different grounding changes, cheater plugs etc. No change. Finally I sent the TP2.0 off to its designer Joseph Chow. As you may know this line of tube preamps is touted for its low noise high S/N so Mr Chow took the hum quite seriously. When the unit returned it had NO hum but instead a steady state (at idle, not affected by volume control) HISSS. When I discuseed this with Mr Chow he referred me to sources of low noise replacement tubes.

Discouraged, I didn't want to shop for tubes. I set aside the TP2.0 and went back to my Parasound P/HP 850 solid state preamp which is very quiet with all my amps and the super sensitive KHorns. Note that the Parasound however could never be set higher in volume than about 9 oclock and very small movements of the continuously variable volume knob ALSO created large changes in volume just like the tube pre - what's going on here...?

This past week I found the above link and read the article. I shopped for inline attenuators but realized I didn't have any idea how much attenuation I would need or even if this would work. Then I got the idea to put my quiet Parasound SS preamp inline after the TP2.0 tube preamp and in that way have a volume control that could reduce the TP2.0's gain because after these readings it seemed I had too much and I was hearing the steady state tube HISS of the preamp.

This past weekend I inserted the Parasound pre after the TP2.0. I am happy to report I am now a believer in the importance of gain matching. With the SS pre volume control under 9 o'clock not only was there no HISS at the listening position but the TP2.0 tube pre now had a very useable volume control range in the 10 -2 o'clock range!

So my experiment works. I proved to my satisfaction the importance of gain matching. Here is where I need forum member help. How do I determine the correct attenuation to buy for inline devices since of course I don't want to leave 2 preamps inline?

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Well you start with the source (disc player, turntable, tape deck
etc.) and test material. The test material will have a particular track with a tone or noise
that drives the device's output amps to maximum playable level. If you
can't procure test material for your source, a satisfactory work-around
is to find the loudest, most dynamically compressed recording you have
in your collection.

Then you use the meter to read the AC voltage at device's output jacks while it's being fed that signal.

That
gives you an idea how much voltage gain is actually required downstream to get
your system to satisfactory levels. It also gives you a number to associate with your system.

You can use a voltage ratio calculator from that point to get an idea of what's actually taking place using decibels.

See here: http://www.net-comber.com/decibel.htm (the part on the middle of the page)

For Example:

You're contemplating buying a -10 dB attenuator

You measure the voltage at you preamp outputs at 1.876V

You enter that into the "input" side of the calculator, then enter an arbitrary number in the "output" side and click "calc"

You keep fudging the value around on the "output" side and clicking "calc" until the resulting "decibels" reads -10, which would be 0.593 in this case.

Two things to take note of are:

1. The inline attenuator is just a couple of resistors in a fancy package. Keep that in mind when you're looking at prices for these things.

2. Your pre-amp output may actually be enough to drive your K-horns to satisfactory listening levels. This is not unheard of. I can power my La Scalas off the headphone output of my walkman. A single volt makes plenty of sound.

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Very helpful Quiet_Hollow thanks!

I'm thinking with the current 2 preamp setup I could run the 0db recorded tone and measure the voltage from the TT2.0 tube preamp output with the volume turned up in the 8-9 o'clock range (as high as I can ever go when it's the only preamp). Then I can measure the output voltage on the SS pre again with tone playing where the SS pre volume is about 9 o'clock and the tube pre about 12 o'clock (dialed in gain matching). If I plug these 2 measurements into your calculator it should give me the amount of attenuation I'll need.

Sound right?

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2. Your pre-amp output may actually be enough to drive your K-horns to
satisfactory listening levels. This is not unheard of. I can power my La
Scalas off the headphone output of my walkman. A single volt makes plenty of sound.

Interesting idea. I may try that but would wonder what would happen to a preamp which is meant to drive an amp input when you instead put a loudspeaker load on it

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I'm thinking with the current 2 preamp setup I could run the 0db recorded tone and measure the voltage from the TT2.0 tube preamp output with the volume turned up in the 8-9 o'clock range (as high as I can ever go when it's the only preamp).

Sounds good, but I'd also respect the Rothwell website's -10 dB attenuator recommendation as they deal with the sales numbers all the time. I'd be inclined to find one for less than $80 though. I see Parts Express has them for ~$20, for example.

Interesting idea. I may try that but would wonder what would happen to
a preamp which is meant to drive an amp input when you instead put a
loudspeaker load on it

Given the build quality of the TP2.0, it'll work, but without a matching transformer, it's not a load you'd want to expose the pre-amp to for any length of time beyond a low-level experiment to satisfy curiosity.

It is a real eye opener to see how the significance of high-sensitivity translates in real terms.

Here's another example:

The TP2.0 has a 600 ohm line-level output impedance. Average maximum line level output is ~1V.

When we take those numbers and place them into an Ohm's law calculator we find that the pre-amp is good for ~0.002 watt

Now take 0.002 watt back over to the power side of the Decibel Calculator (from above) and put 0.002 Watt on the "output" side and enter 0.03 watt on the "input" side. Click "calc" and it reads -11.7 dB

This is where it gets interesting...[:P]

Taking a look at Don Keele's power/SPL charts from the Dope from Hope Vol.16 #1 (1977) and subtracting -11.7 dB from the "Medium Level" yields a respectable 73 dB SPL average from your pre-amp alone. [:|]

As is the case for most of the big Heritage speakers, with any source other than the turn table (which requires the phono stage), the pre-amp is more or less a BIG throttle for the power amp...so much so, that depending on your average listening levels, the power amp could arguably be considered an accessory than a necessity.[:-*]

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This is where it gets interesting...Stick out tongue

Taking a look at Don Keele's power/SPL charts from the Dope from Hope Vol.16 #1 (1977) and subtracting -11.7 dB from the "Medium Level" yields a respectable 73 dB SPL average from your pre-amp alone. Indifferent

...and to bring this SPL number a little closer to home... Here's a video of a pair of Heresy II being driven plum-happy direct off the L&R line level "audio out" from the back of a television set, using roughly the same amount of power as discussed above, if not less.

K-horns would be even louder than that.

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