I Need a Tech

[speakerman1]

Explain why and what Variable gains do and why do I want them?

Larry

 

[Pure_Brew]

I guess it depends on the context of usage? I thought gains were set in push pull amplification to get the waveforms lined up equally on both halves of the sign wave, which I believe is in class A/B. Or, I could be completely wrong. A lot of techs and some good audio engineers hang at diyaudio and are quite responsive, I would ask there and let us know what you find.

Joe

 

[jbeckva]

Variable gain is usually used in compression/expansion schemes. Unless that's your specific purpose, I don't see what else you'd want that for. It's like the "modulation" in AM i.e. amplitude modulation. The transmitter of which would use a variable gain scheme with the audio signal driving it to "vary" the amplitude of the carrier wave.

 

[speakerman1]

TUBES

Larry

 

[mlucitt]

My understanding of Variable Gain Amplifiers (VGAs) is application specific. They are not ideal for audio because they introduce additional noise into the front-end (the most sensitive part) of the amplifier. Let me give you an example of a VGA. When you are standing under a cell tower and your phone is receiving the maximum possible signal, the volume is no louder than when you are five miles from the cell tower and the signal is very weak. This is because the VGA senses the signal strength and boosts the gain. VGAs are used in comms, signal equipment, and sensors, but not so much in audio because our inputs are pretty consistent - CD, DVD, Tuner, Phono, etc.

Does that help?

Mark

 

[laatsch55]

Helps me, now I know.

 

[speakerman1]

Helps me, now I know.

I have to look at the wiring of the gains on the pre. John explained it pretty simple as being able to use it as a way to balance the room with the pre using the speakers thus being able to compensate for the shortcomings of the room. These are VGs but not true VGs in the sense.

I think you will learn more as you build this pre. I think this is the most modded of Roy's kits. So it will be a one and only. I have to buy the tubes today. I can't even see the resistors on the goldpoints. So the others are yours to do as you wish.

Larry

 

[laatsch55]

Really!! HMMMMMM.....................

 

[mlucitt]

When I think of room compensation, I think of a graphic equalizer and there are some very good ones out there that can add or cut at a specific frequency with a 24db slope and make the room very neutral. But you will have to use a calibrated microphone to get the sound right. Even then you may not like it. I have a friend who spent $15K on a mic, equalizer and room treatments to get it "just right". It sounded horrible because it was not what he was used to and the bass and highs were so diminished, the sound rang hollow. Our ears are not calibrated mics, just a bunch of tissue and they do a pretty good job but the current audio systems compensate very well too.

Mark

 

[speakerman1]

What I meant by room compensation was speaker placement. You will never find an EQ in my system. Others can have them I find no need for them. I can't sit right in the sweet spot so by using the gains I can move it by adjusting the speakers through the gains. I can do the same thing with my 700B. That is all I meant.

Larry

 

[mlucitt]

Larry, Got it. Speaker placement is critical. I used to have mine on the floor. Raising them 12" made a huge difference in the bass, no longer booming and able to carry further from the cones. I like to have the tweeters pointed right at my face, they are very directional in the upper frequencies. I can only hear up to about 18kHz, but I have perfect pitch up to then.

Lee and I like to "feel" the music so it is a physical experience. Above a certain sound pressure level the room acoustics start to fade into the background and the only concern about "room compensation" is removing things from the walls and shelves so they don't end up on the floor.

Mark

 

[laatsch55]

What I meant by room compensation was speaker placement. You will never find an EQ in my system. Others can have them I find no need for them. I can't sit right in the sweet spot so by using the gains I can move it by adjusting the speakers through the gains. I can do the same thing with my 700B. That is all I meant.

Larry

In a Lily Tomlin voice--" Well aren't you special."!!!

 

[laatsch55]

Larry, Got it. Speaker placement is critical. I used to have mine on the floor. Raising them 12" made a huge difference in the bass, no longer booming and able to carry further from the cones. I like to have the tweeters pointed right at my face, they are very directional in the upper frequencies. I can only hear up to about 18kHz, but I have perfect pitch up to then.

Lee and I like to "feel" the music so it is a physical experience. Above a certain sound pressure level the room acoustics start to fade into the background and the only concern about "room compensation" is removing things from the walls and shelves so they don't end up on the floor.

Mark

Yep, when the SPL's get above 120 who gives a F%$# about room treatment!!!

 

[speakerman1]

What I meant by room compensation was speaker placement. You will never find an EQ in my system. Others can have them I find no need for them. I can't sit right in the sweet spot so by using the gains I can move it by adjusting the speakers through the gains. I can do the same thing with my 700B. That is all I meant.

Larry

In a Lily Tomlin voice--" Well aren't you special."!!!

Yes the little yellow bus was pretty crowded.

Larry

 

[laatsch55]

Thought I recognized you!!!!

 

[mlucitt]

Lee, I was sitting here with a beer (it is just after Midnight) thinking what you meant by "SPL's get above 120..." Did you mean 120 db? Or did you mean 120 MPH as in how fast your hair is wagging in the breeze? Or is it 120 pascals (Pa)? Of course, you know 20 uPa RMS being the threashold of human hearing, roughly - the sound of a mosquito flying 3 meters away. So, I think 20 Pa RMS is equal to 120 db, but I digress.

I believe a SPL of 194 db is the max for 1 atmosphere here on earth (the largest pressure variation in an undistorted wave). That would be 101,000 Pa RMS, pretty loud. So 120 db is right up there where hearing damage starts at 2kHz, the most sensitive frequency for human hearing. Luckily, we are listening to full spectrum sound so it seems louder but it is not a constant 2kHz at 120 db. They say my M1 Garand rifle will make 168 db at 1 meter, but that does not last as long as Shpongle's Electroplasm.

 

[laatsch55]

Or, the 10 minutes of "Divine Moments of Truth". The Crystal Methgod--Vegas is an ass kickin CD for a good, thumpy bass loaded test. ya might check it nout. My other favorite is "Star Shpongled Banner" , trtack 21 -tales of the Inexpressible.
The Gibsonian 700 Ser 1 has left the building. I believe I am suffering post-partum blues.

 

[nakdoc]

Larry, Back to your original question. You are asking why a tube amp has a chioce of 2 different gains. Two reasons: In a high gain amp, noise from the source is amplified. If you do not need the extra gain, a lower gain improves S/N.
I suspect the real answer though has to do with negative feedback. Typically tube amps use a little negative feedback, primarily to set the gain and keep it stable. There are some how would rather listen to "zero feedback " amps, and quite often the "high gain" position also has zero feedback! There are time dependent distortions that are aggravated by feedback (see slew rate), so a gain switch allows you to play with some of the critical listening parameters.

 

[orange]

Larry, Got it. Speaker placement is critical. I used to have mine on the floor. Raising them 12" made a huge difference in the bass, no longer booming and able to carry further from the cones. I like to have the tweeters pointed right at my face, they are very directional in the upper frequencies. I can only hear up to about 18kHz, but I have perfect pitch up to then.

Lee and I like to "feel" the music so it is a physical experience. Above a certain sound pressure level the room acoustics start to fade into the background and the only concern about "room compensation" is removing things from the walls and shelves so they don't end up on the floor.

Mark

Yep, when the SPL's get above 120 who gives a F%$# about room treatment!!!

Captain I just canna hold her, she's doing the Watusi!