Post by thread:[EE] Stable Audio Amplification

Part of the lab equipment where I work is a tone generation system which needs to make a 1 kHz tone upon demand, at a predetermined volume (usually 85 dB). Typically, we use an off-the-shelf frequency generator and a home stereo amplifier. We're starting to notice that the 85 dB we calibrate with a soundmeter isn't staying at 85 dB. We'll come back later and measure it at 90 or 95 or more. My theory is that when we turn it on and calibrate it, the amp is cold and behaves as such. After it's been on awhile, and especially after it's been in use, it warms up and the actual transistors used to do the amplification start to allow a larger current into the load, as a warm transistor will do (right? That's why parallel BJTs are bad: one heats up, conducts more, heats up more, conducts more, etc., until one has 95% of the load and POOF! Do I have my facts straight?) So, here's the Q: Should we just warm up the amp for a bit, then calibrate it, or should we be looking for amps with a different output stage, or should I be thinking about just building an amplifier? The problems I see are that it may not stay warm enough during use for that calibration to necessarily stay valid. A tone is generated at a random interval of between 15 and 25 seconds, and lasts for 450 ms. That's a lot of cool down time, but not enough for it to be the same as the amp being newly turned on. Building an amp has its appeal; I've never done it, but this doesn't exactly have to be Dolby Digital or THX or anything, and it could allow me to add in a few features. OTOH, it's a lot of work that might not be worthwhile, since I probably won't be around to service a custom device much longer. Mike H.

On Mon, 10 Jan 2005 13:08:10 -0600, Mike Hord <spam_OUTmike.hordTakeThisOuTgmail.com> wrote: > Part of the lab equipment where I work is a tone generation > system which needs to make a 1 kHz tone upon demand, > at a predetermined volume (usually 85 dB). > > Typically, we use an off-the-shelf frequency generator and > a home stereo amplifier. We're starting to notice that the > 85 dB we calibrate with a soundmeter isn't staying at 85 dB. > We'll come back later and measure it at 90 or 95 or more. > > My theory is that when we turn it on and calibrate it, the > amp is cold and behaves as such. After it's been on awhile, > and especially after it's been in use, it warms up and the > actual transistors used to do the amplification start to > allow a larger current into the load, as a warm transistor > will do (right? That's why parallel BJTs are bad: one heats > up, conducts more, heats up more, conducts more, etc., > until one has 95% of the load and POOF! Do I have my > facts straight?) Yes, but that doesn't really count: Your amp surely uses negative feedback, so it's the ratio of two resistors, rather than individual transistor parameters that determine the gain. You can easily measure if the amp is at fault by measuring the AC voltage at the amplifier's output. 85->95 dB is a ten-times change in output power, or a 3X change in output voltage. There's no way your amp is changing in gain by 10dB. And your signal source shouldn't either, if it's at all good (test by watching the voltage at the input of the amplifier) My theory is something is changing acoustically in your setup. Are you moving things around in the room? Maybe adding some reflective surfaces near the transducers? Building amplifiers is a fun hobby, but you probably can't justify it when good ones are so available off-the-shelf. Regards, Mark markrages@gmail -- "We act as though comfort and luxury were the chief requirements of life, when all that we need to make us happy is something to be enthusiastic about." - Einstein

--- Mike Hord <.....mike.hordKILLspam@spam@gmail.com> wrote: > Part of the lab equipment where I work is a tone generation > system which needs to make a 1 kHz tone upon demand, > at a predetermined volume (usually 85 dB). > Ok interesting audio equipement testing perhaps? {Quote hidden}> Typically, we use an off-the-shelf frequency generator and > a home stereo amplifier. We're starting to notice that the > 85 dB we calibrate with a soundmeter isn't staying at 85 dB. > We'll come back later and measure it at 90 or 95 or more. > > My theory is that when we turn it on and calibrate it, the > amp is cold and behaves as such. After it's been on awhile, > and especially after it's been in use, it warms up and the > actual transistors used to do the amplification start to > allow a larger current into the load, as a warm transistor > will do (right? That's why parallel BJTs are bad: one heats > up, conducts more, heats up more, conducts more, etc., > until one has 95% of the load and POOF! Do I have my > facts straight?) > > So, here's the Q: Should we just warm up the amp for a > bit, then calibrate it, or should we be looking for amps > with a different output stage, or should I be thinking about > just building an amplifier? > > The problems I see are that it may not stay warm enough > during use for that calibration to necessarily stay valid. > A tone is generated at a random interval of between 15 and > 25 seconds, and lasts for 450 ms. That's a lot of cool > down time, but not enough for it to be the same as the > amp being newly turned on. > > Building an amp has its appeal; I've never done it, but > this doesn't exactly have to be Dolby Digital or THX or > anything, and it could allow me to add in a few features. > OTOH, it's a lot of work that might not be worthwhile, > since I probably won't be around to service a custom > device much longer. > Home stereo amps have some features you probably don't want. You might want to buy a professional sound reinforcement grade amplifyer by someone like Crown. Crown's D series, the D75A for example is an excelent amp and not rated at enough wattage to kill you if you stand next to the transducer. They are also a bit less expensive than there other stuff. Alesis RA150 or RA300 might be just fine as well. I suggest going to your local professional sound store and ask questions explain what you need and get a suggestion from them. Nice thing is it's all rack mountable and will be nice and neat in your lab. As for building your own amplifyer, well if the amps are FET based the output shouldn't change at all. It really depends on the quality of the amplifyer you have. This is why I suggested some professional gear like the above. You can read pertinant information reguarding them and get peoples experience. The most important thing is often a store can loan you a unit to see if it does what you WANT, before you invest your money into it. I would say that is worth the time and effort. Stephen __________________________________ Do you Yahoo!? Yahoo! Mail - You care about security. So do we. http://promotions.yahoo.com/new_mail

On Mon, 10 Jan 2005 12:54:02 -0800 (PST), Stephen R Phillips <cyberman_phillipsKILLspamyahoo.com> wrote: > > > > Home stereo amps have some features you probably don't want. You might > want to buy a professional sound reinforcement grade amplifyer by > someone like Crown. Crown's D series, the D75A for example is an > excelent amp and not rated at enough wattage to kill you if you stand > next to the transducer. They are also a bit less expensive than there > other stuff. Alesis RA150 or RA300 might be just fine as well. I > suggest going to your local professional sound store and ask questions > explain what you need and get a suggestion from them. Nice thing is > it's all rack mountable and will be nice and neat in your lab. On the contrary, a home stereo amp is more than adequate to amplify a 1kHz tone to 85dB. With a moderate-effieciency loudspeaker, this is about one watt of output to aschieve 85 dB SPL at 1m. So forget about the gold-plated Crown idea. Besides, a Mark Levinson #33 power amplifier would be even nicer and neater in the lab, and can be used as a welder as well. > As for building your own amplifyer, well if the amps are FET based the > output shouldn't change at all. It really depends on the quality of the > amplifyer you have. This is why I suggested some professional gear like > the above. You can read pertinant information reguarding them and get > peoples experience. The most important thing is often a store can loan > you a unit to see if it does what you WANT, before you invest your > money into it. I would say that is worth the time and effort. If the gain of my op-amp circuit was varying by 10dB, would you tell me it is because I didn't use a JFET op amp? In any modern amp*, the gain is set by a feedback voltage divider. [*] I am aware of the no-overall-feedback triode amplifiers the extreme hifi folks use. Regards, Mark markrages@gmail -- "We act as though comfort and luxury were the chief requirements of life, when all that we need to make us happy is something to be enthusiastic about." - Einstein

> > My theory is that when we turn it on and calibrate it, the > > amp is cold and behaves as such. After it's been on awhile, > > and especially after it's been in use, it warms up and the > > actual transistors used to do the amplification start to > > allow a larger current into the load, as a warm transistor > > will do (right? That's why parallel BJTs are bad: one heats > > up, conducts more, heats up more, conducts more, etc., > > until one has 95% of the load and POOF! Do I have my > > facts straight?) > > Yes, but that doesn't really count: Your amp surely uses negative > feedback, so it's the ratio of two resistors, rather than individual > transistor parameters that determine the gain. One would hope. But then, one would hope that repeatedly returning a volume knob to the same spot on an amplifier of reasonable quality would produce the same results, as well. > You can easily measure if the amp is at fault by measuring the AC > voltage at the amplifier's output. 85->95 dB is a ten-times change in > output power, or a 3X change in output voltage. There's no way your > amp is changing in gain by 10dB. And your signal source shouldn't > either, if it's at all good (test by watching the voltage at the input > of the amplifier) We've watched the signal source, and frankly, I doubt an Agilent signal generator is going to have much drift in frequency/amplitude. I'm aware of the scale of the jump here. One or two dB I could buy into, and accept as imprecision in the position of the volume, but 8-10 is unbelievable. > My theory is something is changing acoustically in your setup. Are you > moving things around in the room? Maybe adding some reflective > surfaces near the transducers? Impossible. The "room" is a 1 meter (approximately) cubed box, built to be (within reason) acoustically isolated. Things aren't going in and out of it. The calibration is performed under reasonably repeated circumstances (meter similarly positioned, doors closed, etc.). Besides, the difference in amplitude is easily perceived outside the box, and if it were a measurement difference, caused by a "hot spot" inside the box where the echo was *just right*, the tone would be the same to an observer outside the box even as the measurement changed with moving the meter. I still haven't ruled out observer/user error, since of five similar setups, only one is really having this problem. However, a bad amp could cause that as well. As to buying some high-end professional amps, that's not completely out of the question. It would be a tough sell, though, and some of the features of a home stereo make it very well suited to this indeed (for example, balance control and multiple output channels [front/rear]). Mike H.

> > > You can easily measure if the amp is at fault by measuring the AC > > voltage at the amplifier's output. 85->95 dB is a ten-times change in > > output power, or a 3X change in output voltage. There's no way your > > amp is changing in gain by 10dB. And your signal source shouldn't > > either, if it's at all good (test by watching the voltage at the input > > of the amplifier) > > We've watched the signal source, and frankly, I doubt an Agilent > signal generator is going to have much drift in frequency/amplitude. If Agilent is still using Bill Hewlett's light bulb it shouldn't! ( if you don't know the story, see http://www.hp.com/hpinfo/abouthp/histnfacts/museum/earlyinstruments/0002/index.html) Have you measured the output of the amplifier when it is misbehaving? I suggest using an AC voltmeter on the amplifier output to set the level, rather than relying on a volume pot. Modern Best-Buy-quality hifi tends to have an S-taper "goes to 11" volume control that makes it hard to adjust at low levels. Now my theory is something mechanical in your loudspeaker is intermittently rubbing. Regards, Mark markrages@gmail -- "We act as though comfort and luxury were the chief requirements of life, when all that we need to make us happy is something to be enthusiastic about." - Einstein

Mike Hord wrote: > Part of the lab equipment where I work is a tone generation > system which needs to make a 1 kHz tone upon demand, > at a predetermined volume (usually 85 dB). > > Typically, we use an off-the-shelf frequency generator and > a home stereo amplifier. We're starting to notice that the > 85 dB we calibrate with a soundmeter isn't staying at 85 dB. > We'll come back later and measure it at 90 or 95 or more. Notwithstanding all the other comments -- if you still want or need to build something, maybe it's not an amp but a feedback loop consisting of a microphone and an electronic volume control? In a defined setup, you can probably calibrate that rather easily to a few fixed sound levels. Gerhard

>-----Original Message----- >From: .....piclist-bouncesKILLspam.....mit.edu [EraseMEpiclist-bouncesspam_OUTTakeThisOuTmit.edu] >Sent: 10 January 2005 22:37 >To: Microcontroller discussion list - Public. >Subject: Re: [EE] Stable Audio Amplification > > >We've watched the signal source, and frankly, I doubt an >Agilent signal generator is going to have much drift in >frequency/amplitude. I'm aware of the scale of the jump here. >One or two dB I could buy into, and accept as imprecision in >the position of the volume, but 8-10 is unbelievable. > Don't bet on it. IMO the signal source is a likely cause of the output level changes, I've seen the exact same problems with some quite expensive kit that had to be left switched on permanently to avoid the drift during warm up period. The gain of the amp should not be be varying significantly unless it is faulty. As said, the gain is set by negative feedback, and unless the compoenents responsible for this are faulty, the gain simply should not change. For this reason, the effect of temperature on BJT output stages that you mentioned should not affect gain to any great degree, it simply increases the quiescent current in the the output stage. Any halfway decent BJT based amplifier will have some form of compensation for this anyway. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================

>From Howard: > Can you swap in another home hi-fi amp, just to confirm > that the problem is in the amp itself? We're actually back to the first amp we used, since the swapped-in unit went belly-up. > I used to have an amp that would occasionally go very > quiet - still audible and still a good sound, but very > much quieter. It would happen randomly, but usually > some time after switching it on, and sometimes thumping > the case would correct it - I suspect it had a dry joint > somewhere. This is good news, as a positive indicator it could happen. and from Mark: > Have you measured the output of the amplifier when it is misbehaving? > I suggest using an AC voltmeter on the amplifier output to set the > level, rather than relying on a volume pot. Modern Best-Buy-quality > hifi tends to have an S-taper "goes to 11" volume control that makes > it hard to adjust at low levels. I'm a little confused...I can see using the feedback from the voltmeter, but how would I use that instead of a pot to set the level? Are you suggesting I create a feedback loop, monitoring the output level and adjusting accordingly? > Now my theory is something mechanical in your loudspeaker is > intermittently rubbing. Not a bad theory, but we've observed this behavior with two different speakers now. That was my first thought, as well. Gerhard suggested: > Notwithstanding all the other comments -- if you still want or need to > build something, maybe it's not an amp but a feedback loop consisting of a > microphone and an electronic volume control? In a defined setup, you can > probably calibrate that rather easily to a few fixed sound levels. That would maybe not be too bad an idea. I've toyed with the idea of a microphone, a small amplifier and a PIC to monitor levels, just to be sure it doesn't deviate to far from the accepted value, but never had a real need (until now, I suppose). No real need for the feedback to adjust volume (that would be too complex unless it had the amplifier built into it), but a user alert for over- or undervolume wouldn't be out of line. Mike H.

Obtain or build a class A amp (from a kit) and leave it on all the time (without signal). Alternatively, obtain the schematics for your amplifier, determine the feedback circuit (one of them) and see where a thermistor could be applied to fix your problem. Chaning gain to that extent (10db) over a short time indicates a failing amplifier, maybe dried electrolytic capacitors. If the amp is older than about 10 years from manufacture then this must always be suspected. The fault can be duplicated using a heat gun. You can measure output in db using a normal d'Arsonval meter on db scale with a dummy load. ime an amp in good condition will not change gain by more than about 2db from storage-cold to stage-boiling. Peter