|
|
Post by leonski on Jun 20, 2020 18:11:25 GMT -5
Now you are entering 'calibration hell'.
Just for example? My last Reel-To-Reel, a Tandberg ($$$) had small meters. On record, they were Uncalibrated, but playback was (supposed to be) calibrated to some kind of standard.
I think it was NAB...(national association of Broadcasters)
I have NO idea how many calibrations exist.. Maybe 'A' weighted? or 'C' weighted? Please let me know how your investigations go.
To get your VU meter to work, you'll need to have some 'clipping' reference. Most of these kinds of meters on old-school tape recorders, be they Cassette or Reel would have
a ZERO, maybe 2.5 on your meter, and ABOVE that 'in red' would be up to +10 which indicated.....HEY! You're Distorting!
YOUR trick will be relating the above meter to either CLIPPING or Power Output.
If you can get a schematic for YOUR amplifier? Maybe find a tech who could hazard a guess as to WHERE in the circuit a BIAS meter could be inserted?
I do NOT know of a specific article, but you MIGHT want to check thru the listings on Elliot Sound Products. Than make a run over to DIY Audio and make a posting......
WHo Knows? Someone else might have you in mind when they did a project and can give you some help!
|
|
ttocs
Global Moderator 
I always have a wonderful time, wherever I am, whomever I'm with. (Elwood P Dowd)
Posts: 8,171
Member is Online
|
Post by ttocs on Jun 20, 2020 18:39:20 GMT -5
Now you are entering 'calibration hell'. Just for example? My last Reel-To-Reel, a Tandberg ($$$) had small meters. On record, they were Uncalibrated, but playback was (supposed to be) calibrated to some kind of standard. I think it was NAB...(national association of Broadcasters) I have NO idea how many calibrations exist.. Maybe 'A' weighted? or 'C' weighted? Please let me know how your investigations go. To get your VU meter to work, you'll need to have some 'clipping' reference. Most of these kinds of meters on old-school tape recorders, be they Cassette or Reel would have a ZERO, maybe 2.5 on your meter, and ABOVE that 'in red' would be up to +10 which indicated.....HEY! You're Distorting! YOUR trick will be relating the above meter to either CLIPPING or Power Output. If you can get a schematic for YOUR amplifier? Maybe find a tech who could hazard a guess as to WHERE in the circuit a BIAS meter could be inserted? I do NOT know of a specific article, but you MIGHT want to check thru the listings on Elliot Sound Products. Than make a run over to DIY Audio and make a posting...... WHo Knows? Someone else might have you in mind when they did a project and can give you some help! My amps are Bob Latino M-125 monos which I built last year. The bias test pins are on the front and I set the bias to 0.9V. The meters I ordered are ready to go as is so should be easy. I always wondered if the bias points could be used for metering. What I don't know yet is what the max voltage is at the bias points at full power, never bothered to find out before but that's easy enough to check with a scope, and yes, I'd be using clipping as max power. I'm only doing this purely for the fun of it, and if I learn a little along the way, then, win win. edit: And that Pass info was a great read! Thanks! |
|
|
|
Post by leonski on Jun 20, 2020 22:23:37 GMT -5
Most equipment road tests will specify something like 1% distoriton as the 'limit'.....power produced over that is not generally considered useable.
What is the idle power of your amps? with <1volt bias, I'm calling it as and A/B amp
If you can monitor your amps bias while operating under load? And NO Reason why not, you might be able to get useful information from when the meter begins to drop.
For that? You might be better served by a volt meter than whatever calibration the meter you pictured is...
|
|
KeithL
Administrator
Posts: 10,276
|
Post by KeithL on Jun 20, 2020 23:07:58 GMT -5
If you have a tube amp, or any other amp without auto-bias adjustment, then bias meters are handy. However, while it may vary slightly under different operating conditions, the bias does not track the output power... So, as a power output meter, a bias meter wouldn't be informative, or much fun to watch... (I do seem to recall some amplifiers with output power meters that could be switched to read bias when you needed to adjust it.)
If you want an output power meter you really need an output power meter. And, if you want it to be useful and not boring, you want some circuitry to make its response logarithmic instead of linear. (You really want a nice big pretty analog meter... with a nice microprocessor to actually run it...) And, of course, there are some very cool digital meters that look more or less analog....
(Personally I'm somewhat partial to old-style LED bargraphs - the kind with separate LEDs.)
Now you are entering 'calibration hell'. Just for example? My last Reel-To-Reel, a Tandberg ($$$) had small meters. On record, they were Uncalibrated, but playback was (supposed to be) calibrated to some kind of standard. I think it was NAB...(national association of Broadcasters) I have NO idea how many calibrations exist.. Maybe 'A' weighted? or 'C' weighted? Please let me know how your investigations go. To get your VU meter to work, you'll need to have some 'clipping' reference. Most of these kinds of meters on old-school tape recorders, be they Cassette or Reel would have a ZERO, maybe 2.5 on your meter, and ABOVE that 'in red' would be up to +10 which indicated.....HEY! You're Distorting! YOUR trick will be relating the above meter to either CLIPPING or Power Output. If you can get a schematic for YOUR amplifier? Maybe find a tech who could hazard a guess as to WHERE in the circuit a BIAS meter could be inserted? I do NOT know of a specific article, but you MIGHT want to check thru the listings on Elliot Sound Products. Than make a run over to DIY Audio and make a posting...... WHo Knows? Someone else might have you in mind when they did a project and can give you some help! My amps are Bob Latino M-125 monos which I built last year. The bias test pins are on the front and I set the bias to 0.9V. The meters I ordered are ready to go as is so should be easy. I always wondered if the bias points could be used for metering. What I don't know yet is what the max voltage is at the bias points at full power, never bothered to find out before but that's easy enough to check with a scope, and yes, I'd be using clipping as max power. I'm only doing this purely for the fun of it, and if I learn a little along the way, then, win win. edit: And that Pass info was a great read! Thanks! |
|
|
|
Post by AudioHTIT on Jun 21, 2020 12:29:34 GMT -5
If you have a tube amp, or any other amp without auto-bias adjustment, then bias meters are handy. However, while it may vary slightly under different operating conditions, the bias does not track the output power... So, as a power output meter, a bias meter wouldn't be informative, or much fun to watch... (I do seem to recall some amplifiers with output power meters that could be switched to read bias when you needed to adjust it.)
If you want an output power meter you really need an output power meter. And, if you want it to be useful and not boring, you want some circuitry to make its response logarithmic instead of linear. (You really want a nice big pretty analog meter... with a nice microprocessor to actually run it...) And, of course, there are some very cool digital meters that look more or less analog....
(Personally I'm somewhat partial to old-style LED bargraphs - the kind with separate LEDs.)
My amps are Bob Latino M-125 monos which I built last year. The bias test pins are on the front and I set the bias to 0.9V. The meters I ordered are ready to go as is so should be easy. I always wondered if the bias points could be used for metering. What I don't know yet is what the max voltage is at the bias points at full power, never bothered to find out before but that's easy enough to check with a scope, and yes, I'd be using clipping as max power. I'm only doing this purely for the fun of it, and if I learn a little along the way, then, win win. edit: And that Pass info was a great read! Thanks! So as I remember the history of meters on gear, the metered amplifier examples would include McIntosh and SAE, then most tape decks like TEAC, Tandberg, Revox, etc. The earliest ones had a range of about 23 dB — they went from -20 to +3 — and on amps weren’t much fun to watch because your normal listening was far below -20 of rated power. Starting with the MC2205 (the first Mac amp to conform to the new aesthetic started by the venerable MR78), McIntosh introduced a meter with around 60 dB of range, they were awesome and showed activity most of the time you were listening. Then the LEDs you mention became popular, the first were also limited to 23 dB or so, and again were boring (they probably encouraged people to listen louder ... what?). My first amp with LEDs was the Threshold 4000 which has 23 dB LEDs. Nelson’s next amp, the CAS-1 had 75 WPC and no meters, it didn’t sell well, so it was replaced with the CAS-2 that had a little more power and 43 dB LEDs, once again we could delight in bouncing lights without turning up too loud (I still use that amp in my studio). The question related to this thread would be, how did McIntosh get such wide range meters? Seems early for microprocessors in audio (1974 maybe), were they just very expensive? Edit: I now also remember the 2nd generation Nakamichi cassette decks (like the 1000 Mk II) had a 43 dB VU meter, which was also more useful. |
|
|
|
Post by DavidR on Jun 21, 2020 13:02:15 GMT -5
(Personally I'm somewhat partial to old-style LED bargraphs - the kind with separate LEDs.)
Like this > |
|
ttocs
Global Moderator
I always have a wonderful time, wherever I am, whomever I'm with. (Elwood P Dowd)
Posts: 8,171
Member is Online
|
Post by ttocs on Jun 21, 2020 13:10:30 GMT -5
Most equipment road tests will specify something like 1% distoriton as the 'limit'.....power produced over that is not generally considered useable. What is the idle power of your amps? with <1volt bias, I'm calling it as and A/B amp If you can monitor your amps bias while operating under load? And NO Reason why not, you might be able to get useful information from when the meter begins to drop. For that? You might be better served by a volt meter than whatever calibration the meter you pictured is... Re the distortion testing, I'm not knowledgable in doing that - but I will look that up. Yes, the amps are AB. And yes, it's while under load that I'm interested in seeing on an analog meter. The cheap things I ordered are Voltmeters good for up to 3V which I estimate is good enough for my purpose at the moment. |
|
ttocs
Global Moderator
I always have a wonderful time, wherever I am, whomever I'm with. (Elwood P Dowd)
Posts: 8,171
Member is Online
|
Post by ttocs on Jun 21, 2020 13:14:38 GMT -5
The question related to this thread would be, how did McIntosh get such wide range meters? Seems early for microprocessors in audio (1974 maybe), were they just very expensive? There's a link I posted on the first page of this thread for a circuit that makes metering logarithmic. I think the McIntosh meters make theirs "move" more because they extended the low range of the scale where you'll get more "pop" and pizazz. |
|
|
|
Post by MusicHead on Jun 21, 2020 14:03:32 GMT -5
My first integrated amp was a Technics SU-V5. It had a FL power meter with two switchable scales, so you could make sure to get it moving at any volume level. Gotta have your priorities straight  But it was my 3-head tape deck, the Technics RS-M260, to have the coolest FL meter. It had two colors for below and above 0dB and peak hold. For the teenager me at the time, it was almost hypnotic to see the segments dancing up and down with the music.  |
|
|
|
Post by leonski on Jun 21, 2020 15:16:55 GMT -5
Again? What is the standard? And in the case of the Cassette Deck? Any way to calibrate the meter to the tape? Some tapes, maybe the old TDK SAx formulation was pretty good at 'overload' and might still be OK at +5 on the VU meter. The other issue with meters is that of ballistics. Is your meter a Peak reading type or 'other'? Here is link to Elliot Sound Products article which MAY get a little technical. Just skim thru for the main points which will certainly come thru and be helpful on this subject. sound-au.com/project55.htmSome useful DIY is also presented... |
|
|
|
Post by audiobill on Jun 21, 2020 15:32:14 GMT -5
Why all the agony? If you want nice big responsive meters in patented blue, just buy McIntosh.
|
|
|
|
Post by AudioHTIT on Jun 21, 2020 15:52:35 GMT -5
Why all the agony? If you want nice big responsive meters in patented blue, just buy McIntosh. Exactly, generally people want a meter that looks pretty and dances around when you play music at a normal (read not loud) volume. If +3 was set ‘around’ the point it would clip, that would be useful too, but ‘about’ as much of a standard as you need. So you need a pretty meter with ‘reasonably’ quick response and 40 to 60 dB of dynamic range. |
|
|
|
Post by tme110 on Jun 21, 2020 16:23:48 GMT -5
I don't even remember what generation it was but I did have an Emotiva amp with power meter LCDs on it and I never saw one light up once. I guess my power needs were just much less than what the amp could do.
|
|
ttocs
Global Moderator
I always have a wonderful time, wherever I am, whomever I'm with. (Elwood P Dowd)
Posts: 8,171
Member is Online
|
Post by ttocs on Jun 21, 2020 16:32:47 GMT -5
^^^^^^
Only the indestructible, invincible youth with perfect hearing can tolerate seeing a clipping indicator operate.
|
|
|
|
Post by leonski on Jun 21, 2020 20:16:05 GMT -5
My Carver Cube had a stereo 'ladder' indicator which when completely lit DID have audible distoriton. I suspect a good part of the reason was speaker ALSO reaching redline at the amps power limit.
I tried fixing the internals of the amp once but it was AWFUL and resembled an Electronic Chinese Puzzle Box......
|
|
|
|
Post by AudioHTIT on Jun 22, 2020 0:08:35 GMT -5
Again? What is the standard? And in the case of the Cassette Deck? Any way to calibrate the meter to the tape? Some tapes, maybe the old TDK SAx formulation was pretty good at 'overload' and might still be OK at +5 on the VU meter. ... Nakamichi’s could set both bias and azimuth to the meter, but making the perfect recording is still an art. |
|
|
|
Post by leonski on Jun 22, 2020 13:32:28 GMT -5
Back in maybe '69 or '70, a buddy of mine bought the ORIGINAL Advent / Dolby external noise reduction. This was before this circuitry was included IN a cassette deck.
A real PIA to calibrate and make 'right'.......
There were 2 versions. My buddy bought the least expensive which meant you could not 'monitor' a recording which meant you recorded than played back. Much more time consuming.
My best cassette recordings? Made from early CDs in about '84 or so. On TDK SAx. I think the book with the cassette mentioned that as the 'reference' tape?
|
|
ttocs
Global Moderator
I always have a wonderful time, wherever I am, whomever I'm with. (Elwood P Dowd)
Posts: 8,171
Member is Online
|
Post by ttocs on Jun 22, 2020 14:27:33 GMT -5
leonski, I need your expertise. Just received the little 3V meters. Did a quick hookup and they work quite nicely for permanent bias meters, I can punch a hole in the chassis and install a switch between each pair of tubes. But, here's where I'm lost due to no knowledge. I was "expecting" the meters to move as the power increased, but they don't - much. The bias is set for a little over 0.9V, and at high volume the peaks move the needle up to about 1V. So I hooked up my 25 year old Fluke 87 and put it into record 1ms and sure enough, the peaks were right at 1V, so only a little less than 0.1V movement, let's call it 10% movement. I thought what would happen is that the bias would move as a percentage of the total power output. So the real question is: What does this mean?
|
|
|
|
Post by leonski on Jun 22, 2020 15:07:59 GMT -5
Nelson Pass knows more about this sort of thing than all but a very few. He is a multiple patent holder and, as it turns out, a decent enough writer to be both engaging and legible. www.passlabs.com/technical_article/leaving-class-a/I would recommend reading the above article THAN drift over to Elliot Sound Products for a glance at some of those articles AND some DIY stuff which may be of interest. Simply hooking a meter up? I don't know. The meters characteristics THAN are part of the original circuit and may adversly influence it. That's why the old VOM was not good for some measurments since it was rated in ohms / volt and at low voltages, very low resistance. The VTVM I had was 11 Meg input resistance and would NOT load a circuit down even at fairly low voltages. sound-au.com/class-a.htmi just found a 'cover' article for you to look at. But ESP has a LOT of articles covering everything from PS design to the nuts / bolts of many othe audio related subjects. Bias on your Latino amps is 'fixed' apparently. Only at the highest powers will the PS load down, perhaps, and reduce that value. Some amps have a DYNAMIC bias which rises with signal level. Pass amps seem to be 'fixed' and the meter is mid-scale NORMALLY. His amps will deflect downward at high powers.....indicated you are 'out of class 'A' range'.......as a general rule. |
|
|
|
Post by audiobill on Jun 22, 2020 17:06:14 GMT -5
ttocs, the bias is the dc grid voltage that "biases" the tube into a controlled state of cathode emissions to the plate. The AC musical signal "rides" on this DC voltage, and causes changes in the plate voltage as a result of varying cathode emissions, thereby amplifying the signal.
For your M125s with four output tubes, your bias at 1vdc is correct, and you shouldn't expect to see a lot of variation from your input signal, it hasn't really been amplified at this point.
Bill
|
|
