Just pointing out the facts before it goes out of control
Maybe if everyone pays up for the print version, the extra cash would allow a trip?
I paid already this year.
No problem there, I'd pony up for a subscription. could be a win/win.
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Ampeg SVT-7 Pro TEST
- Thread starter f.c.geil
- Start date
No problem there, I'd pony up for a subscription. could be a win/win.
- Apr 3, 2003
- 35,518
- 19,954
- Disclosures
- Editor-in-Chief, Bass Gear Magazine
- Apr 3, 2003
- 35,518
- 19,954
- Disclosures
- Editor-in-Chief, Bass Gear Magazine
Are you plugging into the instrument input? Are you using the -15dB pad? With an input of .5v (RMS), we are clipping the input (unless we engage the -15dB pad).
Also, are you using any form of switching instrument measurement filter?
Tom Lees just got 62v out of the 7pro into a 4-ohm load (with the pad engaged), but the THD was 16%-18%. Wall voltage was 121-122v at the start of the test, and it is dipping no more than 2 volts during his testing.
Using the power amp in, with 122v at the wall, Tom just got 62.5v (into a 4-ohm load) at 21% THD (wall voltage dropped to 120v).
This was my reason for asking Tekdiver if he was measuring distortion out of the preamp, as well as the power amp.
Yes, I was using the -15 pad. I, too, clipped the input without using the pad, although I did so at .6v.
Yes, a National Instrument AES17 hi/low pass filter.
The difference in THD is very interesting. I wonder if it could be differences in tubes. I wouldn't think it would be, but I don't know.
No, I didn't measure THD from the preamp.
I'm feeling the need to reiterate that I am not an engineer, and I am not trying to "duplicate" or "refute" anyone's results. I am merely trying to find out whether or not the SVT-7 Pro can produce the claimed output, and if so, what is needed to get there. I'm just a regular guy, checking real-world results that I think anyone else could duplicate. Can the SVT-7 Pro produce a "ferocious kilowatt of pure Ampeg tone" as Ampeg claims? Yes. Will it do so under normal use? Maybe. I think the variance between my results and BGM's (when looked at together) are somewhat equivocal, and I would like to see someone else do some additional testing on a separate unit. At a minimum, we would then begin to get an "average" maximum output for these units.
A voltage-responsive clipping limiter, which is what I recall is on the 7PRO, would be a very strong suspect here.... Might not have been the very best choice in this product, done in the way I recall.
There were several op-amps and quite a number of 1% resistors involved (IIRC not all parts in there were shown as 1%, adding a possible variable of 5%), plus some diodes, with temp-related voltage drops, and so on.......
With that setup, I would think there was a substantial risk of sample-to-sample variation. I've tried, even used, but never *liked* that sort of limiter.
OK, then it is a CLIPPING limiter..... it would actually PRE-DISTORT the signal if it were set too low, and would, IIRC do it at a point where the user has no control, "inside" the power amp.
A very possible reason for much higher THD on one sample than another. One may simply have been clipping the power amp input. If that was occurring (and I don't KNOW that) then running up the input level to compensate will raise the rms output, at the expense of substantial distortion.
AND that will require a LOT of input, which as I recall from other posts was a symptom that got some mention.
It is a good idea... same reason I have used that system. But it is dependent on a sort of "difference of large numbers" setup, which I would regard as somewhat problematic. I don't think I would rely of it for a precision limit level, not without very careful "margining" of the design to be sure it works without limiting normal output with the full range of tolerances. A "Monte-Carlo analysis" * is good, but what if you are the one who gets the "worst case" amp?
My strong temptation would have been to add some sort of adjustment to allow setting it correctly at board test, so that it would be knon to allow full power under full power voltage conditions.
Or, if possible, I would want to to use a setup that prevents long pulses, and avoids over-current faults that might mute the amp for a time.
Loud has a reference for that if they dig through the old schematics..... although I assume that as is usual, the previous engineers are regarded as dufuses..... in the very best of the "blame the last guy to leave the company for all problems" style. (to be fair, I do NOT know that they think that way.) There were some good folks up there in Woodinville at the time I left, they could figure this out, assuming they are allowed to.
I thought the china development lab shut down after Jonathan Chen left, but maybe not and the problems were sent back over to them. Chinese engineers are good, but not always experienced...... there can be a large difference.
OK, I am "monday-morning-quarterbacking" this design, and I may be waaaayyy off... But it WOULD explain several apparently related behaviors.........
* Yes, a Monte-Carlo analysis is a real thing.... a probability-based predictor of circuit behavior... The one problem with them is that tolerances MAY not be distributed on a real "bell curve".. sometimes parts are supplied ALL at one end of the range, and then suddenly your "low probability" behavior may occur on over half your products.... OUCH!
This is a pretty cool and interesting thread. I would love to see someone do this for a GK amp.
If not true it would be the exception to the rule even if the mess left behind was the result of bean-counter requirements.
Jerrold Tiers is coming with gold here, but I've been drinking too much red wine to follow all his remarks.. Must read it again tomorrow!
Jerrold Tiers is coming with gold here, but I've been drinking too much red wine to follow all his remarks.. Must read it again tomorrow!
I've found that he generally does, and I really appreciate his unbiased attitude. Being that he no longer works for Ampeg, many people would be either a) an Ampeg fanboy; or b) an Ampeg hater. Jerrold does neither, for the most part, but tries to get to the truth. I like that.
Two things I just noticed about the SVT-7 Pro manual that can currently be downloaded from the Ampeg site:
1) The title of the manual is SW0787 - Rev B and it's dated 09/09 .
So the Wattage spec's given in the manual are for Rev B while Bass Gear Magazine and tekdiver500ft tested a revision C.
2) At the end of the manual it says " Ampeg continually develops new products and improves upon existing ones. For this reason, the specifications and information in this manual are subject to change without notice."
I'm glad that the posts in this thread are steering clear of the blame game and I'm hoping there will be a definitive factual explanation to the measurement discrepancies that have occurred. I'd also like to see a response from Ampeg at some point making their position clear. Maybe the manual will be amended for Rev. C
1) The title of the manual is SW0787 - Rev B and it's dated 09/09 .
So the Wattage spec's given in the manual are for Rev B while Bass Gear Magazine and tekdiver500ft tested a revision C.
2) At the end of the manual it says " Ampeg continually develops new products and improves upon existing ones. For this reason, the specifications and information in this manual are subject to change without notice."
I'm glad that the posts in this thread are steering clear of the blame game and I'm hoping there will be a definitive factual explanation to the measurement discrepancies that have occurred. I'd also like to see a response from Ampeg at some point making their position clear. Maybe the manual will be amended for Rev. C
The rev level of the manual and the rev level of the amp do not have to be the same..... it depends if all the amp revs changed performance enough to need a manual change to explain them. In that case, they *might* be at the same level.....
The manual part number and 09/09 date are at the bottom of page 11 of the PDF manual.
If 09/09 is indeed a date code then could the amp specs in the manual actually be for an older model than Rev B?
At any rate, if the manual hasn't been updated since 09/09 then I'm assuming the wattage spec may or may not apply to the current Rev. C models.
If 09/09 is indeed a date code then could the amp specs in the manual actually be for an older model than Rev B?
At any rate, if the manual hasn't been updated since 09/09 then I'm assuming the wattage spec may or may not apply to the current Rev. C models.
- Apr 3, 2003
- 35,518
- 19,954
- Disclosures
- Editor-in-Chief, Bass Gear Magazine
Back when I worked for the Ohio EPA, we used Monte Carlo analysis and ran 10,000 iterations of our risk modeling simulation, pulling from a large number of data sets which were based upon reasonable, realistic presumptions, and based upon the best peer-reviewed scientific data available. It was a real departure, at the time, for determining cleanup goals. In the appropriate circumstances, Monte Carlo analysis can be a very powerful tool.
To be honest, though, I'm not sure how I feel about using it in the context of determining when the limiter in my amp might kick in...
- Apr 3, 2003
- 35,518
- 19,954
- Disclosures
- Editor-in-Chief, Bass Gear Magazine
We did test the Fusion 550 back in issue #3, and Tom Lees is testing the MB800 right now for a review in issue #8.
- Apr 3, 2003
- 35,518
- 19,954
- Disclosures
- Editor-in-Chief, Bass Gear Magazine
Why does everyone spell "fanboy" that way now?! I don't get it! Is it some sort of reference that I'm not in on?
