Posts contrassegnato dai tag ‘review’

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And some tests…

Recently I bought a couple of second hand Yamaha P7000S in order to driver a total of 4 bass units, built on the RCF L15P200AK-II woofer; like i did for other amplifiers in the past I took a look on on the Italian Mercatino Musicale site and found them in very good condition and for a reasonable price, little more than 700€ both. I was looking for a couple of amplifiers over a single unit of more power in order to have better flexibility in the configuration; I can use them all in stereo @8Ohm or bridged @4Ohm on two bass unit each. Moreover, considering that I currently use a CC4000 to drive the 12” mid basses in both 8 and 4 Ohm setup I can also try different configurations and for example use the CC4000 to drive the 4 bass units in 4Ohm stereo and the Yamahas for the 12”.

Also for this model, like my CC4000, I read several different comments both positive and negative on some forums; as usual in the choice I used my head and decided to give it a chance. Looking and the schematics on the net once again I found and amplifier with:

  • a good number of output devices, you can see a total of 12 per channel; better damping factor, less power per device, and so on …
  • a full transistor schema from the input to the output of the power module, while as usual input, filters and other “accessory” units are IC based. On most amplifiers the input stage of the power module is IC based; the P series is just like the Crest CA all transistor based, and with 1 capacitor through the whole signal path
  • an interesting power supply technology (EEEngine) which seems to be promising in terms of total power consumption and dissipated (= wasted); this translates into less heat generated.

Here is a shot of the internals

P7000S Internal

I haven’t had yet the chance to do a listening test and compare it to both the CA6 and the CC4000,but found some time to put it under a small bench; as usual I used my 8 2Ohm 50W resistors to build up a stereo 8Ohm load of 200W, which is capable of handling up to 1000W with a duty cycle of 5 (1s on 5 off). I then connected a small power distributor to the wall plug with 5m of 3×2.5mm2 cable and powered the unit, which has a 2m supply cable, quite small in my opinion; it’s a 3×0.75mm2 unit and maybe could “eat” some watts (read my CC4000 review for details). Maybe I’ll come back later on this.

With the usual 3s on and 15s off @100Hz the P7000S clipped @675W/8Ohm, with the meter reading 73.5V RMS, the clip leds starting to light up and the wave figure like below

100Hz full power

The amplifier well met it’s specifications, which state 650W RMS for the EU version; yes the EU version (230V) seems to loose almost 50W compared to the other, and looking at the schematic this is due to a couple of inductors put in series on the main supply, which role sincerely is not so clear to me, even if on a forum i read that it should act like a PFC circuit, but PFC is something that is a little more complicated than a simple pair of inductors.

Supported by a video I saw on the net of a modded P7000S, and by a check on the schematic which confirmed me that their removal would produce only gains, I decided to disconnect the terminals of the coils and to put in place a jumper built with some cm of cable and a couple of faston; the non EU version have a jumper built on the board but it is the same.

I tested it again and got back around 740W 8Ohm, with the meter reading 77V RMS and the clip leds flashing lightly

100Hz full power no coils

As You can see the wave is still clean and this is very good; it seems that the removal of the coils not only give more power (65W) but also seems to present a better supply line to the switching modules and finally to the amps.

I then repeated the test @50Hz and the results were the same, if not slightly better (few mV more).

50Hz full power no coils

I currently do not have enough resistors to build  a proper 4Ohm load, so I cannot test such a high power unit without the risk of destroying the resistors, but I can test a 4Ohm bridge configuration with some safety margin; for this amplifier the sheet only report the bridge power for 4Ohm loads (2Ohm stereo) for 20ms peaks, and my objective was to have a unit which, when  bridged @4Ohm, just meet the RMS specs of my CA6 (currently 1650W) with some margin; so when used together they will deliver around 3200/3300W, and most important without pushing hard the amps to the limits; currently with the CA6 driving 2 bass units and with 2 satellites there is a very good sound balance, so with the whole 4+4 configuration the same balance will guaranteed, and also with a reasonable little margin to play a little harder when needed.

Given this I tried some tests with the bridged configuration @4Ohm and finally decided to stop at around 2350W RMS, that is 97V RMS; the two coils are still disconnected.

4Ohm bridged

This is more than enough form me, and the good thing is  that no breaker or other form of protection/limiting was kicking in; just remember that the CC4000 was triggering it’s circuit braker at around 2100W when bridged @4Ohm, so that I had to reduce the cycle to 1s on in order to be able to reach higher power levels.

As usual I also do some tests at higher frequencies to check for the absence of wave artifacts due to missconfigured idle current (higher distortion) or power supply residual; this time I was also curious about the behavior of the EEEngine.

This is the 10KHz wave at 2.83V RMS

2.83V @10K

And this is the same 10KHz wave at 80VPP (28.2RMS, 100W/8Ohm).

They are both clean; I then repeated the tests at 15Khz and the figures didn’t change, very clean the same.

I will order in the next days an additional resistors set in order to build a reliable 4Ohm load and will come back with the results under that load; moreover i think I will play around with the power chord.

Update on 08/12/2015

Today I replaced the power chord with a piece pf 3×2.5mmq cable, like You can see in this photo compared to the original.

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The results were almost the same, with the amplifier delivering 77.7V RMS @8Ohm (755W/8Ohm) at the onset of the clipping, confirming that the P7000S ,and also the P5000S I think, has a minimal sort of “regulation” for small undervoltage; the voltage at the board connectors was around 227V while with the original power chord it was around 222/223V.

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Here are some additional readings of this amplifier taken from the power box I use, just to get an idea of its power consumption

This is the at idle with no other equipment connected to the power box: 60VA

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This is at100W RMS @8Ohm both channels, with a total of additional 200VA used by a lamp, scope, pc and mixer; so around 470VA absorbed

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This is at 200W RMS @8Ohm both channels; around 800VA used

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And this is at full power @8Omh both channels: a total of around 1900VA for 1500W on the load

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a small review but at the moment without bench data…

Some months ago I decided to take an amplifier with higher specifications than the “wonderful” CA6, in order to fully kick my bass units, because when used outside, in bridge mode @4Ohm (~1600 Watts), the “small” CA6 appears to be a little undersized, even if it drives 4Ohm bridged without problems. Furthermore I’d like to not “overuse” a power amplifier, so that I carefully look at all the clip leds in order to have them light for few times during the events; I know, and already described, that the limiter circuits do a good job in keeping distortion at low levels when they’re on, but on the other hand there isn’t much more to obtain from an amplifier when it is already telling you that it reached its limit, that’s my point of view… I bought a second hand CC5500 in very good conditions so that it seems just came out of the box.

I read some very negative comments on the various forum .. “it’s crap”, “economical” and so on, but my impression was that many of them were driven by “fanaticism” for other brands, non technical/on the road opinions and assumptions: on the other hand the Crest Pro series is considered to be one of the best sounding line, so why not the CC5500 which has the same schematic as the Pro serie, and the Peavey CS series, with the CC and the CS having a traditional power supply instead of switching? You can also read very good opinions regarding the CS4080 which is CC5500 with higher supply rails and minimum 4Ohm load and, most important “The things I learned while designing the Crest 9200 carried over to your Peavey CS4080Hz. It’s mostly a Crest 9200 output stage with a linear transformer limited to 4 ohm/ch capability. That in turn morphed into the Crest CC5500 with a two ohm rating. Hence the similarity you noted. We would like to keep the two brands uniquely separated, but I’m not going to burn Hartley dollars reinventing technology. When the price point allows, the Crest gets more stuff inside”, forum words of an ex-Crest engineer… point…

In addition to the above I based my choice also on this review and this one  

Currently in this page You will not find bench data like for other units I own, basically for 3 reasons:

  1. my load resistors ( 4 x 2Ohm in series) are rated at 50W each for continuous drive, so 200W total, and a 5x overdrive capability for a duty cycle of 5s ( 5on and 5off). I already use a longer OFF cycle (10s)  and 2 fans blowing on the heat sink, however their life will be at risk with the CC5500, because they already “cried” with the CC4000.
  2. I always use  “real” home plugs in my tests, currently cabled with 1.5mm2 wires from the general switch having a 10A wall outlet, to which I attach a 3m extension (3×2.5mm2) going to the power distributor on the rack. This is far away from regulated bench supplies and almost reflect real usage scenario.
  3. The results of the CC4000 presented on this pages (for the point above) were misunderstood and interpreted on some forums (or from some people) as the amplifier not meeting its specs, and not as the result of the whole power supply line performance. As You can read here the CC4000 fully reached its 8Ohm specs on a bench supply but started suffering mains voltage drops on lower loads.

My Crest Audio CA6 amp

Pubblicato: 25 giugno 2013 in Elettronica, Musica
Tag:, , ,

A review with some measured data and some modifications to further better this amazing mid-power Pro Amp

I’ve been always fascinated by this line of Pro power amplifiers of the famous brand of Meridian, Mississippi, for several reasons, among which I count

  • Modular construction, there is almost one board for each section: input & and gain, pre/driver & protection, output stage, power supply.
  • “All Transistor” construction, excluding the input like in many Hi-Fi amps, which gives this series a superior sound quality; this is almost an Hi-Fi like amp, and the version with the output binding posts is considered such this way. Moreover there is only 1 ( I say 1) polypropylene capacitor in series with the signal… (I’ll show You later)
  • It uses and advanced (= well engineered ) Modulated Class H output stage, which exhibits no glitches at high frequency during rail switching, also without using RC networks across the switching diode; I simulated the whole circuit with LTSpice and there is always a clean wave and a perfect switch also at 20Khz.
  • Triple Emitter Follower output configuration with a total of 12 output transistors for each channel; this translates into better behavior on lower loads and less distortion, and more reliable operating conditions due to a great number of devices compared to other brands for the same rating, which use typically 8.

(altro…)

And some measured data… (for what my test bench can do)

I bought this amplifier as “second hand” from a nice guy here in Italy, after looking around for an amplifier versatile enough for my setup; I was searching a unit which I can use either with the Ciare PW330 low mids (500W RMS -1000W Peak) or the RCF L15P200AK-II in the bass unit (800W RMS – 1600 Peak), on which I could use it both stereo @8Ohm and bridged @4Ohm, depending on the needs.

(altro…)

Ita 

With some real measures…

I recently bought the EPX4000 to drive my two bass unit built on the CIARE HW380 woofer (yes, it is a “home” component), which specifications are 300W RMS and 500W max; some month before I bought an EPQ2000 to drive an old CIARE PW322, a 12” unit.

These amplifiers belong to a recent line of “traditional” amplifies, with the only difference the the EPQ does not provide 2Ohm stereo and 4Ohm bridge working condition, and they represent the highest model of their series; basically they are the last two lines of “analog” amplifier while the latest is the iNuke one, based on digital amplifiers (which I don’t like).

Before going to the datails of the two amplifiers, particularly to the EPX4000, I would like to comment a little the way Behringer uses to declare the power ratings since some time, which is the result of my experience on less recent models (EP2000) and on discussions with their technical support; first of all I was told that they mean “ideal” test conditions, from which I though they refer to stabilized 230V (in Europe). Second, some models show declarations like “XOhm per channel, stereo” and in this case, confirmed by the tech support, it means max power in stereo configuration with 1 channel driven; to be honest I found myself a little bit disappointed for the EPQ2000.

That being said here are the specifications of the two amplifiers:

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As we can see the EPX4000 is declared for 530W @8Ohm with both channels driven; within a little we will see that this is too optimistic. Going back in time we can see that for the old EP1500 the specifications were in line with the real measures, as explainer in this test. Same thing for the EP2500 tested on the same pages. With later model instead, like the EP2000 I had, Behringer began to drift with the RMS power ratings, and today we can consider a 20% less for model like the EPQ (“XOhm per channel, stereo”) and around 15% les for models like the EPX (“Stereo, both channels driven”). So, when I bought the EPX4000, mindful of the previous experience with the EPQ, I was thinking to an amplifier of about 450W @Ohm compared to the 530W @8Ohm of the specifications.

That said, I can confirm that both models are well made, even if they use electronic components of poor quality for the capacitors (DECON or XUNDA) and fair for the ICs (JRC4580, also used in the Crest CPX); on the other hand the QSCs, which the Behringer seem copy of, use ELNA capacitors and the famous and reliable NE5532. with a minimum expense and a little work with a soldering iron the supply capacitors and the ICs could be replaced, with a consistent quality gain; but this is another story … or another blog 🙂

Here are some images of the internals of the EPX4000

This confirms that the EPX4000 is simply a “renamed” EPX3000, and also the specifications are the same; You can see also an EPX2000 mark (now EPX2800).

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This is the detail of the supply voltages, 65V/100V, which are values with no load; they are almost the same values as the QSC CX-702 and DCA-2422, which are declared for 425W @8Ohm and 700W @4Ohm (keep in mind this values). The EPQ2000 values are 55V/85V ( @idle we have 55V/95V, probably under load the higher one lose about 10V), but I didn’t find an equivalent schema on the QSC site; considering the dimensions and the specifications it could compare to the GX series.

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These are the capacitors on the high voltage supply, 4x2200uF in series two by two, like on many QSC, while the EPQ2000 has only 2x1500uF.

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This is the detail of the supply capacitors of the 65V/100V lines: a total of 16x470uF capacitors while the QSC CX and DCA have 20, but considering the working frequency of the supply the “drive” capacity should be minimum. I imagine what could happen replacing them with some very low impedance PANASONIC … 🙂

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Last, the detail of the cooling tunnel, a little smaller on the exit probably to compensate the lower air velocity at that position

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After this inspection, I did the same day I bought it, I connected the EPX4000 to my scope and each exit to an 8Ohm load built with 4 2Ohm/50W aluminum  resistors, fixed to a heatsink with mounting grease; they count for 200W (each channel), but based on their specifications for a “duty cycle” of 5s (on/of) each package can handle till 1000W. During my tests with sin waves I use a 5s On and 10s Off cycle, so the real limits are a bit higher.

Here are some data I measured; with a 100Hz sin wave (it’s to easy to test @1Khz !) the EPX4000 shows the first clipping signs @59.5V with an 8Ohm load, which is 442W, and this come back to the 425W@8Ohm of the QSCs cited before; this is also the maximum voltage the limiter allow to deliver with the 8Ohm load. A little after I found that during the tests my wife was using a hairdryer … to further confirm the “real condition” operation 🙂

With a 4Ohm load clipping happens @54V, which translates into 730W; this is a further confirmation of the likeness with the specifications of the CX-702 and DCA-2422.

Considering the measured data and the above considerations the EPX4000 seems to ma a good “value for money”: I paid it 325€ here in Milan.

I write also the  8Ohm power which I measured sometime before on the EPQ2000: at the first sign of clipping it delivers 55.5V RMS which translates into 385W vs the 500W declared, but remember how the specifications are;by the way You can now take the EPQ2000 home with around 250€ and because I expect around 550W @4Ohm practically with around 500€, if you don’t have size problems, you can build a stereo amplifier of more than 1000W @8Ohm (using them in bridge mode), but remember to use it only with 8Ohm loads. The same is true for the EPX4000… around 700€ to have the equivalent of a stereo amplifier of more than 1400W @8Ohm (bridge mode), with the option of use it @4Ohm.

I’ll try to update this blog in the future with the 2Ohm test for the EPX4000 and the 4Ohm one for the EPQ2000, in order to complete also the figure of the bridged power.

Let’s see now a small negative note shared by the two amplifiers, which I found while they were under the scope; they are almost without polarization or with too few, and for sure also other model of the same series, the “old” EP and several amplifiers of other brands (except the Crest CPX, the Peavey PV and for sure other models of the same group). This comes to a consistent  crossover distortion as well as high levels of THD at high frequencies.

This is a 15Khz 2,83V RMS @8Ohm taken on the EPQ2000, and the one on the EPX4000 is almost identical; we can see a consistent crossover  distortion.

To confirm this, if we look at the following image, taken from the test of the old EP1500

we can see that at higher frequencies due to the poor polarization the distortion is very high, around 2%; the same happens for the EP2500, which delivers only 260W @8Ohm with 0.1% TDD compared to the 450W at 1Khz and 100Hz.

If we look at the Peavey PV2600 tested on another site (identical to the CREST CPX2600) we note how low is the THD at high frequencies, due to proper polarization.

image

Given this, once opened the EPQ2000 I found the bias trimmer of the output stage, and with a tester connected to one of the output stage resistors I set the trimmer in order to have a minimal polarization (few mA); the result is the following image where the crossover distortion has gone away.

I  currently made this change only on the EPQ2000, while the EPX4000 is still unchanged. Everyone with a minimal practice can try this operation and set a minimal bias current to better in a consistent way the THD figure at high frequencies of these amplifiers, few mV, between 3 and 5, are enough; remember that these are not the only PRO amplifiers suffering of this small “defect”.

And … sorry for my English.

Update on 29-Dec-2012

As stated before today I tested the EPQ2000 in bridge mode @8Ohm, using the same method as above, so 5s On and 10s Off; the RMS voltage reached by each side (so 4Ohm stereo equivalent) at the clipping has been 45,5V, a total of 91V RMS, which translates into a little more than 1030W RMS @8Ohm (around 515W/4Ohm stereo).

The strange thing i noticed during the test was the fact that the RMS voltage dropped to 87/86V (940W/920W) after a couple of seconds.

Update on  01-Jan-2013

After having done a re-cabling of the power string on the rack, as I wrote on the Crest CC4000 article, which allowed to me to gain some W, I tested again the EPQ2000 in bridge mode @ 8Ohm, and I got interesting results

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As You can see clipping happens at 98.6V RMS, which translates into 1215W; with the limiter on the RMS voltage does not go over 96.5V RMS (around 1165W) practically around 10% more power than before and the most important thing was the the RMS voltage didn’t  drop as before after 2s.

The main voltage dropped from 238V to 232V

Con alcuni dettagli di misure “reali” …

Recentemente ho acquistato il finale EPX4000 per pilotare le mie due unità bassi basate sul Woofer CIARE HW380 (si si è un componente della linea Home … ahahah ), le cui specifiche prevedono una potenza RMS di 300W ed una massima di 500W; prima di questo avevo acquistato un EPQ2000 per pilotare un “vecchio” CIARE PW322 da 12”

Questi finali appartengono ad una recente linea di amplificatori “tradizionali”, con l’unica differenza che la EPQ non prevede il funzionamento su carichi pari a 2Ohm stereo e 4Ohm a ponte, e rappresentano i modelli di punta delle rispettive serie; in sostanta sono le ultime due linee di amplificatori “analogici” mentre l’ultima in termini di tempo è la iNuke basata su amplificatori digitali (che non mi piacciono).

Prima di passare ai dettagli dei due amplificatori, con particolare riferimento all’ EPX4000 vorrei spendere alcune parole sul modo che usa behringer per dichiarare la potenza dei sui amplificatori da un po’ di tempo a questa parte, frutto di esperienze su modelli un po’ meno recenti (EP2000) e su una lunga discussione avuta con il loro supporto; prima di tutto mi è stato riferito che le condizioni di test sono “ideali”, per cui ho dedotto che si affidino ai 230V stabilizzati. In seconda istanza alcuni modelli presentano dichiarazioni del tipo “XOhm per channel, stereo” ed in questo caso, confermato dal supporto, si tratta della massima potenza in configurazione stereo con 1 canale in funzione; devo essere sincero che per l’ EPQ2000 mi sono trovato un po’ spiazzato.

Detto questo ecco di seguito le specifiche dei due amplificatori:

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Come si può vedere il’ EPX4000 viene dichiarato per 530W RMS su 8Ohm con entrambe i canali in funzione; a breve vedremo come sia un valore un po’ troppo ottimista; andando un po’ indietro nel tempo possiamo notare come per il vecchio EP1500 le specifiche coincidessero con il dato reale misurato, come su può vedere in questo test. Stessa cosa per l’EP2500, testato più avanti. Con i modelli successivi invece, come l’ EP2000 che avevo, Behringer ha iniziato ad andare alla deriva con le dichiarazioni sulla potenza RMS, e ad oggi si può considerare la potenza reale praticamente il 20% in meno di quell dichiarato per modelli tipo l’ EPQ (“XOhm per channel, stereo”) e circa il 15% in meno per modelli tipo l’ EPX (“Stereo, both channels driven”). Quindi al momento dell’acquisto dell’ EPX, memore delle precedenti esperienze con l’ EPQ, sono partito per l’acquisto pensando ad un ampli da 450W/8Ohm contro i 530W dichiarati

Detto questo posso confermare come entrambe i modelli siano di buona fattura nonostante l’assemblaggio cinese, seppur utilizzino componenti di qualità mediocre per i condensatori (DECON o XUNDA) e discreta per gli integrati (JRC4580, usati anche nella serie CPX di Crest); per contro i QSC, ai quali i Behringer fanno praticamente capo, usano condensatori ELNA e il famosissimo e affidabilissomo NE5532. Con un minimo di spesa ed un po’ di lavoro con il saldatore si potrebbero sostituire i condensatori di filtro sull’alimentazione e gli integrati con un notevole guadagno in termini di qualità, e magari anche qualche integrato; ma questa è tutta un altra storia … o un altro blog Smile

Vediamo alcune immagini dell’ interno dell’ EPX4000

questa in sistanza conferma che l’EPX4000 non è nient’altro che una “rimarchiatura” dell’ EPX3000, ed infatti le specifiche sono praticamente coincidenti; si noti anche il riferimento all’ EPX2000 sulla scheda (adesso EPX2800).

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Qui vediamo il dettaglio delle tensioni di alimentazione, 65V/100V che esprimono i valori a vuoto; si tratta in sostanza degli stessi valori riportati negli schemi dei QSC CX-702 e DCA-2422, dichiarati per 425W/8Ohm e 700W/4Ohm (tenete bene in mente questi valori).  Per l’EPQ2000 i valori sono 55V/85V ( a vuoto le tensioni di alimentazione risultano 55V/95V, probabilmente sotto carico l’ EPQ2000 scende di circa 10V), ma non ho trovato uno schema equivalente sul sito QSC; viste anche le dimensioni e le caratteristiche tecniche dovrebbe ispirarsi alla serie GX.

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Questi sono i condensatori principali sull’alimentazione ad alta tensione, 4×2200 in serie a coppie, come su molti modelli QSC, mentre l’ EPQ2000 ne presenta solo 2 da 1500uF

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Dettaglio dei condensatori di alimentazione sulle due tensioni 65V/100V: un totale di 16 condensatore da 470uF mentre i QSC CX e DCA ne hanno 20 in totale, ma data la frequenza di lavoro la differenza in termini di capacità di pilotaggio è minima. Chissà se cambia qualcosa sostituendoli con dei PANASONIC a bassissima impedenza Smile

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in ultimo il dettaglio del tunnel di raffreddamento, leggermente rastremato all’uscita, probabilmente per compensare la minore velocità del flusso d’aria in quella posizione

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Dopo questa prima ispezione, eseguita il giorno stesso dell’acquisto ho collegato l’ EPX4000 all’oscilloscopo e le sue uscite ad un carico di 8Ohm composto da 4 resistenze da 2Ohm/50W in alluminio montate su un dissipatore, quindi un carico nominale di 200W: data la loro caratterisca con un “duty cycle” di 5s (on/of) il pacchetto può arrivare fino a 1000W. Nei miei test utilizzo per i segnali sinusoidali un ciclo di 5s On e 10s Off per cui il limite si sposta ancora più in alto.

Passiamo adesso ad alcuni dati che ho rilevato: utilizzando una sinusoide a 100Hz (troppo comodo fare i test ad 1Khz!) l’EPX4000 evidenzia i primi segni di clipping a 59.5V RMS con un carico di 8Ohm, il che si traduce in una potenza pari a 442W, e tutto torna con i425W/8Ohm Smile dei QSC citati prima; questa è anche la massima tensione che il limiter permette di erogare su 8Ohm. Poco dopo il test ho scoperto che in casa stava “girando” un phon … ad ulteriore conferma del test in condizioni “reali”.

Passando ad un carico di 4Ohm il clipping avviene a 54V RMS pari a 730W, tutto questo è un ulteriore conferma della coincidenza dell’ EPX4000 con le specifiche del CX-702 e del DCA-2422.

Visto i precedenti dati e le premesse fatte prima l’ EPX4000 mi sembra un buon “value for money”: 325€ da Lucky Music a Milano.

Riporto per completezza anche la potenza su 8Ohm che avevo rilevato tempo prima sull’ EPQ2000: ai primi segni del clipping siamo a 55.5V RMS pari a 385W contro i 500W dichiarati, ma ricordatevi come erano le specifiche; a proposito adesso l’ EPQ2000 lo portate via con 250€ e siccome mi aspetto almeno 550W circa su 4Ohm praticamente con 500€, se non avete problemi di ingombro, vi fate un finale stereo da piu di 1000W per canale (usandoli a ponte), a patto di usarlo solo su 8Ohm. Stesso discorso per l’ EPX4000 … circa 700€ per fare l’equivalente di un finale stereo da oltre 1400W RMS su 8Ohm…

Prissimamente aggiornerò il blog con le prove su 2Ohm per l’ EPX4000 e su 4Ohm per l’ EPQ2000, per completare anche in quadro delle potenze a ponte

Vediamo ora una piccola nota negativa che accumuna i due amplificatori e che ho potuto scoprire mentre li avevo sotto l’oscilloscopio; i due finali in questione, ma sicuramente anche gli altri modelli delle stessa serie, cosi come le “vecchie” serie EP e molti finali di altre marche (tranne i CPX di Crest e i PV di Peavey, che poi sono la stessa cosa e ragionevolmente anche gli altri modelli dello stesso gruppo) sono praticamente senza polarizzazione dei finali, o molto scarsa. Questo si traduce in una consistente distorsione di incrocio ed altrettanti elevati livelli di distorsione alle alte frequenze.

Questa è la sonusoide a 15Khz 2,83V RMS su 8Ohm presa sull’ EPQ2000, e quella dell’ EPX4000 è molto simile; si nota una consistente distorsione di incrocio…

A riconferma di tutto questo se guardiamo l’immagine di seguito, roportata da una prova del vecchio EP1500

ci accorgiamo come alle alte frequenza per effetto della scarsissima polarizzazione la distorsione è molto elevata, quasi il 2%; stesso discorso per l’ EP2500 che a 20Kz eroga circa 260W al limite dello 0,1% contro i 450W a 1KHz e 100Hz.

Se prendiamo ad esempio il Peavey PV2600 provato su un altro sito (praticamente identico al CREST CPX2600) notiamo come per effetto di una corretta polarizzazione la distorsione alle alte frequenze rimanga contenuta

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A fronte di tutto questo una volta aperto l’ EPQ2000 ho individuato il trimmer per la regolazione della corrente di riposo e con un tester collegato alle resistenze poste sui finali ho regolato il trimmer per avere una minima polarizzazione dei finali stessi: il risultato ottenuto è praticamente l’immagine seguente, dove la distorsione di incrocio è praticamente sparita

Questo intervento al momento l’ho fatto solo sull’EPQ2000, che viene utilizzanto nella gamma 80Hz-1,8KHz, mentre sull’ EPX4000 che pilota due unità bassi non ho ancora effettuato la modifica. Chi avesse una minima dimestichezza in elettronica può cimentarsi in questa operazione e settare un minimo di corrente di polarizzazione per migliorare in modo consistente la figura della distorsione alle alte frequenze di questi due amplificatori, bastano pochi mV, tra 3 e 5; ripeto non sono gli unici a soffrire di questo piccolo “difetto” …

Aggiornamento del 29/12/12

Come anticipato oggi ho misurato la potenza a ponte dell’ EPQ2000 su 8Ohm, utilizzando lo stesso metodo visto sopra, quindi 5s On e 10s Off; la tensione RMS raggiunta da ogni ramo (quindi l’equivalente di un carico di 4Ohm) prima del clipping è stata di 45,5V, quindi 91V RMS totali, equivalenti a poco piu di 1030W RMS su 8Ohm o circa 515W RMS su 4Ohm.

La cosa che ho notato durante il test è che con il limiter inserito con un minimo di “overdrive”, giusto per farlo intervenire, la tensione RMS dopo circa 2s scende a 43,5/43V quindi a circa 470/460W su 4Ohm o 940/920W a ponte su 8Ohm.

Un’ ulteriore piccola nota; la prova è stata eseguita usando una prolunga da 3m (cavo 3×2,5 mm2) collegata ad una ciabatta da rack (con cavo da 1,5m) che poi alimenta l’EPQ2000, quindi di nuovo in condizione d’uso reale. Inoltre ho notato che per tutta la durata del test, anche a varie potenze ma sempre abbastanza sostenute la ventola non ha aumentato ti velocità e l’aria in uscita era tiepida.

Aggiornamento del 30/01/13

l’ EPQ2000 da oggi è in vendita per passaggio a finale di potenza superiore: chi fosse interessato può contattarmi per i dettagli.

Nel frattempo, dopo avere ricablato l’unità di distribuzione dell’alimentazione presente nel rack, come descritto nell’ articolo sul Crest CC4000, ho rieseguito i test a ponte su 8Ohm ed i risultati sono stati interessanti, come si vede nella foto allegata.

Come si vede ai primi segni del clipping siamo a 98.6V RMS che corrispondono a 1215W (600W/4Ohm Stereo);con in limiter inserito la tensione in uscita viene bloccata praticamente a circa 96,5V, corrispondenti a circa 1165W (580W/4Ohm Stereo); praticamente circa il 10% in più di prima dell’intervento sul distributore, con la tensione di rete che è scesa da 238V a 232V misurata su una delle uscite del distributore stesso. Vorrei ricordare inoltre che la presa nel muro è comunque da 10A e sicuramente cablata con un cavo da 1mm2