BGW 750 amplifier power output - increased.. - modified filter caps??? - diyAudio
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Old 16th April 2004, 12:42 AM   #1
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Default BGW 750 amplifier power output - increased.. - modified filter caps???

I heard somewhere that if you replace the filter caps on a BGW 750, the power output will increase substantially. Heres the story: A few years back I bought a BGW 750 B, and when I hooked it up to 2 JBL-SR25's in stereo, it was a lot louder and crisper (with both amp channels fully cranked) than any other 750 B that I own. In total, I own 8 BGW 750 B's, and every single one seems to have the same performance in terms of loudness. As a test measure I tried each of the 750 B's with both amp channels fully cranked, and with the master volume level, and the line level topping out at 0 db on my Pioneer DJM-600 mixer. Every one of the amps had the same loudness and excellent sound quality (less than 0.1% THD of course....), except for this particular 750 B that I bought a few years back. The thing is a heck of a lot louder (with the same sound quality) as opposed to the other 7 amps. ?????? I couldn't figure out why......
So since then I have been investigating and trying to look for any information I could about this phenomena. A short time after, I heard something about a trick that that can be done with the BGW 750, -by replacing the filter caps with bigger ones, the amp can achieve an output of relatively 375 watts continuous into 8 ohms. The normal specs from BGW state that the 750 B/C series operate at 225 watts continuous into 8 ohms. I can say that this particular 750 B amp that sounds louder than the rest of the ones that I own, is DEFINITLY A LOT MORE THAN 225 watts.
So, I think I may have one of these "modified", or "suped-up" 750's. I haven't opened up the case yet, so I have know idea what has been changed in it. I would guess that probably some repair tech fixed the thing a while back and added the caps (or whatever else needed to be added) to get the louder performance. So, does anyone out there know about this performance upgrade, and what exactly needs to be done? Is it the filter caps? .... ??? etc. any useful information that anyone could give me would be greatly appreciated.

Thanks,
Robert
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Old 16th April 2004, 09:21 AM   #2
djk is offline djk
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A lot of people confuse gain with power.

Someone has changed the gain on your odd-ball BGW.

26dB was a standard figure for gain for many years, it just wasn't enough for large PA power amps.

Newer designs run as much as 40dB.

I used to go in and change the gain, but this can cause stability problems.

If it was my odd-ball BGW, I would change the gain to match all my other stock BGW amps.

You want it to really play louder?

Caps won't do anything, a new power transformer will. I could turn your 750 into a 1000, don't ask how much it would cost.

Buy a bigger amplifier.
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Old 16th April 2004, 12:36 PM   #3
AJT is offline AJT  Philippines
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Quote:
Newer designs run as much as 40dB.
with amps such as that, you can put a buffer with a gain of x2 to cope up.....
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Old 16th April 2004, 03:18 PM   #4
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Default It probably IS in the power supply...

A lot of 70's era amplifiers were rather skimpy with the power supply capacitors, and sometimes also the transformer. They were designed with components not intended to provide 3-5% regulation, such as a properly specified Plitron power transformer, but with less costly transformers and caps, but running at a higher initial voltage. Under steady load, the rails sag. Heck, under high music demands the rails sag.

By way of illustration, Phase Linear amps of that timeframe used only about +/- 10,000 uf in the power supply, a total of 20kuf. The "revered Dyna stereo 400 was the same, re power supply caps, but at least it had a pretty good tranformer.

In the mid seventies I built a beast of an amplifier, by starting with a Dyna 410 Kit, (the circuit was a typical Borngiorno cascode output using two series lower voltage, high current bipolars in series for each polarity of signal swing), doubling the number of output transistors, and adding 8 more 10,000 uf power supply caps, plus 100uF of polypropylene HF bypass. I didn't just change the size of the electrolytic caps, because ESR is a big factor on current delivery, and most of the available single caps at that time didn't have very spectacular ESR. Paralleling caps brings the ESR value down. Plus, the larger value electrolytics tended to be more inductive, with a lower self resonant frequency- another reason to parallel "smaller" caps. The polypropylene caps reduced the HF impedance of the power supply further.

With the extra supply capacitance, the rails were much stiffer, and sagged less under load. It would deliver 280 watts per channel on sine wave testing without clipping, and it was effortless in doing so. A stock 410 clipped at about 220 watts/ch.

This amp was compared against a big SAE (don't remember the model), a Luxman M4000, a Luxman M6000 (rated 300 watts per channel), and a Stax DA-150. The others sounded strained and "solid-state" in comparison at high levels (short of clipping); the Dyna on steroids always sounded effortless, due I think to lack of rail modulation from load. This was driving speakers like B&W's top of the line at that time DM6.

Moral of the story? An amplifier is just a way of connecting the energy from the power supply to the speakers. The better the power supply, the better the amplifier will be, as most real world circuits don't have infinite PSRR.

Did the same thing once with a Phase 700. Scary.... 450 watts+ per channel at 8 ohms.

~Jon
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Old 16th April 2004, 03:42 PM   #5
markp is offline markp  United States
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Old BGWs had small power supplies and by now the caps are dried out too. Upgrade the ps caps and bypass them with a film cap and you will be very happy with the result. Much more continuous power will be available as will more peak power. Be careful not to overheat the amp as its heatsinks are not designed to handle more power than stock.
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Old 16th April 2004, 03:52 PM   #6
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Default Specification BGW750C 750B

BGW 750 C B
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Old 24th August 2006, 10:01 AM   #7
tt398 is offline tt398  United States
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Question 750 and 500 Manuals

I've picked up some used BGW 750 C and 500 D amps = without owners manuals. I've tried to get them from BGW - but they don't seem to answer e-mail.

Do any of you guys know where I can get some soft (acrobat) copies of these manuals?

Thanks!
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Old 24th August 2006, 11:16 AM   #8
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by djk


Someone has changed the gain on your odd-ball BGW.
26dB was a standard figure for gain for many years,
it just wasn't enough for large PA power amps.
Newer designs run as much as 40dB.
I used to go in and change the gain,
but this can cause stability problems.

Hi,

Increasing gain will not cause stability problems, reducing it will.

For stability you are interested in the gain margin, increasing
gain increases it and reducing gain reduces it, so adding 10dB
gain to a power amplifier should not cause any problems at all.

/sreten.
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Old 24th August 2006, 11:54 AM   #9
AJT is offline AJT  Philippines
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Quote:
Do any of you guys know where I can get some soft (acrobat) copies of these manuals?
i have the original article that i posted here: http://pinoydvd.com/board/index.php?topic=47979.280

it's a local site, i am not sure if it is allowed here....

i have them on my hd but i am using dial-up so sending them is a big problem...
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Old 24th August 2006, 05:34 PM   #10
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Default manual for 750 c

tt398, you can get a copy of the owners manual and spec sheet (47 pages long) from www.musicparts.com I think it cost me $20 when I ordered mine.
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