Fans are a potential source of noise, but if your using a pro amp in a consumer setting and will not be overdriving them, you could disable the fan and it still function reliably. As for noise floor and sound quality, my favorite amp was always Crown, but then below the Crown, I would rate the QSC and AB amps next followed by Carver then Peavy, just to name a few that I have used in pro-sound reinforcement days. You would likely be well served by the Crown or QSC as long as you get the ones that are stereo 4ohms. Bridging or running 2ohm loads will increase the THD and your noise level accordingly. I have had 30,000 watt arrays that were whisper quiet with the Crown MacroTechs that we used. To respond to your point about output transistors, I just remembered another point, there are various types of amplifier IC's and generally, the ones like in the Outlaw are the small plastic encassed units. But that made me remeber the type in the pro-amps are the large metal encassed units. Accordingly, don't compare number of IC's in these because they are two different types.

There are many amps that use Tripath controller i.c.s, some of which have received rave reviews by Stereophile and other magazines. Of course, they are very expensive ($3k+), but not outrageously so compared to some other high-end amps ($20K+).

I have a couple of Crown K1 amps. I believe that they use Tripath controller i.c.s also. In my opinion the K1 (and the more powerful K2) makes a superb subwoofer amp. I used it briefly to drive my Acoustat 2+2s just to find out how well (or bad) it would do. It sounded OK but not great working as a full-range amp.

So, depending on your application, a digital amp may work for you. Especially in situations where ventilation (or lack of) and heat buildup are a concern.

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the 1derful1

Thank you gentlemen for all your inputs. I looked into pro amps and "high-end" amps and decided to take the path of least resistance to my wallet. I decided to upgrade/modify my own Adcom GFA 555 II for less than half of all the other options. I am having the following modifications done.

1. Replace a 675 VA input transformer with two 1KVA, making it dual mono.

2. Replace 16 old, 150 watts/180 volts output tansistors with (still) 16 new ones rated 250watts/250volts.

3. Replace four 15k uF capacitor to 33k uF ones.

4. Replace 25Amp rectifiers with 50Amp ones.

Along with these main changes, some minor ones will be done as well -- drivers are being updated and bypass caps are being added near the output for supposedly "faster recharging."

I don't know if the modified amp will drive the VMPS woofers well or not, but I am hoping that this least expensive path will significantly improve the bass response of these speakers. My main concern is whether the existing heat sink will be able to handle the increased capacity, but I am hoping that driving only the bass (2 10" cones and a passive radiator each speaker) won't heat up the amp very much. Plus I don't blast music (mainly orchestral, operatic, jazz and blue grass music) and Adcom GFA 555 II is as big as Outlaw 770 and has impressive fins on the back.

Thanks again for the advices.

Unless the supply rail voltages are increased (I would tend to think not - you can't just do this without some serious re-design work of the circuitry), you are not going to get more power out of your amplifier. This mod looks impressive on paper, but all you are really doing with this mod (unless the rails are increased) is to make the power supply capable of more current at low impedances, but not more usable power. This mod will only help with very low impedance loads, which I don't think is your problem. I seriously do not think this will improve your situation. I would ask some really hard and direct questions of the people doing this work before plunking down your money. Unless your amplifier is clipping now, I would just save your money or buy a higher powered amplifier - the largest power demands are in the bass, but if your subs are not an unusual load, the extra current capacity will be wasted.


[This message has been edited by soundhound (edited February 09, 2004).]

Soundhound,

Thanks for the information. This is the reply I got form the mod person.

Increasing transformer alone WILL mean increase in power because the
voltage rails will not sag during load as they would with a smaller
Toroid of the same voltage.

Case in point, the Aragon 4004's and 8008's pump out 200wpc using
voltage rails of +/- 68, yet Adcom gives out the same power using +/-
82volts why?

If your friend was accurate then this would not be possible. The thing
is, the Aragon uses 2 800VA toroids in the 4004's (1.6kva) while the
Adcom uses a single 675VA Toroid.

However that alone is not sufficient to increase the power to the levels
we are looking for. For that, I am increasing the voltage rails to +/-
87 vdc from the stock +/- 82 vdc.

Extrapolating from the Aragon (which actually clips at 240 watts into 8
ohms), if 136 volt rails (68*2) gives us 44 volts RMS, then the upgraded
Adcom will give us 44/136*174= 56.29 volts RMS or 396 watts into an 8
ohm load, give or take.

In actuality with a good power supply, to get 400 watts, all you need
are rails of +/- 85vdc. On the other hand for cheap amplifiers with
small power supplies, you would need +/- 105 volt rails to get the
same... so while voltage is one variable, current is the other.

I would appreciate your take on this explanation.

Thanks

I still think that this mod is not a terribly cost-effective path to persue. If your amp is rated for instance at 400 watts continuous, increasing the current capacity of the power supply will do nothing for this static power output. It will effect the ability of the amplifier to deliver it's normal transient power for a longer period, but in my opinion, this relatively slight benefit is not as cost effective as simply buying a higher powered amplifier.

Of course the person doing the mod wants to make some money. I don't know how much the labor and parts cost of this mod is, but I really think you would be better served by buying a new amp that comes with a factory warranty in case anything goes wrong. I seriously doubt that your amplifier would be reparable by the original manufacturer in modified form, nor would it's resale value be as high.

It sounds as though these mods, which are mostly taking a real-world power supply and makeing it somewhat more ideal and improving the output stage inasmuch as the output transistors will drop less voltage when they are at or near saturation, will mostly increase the average (long term, misnomered RMS) power output of the amp, moreso at low impedances.

If you are mainly concerned in how well the WOOFERS will be driven, then I think that these mods will have benefits that are mostly in the area of bass reproduction and may work for you. As to whether the benefits are worth the cost, I have no idea.

I do not think that these mods will help in areas of soundstaging, musicality, air, and other areas of "high-end" concern. To improve the sound quality in these areas will take more than improving the brute-force aspects of the power amp. A knowledge of ciruit design and the relative benefits of various topologies is needed. Effective mods in this regards are a function of delicacy and finesse as well as brute force.

Aside from the case (chassis), which on some high-end amps can cost a fortune just for the fancy machining, the transformer(s), main filter capacitors, and output devices of a power amp are some of the most expensive parts in an amp (if you ignore the $700 high-end power cord which you must have if you are to worship at the high-end altar).

Please keep us informed of you decisions and concerns.

Paul



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the 1derful1

Thank you for the feedbacks.

I just noticed that I have been promoted to a gunslinger! Funny, how I don't feel any wiser on the topic of amplifiers......

I have decided to go ahead with this modification of Adcom GFA 555 II. Here are the reasons.

1. I already own one and, thus, effectively cost me nothing.

2. Ampzilla 2k monos that are the choice amps for the speaker designer Brian Cheney have rating of 400 watts into 4 ohms. Other ones he has used with good results are all class A monsters (Pass Labs and Plinius)with ratings no higher than 400watts into 4 ohms. From this I am guessing that "high current" is more important than "high power".

3. I can't afford any class A amps with enough power.

4. A decent Pro amp would cost me more than the cost of this mod and the "midfi" amp put together.

5. Since I am on Outlaw forum, I will also say that 2 M-blocks were considered but ultimately decided against. The reason being that 2 separate channels of 770 didn't cut it. If Adcom(unmodified) didn't have enough current, I did not think that M-Block in that slim casing would have big enough power supply to get the kind of current I am looking for.

These are the reasons I am doing this mod hoping to get an amp that will drive the woofers with authority. I think this person offering the modification is honest, reasonable, and believes in what he does. At least, I can understand the reasons for his mods, which are simply upgrading parts to get the most power/current the original equipment layout can handle. I am not holding breath waiting for some electronic Fen Shui that some outfits promise in "signature" mods costing k's of dollars for that illusive "high end" musicality.

I feel better knowing that all this mods are costing me no more than what a "highend" power cord costs. I will post updates, if only to let others see what not to do......

Gentlemen,

I have just received the Adcom GFA 555 II back from being modified as stated before. Arif, the modifier, threw in for free addional tweaks like "snubbing" capacitors on the bridge rectifiers and new AC socket because he got carried away with the modification process.

I hook the amp up to drive the woofers on VMPS RM40 speakers in a biamp configuration. Wait 30 minutes for it and the tubes in the other amp and preamp to warm up. The needle hits the groove, and voila, the sound is to so seductively good that I stay up and listen to every record I have......

Actually, it is more a function of biamping then mods on the Adcom amp. With just 100 watts tube amp, everything was just too thin. Bob Dylan sounded like what Robert Zimmerman must have sounded before puberty. 100 watts just is not enough to move the huge magnets on RM40. Good 300 or 400 watts of steady power are needed to drive the woofers well.

What I noticed with the modification is that it is in absolute control of the woofers -- no blaring booms. There is no hint of rolling off or muddied sound. Rock solid bass really improves the overall tonality. In addition, the adjustment on the sensitivity really helps. I had the speaker level control of the highs and mids way down to keep with the bass before. Now, they are in neutral positions and sound better.

I am sure it will take time to make the right adjustments, and the amp also needs burn in time. For now, it sounds very promising. I am going to listen this weekend with just the Adcom amp to see if it has "acquired" some "audiophile airs", but I am pretty sure I will get back to biamping with tubes for the voices. I will keep you informed if anybody is interested.....