How to remap your amplifier.

Since our first post we have received three requests with regards to boosting amplifier performance from our younger readers. By boosting amplifier performance we are talking here wholly about power output, not quality of course. Well this can be done of course, usually at some exorbitant price.

The Sound Wars Vs Whisky in the Jar

The idea of increasing output power of any amplifier lends itself to the fact that we usually need this power after a few jugs or to impress.  Many years ago with a tube 15W per channel amplifier and paper coned speakers sound volume was the last thing on our mind.  Amplifiers just did not come more powerful, for a price at least. Solid state engineering and the marketing machine put paid to this.

Behringer EP4000 – a convincing best seller

Bridging your amp

For starters, the easiest way to get more power from an amplifier is to bridge-mode the output. This means the speaker is connected between the (+) terminals of each amplifier and the inputs are driven out of phase with each other. The restriction is speaker load. This effectively should double the amplifier power at twice minimum speaker impedance as suggested or warned by manufacturer.  However we have now lost stereo.  And I don’t like bridging- this is something which should only be done by an experienced audio or electronics person that understands SOA and loading.

The connection between high voltage rails and low impedance loads?

So what to do, what to do? In one of our mails our newbie pointed out that he had a Sony STR-DG600 which he wanted to modify to drive a 4 Ohm load by adding extra output devices.   I am familiar with the 500 and 600 series and whilst pretty good home theatre amplifiers they are prone to switching off in high humidity (if you live in Sea Point or Durban you may know the feeling).

They are both rated about 100W per channel (in stereo mode, which I doubt). The output stages of these amplifiers should be able to put out the advertised 100W per channel into 8 Ohm loads but not with the mains transformer used – it’s essentially a home theatre amp after all and according to my own calculations the transformer is only about 200VA max.  Transformers are very expensive.

Behringer EP4000 Rear

Playing into a 4 Ohm load may double the power dissipation but will fuse the input to the transformer or take out the power transistors – this is not a possibility but a definite. (for continuous periods at high volume control settings).

The Sony does not have the real estate to allow for higher powers, both in cooling capacity and a more powerful transformer.

Recommendation

Unfortunately modern integrated amplifiers (pre-amp and power) don’t lend themselves easily to modifications and it’s advised to leave as is. A common modification however would be to increase supply rail filter capacity, replacing the rectifier with fast switching diodes and adding fan cooling (not always a great idea for home use because of the noise).

Attenuation – from power amp to power amp

The STR600 is no exception but I wouldn’t worry at all about any changes to the supply or rectifier.  What one could do is of course reduce loading on the two front channels which would give the power supply room to breathe.  This would mean a resistive potential divider chain, usually a 50K and 10K resistor in series, the 50K leg going to the (+) speaker terminal and the lower 10K to (-) with the mid connection feeding another power amplifier.

This will reduce the loading on the two channels, less hum and noise – and less distortion at higher levels of volume.

Note:  The attentuator schematic above is for conventional amplifier outputs and not a bridged output.

Budget power amplifiers – what the hell does budget mean anyway?

As a budget amplifier the Dixon MA-200 or MA400 are pretty good. Don’t be put off by the sneer brigade,  the resellers have a name to protect and if one had to actually open the chassis and peer inside you’d be surprised at the size of the toroidal. On our South African market that would easily set you back at least R 1 200.00.  I don’t like the cooling though but it suffices and of course this applies to many other amps on the market. Will cover this in another article. In any event, these amplifiers can be purchased for about R 2 000.00 second hand, even through Cash Crusaders.

Behringer EP4000 (or EP2500)

My personal favourite though is the Behringer EP4000 which for home use may be total overkill.  The price is a problem however, they are also very much in demand on the 2nd hand market.  Sticking to a R2 000.00 budget with the MA-200 would be cheaper than what it would cost to modify most 100W amplifiers with a small power supply.

Even the power supplies used in some stereo amplifiers from the “snob lineage” can be pretty meagre which is a great disappointment. NAD as a rule don’t market their products without the recommended USA power approved method and this will cover supply rating as well.  In our sister-website, Parts-Ring I tested a 7200 which is rated at about 50W p.c. and although it uses a now fairly common type voltage shift supply the power output did not have the punch of the MA-200.

I have tested the MA-200 on the STR500 and the results were impressive.  I don’t have the scope tests at hand (analogue scope) but there was no sign of supply droop at 100W per channel, unlike the original load testing. Distortion was near non existent at 28V across the 6 Ohm speaker load.

Attention: No modification required

In my opinion most home theatre amplifiers shouldn’t be modded to get extra power, the entry level units, even some high end stuff,  were just not designed for continuous flat out music reproduction.  The mains transformer definitely not. A professional category amplifier may be better for this purpose with the added precautions*.  A 100W amplifier does not sound ten times as loud as a 10W amplifier and therefore it stands to reason if one really wanted to beef up an H.T.  100W amplifier the time and effort required does not justify the cost.

A practical solution

The best solution here, my recommendation and opinion here of course, an attenuator and MOSFET amplifiers such as at Rod Elliott or Yebo. Guaranteed to fly your kite and at the same time, can be used as a proper “musical” sounding amplifier by switching inputs to a first class pre-amplifier.

*Added precautions

The best amplifier I have ever heard used a MOSFET output stage.  This is an opinion.  Separate power and pre-amplifiers are better than an integrated solution. Another opinion. Professional audio uses more robust connectors.  A fact.

Professional series power amplifiers are designed for long duration high power usage. My opinion.

Professional does not mean one can throw the kit around. My opinion.

There are so many opinions and facts out there that it has become difficult to sift through marketing ploys, opinions, Kudu droppings and what we may see as “real” fact. For the novice, coming into the audio arena this can be challenging.

The article above serves to lay out the rationale, in my opinion, why it is best not to adopt an “upgrade” approach to modern audio circuits, more the home theatre variety.  It’s just not worth the time and effort (and financial outlay) for simply put, a higher output into a pre-determined load.

Better to build from scratch and use your existing gear as a driver.

Consumer goods, e.g. HT DSP preamplifers which have XLR preamplifier outputs are prohibitively expensive. It numbs the mind. I have personally modified a Sony 600, ripped out the output stages and wired to XLR outputs to feed a Behringer NU4-6000 and MA-200. The resulting reproduction, in my opinion, was phenomenal. I put this down to limitations in the headroom of the Sony.  It’s not a kW ear-burster.  But using an old Pioneer stereo amplifier as a driver to the Behringer amplifier was even better. Because it was designed for audio and not HT. I prefer “analogue style” tone controls.

Because many will not feel inclined to blow a substantial amount on an amplifier such as one in the iNuke series, a better bet is the MOSFET amplifiers advertised by Yebo and on Rod Elliott’s pages.  I have nothing against the iNuke, MOSFET amplifiers just have such an incredible dynamic range. 

In all my tests I used Eltax Millenium 500s. Why? Because they handle vast amounts of power.  Power pundits like these speakers for this exact reason. And no, they sound awesome. My opinion. 

I promote Behringer products because for me they are the underdogs of the musical world. Criticism, rubbishing and just plain ignorance are the order of the day. I could go on and on.  Since more Behringer gear gets used than any other manufacturer they must be doing something right.  Also how many permutations of an audio amplifier do we actually get?

[Ed’s note:- although mention is made of the MA-200 and MA-400 we are not affiliate to Cash Crusaders through any means. Likewise Behringer of course].

Next:  Sony amplifier’s MN2488 and MI1620 Power Darlingtons. (2SD2488 and 2SB1620)