Graham Slee Power Amplifier

 

Analog Kid wrote:   All of them?  Pure class A must be an illusion then. There is one Japanese HiFi company that manufacturers a 2 x 60 watts (@ 8 ohm) "pure" class A stereo poweramplifier. It has the capability of operating in bridged mode, where one stereo poweramplifier becomes a 240 watt mono block. I would have assumed in bridged mode the power would double up to 120 watts at the very most, when each side is combined. I find it hard to believe the amp can pump out 240 watts of real class A power. Sounds like it secretly switches to class AB operation when used in bridged mode. What you think? I can send you a link to the PDF brochure in a private message, if you want a closer look.

I think I may know what you are talking about and I don't think you have to be too circumspect, HiFi News reviewed the Luxman 600 series amplifiers where the reviewer commented on on how much more powerful sounding the power amplifier sounded than its 30w (8ohm) Class A spec suggested. The technical review (by Paul Miller) pointed out that the only the first 30 watts was Class A where upon it switched to class AB to reach a maximum of 115 watts (8ohm)

Interestingly he also mentioned that the power consumption also went down by 100+ watts after an hour of use.

 

Edited by ServerBaboon - 12 Mar 2011 at 8:57pm

ServerBaboon wrote: I think I may know what you are talking about and I don't think you have to be too circumspect, HiFi News reviewed the Luxman 600 series amplifiers where the reviewer commented on on how much more powerful sounding the power amplifier sounded than its 30w (8ohm) Class A spec suggested. The technical review (by Paul Miller) pointed out that the only the first 30 watts was Class A where upon it switched to class AB to reach a maximum of 115 watts (8ohm)

They're all sliding bias ("ana-slog" amps that is) - they all operate class A to a greater or lesser degree

ServerBaboon wrote: Interestingly he also mentioned that the power consumption also went down by 100+ watts after an hour of use.

Probably sounded all over the place in the first hour too?

I make it 15 but I could be wrong - we'll soon find out though.

One piece of test gear required to test a power amp is the "dummy load". Obviously a signal generator and analyser is also required but we have these. But as the last power amp I tested was some 13 years ago in somebody else's factory, I don't have a dummy load, until today that is...







So here we have it!

I have no idea what the heatsink is rated at but would guess less than 1 degree K/watt. The best match to 8 Ohms has to be made up from available values, and here I've used 3R3 and 4R7 in series. The resistors are rated 50W but in use will probably derate to 25W each due to temperature rise. They are advertised as non-inductive (see http://docs-europe.electrocomponents.com/webdocs/0aad/0900766b80aad9a6.pdf). Although they have 5% tolerance, after measuring a short to subtract meter inaccuracy, you can see they're bang-on 8 Ohms per channel. As I'm working on a monobloc I can "piggy-back" the channels together and get a 4 Ohms load.

By choosing non-inductive resistors I can measure what the amp does into a purely resistive load. Then I can introduce some capacitance to mimic real cables. However, some "real" cables are very capacitive - the "snake-oil" types I mean. These can exhibit 2,000 - 3,000 pf and can do real damage to power amps. I'll test the monobloc to 2,000 pf which I tried on the simulator. After that who knows what will happen? I could put extra inductance in the output but that usually causes instability with good speaker cables... You can't win with "snake-oil"!

OK, here are the photos of the board after checking. I found one or two mistakes and the "eagle-eyed" may spot one after refering to earlier pictures...