Simple Killer Amp - Listening impressions

[Stuey]

Well, I finally got the parts! Good old Aussie Post!

I won't be building until I've finished my preamp though.

Stuey

 

[float]

I'll have to think seriously about a single trafo - I was going dual mono....

If you were thinking dual mono, I'd stick with that. I've tried both ways, and monos are clearly better.
The extra horsepower bring better bass control -as you'd expect, but also better focus, separation and soundstaging.
It's a great amp anyway, even with just a single 300va.

 

[Stuey]

Well, Float, I may well do that. I haven't had anyone clearly state that it is better, but now you have!

What VA rating did you use, each?

I'm intending this to be my last power amp for a while (limited budget - young children) as I want to upgrade my turntable and DIY speakers, so I guess it should be as good as possible.

Thanks for the input.

Stuey

 

[float]

My PS evolution as follows;

single 300va, single bridge, one pair 22000uf caps

changed trans for 500va - slightly better bass when cranked up

double rectifier bridges - theoretically better, can't honestly say

2 x 500va + 4 bridges + pair 22000uf per channel - fundamental improvements across the board.

So dual mono is clear winner, no surprises really. I'm not using Gregs PS boards, but I'd expect the same benefits from those.

 

[KLe]

My PS evolution as follows;

single 300va, single bridge, one pair 22000uf caps

changed trans for 500va - slightly better bass when cranked up

double rectifier bridges - theoretically better, can't honestly say

2 x 500va + 4 bridges + pair 22000uf per channel - fundamental improvements across the board.

So dual mono is clear winner, no surprises really. I'm not using Gregs PS boards, but I'd expect the same benefits from those.

Thanks Float
You know that you have effectively done the equivalent of 1 x 1000VA + 2 bridges + 2 pair 22000uf per channel

So, I wonder if using a 1KVA tranny + 2 bridges + 2 pair 22000uf per channel would be equal to 2 x 500va + 4 bridges + pair 22000uf per channel ...

ciao

 

[float]

Thanks Float
You know that you have effectively done the equivalent of 1 x 1000VA + 2 bridges + 2 pair 22000uf per channel

So, I wonder if using a 1KVA tranny + 2 bridges + 2 pair 22000uf per channel would be equal to 2 x 500va + 4 bridges + pair 22000uf per channel ...

ciao

Aaaahhhh, I see I've been careless in my description. What I mean is actually one 500va dual sec per channel with a bridge and one 22000uf cap for each secondary.
So the total 2-channel amp has (only) 2 trannys, 4 bridges and 4 caps.
Sorry for confusion. I have considered using one transformer per phase though; if I ever think my back could take the punishment of lifting the thing, I might try it one day.

 

[float]

I should probably also say that I started with a single 300va transformer, and it was like this when I found it comparable to (and better than some) £3k commercial amps. More PS grunt is icing on the cake, not a necessity for good sound.

Be aware that 1kva of transformer can/will blow fuses, trip MCBs and mangle soft starts if you're not careful.

 

[KLe]

I should probably also say that I started with a single 300va transformer, and it was like this when I found it comparable to (and better than some) £3k commercial amps. More PS grunt is icing on the cake, not a necessity for good sound.

Be aware that 1kva of transformer can/will blow fuses, trip MCBs and mangle soft starts if you're not careful.

Hi Float
Yes, you are right I have listened to quite expensive amps and I am quite happy with my GB300D

I only use a 500VA for my 200watt/channel stereo GB300 and the sound is terrific, but a 1KVA would be to try ... and Greg's got a really nice soft start kit that would handle it without any problems

ciao

 

[jacco vermeulen]

Excellent is not to use a transformer with a VA rating higher than the total Pd number of the output stage, even more so for toroidals.
Which should be pretty obvious, money, power and otherwise.

 

[AndrewT]

mangle soft starts if you're not careful.

how do we avoid this?

or is it made up?

 

[float]

how do we avoid this?

or is it made up?

I'm just trying to pass on advice of others. Soft starts may be designed for various loads, and if your pcb tracks are too fine or your resistor/s too low rated then something may blow when you hit the switch. Common sense says to me that the soft start should be rated for the load involved.

 

[KLe]

Excellent is not to use a transformer with a VA rating higher than the total Pd number of the output stage, even more so for toroidals.
Which should be pretty obvious, money, power and otherwise.

Hi jacco

what is "Pd" ...

ciao

 

[Bratislav]

Hi jacco

what is "Pd" ...

ciao

Total power output transistors can dissipate(*). If output transistors can't sustain more than X watts, doesn't make sense to have toroid any bigger. In fact transformers are much more tolerant to overloading, typically they can be dimensioned at less than Pd (in some cases quite a bit less).

(*) this figure is significantly smaller than theoretical (catalog) figure. Case in point, MJ15003/4, rated 250W/16A. In a typical power amp I would feel uneasy giving them more than about 20W continuous duty ... and that is if heatsink is BIG.

 

[KLe]

Thanks Bratislav ...

 

[jacco vermeulen]

Yeah, i meant the total Max Pd of the output stage.

The total Max Pd number needs to be a multitude of the continuous output power for a Class AB output stage to perform ok.
A common sensible number is 4 times, which is also subscribed by Mr G Bally.
For a Class A output stage it'll be double that number, at least.
Of course, if you look at mass consumer hardware the multiplier is lower than 4, with soundraping protection circuitry sometimes even as low as 2 times the output in 8.

Raising the factor significantly above 4 only makes sense for driving really low impedance loads, such as killer ribbons and ESL's, same story for the transformers and PS lytics.
But as Bratislav mentioned, toroidal transformers can easily sustain twice the nominal load for longer periods, and toroids deliver much more oompf than EI's for peak output power.

Example of a biggy transformer amp : the late 80s/early 90s Solitaire power amp by your fellow Greek/Aussie Costas Metaxas.
The first generation 150W/8Ohm Solitaire started with 8 x 150W Pd Toshiba MT200 ringemitter output stages which were fed by a 1600VA EI transformer, = 67% of the total Max Pd number.
Costas buddy Gassmann in Switzerland dropped in a 2KVA transformer for the 220 Vac Euro models in the early 90s, enabling the Solitaire to peak at 700 watts continuous in 1 Ohm loads, still a block transformer, = 84%.

Availability of the Toshiba RETs stopped in the first half of the 90s, so MAS switched to 50/60MHz TO3P NEC 120Watt Pd devices in the output stages of the Solitaire. (the NEC A1227/C2987)
Consequently, the total max Pd number for the 2 output stages became 2x8x120= 1920 Watt.
The transformer rating for the NEC version was limited to 1.6KVA, = 84% again.
A bunch of the Solitaire MKII models came with a 1KVA transformer, feeding both output stages means 52%.

On a monaural Class AB GB150d there's no gain in going higher than a 500VA toroid transformer.
Real biggies only make sense in Full Class A amps and monster output stage Class AB's.
Such as the 2.5KVA toroid in the monaural RR Model 7, which is about 6 times the nominal continuous output in 8 Ohm but still only half of the output stage total Max Pd number.

 

[KLe]

Thanks jacco ... excellent summary.

For a stereo Class AB GB150D would you go as high as a 1000VA?

What would you use for a stereo Class AB GB300D?
It has about twice the Pd of the GB150 ...

I am currently using a 500VA for my GB300D, but I have a 1000VA that I am going to drop in ...
It sounds like a 1000VA is only just large enough ...

ciao

 

[Cobra2]

Drop it in...there is no such thing as a too large trafo. (or freezers, or harddrives...).

A bigger trafo keeps its regulation much tighter, and with all amps I have built, it seems that they sound better with an oversized trafo. Not always the same story with adding caps...

Arne K

 

[jacco vermeulen]

there is no such thing as a too large trafo.

There is a thing called sanity.
Me, even in a stereo encasing i favor 1 amp on 1 transformer, rather 2 times 400VA than a single 1KVA.
A single 1KVA for two GB300s is a bit on the lean side.

 

[KLe]

Thanks Cobra2, yes PS Caps are another issue ...


Thanks jacco, I thought that 1000KVA was a little low, but I have 40-0-40 available for use ...
I am always amazed by just how good the GB300 sounds with the 500VA that I am currently using. So, there should be immediate improvements when I drop in the 1000VA ... amazing really ...

ciao

 

[KLe]

GB150D PSRR Tweak

Hi all 150D'iyers

Greg’s found a tweak for it, which replaces the Zener Mod. It increases the PSRR by 10dB in the range 100Hz to 10KHz to better than -130db.

You find all the details here ... “Simple New Tweak for GB150D” … http://ska-audio.com/Forum/YaBB.pl?num=1197570631/0

Have a look and let me know what you think?

ciao