Serial or parallel

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Serial or paralllel

K-amps said:
Jacco/Andrew/Stuart and others:

(Pinky feel free to delete this post as it does not relate to the KSA50 but then where will I find such talent in one thread. :devilr: )

I have a couple of Nelson's old Forte 3's. (200wpc -AB) they are running a 2 ohm load is taxing them (Sonically). Using just one channel of each amp... other not loaded.

I cannot help but think this is not efficient use of 2 power amps.

I have 2 options: Parallel bridge 2 channels to make one hefty channel. (By decoupling one channel's input VA stages and running both OP stages by the other channel's input/VA stage.)

Option 2: Run the Trafo primary in series, reducing the rails by 50% and running both channels serial bridged.


Questions:

1) Under which arrangement will I be able to get more class-A biasing (I am guessing parallel)
2) Will the serial bridged OP be able to run a 2 ohm load?
3) What kind of class-A levels can I expect from the serial bridge arrangement.

The current OP stage is 4 pairs of the 3281/1302's running off 75 vdc rails. After serially bridging them there will be 8 pairs running off 37.5 vdc rails.

I am leaning towards bridging because i have always liked the less grainy sound of many bridged amps (as long as the loads were not hairy and the rails were not high).

Once again apologies for sticking this question in this thread. you guys can reply on my personal email.

Best,
K-fed-up


jacco vermeulen said:
K,

How do you spot a con-artist ?
Easy, wooden shoes don't have laces !

Seriously,
if your Forte Audio M3 didn't hit that thermal braker on the heatsink and those nice Toshies didn't turn Central-Africa then the PS saved them because it's anaemic at 2 ohms use.
Which is not a miracle, i had one of the earlier Class A Forte models.
Worked great in low imp loads, but had about the same layout with way lower rail voltages.
(i used to have dreams about glueing a piece of paper on the front with SA-Slash printed on it)
Those big wattage Forte amps were supposed to run normal loudspeakers, not the crematory.

Whether you half the rail voltages and bridge it, or place the output stages in parallel, makes no difference.
Whether half rail voltage/number of parallel devices or full voltages/number can make a difference for the Tosh SOA.
But you'd still have the 2 Ohm load, and 75V across it.
=> same peak current=> same load the PS can't deliver.

Class A bias level makes little difference, whether parallelled or bridged. Expect a lower clean power figure when bridged, see my response to Stuarts post on bridging.

You can place a bull behind a cow, but if he did not have his cereals there be no flaggin the queen.
If you need more current:
- bigger toroid
- lower the effective voltage across the load

The pic down below is a bridged 100/8 monaural amp that had those Toshiba numbers.
It could be set to 2/1 ohms use through a lower PS voltage, 100Wattsthat in 2 and 160W in 1 Ohm.
1/2 the rail voltage and 2 times the output current=> same VA rating for the transformer.
That musician/designer guy from the Old Reich again
 
Well Jacco:

The PSU while Anemic is capable of more (Or I can change the tarfos no sweat).

Issue is: I think I am running the hfe's of the OP stage into non-linearity.

Tying the 2 channels will give me a couple of advantages:

1) Gain of the OP stage will be enough to drive 2 ohms... maybe?
2) Additional heatsinking available means I can bias it to some extent into class-A, making the rails a little more ridgid thereby giving me taut bass.
3) The extra pair of power caps that are loafing around are now in use.

The load is not tru 2 ohms... more like 2.66 ohms resistive with a 0.43 ohm inductor in series. (All resistive).

I am open to changing the trafos ... what i am trying to get at is a PSU and OP stage that is loafing when driving 2 ohms...
 
Of course it is, it's a crying shame to use only one channel.
And you can get a higher bias level, just think you need a fatter donut.

btw: overhere it was/is common to use 4 of the Toshibas for power levels up to 150 watts/8.
For Euro trash standards 8 of those per channel is a thick booty amp.
 
K-A,

btw:
The insides of the Threshold T-series give the impression of a beefed-up version of Forté Audio amps.
At least the ones i've seen, wouldn't surprise me one bit if the same board layouts were used.
I gathered that the Forté amps were manufactured elsewhere, but inspection took place at the Threshold location.
Would seem logical that when Mr Pass sold the shop in '91 and left the building with a mula suitcase, the Forté rights remained in the Threshold drawers.
Anyhow, i remember that Threshold issued an update package with Toshiba IGBTs to replace the T-series bipolar output devices.
The original output stage seemed prone to blow, not surprising if the T-series were Forté designs with a much chunkier powersupply in a fancier case.

You could take a look which drivers were used in your FAs.
The Forté model i had employed an output stage with 8/ch MT200 Toshibas(Sankens later i gathered) and TO220 Toshiba drivers that were connected to the heatsink through metal stand-offs.
With just a thermal sensor glued to the heatsink, that's it. Most boring PCB i've seen at the time, only interesting part was the 2 square inches area that contained the ccs-d differential and gain stage. PCBs weren't even attached secure to the heatsinks.
If you could replace the drivers by higher hFe versions you wouldn't have to be concerned that the output stage can not deliver enough current.
 
need schema...

Hi,

The chances of everything working perfectly with half the supply voltage and no other adjustments is not nil, but unlikely...

Once you have the schematics you should be able to tell which parts if any will need adjustment to keep currents the same etc,

I have never seen or heard a Forte, so I have no clue as to the degree of voltage independence that will be part of it's design, but if it is an NP design he would be a good person to ask...he is very generous with his time and replies to emails if you don't manage to attract his attention with this thread...

If you go for the lowering of the voltage, halving it may be too far, going for 2/3 voltage will probably get you closer to what you need, but not quite as simple to arrange, since the primary and secondaries are unlikely to allow it trivially. I've only done thumbnail calcs, but as far as I can see to go from 4ohm to 2 ohm 'competence' you want to drive 40% more current with 40% less voltage...giving you the same peak power into half the load...

HTH

Stuart
 
Thanks!

Good info on the 40%, I was also going to go with 60% rails. I do not have the schema yet, but I'd guess it is similar to the Adcom 555ii. they were produced around the same time give or take and both were Nelson designs. Ofcourse this is aguess.

No DC servos and newer parts than the GFA-555ii, i.e. 1302/3281 instead of 2sd424/B554.
 
K,

Just ran across this oldy on hFe, thought you might like to see just how good the hFe of the 1302/3281 is :

www.diyaudio.com/forums/showthread.php?postid=13938#post13938

The hFe comparison table is not out of the guy's imagination, but X'd from an oldy.

Asking the big guy himself by reposting in the Pass section is the best suggestion so far. He's given some assistance for questions on Forté amps before.
Having the output stage on half the rail voltage and rails for the front end with high enough voltage to get it going is another option, but a risky one.
 

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Sweet!

Almost like a horizon.


By the way (again I am guessing): but the Model 3 was designed to be interchangeable with Model 1? The 50 watt class-A version which was pretty much identical in every way (parts wise) to the 3.

Difference was, on the 3, Neslon had the trafo prims wired in paralell for higher rails and in the Model 1 had it wired in series for lower rails.

Also the main caps were 45v version not 90v wired in series for the model 3. Had he optimized the amp for 200w AB, He would not have used two 45v Caps in series but 90v versions.

That tells me the circuit had the same BOM, optimized for 38v rails (that could run on higher Vce's).

Additionally the buss bars that hold the caps have holes such that installed at a 90 degree, they can connect the caps in parallel vs. series for higher volts... too many clues ;)
 
Which brings me to trafos...

I have a 1kva Plitron 33-0-33vac and also have a couple of 1kVA (single tap) 43-0 's that together form a 2kVA 43-0-43.

I can use either one (leaning on the the dual 43-0's) but issue is, I have just one of each combo...


Anyone wanna trade??? :)
 
K-amps said:
the Model 3 was designed to be interchangeable with Model 1?

The Model 1-A is my guess.

The M-1 i had did 40 in A, managed slightly over 45/8 in class AB.
It was succeeded by the 1-A, with 60/8.
Boils down to the same thing though.
Only seen pics of the insides of the model 3, looked identical to the class A numbers.
I've always been impressed by Nelson Pass's cost-cutting skills.
The Forté brand was a budget entry line, the way they were set-up for swift manufacturing makes the universal approach obvious imo.
That approach looks recognisable in the First Watt amps, especially with the FW revamped Aleph-J amplifier.
I'm never quite sure what to find the most interesting, P's designing skills or his lean and mean business talents.
 
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