Serial or paralllel
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...
By the way does anyone have the schematics of the Forte model 3?
At 7 amps output current the old Toshibas still had hFe values above 100 at operation temperatures.
With 4 devices in parallel that's 28 amps, enough to push to above 50 volts continuous in 2 ohms.
I doubt that NP designed the drivers for higher hFe levels.
So whats the verdict? It's not worth messing with?
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.
Serial or parallel?
Serial is much easier to implement plus I can get more power into 8 ohms for other applications. and i don't rip out the original circuitry, just a resistor in the right place and Bias reset and i am done.
At 70Vce the Toshibas do:
10 mS-10 Amps
100mS- 3 amps
10 mS-30 Amps
What do you reckon ?
Depending on which donuts I get, the rails could be 50-60Vce.
While a bridged apparatus will suck twice the current (offset by the 2x OP stage) I reckon I go "balanced" :)
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.
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