Eden WT800 bridged mono not inverting

[Mick Bailey]

I have an early Eden WT800 where in bridged mono mode both power amps are in phase. Looking at the schematic on sheet 4 there's a signal on connector P3.4 called 'Invert Signal'. Anyone know where this goes?

Here's the schematic - poor, but it's the one that Eden sent me and I can't find a better version.

eden_wt500_wt600_wt800_sch.pdf

[MarkusBass]

My explanations were incorrect. Page 8 is exactly the same as page 4. Several pages are repeated in the schematic. Its quality is so low :-(.
I would look at page 14 - the block diagram. P3.4 is connected (page 13) through 12k resistor to LEFT SPEAKER OUT and through 27k resistor to FEEDBACK on RIGHT POWER AMP.

Mark

[J M Fahey]

I have an early Eden WT800 where in bridged mono mode both power amps are in phase. Looking at the schematic on sheet 4 there's a signal on connector P3.4 called 'Invert Signal'. Anyone know where this goes?

Here's the schematic - poor, but it's the one that Eden sent me and I can't find a better version.

Well, they shouldn't.

Normal mode of operation is both amps in phase, in parallel, fed the same signal and each drives its own speakers, a.k.a parallel mono, or stereo, or biamplified, or bridged mono, but in this case one needs an inverted signal and output jack goes from hot to hot.

Very poor schematic drawing, but I *suspect* they may use the inverted Line Out signal for the task and save 1 Op Amp ..... I would

[Jazz P Bass]

Here is the WT800C schematic.

It certainly is clearer.

[MarkusBass]

Do I understand correctly that there is no signal inverter in the preamp? The preamp provides signals in the same phase to both power amps. When the BRIDGE switch is ON, the right amp input is muted and the signal is provided by 12k+27k resistors from LEFT OUTPUT to FEEDBACK input of the right power amp, which is inverting. So either the switch has failed (and it is switching the LED only), or there is something wrong with the 27k resistor. But it is rather the switch.

[J M Fahey]

Interesting, congratulations on catching that obscure detail.
Yes, feeding one amp speaker output into the other one's NFB output, through a same (or *slightly* reduced) resistormakes the second one act as a unity gain inverter.
And you save an Op Amp

As we say here:"los centavos hacen los Pesos" meaning: "save cents today, they will eventually add into $$$ "

[MarkusBass]

If my findings are correct, this would be quite tricky way to bridge amps. I would check also the muting of the right amp. It seems to be a very simple problem but very difficult to find due to the quality of the schematics. I play bass and WT800 is considered a very good bass amp. But the quality of the schematics and the way it is manufactured is veeeeeeeeeeery far from perfect. I can only mention resistors in the power supply that heat up so much that they desolder themselves from the PC board.

Mark

[Mick Bailey]

This is where an extra pair of eyes is really helpful - I'd missed the way the power amps are coupled. I was expecting to see a more conventional inversion off the preamp output, so well spotted MarcusBass. I'm taking today off to go out on my motorcycle to catch a few last rays from a dying sun, but I'll be picking this up first thing tomorrow.

It is a good sounding amp, though this particular one has been serously damaged in the past and has had a lot of tracks on the power amps badly repaired. It was working though, so this fault is something new.

[J M Fahey]

It bit me too.
I felt sneaky imagining they were so cheap as to use the XLR out phase inverter also for that but they beat me, they were even cheaper

I shouldn't be surprised, I used the same trick in my old 12V battery powered amps, with 2 bridged TDA2003, go figure, where changing one jumper turned them from stereo 2 x 10W @ 2 ohms into bridged mono 20W @ 4 ohms ... just that I thought that too lowly a trick for "real" amplifiers .

As of Eden amplifiers, I'm certain they are a typical "inventor/designer" product, not something fully commercial like, say, Fender or Peavey.

So besides creating killer amps, Mr David "Eden" Nordschow must design PCBs himself (they have lots of flying wires where others would have tweaked the design to use printed tracks), front panels , hand draw schematics, etc. plus manufacturing in a relatively small scale.

Kudos to him, of course, for his creativity.

[Mick Bailey]

Fixed!

The switch intermittenty worked according to how fast/slow it was operated. The connections to the IDC connector were all bad and the braiding on the screened cable was intermittently shorting under slight flexing due to how the cable had been trimmed back. So, cleaned the switch, tidied the wiring and soldered up the connections to the IDC.

Much appreciated, thanks.