My First SS Amp (P101) - Transformer Advice Needed

[seabass23]

I'm planning to build a 2-Channel ESP P101 amp.

I have a broken NAD 2100 Amp from which I hope to salvage the case, transformer, heat sink, line-in connectors, and speaker connections.

I need some advice on re-using the transformer. I have no info on the transformer. The markings say "EI-96" but that's about it. I Googled all other markings but found nothing useful. This transformer has 3 pairs of output wires. Aside from the 120V input pair, it has a pair that goes to the impedance switch which must be used to short circuit the windings. From the output pairs I took the following AC voltage measurements.

Impedance Switch Position___4 Ohms___8 Ohms
----------------------------------------------
Purple/Purple________________55VAC_____55VAC
Blue/Blue___________________85VAC_____109VAC
Red/Red____________________58VAC_____72VAC

There is a green wire that emerges from the transformer at about the same place as the purples. Each purple to green gives me 27VAC. Can I properly interpret this as a 27-0-27 transformer if I use the purple and green wires and ignore the blue and red? Should I be using a different output pair?

Your expert advice is greatly appreciated!

Matt

[jaycee]

The service manual for the amp will probably help you:

Categorized Schematics and Service Manuals for free download I-N

Page 10 shows the power supply arrangement.

It's a little confusing - the impedance switch seems to change where the ground tap is connected. It looks like this amp may be a Class G design and so as well as the impedance switch, it has low and high voltage rails. You may find the high voltage rails are used to drive the frontend, and the low ones used to drive the output stage.

Looking at the PCB, green seems to be the centre tap for the violet winding. Those windings seem to be fused to 500mA, which feeds the lower voltage part of the amp. Not really suitable for power use.

I'd try and figure out where the start and end of each power winding is. The wires to the impedance switch should give you the info. Switch it to the 8 ohms position and find out which wires from the transformer are being used for ground. Then test from this point, to the other windings. The Violet-Green-Violet windings are seperate and you should ignore those.

If i had to guess, in 8 ohm mode, the supplies will be something like 70VDC symmetric, 48VDC symmetric.

[seabass23]

Thanks for your help!

I did some more investigating and came to the same conclusion about the purple pair. They do seem to power the lower voltage stuff so I'll ignore those.

I'll try search out the ground with the Impedance Switched to 8 ohms.

Based on the measurements I already took, I know that the (in the 8 ohm setting) the blue pair will give me 109VAC (-55VAC,+55VAC) and the red pair will give me 72VAC (-36VAC,+36VAC). Assuming I locate the wire used as the ground for each pair, which one should I use to power my amplifier? My understanding of the P101 design is that either supply voltage will work. I guess since I don't know the VA rating of the transformer, maybe I should stay to the lower voltage option?

I'm also unsure as to whether I should keep the Impedance switch in my new amplifier so I can toggle the output voltage, or should I hardwire to keep it in either the 8 ohm or 4 ohm setting?

Sorry for all the questions, I'm just glad to be getting some help!
Matt

[jaycee]

Personally I would use the 36VAC taps which will get you 50VDC unloaded when rectified and smoothed. This is about 90W of output power into 8 ohms, more than enough for domestic use IMO.

I wouldn't bother with the impedance switch - IMO this is a sign that NAD under-engineered the output stage and you had to set 4 ohms in order to prevent the thing blowing up!! The output MOSFETs used in P101 are quite rugged.

[seabass23]

Thanks for the advice! I'm going to go break out the multimeter and find a way to make this happen!

[seabass23]

Thanks to your advice, I think I figured it out.

There are Orange and Yellow wires going to the Impedance switch. With the switch in the 8ohm position, the yellows are shorted and the oranges are open. I believe the yellow is the center tap. I confirmed this by measuring voltage from Red to Yellow. As expected 36VAC on each!

I performed a very rudimentary load test by hooking up the red wires to a 100W incandescent bulb. Under this load, I had a voltage drop from 72.8VAC to 71.8VAC (this doesn't seem like much to me). I also ran the bulb for a good half hour and couldn't notice any heat buildup on the transformer.